ectonic - iewarchiv.uni-graz.atiewarchiv.uni-graz.at/abschluss/files/Diss_Andreas... · 76 7.1.4...

Te toni Setting ofearly- to late Pan-Afri an Stru turesfrom the Mozambique Belt inSE Kenya and NE TanzaniaDissertationzur Erlangung desDoktorgradesder Naturwissens haftli hen Fakult�atan derKarl{Franzens{Universit�atin Grazvorgelegt amInstitut f�ur Geologie und Pal�aontologievonAndreas BauernhoferGraz, im Juli 2003

ContentsZusammenfassung 11 Regional Geology 31.1 The Pan-Afri an Orogeny . . . . . . . . . . . . . . . . . . . . 31.1.1 Timing of events . . . . . . . . . . . . . . . . . . . . . 41.1.2 Ophiolites . . . . . . . . . . . . . . . . . . . . . . . . . 72 The northern Mozamibque Belt 112.1 Geologi al overview(Kenya - NE Tanzania) . . . . . . . . . . . . . . . . . . . . . . 112.1.1 Introdu tion . . . . . . . . . . . . . . . . . . . . . . . . 112.1.2 Stru tural domains . . . . . . . . . . . . . . . . . . . . 112.1.3 Te tono-metamorphi evolution . . . . . . . . . . . . . 143 SE Kenya - NE Tanzania 193.1 Study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.1 Lo ation . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.2 Previous work . . . . . . . . . . . . . . . . . . . . . . . 193.1.3 SE Kenya (re ent work) . . . . . . . . . . . . . . . . . 213.1.4 NE Tanzania (re ent work) . . . . . . . . . . . . . . . 264 Stru tural investigations 314.1 Classi� ation and des ription . . . . . . . . . . . . . . . . . . 314.1.1 Te tonostratigraphy . . . . . . . . . . . . . . . . . . . 314.1.2 Field observations . . . . . . . . . . . . . . . . . . . . . 315 Geo hronology 475.1 Previous dating . . . . . . . . . . . . . . . . . . . . . . . . . . 475.1.1 Taita Hills and adja ent areas . . . . . . . . . . . . . . 475.1.2 Pare-Usambara mountains . . . . . . . . . . . . . . . . 485.2 Present study . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

6 CONTENTS5.2.1 40Ar/39Ar - Sm/Nd system . . . . . . . . . . . . . . . . 495.2.2 Cooling history . . . . . . . . . . . . . . . . . . . . . . 546 Strain estimations 596.1 Purpose and method . . . . . . . . . . . . . . . . . . . . . . . 596.1.1 Geographi al s ope . . . . . . . . . . . . . . . . . . . . 596.1.2 Field method and restri tions . . . . . . . . . . . . . . 596.1.3 Evaluation of data . . . . . . . . . . . . . . . . . . . . 616.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 626.2.1 Galana East (I) . . . . . . . . . . . . . . . . . . . . . . 626.2.2 Galana West (II) . . . . . . . . . . . . . . . . . . . . . 626.2.3 Transition zone (IV) . . . . . . . . . . . . . . . . . . . 636.2.4 Taita Hills (III) . . . . . . . . . . . . . . . . . . . . . . 646.3 Convergent te toni s and strain patterns . . . . . . . . . . . . 666.3.1 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 666.3.2 Comparison . . . . . . . . . . . . . . . . . . . . . . . . 677 Magneti fabri studies 757.1 Purpose and method . . . . . . . . . . . . . . . . . . . . . . . 757.1.1 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . 757.1.2 Theory and de�nitions . . . . . . . . . . . . . . . . . . 757.1.3 Measurements . . . . . . . . . . . . . . . . . . . . . . . 767.1.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 798 Geo hemistry 878.1 Metabasites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 878.1.1 O uren e . . . . . . . . . . . . . . . . . . . . . . . . . 878.1.2 SE Kenya . . . . . . . . . . . . . . . . . . . . . . . . . 878.1.3 NE Tanzania . . . . . . . . . . . . . . . . . . . . . . . 958.2 Orthogneisses (metagranitoids) . . . . . . . . . . . . . . . . . 988.2.1 Galana River pro�le - Taita Hills . . . . . . . . . . . . 988.2.2 Con lusions . . . . . . . . . . . . . . . . . . . . . . . . 1009 Dis ussion 1139.1 Voi area - Pare-Usambara mountains . . . . . . . . . . . . . . 1139.1.1 General onsiderations . . . . . . . . . . . . . . . . . . 1139.1.2 Te toni evolution (early Pan-Afri an) . . . . . . . . . 1169.1.3 Shear te toni s (late Pan-Afri an) . . . . . . . . . . . . 120Danksagung 125Referen es 127

ZusammenfassungDie vorliegende Dissertation war Teil des Projektes P-12375-GEO "Geo-dynami setting of the Panafri an Orogeny in Eastern Afri a", das Gebie-te in S�udost-Kenya (Taita Hills - Tsavo East National Park) und Nordost-Tanzania (Pare-Usambara Mountains) zum Thema hatte. Die Finanzierungdes Vorhabens wurde dur h den den �osterrei his hen "Fonds zur F�orderungder wissens haftli hen Fors hung (FWF)" si hergestellt. Ein weiterer Berei hder Projektarbeit umfasste eine Kooperation der Institute f�ur Geologie undPal�aontologie bzw. Mineralogie und Petrologie der Universit�at Graz mit denUniversi�aten Nairobi (Kenia) und Dar es Salaam (Tanzania).Zielsetzung war, in Anlehnung an fr�uhere Arbeiten, Daten f�ur die tek-tonostratigraphis he Gliederung sowie tektonis he Entwi klung und -Stellungdieser Areale innerhalb des Mozambique Belt Ost-Afrikas zu erheben. Zusam-men mit dem n�ordli h gelegenen Arabis h-Nubis hen S hild bilden die Restedieses Gebirgszuges das Kernst�u k des ostafrikanis hen Orogens, wel hes zuBeginn des Phanerozoikums zur Bildung des Gro�kontinentes Gondwanaf�uhrte. Die Fragestellung gliederte si h in Detailstudien mit S hwerpunktenzur kinematis he Entwi klung, Verformungs- und Gef�ugeanalyse, Geo hrono-logie und Geo hemie.Informationen �uber kinematis he Aspekte der Deformation einzelner tek-tonis her Zonen wurden im Gel�ande erfasst. Ebenso wurde versu ht anhandvon in situ Messungen die generelle Verformungsgeometrie von strukturellabgrenzbaren Berei hen abzus h�atzen. Erg�anzend dazu sollten Gef�ugeunter-su hungen mittels der Anisotropie der magnetis hen Suszeptibili�at (AMS)Aufs hlu� �uber Mineralregelungen geben und etwaige Korrelationen zu denVerformungsdaten aufzeigen. Eine zeitli he Reihenfolge von Metamorphoseer-eignissen, zum Teil �ubereinstimmend mit der tektonis hen Gliederung, konn-te dur h die Bestimmung von Mineralabk�uhlaltern (40Ar/39Ar-Datierung)na hgewiesen werden.Den Abs hlu� dieser Arbeit stellten geo hemis he Betra htungen dar.Untersu ht wurden Orthogneise und Metabasite, die Hinweise �uber die geotek-tonis he Position des Gebietes w�ahrend der Orogenese liefern sollten. Die1

2 Zusammenfassunggegenw�artigen Ergebnisse lassen die S hlu�folgerung zu, dass dieses Gebiet,unter Ber�u ksi htigung einer m�ogli hen Ver�anderung der prim�aren Signaturder Gesteine (z. B. ho hgradige Metamorphose), die Ges hehnisse rund umdie Enstehung eines Kollisionsorogens widerspiegeln.

Chapter 1Regional Geology1.1 The Pan-Afri an OrogenyThe Pan-Afri an Orogeny is thought to be one of the most important orogeni events whi h united the ontinental mass Gondwanaland by amalgamationof several preexisting rustal domains. Kennedy (1964) introdu ed the term"Pan-Afri an Thermo-Te toni Episode" to hara terize the stru tural dif-ferentiation of Afri a into stable ratons and mobile belts during the late Pre- ambrian and the early Paleozoi . Kr�oner (1984) proposed a time span from950 - 450 Ma for these thermo-te toni events whi h are present throughoutGondwanaland and Laurasia (Fig. 1.1). The Pan-Afri an belt ontains a lotmore of features typi al of plate te toni pro esses like abundant ophiolites,nappe empla ement, al -al aline batholiths, vol ani sequen es and imma-ture lasti sediments than the older rust formed between 1.8 and 1.0 Ga.Burke and Dewey (1972) reported granulite o uren es suggesting rustalthi kening as a result of plate ollision.M Williams (1981) and Stern (1994) argued that the assemblage of Gond-wana during the Neoproterozoi mainly onsists of two fragments - East-and West Gondwana - whi h ollided along the Mozambique belt of EastAfri a and its ontinuation to the north, the Arabien Nubian Shield (ANS)(Fig. 1.1). The term "Mozambique Belt (MB)" was introdu ed by Holmes(1951). Stern (1994) suggested the term "East Afri an Orogen" for the en-tire belt of the ANS and the MB. The ophiolites, granulites and stru tures ofthe East Afri an Orogen are onsidered to be fossile fragments of a neopro-terozoi Wilson y le, representing the opening and losing of a o ean basin(Mozambique Ozean) whi h was bordered by the older rustal blo ks of Eastand West Gondwanaland. 3

4 CHAPTER 1. REGIONAL GEOLOGY

Figure 1.1: Gondwana at the beginning of the Pre ambrian (from Kr�oner(1991)).1.1.1 Timing of eventsRiftingThere is mu h eviden e for a rifting stage within the Pan-Afri an regions.This is usually interpreted as the break-up of the pre eeding super ontinentRondinia (Fig. 1.2) as outlined by several authors (Moores (1991), Ho�-man (1991), Dalziel (1991)). Aula ogen-like stru tures o ur in south- entralAfri a (Zambezi belt) whi h are dated at 880 Ma (U-Pb on zir ons, Hansonet al. (1989)) and in Sudan the Atmur-Delgo suture, dated at 750 Ma withthe Sm-Nd isotopi system (Denkler et al., 1993). These data were inter-preted as a minimum age for a rifting event in northeast Afri a. Passive ontinental margin sediments like quartzites, semipeliti gneisses and ar-bonates whi h o ur in Kenya are assumed to be deposited during rustalextension between 840 and 770 Ma (Key et al., 1989). U-Pb zir on ages onbimodal vol ani s from the Zambezi Belt (Wilson et al. (1993), Hanson et al.(1993)) of 879 � 19 Ma are interpreted together with lasti sediments andshallow marin arbonates to represent extension and thermal subsiden e.CollisionIt is ommon opinion to onsider the Mozambique Belt of Central Gond-wana as a fossile remnant of a ontinent- ontinent ollision (e.g. Burke and

1.1. THE PAN-AFRICAN OROGENY 5

Rock

elides

Moza

mbiq

ue

Bel

t

African

West

LufilianArc

Gabon Congo

Angola

W.

Congo

Gab

on

Zimbabwe

Zam

bia

Kasai

Tan

zanian

KapvaalFigure 1.2: Con�guration of Rodinia at 750 Ma and an ient ratoni domainsof Afri a (from Meert and Van der Voo (1996); Dalziel (1992)).Dewey (1972)). However, onstellations of ontinents and number of te toni - metamorphi events responsible for the formation of the Mozambique beltitself and an united Gondwana are not that straight forward.

6 CHAPTER 1. REGIONAL GEOLOGYHo�man (1991), suggested a fan-like losure of an assumed MozambiqueO ean about a pole to the south (today South-Afri a). This would result inthe produ tion of large areas of o eani rust in the north and only smallerareas in the south. This is in a ordan e to �eld observations that ma� -to ultrama� sequen es are abundant in the ANS and less frequent in EastAfri a. Stern (1994) proposed a major ollisional event between 850-600 Mafollowed by es ape te toni s until 550 Ma. Dalziel (1992) indi ated the ex-isten e of a se ond short-lived super ontinent formed by rifting events and rustal separation (600-550 Ma) in eastern Laurentia (Bond et al. (1984),Williams and His ott (1987), Meert et al. (1994)). This on�guration existedbetween 650 and 550 Ma on the eastern Margin of Laurentia together withan united Gondwana. In these models the Mozambique Belt is seen as theresult of ollision between fragments of West Gondwana and a oherentlytreated East Gondwana. However the on ept of a "unique" East Gondwanahas been questioned (e.g. Kr�oner (1994), Wilson et al. (1995)).Another model (e.g. Meert et al. (1995), Meert and Van der Voo (1996))favours a polyphase a rretion of rustal domains to onstru t an united Gond-wana. On the basis of paleomagnetism and isotopi ages these authors on- luded that the assamblage of the eastern portion of Gondwana o ured dur-ing two separate orogeni events be ause of a non ommon apparent polarwander path for East- and Central Gondwana prior to 550 Ma (Fig. 1.3). The�rst, denoted as East Afri an Orogeny (sensu Stern (1994)), lasted from 800to 650 Ma and resulted in a ollision between the Congo raton and the IM-SLEK terranes (India, Madagas ar, Sri Lanka, Enderby Land, and the Kala-hari raton). Based on paleomagneti eviden es and widespread granulite fa- ies metamorphism at about 550 Ma (Kr�oner (1994), Shiriashi et al. (1994),Kriegsman (1995)) in Sri Lanka, Madagas ar and parts of Antar ti a (En-derby Land), the authors propose a distin t event, the Kuunga Orogeny, rep-resenting the ollision of the earlier East Afri an Orogen (Congo-IMSLEK)with Australo-Antar ti a.These di�erent per eptions of the te toni evolution of the Pan-Afri anOrogen - prolonged y le or polyphase development - are re e ted in theinterpretation of related metamorphi events. Appel et al. (1998) and M�olleret al. (2000) dis ussed the ne essity of a ontinent- ontinent ollision to pro-du e high grade metamorphism earlier than 600 Ma in the eastern granuliteterranes of the entral Mozambique Belt in Tanzania. They on luded thatgranulite fa ies metamorphism was related to magmati underplating pro- esses. If this is true, then the time of peak metamorphism would not be asyn hronous measure of a ollisional event (M�oller et al., 2000).


Figure 1.3: Early Cambrian path of Gondwana and plate te toni re onstru -tions (from Meert and Van der Voo (1996).1.1.2 OphiolitesO urren eOphiolite belts are believed to represent losed o ean basins or the sitesof an ient destru tive plate margins. Areas with abundant ophiolites arepresevered in the ANS (Fig. 1.4) with ages ranging in between 880 to 690Ma. They are often pla ed as nappes (Shanti and Roobol (1979), Chur h(1988), Abdelsalam and Stern (1993)) and asso iated either with suturingbetween ar terranes or juvenile rust (Wallbre her et al. (1993), Fritz et al.(1996), Abdelsalam and Stern (1996)) of the ANS and older rust to thewest. Together with related vol ano- lasti and immature sediments they

8 CHAPTER 1. REGIONAL GEOLOGYare on luded to indi ate ba k-ar or for-ar , or ar settings (Pri e (1984),Pallister et al. (1988), Berh�e (1990), Rahman (1993), Neumayr et al. (1996))with a strong "supra-subdu tion-zone" geo hemi al signature (Pri e (1984)).Although there is an abundan e of possibly related ma� - to ultrama� ro ks in the ANS, these ro ks are s ar e in East Afri a. In the northern partsof the Mozambique belt (Kenya, Ethiopia) several o uren es are reported.Geo hemi al investigations on the Adola-Moyale (SE Ethiopia/NE Kenya),Baragoi (Central Kenya) ophiolite omplexes (Berh�e (1988), Berh�e (1990))and Sekerr and Yubdo omplex (Pri e, 1984) are onsistent with the datafrom the entral part of the Arabian Nubian Shield. The umulates, island-ar tholeiites, MORB- and al -al aline basalts are asso iated with deepwatermetasediments, vol ano lasti s and ontinental shelf sediments on�rming anar - to ba k-ar environment. More to the south in the Mtito Andei - TaitaHills area (SE Kenya) ophioliti remnants (Pohl et al. (1980), Fris h and Pohl(1986)) have been identi�ed, too. Just a ross the border in northern Tanzanias attered isolated ma� to ultrama� bodies are embedded in the granulites ofthe Pare Mountains (Bagnall, 1960). Considering geo hmi al onstraints andthe deep rustal environment, Prohaska and Pohl (1983) argued against anophioliti asso iation, but suggested a sub- ontinental origin of these bodies.SuturesRelationships between ophiolite belts and the evolution of the ANS are dis- ussed on numerous works (Bakor et al. (1976), Garson and Shalaby (1976),Gass (1977), Sha kleton (1979; 1986) Sha kleton (1981), Vail (1983; 1985),Camp (1984), Kr�oner (1985), Stoeser and Camp (1985), Abdelsalam andStern (1996)). Camp (1984) gave the �rst ompilation of suture zones andophiolite belts in Saudi Arabia. In East Afri a, however, the stru tural on-ne tions between di�erent ophiolite o uren es were not always do umented.Berh�e and Rothery (1986) attempted to ombine �eld data and remote sens-ing te hniques for areas in Kenya, Ethiopia and Sudan. Berh�e (1990) tra edout 5 possible sutures extending from the ANS to East Afri a (Fig. 1.4) and a6th between India and Madagaskar. Sha kleton (1996) dis ussed the lo ationof a �nal suture between East- and West Gondwana and suggested a olli-sional belt (a tive between 680 to 550 Ma) ontinuing from the Nabiath zonein Arabia to the Baragoi ophiolite in north entral Kenya ( omparable to (4)in Fig. 1.4) and via the Cobu�e (northeastern Mozambique) and Mani a Belts(along eastern margin of the Zimbabwe raton) to Dronning maud Land inAntar ti a.But there also have been riti al omments on the geote toni position ofophiolites. Based on observations from the Arabian-Nubian Shield, Chur h


Figure 1.4: Ophiolite belts in NE and E Afri a. Saudi Arabia and Mada-gas ar in pre-drift re onstru tion (from Berh�e (1990)). Inset: (1) SolHamed Wadi (2) Port Sudan-Qala al Nahal- Ingessana-Moroto (Afri a),Jebel Thurwah (Arabia); (3) Barka-Yubdo-Sekerr-Itiso (Afri a); (4) Baragoi(Turkana)-Mpwapwa (Afri a); (5) Adola-Moyale-Taita-Pare- ?Na hingewa-Morrola (Afri a), Idsas (Arabia); (6) Madagas ar-?Mait (Afri a). TRMTanganyika-Rukwa-Malawi-lineament.(1988) doubted the usefulness of ultrama� ro ks to indi ate fossile suturezones. Many ophiolite o uren es in NE- and East Afri a appear to be de-ta hed and transported. Eviden e for the strong in uen e of horizontal te -toni s are reported espe ially in Kenya and Tanzania (Mosely (1993), Sha k-

10 CHAPTER 1. REGIONAL GEOLOGYleton (1986, 1993a) Key et al. (1989), Hepworth (1972)). Sha kleton (1988)stated that many ophiolites have been transported far from the site of theoriginal sutures.A lot of of du tile shear zones and faults (Fig. 1.4), Fig. 2.1) importantfor the te toni evolution of rustal domains and ophiolites in the ANS andthe higher grade basement areas of East Afri a trend in a NW-SE array.The Nadjd fault system (Saudi Arabia and Egypt) formed between 630-560Ma (Fle k et al. (1979), Sta ey and Agar (1985)) was generally assumed toindi ate a late Pre ambrian ollision event between the Arabien Shield anda rigid indenter east of the Idas suture (S hmidt et al. (1979), Fle k et al.(1979), Davies (1984)). In ontrast, Stern (1985) related the nearly paralleloriented Najd faults to a major episode of extension in northernmost Afro-Arabia postdating a ollisional event. By reason of the presen e of NE-SWoriented faults Berh�e (1988) onsidered the NW-trending faults and shearzones to be a a onjugate set of strike-slip faulting aused by ontinent- ontinent ollision.

Chapter 2The northern Mozamibque Belt2.1 Geologi al overview(Kenya - NE Tanzania)2.1.1 Introdu tionThe Mozambique Belt of East Afri a, as de�ned by Holmes (1951), extendsover more than 5000 km from Mozambique to Sudan and Ethiopia (Fig. 1.1)and onsists of high grade gneisses, migmatites, amphibolites, marbles ands hists intruded by granitoids. The main sedimentary a umulation of anassumed Mozambique basin is thought to have o ured between 1200 and820 Ma. In luding orrelations of Baker (1965) and Key et al. (1989), thereare three main groupings (Mosely (1993)). An initial arena ous ontinental lasti su ession apparently extended a ross a thinned and downwarpingbasin or rift site oevally inter alated with basal shales and oarser lasti units. Su eeding shelf sediments like limestones, sandstones with arbona- eous limestones and aluminous shales developed later on a ross part of thebasin.At least two ollisional events (e.g., Sha kleton (1993b)) are thought tohave a�e ted the late Proterozoi Mozambique belt do umented in di�erentte toni styles and metamorphi overprints.2.1.2 Stru tural domainsKenyaBased primarily on stru tural riteria, Mosely (1993) proposed three se torsthroughout the basement ro ks in Kenya separated by late-stage steep, up-right, du tile shear zones (see Fig. 2.1), often with a sinistral shear sense. De-11

12 CHAPTER 2. THE NORTHERN MOZAMIBQUE BELTSudan

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Brittle/ductile fracture, inferred

Aproximate position of kyanite/sillimaniteisograd

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Figure 2.1: Te toni sket h of some Pre ambrian stru tures of Kenya (sim-pli�ed after Mosely (1993)).spite of di�erent stru tural styles (fold vergen e and kinemati indi ators),lithologi al units remain similar and possibly repeated throughout with ageneral on ordan e in the metamorphi history between ea h of these se -tors.In the western se tor, W - to NW verging thrust piles with te toni allyrepeated sequen es prevail (Fig. 2.1). NW to SE dire ted stret hing lin-eations and asso iated displa ements are ommon. Sanders (1965) showed

2.1. GEOLOGICAL OVERVIEW (KENYA - NE TANZANIA) 13the strongly te tonised and intersli ed nature of the Mozambique belt ro ksand the older granite-greenstone terrane of the Nyanzian Shield (around lakeVi toria). Large s ale duplexes and omplex fold patterns are do umented inthe Loita hills (Fig. 2.1) and Masai Mara where early re umbent folds withNE-trending hinges beeing refolded on NW axes. Mosely (1993) argued thatthe varian e in attitude of re umbent folds may indi ate a lo al gravitational ollapse.The entral se tor (Fig. 2.1) o urs as a narrow zone in the southwestbordered by the Athi-Ikutha- and Mutito shear zone whi h widens towardsthe northwest until the Sekerr-Elgeyo lineament. Flat-lying neararly horizon-tal thrust sta ks with generally ESE-WNW trending axial tra es (Saba hian;Key et al. (1989)) are dominant. They are refoliated by two su eeding events(Baragoian and Barsaloian, Fig. 2.1) whereas the latter appears as uprightdu tile shear zones. To the east of the Barsaloian zone, W-verging, E-dippingthrusts show a te toni ally repeated sequen e of paragneisses lo ally inter-digitated with a migmatiti basement of Kibaran age (�1200 Ma) .The NW-SE trending Mesozoi Anza Graben (Fig. 2.1) in northeastKenya obs ures the relationships between the eastern se tor and ro ks of the entral and western se tor. Close to the Ethiopian and Somali borders similarlithologies o ur and di�er from the rest of Kenya by the abundan e of gran-ites and granitoid gneisses. Flat lying east verging nappes are hara teristi for the te toni style. The N-S trending and steeply westward dipping Bunashear zone (Williams and Matheson, 1991a), southeast of Moyale, marks thewestern border of the eastern se tor. It extends northwards into Ethiopia andis probably related with the Mutito shear zone to the south (Mosely (1993)).A vol anosedimentary sequen e and parts of ultram� s spreading around theKenya-Ethiopia border (Fig. 2.1) are thought to have been thrust from eithernortheast or northwest (Beraki et al. (1989), Bogliotti (1989)) over a gneissi basement. This is onsistent with the assumption of a te toni inter�ngeringof the ANS and the Mozambique Belt ro ks.TanzaniaA regional approa h to tra e out the di�erent te toni styles in northeasternTanzania was taken by Hepworth and Kennerly (1970) and Hepworth (1972).They established four domains re ognizable throughout the orogen. Three do-mains, regarded as Kondoan, Bongan and eyed-folds (superimposed folding),are onsidered to be of Mozambiquian age (Pan-Afri an), the Parangan isrelated to a pre eeding event.The Parangan domain in ludes a group of amphibolite fa ies graniti gneisses, amphibolites and quartzites and extends from the Tanzania ra-

14 CHAPTER 2. THE NORTHERN MOZAMIBQUE BELTtoni margin about 100 km to the east. A northeasterly, west dipping trendis retained in the north and related to two phases of nearly oaxial folding.To the south the trend turns gradually into a ommon N-S trend. Superim-posed Kondonan te tonism reates a wide s atter of S-surfa es and fold axesarranged in a subhorizontal girdle with maxima trending NE. The presen eof the Kondoan deformation is do umented in a se ond girdle with a shallowSE plunging maximum of axes.The following Kondoan, onsidered as the main Mozambiquian phase ofdeformation (Hepworth, 1972), aused reworking of the Parangan domainsinto L-te tonites and aggy gneisses where deformation was most intense.The domain is re ognizable in the same range as the Parangan. A strongre lined folding about southeasterly plunging axes with at or east dippingplanar foliations is typi al for the Kondoan. Asymmetri folds are frequentand a strong lineation due to interse tion plunges to the southeast.Late stage folding upon NE or NNE axis (Bongan) o ur only in restri tedareas near the Ar hean raton to the west. The te toni style is dominatedby exure folds with axial planes either upright or overturned to the west.2.1.3 Te tono-metamorphi evolutionKenyaIn north and entral Kenya (Fig. 2.1) four te tonothermal events of Pan-Afri an age have been re ognized (Key et al. (1989), Charsley et al. (1984))by means of detailed mapping, stru tural investigations and ombined agedating on Rb/Sr and K/Ar isotopi systems of syn- to late te toni gran-itoids, dykes and ma� intrusives (Fig. 2.2). The a ompanying metamor-phism rea hed amphibolite to granulite fa ies exe ept for the last asso iatedwith greens hist fa ies onditions.The Samburuan-Saba hian (� 830� 800 Ma; see western se tor, Mosely(1993)) event reated upper amphibolite- or granulite fa ies mineral assem-blages responsible for the oarse gneissi fabri of all upper rustal ro ks.Dis ordant quartzo-feldspathi veins form open ylindri al folds. A silliman-ite/kyanite isograde is present with kyanite on�ned to the east. After thethermal peak, the ro ks were deformed by major re umbent folding and re-lated low-angle thrusting to the SE, with gentle westerly plunging axial tra estrending in the ar NE-SW to ESE-WNW. S, SL and L fabri s are frequent.Close to major thrusts, underlying the ophioliti slabs, the growth of kyaniteis hara teristi in appropriate lithologies. The empla ement of vol ani sli esas allo hthonous oherent slabs an be regarded as an important e�e t of theSaba hian event whi h is related to plate ollision (Key et al., 1989) oblique

2.1. GEOLOGICAL OVERVIEW (KENYA - NE TANZANIA) 15

Figure 2.2: Correlation of te toni style and ages of Pan-Afri an ro ks inKenya (from Sha kleton (1993b)).to the orogeni strike (Key et al. (1989), Sha kleton and Ries (1984)), Berh�e(1990)).In the intervall from 780-730 Ma ages are mainly reported from Ethiopiaand represent a period of magmatism. An age date of 766 � 40 Ma (Suwaet al., 1979) is onsidered as intrusion age of a granite followed by deformationand metamorphism. The su eeding period (� 100 Ma) is poorly onstrainedby metamorphi and stru tural events. In Tanzania ages of about 700 Ma areoriginally related to granulite fa ies metamorphism ((Maboko et al., 1985)but later re-interpreted as intrusion ages (M�oller et al., 2000). Continued rustal shortening (� 630� 570 Ma) resulted in trans urrent shearing on a ontinental s ale reating losely spa ed, steep upright N-S or NNW-SSE du -tile shear zones with dominant left lateral shearing. Asso iated L-te tonitefabri s are ommon as well as partial melts due to high grade metamorphism(Mosely (1993)).Upright or gently in lined folds with NNW-SSE to NW-SE plunging ax-ial tra es are asso iated to Baragoian (� 620 Ma) te toni s. They re e t aENE-WSW ompression a ross the orogen. Small s ale stru tures are similar, on entri , harmoni and disharmoni folds, and minor renulations of gneis-sosity. Interferen e patterns exist between minor Saba hian and Baragoianfolds. O uring sinistral shear zones are either lo alized stru tures in�lled byfelsi veins and/or ma� to ultrama� pods or do umented in large s ale by

16 CHAPTER 2. THE NORTHERN MOZAMIBQUE BELTdispla ement in the range of kilometres. S, SL and L fabri s are ommon.The following Barsaloian event (� 580 Ma) generated domains whi hare up to 20 km wide and several hundred kilometres in length. They forman en �e helon array of generally N-S trending shear or straigthening zones(Hepworth, 1967). Verti al planes lo ally de�ne upright, tight to iso linalfolds. Lineations plunge gently to the north, and in a few parts, to the south.The sense of shear is inferred to be dextral. Auto hthonous migmatites andparauto hthonous graniti melts o ur lo ally.Late stage deformation (post 580 Ma), a ompanied by Greens hist fa- ies metamorphism, is do umented as open folding on NE-SW- trending ax-ial tra es (Loldaikan). Lo ally parallel and SSE-NNW trending shears area entuated by narrow migmati zones and show sinistral displa ement. Fi-nally, E-W trending asymmetri open folds with synkinemati metadioriteand metadolerite dyke and sheet empla ement are regarded as Kipsingianevent, sporadi ally a ompanied by small s ale thrusting. A retrogressivegreens hist fa ies metamorphism aused overgrowth of biotite in the dykesand seri itisation and hlorite growth in the ountry ro ks.NE-TanzaniaRe ent work fo using on age relations within the Mozambique Belt in Tanza-nia (M�oller et al. (1998)), revealed three di�erent rustal age domains usingNd and Sr model ages and Pb isotopes on lea hed feldspars. 3.3-2.7 Ga areobtained from metagranitoids from the Tanzania raton and e logites andamphibolite fa ies ro ks from the adja ent Usagaran belt. The granulites ofNE Tanzania (Pare-Usambara mountains, Umba- Wami river) are explainedas juvenile rustal additions ranging from 1.5 to 1.1 Ga. This interpretationis orroborated by data of Muhongo et al. (2001) and Maboko and Naka-mura (2001) partly displaying younger ages. A third group, restri ted to theUluguru Mountains yields ages of 2.6-2.1 and may represent a mixing of rustal material of di�erent ages (M�oller et al., 1998). Maboko (2000) arguedfor the onsisten e of neoar hean ages (�2.5-2.8) Ma throughout the northernMozambique Belt in Tanzania ex luding the northeastern granulites. Similarages (U-Pb, zir on) were eviden ed for the Wami River region by Muhongoet al. (2001).Datings whi h aim at metamorphi overprints of distin t domains show� 2550 Ma (Bell and Dodson (1981), Rb-Sr whole ro k) for the ar hean raton, ages of � 2000 Ma (Lenoir et al. (1994), U-Pb zir on, M�oller et al.(1995), U-Pb monazite) for the adja end sited Ubendian- and Usagaran Beltsand a Pan-Afri an high grade event of 610-655 Ma (Coolen et al. (1982),Muhongo and Lenoir (1994), U-Pb zir on (bowth); M�oller et al. (2000), U-

2.1. GEOLOGICAL OVERVIEW (KENYA - NE TANZANIA) 17Pb monazite; Muhongo et al. (2001), Shrimp and evaporation te hnique).Similarities on erning metamorphism, stru ture and lithology of di�erentPan-Afri an granulite distri ts in eastern Tanzania are eviden ed by Coolen(1980) and Appel et al. (1998). Peak metamorphi onditions indi ate 810�40ÆC and pressures of 9.5 to 11 kbar throughout the areas and a ommonmetamorphi history was suggested by an anti- lo kwise P-T-t path (Appelet al., 1998).From Kenya both lo kwise and anti- lo kwise P-T-t paths are reported(Mosely (1993); Fig. 2.1). In the western se tor the abundan e of kyanite andsubsequent overprinting by sillimanite is in a ordan e with a ollisional eventresulting in large s ale thrust te toni s. High pressure ro ks are ommon inthe western part of the western se tor, indi ated by the abundan e of kyanite(Temperly, 1953) and spinel simple tites (high pressure metabasite oronas),a broadening of the garnet stability �eld a ross all lithologies and the lo aldevelopment of rutile in titanium-ri h mineralogies (Temperly (1953), Bear(1955)). Variable metamorphi onditions are do umented by temperaturesof 500-700ÆC and pressures of 4-10 kbar (Key and Hill (1989), Suwa et al.(1979)).The entral se tor di�ers qualitatively in the high-temperature hara terof metamorphism (sillimanite grade and higher). Fabri s are generally more oarse grained and granulite fa ies metamorphism is rea hed in restri tedareas.Both types of P-T-t paths are inferred to exist in the eastern se tor. Inthe northeast, ultram� s around Adola (Ethiopia, see Fig. 2.1) and their on-tinuation to Moyale (near the Ethiopian border) form a thrust sta k togetherwith a gneissi and migmati "basement" and a low grade metamorphi se-quen e. The indi ated P-T-t path is lo kwise and ontrasts with eviden esfrom sillimanite-bearing gneisses of the Buna area (Fig. 2.1). Williams andMatheson (1991b) showed two phases of sillimanite growth. The minor on-tent of kyanite in these paragneisses was related to an anti- lo kwise P-T-tpath (Bohlen, 1987).Paleo- to Mesoproterozoi ages are hardly reported throughout the Mozam-bique Belt of Kenya. Nd model ages Harris et al. (1984) indi ate Ubendianages (� 1940 Ma) for the Turbo migmatites and the Mari h granite in westernKenya near the Ar hean raton. In north- entral Kenya a Kibaran basement(Mukogo Migmatites, �1200 Ma, Rb/Sr) was suggested to form the oor ofthe Mozambique Belt (Key et al., 1989).

18 CHAPTER 2. THE NORTHERN MOZAMIBQUE BELT

Chapter 3SE Kenya - NE Tanzania3.1 Study area3.1.1 Lo ationThe study area is situated within a re tangle extending about longitude37Æ30'00" - 39Æ00'00" E and latitude 3Æ00'00" - 5Æ00'00" S. In southeasternKenya it omprises predominantly the Taita Hills (near Voi), bordered by theTsavo West National Park and the main road Nairobi-Mombasa as well asparts of the Voi-South Yatta area along the Galana River within the TsavoEast National Park (Fig. 3.1). In Tanzania the work on entrated on parti -ular distri ts of the Pare-Usambara mountains and the western peneplain aswell as parts of the Umba Steppe to the east (Fig. 3.5).3.1.2 Previous workEarly travels and reportsOne of the earliest foreign visitors of the area were the missionaries Rebmann(1847) and Krapf (1849 and 1851) who journeyed to the Taita Hills andUsambara mountains and Ukambani. During his �rst visit in 1847 Rebmandes ribed the solitary Kasigau mountain (Fig. 3.5) as "stret hing about oneleague and a half from south to north and near its entre rea hing its highestsummit whi h onsists of an enormous mass of ro k and is, for the mostpart, ompletely perpendi ular" (Krapf, 1860). R. Thorton (1862) des ribedin a letter to Sir R. I. Mur hison the Kasigau mountain as "a high, narrowpre ipitous mountain omposed of old, rystalline metamorphi ro k in thi kbeds dipping to the east at about 5Æ".Charles New (1871) and J. Thomson (1883) were among the next to om-19

20 CHAPTER 3. SE KENYA - NE TANZANIA

Orthogneiss

Lugard’s F

Am-Migmatiteand amphibolites

Migmatites andgneisses

Metapelite

Marble

Amphibolite

Red Soil

gn

amg

amg

am

am am

mp

mb

mb

mb

mb

mbgn

amg

amg

am

am

am

am

am

mp

mp

mp

mp

mp

mb

mb

mb

mb

10 km

Galana Westn: 91 max: 26.0

1.02.14.69.7

20.8F

n: 90 max: 19.0

1.02.03.97.7

15.2L

Transitionzone

n: 31 max: 9

1.01.62.74.47.2 L

n: 29 max: 19

1.02.03.97.7

15.2F

n: 183 max: 34.2

1.02.35.2

12.027.4

n: 174 max: 30.5

1.02.24.9

11.024.4F L

Taita Hills

alls

Ithangethi

max: 21.0n: 69

1.02.04.18.3

16.8

n: 66 max: 14.0

1.01.83.36.1

11.2F L

Galana East

mp

Figure 3.1: Geologi al overview-map of the Voi area (Hauzenberger et al.,2003) ompiled from Sanders (1963), Pohl and Niedermayr (1979) and Horkelet al. (1979). Inserted diagrams show ontoured orientations (multiples ofeven distribution; equal area, lower hemisphere) of lineations and foliationpoles for di�erent te toni domains. Sample lo ations of geo hemi al dataare indi ated as open (orthogneisses)- and �lled (amphibolites) ir les.

3.1. STUDY AREA 21ment on the dry and sparsely inhabited area between the oast and MountKilimanjaro (Forbes-Watson (1951), Loftus (1951)). A physiographi al de-s ription and reasonable a urate map of some topographi heights of thearea (e.g. Sagala Hills, see Fig. 3.1) was given by H�ohnel (1890). Other visi-tors of this region in luded Kersten and Baron von der De ken (1879), Hilde-brandt (1879) and Meyer (1890).Various ro ks types su h as Duruma sandstones, rystalline limestones, bi-otite and hornblende gneisses and pegmatites were olle ted during di�erentvisits. Roth (1864), Beyri h (1878) and Shearson (1889) gave petrographi aldes riptions on numerous of these samples. Von Rei henba h (1896) summa-rized the results of the early travellers to this part of Kenya, but did nothimself visit East Afri a.In his journey from the oast to Lake Vi toria, Wal ot Gibson (Gibson,1893) passed through the area and onsidered there was an un onformity(whi h later turned out to be a fault onta t) between the oastal sedimentaryro ks and the metamorphi basement. Hobley (1895) ommented on ro kyridges of pink gneiss traversing the ountry in a north-south dire tion in theTsavo Valley and limestones and metamorphi ro ks west of Voi. Prior (1903)des ribed some geolgi al samples whi h were taken by Gregory (1894, 1896)along the Galana River (Gregory, 1894). In 1903, Walker reported on parts ofthe area and was followed by Mu� (Maufe) who mentioned the hornblendi and garnetiferous ro ks of Voi in his general des ription of gneisses of thispart of Afri a (Mu�, 1908).In 1919 Gregory returned to Kenya and made further journeys in theColony in ompany with C. W. Hobley, W. M Gregor Ross and H. L. Sikes.Two years later a geologi al a ount of the Galana River se tion was pub-lished (Gregory, 1921) and parts of the Voi distri t are noted by Krenkel ina geologi al summery of Afri a (Krenkel, 1925).3.1.3 SE Kenya (re ent work)Area south of the Taita HillsTo the south of the Taita Hills Walsh (1960) mapped a su ession of para-and orthogneisses in luding marbles, graphite gneisses and s hists, and bi-otite gneisses. The inferred metamorphi history in ludes amphibolite fa- ies onditions followed by abundant alkali metasomatism and granitization.Stru turally the sequen e form a uniformly west verging and ENE dipping(15-35Æ) overturned fold pattern with a gentle (10Æ) northerly dip of axes.Lineations measured throughout the area oin ide with the axial data andthus represent b-lineations. Walsh (1960) related this folding to a single or

22 CHAPTER 3. SE KENYA - NE TANZANIAdi�erent events with similar pressures dire ted from the east. The all overnorthward dipping of lineations and fold axes, ommon within this part ofKenya, were explained by the o uren e of east-west faults of an unknownevent of late Pre ambrian age. Although not mapped in the study area Walsh(1960) referred to a stru ture lo ated at the southern border of the TaitaHills. Parkinson (1947) already mapped an east-west fault in the Mtito-AndeiTsavo area and related this to a possible overthrust from the north.Voi-South Yatta areaSanders (1963) mapped and des ribed the geology east and northeast of theTaita Hills hoosing a pro�le se tion along the Galana River whi h trans-se ts the lithologies at high angle in an east-west dire tion. The easternmostpart of the area is overed by the upper Palaeozoi Duruma sandstones, asequen e of arkoses with lo ally o uring sandstones, shales and limestones.They are faulted against the eastern portions of the Basement system, theSobo formation, omprising a su ession of gneisses predominatly of metased-imentary origin whi h was metamorphosed under amphibolite- to granulitefa ies onditions. Garnetiferous s hists and granulites, thin quartzites, oarsegrained marbles, �ssile hornblendi gneisses and dark garnet amphibolites aremost frequent and show an average SW-NE plunge of 20Æ-40Æ. Migmatitesare subordinate ompared to the western parts of the area. At Ithangethithe onditions hange suddenly. An east verging thrust onta t gives way toa NNW-SSE striking steep westerly dipping omplex dominated by bandedand ontorted migmati biotite-hornblende gneisses whi h en lose a variableamount of massive pegmatite segregations (Lugard's Falls belt). Migmatizedgarnetiferous and diopside bearing gneisses and to a minor extend marblesare inter alated. The ro ks were subje ted to intense plasti deformation un-der high grade metamorphi onditions, indi ated by ow folds and pin hand swell stru tures of pegmatites.The western ontinuation of these zone in ludes iso linally folded gar-netiferous and graphiti gneisses with inter alated strips of thin hornblendi and hornblende-garnet gneisses. Following along the strike the sequen e anbe found agagain at the Sagala Hills to the south (Fig. 3.1). Here the gneis-ses often ontain diopsidi lenses and s hlieren pointing at a more al areous omposition of the protoliths.Kasigau-Kurase areaTo the southeast Saggerson (1962) divided the mapped basement ro ks intoa Kurase- and Kasigau series whi h were later rede�ned as Kurase- and Kasi-

3.1. STUDY AREA 23gau group (Pohl and Niedermayr, 1979). The �rst group mainly onsists ofkyanite and graphite bearing quartz-ri h peliti gneisses and is hara terizedby numerus marbles. The overlying Kasigau group in ludes peliti , al are-ous and semi- al areous ro ks like biotite-hornblende-, kyanite-garnet- andkyanite-biotite-gneisses, quartz-feldspar- and granitoid gneisses. Both unitswere metamorphosed under amphibolite fa ies onditions, granulite fa ies iseviden ed by al -sili ate- and garnet bearing para-granulites. Migmatizationis lo ally important whereas alkali metasomatism is ommon.Two deformation events, inferred from di�erent style of folding and s at-tering of b-lineations, are suggested to have a�e ted the whole area. The�rst, dominant in the Kurase group, produ ed a number of re umbent andoverturned west-verging folds with NNE plunging fold axes. The average dipof foliations is about 30Æ to the east. Open folding with northerly plungingfold axes hara terizes the se ond event, whi h is typi al for the Kasigaugroup. The absen e of re umbent and overturned folds ontradi ts an ear-lier deformation of the Kasigau group but may be proved by examination ofsmall s ale stru tures (Saggerson, 1962). Alternatively, disharmoni foldingwas onsidered to a ount for this distin t fold pattern, as no dis ordan ewas observed between the metasedimentary units. Common features of de-formation are rotated boudins, shear bands, and pin h and swell stru tures.Taita Hills (Mtito-Andei)Parkinson (1947) gave a �rst geologi al des ription of the Taita Hills (in lud-ing a map of s ale 1:250.000) and divided the Basement system, relating totheir sedimentary protoliths, into an arena eous and an argilla eous group.Farquhar (1960) on entrated his geologi al studies to the northern TaitaHills with parti ular interest in asbestos mineralization and the related ul-trama� host ro ks.During the mid-seventies the geology of the Taita Hills and surroundingareas were mapped and des ribed (e.g., Pohl and Niedermayr (1979), Horkelet al. (1979)) within a ooperation proje t of AUSTROMINERAL and theKenyan government with parti ular interest in gemstone exploration.Referring to Saggerson (1962) the lithostratigraphi al subdivison into aKurase and Kasigau group was kept (Pohl and Niedermayr (1979), Horkelet al. (1979)) and the latter divided into numerous formations. A te toni allyempla ed older harno kiti to granuliti basement was thought to serve asshelf for mio-geosyn linal metasediments and basi vol ani s (Kurase group),on whi h arbonates were sedimented mainly on swells and peliti sequen esin en losed basins (Pohl et al., 1980).The overlying Kasigau group indi ates a marked hange of sedimentary

24 CHAPTER 3. SE KENYA - NE TANZANIA onditions. Monotonous eu-geosyn linal metasediments now present as Qtz-Fsp-Bt-Hbl-gneisses (formerly metagreywa kes) and inter alated amphibo-lites were asso iated with a ontinental margin or a fast downwarping basin.Repeated horizons of serpentinites, pyroxene-tal -amphibole ro ks andamphibolites (derived from dunites, peridotites and pyroxene-peridotites) o - ur as remnants of metamorphosed o eani rust.Metamorphism attained upper amphibolite fa ies onditions with slightlydi�erent grade (630-700ÆC) for Kurase and Kasigau group. The onsiderablevariation of pressure (3.5 to 7.5 kbar) was related to di�erent metamorphi onditions within the groups or two metamorphi events (shifting from kyan-ite to the stability �eld of sillimanite). Migmatization is ubiquitous as wellas asso iated anate ti pegmatites and segregations.A polyphase stru tural evolution (Pohl and Niedermayr (1979), Horkelet al. (1979), Pohl et al. (1980)) was inferred to embra e the area (see Fig. 3.2)starting with open or iso linal exural slip folds (F1), mainly dipping to theB3

B1 B2

p1 p 2

sf1

sf2Figure 3.2: Three phases of folding identi�able in the Taita Hills (from Pohlet al. (1980)).NNW. They are widespread in adja ent areas (e.g. to the north) but almost ompletely erased by su eeding deformation in the Taita Hills. Possibly orresponding overturned to tight re umbent folds with the same orientationof fold axes are frequent to the south (e.g. Mwatate area), where additionally ross-folding an be found lo ally. Contemporary ultrama� s, harno kitesand granulites were assumed to have been empla ed.The following deformation (F2), asso iated with abundant migmatization, aused shear-, ow- and ptygmati folds, boudinage of anate ti segregations(pegmatoids) and o asionally fold interferen e patterns. Fold axes and lin-eations (mainly b-lineations) plunge gently to the NNE. As a result thrustinggenerated intense deformation and realignment of ultrama� omplexes alonglithostratigraphi al boundaries.Large open folds (F3) and gentle exures subparallel to F2 fold axes repre-sent a late stage deformation and ontrol the joint pattern and empla ementof pegmatites.

3.1. STUDY AREA 25Most re ent petrologi al- and thermobarometri studies (Fig. 3.3, Fig. 3.4)as well as age datings (Hauzenberger et al. (2000), Hauzenberger et al. (2003)eviden ed the multiple metamorphi history of the Taita Hills and the GalanaRiver area. Granulite fa ies is attained throughout both domains with peakP

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T [°C]Figure 3.3: P-T-t path for Galana River area (Hauzenberger et al. (2003).metamorphi onditions of 700-800ÆC and 8-10 kbar for the eastern GalanaRiver (Sobo Formation) and parts of the Lugard's Falls belt (Galana Rivershear zone). Similar temperatures (800-820ÆC) but notably di�ering pres-sures of 10-12 kbar were indi ated for the western belt (in the followingtermed Transition zone) and the Taita Hills. A lower amphibolite fa ies (upto 570ÆC and �5 kbar) was only re orded for the Taita Hills, e.g., as a tino-lite rims around hornblende of amphibolites. Possibly, this ould be relatedto the in uen e of the later metamorphism in the Galana River area. Basedon the breakdown rea tion of kyanite to sillimanite, assumed to have hap-pened at peak metamorphism, and P/T data of retograded metasediments,Hauzenberger et al. (2000) proposed a lo kwise P-T-path.

26 CHAPTER 3. SE KENYA - NE TANZANIAP

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T [°C]Figure 3.4: P-T-t path for the Taita Hills and Transition zone (Hauzenbergeret al. (2003).3.1.4 NE Tanzania (re ent work)Pare - Usambara mountainsThe granulites and granuliti gneisses of the North-Pare mountains (Bagnall,1960) and the adja ent South-Pare and Usambara mountains represent athi k sequen e of enderbiti to harno kiti gneisses (Appel et al. (1998),Fig. 3.5) with inter alated arbona eous and al areous layers.Common types in the North-Pare mountains are hornblende- and py-roxene granulites, quartz-feldspar- and al -sili ate granulites. Asso iatedlithologies are kyanite-, graphite- and biotite gneisses and s hists, quartzites,marbles and amphibolites, whi h often form thin on ordant strips or lensesof variable lateral extension.Metamorphosed basi and ultrama� ro ks o ur as single dyke like bod-ies of di�erent size omprising metagabbroids and meta-anorthosites, py-

3.1. STUDY AREA 27

Same

North

South

Pare M

ountain

s Usam

bara Mountains

Pare

-

Mkomazi

Lushoto

Mombo

Kitumby quarry

Anorthosite - an

Marbles and calcsilicate rocks - mc

Metapelites - mp

Umba River

Mwaki-jembe

Daluni

Umba

Tsavo

Galana River

m1m2

m3

mp

an

mc

mb

Metabasites/ultramafites - mb

Granulites - gr

gr

Mesozoic/Cenozoic cover

Non-differentiated Basement (mainlymigmatic Orthogneisses ± granulites)

Thrust

Shear zone boundary

Shear zone

Faults (observed)

lithostratigraphical boundary

Strike-slip tectonics (map scale)

Faults (nature unkown)

cross-striking zones(with folding)

Voi

Kasigau

Sagala Hills

Kasigau group

Kurase group

Mto

nga - K

ore

charn

okite co

mplex

Kinyiki H

ill

Dembwathrust

Wundanyi

Dembwa

Mwatate

Wanjalathrust

MgamaRidge Nyangala

20 km

Taita Hills

n: 75 max: 18.0

1.01.93.87.4

14.4

62 : 13.0

1.01.83.25.8

10.4

n: max

n: 37 max: 9.0

1.01.62.74.47.2

n: 46

F L

max: 7.0

1.01.52.43.65.6

cross-strikingzones

F L

UsambaraMoun-tains

Manyani

n: 58 max: 12.0

1.01.83.15.59.6

n: 63 max: 24.0

1.02.14.49.2

19.2F L

PareMoun-tains

gwr 583.7 8.1±

gwr 586.2 6.1±

hbl 559.1 5.4±

hbl 563.7 6.1±

bt 548 9.1±

Mbt 556 3.0±(excess Ar)

Mhbl 518 15±

hbl 524.7 5.0±hbl 519.3 5.1±

Mbt 490.0 3.0±

Mhbl 530.0 3.0±

Mhbl 542.0 8.0±

bt 500.9 4.9±

gwr 537.0 19.0±

ms 505.6 5.1±

Lugard’sFalls

?

?

hbl 524.5 4.9±bt 490.4 4.7±

hbl 570.1 5.0±

gwr 547.6 5.9±

n: 13 max: 12.0

1.01.83.15.59.6

n: 28 max: 14.0

1.01.83.36.11.2

F L

UmbaSteppe

1

gwr 529.0 6.0±

Figure 3.5: Study area with Pare-Usambara mountains and Umba Steppe. Re-drawn (in luding some stru tual data) after Bagnall (1960), Dundas (1965),Bagnall et al. (1963), Hartley and Moore (1965), M�oller et al. (2000). Bound-ary of Kurase-Kasigau group from Saggerson (1962) and Pohl et al. (1980).Not all isolated out rops of the Umba Steppe and the western peneplain areshown. Ar/Ar ooling ages of hornblende, biotite and mus ovite (hbl; bt;ms) are indi ated. Data of M�oller (1995) are denoted by pre�xed M. Sm/Ndgarnet-whole ro k ages (Hauzenberger et al. (2003); pre�x gwr) are presentedadditionally.

28 CHAPTER 3. SE KENYA - NE TANZANIAroxenites and serpentinites. Where exposed they often appear as lenti ularshaped and massive at out rop s ale.Throughout the area pegmatites of di�erent age (Bagnall, 1960) an befound. Con ordant and foliated pegmatites whi h were later ut by dis ordantand unfoliated types are oldest. Late dyke-like pegmatites are verti al or westdipping and trend in a north-west dire tion.The main foliation of the granulites was asso iated with a phase of re- umbent folding about northeast trending axes. A later episode of lo alized ross-folding reated axes of north-westerly strike and was probably relatedto thrust te toni s.The most frequent ro k types of the South Pare mountains (Dundas,1965) are banded garnet, pyroxene and hornblende granulites. O asionallygraphite gneisses are inter alated. Lo ally metasomatism aused a oarse-grained, poorly foliated and massive appearen e of the granulites.At Ikongwe a metamorphosed gabbroi anorthosite (Fig. 3.5) forms asto k-like body. It grades from a marginal planar stru ture, indi ated bydis ontinous laminae and lenses of ma� onstituents, into a massive type hara terized by oronal stru ture of garnet surrounding pyroxene and horn-blende. Small pods of ultrabasi intrusions are s attered over the area andmay represent the ma� equivalent to anorthosites (Dundas, 1965).Generally the granulites dip to ENE at moderate angles (up to 40Æ).About 15-20 km west of Same a gently NE plunging re umbent fold (notshown) reminds of the stru tural style present to the north. Related minorfolds and lineations plunge also to the NE. Numerous small s ale folds andlineations s attering within a NE-ESE dire tion are typi al throughout thearea.To the south the mountain range ontinues with the Usambara mountains(Bagnall et al., 1963). Again hornblende, pyroxene and quartzo-feldspati granulites form the main mass of the ro ks with sporadi ally embedded al -sili ates, metapelites and amphibolites. Distin t bands of hornblende pyrox-ene granulites are frequent. Biotite gneisses are ommon, but as in the Paremountains, better exposed in the western plain areas. They were probablyformed by metasomatism of the granulites whi h led to a retrogression toamphibolite fa ies onditions. Migmatites o ur along fault zones within themountains. Pegmatites are widespread and orrespond to the pattern re og-nized in the North-Pare Mountains.Tight re umbent folds are dominant stru tues. They were subje ted tosevere axial plane shearing and display shallowly NE plunging axes. Minorwarping along NNW axes was asso iated to a su eeding event of ross fold-ing. Widespread are NNW striking faults with onjugated NNE faults. Aseries of NE trending wren h faults appear to o�set the NNW faults.

3.1. STUDY AREA 29The eastern part of the Usambara mountains and the area to the north-east, the Umba Steppe, (Hartley and Moore, 1965) outlines the easternmosto uren e of pre ambrian ro ks in the study area. Metapelites, al sili atesand marbles are the main lithologies of the Umba Steppe wheras typi alUsambara-type granulites or gneisses (varying hornblende-pyroxene ontent)are almost absent. Ex ept for a reasonably sized serpentinite body (and asso- iated amphibolites) west of Gerevi Hills (Fig. 3.5) ma� to ultrama� ro ks an only be found as small isolated bodies, as usual for the whole area.Foliation planes strike predominantly north-south with a gradual hangeof a onspi uous lineation trend from 45Æ in the south to �15Æ in the north.Re umbent iso linal folds are frequent and two periods of folding were as-sumed to a�e t the area as reported from adja ent domains. Faults are om-mon espe ially in the easternmost parts of the Usambara mountains whereprominent NNE faults form parallel oriented valleys. Another fault systemstrikes north-south.Sha kleton (1993a) dedu ed an upward transition and strain gradientfrom intense planar shearing and asso iated linear fabri s in the granulitesto a more in lined fold and thrust deformation in stru tural upper levels(and overlying metasediments of the Umba Steppe) whi h was assumend tohave happened later. He related the present stru tures to the early Samburan-Saba hian event of north- entral Kenya (Key et al., 1989) with a likely trans-verse motion following the widespread E-W lineation trend of the Mozam-bique Belt (see Fig. 3.6). Parallel oriented folds of di�erent s ale are on- luded to be a ute sheath folds. The slow bending of lineations into a on-spi ous N-S trend north of the Pare-Usambara mountains may demonstratein remental strains rooted in strong late stage Mozambiquian deformationwhi h su essively rotated the earlier re umbent folds (Sha kleton, 1993a).Petrologi al studies (e.g. M�oller (1995), Appel et al. (1998)) display thatgranulite fa ies metamorphism was a ompanied by high pressures and slight-ly lower onditions for the Umba Steppe but a similar geothermal gradient.Peak metamorphism was dated to take pla e between 610-655 Ma (M�olleret al. (2000), Muhongo et al. (2001)). Slow ooling rates of 2-5ÆC (M�olleret al., 2000) and an observed anti- lo kwise P-T-t path (Appel et al., 1998)support the opinion that intrusions (magmati underplating) into the lowerand middle rust originated high grade metamorphism prior to a ollisionalevent.

30 CHAPTER 3. SE KENYA - NE TANZANIA

Figure 3.6: Gradual hange of stret hing lineations from E-W to N-S in thenorthern Mozambique Belt (from Sha kleton (1993a)).

Chapter 4Stru tural investigations4.1 Classi� ation and des ription4.1.1 Te tonostratigraphyThe area near Voi (Fig. 3.1, Fig. 4.1) omprises three te tonostratigraphi units: Galana East- and West (I, II), to the east within the Tsavo EastNational Park and the Taita Hills (III), lo ated some km west of Voi. Anarrow zone (IV) indi ates the transition between the Taita Hills and GalanaWest.This subdivison is based on di�erent prevailing te toni styles in ea h ofthese domains (see Fig. 4.1) and orresponds to the lithostratigraphi Soboformation (Sanders, 1963) in the eastern Galana se tion (Galana East), theLugard's Falls belt (Galana West) and parts of the Kurase-Kasigau group inthe Taita Hills (Pohl et al., 1980) to the west. A possible orrelation of Soboformation and Kurase group was supposed by Saggerson (1962).A brief omment on stru tural observations of the neighbouring Pare-Usambara mountains and the Umba Steppe will omplete the studies andshould allow a more omprehensive view of the eastern portions of the Pan-Afri an Mozambique Belt in this area. Unlike in SE Kenya this is not givenin terms of a te tonostratigraphy as work depended mu h more on spe i� se tions. Stru tural relationships of domains were not studied in parti ular.4.1.2 Field observationsExposuresUsing the maps of Sanders (1963) and Horkel et al. (1979), �eld work on- entrated on parti ular se tions throughout the area. Unit I-II were studied31

32 CHAPTER 4. STRUCTURAL INVESTIGATIONS

Taita Hills (unit III)

Transition zone (IV)

Galana West (unit II) Galana East (unit I)

Taita Hills - Galana River

Figure 4.1: Simpli�ed stru tural blo k diagrams representing the te tonos-tratigraphi units near Voi. Variably sized and deformed graniti (pegmatiti )- to migmatiti and ma� veins are typi al. Shear ( ow) foldsand -bands are ommon stru tures. Thrusts and folded strata (marbles) asindi ated for unit I are based on the pro�le of Sanders (1963).The boudi-naged bla k layer in the Taita Hills marks the border between Kurase- andKasigau group.mainly along the Galana River, and to the South following motorable tra ks.unit III (Taita Hills) by following road uts whi h often follow the main li�sin a north-southerly dire tion. The most ommon ro k types in all units aremigmati Am-Bt-Fsp-Qtz�Grt�Cpx�S ap orthogneisses with inter alatedamphibolites (Hauzenberger et al., 2000). Metasedimentary ro ks in lusive oarse grained graphite bearing marbles, Cpx-Grt-Am-Pl-C �Wo al sili- ates and Grt-Bt-Ky-Sil-Fsp-Qtz-Rt metapelites are frequent in units I andIII ((Hauzenberger et al., 2003), see also Sanders (1963); Horkel et al. (1979)).In NE Tanzania the work was based on maps of Bagnall (1960), Dundas(1965) and Hartley and Moore (1965)) in luding pro�les from the peneplainup to the mountains (e.g. Mombo-Lushoto), exposures along the NNE-SSWtrending main road from Arusha to Tanga and roads of similar trend at thebase of the prin ipal li�s of the Pare-Usambara mountain range. Two shortE-W pro�les (15-20 km) were taken in the western peneplain and one E-Wpro�le was investigated along the Umba River from Mwakijembe to Umba

4.1. CLASSIFICATION AND DESCRIPTION 33(Fig. 3.5).Galana East(I)Typi al stru tures in the easternmost basement ro ks are moderateley (upto 40Æ) south-westward and gently or even subhorizontally to the NE (E)dipping foliation planes. They reate an open (gentle) folded appearan e ofthe largely metasedimentary sequen e (Fig. 4.1). At out rop s ale folds oftenemerge as losed or tight stru tures, iso linal (e.g. migmatized layers) and re- umbent examples an be found additonally (Fig. 4.2, Fig. 4.3). Asymmetri folds with layer-parallel limbs, partly attened, are numerous and indi ateENE (NE)-WSW (SW) shortening with a likely NNE-NE (E) dire ted senseof shear. Boudins o asionally display an internal fabri whi h substantiatesthe in uen e of non- oaxial deformation.Looking at the lineation pattern (Fig. 4.1) the situation be omes moreun lear. Aside from a luster oriented NNW (NW)-SSE (SE) we also �ndsubordinate NNE-SSW and E-W trends. Measured fold axes oin ide withthe NW-SE strike whi h also forms the �-pole to the distribution of folia-tion planes. The nature of the minor trends, however, is less evident. Theymay either represent remaining b- or stret hing lineations of an earlier event(thrusting?) or ould be related to the a tual Pan-Afri an te toni s. Our �eldobservations do not allow an unequivo al determination as thrusts, if theyexist, might be in onspi ous (Sanders, 1963). Kinemati interpretation fromthin se tion studies often stay, like in other domains, ambiguous as oarsegrained re rystallized fabri s dominate as a result of high grade metamorphi onditions.Galana West (II)Immediately to the west, at Ithangethi (Fig. 3.1), a nearly 30 km eastward ex-tending domain of predominatly steep (�70Æ) WSW dipping foliation planesmarks the Galana River shear zone, an integral part of prin ipal lineamentstransse ting the Mozambique Belt of Kenya in a N-S to NNW-SSE dire -tion (Fig. 2.1). Common ro k types are migmati Am-Bt-orthogneisses ofgranodioriti -tonaliti omposition whi h were ut by variably oriented peg-matiti (granitoidi ) veins. Marbles and para-gneisses ontaining garnet arerare in ontrast to unit I. Amphibolites and possibly also metagabbros (�Grtbearing), sporadi ally inter alated as on ordant strips or small pods, be omemore frequent in the western part.Lineations (Fig. 3.1) show a strong preferred orientation trending NNW-SSE with a gentle (�15Æ), more regular dip to the NNW. Steeper dipping


Figure 4.2: strongly deformed and iso linally folded migmati gneiss (aboutone km east of Hippo point). Height of stru ture about 1 metre.lineations are less abundant, however, diverge along a great ir le ontain-ing the major trend ex ept a SSW plunging submaximum, restri ted to thetransition of the shear zone and Galana East.Intense deformation is ubiquitous at most exposures. At �rst sight thisis onspi ious looking at folded and boudinaged graniti to pegmatiti veins.An ex ellent example an be found at Lugard's Falls, one of the most im-posing out rops of the area (Fig. 4.4). Veins vary in thi kness from m tohalf-metre s ale and are often on ordantly aligned to the regional strike anddip. Pin h and swell stru tures are spe i� for felsi veins while more om-petent ma� layers usually form barrel (irregular) shaped boudins. Fold axesof minor iso linal folds plunge gently NNW and o ur in orthogneisses aswell as in amphibolites. Besides parti ular high-strain domains a non- oaxial omponent of deformation is less obvious but indi ated by asymmetry of

4.1. CLASSIFICATION AND DESCRIPTION 35

Figure 4.3: re umbent fold (marble) from unit I at the Galana River (latitude38Æ50'18"). The base of the stru ture forms an amphibolite layer. Height ofstru ture about 1 metre.swells or rotated boudins (Fig. 4.6), strain shadows of ma� lasts (�- to Æshapes) and shear ( ow) folds (Fig. 4.5). Distin t shear bands are frequentlyin�lled with a idi melts or migmatiti (pegmatiti ) segreations.Despite of omplex relationships, possibly un onsidered but substantialfor interpretation, stru tural observations suggest predominant left lateraldispla ement during high grade (amphibolite fa ies) metamorphi onditions.A dextral overprint ( ontinous event?), most obvious from shear bands, spo-radi ally interse ts the fabri and dis rete stru tures (e.g. pin h and swellveins or folds, Fig. 4.7) partly indi ating onsiderable displa ement. The hara ter of this later shear bands an not be spe i�ed exa tly be ause it isonly noted on subhorizontal erosion surfa es and a potential omponent ofverti al displa ement remains unknown. Moreover, parti ular sets of shearbands may be related to hook shaped folds (Hudleston, 1989) or ankingfold stru tures (Pass hier (2001), Grasemann et al. (2003)) whi h then wouldre ord syn- or antitheti slip (rotation) during progressive myloniti ow. Inaddition, importan e has to be atta hed to the fa t that steep uts re ordverti al extension of material as well (e. g. boudinaged veins). We will laterreturn to this topi when dis ussing strain patterns related to su h a type ofdeformation.


Figure 4.4: Common a idi pin h and swell veins and boudinaged amphi-bolites o uring in the Galana River shear zone (Lugard's Falls; latitude38Æ41'26").Transition zone (IV)Isolated ridge-shaped orthogneiss bodies parallel the shear zone and de�nea narrow domain stret hed out to several tens of km north of Voi (Fig. 3.1,Fig. 4.1). The size hanges from regional (Sagala Hills), more then 10 km inlength and several km in width, to km s ale or even smaller. Outside the studyarea the zone ontinues northwards as one an note on the main road fromVoi to Nairobi. Gr-Am-Pl-Qtz�Bt-orthogneisses arise as the predominantlithology. Inter alated amphibolites are present either as thin strips, isolatedlenses and nodules, or rarely as massive garnet-amphibolites.The biggest stru ture of the zone is lo ated immediately south of Voi.Sanders (1963) des ribed the Sagala Hills, although ompli ated in detail, as


Figure 4.5: Folded pegmatiti vein ( entre) and sigma last (ma� nodule;right upper orner) denoting asymmetri deformation ( a. 150 m east ofLugard's Falls).an open anti linal fold pervaded by a series of high-angle faults downthrowingto the east. Field measurements of the inner part of the stru ture supportthe former opinion sin e poles to foliation planes s atter upon a great ir lewith a steep (> 50Æ) average ENE plunge of planes (Fig. 3.1). Gently NNW-SSE dipping lineations ontain the �-pole (346Æ/14Æ) of foliation planes and orrespond to the fold axis.Towards the north, the zone approa hes the Taita Hills and allows a moredetailed study of stru tures at some out rops. The bodies are usally smallerand spatial relationships an be observed at on e, however, spheri al erosionsurfa es ompli ate interpretation as they usually border the entire exposure.Two onspi ious sets of shear zones (partly myloniti veins), either steeply(up to verti al) or more regular shallowly dipping (Fig. 4.8), are dis ernableat ertain lo alities. Measurements yield steep (> 70Æ) easterly dip or s at-tering values of NNW-SE plunge less then 20Æ. Corresponding lineations �tto the ommon trend. Details of deformation an be gained espe ially fromfolded ma� veins viewed at di�erent surfa es. Pre ipitous uts (parallel tolineation) show apparent northerly displa ement of dyke segments whereassubhorizontal top surfa es (parallel to lineation) repeatedly indi ate dextralstrike slip movement (Fig. 4.9) although opposed shear senses an also befound. High-angle (�perpendi ular to lineation) surfa es are instru tive too


Figure 4.6: Sinistrally displa ed boudin. Note the internal fabri roughlyparallel to the long axis of the stru ture (latitude 38Æ37'2" at the GalanaRiver).as they illustrate the various orientation of folded (ptygmati ) veins anddistin t shear zones.O asionally tight folded layers emerge ontaining re tangular boudins. Itis not lear whether the development of the shear zone rules the te toni style(e.g. formation) of this domain or simply in uen es a preexisting stru ture(e.g. reorientation). The on ordant alignment of both domains ould supportthe former assumption, however, a on urren e of the me hanisms would alsobe quite likely.Taita Hills (III)A gently (in average < 20Æ) NNE dipping domain of metavol ani s and meta-greywa kes (Kasigau group) and underlying shelf sediments (Kurase group)make up the Taita Hills, extending over more than 800 square km west ofVoi (Fig. 3.1). Lineations s atter about NNW to NE with a lear maximumat 12Æ=14Æ (Fig. 3.1). The pa kage appears to be imbri ated by S-SW verg-ing low angle thrusts de�ning in part the Voi suture zone (Fris h and Pohl(1986), Pohl et al. (1980)), an array of ultrama� (ophioliti ) lenses pla ed atthe onta t of Kurase and Kasigau group (e.g. Wanjala thrust, see Fig. 4.1).An ex ellent example of a thrust is exposed near Dembwa (opposite to


Figure 4.7: Dextrally displa ed pin h and swell vein (Galana River, latitude38Æ41'26").Murry girls high s hool) about 6 km north of Mwatate (Fig. 4.1). S-C fab-ri s and small s ale duplex stru tures indi ate sta king towards S-SW (seeFig. 4.10). The banded (myloniti ) nature of the ro ks (Fig. 4.10) and sharpboundaries on top of pha oidal shaped fragments probably denoting syn-theti shear domains are onspi ouous. Lineations dip to the N-NNE andindi ate, in this ase, most likely a similar oriented movement dire tion.Trusting ommen ed under high grade metamorphi onditions as shown,e.g., by metapeliti samples. One of several sillimanite generations (Hauzen-berger, submit.) de�nes in part the foliation and also shear bands. Sporadi- ally re umbent folds are observable and where spatially exposed, b-lineationsplunge shallowly (< 20Æ) to the NNE. At the eastern slopes of the Hills one an �nd foliations des ribing an open north plunging (< 10Æ) anti line whi hgradually are tightend when approximating the Transition zone.


Figure 4.8: Detailed view of a small gneiss body of unit IV with myloniti horizons utting through shallowly (SW of Manyani).Pare-Usambara mountainsThe NNW-SSE trending mountain range of the Pare-Usambara mountains(Fig. 3.5, Fig. 4.11) forms the northernmost granulite terrane of eastern Tan-zania. Foliation planes (Fig. 3.5) of the Usambara mountains plunge gently(�20Æ) to the E with a lineation maximum at 63Æ/15Æ. A similar pattern isindi ated in the Pare mountains with a maximum of foliations and lineationsyielding 81Æ/24Æ. A remarkable feature of lineations of both areas (see alsoTaita Hills) is an obvious s attering along great ir les. To a lesser extendthis an be dete ted also in the Taita Hills. Be ause of polyphase deforma-tion an a urate subdivison into stret hing (mineral)- or b-lineations mayoften remain ambiguous. Lineations originally representing b-lineations mayhaven been rotated due to su eeding deformation and are now lo ated in the


Figure 4.9: dextrally deformed ma� vein on subhorizontal errosion surfa eon top of a gneiss ridge of unit IV (SW of Manyani).extension �eld of �nite strain. Late Mozambiquian deformation (Sha kleton,1993a) reoriented preexisting stru tures and strain geometry itself (e.g. at-tening) may also ause a spread of "true" stret hing lineations. (Sha kleton,1993a) des ribed extensional strains of X:Y:Z=30:2:1 for a spe imen from theNorth-Pare mountains and generally argued the importan e of sheath folds.However, ross folding as des ribed by Bagnall (1960) probably ontributedto spread of lineations as well. Two ross striking zones asso iated with lo- alised folding (Fig. 3.5, Fig. 4.12) were observed in the Pare mountains andthe western peneplain.Poles to foliation planes outline a fold stru ture with a moderate northerly(6Æ/45Æ) dipping maximum. Lineations vary in plunge from ENE to SE withan E dipping maximum at 93Æ/21Æ nearly �tting to the �-pole of foliationplanes (104Æ/17Æ). A few steeper lineations (Fig. 3.5) may be related to laterte toni in uen e like strike slip movement on steep (N)NNE-SSW dippingfoliation planes. At some out rops the latter deformation an be assumedprobably to balan e imposed shortening. Shallowly SSW plunging lineations an be found, too.Returning to explanations of Bagnall (1960) and Walsh (1960), who sug-gested superimposed thrust te toni s to explain either lo alised ross foldingor gently northward plunge of west verging overturned folds and ombinethis with data of Pohl and Niedermayr (1979), Horkel et al. (1979), and


Figure 4.10: Detailed view of a thrust within the Taita Hills. Asymmetri displa ed myloniti layer ( entral) hint at transport towards S-SW.our own observations, one may on lude that thrusting as re ognized in theTaita Hills an be found as well to the south, at least in the Pare moun-tains. At the northern foothills of the N-Pare mountains a lenti ular bodyof ma� to ultrama� ro ks (m3, Fig. 3.5) seems to be subje ted to S-SWdire ted thrusting. Numerous pha oidal shaped fragments, often embeddedin pegmatoidi segregations, o ur. Fabri measurement of a fragment showvalues of 35Æ/35Æ and 25Æ/20Æ for foliation and lineation. The in uen e ofsu eeding shear te toni an be inferred from steep to subverti al northerlytrending zones frequently representing thin ( m - dm s ale) pegmatiti veins.A well exposed stru ture (260Æ/80Æ) indi ates dextral displa ement but witha signi� ant oblique-slip omponent.The internal fabri of other ma� pods (m1, m2 in Fig. 3.5), however,is more on ealed. Some tens of km ESE of Same (m2) foliation of ro ks is


Figure 4.11: Main li�s of the North Pare mountains (viewed from the west).hardly re ognizeable and varies in dip from steep (50-60Æ) to SSE to subver-ti al towards E or W while hosting orthogneisses plunge gently (< 10Æ) tothe E. SE of Same (m1) one an �nd steep WNW-ESE striking faults andstrongly weathered northerly trending zones within the pod whi h may be omparable to lo alised shears north of the Pare mountains (see above).Additionally a short visit was payed to the anorthosite body at Ikongweabout 20 km east of Same (Fig. 3.5). Yet observations have to be handledwith are be ause it was only glan ed over some out rops but a lot of stru -tures and perhaps the fabri may be attributed to empla ement while otherinhomogeneities favour ontemporaneous deformation of the body. Lo alized( m - dm s ale), steep westerly dipping, NNW-SSE striking du tile shearbands (e.g. 240Æ/80Æ) are demonstrative. They ross ut the fabri and mostlikely do ument the widspread in uen e of postdating te toni s as evin edby the Galana River shear zone.Umba SteppeCompared to the mountain range of the Pare-Usambara mountains, ro ks ofthe Umba Steppe are less exposed and out rops are more sparse. Predom-inant lithologies are high grade metapelites, marbles and al sili ate ro ks.Good exposures an be found along a tra k following the general E-W trendof the Umba River from Mwakijembe to Umba (Fig. 3.5) and a southern


Figure 4.12: Detailed view of an out rop of a ross-striking domain SW ofSame (Fig. 3.5). The folded vein shows the steep foliation (�ESE trending) ontrasting from the more northerly trend of surrounding ro ks plungingmoderate to shallowly. ontinuation from Umba to Daluni.Frequently foliations dip to the E at moderate angles (< 40Æ) with amaximum lo ated at 81Æ/27Æ. Steep foliations (55Æ-80Æ) o ur sporadi allyand were observed in marbles east of Mwakijembe and in the western part(N-S se tion Daluni-Umba). Pronoun ed lineations, although rarely noti ed,indi ate strong preferred orientation (15Æ/8Æ).If we attempt to relate ro ks of the Umba Steppe with those in Kenya wemay �nd a orrelation with the Kurase group (Saggerson, 1962) and proba-bly with the Sobo formation (Sanders, 1963). The Galana River shear zone,however, is not exposed. Given that the stru ture ontinues it is assumedthat it trends east of the Umba Steppe, now overed by the upper Paleozoi

4.1. CLASSIFICATION AND DESCRIPTION 45Duruma Sandstones.

Chapter 5Geo hronology5.1 Previous dating5.1.1 Taita Hills and adja ent areasK/Ar - Rb/Sr systemTo the southeast at Maungu and Nyangala (Fig. 3.5) ro ks of the Kasigaugroup yielded ages of 490�25 Ma and 425�25 Ma for feldspar porphyroblastsof a migmatite and a granitoid gneiss, respe tively (Saggerson, 1962). Holmesand Cahen (1955) dated potash feldspars from a pegmatite south of Tsavo(Fig. 3.5) showing 560�50 Ma.Ages of 498�15 Ma on biotite and 519�16 Ma on hornblende were de-termined by Shibata (1975) from ro ks of the Taita Hills. Fris h and Pohl(1986) dated biotites and K-feldspars from several lo ations throughout thearea (Fig. 5.1). Biotite ages of 528-489 Ma were derived from an amphibo-lite, biotite gneiss and pegmatoid layers at Wundanyi (Fig. 3.5), whereasbiotites from a pegmatoid layer north of Dembwa (Fig. 3.5) indi ated 511.3Ma. A biotite age from a mi rogranodiorite dike at Manyani quarry (Fig. 3.5)is a ordant with 497.2 Ma. The biotite ages ranging from 490 - �530 Mawere explained as resetted ooling beneath 300ÆC aused by a thermal andstru tural overprint (see also Kennedy (1964)).K-feldpar ages of 456 Ma and 463.2 Ma (Fris h and Pohl, 1986) wereinterpreted as losing ages for the K/Ar system, where the former was derivedfrom the sample north of Dembwa and the latter represents a pegmatite fromthe harno kiti Mtonga-Kore omplex (Fig. 3.5).Two Rb/Sr whole ro k datings are available but show ontrasting values.Shibata (1975) al ulated an Rb/Sr-error hron of about 827�55 Ma whi hwas onsidered to be not very reliable, however it agrees with a main Mozam-47

48 CHAPTER 5. GEOCHRONOLOGY

Figure 5.1: Cooling ages of Kurase- and Kasigau group (from Fris h and Pohl(1986)).biquian metamorphism of about 840 (Cahen et al., 1984) as noted by Fris hand Pohl (1986). Cambrian to ordovi ian ages (Fig. 5.1, Wundanyi samples,biotite gneiss and pegmatoid layer) are lose to ooling ages of biotite andeven K-feldspar and thus onsidered to be of no signi� an e.5.1.2 Pare-Usambara mountainsU/Pb systemConstraints on the ooling history of Pan-Afri an granulites of eastern Tan-zania were eviden ed by M�oller et al. (2000) applying U/Pb isotopi systemto various minerals (monazite, zir on, rutile, titanite) and supplementaryuse of K/Ar data (hornblende and biotite, Cahen and Snelling (1966)) andone zir on age (Muhongo and Lenoir, 1994). Correlative ooling paths forthe Pare-Usambara mountains and Umba Steppe (M�oller et al., 2000) areindi ated in Fig. 5.4 displaying slow ooling rates of 2-5Æ/Ma valid for mostof the o uren es. Peak metamorphism, dedu ed from monazite and zir on losing temperatures, veri�es di�eren es in time of granulite fa ies metamor-phism for the distin t areas. While the Usambara and Umba Steppe dataoutline a nearly linear ooling path, the Pare mountains deviate with aninitial rapid ooling of �10ÆC/Ma and su eeding slow ooling (<5ÆC/Ma)until a. 400ÆC then the paths follow a similar trend.

5.2. PRESENT STUDY 495.2 Present study5.2.1 40Ar/39Ar - Sm/Nd system40Ar/39Ar age dating (this study) were performed in order to gain insightinto the ooling history of the di�erent te tonostratigraphi units. Thirteensamples were produ ed in luding migmatiti orthogneisses, amphibolites andone sample of pegmatite (mi a), thereunder 5 pairs of amphibole and biotiteea h from the same sample. Three samples yielded single fra tions of amphi-bole, biotite or mi a. Dating were done by Robert Handler at the Universityof Salzburg, Austria. Two sets of grain sizes were separated ranging between> 140 < 180�m and > 180 < 250�m. To minimize errors resulting from mix-ing of amphibole and biotite grains (espe ially where pairs were separated)the fra tions were hand-pi ked. A orrelation with Sm/Nd garnet-whole ro kages (Hauzenberger et al., 2000) were attempted to appraise reliability of thedata and to draw near a possible ooling path.Galana East(I)A pair of amphiboles and biotites (AB-F2, Fig. 5.2, Fig. 5.3) derived from anintensely deformed ma� migmatite, was analysed yielding a sound plateauage of 519.3�5.1 Ma for amphiboles, although a small amount (�3.6%) ofex ess Ar is re ognizeable within the �rst 6 steps of the run. A noti ablehigher age of 537.9�7.7 Ma and poorly de�ned plateau is shown by thebiotites.Applying the on epts of losure temperature (Dodson, 1979) and ali-brations for amphiboles (e.g. Harrison (1981), Villa (1998)) and biotites (e.g.Harrison et al. (1985),Villa and Puxeddu (1994)) one must doubt about the orre tness of the biotite age most probably re e ting major in uen e ofex ess argon. This highlights a problem important for the study area as awhole (see also M�oller et al. (2000)) and beyond (e.g. Maboko et al. (1989))be ause petrologi al and stru tural studies, as indi ated above, point learlyto a polyphase evolution of the area.Ex ess Ar may re e t oddment of an older metamorphism or an beassimilated by a mineral grain during a su eeding te tonothermal event.DiÆ ulties in interpretation of Ar-Ar ages generally arise from phenomenasu h as re rystallisation (e.g. Villa (1998), Dallmeyer and Ibargu hi (1990)), uid ir ulation and strain (e.g. Villa (1998)) as well as hemi al ompositionof minerals (Lee, 1993). Analyti al ompli ations, e.g loss of 39Ar by re oilduring irradiation (Faure, 1986), also e�e t apparent mineral ages.Even seemingly at plateaus are not a proof of geologi al relevan e as


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Figure 5.2: 40Ar/39Ar ooling ages of hornblends from various te tonostrati-graphi units.indi ated for biotites ex eeding known ro k ages (Pankhurst et al. (1973),Roddi k et al. (1980), Foland (1983)). This behaviour was attributed to ahomogenous distribution of ex ess Ar with respe t to K in a sample (Faure,1986) or dehydroxylation and delamiantion during heating in the va umextra tion line (Hanson et al., 1975). Moreover, hloritization of biotites givesrise to old apparent ages.

5.2. PRESENT STUDY 51AB-F2

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Figure 5.3: 40Ar/39Ar ooling ages of biotite (mus ovite) from variouste tonostratigraphi units.

52 CHAPTER 5. GEOCHRONOLOGYGalana West(II)Two pairs of amphibole and biotite (AB-KGB35LF, AB-KGB49C, Fig. 5.2,Fig. 5.3) both des ending from migmatiti orthogneisses were analysed tosurvey a feasible onsisten y of ooling throughout the shear zone. This as-sumption was on�rmed by well de�ned plateaus and identi al integratedapparent amphibole ages of 524.5�4.9 Ma and 524.7�5.0 Ma. Irregularitieswithin the release spe tra (Fig. 5.2) at the beginning and end of the runonly a�e t small proportions of the released 39Ar. In the ase of sample AB-KGB35LF it may re e t ex ess argon whereas sample AB-KGB49C indi atesloss of Ar (possible overprint?) at the beginning (�rst six steps) and ex essAr only at the last step.Reliable ages of 490.4�4.7 Ma and 500.9�4.9 Ma are evin ed by orre-sponding biotites (Fig. 5.3) where only sample AB-KGB49C shows loss orex ess Ar (one step) within the �rst seven in rements of the release pattern.Transition zone (IV)Biotites (AB-KGB85) from a strongly deformed and shallowly interse tingmylonite (< 20 m, Fig. 5.3) were pi ked and dated. Compared to biotite andamphibole ages of the shear zone a marked hange is observable displayingan integrated age of 548.8�9.1 Ma. From the 39Ar release pattern it be omes lear that ex ess Ar was released during the �rst steps of the heating ex-periment. As no amphiboles were dated from this zone the validity of themeasurment annot be stated at �rst but will be examined on dis ussing thewhole data.Taita Hills (III)Six ages were produ ed from the Taita Hills, among them two pairs of amphi-bole and biotite (AB-MYl, AB-KTB60, Fig. 5.2, Fig. 5.3) from migmatiti orthogneisses of di�erent te tonostratigraphi levels of the Kurase group(Fig. 3.5). Two fra tions of hornblende (amphibolite, AB-KTB55) and mi a(pegmatite, AB-PGL) were singled out from the Kasigau group.Looking at integrated ages of amphiboles we �nd a variation of 570.1�5.0Ma to 559.1�5.4 Ma inside the Kurase group. The appendant amphibole agefrom the Kasigau group shows 563.7�6.1 Ma. A slight e�e t of ex ess Ar anbe dedu ed from the release patterns of AB-MYL and AB-KTB55 (Fig. 5.2)but it pertains only inital steps of the low temperature release of 39Ar.When evaluating the appropriate biotite data one en ounters diÆ ul-ties. Integrated biotite ages of AB-MYL and AB-KTB60 (Fig. 5.3) exhibit558.6�6.3 Ma and 580.3�6.2 Ma and then e equal (e. g. AB-KTB60) or even

5.2. PRESENT STUDY 53exeed (e. g. AB-MYL) amphibole ages while representing un onspi uous re-lease patterns. This may be interpreted, as aforementioned, in terms of evenlyspread ex ess Ar. Yet within the margin of error they are not distinguishablefrom the amphiboles and ould alternatively signify a period of fast uplift.The only mi a sample whi h was measured stems from a quartz-ri h peg-matite of the northeastern Kasigau group and reports an upper Cambrianage of 505.6�5.1 Ma.Di�erent metamorphi domains were also dete ted by means of Sm-Ndgarnet-whole ro k dating (gwr-values in Fig. 3.5) of migmatiti orthogneisses.Late proterozoi ages of 586.2�6:1 Ma and 583.7�8:1 Ma were observed onlyin the Taita- and Sagala Hills whereas Cambrian to Cambrian/Pre ambrianages of 529.0�6:0 Ma and 547,6�5:9 Ma an be found whithin the shear zoneand the Sobo Formation towards the East. A Cambrian age of 537.0�19:0Ma, derived from the NE orner of the Taita Hills, display a larger error andhas to be onsidered with pruden e as it a ord with the late Proterozoi domain, although it might indi ate their dis ontinuity.Combining the results gained by Ar/Ar and Sm-Nd isotopi systems we an �nd a orrelation in that two domains, Taita Hills and Galana River area,di�er signi� antly in age and orroborate stru tural investigations whi hpoint out a polyphase development ulminating in formation of the GalanaRiver shear zone. In ontrast to Sm-Nd dating whi h revealed di�erent ool-ing of Galana East and West, Ar/Ar dating does not mirror this but may hintat a ommon history postdating peak metamorphism. Possible ex ess Ar ofbiotites from the Taita Hills, if geologi ally onstrued, may evin e the in u-en e of metamorphism a ompanying te toni s of unit. Another te toni allyfounded reason ould be a fast uplift due to the main period of thrusting inthe Hills. The biotite age of the Transition zone is older than amphibole agesof the shear zone and Galana East and younger than the amphibole ages ofthe Taita Hills. In view of a possible overprint it may represent an una�e teddomain or, sin e the grains were derived from a pronoun ed myloniti hori-zon, ould be onne ted with a late stage in rement of deformation at thattime.Considering our analyses as reliable we have to debate them in relationto existing data. Biotite ages (K/Ar) of 528-498 Ma (Fris h and Pohl (1986),Shibata (1975)) are substantially younger than our estimates. Supposing thatour results re e t ex ess Ar we annot ex lude this likewise for previous K/Arages yielding no information on spe i� heating in rements. An explanationmay be given by apparent ages of sample AB-KTB60 (Fig. 5.3). While theintegrated age shows 558.6 Ma, the total gas age suggests 539.6 Ma and thus omes near to the upper age observed by Fris h and Pohl (1986). Yet, thebiotites of dated pegmatites are onsistent with our estimates for for biotites

54 CHAPTER 5. GEOCHRONOLOGYof gneisses of unit II. This ould indi ate a relation between late thermala tivity of unit III and the formation of unit II.The hornblende age of 519 Ma (Shibata, 1975), on the other hand, ismore diÆ ult to onstrue as it mat hes datings indi ated by the shear zoneand Galana East. Although a metamorphi imprint aused by the shear zonewould not be unlikely it is not lear whether orresponding stru tures arestrike slip faults (not observed) or thrusts. Exhumation related deformationsu h as normal faults may be unobtrusive be ause it ould rea tivate thepreexisting low angle thrust fabri . Nevertheless we on lude that amphiboleages of 560-570 Ma demonstrate subsequent ooling to a major te tonometa-morphi event responsible for the stru tural style whi h is most obvious inthe Taita Hills.5.2.2 Cooling historyIn general it is diÆ ult to make statements about ooling of orogens evenwhen petrologi al and stru tural development are "suÆ iently" do umented.Estimation of ooling rates often remains un ertain as the e�e t of deforma-tion or re rystallization on minerals (di�usivity) may not be transparent. Forthe Taita Hills and the Pare-Usambara mountains analogi al P-T ondidionsand time of peak metamorphism (Hauzenberger et al. (2003); Appel et al.(1998)) are stated but with opposed P-T-t paths (see above). Be ause it isassumed that both areas are te toni ally linked we ompare our data withthe ooling paths proposed by M�oller et al. (2000) for the Pare-Usambaramountains and Umba Steppe to reveal possible similarities.Taking variations of grain sizes and experimental data into a ount weutilized two alibrations of losing temperatures for amphiboles (450-500ÆC,e.g. Harrison (1981); 600ÆC, Villa (1998)) and biotites (300 ÆC, e.g. Harrisonet al. (1985); 450ÆC, Villa and Puxeddu (1994)) at ea h ase.Solid lines in Fig. 5.4 mark ooling paths for Pare-, Usambara mountainsand Umba Steppe. To avoid a mix-up, the data of M�oller et al. (2000) andof the authors re al ulated K/Ar data (Cahen and Snelling (1966); subindex2) were pi tured as ellipti - to sub- ir ular �elds. Below several possibilitiesare outlined to show how the data ould be joined. Dashed lines and solid ir les denote hypotheti al linear ooling paths (e.g. 1) for lower temper-atures. Dotted lines- and ir les refer to hypotheti al linear ooling paths(e.g. 1a) resulting from higher temperature alibration. Sm-Nd whole-ro kestimates (Hauzenber, in prep.) are assumed to indi ate �rst ooling afterpeak metamorphism (�650ÆC) and were plotted additonally.Cooling at rates of 8.5-6ÆC/Ma would stem from paths 1 (in ipient step;Grt to Hbl losure) and 4 valid for the Taita Hills-Transition zone and Galana

5.2. PRESENT STUDY 55

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Figure 5.4: Comparison of ooling ages of di�erent areas throughout the studyarea. Diagram modi�ed after M�oller et al. (2000). GE (Galana East), GW(Galana West), Tz (Transition zone), T (Taita Hills). Cooling for lower (a)and higher (b) temperature alibrations. See text for dis ussion.East, temperatures of 11ÆC/Ma would be gained from the lower part of path1. If we onsider a ommon ooling for Galana West and the Taita Hills(ex luding the Transtition zone biotite) at lower temperature (2) slow oolingof 3ÆC/Ma would result. In the ase of path 3, we would observe reallyfast ooling of 30-40ÆC/Ma at the inital step and subsequently slow ooling(6ÆC/Ma).If we adopt higher losing temperatures for amphibole and biotite wewould obtain slow ooling rates of 2-3ÆC/Ma for path 1a ((in ipient step;

56 CHAPTER 5. GEOCHRONOLOGYGrt to Hbl)) and 4a and and fast ooling at lower temperatures similar topath 1 (11ÆC/Ma). The same slow ooling (2-3ÆC/Ma) would be observed ifwe assume a mutual history for the Taita Hills and Galana West (2a). Fast ooling of 11-12ÆC/Ma would o ur for Galana West (GW; 3a) at highertemperature followed by a moderate gradient of 5ÆC/Ma at the lower part ofthe same path.Besides ooling rates, ne essarily remaining hypotheti al linear estima-tions either between pairs of garnet and hornblende or hornblende and bi-otite, the plots learly show that the Taita Hills amphiboles �t into the ooling history as proposed for the NE granulites of Tanzania. For elevatedtemperatures they meet the paths of the Usambara mountains and UmbaSteppe, for lower temperature alibration they agree with those for the Paremountains. Di�eren es in ooling are obserable at lower temperature (2a)whi h would imply a thermal overprint on the Umba Steppe or Usambaramountains.Basi ally ooling for Galana East and West has to be treated separatelybe ause of signi� ant dis repan ies in age of peak metamorphism and subse-quent ooling. However, if we onsider the lower parts of fairly rapid or alle-viated gradient of (3; 3a) either approa h the paths of Usambara and UmbaSteppe at temperatures of upper greens hist- to greens hist fa ies onditions.This may be asso iated with lo alized low temperature stru tures (variablyoriented faults and shears) observed within the northeastern granulite areaof Tanzania. Regardless of di�erent alibrations the qui kest time span be-tween post-peak metamorphism and hornblende losure temperature ( 5 Ma)is visible for the Galana river shear zone. Strain estimations suggest intense�nite attening for the stru ture with onsiderable verti al stret h. Adve -tive heat transfer supposedly a ompanying the rise of material ould explainfast ooling during this time of development of the stru ture. On e the hotro ks were brought to shallower rustal levels a steep ooling gradient willfollow (e.g. Thompson et al. (1997)) abating afterwards (see 3, 3a) where25-35 Ma were requrired from hornblende to biotite losure. As an importantresult for the older Pan-Afri an domains we on lude that they ould haveexperien ed a omparable history at an advan ed stage of ooling possiblyfounded in a ommon te toni overprint. This is orroborated if we employlower temperature alibration for biotite (Harrison et al., 1985) as used byM�oller et al. (2000)). In this ase we even annot distinguish between latestage ooling of the early and late Pan-Afri an domains (Fig. 5.4).Some of 40Ar/39Ar (labeled Mhbl, Mbt) data produ ed by M�oller (1995)are pi tured in Fig. 3.5. Yet, as pointed out by the author, they have to belooked at pre autiously sin e they might be in uen ed by atmospheri andex ess Ar. Nevertheless one may ompare the data to results of SE Kenya

5.2. PRESENT STUDY 57allowing for polyphase metamorphi - and stru tural development as in SEKenya.A hornblende ooling age of 518.0�15 Ma from the N-Pare mountains(Fig. 3.5) displays a larger error of 15 Ma but omes lose to estimations ofGalana East and West (or ontrary to hornblende age of Shibata (1975) forthe Taita Hills) and ould signalise an overprint. A ording to Bagnall (1960),shear zones (not spe i�ed) exist in this area, and this is on�rmed by ownobservations, too. A biotite ooling age of 556.0�3:0 Ma from the same areaobviously exhibits ex ess argon and is omparable with likewise interpretedbiotite from the Taita Hills (see above). No oin iden e with a hornblendeage of 542.0�8:0 Ma from the S-Pare mountains is dete table but may be ollated to the Taita Hills age domain. Biotites from a metapelite of thewestern Usambara mountains yield 490.0�3:0 Ma and represent an identi alage as biotites from the Galana River shear zone. South of the Usambaramountains (Fig. 3.5) a hornblende was determined with 530.0�3:0 Ma whi hwould be slightly older than hornblende estimates for Galana West. Furthersouth, from the eastern Uluguru mountains (not shown) the author reportedhornblende ages of 550�4 Ma and 527�8 Ma also a ordable with mainte tonostratigraphi units (II, III) in the SE Kenyan pro�le.

Chapter 6Strain estimations6.1 Purpose and method6.1.1 Geographi al s opeThe following onsiderations are valid for parts of the Taita Hills, GalanaRiver and the Transition zone. They were arried out to get a further dis-tin tion between di�erent te tonostratigraphi units. A short overview shouldallow a general statement of strain patterns and their possible meaning forthe stru tural development of the area. In parti ular this refers to the GalanaRiver shear zone (unit II) whi h has to be lassi�ed, by style of deformation,as a transpressional system.6.1.2 Field method and restri tionsHigh grade gneissi terranes usually ontain a lot of pegmatiti veins ormigmatiti segregations and may therefore be suitable for in situ strain esti-mations tending to obtain information on strain a umulated during defor-mation and metamorphism. For this purpose, a method was applied whi huses variously oriented sets of shortened (folded) and elongated (boudinaged)veins, as suggested by Talbot (1970). Out rops with varyingly dipping erosionsurfa es at high angle relationships are most suitable. Likewise this dependson distan es between suitable out rops or variations of lithologies. Hetero-geneity o urs either due to strain gradients observable as lo alized zones of on entrated deformation (e.g. Fig. 6.1) or hange of mineralogy (e.g. ul-mination of ma� minerals) within gneiss domains otherwise regarded ashomogenous. Inhomogeneity, of ourse, is aused by generation of dis retestru tures (e.g. folds) Limitations of the method, as pointed out by Talbot(1970), are also problemati in order to obje tify the data. This on erns59

60 CHAPTER 6. STRAIN ESTIMATIONS

Figure 6.1: High-strain domain within the Galana River Shear zone(�horizontal erosion surfa e, subverti al dipping foliations). A detailed viewof the felsi boudin ( entre) is given in Fig. 4.6.initial orientation and time of development of veins as well as ompeten yratios and a ura y of olle ted �eld data. Morover a possible presen e of vol-ume hange during deformation is relevant for interpretation, although thisis assumed to have no substantial in uen e on our onsiderations. On thesea ounts, the following des ription and dis ussion of o uring strain patternsmust be onsidered as a generalized strain geometry for ea h te tonostrati-graphi domain omitting inhomogeneities and lo alized strain a umulationzones.

6.1. PURPOSE AND METHOD 616.1.3 Evaluation of dataThe te hnique is based on the assumption that omparison of inital (un-deformed ir le) and deformed state (strain ellipse) an de�ne �elds, whereveins appear as shorter or longer than their original length. These �elds aresepearated by lines whi h experien ed no �nite deformation (Flinn, 1962)or lines of no �nite longitudinal strain (Ramsay, 1967). In three dimensionsthese lines make up the surfa e of no �nite longitudinal strain (s.n.f.l.s, Tal-bot (1970)) hara terizing either two oni al (oblate or prolate strain) orellipti al shapes (plain strain). They are symmetri ally arranged to the prin- ipal axes. When dire tions of shortening and extension are plotted in anequal area proje tion these lines de�ne ellipti al or ir ular- to sub- ir ularboundaries. In mathemati al terms these shapes an be des ribed by esti-mation of the api al angle of interse tion with the prin ipal planes of the�nite strain ellipsoid, de�ned as XY Æ, XZÆ and Y ZÆ (Talbot, 1970) or�01, �02 and �03, Ramsay and Huber (1983). Two angles, respe tively, are re-quired for oblate ( XZÆ � Y ZÆ; �02 � �03) and prolate ( XY Æ � XZÆ;�01 � �02) ellipsoids, a single value ( XZÆ; �02) is suÆ ient for plane strainsin e XY Æ (�01) = 90Æ and Y ZÆ (�03) = 0Æ. Uniaxial ellipsoids are gainedwhen orrespondent angles oin ide.Solving equations spe i�ed Talbot (1970) or Ramsay and Huber (1983)),one obtains an estimate of minimum �nite strain in terms of axial ratiosof prin ipal dire tions of the ellipsoid. Depending on these relationships aniterative routine was used written by W. Unzog (pers. omm.). AlternativelyE. Wallbre her (pers. omm.) suggested a graphi al solution resting uponeigenvalues of shortened or extended element (veins) distributions.Results are instru tively pi tured in a Flinn-diagram (Flinn, 1962), whereordinate and abs issa represent a(x/y) and b(y/z) ratios (x�y �z) at ea h ase and a k-value, stated as k = a�1=b�1, indi ates the slope of data pointswith respe t to the abs issa. K-values < 1 denote attening, those > 1 areasso iated with prolate ellipsoids and plane strain onditions are assessedby the relation k = 1. Ramsay and Huber (1983) proposed the notationof apparent attening or - onstri tion taking into a ount unknown volume hange. These terms will be adopted here as well.Details of deformation history might be dedu ed by subdivision of exten-sion and shortening �elds into ategories like shortened only, �rst shortened -then boudinaged (boudinaged folds) and extended only (e.g. Pass hier et al.(1992), Pass hier and Trouw (1996)) whi h would allow statements on type( o- or non- oaxial) or even measure of non- oaxiality (e.g. kinemati vorti -ity numberWn; Means et al. (1980)) of the ow. Su h information annot bea hieved by our data as too little observations were made to allow a reliable

62 CHAPTER 6. STRAIN ESTIMATIONS lassi� ation.6.2 Results6.2.1 Galana East (I)The strain estimate of unit I just slightly deviates from plane strain ondi-tions (Fig. 6.2) whereby the prin ipal plane ontaining the main extensions(x; y) gently dips to the SW (235Æ/12Æ) whi h is identi al with the x-axisas y is horizontally arranged (Fig. 6.2, Fig. 6.3). Hen e the main shorten-ing axis (z) dips steeply towards NE (Fig. 6.2). In that we have a guessabout orientation of prin iple axes, we an ompare to stru tural observa-tions and �nd a reasonable orrelation of intermediate strain axis, main lin-eation trend (Fig. 3.1) and measured hinge lines of folds. Consequently thistrend an be related to b-lineations whi h is orroborated by the fa t, thatthe guess for extension in dire tion of y equals � 1 . The prin ipal exten-sion (x-axis, Fig. 6.2) is pla ed between SSW-NNE to WSW-ENE orientedsub-maxima with an approximate deviation of a. 20Æ. These minor trendsmight be seen as true stret hing lineations possibly generated as fold axes orpertinent renulations. In omparison with neighbouring domains (Fig. 6.2) al ulated mean onditions of minimum strain are lowest for unit I althoughintense deformation is obvious from most out rops. Assuming that our datare e t the latest deformation event this may be explained as a �nite straingradient a epting a ommon deformation history with unit II. Origin andrelevan e of this event will be dis ussed later when omprising the entire datafor the Galana River pro�le.6.2.2 Galana West (II)Axial ratios dedu ed from veins of the adja ent shear zone indi ate the dom-inan e of distin tive attening with respe t to minimum strain onditions(Fig. 6.2, Fig. 6.3). Determination of the dire tion of prin ipal elongation(steep or shallow regarding the strike of the zone) is vital for su h pat-terns. When strains approa h uniaxial attening the angles XZÆ and Y ZÆadopt similar values (Fig. 6.2). Therefore a de�nite restri tion of bound-aries (s.n.f.l.s) between shortening and extension �elds be omes riti al. The hange of prin ipal dire tions depends, in this ase, on a few degrees andinterpretation may additonally rely more on �eld eviden e.Disse tion of the data set yielded angular values of XZ = 23Æ and Y Z = 19Æ whereas the larger angle is lo ated at (near by) the ir umfer-

6.2. RESULTS 63en e (Fig. 6.3) of the equal area proje tion. Then the major extension is(sub)horizontally and aligned with the strike of the zone boundary and maintrend of foliation planes. This is onsistent with the frequen y of the lin-eation pattern (Fig. 3.1) showing a gently NNW plunging luster, although,from state of apparent attening, all other appendant (steeper) lineations(Fig. 3.1) ould be asso iated with stret hing too (as arranged in the overallextension �eld).Supposing onstant volume, a generalized minimum elongation (referringto veins) an be estimated for all prin ipal dire tions. Starting from the unitsphere (r = 1) where the radius equals the undeformed length, lo, and regard-ing the semi-axes of the �nite strain ellipsoid as deformed lengths (ldi), theelongation ei (i = x; y; z) is determined as ei = (ldi� lo)/lo. For the prin ipaldire tion of shortening, ez (Fig. 6.3), this omes to � 50% whi h has to bebalan ed by oblique- to orthogonal (ey � 30%) and horizontal extension ofthe zone (respe ting the strike of boundaries). An estimate of the horizontalratio, ex, shows � 60% extension partially supported by single observationsof boudinaged ma� veins on subhorizontal erosion surfa es. Debarring mean onditions, however, boudinaged layers are expe ted to display greater ex-tension, up to twi e as mu h and even multiple if onne ted with high straindomains (e.g., see Fig. 6.1).6.2.3 Transition zone (IV)Contrary to other units, olle ted strain data from the transition zone ares anty. On the one hand this is due to dense vegetation overing exposures(or the surroundings) or to the identi� ation of suitable lo ations (abundan eof veins). On the other hand, the main fo us was not turned to this domain.Stru tural- and geo hronologi al investigations (see former hapters and alsogeo hemistry) suggest the importan e of this region as a boundary ( hange)of varying te toni styles and age domains. For these reasons it is apposite todis uss the zone referring to state of strain but yet in a broader sense thanfor adja ent areas.The major di�eren e to adjoing units II and III (Fig. 4.1) is exempli�edby a markedly prolate strain geometry (LS-fabri s), sometimes dete table inthe �eld as vigorous L-te tonites la king any foliation. As shown in Fig. 6.3no extensions (apart from single observations of boudinaged ma� veins) havebeen re orded in the �eld. This may partly be explained in that relevant out- rops represented almost perpendi ular uts to assumed dire tion of prin ipalextension (�longitudinal axes of orthogneiss bodies). Hen e shortened veinspreferentially appear on these surfa es and boudinaged or elongated layersmay be in onspi uous. Delimitation of the extension �eld (Fig. 6.3) is on-

64 CHAPTER 6. STRAIN ESTIMATIONSsidered as reliable around the south pole of the equal area proje tion. Byreason of plunge of inspe ted erosion surfa es, only two measurements ofshortening plot near the north pole. To larify the boundary on the oppo-site side, some data were mirrored (open ir les). Despite of the requirementof symmetry of �elds, this remains spe ulative. The extension �eld mightbe slightly enlarged as stru tural observations indi ate a shallow northerlyplunge of stru tures (� 14Æ). However, a good orresponden e is suggestedwith the trend of stret hing lineations o upying a luster at 357Æ=9Æ. There-fore and on the basis of magneti fabri analysis ( hapter 7), it is on luded,that mean onditions may be des ribed by su h an approximation.Expe ting larger strain ratios it might be more informative to expressrelative values as stret h, Si (i = x; y; z), al ulated from ratios of deformedversus undeformed lengths (Si = ld=lo). For the suggested prolate geometrySx � 4:6 illustrates the greatest magnitude of all estimates of the area. Asabove-mentioned, strain estimates of this domain are based on a few appro-priate exposures and the stri t boundary, outlined by the extension �eld,might be signi� ant only for several lo ations. To allow for a variation instrain magnitude of the zone a se ond solution (x0; y0; Fig. 6.3) was on-sidered. Without a ting arbitrarily we an hange the angle XY Æ whi henlarges the extension �elds towards the SE (NW; Fig. 6.3) and, a ordingly,redu es the strain ratio (Sx � 3:5). On the SSW (NNE) side we keep theprimary solution and thus originate a shift of �elds, out of beeing symmetri about the north-south pole of the proje tion, to the NNW-SSE. In so doing,we en ompass more of the lateral portion of the lineation trend (Fig. 3.1)and generally follow the strike of the domain as evin ed by stru tural data.The widely prolate geometry of gneisses, forming this zone, ould possiblybe explained by folding of a pre-existing gneissi fabri , although at out rops ale not always re ognizeable. Progressive axial elongation may then pro-voke onstri tional strain. If, for instan e, representing major fold losures,this may have aused migmatization and plentifulness of veins at spe i� lo ations. Otherwise the fabri may have been re eived on omitant to em-pla ement of bodies whi h might be of importan e for smaller leu o rati varieties.6.2.4 Taita Hills (III)In the Taita Hills attening geometry, as obvious from strain estimates, ismost pronou ed. Hills (Fig. 6.2). The prin ipal plane of extension dips sub-horizontally (�3Æ) to the north whi h an be assigned to the present hor-izontal te toni s. This is identi al to the plunge of the prin ipal extensionbe ause the y-axis remain horizontal. On grounds of fo used work and out-

6.2. RESULTS 65 rop situation the strain pattern is valid, stri tly speaking, only for parts ofthe Kurase group ranging from Mwatate up to the Wanjala thrust (Fig. 3.5).To the same extend as in the shear zone, onvergen e of angles XZÆand Y ZÆ ompli ates �xing of main stret hing dire tions. Sin e here thedis repan y is still less we an onsider another eventuality using the orien-tation of lineations (Fig. 6.3), together with the a urate geometri result.Stru tural observations (see hapter 4) yield a good reason to assume S-SWdire ted thrusting and hen e a similar oriented prin ipal extension. The lin-eation trend maximum whi h di�ers about 20Æ from the determined �nitex-axis (Fig. 3.1, Fig. 6.3) of the strain ellipsoid is presumed to belong to thisevent. Availing the appropriate azimuth dire tion as a se ond option a slightadjustment of the prin ipal plain of extension would happen. Be ause of theoblate geometry (see above) it is possible to as ribe more s attered and lessabundant dire tions to stret hing as well. Bearing polyphase evolution of thearea in mind, this implies reorientation of earlier fold axis and lineations tobe ome parallel with the main trend and the major extension. In luding pre-mentioned restri tions, minimum ratings of �nite strain an equally be quotedfor the Taita Hills. Within the prin ipal plane of extension this amounts to anelongation of about � 80% and � 60% in x- and y-dire tions while demand-ing over 60% verti al shortening. Again, these estimates indi ate generalized onditions and annot give information on lo alized stru tures (e.g. spe i� thrusts and myloniti horizons).As for other domains we have to form an opinion about the ompatibilityof this strain pattern with the observed stru tural style. We might expe tsimilar onditions to that of unit I (near plane strain) for this thrust envi-ronment, too. However, distin t attening is illustrated for the lower partof the pile. This dissimilarity in strain magnitude might derive from deeper rustal environment (� 12 kbar ompared to � 9 kbar (unit I)) and relatedrheologi al behaviour.Results of theoreti al modelling (Ramberg, 1975) may also aÆrm su h asituation. Assuming that nappes, while propagating, are attended by theirown weight or undergo gravitational ollapse (Ramberg, 1980), onditionswithin the lower parts of thrust sta ks an be des ribed as ombination ofpure shear with horzontal extension and a simple shear omponent on or-dantly aligend with the dire tion of nappe movement. Similar �ndings werea hieved, e. g., by Sanderson (1982), who suggested the developement of lowamplitude sigmoidal urvature of leavage as a typi al feature when oin id-ing with the xy-plane of the �nite strain ellipsoid (for shear parallel stret h,�, with � > 1). As the maximum angle (�0) between leavage and the thrustplane depends on �, this urvature may not always be dete table. For valuesof � > 2:5, �0 equals 5Æ and thus be oming an unobtrusive feature if out rop

66 CHAPTER 6. STRAIN ESTIMATIONS uts are not favourable. For the Taita Hills this was not re orded but mayexist in the form of o asionally visible stru tures resembling S-C fabri s.Another problem of ompatibility arising from nearly uniaxial atteningis that also lateral stret h has to be ompensated (e.g. � 60% by the inter-mediate strain axis). Potentially this an be a hieved by folding or prolategeometry along the margins or at parti ular domains internal to the pile.Within the Taita Hills this was not observed even though sheath fold geome-tries might o ur. East of the main li�s open folded se tors an be foundbetween Mwatate and Voi (Fig. 3.5) but these were not studied in detail. No-tably, this on erns the western border. Only a view out rops were visitedverifying the ommon situation sin e possibly not seated at the hange tothe neighbouring area. Towards the north, where the transition zone nearsthe foothills of the ulmination, folding may grow more intense and poses thequestion if this domain itself is in uen ed by the strain pattern of unit IIIas parts of both ontemporary share high-grade metamorphism (see hapter5).6.3 Convergent te toni s and strain patterns6.3.1 ModelsIn this se tion we try to �nd out whether the strain geometries dedu edfrom the Galana River (mostly unit II) are suitable for model al ulationsdealing with wren h te toni s as a result of onvergent plate motion. Belowwe will attempt to on�ne possible dire tions of onvergen e be ause this isof parti ular interest in view of late Pan-Afri an orogeny.Harland (1971) introdu ed the term transpresson for obliquely approa h-ing plates involving large-s ale hanges in style of deformation along per-sistently distorting margins. Inherently, this on ept is of great omplexity,sin e it a ounts for many pro esses of ollisional te toni s varying in time(e. g. thrusting and strike-slip movements) or spa e (e. g. lateral hange ofdeformational domains aused by irregular plate fringes).Sanderson and Mar hini (1984) modelled transpressional behaviour of ahomogenously deforming material ( onstant volume) bounded by steep andparallel planes implementing a fa torization of the deformation (pure shearfollowed by simple shear) su h that the shear strain ( ) is aligend with thestrike of the zone (Fig. 6.4a). Sin e limited at the base and laterally, thedu tile material an onlyes ape against the upper free surfa e.Heterogenous strain onditions at spe i� "press" (perpendi ular) and"trans" (parallel) omponent ratios ( on erning boundaries) were onsidered

6.3. CONVERGENT TECTONICS AND STRAIN PATTERNS 67by Robin and Cruden (1994) allowing no free slip at any bordering plane ofthe deforming material (Fig. 6.4b). Important onsequen es of their studiesare a ommonly observed tri lini deformation geometry and systemati allyvariying lineation dire tions within the zone.Presuming steady state and syn hronously a ting pure- and simple shear,Fossen & Tiko� (1993) and Tiko� and Fossen (1993) demonstrated the useful-ness of this treatment investigating strain paths related to transpresson (e. g.boundary onditions analogous to Sanderson and Mar hini (1984). Followingthis approa h, Tiko� and Teyssier (1994) investigated e�e ts of strain parti-tioning (Fig. 6.4 ) and attributed this behaviour to disagreement of instan-taneous and �nite strain ellipsoids during transversal plate onvergen e. Forhomogeneous steady-state deformation they derived a relationship betweenthe angle of relative plate motion (�) and instantaneous strain properties (seealso Fossen and Tiko� (1993)) distinguishing between wren h (� < 20Æ)- andpure-shear (� > 20Æ) dominated types.Jones et al. (1997) removed lateral and basal restri tions of the Sander-son and Mar hini model permitting material to ow in these dire tions aswell. Consequently, they formulated un on�ned transpression (Fig. 6.4d) by hanging the on�guration of the latter authors. The plane-strain pure shearwas repla ed by a triaxial pure shear whi h an be fa torized further into twoorthogonal plane-strain omponents. A kinemati ne essity of this model isantitheti strike-slip shear additionally a tive to a ommodate lateral es apeof material.The in uen e of horizontal material stret h on the fabri stability weredebated by Teyssier and Tiko� (1999). In reasing along strike elongation, ini-tiated by an identi ally oriented oaxial shearing, were found as a stabilizingfa tor for horizontal lineations and verti al foliations during oblique trans-pression. For strike-parallel simple shear, equated oaxial (horizontal=verti al) omponents and 50% extrusion in either dire tions (a ording to their bound-ary onditions) they proposed a horizontal prin ipal �nite strain axis (see alsoJones et al. (1997)) independently of the angle of onvergen e and strainmagnitude.6.3.2 ComparisonBased on stru tural re ords and strain estimates we an test whether ourobservations are a ordable with di�erent aspe ts or requirements of mod-elling ne essary to ounterbalan e imposed deformation. This should help to�nd independent indi ations for interpreting our data and provide an insightin ertain stages during development of the area. An important fa et of the�nite strain estimate for the Galana River shear zone an be derived from

68 CHAPTER 6. STRAIN ESTIMATIONSlateral stret h of material o urred during deformation or not. All homoge-neous transpression models an make pronoun ed attening, but the strainpattern alone is no eviden e for boundary onditions. Antitheti shearing, asproposed by Jones et al. (1997), an be indi ative when it belongs demon-strably to the same event. Shear bands (see hapter 4) may be helpful toprove a progressive deformation event and then may arry information on thetype of ow (e.g. Platt and Vissers (1980)). Re ent modelling (Grasemannet al., 2003), lassi� ation and dis ussion in relation to anking stru tures(see Pass hier (2001), Grasemann et al. (2003)), however, show that ommontypes an readily be onfused and may o ur under a wide range of ow typesdepending on their initial orientation.Angles between tra es of re orded shear bands (mostly not lassi�ed butwith dis ernible displa ement) and foliation, observed on subhorizontal ero-sion surfa es, vary frequently between 20Æ - 35Æ. Less abundant are anglesof < 20Æ - 10Æ as well as orientations > 40Æ. Generally, their strike hangesfrom 350Æ to 10Æ (maximum is aligned north-south) and more seldom NNEor NNW orientations were observed. Besides normal shear bands and n-type(no displa ement) anking folds, some of the stru tues ould be lassi�ed asa-type related (antitheti displa ement), e.g., in the vi inity of high strain do-mains (Fig. 6.5). S-types (syntheti displa ement) were not learly identi�edalthough single observations hint at their presen e.Perhaps the most easely on eivable and well do umented setting for lat-eral extrusion is indi ated by a steep onvergen e of plates (e.g. Molnar andTapponier (1977)) or pure-shear dominated transpression (Fossen and Tiko�(1993), Tiko� and Teyssier (1994), Jones et al. (1997)). If we ontemplate thispossibility for the Galana River area we have to �nd stru tural argumentsfor this assumption. Grasemann et al. (2003) stated the stability of normalshear bands under general ow onditions (Wn = 0:6 � 0:8) whi h woulda ord with an angle of 40� 50Æ between the eigenve tors of entran e- andexit ow (parallel to shear zone boundaries). Using the relationship betweenrelative plate motion ve tor ( ontradi tional ow apophysis) and instanta-neous strain quantities (Tiko� and Teyssier, 1994) in a qualitative way wewould obtain a onvergen e angle of about 30� 60Æ (dashed lines, Fig. 6.4)for a sinistral transpression hara terized by ombined attening and simpleshearing. The Wk value will be lowered as a result of the present 3D straine�e ts. Yet, many �eld examples ategorized as normal types (syntheti dis-pla ement to bulk non- oaxial ow and in lined towards the shear dire tion,Fig. 4.1) are oriented in a manner that they would re e t a dextral non- oaxial ow. This may evin e a omponent of lateral stret h assuming, e.g.,that left lateral shear was relieved at �rst during oblique onvergen e. Com-pensation of orthogonal shortening at an advan ed stage of deformation may

6.3. CONVERGENT TECTONICS AND STRAIN PATTERNS 69then have a tivated strike-parallel extension in a un on�ned way a entuatedby distin t shear bands. The frequently observed shallowly dipping lineationswould also be onsistent sin e favoured by horizontal material es ape. Thismeans that if we stri tly attribute the strain pattern of unit II to un on�nedtranspression, the onvergen e ould take pla e within a se tor ranging be-tween a ute and nearly perpendi ular angles to the zone boundary (dottedlines, Fig. 6.4). All eventualities would lead to horizontal or gently dippinglineations and ould give the impression of a purely wren h-dominated event.Basi ally, this annot be ex luded unless we have information on the degreeof strain partitioning. As mentioned earlier, the strain pattern of unit I maybe related to a parti ular step during the development of the shear zone.Thus it ould be a ounted as a partitioned oaxial omponent of deforma-tion whi h is reinfor ed by similar Ar/Ar ooling ages (see hapter 5). Withinthe shear zone partitioning in terms of dis rete stru turally bound domainswere not as ertained but an rather be found as narrow zones of on entratedstrain (heterogeneity). The �rst mentioned possibility may relate to vis osity ontrasts of alternating lithologies, although their rheologi al behaviour mayapproximate under high grade metamorphi onditions. Likewise, the latteris typi al for deep rustal environments. Sin e until now we onsidered onlyhorizontal stret h of material, a short omment should be addressed to theverti al portion. In this ontext we have to ask for a releationship betweensporadi ally observed steepening of stret hing lineations and spe ial domains.Su h regions ould denote a prevailing dip-ass o iated pure shear omponentessential to ompensate �nite strain variant from strike dire tion. Otherwise,but not ex luding the prementioned, it is on eiveable that they indi ate anon- oaxial dip-slip omponent ontributing to steeper lineations whi h alsowould be re on ilable with a pure-shear dominated transpression. A singleobservation within the shear zone suggests the availability of this option. Averti al erosion surfa e, visible at an obtuse angle to the steeply plunging fo-liation, displays a strait zone of a learly asymmetri fabri (Fig. 6.6). It hasto be noted, however, that the ir umstan es did not admit the measurementof lineations and hen e a orrelation annot be proven.Furthermore, an alternative way has to be dis ussed how the present situ-ation an be a hieved. Considering the equivo al distribution of lineations ofGalana East we ould impli ate this with superimposed te toni s, as statedfor adja ent areas (e.g. Saggerson (1962)), or varian es in the mode of defor-mation. A gradually hanging plate motion and reoriented stress �eld ouldhave formed the oblate strain geometry of the shear zone, too. Granted thatthe formation ommen ed with a greater left-lateral wren h omponent andresulted in a system of emerging oaxial deformation this would be a om-panied by a ounter lo kwise rotation of the onvergen e ve tor (su essive

70 CHAPTER 6. STRAIN ESTIMATIONSenlargement of �).

6.3. CONVERGENT TECTONICS AND STRAIN PATTERNS 71

1

2

2 3

3

4

4

5.5

8.9

5

Galana West

I IIIII

IV

Galana East

Transition zone

Taita Hills

k=1

k=¥

k=0

XY

YZ

(

(

)

)

a

b

a b( )F1’yXY° ( )F2’yXZ° ’( )F3yYZ°

36°

23°

17°

30° 25°

38° 26°

16°

19° 1.2

1.1

8.9

5.5

4.6

1.2

1.4

2.7

15° 1.2 1.4I

II

III

IV

W Galana East (unit I)Galana est (unit II)

Taita Hills (unit III)

Transition zone (IV)y

x’

y

z

xz

z

z’

x

y

x

y

x

yz

y

x

z

y

zx

z

y

x

Figure 6.2: Flinn-diagram and angular values for di�erent units (top). Belowappendant orientation of prin ipal strain axes for estimates from the TaitaHills-Galana River area. Apostrophized notations indi ate that axes appearas shorter or longer on a horizontal se tion.

72 CHAPTER 6. STRAIN ESTIMATIONSGalana East (I)

n=52 (e )n=50 (s )

n=55 (e )n=60 (s )

n=42 (s )

n=35

n=44

n=121 (e )n=121 (s )

Galana West (II)



Taita Hills (III)

n=27

Galana East (I) Galana West (II)

Taita Hills (III)

x

x

x’ x x x

y

y

z

zz (z’)

y’

y y

y

a

b

z

n=34

Figure 6.3: Equal area proje tion (lower hemisphere) of a) in situ strainmeasurements and b) fabri orientations. (a) delimitation of shortening (grey)and extension �elds (white). (b) plunge dire tions of prin ipal AMS axeslabeled as solid squares (kmax), solid triangles (kmean), and solid ir les (kmin).

6.3. CONVERGENT TECTONICS AND STRAIN PATTERNS 73

Y

d

a

y

z

x

simpleshearing

pureshearing

zsimpleshearing

pureshearing

x

Ya

yc

xz

simpleshearing

pureshearinga

yb

simple

x

y

Y

zshearing

pureshearing

a

a

e

Figure 6.4: Transpression models from (a) Sanderson and Mar hini (1984);(b) Robin and Cruden (1994); ( ) Tiko� and Teyssier (1994); (d) Jones et al.(1997). e) Possible angles of onvergen e onsidering stability of shear bands(dashed lines) and lateral extrusion of material (dotted lines). See text fordis ussion.

74 CHAPTER 6. STRAIN ESTIMATIONS

Figure 6.5: Assumed a-type anking fold with opposed (dextral) displa ementto overall sinistral sense of shear. (Latitude 38Æ37'2" at the Galana River).

Figure 6.6: Subverti al erosion surfa e with asymmetri extensional fabri hinting at oblique- to verti al movement. The ba kground of the pi ture isformed by paleogene phonolites of the Yatta plateau. (Latitude 38Æ41'26" atthe Galana River).

Chapter 7Magneti fabri studies7.1 Purpose and method7.1.1 OutlineAgain we entre on the te tonostratigraphi units of SE-Kenya. Complemen-tary to strain estimations the fabri of high-grade metamorphi gneisses wasalso hara terised using magneti properties. Based on these results informa-tion on the fabri development of the di�erent units was dedu ed.7.1.2 Theory and de�nitionsFor low magneti �elds (Collinson, 1983) the magneti sus eptibility (k) anbe des ribed as a se ond rank-symmetri tensor relating an indu ed magne-tization (M) of a material to an applied external magneti �eld (H) so thatM = kH. A visualization of k is possible by its eigenvalues and -ve tors whi hde�ne an ellipsoid (kmax�kint�kmin) representing the anisotropy of magneti suszeptibility (AMS) or the magneti fabri (e.g. Hrouda (1982)). Depend-ing on atomi properties and orresponding magneti behaviour, most of thero k forming minerals are lassi�ed as dia- (e.g. quartz, feldspar, al ite) orparamagneti (e.g. mi as, amphibole, pyroxene). A third relevant group, of-ten asso iated with the a essory portion of ro ks, are ferrimagneti minerals(e.g. magnetite, haematite). As a onsequen e, the overall suszeptibility re-sults from the totalised volumetri ontent of single mineral suszeptibilities omposing a volume of ro k.To des ribe the magneti fabri several parameters are used on erningthe magnitude of anisotropy or the shape of the suszeptibility ellipsoid. Anindi ator for the sus eptibility of a ertain volume of a material is quoted as75

76 CHAPTER 7. MAGNETIC FABRIC STUDIESthe mean (average) sus eptibilitykmean = (kmax + kint + kmin)=3A measure of the degree (magnitude) of anisotropy (e.g. Nagata (1961)) isdetermined as P = kmax=kmin or stated in the form of the orre ted anisotropydegree (Jelinek, 1981)P 0 = expq2 [(lnkmax � km)2 + (lnkint � km)2 + (lnkmin � km)2℄with km = (lnkmax + lnkint + lnkmin)=3Lineation (L) and foliation (F ), in terms of magneti anisotropy, are ex-pressed as L = kmax=kint (Balsley and Buddington, 1960) and F = kint=kmin(Sta ey et al., 1960). Integrating all lineation and foliation fa tors, Jelinek(1981) suggested a shape parameter (T ) de�nded asT = [2(lnkint � lnkmin)=(lnkmax � lnkmin)℄� 1Prolate shapes are labeled by T-values between -1 and 0, oblate fabri s showvalues between 0 and 1.7.1.3 MeasurementsTe hnique and mineralogyIn the �eld oriented hand-spe imen were ut in a way that small ubes of2 m side length were produ ed in losing the foliation as a referen e plane.The measurements were a omplished at the Paleomagneti Laboratory ofthe Institute of Geophysi s (University of Leoben) in Gams, Austria. Bulksuszeptibilities (kmean) and anisotropy parameters (see above) were al u-lated applying a Kappabridge KLY-2 (Geofyzika Brno) and the softwareANISO20.The majority of ro ks utilized for the study are migmatiti Am-Bt-Fsp-Qtz�Grt�Cpx�S ap gneisses with small- to a essory ontent of magnetite,haematite and ilmenite. About 30-40 (totally 140) samples were measuredfrom ea h te tonostratigraphi unit. Additionally, a few samples of all unitswere demagnetised with AF (alternating magneti �eld) te hnique by RobertS holger (University of Leoben) proving among others the existen e of mag-netite and hematite.A pre ise allo ation of all magneti omponents was not arried out andthe following dis ussion relies mu h on �eld (large-s ale te toni style)- and

7.1. PURPOSE AND METHOD 77fabri observations (meso- and mi ros opi ) ompared to magneti propertiesof spe imens pertaining to a unit. An average of about the half of the datasetshows mean suszeptibilities ranging between � 1 � 10x10�4SI, one thirdindi ates values of � 1� 9x10�3SI and around one seventh displays a kmeanof � 1� 4x10�2SI.Two parameters ontrol the anisotropy of single grains, namely the rystal-line- and the shape anisotropy. Inter rystalline dire tions (e.g. planes, axes) ause a orrelative arrangement of the magnetization and determine the rys-talline anisotropy whereas shape anisotropy is bound to the longest dimensionof a grain. Both, together with the alignement of individual grains, form themagnitude of the anisotropy. In matters of the magnitization this means thatdia- and paramagneti minerals, as well as parts of the ferrimagneti (e.g.haematite), are mainly a�e ted by the rystalline anisotropy. In the ontrary,shape anisotropy is most distin t for magnetite. From mean suszeptibilitiesof our data a umulative in uen e or dominan e of the ferromagneti propor-tion is evident. Basi ally this is important as smallest quantities of magnetitebias or may de ide on (> 1%) the degree of anisotropy (de rease) and shape(prolate) of a fabri versus predponderating dia- and paramagneti mineralsmake up the matrix (e.g. Borradale (1988)).Chara terization of fabri sSubsequently we will avail AMS hara teristi s to �gure out eviden es helpingto on�rm or re onsider informations gained from the �eld. A fundamentalpremise valid for all onsiderations involved is that preferred orientation ofmeasurements mostly should stem from te toni events whi h gave rise tothe varying appearan e of domains. This assumption is supported by straindata ( hapter 6) and in situ observations pointing to intense deformation atthe majority of out rops.A prevalent phenomenon in deformed ro ks is the onformity of dire tionsof magneti anisotropy fabri s and those te toni al indu ed su h as myloniti lineations as well as regional stru tures (e.g. Goldstein (1980), Klig�eld et al.(1977)). By omparing our magneti - and �eld data (lineations and foliations)we strike for the same goal. We examine a orrelation and, if so, attempt todes ribe a possible meaning for fabri development or deformation. The pre-mentioned an be on luded from a summing-up of Fig. 7.1 and Fig. 3.1.Equal area plots (lower hemisphere) show an agreement of ontoured folia-tions - lineations of di�erent units and the frequen y of appendant magneti equivalents marked as solid squares for lineation (kmax), triangles (kint) and ir les for foliation poles (kmean). Hen e, we may address to the latter ques-tion.

78 CHAPTER 7. MAGNETIC FABRIC STUDIES

Taita Hills (III)

2,5

-1

-0,5

0

0,5

1

1,5 2

P'

T

Galana West (II)

T

-1

-0,5

0

0,5

1

1,2 1,4 1,6 1,8 2

P'

-1

-0,5

0

0,5

1

1,5 2 2,5

P'

T

Galana East (I)

-1

-0,5

0

0,5

1

1,5 2 2,5

P'

T


Figure 7.1: Orientation of AMS prin ipal dire tions (equal area; lower hemi-sphere) and P'-T diagrams (Jelinek (1981)), Hrouda (1982)) for di�erentte tonostratigraphi units. Marked areas (dashed lines) indi ate lusteringof data whi h orresponds to the abundan e of lineations measured in the�eld.A pro edure was hoosen whi h allows graphi al presentation of dire tionsand distributions of magneti data. This is a hieved by the use of eigenve -tors and -values plotted into a S hmidt net and a two-axis plot (Wood o k,1977), respe tively. Starting from P'-T diagrams (Jelinek (1981), Hrouda(1982), Fig. 7.1) we have to �nd a riterion that enables us to identify spe- i� subgroups within measurements of a domaine. The shape parameter Tis adopted for this purpose following a study on greens hists of the varis an

7.1. PURPOSE AND METHOD 79Start-Complex in SE England. S hobel (1998) dedu ed a quantitative re-lationship between fabri evolution ( hanging T parameter and anisotropydire tions, in reasing magneti fabri strength) and intensity of deformationduring progressive te tonism.For various reasons it is more spe ulative to arrive at de�nite on lusionsin our ase. Polyphase te tonothermal events were reported (e.g. Saggerson(1962), Pohl et al. (1980)) for parts of the study area and own observationsmay also permit this interpretation for other se tions. As a result, unexpe teddistributions may have been generated diÆ ult to onstrue. In all, the datasetis not homogeneous in referen e to kmean (see above) and anisotropy degrees(P 0, see Fig. 7.1) denoting modi� ation of mineralogy within the gneisses ordi�erent lithologies. Another ompli a y is due to the number of spe imenutilized. Originally, this study was not s heduled and sampling was "arbi-trary" for the subje t but may thereby argue for the �ndings. Consequentlythe su eeding onsiderations essentially stand for a general survey like statedfor strain estimations.7.1.4 ResultsAt �rst we look at the entire disributions of all measurements. From P'-T- orFlinn-diagrams (Fig. 7.1, Fig. 7.2) one an �nd that units I and III predom-inantly show oblate fabri s whereas the other domains ontain a noteableamount of prolate types. Sin e deformation is assumed to be the main ausefor o uring fabri s we have to assess the in uen e of mineralogy. About onethird of the kkmean values exeed 2x10�3SI and demonstrate the signi� antpresen e of ferrimagneti grains. The Transition zone, Galana West and -East are espe ially a�e ted. Up to mean suszeptibilities of � 2x10�2SI theratio of oblate to prolate fabri s is in favour of positive T parameters, be-yond (seven samples) only negative T values are dis ernible. Despite of theoverwhelming dominan e of magnetite in these spe imens belonging to theTransition zone (kkmean > 2 � 1x10�2SI) mesos opi examination indi atesa L(S)-te tonite appearan e. Thus the mineral property is not de isive forthe prolate shape. Other eviden es are strain estimations of the domain sug-gesting a ommon onstri tional pattern (see hapter 6). On that s ore, therole of deformation as a shape forming property is reinfor ed and supposedto apply as well to low suszeptibility (paramagneti ) fabri s where, onver-sly, oblate types are more abundant (e.g. Taita Hills; Fig. 7.1). There thesame argumentation an be asserted sin e deformation was a ompanied bypronoun ed attening (Fig. 6.2).Like strain- and stru tural data, distributions of AMS prin ipal dire -tions re e t the di�eren es in te toni styles. In order to dete t whether the

80 CHAPTER 7. MAGNETIC FABRIC STUDIES0

0.1

0.2

0.25

0 0.2 0.4 0.6 0.8

k = 1prolat

oblat

ln(k

1/k

2)

ln(k2/k3)

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

k = 1prolat

oblat

ln(k

1/k

2)

ln(k2/k3)

0

0.1

0.2

0.25

0 0.2 0.4 0.6 0.8

k = 1prolat

oblat

ln(k

1/k

2)

ln(k2/k3)

0

0.1

0.2

0.3

0 0.1 0.2 0.3 0.4 0.5

k = 1prolat

oblat

ln(k

1/k

2)

ln(k2/k3)

Taita Hills (IV) Transition zone (IV)

Galana East (I)Galana West (II)

Figure 7.2: logarithmi Flinn-diagrams for ro ks of di�erent te tonostrati-graphi units.magneti fabri s are aligned with the strain geometry the measurements wereplotted together with the estimated s.n.f.l.s (see hapter 6) for ea h te tonos-tratigraphi unit (Fig. 6.3). The larger part of the dataset indi ates a losemat h sin e magneti foliation poles (kmin-) and -lineations (kmax axes) arepreferentially restri ted to the �elds of shortening and extension (Fig. 6.3).More often, the intermediate axes (kint) an be found in both domains. Inpart the deviation of all prin ipal AMS dire tions respe ting the strain se -tors an be explained by the orientation of spe imens in the �eld and applyto foliations whi h dip steeper or shallower than the situation re orded in av-erage. Another reason for non- onforman e might be a disregarded su essive hange of the strain geometry within a domain. For instan e, variably dippingmyloniti horizons o ur at some out rops in the Transition zone (Fig. 4.1)possibly belonging to spe i� steps during the stru tural development (e.g.easterly plunging kmax axes in Fig. 6.3b). In prin iple, however, the issueis aused by problems like homogeneity and s ale, and the fa t that there

7.1. PURPOSE AND METHOD 81exists no "one-to-one" relationship between deformation and the anisotropyof magneti suszeptibility (e.g. Tarling and Hrouda (1993), Borradaile andHenry (1997)).As yet we have as ertained an analogy between fabri orientation andstrain patterns. Before the data an be systematised statisti ally we haveto readdress to the spread of P 0 fa tors and mean suszeptibilites. Abovewe attributed the magneti fabri to the style of deformation and pointedout a likely independen e from mineralogy as noti able from kmean values.The same must be valid for the orre ted anisotropy degree. This was testedgraphing the two parameters (not shown here). A linear dependan e of P 0and kmean exists within a range of � 1:2 � 1:9 and � 1 � 9x10�3SI for themajority of measurements. Suszeptibilites whi h ex eed this range are weaker orrelated with P 0 and those falling below are uniform (P 0 � 1 � 1:2) andthus independent. Relating to our advisements this means that P 0 annot beappli able here as a gauge sin e seemingly unasso iated with deformation ordependent on kmean (Borradaile and Henry, 1997).For statisti al treatment the span of shape parameters was subdividedinto three intervals (Fig. 7.4). All units, other than the Taita Hills for whi honly oblate fabri s were used, ontain two oblate (T2, T3) and one prolate(T1) group. The disparate arrangement of intervals is broadly predeterminedby the primary distributions of fabri s (Fig.7.1) whereas their number alsohinge on the amount of samples available. The numeri al bound for mostintervals is between � 0:3 � 0:4 and arise in our example from small steps,presumably be ause of a la k of �eld information, inside a distribution. Inthe main, the in rements an also be mu h smaller but then imply a suÆ ientquantity of samples.Cal ulations of eigenvalues and -ve tors were performed with an updatedversion of the software quoted in Wallbre her (1986). In doing so the au-thor's notation was retained denominating the order from greatest to leastwith number 3 to 1 (see also Wallbre her (1979)). The statisti al signi� an eof T- ategories were veri�ed with the eigenvalue test suggested by Wood o kand Naylor (1983). Exe pt for a subgroup of kint-axes of Galana East (unitI), anyway pi tured for the sake of ompleteness, all other meet the 95% on�den e level appropriate for small datasets adding up to 6-20 measure-ments per interval. About 15% (22 AMS readings split onto all units) of theprimary dataset were omitted from the analysis in lusive samples of deviantorientation already spoken to. Normally they emerge as single outliers inequal area proje tions of orrespondent subgroups but may, anyhow, om-pose a subfabri type. Half of the left set dese ends from the Transition zone(unit IV) and together with mylonites (see above) omprise prolate fabri s,too little for an own grouping, with shape parameters more negative than

82 CHAPTER 7. MAGNETIC FABRIC STUDIESin luded in T1.Regarding our assumptions a �rst hint for the usefulness of the methodand resultant fabri orientations (Fig. 7.3) is obvious from unit I (GalanaEast). The eigenve tor of prolate fabri s of subgroup T1 (Fig. 7.3) is on-

1

1

1

2

2

2

3

3

3

Galana West (II)

n=29 x

y

z

1

1

1

2

2

2

3

3

3


n=24 x’ x

z (z’)y’

y

Taita Hills (III)

1 1

1

2

2

2

3

3

3

n=41

y

x

z

y

x

1

1

1

2

2

2

3

3

3

Galana East (I)

n=24

z

x

y

Figure 7.3: Comparison of eigendire tions (open symbols; squares (kmax), tri-angles (kmean), ir les (kmin)) and position of �nite strain axes (�lled squares)for di�erent units (equal area, lower hemisphere). Numbers orrespond to par-ti ular T-subgroups. Statisti al insigni� ant values are indi ated by dashed ir les.form with the most distin tive ontoured submaximum (SSW-NNE oriented)of �eld lineations (Fig. 3.1). Conversely, those of the oblate groups (T2, T3;Fig. 7.3) approa h the maximum of �eld lineations (Fig. 3.1 ) whereby themore oblate group plot losest. Altogether, from the a ordant eigenvalueratios we preferentially onsider the varying magneti foliation poles to ex-emplify the fabri s (e.g. Par�es et al. (1999); Fig. 7.4).To strive for an identi� ation of a lear tenden y (as outlined by dotted urves of Fig. 7.4) of di�erent units is tri ky. Most of the data, other thanthe shear zone (unit II), depi t, if onjoined, expli it loops so that a hangeof position k = (ln(e3)� ln(e2)) = (ln(e2)� ln(e1))

7.1. PURPOSE AND METHOD 83

T3 = 0,3 - 0,6

T2 = 0,0 - 0,29

T1 = -0,48 - -0,15

T3 = 0,35 - 0,7

T2 = 0,0 - 0,28

T1 = -0,42 - -0,05

T2 = 0,23 - 0,59

T3 = 0,65 - 0,98

T1 = -0,35 - 0,07

T3 = 0,7 - 0,9

T2 = 0,45 - 0,69

T1 = 0,0 - 0,39

Galana West (II)


Galana East (I)

Taita Hills (III)

girdles

clustersk = 1

0 0.5 1 1.5 2 2.5 3 3.5 4

ln(e2/e1)

ln (

e3/e

2)

girdles

clustersk = 1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

ln(e2/e1)

ln (

e3/e

2)

girdles

clusters

k = 1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

ln(e2/e1)

ln (

e3/e

2)

0

00

0.5

0.50.5

1

11

1.5

1.51.5

2

22

2.5

2.52.5

3

33

3.5

3.53.5

4

44

k = 1

girdles

clusters

0 0.5 1 1.5 2 2.5 3 3.5 4

ln(e2/e1)

0

0.5

1

1.5

2

2.5

3

3.5

4

ln (

e3

/e2)

Figure 7.4: Orientation distributions for the various units. Di�erent shadingsof symbols (here eigenvalue ratios of prin ipal AMS axes) denote varyingT-subgroups.(Wood o k, 1977)) and strength ( = ln(e3=e1), Wood o k (1977)) is not ombined with the order of T-intervals as anti ipated for a gradual progres-sion. Hen e, in these ases, we have to investigate how far it is feasible toderive a development. Further on, deformation has to be in orporated sin eheld responsible for fabri o urren e and should largely ontrol distributions

84 CHAPTER 7. MAGNETIC FABRIC STUDIESas well as orientations.The hypotheti al path of unit I (Fig. 7.4) ommen es at the girdle �eldwith T1 and prolate shapes and shifts into the the luster domain in whi hT3 manifests the most distin tive tenden y. In this regard it ould be on-sidered as trend ending in a k > 1. This is more ambiguous if we fa torin the strength (a retive with distan e from the origin) greatest for T2.It is not lear whether this originates in deformation, arrangement of dataor in omplete sampling. A gradual forming of luster distributions of kminwould be on lusive rese ting the assumed oblate strain geometry (see hap-ter 6) of unit I. (Fig. 7.3) A potential interrelation of fabri developement(orientation) and deformation might be signi�ed in that the eigenve tors ofmagneti foliation poles (oblate subgroups T2, T3) are lo ated nearer to the�nite shortening axis z than the prolate subgroup T1 (Fig. 7.3).The learest development ould be obtained for the shear zone (unit II).The varian e of positions within the two-axis plot (Fig. 7.4) from a pro-noun ed girdle- to a transitonal girdle- luster distribution, is orderly oupledwith an enhan ement of strength but with a smaller gradient between T2and T3. A ordant orientations of kmin axes (Fig. 7.3) display an in reas-ing dip of the fabri towards oblate shape parameters whereupon T3 plot in lose vi inity to the �nite shortening dire tion z. A similar behavior is visi-ble from distributions of kmax axes whi h be ome more in lined with risingT-invervals. This may hara terize a transpressional regime, e.g., tenden yto �nite attening geometry or steepening of stret hing lineations.The ommon prolate or folded nature of the Transition zone (IV, Fig. 7.4)appears as marked by girdle distributions of subgroups T1 and T2. Alike forthe other units a feasible path should be interpreted along with strain esti-mations ( hapter 6) and, orrespondingly, evin e onstri tional deformation.Then the urve (or straight line) should progress from (near) the luster-into to the girdle �eld as partly ful�lled by our data. Again, the strengthof the fabri is greatest for grouping T2 and redu ed here for T1, the mostprolate fra tion, at onstant slope. Appendant eigenve tors of T2 and T3(Fig. 7.3) do ument a shallow to moderate westerly and north-easterly dip-angle (� 19 � 33Æ) of fabri s, T1 dips steeply towards the west. The leastangle to the now verti al z dire tion is formed by T2, yet the other dire -tions are not systemati ally spread (no enlargement of angles with de reasingstrength). The kmax dire tions are pla ed around the di�erent solutions of xand x0 (see hapter 6) with T2 most variant from x.All eigenve tors of magneti foliation poles of the Taita Hills (unit III,Fig. 7.3) are more lustered than for other units. Possibly, this is linked upwith the instan e that oblate fabri s were preferably olle ted owing to the onspi uous attening strain expressed in the two-axis plot (Fig. 7.4) as val-

7.1. PURPOSE AND METHOD 85ues lose to or surmounting the gradient k = 1. Aside from a turn betweeninterval T2 and T3, the strength of the groupings ould be viewed as evo-lution gradually advan ing. Moreover, the ontoured maximum of foliations(Fig. 3.1) and that of T3 (dotted plane) nearly oin ide unless a few de-grees in strike whi h is true also for the shear zone data (not indi ated).The magneti lineations (T1-T3) are stret hed a ross the highest densityarea of ontoured lineations where T2 overlaps with the statisti al maximum(Fig. 3.1, Fig. 7.3).Signi� an eSumming up the results we an note that generally AMS dire tions tend tomirror the average stru tural onditions (deformation geometry) of di�erentte toni domains and might support their tenability. Classi� ation into in-tervals and following analysis provides an insight into fabri hara teristi s(orientation, distribution and strength), their development and eventually re-lated deformation (�nite strain). A methodi varian e of all qualities is mostobious from the shear zone (unit II).There, in addition, the �nite shorten-ing axis (z) and subgroups T2 and T3 plot at lose quarters pointing at aninterdependen y of attenig geometry and oblate fabri s. The relevan e ofT1 (prolate shape parameters) ould be onstrued, e.g., as o uring prolatedomains or evolutionary as re orded remnants of an earlier folding stage.For other domains more general tenden ies an be inferred. Alike for unitII eigenve tors of T2 and T3 of unit I approa h the z axes of the �nite strainellipsoid. Those asso iated with magneti lineations (kmax of T1, T2 andT3) evin e a greater variation from the x- and y dire tions ompared to theother units. This is potentially on erned with the te toni situation or la kof information (number of samples). Distributions of kmin axis of the TaitaHills (unit III) are hara terised by prevailing luster omponents omplyingwith deformation. Equal area proje tions of all T groupings ome near thez axis but due to lustering an angular separation (10-14Æ) emerges moredistin t than for other units. This might be onne ted with thrust te toni swhi h is proved to have a major e�e t on unit III ( hapter 4). Statementsabout the Transition zone are more limited. The prolate habit of the domain an be reprodu ed by girdle distributions and eigendire tions. An intera tionof the statisti parameters as a unique trend is unapparent, however, mightbe expli able by polyphase deformation whi h is of importan e also for unitI and III. In ontrast, absent overprint might a ount for the results of theshear zone (youngest stru ture). An improvement of �ndings ould de�nitelybe attained enlarging the number of samples (T intervals) thus permitting amore detailed view on spe i� lithologies or domains.

86 CHAPTER 7. MAGNETIC FABRIC STUDIES

Chapter 8Geo hemistry8.1 Metabasites8.1.1 O uren eSeven o uren es were sampled omprising ro ks from the Galana River andTaita Hills in SE Kenya (Fig. 3.1) and three lo alities in NE Tanzania at thenorthern foothills (m3)- or within the Pare Mountains SE to ESE of Same(m1, m2; Fig. 3.5). In SE Kenya amphibolites and garnet bearing varietieswere pi ked from small lenses and thin bands. The Tanzania samples arepredominatly ultrama� and appear as isolated lentiform bodies whereasseveral ro k types were spe i�able at at all out rops.Major and tra e element analyses of all metabasi and gneissi sampleswere performed at A tivation Laboratories LTD in An aster (Ontario), Can-dada, using ICP-AES and ICP/MS te hniques (Table 8.1). The sampledro ks were subje ted to severe deformation and metamorphosed under gran-ulite fa ies onditions (Hauzenberger et al., 2003) a ompanied by uid in�l-tration during evolution of the high-grade gneisses. Therefore element mobil-ities aused thereby must be onsidered. This ould geneti interpretationsput into perspe tive.8.1.2 SE KenyaFour spe imen fromGalana East (unit I) and -West (unit II) at ea h ase wereinvestigated. Those from unit I stem from four out rops (Fig. 3.1) lo atedat the eastern, entral and western part. Two exposures ontribute to thedataset of the shear zone situated more easterly and entral in respe t to theborders. Supplementary three amphibolites were analysed from the Taita87

88CHAPTER

8.GEOCHEMISTRY Sample

SiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

TiO2

P2O5

LOI

Total

Mg/(Mg+Fe)

ACNK

Sc

Be

V

Cr

Co

Ni

Cu

Zn

Ga

Li

As

Rb

Sr

Y

Zr

Nb

Mo

Ag

In

Sn

Sb

Cs

Ba

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

Ta

W

Tl

Pb

Bi

Th

U

m1 m2 m3 am1

147-1 147-2 147-3 147-4 147-5 149-1 149-2 149-3 149-4 152-0 152-1 152-2 A 14 KTB 98

43,61 45,46 40,58 42,35 49,25 36,73 37,47 38,11 39,28 41,73 46,83 47,79 46,33 48,57

16,38 16,85 23,65 12,01 21,11 15,96 14,44 13,75 17,36 15,93 12,73 13,83 13,28 13,20

10,47 9,21 9,71 11,17 7,01 19,87 19,61 17,58 16,99 15,16 5,26 3,48 15,45 15,50

0,16 0,17 0,25 0,20 0,17 0,29 0,32 0,28 0,26 0,23 0,11 0,11 0,28 0,23

12,21 12,11 3,83 11,09 3,26 8,98 6,09 8,20 7,84 8,36 16,97 10,39 5,37 5,26

12,59 12,90 19,54 20,67 12,00 14,48 12,87 12,52 12,97 13,87 16,44 22,10 8,80 9,03

2,10 1,78 0,62 0,71 3,96 1,60 1,41 1,93 1,70 1,59 0,64 0,36 2,96 2,60

0,38 0,44 0,17 0,14 0,37 0,26 0,83 0,86 0,67 0,28 0,10 0,22 1,74 1,24

0,29 0,07 0,70 0,26 0,60 1,03 4,13 4,18 0,91 1,28 0,09 0,09 2,98 2,17

0,02 0,02 0,04 0,01 0,21 0,10 0,41 0,70 0,08 0,12 <0,01 0,03 0,63 0,30

1,40 1,23 0,34 1,50 0,44 0,62 <0,01 0,34 0,54 0,02 1,31 0,49 0,87 0,43

99,60 100,24 99,42 100,11 98,38 99,92 97,59 98,45 98,59 98,57 100,48 98,88 98,69 98,53

0,70 0,72 0,44 0,66 0,48 0,47 0,38 0,48 0,48 0,52 0,86 0,86 0,41 0,40

34,00 59,00 22,00 63,00 20,00 72,00 49,00 34,00 59,00 57,00 41,00 53,00 51,00 39,00

<1 <1 <1 <1 1,00 <1 1,00 2,00 <1 <1 <1 <1 1,00 1,00

398,87 267,95 126,11 270,48 100,24 487,77 563,32 331,64 433,71 360,14 115,89 124,53 361,03 333,27

23,56 89,56 <20 491,98 <20 62,76 64,15 444,64 62,06 315,51 789,26 246,83 64,65 92,58

80,87 62,51 19,84 65,09 16,50 70,58 58,03 69,68 59,55 84,32 64,33 27,83 48,35 68,88

41,97 61,52 <20 103,22 <20 52,67 82,45 807,65 57,78 131,46 296,09 94,34 38,14 57,86

17,80 12,77 <10 14,17 <10 12,75 17,71 11,30 10,92 24,97 24,23 16,09 43,92 68,06

<30 <30 <30 60,58 44,31 75,11 <30 122,96 46,66 36,39 35,31 <30 124,72 42,26

12,69 6,04 17,87 11,54 16,58 8,70 16,73 24,25 13,01 18,24 7,30 6,70 18,43 18,06

1,30 1,41 0,60 2,52 0,59 <0,5 0,61 1,24 0,73 1,69 1,05 0,73 0,87 1,43

<5 <5 <5 <5 41,58 <5 <5 <5 <5 <5 <5 <5 <5 <5

2,10 6,75 4,61 3,64 3,32 2,82 7,35 5,34 8,40 2,28 2,21 4,72 35,54 27,51

210,03 272,13 2260,00 364,04 1090,00 465,08 378,07 484,77 893,28 165,58 42,55 73,89 256,47 219,19

1,54 1,09 38,86 5,96 28,00 30,48 61,72 38,75 26,58 28,18 3,72 2,80 53,07 50,53

13,06 11,55 178,58 13,45 149,16 43,17 144,92 293,61 33,26 61,06 2,91 12,48 166,11 156,28

0,35 0,29 7,67 0,71 10,31 0,50 12,18 53,18 1,47 3,18 0,25 0,44 5,18 10,08

<2 <2 <2 <2 <2 <2 2,58 <2 <2 <2 <2 <2 <2 <2

<0,5 <0,5 0,83 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5

<0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1

<1 <1 1,17 <1 <1 <1 1,40 2,63 <1 <1 <1 <1 1,10 1,53

<0,2 0,28 0,48 0,99 0,50 <0,2 0,65 0,29 0,24 0,51 <0,2 0,49 <0,2 <0,2

<0,1 0,27 0,12 1,12 0,23 <0,1 <0,1 <0,1 <0,1 0,16 <0,1 <0,1 0,37 0,11

42,90 36,70 663,53 56,55 1200,00 195,97 386,49 241,40 699,63 72,68 18,12 146,72 993,93 267,41

0,93 0,98 31,08 2,49 20,39 2,20 20,09 51,14 2,27 4,87 0,54 0,63 13,90 17,44

1,59 1,25 58,34 6,52 42,21 6,01 49,01 107,90 5,75 12,88 1,00 0,96 37,65 42,06

0,18 0,11 6,79 0,79 4,43 0,89 6,12 11,44 0,81 1,65 0,15 0,11 5,32 5,09

0,76 0,42 28,14 3,72 19,61 5,64 30,11 51,23 4,79 8,48 0,73 0,51 28,86 24,55

0,16 0,10 5,99 0,95 4,36 2,37 8,42 10,81 1,93 2,85 0,32 0,22 8,31 6,72

0,17 0,12 1,67 0,21 1,33 0,97 3,48 3,79 0,83 1,13 0,19 0,12 2,97 2,22

0,22 0,13 5,63 0,97 4,46 3,30 10,27 9,34 2,97 3,65 0,42 0,32 9,67 7,94

0,03 0,02 1,02 0,16 0,72 0,70 1,72 1,40 0,58 0,67 0,09 0,06 1,58 1,35

0,24 0,18 5,91 1,03 4,36 4,93 10,50 7,83 4,06 4,44 0,66 0,48 9,60 8,53

0,05 0,04 1,20 0,21 0,91 1,08 2,14 1,39 0,92 0,98 0,14 0,10 1,94 1,80

0,16 0,14 4,19 0,67 2,93 3,42 6,34 3,73 2,86 3,01 0,43 0,31 5,75 5,45

0,02 0,02 0,69 0,11 0,49 0,57 0,95 0,53 0,46 0,48 0,06 0,04 0,81 0,82

0,18 0,15 4,56 0,74 3,12 3,55 5,75 3,14 2,96 3,12 0,40 0,30 4,84 5,11

0,03 0,02 0,72 0,10 0,49 0,53 0,86 0,40 0,47 0,50 0,06 0,04 0,70 0,75

<0,1 <0,1 5,16 0,14 4,01 1,38 4,27 7,70 1,15 1,69 <0,1 0,12 4,49 4,67

0,97 0,61 1,62 0,45 0,97 0,42 1,73 4,52 0,31 1,05 0,67 0,47 0,77 1,69

334,33 223,66 215,09 181,96 102,41 173,58 188,63 136,95 113,22 375,55 224,96 156,65 181,09 338,12

<0,05 <0,05 <0,05 0,06 <0,05 <0,05 <0,05 0,06 <0,05 <0,05 <0,05 <0,05 0,17 0,09

<5 <5 <5 8,08 14,06 <5 <5 <5 <5 <5 5,55 <5 <5 <5

0,16 <0,1 <0,1 0,51 0,38 0,79 <0,1 0,34 0,45 0,10 <0,1 <0,1 <0,1 <0,1

0,17 0,38 5,07 0,33 3,45 0,15 1,95 5,52 0,21 0,49 0,10 0,27 0,68 1,30

0,07 0,14 0,90 0,09 1,18 0,07 0,88 1,43 0,08 0,12 0,05 0,09 0,40 0,54

am2

K57 F2 KGB 27 K 108

45,82 54,54 46,26 46,61

12,60 16,49 13,64 11,33

20,94 7,79 14,08 10,97

0,28 0,17 0,21 0,19

5,62 5,55 7,94 13,20

8,70 7,83 11,99 15,01

1,51 4,68 2,20 0,89

1,43 1,32 0,52 0,59

3,00 0,70 1,15 0,68

0,40 0,18 0,03 0,06

0,31 1,58 0,83 0,77

100,61 100,83 98,85 100,30

0,35 0,59 0,53 0,70

44,00 25,00 50,00 66,00

3,00 <1 <1 <1

625,06 169,12 331,83 251,46

41,17 130,10 241,18 491,01

94,16 64,82 65,63 78,86

130,27 37,71 97,19 129,78

156,60 <10 66,09 27,26

161,32 75,18 61,99 <30

25,25 17,28 15,82 7,00

2,34 1,49 1,22 <0,5

<5 <5 <5 5,76

34,35 22,27 9,19 14,13

122,75 645,27 119,13 388,85

61,68 13,65 23,20 15,08

227,27 66,15 28,66 30,47

12,59 2,22 0,55 1,05

<2 <2 0,50 <2

<0,5 <0,5 <0,5 <0,5

<0,1 <0,1 <0,1 <0,1

2,81 <1 <1 <1

1,39 0,23 0,21 0,53

0,20 0,19 0,16 0,26

327,70 615,49 306,67 133,93

20,09 13,35 5,06 3,48

47,31 29,71 9,56 9,34

6,14 3,58 1,07 1,36

28,46 15,36 5,51 7,84

7,55 3,27 2,18 2,53

2,76 1,29 0,97 0,87

8,95 2,92 3,16 2,70

1,71 0,43 0,61 0,46

10,76 2,56 4,03 2,68

2,13 0,48 0,88 0,52

6,81 1,46 2,66 1,51

1,03 0,19 0,42 0,23

6,85 1,32 2,55 1,35

1,00 0,20 0,38 0,18

6,67 1,96 0,95 1,00

2,03 1,35 0,49 1,01

361,74 385,97 184,64 333,99

0,16 0,11 0,05 <0,05

7,54 7,95 8,38 <5

0,21 <0,06 0,11 <0,1

1,85 1,21 0,68 0,41

0,67 0,41 0,55 0,22

8.1.METABASITES

89Mg/(Mg+Fe)

Sample

SiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

TiO2

P2O5

LOI

Total

ACNK

Sc

Be

V

Cr

Co

Ni

Cu

Zn

Ga

Li

As

Rb

Sr

Y

Zr

Nb

Mo

Ag

In

Sn

Sb

Cs

Ba

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

Ta

W

Tl

Pb

Bi

Th

U

gn3 gn4

MDB KTB 46A K37 K115 K120 K154 K209 KGB 174

62,26 68,34 65,82 66,53 75,84 73,94 67,43 63,07

16,36 14,95 17,90 14,98 12,25 13,04 14,66 15,94

5,82 3,36 3,92 7,95 3,89 5,03 6,31 6,76

0,08 0,06 0,05 0,17 0,09 0,14 0,14 0,11

2,86 0,86 1,10 1,32 0,55 0,56 1,40 2,75

4,83 2,42 3,20 5,42 3,47 3,56 5,64 5,67

4,30 3,87 4,81 3,19 3,46 3,91 3,22 3,65

1,76 4,24 2,74 0,66 0,57 0,24 0,32 1,32

0,92 0,58 0,68 0,37 0,22 0,23 0,35 0,80

0,19 0,15 0,20 0,14 0,08 0,08 0,10 0,18

0,95 0,41 0,41 0,05 0,22 <0,04 <0,07 0,41

100,33 99,24 100,83 100,78 100,64 100,73 99,57 100,66

0,92 0,97 1,07 0,95 0,97 0,99 0,92 0,90

14,00 7,00 6,00 29,00 17,00 17,00 22,00 18,00

1,00 2,00 2,00 <1 <1 <1 <1 1,00

94,14 31,67 42,95 56,55 25,47 17,93 66,93 84,34

34,94 <20 <20 <20 <20 <20 <20 30,86

88,45 42,03 78,48 85,15 99,03 74,00 84,06 80,59

28,42 <20 <15 <15 <15 <15 <15 <20

11,29 <10 <10 98,49 <10 <10 <10 14,37

88,11 30,23 49,08 108,74 42,69 54,73 77,26 <30

20,41 17,78 22,55 17,33 12,86 16,45 14,10 17,87

1,14 0,62 0,84 1,64 1,21 1,54 1,46 0,93

<5 <5 <5 <5 <5 <5 <5 <5

49,59 74,28 58,55 4,60 8,83 <2 <2 48,39

443,24 269,05 396,93 384,22 118,30 165,75 122,30 296,00

19,52 25,36 13,07 16,37 36,03 46,02 23,22 23,66

235,61 342,09 352,88 24,53 91,63 85,37 40,41 95,39

4,80 14,59 11,12 1,78 1,80 1,21 1,39 4,35

<2 2,44 <2 <2 <2 <2 <2 <2

<0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5

<0,1 0,10 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1

1,57 1,66 <1 <1 <1 1,16 <1 1,67

<0,2 <0,2 <0,2 <0,2 <0,2 <0,2 0,21 <0,2

1,00 0,78 1,43 <0,1 <0,1 <0,1 <0,1 1,47

610,53 927,54 834,84 220,44 279,62 111,23 69,05 414,99

41,88 57,72 41,70 2,88 8,64 6,65 2,78 11,78

86,60 106,31 80,77 7,19 18,57 14,64 7,63 26,53

8,42 9,58 8,47 1,05 2,61 2,22 1,18 3,07

30,17 35,61 30,46 5,45 12,44 11,14 6,21 13,60

4,86 6,50 5,13 2,03 3,41 3,39 2,10 3,57

1,53 1,34 1,56 1,09 0,84 0,91 0,83 1,07

3,82 5,24 3,86 2,60 4,27 4,88 2,70 3,72

0,63 0,81 0,53 0,50 0,87 1,05 0,55 0,61

3,63 4,58 2,69 3,05 5,55 7,13 3,59 3,77

0,71 0,89 0,46 0,60 1,16 1,53 0,78 0,77

2,08 2,54 1,36 1,87 3,84 5,28 2,67 2,39

0,31 0,39 0,17 0,28 0,59 0,83 0,41 0,36

1,94 2,36 1,17 1,98 3,99 5,65 2,86 2,27

0,29 0,31 0,17 0,31 0,60 0,84 0,44 0,36

5,83 9,12 8,93 0,71 2,91 3,23 1,36 3,00

2,45 1,94 2,73 2,48 3,55 2,17 2,70 2,29

750,49 435,46 734,65 835,23 1040,00 779,81 896,57 773,38

0,20 0,23 0,26 0,06 <0,05 <0,05 <0,05 0,17

8,70 6,59 15,57 14,93 <5 <5 <5 <5

<0,06 <0,1 <0,06 <0,06 <0,06 <0,06 <0,06 <0,1

6,92 9,05 8,09 0,65 1,16 0,34 0,07 2,44

0,86 1,00 1,43 0,36 0,39 0,17 <0,05 1,78

am2 am3 gn1 gn2

KGB 173 KGB 49A KTB60 KTB 175 AKTB55 K13 MB 153 KGB 170 KGB49C KGB35LF

46,54 43,26 43,83 53,69 49,39 61,52 58,56 63,26 59,05 62,77

9,19 14,66 14,87 15,86 15,58 17,08 17,22 16,38 16,75 15,59

10,93 12,73 16,09 9,67 14,53 7,55 6,14 5,44 7,00 5,29

0,19 0,20 0,22 0,13 0,20 0,17 0,11 0,09 0,11 0,08

14,85 8,90 6,58 6,32 5,81 1,87 3,38 2,04 3,64 2,41

16,03 13,64 7,13 9,33 9,51 4,18 6,35 5,10 6,11 4,46

0,87 1,88 2,65 2,48 3,04 3,83 4,81 4,60 3,96 3,97

0,17 0,85 3,19 0,47 0,48 2,21 1,38 1,69 1,79 2,76

0,75 1,10 3,56 1,39 1,29 1,21 0,68 0,84 0,98 0,58

0,09 0,09 0,65 0,26 0,18 0,36 0,33 0,19 0,35 0,23

0,64 0,91 1,26 0,53 0,84 0,95 0,83 0,46 0,93 0,89

100,25 98,22 100,03 100,13 100,85 100,93 99,79 100,09 100,67 99,03

0,73 0,58 0,45 0,56 0,44

1,05 0,82 0,88 0,86 0,88

82,00 52,00 20,00 22,00 38,00 20,00 13,00 13,00 15,00 12,00

<1 <1 2,00 1,00 <1 1,00 2,00 2,00 2,00 2,00

253,45 413,92 245,32 165,33 355,28 120,85 102,87 99,70 137,62 89,86

715,87 81,97 29,09 300,61 44,21 41,13 63,09 22,90 115,37 41,37

76,65 60,04 58,34 80,39 88,62 56,92 39,23 53,57 86,07 62,26

157,99 61,85 79,53 179,64 69,17 25,89 30,39 <20 58,37 19,13

43,35 10,45 22,26 55,76 40,24 66,28 12,34 41,16 49,66 <10

<30 <30 173,70 <30 106,31 160,49 <30 <30 95,09 66,14

10,58 16,35 20,94 13,94 19,25 20,96 17,92 15,30 22,40 18,67

1,76 1,37 1,46 <0,5 1,53 1,34 0,82 0,86 1,30 1,08

<5 <5 <5 <5 <5 <5 <5 <5 <5 <5

4,86 19,24 63,24 2,46 5,07 55,15 26,77 35,65 77,94 87,27

350,96 479,35 500,34 765,55 227,97 368,99 1050,00 619,41 784,99 860,05

15,92 15,41 29,41 12,22 31,26 37,90 16,97 22,43 19,80 14,99

30,86 24,24 215,79 65,00 85,38 245,87 151,06 197,50 205,24 179,43

0,88 1,15 28,94 8,58 2,51 10,46 8,29 6,45 5,95 7,77

<2 <2 3,04 <2 <2 <2 <2 <2 <2 <2

<0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5 <0,5

<0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1 <0,1

<1 <1 2,53 <1 <1 1,55 <1 2,10 1,39 1,01

0,22 0,31 <0,2 0,33 <0,2 <0,2 0,50 1,02 <0,2 <0,2

<0,1 0,11 1,57 <0,1 <0,1 1,08 0,29 1,07 1,60 1,17

98,89 128,74 708,34 91,25 153,70 510,66 837,89 509,02 647,18 1450,00

3,92 4,75 22,94 8,47 5,20 28,33 39,22 27,69 24,21 36,05

10,05 12,21 56,16 22,59 11,43 62,92 78,31 59,51 53,47 64,91

1,65 1,67 7,51 2,91 2,18 7,37 8,20 6,50 6,36 7,04

9,64 9,17 31,39 14,39 11,44 30,37 33,83 27,24 26,22 24,40

3,01 2,84 6,45 3,54 3,37 6,34 5,91 5,79 5,16 4,12

1,02 1,17 2,83 1,50 1,44 2,02 1,85 1,48 1,70 1,13

3,27 3,01 5,83 3,21 4,24 6,26 4,59 4,96 4,69 3,24

0,51 0,48 0,94 0,46 0,83 1,07 0,57 0,73 0,70 0,45

2,94 2,88 5,09 2,55 5,28 6,47 3,11 4,22 3,70 2,44

0,56 0,55 0,99 0,46 1,06 1,28 0,58 0,82 0,67 0,49

1,61 1,58 2,77 1,22 3,38 4,13 1,64 2,34 2,03 1,41

0,23 0,24 0,36 0,16 0,48 0,63 0,23 0,34 0,26 0,19

1,40 1,48 2,31 0,94 3,19 3,82 1,42 2,11 1,70 1,25

0,19 0,20 0,34 0,12 0,47 0,59 0,20 0,29 0,25 0,18

1,12 0,75 4,89 1,93 2,41 6,34 3,70 6,04 5,31 4,82

0,82 0,72 2,55 2,07 1,43 1,99 1,12 1,86 2,34 2,18

290,52 210,33 134,29 564,49 457,53 428,68 284,78 502,59 609,87 530,31

<0,05 <0,05 0,28 <0,05 <0,05 0,31 <0,05 <0,05 0,26 0,47

<5 <5 9,10 <5 <5 11,34 <5 <5 12,85 19,06

<0,1 <0,1 0,07 <0,1 <0,06 <0,06 <0,1 <0,1 <0,06 <0,06

0,24 0,35 1,70 0,67 0,44 4,03 1,05 10,37 3,48 14,05

0,29 1,54 1,65 0,52 0,38 0,88 0,44 0,53 1,44 1,85 Figure8.1:Geo hemi a

lanalysesofro ksofthe

studyarea.Abbreviatio

nsamandgnr

efertoamphibolitesand

gneisses,mmarkma� -

toultrama�

ro ksoftheParemount

ains.Numbersindi ate

appendantte tonostrati

-gaphi units

.

90 CHAPTER 8. GEOCHEMISTRYHills (unit III). Two samples were taken from the upper Kurase- and onefrom the northern Kasigau group (Fig. 3.1).Chara terizationOther than two amphibolites (>45 wt% SiO2) all are basi to intermediate(>45 -�55 wt% SiO2; Table 8.1). In the TAS diagram (Le Bas et al. (1986),Fig. 8.2) they plot into the �elds of basalt (majority), basalti andesite, or

35 45 55 65 750

5

10

15

SiO2 (wt %)

Na2O

+ K

2O

(w

t %

)

Foidite

Phonolite

Basalt

BasalticAndesite Andesite

BasalticTrachy-andesite

Trachy-andesite

Trachyte

Dacite

Rhyolite

Picro-basalt

Tephrite-Basanite

Zr

/T

iO2

Andesite/Basalt

.01 .1 1 10.001

.01

.1

1

5

Nb / Y

SubAlkaline Basalt

Phonolite

Trachyte

Rhyolite

Rhyodacite/Dacite

TrachyAnd

Alk-Bas

Com/Pant

Bsn/Nph

Andesite

Figure 8.2: Major and tra e element dis riminations of amphibolites after LeBas et al. (1986) and Win hester and Floyd (1977) (below). Open triangles= Galana East; open ir les = Galana West; open squares = Taita Hills.basalti tra hy-andesite. They are similar a ording to tra e elements (e.g.,Win hester and Floyd (1977)) and mostly range from sub-alkaline basalt toandesite/basalt with exeptions of the Kurase group amphibolites whi h dis-play alkaline types. A similar result is obtained using major elements (Irvineand Baragar, 1971) where in addition a tholeiiti trend an be as ertained for

8.1. METABASITES 91most of the ro ks. Sele ted tra e element variations (Fig. 8.4, Fig. 8.5) reveal

Na2O + K2O MgO

FeO*

Tholeiitic

Calc-Alkaline

35 40 45 50 55 60 65 70 75 80 850

2

4

6

8

10

12

14

16

18

20

SiO2 (wt %)

Na2O

+ K

2O

(w

t %

)

Alkaline

SubAlkaline

Figure 8.3: Chara terization of amphibolites after Irvine and Baragar (1971).di�erent groups for Galana East (open triangles) and -West (open ir les)barring one sample lying in between together with those from the Taita Hills(open squares).This distin tion an be noti ed also from hondrite-normalized REE pat-terns (Fig. 8.6). Amphibolites of unit I show LREE enri hment (similar toE-type MORBs) and overall multiple element on entration ompared to theshear zone ro ks (unit II). The latter are hara terized by several hangingslops from L- to HREE a ross the spe trum too. An intermediate position ismirrored by the sample of unit I pla ed among both groups. The Taita Hillsamphibolites obviously show two trends: LREE enri hment and slightly pos-itive Eu anomalies are typi al for the spe imens of the upper Kurase groupwhereas the Kasigau sample evin es an uniformly augmented plain pattern.In general all analyzed amphibolites are assumed to be of magmati origin.This is on�rmed, e.g., by a Niggli -mg plot (Leake (1964); Fig. 8.7). Two

92 CHAPTER 8. GEOCHEMISTRY

10 100 500.01

.1

1

10

100

Zr (ppm)

Nb

(p

pm

)

10 100 500.01

.1

1

10

Zr (ppm)

TiO

2 (w

t %

)

Figure 8.4: Sele ted tra e element variations for the Taita Hills-Galana Riverarea.data from the shear zone are deviant, but high Cr values (>450 ppm) andstru tural appearan e of the ro ks almost ex lude a sedimentary pre ursor.This is supported by tra e element dis rimination (e.g. Win hester et al.(1980)) in whi h all amphibolites plot below the sedimentary �eld (Fig. 8.14,shear zone ro ks are not shown as falling beneath the abs issa).SettingIn order to on�ne the te toni environment, MORB-normalized plots (e.g.Pear e (1982)) were applied for all units. For the shear zone samples (unitII) a signi� ant depletion of HFS elements like Nb, Hf or Zr an be found aswell as onsiderable enri hment of the LIL se tion (Sr to Th; Fig. 8.8).More di�erent patterns are shown for Galana East (unit I). Generally theelement enri hment is greater than for the previous examples. On the one

8.1. METABASITES 93

10 100 500.001

.01

.1

1

Zr (ppm)

P2O

5 (w

t %

)

10 100 5001

10

100

Zr (ppm)

Y(p

pm

)

Figure 8.5: Sele ted tra e element variations for the Taita Hills-Galana Riverarea.hand, a moderate- to small gradient from LIL to Nb and Ce to S is visiblefor two samples. The others indi ate a more sele tive enri hment within theLIL range (Ba) and, in one ase, are ombined with a sudden de line from Thto Nb. Varieties exist as well for the Taita Hills (unit III). The amphibolitefrom the Kasigau group displays a representative MORB-like signature fromCe to S (Fig. 8.9) apart from slighly enri hed P2O5. The only LIL withhigher abundan e is Ba, a small Nb anomaly an also be seen. The Kurasepatterns di�er in part by a greater element abundan e from Sr to P2O5 orTiO2 and are depleted from Y to S . A striking di�eren e is the einri hmentof Nb,one of the most in ompatible elements. Thus from normalized plots itappears that several sour es- or te toni settings might be involved in the for-mation of the amphibolites. The prominent HFS element depletion oupledwith parti ular enri hment of LIL for the shear zone ro ks and samples ofunit I suggests a relevant in uen e of subdu tion. For the remaining amphi-


La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu1

10

100

500

Rock

/Ch

on

dri

te

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

10

4

100

Rock

/Ch

on

dri

te

Figure 8.6: REE distributions of amphibolites of di�erent units. Norm valuesfrom Boynton (1984).bolites of unit I the latter omponent seems to fall o�. Tentatively this anfurther be investigated using a hondrite-normalized La/Sm-Ba/La diagram(Basalti Vol anism Study Proje t (1981); Fig. 8.10) where the samples ofunit I plot in the overlapping �eld of MORB and island ar s but near theborder of MORBs. The ro ks from the upper Kurase group do not show hem-i al hara teristi s of subdu tion and di�er also from MORBs in their moresele tive element pattern from Nb to Cr. Various within-plate types an beveri�ed, e.g., by means ofNb=Y versus T i=Y ratios (Fig. 8.10; Pear e (1982))whi h put forward a transitional environment ommon for ontinental rifts.This supports the �ndings of Fris h and Pohl (1986) and demonstrates thatexpli it rift related metabasi s o ur. It would be on eivable that some ofthe ultrama� lenses in the Taita Hills are asso iated with ontinental riftingor display a transition to MORB ambien e. One of the Kurase group sam-ples were taken lose-by a strongly wheathered lense o uring at the westernslopes (Fig. 3.1). The shear zone samples are quali�ed as vol ani ar basalts(one sample not shown as situated left of the ordinate) and those from unit

8.1. METABASITES 95Dolomite

Karoo trend

10

20

30

40

50

60

0.1 0.20

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

mg

c

Pelite and semi-pelite

Variouspelite-limestone

mixtures

Pelite-dolomitemixtures

Figure 8.7: Plot using Niggli and mg (after Leake (1964)). Note that thesamples plot near the magmati Karoo trend.I and the Kasigau amphibolite fall mainly into the ommon �eld of vol ani ar basalts to MORBs (Fig. 8.10). This an be noted also from a te toni dis rimination (Th-Hf-Nb) after Wood (1980) (Fig. 8.10 (bottom)). As a re-sult of subdu tion in uen e the shear zone ro ks learly plot as vol ani ar basalts. Less to slightly a�e ted ro ks su h as exempli�ed from Galana Eastlie marginally or between vol ani ar and MORB domains.8.1.3 NE TanzaniaChara terizationThe visited out rops in the Pare mountains in lude gabbroi ro ks and am-phibolites with variable ontent of amphibole, ortho- and linopyroxene, pla-gio lase, and o asionally spinell, s apolite and epidote. More a essory min-erals are apatite, ilmenite and quartz. Additionally garnet-amphibolites anbe found at all out rops. All but ro ks of lense m2, regularly ontain less than40 wt% SiO2, are ma� to ultrama� (�40-50 wt% Si02; Table 8.1). A totalof 12 samples were olle ted among �ve from site m1, four from site m2 andthree from site m3 (Fig. 3.5). Chondrite-normalized REE plots visualize thatthe o uren es partly are made up of or represent layered sequen es. Threedi�erent patterns, e.g., yield a ma� to ultrama� body (m1; Fig. 8.11) SEof Same.Tow samples are enri hed in LREE and at di�erent fa tors throughoutthe whole pattern de lining until Ho and slightly rise towards Yb. Moreovera negative Eu anomaly emerges for one sample. A third type (two sam-ples) shows enri hment only from La to Pr (Nd) with a distin t Eu peakand appre iable in rease in values up to Lu. More uniformly enri hed pat-

96 CHAPTER 8. GEOCHEMISTRYSr

K2ORb Ba Th Ta Nb Ce

P2O5Zr Hf Sm

TiO2Y Yb Sc Cr

.1

1

10

100

Rock

/MO

RB

SrK2O

Rb Ba Th Ta Nb CeP2O5

Zr Hf SmTiO2

Y Yb Sc Cr

.1

1

10

100

Rock

/MO

RB

Figure 8.8: MORB-normalized graphs (after Pear e (1982)) for amphibolitesof Galana West and -East.terns were derived from the northernmost o urren e m3 (Fig. 3.5; Fig. 8.11(bottom)). Gabbroi spe imens, sporadi ally revealing �ne-grained dynami- al re rystallization, plot near the hondriti referen e line (Fig. 8.11; (bot-tom)). Small LREE troughs and positive Eu anomalies are hara teristi .Tenfold enri hement is indi ated by a garnet-apmphibolite of the same lo a-tion. Mixed sour es are quite likely manifested in the lense ESE of Same (m2,Fig. 3.5; Fig. 8.11 (mid)). While two examples resemble MORB signatureswith slightly depleted LREE those beeing left are signi� antly enri hed inthe latter having moderate to shallow slopes a ross the distribution. In prin- iple there are ertainly more ro k types or fra tionates dete table at ea hout rop whi h an enlarge or omplete the available patterns.SettingFor further lassi� ation of the ro ks we plot the dataset in an AFM dia-gramm as applied by Coleman (1977) (Fig. 8.12). The amphibolites from SE

8.1. METABASITES 97

SrK2O


Zr Hf SmTiO2

Y Yb Sc Cr

.1

1

10

100

Rock

/MO

RB

SrK2O


Zr Hf SmTiO2

Y Yb Sc Cr.1

1

10

100

Rock

/MO

RB

Figure 8.9: MORB-normalized graphs (after Pear e (1982)) for amphibolitesof the Taita Hills.Kenya are plotted ollaterally. Four samples from the Pare mountains ouldbe related with ophioliti remnants in luding ro ks of exposures m3 and m1.Those of site m3 des ending from the northern slopes �t in well into the�eld overing ophioliti sequen es and show typi al Mg=Mg + Fe values of�0.86. Cumulates of m1 as well as two samples of SE Kenya (both groupswith Mg=Mg + Fe = 0:70� 0:73) are situated at (near) the border overlap-ping with the array of a mean komatiiti hemism. Most of the other ro ks(e.g. m2) are lo ated lose to the early Skaergaard liquid trend.For less de�nite attributable ro ks of m1 and m2 we employ extended REEdiagrams normalized over the primordial mantle (e.g. Sun and M Donough(1989)) and ompare it to amphibolites from SE Kenya or the umulates ofm3. The N-types of m2 (see Fig. 8.11) were ompared with the amphibo-lite from the Kasigau group and a more depleted one from the shear zone(Fig. 8.12; below). Although of ourse di�ering in absolute on entrationsthe enri hment (depletion) pattern of elements largely agree whi h ould, asin the Taita Hills-Galana River area, indi ate a subdu tion in uen ed en-

98 CHAPTER 8. GEOCHEMISTRYvironment. Di�erent onditions are obvious when examining enri hed typesfrom the same out rop (Fig. 8.11; Fig. 8.13 (top)). They orrespond ratherto the within-plate setting as determined for the Kurase group. A reason-able mat h an be found also between umulate varieties of m3 and thoseof m1 implying that ro ks of o eani aÆnity are present. Summerizing the�ndings it is proposed that in the Pare and possibly Usambara mountains atransition from rift related within-plate to MORB (e.g. ba -ar basin?)- andsubdu tion ambien e is do umented. This is supported by published resultsof the neighbouring Taita Hills (Fris h and Pohl (1986)) and our own data.Alternatively, Prohaska and Pohl (1983) suggested a sub- ontinental originapplying to some o uren es whi h would not be debarring as intense de-formation during a ollisonal event an in oroporate or juxtopose ro ks ofdi�erent sour es. In addition, Appel et al. (1998) suggested a magmati un-derplating model for the area whi h also might a ount for ma� intrusionsand a ord with an island-ar setting (see hapter 9).8.2 Orthogneisses (metagranitoids)8.2.1 Galana River pro�le - Taita HillsThirteen samples were analyzed in whi h Galana West (unit II), the Transi-tion zone (unit IV) and the Taita Hills (unit III) ontribute respe tively four,�ve and three samples. From the metasedimentary dominated unit I (GalanaEast) a single spe imen were analyzed. All ro ks were taken from migmatiti orthogneisses ommon throughout the study area. Main onstituents areamphibole-biotite-plagio lase-feldspar-quartz and �garnet�iron-oxide. S a-polite, linopyroxene ilmenite and �sphene o ur additionally in the Transi-tion zone and the Taita Hills.Chara terizationThe investigated gneisses generally are intermediate to slightly a idi (�59-66 wt% SiO2; Table 8.1). Those from the Transition zone are a idi by themajority with an average of�67-76 wt% SiO2. Indi ations about the origin ofgneisses an be gained utilizing, e.g, the SiO2-TiO2 diagram of Tarney (1977)(Fig. 8.14). One-third of the data plot into the sedimentary �eld or along theborder dividing sedimentary and igneous domains. Only the gneiss of unit Ishow a higher TiO2 value putting forward a sedimentary pre ursor. A sim-ilar attempt as for amphibolites an be be made purely on tra e elements(Win hester et al. (1980), Fig. 8.14. Many gneisses show Ni values below the

8.2. ORTHOGNEISSES (METAGRANITOIDS) 99dete tion limit (15-20 ppm, Table 8.1, shaded symbols in Fig. 8.14). Consid-ering a shift towards the origin of the graph the issues would also favour amagmati origin for most of the samples although a rustal ontamination an not be ex luded. Adopting the A/CKN riterion of Chappel and White(1974) all gneisses are I-type granitoids (Fig. 8.15) and meta- to weakly per-aluminous (Shand, 1927). A ording to the lassi� ation of Debon and Fort(1983) they vary in omposition mainly between tonaliti to granodioriti ,subordinate are quartz diorite (two samples of unit II) and adamellite (onesample of unit III). A orrelative tra e element subdivision (e.g. Win hesterand Floyd (1977), Fig. 8.15), intended for vol ani ro ks but also appli able toplutonites, indi ates predominately dioriti to granodioriti /tonaliti types.All gneisses are sub-alkaline Irvine and Baragar (1971) and in an AFM graphfollow widely a al -alkaline trend (Fig. 8.15). Ex eptions are some gneissesof the Transition zone whi h learly represents tholeiiti di�erentiates. Thesevarian es are also illustrated by hondrite-normalized REE plots (Fig. 8.16).Similar enri hment patterns exist for the Taita Hills and shear zone gneisses,a higher ontent from Tb to Lu is statable for the sample of unit I. TheTransition zone di�ers noti eable in lower LREE on entrations splitted intotwo groups whereas the M- to HREE se tion is more irregular. Contrary tothe gneisses of the other domains the two show either negative or positiveEu anomalies. The �rst group (bla k diamonds) more onsistently enri hedare akin to N-type MORBs and exhibit, for instan e, an Eu peak. BesideEu troughs hara teristi for the other group one sample evin es a possible hange to the other domains or te toni heterogenity of the zone. An anomalyis la king, LREE are most enhan ed and the middle to heavy portion followsthe gneisses of unit II and III.SettingFor omparision o ean ridge-normalized graphs were used in luding data ofwell-known environments (Pear e et al. (1984)). The gneisses of the shearzone and the Taita Hills (Fig. 8.17) demonstrate vol ani -ar resemblan eas an be noted, e.g., from granitoids of Chile and Jamai a (dashed linesand shaded diamonds). This setting is true also for ro ks of the Transitionzone (Fig. 8.17) whi h broadly di�er in lower HFS on entrations like Nb,Zr and Hf. As a referen e an example of Oman (shaded diamonds) and aM-type granitoid (Whalen et al. (1987); rosses) were quoted. The ex ep-tional position of these gneisses is shown also in a multi ationi diagramof Bat helor and Bowden (1985); (Fig. 8.18). Unlike a pre- ollisional envi-ronment proposed for the plurality of gneisses those of unit IV prove to bemantle fra tionates. Moreover, their fra tionated nature an be noted from

100 CHAPTER 8. GEOCHEMISTRYthe riteria suggested by Whalen et al. (1987); (Fig. 8.18). Finally, a te toni restri tion was attempted by a Y � Nb plot (Pear e et al., 1984) and avail-ing Zr � (Nb=Zr)N ratios (Thi�eblemont and Tegyey (1994), Thi�eblemont(1999)); Fig. 8.19).The former dis rimination displays a vol ani ar or syn- ollisional am-bien e for the gneisses. One sample is pla ed just within the area of o eanridge granites but is assumed to belong to the ollisional types (see normal-ized plots). In the omplementary Rb� (Y +Nb) graph (Pear e et al. (1984),not shown) all data plot in the vol ani ar �eld. Anyway gneisses of unit IVde�ne an own group either by lower Rb or Y values. A mainly subdu tionindu ed origin of the ro ks is also obvious from Fig. 8.19 as proposed bythe aforementioned authors. Due to a low Zr abundan e one of the Tran-sition zone gneisses takes a somewhat outlying position inside the �eld ofperalumious leu ogranites.8.2.2 Con lusionsAmphibolites analyzed from the Taita Hills-Galana River area strongly hintat a vol ani ar environment with variable in uen e of subdu tion. The learest subdu tion omponent is suggested for the Galana River shear zoneand seems to diminish towards the western part of unit I. There MORBrelated amphibolites are o urring. Still further east the subdu tion ompo-nent ould rise again indi ated by a more sele tive pattern of LIL- and HFSelements. In the Taita Hills a hangeover from a rift- to a subdu tion stagewas do umented by previous work. Our data reinfor e this per eptions andindi ate that ma� melts might be present typi ally for an initial ontinen-tal rift. This transition, maybe more vague transparent, is assumed to beobservable also from isolated lenses in the Pare-Usambara mountains of NETanzania. Possibly, ophioliti remnants are also preserved there. In generalthe bodies might be asso iated with layered sequen es as in our examplesalways several REE patterns are distinguishable from the same out rop. Or-thogneisses, all stemming from the Taita Hills - Galana River area, aÆrmthe vol ani -ar setting as proposed from ma� ro ks. In addition gneissesfrom the Transition zone learly suggest a juvenile rust generated due tosubdu tion of an o eani slab. A rustal ontamination of the gneisses annot be ex luded but if to a minor degree for those of unit IV.Several important onsequen es may arise from our results if we onsiderthe global te toni frame of the Mozambique Belt. The rift based ro ks withinthe Taita Hills - Pare-Usambara mountains might be linked to the break-upof Rodinia (see Fig. 9.2), a pre eding ontinent onstellation existent from�1100-750 Ma. The huge amount of island-ar material present in the study

8.2. ORTHOGNEISSES (METAGRANITOIDS) 101area ould be likened to those of the Arabian-Nubian Shield. There ar -ar -to ar - ontinental sutures were formed between �830-�620 (e.g. Abdelsalamand Stern (1996)). In southern Ethiopia Teklay et al. (1998) showed threeevents of granitoid magmatism at � 850, � 750 � 700 and � 650 � 550Ma. The intrusives were lassi�ed as juvenile rust by virtue of Nd and Sristope hara teristi s. Empla ement ages of orthogneisses (U-Pb, zir on) or-responding approximately to the �rst and last period were re ently reportedfrom the study area (Hauzenberger et al., in prep.). This might allude toa southward propagation of these sequen es from the ANS into the entralMozambique Belt. In part, the gneisses demonstrably reveal subu tion vi in-ity (see above). Thus they might signify remnants of an in ipient onvergen e(ar -ar ) leading over to a retion (ar - ontinental) or ontinent- ontinent ollision. The position of the Galana River shear zone and Galana East isnot de�nite in this order of events. Geo hronologi al data (see hapter 5)prove the latter domains to be learly younger than the Taita Hills and theTransition zone. Stru tural onsiderations support this �ndings and togetherthey ould imply a di�erent y le a the end of the Pre ambrian.


MORB

Island

Arcs

10

0

1

2

2 3

3

5

6

7

8

9

4

4

La/Smn

Ba/L

an

thol. WIP trans. WIP

alk.WIP

VAB

MORB+ VAB

MORB

1 2 30.03 0.1100

1000

Ti/

Y

Nb/Y

Th Nb / 16

Hf / 3

N-typeMORB

E-typeMORB +thol. WIP

alkalineWIP

destructivePlate marginbasalts(VAB)Figure 8.10: Te toni dis riminations of the amphibolites as suggested by theBasalti Vol anism Study Proje t (1981) (top), Pear e (1982) (below) andWood (1980) (bottom).

8.2. ORTHOGNEISSES (METAGRANITOIDS) 103


10

100

1000

Rock

/Ch

on

dri

te

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu.1

1

10

100

500

Rock

/Ch

on

dri

te

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu.1

1

10

100

Rock

/Ch

on

dri

te

Figure 8.11: REE patterns of isolated ultrama� - to ma� bodies of NETanzania (m1 to m3; top down). Norm values after (Boynton (1984)). Forlegend see Fig. 8.12.


Na O + K O2 2 MgO

FeO*

Metamorphic Peridodites

MAR

Mafic and UltramaficCumulate OphioliticRocks

Komatiite

Skaergaard Liquid Trend

CsRb

BaTh

UTa

NbK

LaCe

PbSr

PNd

HfZr

SmEu

TiDy

YYb

Lu

.1

1

10

100

1000

Rock

/pri

m. M

an

tle

.1

1

10

100

1000

CsRb

BaTh

UTa

NbK

LaCe

PbSr

PNd

HfZr

SmEu

TiDy

YYb

Lu

Rock

/pri

m. M

an

tle

Figure 8.12: AFM diagram (after Coleman (1977)) ontaining all metabasitesof the study area. Filled triangles = m1; Filled ir les = m2; Filled squares= m3. Below mantle-normalized graphs of N-type ro ks from site m2. Normvalues after Sun and M Donough (1989).


CsRb

BaTh

UTa

NbK

LaCe

PbSr

PNd

HfZr

SmEu

TiDy

YYb

Lu

.1

1

10

100

1000

Rock

/pri

m. M

an

tle

CsRb

BaTh

UTa

NbK

LaCe

PbSr

PNd

HfZr

SmEu

TiDy

YYb

Lu

.01

.1

1

10

100

1000

Rock

/pri

m. M

an

tle

Figure 8.13: Mantle-normalized graphs of enri hed types from m2 (top) and umulates of site m1 (bottom). Norm values after Sun and M Donough(1989).


igneous amphibolites

sedimentaryZ

r/T

iO2

Ni (ppm)

5 10 50 1000.005

0.01

0.05

0.03

0.1

SiO2 wt%

TiO

2 (

wt%

) 0.9

75 80

1.2

60

1.5

55

0.3

0.6

07065

igneous

sedimentary

Figure 8.14: Dis rimination between sedimentary and igneous origin of gneis-ses after Tarney (1977) (left) and Win hester et al. (1980) (right). Unit I(open triangle); unit II (open ir les); unit III(open squares); unit IV (openand �lled diamonds).


SiO2 wt%

0,6

0,7

0,8

0,9

1

1,1

1,2

1,3

55 60 65

Al/

(2C

a+

Na+

K)

(mola

r)

70 75 80

I-type

S-type

Phonolite

Trachyte

Rhyolite

Andesite/Basalt

Rhyodacite/Dacite

SubAlkaline Basalt

TrachyAnd

Com/Pant

Bsn/Nph

Andesite

Alk-Bas

.01 .1 1 10.001

.01

.1

1

Nb / Y

Zr

/T

iO2

Tholeiitic

Na2O + K2O MgO

FeO*

Calc-AlkalineFigure 8.15: Classi� ation of gneisses after Chappel and White (1974) (top),Win hester and Floyd (1977) (below) and Irvine and Baragar (1971) (bot-tom).



1

10

100

500

Rock

/Ch

on

dri

te


10

100

1000

Rock

/Ch

on

dri

te

2


10

100

Rock

/Ch

on

dri

te

Figure 8.16: REE distributions for gneisses of unit I and II (top) and unit II(mid). Patterns for the Transition zone are shown below. Norm values fromBoynton (1984).


K2O Rb Ba Th Ta Nb Ce Hf Zr Sm Y Yb.01

.1

1

10

50

Rock

/OR

G

K2O Rb Ba Th Ta Nb Ce Hf Zr Sm Y Yb

.1

1

10

100

Rock

/OR

G

K2O Rb Ba Th Ta Nb Ce Hf Zr Sm Y Yb.01

.1

1

10

100

Rock

/OR

G

Figure 8.17: ORG-normalized plots (Pear e et al., 1984). Unit I and II arepi tured with granites of Jami a and Chile, unit III with the Jamai a granite.Unit IV is ompared with a granite from Oman and data of Whalen et al.(1987).


100 100050 50001

10

100

500

Zr + Nb + Ce + Y (ppm)

FeO

* /

MgO

FG

OGT

0 500 1000 1500 2000 2500 3000 35000

500

1000

1500

2000

2500

R1 = 4Si - 11(Na + K) - 2(Fe + Ti)

R2 =

6C

a +

2M

g +

Al

1

1 - Mantle Fractionates

2

2 - Pre-plate Collision

3

3 - Post-collision Uplift

4

4 - Late-orogenic

5

5 - Anorogenic

6

6 - Syn-collision

7

7 - Post-orogenic

Figure 8.18: Subdivsion of gneisses after Bat helor and Bowden (1985) (top)and Whalen et al. (1987). OGT = unfra tionated M-, I- and S-type granites.FG=fra tionated felsi granites.


1 10 100 10001

10

100

1000

Y (ppm)

Nb

(p

pm

)

ORG

WPG

VAG +syn-COLG

Zr (ppm)

(Nb

/Zr)

N

0,1

1

10

100 100010

B

C

D

A

A: subduction related series

B: calc-alkaline to alkaline Collision-related lavas and plutons

C: intraplate alkaline felsic lavasand granitoids

D: peraluminous leucogranitesFigure 8.19: Te toni setting of granitoids after Pear e et al. (1984) andThi�eblemont and Tegyey (1994) (Thi�eblemont (1999)).

Chapter 9Dis ussion9.1 Voi area - Pare-Usambara mountains9.1.1 General onsiderationsStru tural and geo hemi al results as well as geo hronologi al data (thisstudy; Hauzenberger, in prep.) and thermobarometri results (Hauzenberger,in prep.) indi ate lose releationships between the high grade basement ro ksof SE Kenya and those of NE Tanzania. The te tonometamorphi evolutions,however, are interpreted di�erently. In SE Kenya investigations point to a lo kwise P-T-t path as dis ernable in metapelites by peak metamorphi kyanite breakdown into sillimanite (�brolite); whereas, in the Pare-Usambaramountains and further south in East Tanzania a reverse path (e.g post peakmetamorphi kyanite growth) is seen (Appel et al., 1998). In the Voi area(Taita Hills - Tasavo Park) a suture was lo ated (Pohl et al. (1980), Fris hand Pohl (1986)) whi h displays a hange from rift related (Kurase group)within plate- to subdu tion ambien e of a vol ani ar (Kasigau group) tomid-o ean ridge environment to the north (Mtito Andei). Geo hemistry onamphibolites and orthogneisses on�rm these assumptions and show thattransitional WIP basalts even o ur in onta t with ultrama� lenses of thesuture in the Taita Hills (unit III). Integrated isotope ages (40Ar/39Ar) ofamphiboles of both groups vary from 559:1 to 570:1 Ma and are supposedto re e t postmetamorphi ooling to a te tonothermal event transformingthe Taita Hills into the present S-SE imbri ated nappe sta k. A similar am-phibole age was reported from east of the Surongo thrust in western Kenya(Shibata et al., 1996) implying the onsisten y of these late pre ambrian agesa ross the western se tor (Mosely (1993)) of the metamorphi belt. A dif-ferent te toni style is o urs immediately east of the Hills where a narrowtransition zone (unit IV) of ridge-shaped often strongly folded orthogneiss113

114 CHAPTER 9. DISCUSSIONbodies (partly of M-type signature) shades o� into the Galana River shearzone (unit II), a more than 20 km wide lineament ontinuing probably inthe Athi-Ikutha shear zone. A stru tural break leads to the easternmost do-main (unit I) hara terized by abundan e of marbles and more metasedimen-tary lithologies. Here subhorizontally to moderately (�40Æ) dipping foliationsprevail. The majority of lineations trend NNW-SE and they are frequentlyasso iated with fold axes.With the beginning of the shear zone the metamorphi onditions hange,too. Granulite fa ies onditions are rea hed throughout the whole area butwith notably lower pressures in the Galana River se tion (I, II; Fig. 4.1,Hauzenberger, in. prep.). Additionally 40Ar/39Ar ooling ages are signi�- antly younger. Amphiboles show ages of 524.5 and 524.7 Ma for the entraland the western part of the shear zone and present uniform ooling a rossthe stru ture. This is orroborated by suitable biotite ages of 490.4 and 500.9Ma. The lower age stems from the entral part of the zone. Sin e the shearzone is assumed to represent the latest event, the biotites are not resettedas indi ated from the Taita Hills. An amphibole age of of 519.3 Ma of theeasternmost domain (unit I) a ords to the shear zone but an unreliable bi-otite (inherited Ar) point at an older history of unit I overprinted by theevent forming the shear zone. Sm-Nd garnet-whole ro k ages (Hauzenberger,in prep.) representing the early ooling after peak metamorphism allo atethe stru tural subdivison and the Ar ages of the area. For the Taita Hillsand the Transition zone they rea h 586.2 and 583.7 Ma and for the shearzone and unit I to 529.0 Ma and 547.6 Ma.Peak metamorphism in the granulites of NE Tanzania are dated between610-655 (M�oller et al. (2000), Muhongo et al. (2001), Muhongo and Lenoir(1994). A omparison of the isotopi data from SE Kenya (all units; Sm-Ndand Ar-Ar) with the ooling path established by M�oller et al. (2000) (U-Pbsystem; Fig. ) shows that the two data sets mat h with the proposed path ex- ept for the shear zone whi h represents a later te tonism. At the lower partof the ooling path ( losure temperature of biotite) the shear zone and thePare-Usambara mountains display similar or identi al ooling (dependent onadopted losure temperatures). Nd model ages (M�oller et al., 2000) of 1.5 -1.1 Ga for the Pare-Usambara mountains and those of 1.8 - 1.1 Ga (Hauzen-berger, in prep.) for the Taita Hills-Galana River area may generally alsoemphasize geneti onne tions of SE Kenyan and NE Tanzanian granulites.Stru tural asso iations between the areas are the obvious S-SW thrust-ing of the Taita Hills whi h also in uen es at least the Pare Mountains, asdo umented by ross folding (own observations), and an identi�ed thrustwith ophioliti remnants at the northern foothills (Fig.3.5) of the North Paremountains. Geo hemistry on two basi to ultrabasi lenses of the Pare moun-

9.1. VOI AREA - PARE-USAMBARA MOUNTAINS 115tains indi ate alike in the Taita Hills a transition from rift related within-plateto MORB or subdu tion related intrusives. Maboko and Nakamura (2001)re ently reported a Sm-Nd whole ro k iso hrone age of 815�58 Ma from or-thogneisses of the Usambara mountains, as ribed to al -alkaline magmatismof a onvergent setting. An U-Pb zir on age (Hauzenberger, in prep.) of anorthogneiss of the Transition zone in SE Kenya (Fig. 3.5) show �850 Ma andis interpreted as time of intrusion of the body. As these gneisses are assumedto be derived from the overlying mantle of subdu ted o eani rust, this dat-ing ould represent a starting age of subdu tion. Similar ages of al -alkalineplutonism (Kr�oner (1990), U-Pb zir on) were shown in the northernmostArabian-Nubian Shield to range between 780-760 Ma with island ar forma-tion at about 810 Ma. Also, Stern and Abdelsalam (1998) noted an older"transient" y le of vol ani ar granitoids from the Nakasib suture (NE Su-dan) with rifting at about 790 Ma and starting of ollision prior to 750 Ma.Three events of rust formation were shown, e.g., for southern Ethiopia withgranitoid empla ement at � 850, � 750� 700 and � 650� 550 Ma (Teklayet al., 1998). Younger episodes of subdu tion-related island ar magmatismbetween 690-670 Ma (Bregar et al., 2002) and 660-580 Ma (Loizenbauer et al.,2001) were re ently reported for the Central Eastern Desert of Egypt, too.The earliest deformations in the Pare-Usambara mountains ((Bagnall,1960), (Dundas, 1965), ((Bagnall et al., 1963); Fig. 9.1) were asso iated withre umbent folds (NE trending to shallowly NE plunging) and severe axialplane shearing with a later epsiode of ross folding (North Pare mountains)or minor warping (Usambara mountains) about north-westerly (impli atedwith thrusting) to NNW striking axes. The same ir umstan e of frequentre umbent iso linal folds and two periods of folding were observed in theestern part of the Usambara mountains and the area to the northeast, theUmba Steppe, (Hartley and Moore, 1965). Here additionally the lineationpattern hanges on N-S trending foliations from NE plunge in the south toNNE plunge in the north.Folding is also ommon in the areas to the E and NE (Fig. 9.1). In theKurase-Kasigau distri t (Saggerson, 1962) two events of folding were de-du ed. Again, the �rst, marking for Kurase group, are re umbent folds over-turned to the W with NNE plunging axes. The overlying Kasigau group wassubje ted to open folding with northerly plunging axes. To the north Walsh(1960) mapped west verging overturned folds with northerly plunging axesand lineations. Finally, in the Taita Hills (Pohl et al., 1980) three stages offolding were re ognized on NNW (F1)-, NNE (F2)- and northerly trendingaxes (F3; late te toni ). The fold and lineation pattern of Galana East (unitI) has to be dis ussed separately as it is presumably linked with the latershear te toni s.

116 CHAPTER 9. DISCUSSION

F1

F3

F2

D1

D2

D3

Taita Hills

F1F2

D1D2

North Pare Mountains

D1

F1F2 D2

Kurase - Kasigau group

D1(D2)

F1(F2)

S of Taita Hills

D1?

F1?F2

D2

Galana East

Figure 9.1: Re orded folding events (F) from di�erent areas in SE Kenya andNE Tanzania. Solid (from literature) and dashed lines (own observations)indi ate strike of mean axial dire tions with shallow to moderate plunge.Assumed deformations (D) are indi ated. Only the NNW trend an be learlyrelated to folding in Galana East (unit I). See text for dis ussion.9.1.2 Te toni evolution (early Pan-Afri an)Various data (ages, stru ture, geo hemistry) from SE Kenya and NE Tanza-nia give strong eviden e that the starting of Pan-Afri an ollision (prior to800 Ma) is do umented in both areas. Key et al. (1989) argued for a NW-SEthrusting and southeastward movement of upper plates during plate olli-sion beginning at �820 Ma. The a ompanying Samburuan-Saba hian event reated re umbent folds with axial tra es ranging in plunge from NE-SW(subordinate) to ESE-WNW. Early nappes and thrusts indi ate ENE-WSW

9.1. VOI AREA - PARE-USAMBARA MOUNTAINS 117transport. However, folding and thrusting (see on lusions) were attributedto an orogen wide extension. Conversely, Sha kleton (1993b) proposed a on-vergen e at that time based on stru tural eviden es and granitoid geo hem-istry yielding island- to ontinental or vol ani ar setting. For the main ENE-WSW of trend of lineations he favoured a sheath fold interpretation whoseimportan e was also indi ated for the Pare-Usambara mountains. Smallerfolds and many of the main stru tures formative for the dominant �NE-SW trend, early fabri and te tonism were interpreted so that they would orrespond to a ENE-NE onvergen e.In the following two possibilities are dis ussed (Fig. 9.2). Firstly, it is on luded that the te toni style present from the Pare-Usambara moun-tains up to the Taita Hills ould be mainly aused by the superposition ofthe Samburuan-Saba hian (�820-800 Ma)- and the su eeding Baragoian-Barsaloian (�620-570 Ma; Key et al. (1989)) te tonothermal events. In SEKenya the latter deformation is possibly more apparent whereas in NE Tan-zania the former is still re ognizable. The NE trending fold axes of re umbentfolds in the Pare-Usambara mountains in onne tion to shortening (not assheath folds) during the losure of an o eani domain then an be assumedas a onsequen e of the Samburuan-Saba hian as ollisonal event, or refer-ring to Gondwana, the starting of the Pan-Afri an Orogeny (Stern, 1994).The dire tion of onvergen e in that ase should be dire ted from SE-E. Inthe Kurase-Kasigau area both fold axes sets trend more northerly, and inthe Taita Hills and adja ent areas the earliest deformation was inferred tobe related with NNW plunging axes. Tentatively, this hange from NE toNNW plunge of the early re umbent or overturned folds might be explainedby a marginal reorientation of the onverging plate(s). Irregular plate fringesmight also be responsible for various orientations.The stru tural interpretation of the Transition zone is of importan e. It isstret hed out (NNW-SSE strike) along the Galana River shear zone (Fig. 3.1)and shows some notable prolate strain that di�ers from the pronoun edly at-tened and thrust dominated Taita Hills. Both, however, ontemporaneouslyshare high grade metamorphism and similar post peak-metamorphi ooling(Hauzenberger, in prep.). It is proposed that the two domains express vari-able aspe ts of the same onvergent setting and deformation whereby theTaita Hills a ounts for the a retionary portion. If the S-SE thrusting of theTaita Hills re e ts plate movement (NNE plunging lineations are onstruedto be stret hing lineations with similarily oriented te toni transport), thenit should be asso iated with a plate approa hing from NNE-NE and would on i t with the assumption for NE Tanzania. In the Transition zone are fre-quent dextral shear senses whi h would onform with the thrust kinemati sof the Taita Hills. There is no doubt that sheath folds o ur in these highly

118 CHAPTER 9. DISCUSSION

Tanzania craton

Tanzania craton

Tanzania cratonTanzania craton

Followed by Baragoian-Barsaloianimprint (closure of an ozeanic domain?)

parts ofGalana East

GalanaWest(+ parts ofGalana East)

NeoproterozoicIsland arcrocks (+ pre-neoproterozoiccrust) (Neoproterozoic)

Island arcrocks (+ pre-neoproterozoiccrust)

+/- island arc rocks(neoproterozoic ?)and abundant meta-sediments (+ pre-neoproterozoic crust)

followedby collisionand wrenchtectonics

Accretion of (I) - (III)(or I+II+III) andophiolitic remnants

Continuoussequence?

Continuoussequence?

? ?

Parts of EastGondwana(Madagascar?)



Consolidated archeancrust (e.g. Congo- and relatedcratons) and mobile belts (or domains) upto Kibaran age (-1200 Ma)

(Consolidated) archeancrust (e.g. India and Madagascar)

and crustal domains (mobile belts?) up tomesoproterozoic ages

Parts ofWest Gondwana

Parts ofEast Gondwana

Starting of Rodinia Rift

Subduction(s)and island arc(s)formation

~900 - ~740 -? Ma

Arc collision(s), -accretion(s)and collision

Arc collision(s), -accretion(s)(+ collision with a pre-neo-proterozoic fragment?)

Collision of (A) with parts ofEast Gondwana involving theGalana River area

Followed by wrench tectonics

- ~ 620 Ma

- ~620 Ma

~620 - ~580 Ma - < 530 Ma

550 Ma(>)

Followed by wrench tectonics(covers rocks of Galana West= Galana River shear zone)

- < 530 Ma

>900 Ma

(Rift basalts in the Taita Hills- Pare mountains?)

Taita Hills-Transition zone(parts of the Galanasection?) -Pare-Usambaramountains -(E-SE Tanzania?)

?

?

?

?{

{

(I)

(III)

(II)

(I + II +III)

a)

(A)

b)

(B)

Figure 9.2: s hemati sket h of di�erent stages possibly relevant for the evo-lution of the study area. Formation of o eani rust and island ar s endingin a single event of ollision and subsequent shear te oni s (a). Polyphasedevelopment of the area (b) with a late Pan-Afri an orogeny at the end ofthe Pre ambrian. See text for details.

9.1. VOI AREA - PARE-USAMBARA MOUNTAINS 119strained lower rustal ro ks but our rare observations of re umbent folds inthe Taita Hills do not support this geometry although both types might ex-ist. Fold axes of observed stru tures (all from the upper Kurase group) arealigned with the main NNE lineation trend or plunge more north-easterly(> 30Æ) and �t the on epts of Saggerson (1962), Horkel et al. (1979), Pohlet al. (1980). Sporadi ally NNW plunging axes an also be found as remnantsof the earliest deformation (Horkel et al. (1979), Pohl et al. (1980)). Theseleavings might omply with the orientation of the Transition zone and, inturn, with the situation des ribed by Walsh (1960) and Saggerson (1962) tothe south and southeast. A SE-E shortening e�e t in the ro ks of the Tran-sition zone during gneissi� ation (before folding) an not be ruled out. Thestru tures seem to be more omplex than onsidered at �rst sight.The bending of lineations in Kenya and Tanzania (Sha kleton, 1993a), as- ribed to a modi� ation from early (transverse)- to late Pan-Afri an (N-S),is well preserved in the study area. Our observations validate this s atteringlineation patterns espe ially for NE Tanzania (Fig. 3.5). In the Usambaramountains lineations more frequently plunge to the �NE and there is a moreeasterly oriented maximum in the Pare mountains. Furthermore, an obviousSE submaximum is assumed to be related to ross folded (striking) domains(Fig. 3.5). If these domains indi ate, as hypothesized, an independent eventit should be initiated by a Barsaloian-Baragoian deformation possibly man-ifested in the Taita Hills as thrusting and distin t attening geometry. Su han overprint ould involve and explain a reorientation (or distribution) ofpreexisting fold axes and lineations to spread nearly along total great ir les(Fig. 3.5). Sha kleton (1993b) dis ussed a twofold a retionary history for theMozambique Belt in East Afri a with a se ond losure of an o eani basinduring the Baragoian-Barsaloian deformation. It is un lear whether this ap-plies to the assumed Baragoian-Barsaloian imprint in the Voi area and NETanzania or not, if so, it would not represent intra- rustal reworking aloneas indi ated by Key et al. (1989).Another possibility, that of a single ( ontinuous) event e�e ting the studyarea during the Neoproterozoi (>800 - <600 Ma), also seems to be plausi-ble. In this ase, however, a hange of the stress system (e.g. reorientationof plates) would be ne essary if the interpretation of the early stru turesespe ially in the Pare-Usambara mountains is ogent. Otherwise, all of the ollisional and subsequent te toni s must have been subje ted to a steady onverging plate motion from the NE and sheath folds must be a substantial omponent of early Pan-Afri an deformation.Independently from the aforementioned kinemati aspe ts it is very likelythat the suture proposed for the Voi area exists also in the NE granulites ofTanzania, resulting from early Pan-Afri an ollison. Consequental the NE

120 CHAPTER 9. DISCUSSIONgranulites in Tanzania and the SE granulites in Kenya ontain a onsid-erable amount of neo- to mesoproterozoi ro ks (see also Stern (2002)) ofvol ani ( ontinental?) ar type. These ro ks are te toni ally inter�ngered ashigh-grade metasediments and orthogneisses. This zone probably ontinuessouthwards (e.g. Wami River, Uluguru mountains?). In all domains theselithologies are enmeshed in late are hean to early proterozoi sequen es (e.g.Muhongo et al. (2001)) and over mu h of the area towards the western ra-ton as shown by Nd model ages ( e.g. (M�oller et al., 1998), Maboko (2000)).The suturing is likely to be a remnant of ar - ontinental (ar -ar ) nature asproposed for the Arabian-Nubian Shield (see Abdelsalam and Stern (1996))en ompassing a time span of �830-�620 Ma for the losure of the Mozam-bique o ean basin. The most striking di�eren e between the Voi area andnortheastern and eastern Tanzania are opposite P-T-t paths whi h indi atesdi�ering te tonometamorphi histories. Appel et al. (1998) re ommended amagmati underplating-loading model with ma� intrusions at the rustalbase and simultaneously addition of melt into the rust. This would be on-sistent with rustal growth as reported for many Phanerozoi ar s in olli-sional environments (e.g. Hamilton (1995)). If so, the present erosion leveldoes not un over these lower parts of the rust implying onsiderable thi k-ening although some of the ultrabasi lenses s attered throughout the Pare-Usambara mountains might typify up-thrust se tions. On the other hand,Hauzenberger (in prep.) proposed a lo kwise path and assigned the moder-ate geothermal gradient (20-25Æ/km; see also Appel et al. (1998)) establishedin both areas to ollisional thi kening whi h is in a ordan e with observedthrust te toni s. High-grade metapelites from the Taita Hills show that sil-limanite is aligned with the foliation and that the thrusting there mighthave o urred after a thermal peak under sillimanite stability onditions. Amore sporadi ally do umented late stage overprint (sillimanite growth) wasreported by Appel et al. (1998). It was asso iated with a supposably fastuplift. This, together with stru tural eviden e supporting a ommon eventand orresponding ooling ages, ould re on ile the disparate paths at anadvan ed stage of te toni evolution. Basi ally the inhomogeneity of P-T-tprogression ould be attributed to extensive te tonometamorphi historiesof ar systems ending with ontinental a retion. They an vary espe iallyalong the strike (Hamilton, 1995). Perhaps they mark spe i� regions withina system now veiled or juxatposed due to unkown strong deformation.9.1.3 Shear te toni s (late Pan-Afri an)The latest te tonomorphi event identi�able as an independent stru turaldomain is linked with the Galana River shear zone. Geo hemi al investi-

9.1. VOI AREA - PARE-USAMBARA MOUNTAINS 121gations on amphibolites and orthogneisses show that many of these ro kshave vol ani ar signatures with a broad subdu tion omponent (e.g. lowHFS element on entrations). A pivotal question arising from these results iswhether this sequen e belongs to early Pan-Afri an te tonism as indi ated bythe Transition zone or to a subsequent event. At present no ages (intrusionof granitoids) are available whi h ould larify this ex epting a pre-Kibaranage (�1300 Ma; Hauzenberger, in prep.) revealing that pre-Pan-Afri an base-ment was integrated during shear te oni s and probably also during earlierPan-Afri an history. The a tual dating (see above), aimed at peak metmor-phism and ooling, proves the shear zone to be one of the latest events in theGondwana assembly but it does not expound whether or not this only indi- ate a late reworking of older ar material. An argument for the latter is that,if orthogneisses of the Transiton zone are mantle derivatives delineating sub-du tion vi inity, there should be more ro ks of ar aÆnity also towards theeast. On the other hand ooling ages of biotites, whi h are in prin iple highlysensitive for inheritan e of older events as reported from many areas in theMozambique Belt, seem to be reliable. This ould suggest that orthogneissesin the shear zone, if not masked by metamorphism and deformation whi h ould have ompletely erased a previous history, are a late juvenile additon.A ru ial argument for a late pre ambrian event is the eastern ounter-part of the shear zone, Galana East (unit I) showing post peak-metamorphi (granulite fa ies) ooling of �548 Ma. Peak metamorphism in the shear zoneo ured at 15-20 Ma later (Hauzenberger et al., 2003). Widespread granulitefa ies metamorphism at �550 Ma was also veri�ed from Sri Lanka, Mada-gas ar or Antar ti a (e.g. Kr�oner (1994), Shiriashi et al. (1994), Kriegsman(1995)). Meert et al. (1995) and Meert and Van der Voo (1996) related theseto a late Pan-Afri an Kuunga Oorogeny whi h aused a ommon apparentpole wander path of East and West Gondwana. It is thinkable that the Galanase tion is related to this y le but then the shear itself would denote a suturedi�erent from those proposed by Fris h and Pohl (1986). Amphibolites fromthe western and entral part of unit I (Fig. 3.1) are, in ontrast to the shearzone, more MORB types, whereas those at the eastern end of the pro�le aremore in uen ed by subdu tion. As for the Taita Hills and the Transition zoneit is suggested that Galana East and West are, again, di�erent expressions ofthe same te toni event. Strain estimations and kinemati onsiderations onthe shear zone indi ate a pronoun ed attening geometry with onsiderableshortening a ross the strike (�50%) during sinistral wren hing, possibly asresult of steep onverging plates (e.g. pure shear dominated transpression,Tiko� and Teyssier (1994)). A dextral overprint sporadi ally obivous as dis-tin t shear bands ould evin e a omponent of lateral extrusion (e.g. Joneset al. (1997)) during ontinuous deformation and would even allow for higher

122 CHAPTER 9. DISCUSSION onvergen e angles (�70-80Æ). The strain geometry of Galana East (unit I)show almost plane strain ondition, but with learly lower magnitude thanfor all other units. This pattern is onsidered to show a partitioned om-ponent to strike-slip deformation appearing stru turally as folding on NNEaxes and o asionally as thrusts (Sanders, 1963). However, older te toni smight have been overprinted as suggested by the s attered lineation trend. Itwould be on eivable that, as proposed for the early Pan-Afri an domains,Baragoian-Barsaloian deformation dire tly pre eded or was among previousevents.The o urren e of MORB related amphibolites east of the shear zone oupled with the sudden hange of the stru tural style and lithology (moremetasedimentary) ould �t to the assumption of a shear zone related suturedisappearing towards the SE beneath the upper paleozoi Duruma sand-stones. This ould be on eiveable provided that the amphibolites of GalanaEast are not reoriented vestiges of an older event. To what extent the earlyPan-Afri an parts of the study area were in uen ed by this te tonometamor-phi event is not ertain. In all probability, in addition to obvious strike-slipshear, the shear zone a�e ts the exhumation history of the older domains. Sin-gle stru tures within the domain leave open the possibility of steep movementto ounterbalan e the predominant �nite attenig geometry. The large di�er-en e in Ar/Ar ages (hornblende) between Taita Hills and the Galana Riverarea and the presen e of the same metamorphi onditions for Galana Eastand the shear zone show mu h uplift previous to shear te toni s (e.g. thrust-ing). Lo ally, NNW trending shears were observed in the Pare mountains andin the Umba Steppe, sometimes with noteable oblique-slip portion, and atthe base of the Kasigau mountain. Saggerson (1962) mapped a sinistral shearin the Kurase distri t (Fig. 3.5). Whether asso iated thrusting o ur east ofthe shear zone has not been investigated. The bending of the lineation trendin the Umba Steppe ould be related. Also an isotopi resetting, espe ially ofbiotites, may be widespread (e,.g. Taita Hills). In north- entral Kenya (Keyet al. (1989)) resetting of K-Ar ages during a 520-530 Ma event along withbrittle/du tile shears (Mosely, 1993) are ommon and mirror the importan eof late stage shear zones like that of the study area. The ontinuation of theGalana se tion to the south or southeast is un ertain, but if it exists, it shouldbe found in eastern Tanzania (see geo hronology) and southern to southeast-ern Madagas ar. Several N-NNE to NW trending metamorphi belts boundby shear zones (Windley et al., 1994), under a shelf sequen e of interthrustedorthogneisses and ophioliti remnants display metamorphi (and sy hronousmelt empla ement) ages between �523 and �588 Ma (e.g. Paquette et al.(1994), Andriamarofahatra et al. (1990)). Martelat et al. (2000) evin ed astrain interferen e pattern of two premsumably interrelated high-temperature

9.1. VOI AREA - PARE-USAMBARA MOUNTAINS 123events about for the same time frame in southern Madagas ar. A at-lyinggranuliti foliation ontaining E-W stret hing lineations, asso iated iso li-nal folds and stratoid granites (D1; 590-530 Ma) were steeply refolded bythe formation of verti al, parti ulate pure shear dominated, transpressionalshear zones (D2; 530-500 Ma). They limit blo ks of enormous pressure gra-dients of 12 (southwest)- to 5 kbar (east and north) with appendant P-Tpaths signifying high-temperature de ompression. Moreover, in the westernand entral areas of the northern island a period of high-grade metamor-phism and magmatism (580-520 Ma; Tu ker et al. (1999)) were related toa Gondwana ollison also spanning the time interval of peak-metamorphism(Hauzenberger et al., 2003) in the Galana River se tion.

DanksagungErneut bedanke i h mi h bei all jenen, die mir ihre Hilfe angedeihen lie�enund zur Fertigstellung dieser Dissertation beigetragen haben. Eine detaillierteAu istung w�urde diesmal bestimmt mehr als ein Kapitel f�ullen and davonm�o hte i h, vor allem au h im Interesse der Betro�enen, absehen.I h danke dem "East Afri an Team", bestehend aus meinem BetreuerProf. E kart Wallbre her, Prof. Georg Hoinkes, Prof. Harald Fritz, Prof.Aberra Mogessi, Prof. Sospeter Muhongo, Dr. J�urgen Loizenbauer, Dr. ElliotMathu, Dr. Norbert Opiyo-Ake h und "last but not least" Dr. Christoph An-ton Hauzenberger, der, insbesondere w�ahrend der Gel�andearbeit, stets auf-munternde Worte fand und Spei herplatz und Re henleistung kostenlos zurVerf�ugung stellte. Den Wildh�utern des Tsavo East National Parks dankei h f�ur das si here Geleit w�ahrend unseres Aufenthaltes in diesem Tier-s hutzreservat. Daran ans hlie�end danke i h dem Fonds zur F�orderung derwissens haftli hen Fors hung (FWF) f�ur die Genehmigung und Finanzierungdes Projektes P-12375-GEO "Geodynami setting of the Panafri an Orogenyin Eastern Afri a" in dessen Rahmen diese Arbeit verfasst wurde.Weiters danke i h Dr. Wolfgang Unzog, Dr. Franz Bernhard, Dr. KurtKrenn, Dr. Christian Biermeier, Dr. Christine Latal, Dr. Ana-Voi a Bojar,Dr. Vroni Ten zer, Mag. J�org Robl, Prof. Robert S holgar, Prof. Alois Fen-ninger, Dr. Robert Handler, Dr. Karl Ettinger, Mag. Andreas "Andi"W�ol er,Mag. Mourad Greiss, Mag. Angelika Reiter, Mag. Bertl Rabits h, Mag. Se-bastian H�ansel, Hrn. Christoph Erhart, Hrn. Heiko Gai h und Hrn. ChristianSteidler. Ein spezieller Dank gilt Dipl. Geol. Frank S hobel.Ebenfalls danken m�o hte i h Prof. Werner Piller, Fr. Elisabeth Murtinger,Fr. Gertraud Bauer, Fr. Claudia Pus henjak, Hrn. Georg Stegm�uller, Hrn.Erwin Kober, Hrn. Gerhard Zmugg und Hrn. Franz Ts herne und allenAngeh�origen der Institute f�ur Geologie/Pal�aontologie und Mineralogie/Petro-logie, die meine Arbeit unterst�utzt haben.Den allergr�o�ten Dank s hulde i h meinen lieben Eltern die mir immervorbehaltlos und in jedweder Hinsi ht zur Seite standen und dadur h erstmeine Ausbildung erm�ogli hten. 125

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