Precambrian Research 164 (2008) 201213Contents lists available at ScienceDirectPrecambrian Researchj our nal homepage: www. el sevi er . com/ l ocat e/ pr ecamr esGeochemistry of ne-grained clastic sedimentary rocks of the NeoproterozoicIkorongo Group, NE Tanzania: Implications for provenance andsource rock weatheringCharles Kasanzu, Makenya A.H. Maboko, Shukrani ManyaDepartment of Geology, University of Dar es Salaam, P.O. Box 35052, Dar es Salaam, Tanzaniaarti cle i nfoArticle history:Received 2 November 2007Received in revised form 23 April 2008Accepted 27 April 2008Keywords:Ikorongo GroupMudrocksGeochemistryWeatheringProvenanceabstractThe Neoproterozoic IkorongoGroup, whichlies unconformablyonthe late ArchaeanNyanzianSupergroupof the Tanzania Craton, is comprised of conglomerates, quartzites, shales, siltstones, red sandstones withrare agstones and gritstones and is regionally subdivided into four litho-stratigraphic units namely theMakobo, Kinenge, Sumuji and Masati Formations.We report geochemical data for the mudrocks (i.e., shales and siltstones) from the Ikorongo basin in anattempt to constrain their provenance and source rock weathering. These mudrocks are compositionallysimilar to PAAS and PS indicating derivation from mixed macfelsic sources. However, the siltstonesshow depletion in the transition elements (Cr, Ni, Cu, Sc and V) and attest to a more felsic protolith thanthose for PAAS and PS. The Chemical Index of Alteration (CIA: 5282) reveal a moderately weatheredprotolith for the mudrocks. The consistent REE patterns with LREE-enriched and HREE-depleted patterns((La/Yb)CN =7.338.3) coupled with negative Eu anomalies (Eu/Eu* =0.71 on average), which character-isticsaresimilartotheaveragePAASandPS, illustratecratonicsourcesthatformedbyintra-crustaldifferentiation.GeochemicalconsiderationsandpalaeocurrentindicationssuggestthattheprovenanceoftheIko-rongo Group include high-Mg basaltic-andesites, dacites, rhyolites and granitoids from the NeoarchaeanMusoma-Mara Greenstone Belt to the north of the Ikorongo basin. Mass balance calculations suggest rel-ative contributions of 47%, 42% and 11% from granitoids, high-magnesium basaltic-andesites and dacites,respectively to the detritus that formed the shales. Corresponding contributions to the siltstones detritusare 53%, 43% and 4%. 2008 Elsevier B.V. All rights reserved.1. IntroductionInclasticsedimentaryrocks, thetraceelementssuchasrareearthelements(REE)andTharesaidtoberelativelyinsolubleandasaresulttheiroriginalcompositionsarenotupsetduringweathering, erosion and transportation from the parent rocks todepositional environments (Taylor et al., 1986). Although, pro-cesses suchas weathering, hydraulic sorting, and post-depositionaldiagenesishavebeenreportedtodistort geochemical informa-tionaboutthesourcearea(e.g., NesbittandYoung, 1982), yet,ratiosoftheimmobiletraceelementsnormallyreectthoseofthe source rocks rather than the effects of sedimentary processes(Taylor and McLennan, 1985). It is on this basis, therefore, that thechemistry of ne-grained clastic sedimentary rocks has been longutilized for making inference on source rock compositions, palaeo-Corresponding author. Tel.: +255 784 77 56 56; fax: +255 222 41 00 78.E-mail address: kcharls16@yahoo.com (C. Kasanzu).climaticconditionsandtectonicsetting(TaylorandMcLennan,1985).The geology of Tanzania (Fig. 1) can generally be subdivided intove tectono-stratigraphic units: (1) Archaean cratonic rocks (i.e.,theDodomanBelt, NyanzianandKavirondianSupergroups);(2)early to late Proterozoic sedimentary covers and metamorphic ter-ranes (e.g., the Karagwe-Ankolean; the Bukoban Supergroup, theUbendian-Usagarani Supergroups;andtheIkorongoGroup);(3)Pan-African metamorphic rocks (i.e., the Mozambique Belt) locatedin the eastern margin of the Tanzania Craton; (4) Phanerozoic sed-imentarybasins(e.g., TheKaroo);(5)NeogenevolcanicrocksofnorthernandsouthernTanzania(Cloutieretal., 2005andrefer-ences therein).The Ikorongo Group of clastic sedimentary rocks, which is thefocusof thisstudy, hasbeencorrelatedtotheNeoproterozoicBukobanSupergroup(e.g., Shackleton, 1986)whichoverliestheTanzaniaCratoninwesternandnorthwesternTanzania(Fig. 1).Geochemical studies in Tanzania to date have mainly concentratedontheArchaeancrustal rocks(e.g., Messo, 2004;Manya, 2005;0301-9268/$ see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.precamres.2008.04.007202 C. Kasanzu et al. / Precambrian Research 164 (2008) 201213Fig. 1. Generalized tectono-stratigraphic map of Tanzania (modied fromASGA/UNESCO, 1968). The regional setting of the study area shown in Fig. 2 is indicated in the insetbox.Manyaetal., 2006, 2007a,b; Mtoro, 2007)andthePan-Africanorogeny(e.g., Maboko, 1995;Muhongoetal., 2001)inthecon-text of deciphering the geological evolution of the Tanzania Cratonand the neighboring mobile belts. On the other hand, sedimentarycovershavereceivedlittleornoattention. Inthispaper, there-fore, we present major and trace element geochemical data of thene-grained sedimentary rocks (i.e., siltstones and shales) fromtheIkorongo basin. The purpose of the study is to constrain source(s)of the Ikorongo clastic sedimentary rocks and to understand theweathering conditions in the source region(s).2. Geological settingTheIkorongobasinliesinthenortheasternpartof theTan-zania Craton (Fig. 1). It is comprised of clastic sedimentaryrocks which unconformably overlie Archaean basement rockscomposedofhighlydeformedmetamorphicrocks, granitesandgreenstone sequences whichformpart of the NyanzianSupergroup(Stockley, 1936). TheIkorongoGroupis composedpredominantlyofconglomerates, quartzitic sandstones, brown and green shales, silt-stones, red sandstones and subordinate agstones and gritstones(Macfarlane, 1965; Kasanzu, 2007).The general geology of the IkorongoGrouphas beendiscussedinPickering et al. (1959) and recently summarized in Kasanzu (2007).Regionally, the group is comprised of four litho-stratigraphic for-mations namely, frombottomto top, the Makobo, Kinenge, Sumujiand Masati. However, the Makobo Formation is not exposed in thestudyarea. TheKinengeFormation, whichunconformablyover-lies basement rocks, is comprised of quartzites and conglomeraticsandstones that are overlain by shales and siltstones of the SumujiFormation(Fig. 2). TheSumuji Formationisoverlain, throughagradual transition, by thick bedded, ne- to medium-grainedcross-bedded red sandstones of the Masati Formation. On the basis oftheirgeographicaldistribution, therocksoftheIkorongoGroupwere most likely derived from weathering, erosion and depositionof rocks that constituted the exposed upper continental crust of theTanzania Craton during the late Proterozoic (Kasanzu, 2007).3. Sampling and analytical methodologySamples presentedhereincludeshales andsiltstones (heretermed as mudrocks) collected fromthe Sumuji Formation in viewof the fact that ne-grainedclastic sedimentary rocks are more use-ful in geochemical studies than the coarser ones (e.g., Taylor andMcLennan, 1985). Mudrocks are ne-grained siliciclastic rocks richinclayminerals(Yong, 2002). Clayspreservesourcerockchem-ical signatures due to the fact their mineralogy is rarely affectedduringdiagenesisandmetamorphosis(Weaver, 1989). Fifty-fourmudrock samples collected after careful geological mapping weretrimmed to remove weathered surfaces and subsequently crushedin a jig-saw crusher for size reduction. The particles were washedand oven-dried at 70C overnight. The dried samples were thenleft to cool for 24h. The samples were pulverized in an agate plan-etary mill to a grain size of Na2O, it was assumed that the concentration of CaOequals that of Na2O (Bock et al., 1998). However, only one brownshale sample (IK 67) showed CaO contents higher than Na2O.The calculated CIA values for the Ikorongo mudrocks are pre-sented in Table 1. The brown and green shales have comparable CIAvalues which range between 59 and 80 with a mean of 75 whereasinthesiltstonesthevaluesrangefrom57to73withameanof66. The average CIA values in the shales are higher than that forPAAS(CIA=69;noCIAdataforPS)andattesttoamoreweath-ered source. In the shale samples, the elements Ca and Sr are moredepleted relative to PAAS than Al, K, Cs and Ba. These discrepanciescan be explained by two possible processes. Firstly, since Ca and Srare contained in minerals that weather rapidly than those whichcontain K, Al, Cs and Ba (e.g., Roddaz et al., 2006), their depletion(i.e., Ca and Sr) in the shales, therefore, could be caused by weather-ing of parent rocks. Secondly, suchdepletions canalso be attributedto source rocks being poor in plagioclase since both Ca and Sr arecontained in plagioclase (e.g., Roddaz et al., 2006).ThefactthatsiltstonesarecharacterizedbylowerCIAvaluesthan PAAS suggests that the Ikorongo siltstones were fromthe ero-sionof a less weatheredsource thanthat for PAAS. This is supportedby the lower contents in both Al and K relative to PAAS. Further-more, the depletioninCaOinthe siltstone samples, relative toPAAS,is suggestive of parent rocks being poor in plagioclase. Therefore,besides weathering, it seems that these rocks were derived fromthe erosion of plagioclase-poor rocks.In a plot of Al/Na ratios versus CIA of Servaraj and Arthur (2006)all shales and most siltstones plot, together with PAAS, in the inter-mediate weathering eld except for a few samples which plot inthe low weathering eld (Fig. 6). Of interest is the coexistence ofthe shale samples with variable Al/Na ratios but displaying similarCIA values (Fig. 6). This trend could be caused by diagenetic loss ofNa+(e.g., Nesbitt and Young, 1982).Most authors favor the use of Al2O3(CaO* +Na2O)K2O(ACNK) ternary plot inevaluating the chemical weatheringtrendsthansimplecomparisonofnumericalvalue(e.g., Nesbittand Young, 1984, 1989; Roddaz et al., 2006). The ACNK plot isalso useful inindicating post-depositionmetasomatic modicationofmajorelementcompositions(Fedoetal., 1995). Insuchdia-grams, plots of mudrocks commonlyformaweatheringtrendthat isalmost parallel to the ACNjoint. On the Al2O3(CaO* +Na2O)K2O(ACNK) diagram, the siltstone samples plot sub-parallel to theC. Kasanzu et al. / Precambrian Research 164 (2008) 201213 207Fig. 6. Scatter plot of Al/Na ratios versus Chemical Index of Alteration (CIA) for theIkorongo mudrocks. Note the increase of Al/Na ratios at nearly constant CIA valuessuggesting post depositional loss of Na+. Fields are fromServaraj and Arthur (2006).Symbols as in Fig. 3.ACNaxis scattering towards the area between illite and muscoviteand, yet, dene an ideal trend for weathering of a primary sourcewithagranodioriticcomposition(e.g., Fedoetal., 1995). Onthecontrary, shale samples cluster in a tight group near the AK axis(Fig. 7), attesting to a uniformly weathered source. The fact thatthe siltstone samples display variable CIA values (Fig. 6), coupledwiththeirscatteringintheACNKdiagram, demonstratethatthese samples might have been affected by other processes thanweathering, mostlikelyK-metasomatism(Fig. 7). Similartrendshavebeenreportedinancientweatheringproles(e.g., Nesbittand Young, 1989) and in Archaean shales (e.g., Fedo et al., 1995),and are interpreted to be caused by diagenetic modication of K bymetasomatism.5.2. Mineral sortingOf particular importance, sedimentary processes suchas sortingmay modify the mineral abundances and consequently the abun-danceofspecicelements. McLennanetal. (1993)usedaTh/Scversus Zr/Sc plot to distinguish between the contrasting effects ofsourcecompositionandsedimentaryprocessesonthecomposi-tion of clastic sedimentary rocks. An addition of zircon by mineralsorting and/or recycling to samples would result in an increase inZr/Sc ratios (McLennan et al., 1993). On the Th/Sc versus Zr/Sc dia-Fig. 7. Al2O3CaO* +Na2OK2O(NesbittandYoung, 1984)diagramforIkorongomudrocks. The scattering of the siltstone samples is suggestive of K-metasomatism.Also included is the position for an original granodioritic source (from Fedo et al.,1995). Plg: Plagioclase; Ksp: K-Feldspar.Fig. 8. Th/Sc versus Zr/Sc diagramfor the Ikorongo mudrocks (after McLennan et al.,1993). Trend 1 represents sediments derived directly from igneous rocks that havebeen least affected by sedimentary sorting and recycling. Heavy mineral accumula-tion by sediment sorting and recycling would result in Zr enrichment relative to Thas dened by Trend 2. Symbols as in Fig. 3.gram, two trends, one showing direct contribution from primarysource rocks (marked 1), and the other showing the inuence ofsedimentaryprocesses (marked2) canbe distinguished(Fig. 8). Thetrenddepicting sedimentary processes reects the effect of mineralsorting and/or sediment recycling, the effect of which is the prefer-ential Zr enrichment in the sediments. On the Th/ScZr/Sc diagram(Fig. 8), all shale samples cluster in the eld sub-parallel to Trend1 suggesting compositional homogeneity and minimal inuence ofheavy mineral sorting. Onthe other hand, the siltstones display twotrends with some samples plotting along Trend 1 which is indica-tiveofminimalinuenceofmineralsortingandothersplottingalong Trend 2 which is indicative of heavy mineral accumulationby sediment recycling and sorting (Fig. 8).Mineralsortingnormallytendstoincreasetheabundanceofnonclay detrital minerals at the expense of clay minerals (NesbittandYoung, 1984). Therefore, thefactthatthesiltstonesplotonboth trends in Fig. 8 suggests that the relative differences in theabundances of some trace elements might have been attributed tomineral sorting. On the other hand, the lower contents in the ele-ments Cr, Ni, Sc and V could be attributed to the lesser amounts ofAl2O3 which have been reported to signicantly control trace ele-ments distribution (e.g., McLennan et al., 1983; Asiedu et al., 2000).Thisfact issupportedbythepositivecorrelation(r2) betweenAl2O3 andmost of the ferromagnesiantrace element abundances inthe siltstones (e.g., Al2O3Sc =0.9; Al2O3Cr =0.4; Al2O3Ni =0.6).We also suggest that the lower abundances of the trace elements,particularly the REE and ferromagnesian trace elements, in the silt-stones couldbeattributedtoa dilutioneffect of quartz. For instance,a siltstone sample IK 80 contains higher contents of SiO2 (92wt%)thanallsamples, consequentlyitsREEcompositionsaresigni-cantly diluted (Fig. 5).Nonetheless, REEpatterns of all Ikorongosiltstonesamples(except sample IK 80) are similar to those of PAAS and PS, suggest-ing that they have not been intensively affected by factors, in thiscase mineral sorting, that could disrupt source rock information.5.3. ProvenanceAmongthefactorsthatcontrolthegeochemicalcompositionof clastic sedimentary rocks include source rocks, weather-ing/recycling, andpost-depositional diagenesis(e.g., Taylorand208 C. Kasanzu et al. / Precambrian Research 164 (2008) 201213Fig. 9. Plot of Th/U versus Th for the Ikorongo mudrocks. The grey box shows thetypical range of upper crustal protoliths. The arrowstands for an idealized weather-ing trend for sediments derived from upper crust (McLennan et al., 1993). Symbolsas in Fig. 3.McLennan, 1985; McLennanet al., 1993). As shownabove, nonethe-less, these processes can only be responsible for minor variationsin major and trace element contents in the Ikorongo mudrocks andrather the chemical characteristics reect the composition of thesources.The major and trace element compositions for the brown andgreen shales (Table 1; Figs. 4 and 5) are highly comparable and aresuggestiveofasimilarprotolith. HoweverthedepletioninTiO2,Al2O3 and the transition trace elements particularly Co, Ni, Sc andV, which are normally enriched in mac rocks (Rollinson, 1993), inthesiltstones, couldprobablyindicaterelativelymorefelsic detritusthan that for the shales.The Th/U ratios are very useful in determining the source char-acteristics of clastic sedimentary rocks (Roddaz et al., 2006). Thepresent day average crust has Th/Uratios of 4.254.30 whereas thevalues for upper andlower mantleare2.6and3.8, respectively(Pauletal., 2003andreferencestherein). AlthoughsometimeshigherTh/U ratios have been related to oxidative weathering and removalof U, yet, clastic sedimentary rocks derived from the upper crustarecharacterizedbyratiosequal toorgreaterthan4whereasratioslowerthan4havebeenrelatedtoamantlecontribution(e.g., Roddaz et al., 2006). The Ikorongo siltstones, brown and greenshales show mean Th/U ratios of 5.71, 6.90 and 5.86, respectively,whichcharacteristicsaresuggestiveof uppercrustal parentage.These ratios are, however, higher than the values for PAAS (4.70)and PS (4.21) (Taylor and McLennan, 1985; Condie, 1993). The ele-vatedTh/UratiosintheIkorongomudrockscouldbeattributedto either increased weathering intensity or variation in oxidationstate during deposition which would permit U mobility (Roddaz etal., 2006 and references therein). On the Th/U versus Th diagram,all mudrock samples from the Ikorongo basin follow the idealizedweathering trend (McLennan et al., 1993) expected for sedimentsderived from the upper crust (Fig. 9).The geochemical variations between elements such as Th and La(indicative of a felsic source) and Sc (indicative of a mac source)havebeenusedtodistinguishbetweenfelsicandmacprove-nances by various authors (e.g., McLennan et al., 1980). Th/Sc ratiosareuseful indicators of sourcerocks processes andareunaffectedbysedimentary processes (Taylor and McLennan, 1985). The Th versusSc plot (Fig. 10), adopted from McLennan et al. (1993), reveals twodominant sourceareas, acontinental sourcewithTh/Sc ratios near 1for siltstones and an almost 5050 mix of continental and interme-diate component for both brown and green shales (Fig. 10; see silt-Fig. 10. Thversus Sc diagramindicating felsic andmac provenance for the Ikorongomudrocks. Notethesiltstonesamples demarcatedbydottedline. Symbols as inFig. 3.stone samples demarcated by dotted line). This observation furthersuggests that the siltstones were formed by more felsic detritus.OnaLaThScternarydiagram(Fig. 11), whichis usedtoprecisely discriminate felsic and mac provenance of clastic sed-imentaryrocks(e.g., TaylorandMcLennan, 1985), theshaleandsiltstone samples are almost indistinguishable and cluster in theeldformixedsourcesclosetoPAAS-andPS-likeprovenance.Morethanftysamplesgenerallyfall betweentheTaylor andMcLennans (1985) approximation of the upper continental crust(Fig. 11). Therefore, traceelement datafortheIkorongoGroupmudrocks (Table 1; Fig. 11) strongly demonstrate an upper crustprotolith, a feature also revealed in the Th/U plot (Fig. 9).Palaeocurrent directions, alongside geochemical data, can alsohelptohighlightthepossibleprovenanceoftheIkorongorocks.Twosets of palaeocurrent directions weredocumentedinthebasin:north-north-west/south-south-east and north/south. According tothegeologicalsettingoftheIkorongobasin, possiblecandidatesfor provenance include older felsic and mac igneous successionswhich are a major assemblage in the neighboring Nyanzian Super-group of the Archaean Tanzania Craton. The compositions of rocksin the terranes bordering the Ikorongo basin have been well con-strained in Messo (2004), Manya (2005), Manya et al. (2007a,b),Mtoro (2007) and Elisaimon (unpublished data). Lithological unitsstudiedinthe area, whichare more likelytohave fedsediments intoFig. 11. Ternary plot of LaThSc concentrations (after Taylor and McLennan, 1985)for the Ikorongo mudrocks. Symbols as in Fig. 3. UC=upper crust (data from Taylorand McLennan, 1985). Symbols as in Fig. 3.C. Kasanzu et al. / Precambrian Research 164 (2008) 201213 209Fig. 12. Major greenstone domains of the Nyanzian Supergroup showing the setting of possible provenance regions (modied from Borg and Shackleton, 1997).the Ikorongo basin, include basalts, dacites, rhyolites, rhyodacites,granites, TTGs, basaltic-andesites and basaltic-trachyandesites.Therefore, in the light of the two sets of palaeocurrent directions,possible candidate sources include the felsic and mac volcanics ofTarime and, possibly Suguti and Ikoma (Fig. 12). Both Tarime andSugutiformpartoftheMusoma-MaraGreenstoneBelt(MMGB)whereas Ikoma belongs to the Kilimafedha Greenstone Belts (KGB)of the Nyanzian Supergroup (see Fig. 12 for locations).Intheir comprehensivestudies onsedimentaryrocks, Taylor andMcLennan (1985) indicated the signicance of relying on the ele-ments that are least mobile under the expected range of geologicconditions in provenance determination. This observation is basedonthe fact that during weathering, the alkali and alkaline earthele-ments are quite soluble whereas elements like Al, Zr, Hf, Sc, Y, Nb,Th and REE are relatively immobile (Taylor and McLennan, 1985;NesbittandYoung, 1982). ThereforetheabundancesofREE, Th,and the transition trace elements, especially Sc, and their respec-tive ratios are the best proxies for provenance studies (Taylor andMcLennan, 1985). Table2compares immobiletraceelements ratios,La/Sc, Co/Th, Cr/Th, andTh/Sc for the Ikorongomudrocks withthoseofpossiblesourcerocks, PAAS, PSandwell-establishedratiosofsands derived from mac and felsic rocks. Although variations areevident, La/ScandSc/ThratiosfortheIkorongorockspointtoafelsic dominated-detritus (Table 2). When compared to PAAS andPS, La/Sc, Sc/Th, Cr/Th and Co/Th ratios in the Ikorongo mudrocksindicate derivation from a more felsic source than the PAAS and PSsources.Thesiltstones showrelativelyhigher proportionof felsic detritusthan shales do (see Table 2), an observation which is also supportedby the multielement variation diagram in Fig. 4, which shows thedepletion of compatible elements such as Cr, Ni, Sc and V which areusually regarded as mac components (Asiedu et al., 2000).The La/Sc and Co/Th ratios of the mudrocks are similar to thoseof TTGs from the Tarime segment of the MMGB to the north of theIkorongo basin suggesting that the TTG supplied bulk of the felsiccomponent to the Ikorongo basin. In particular, the Sc/Th ratios ofthe siltstones are very similar to those of the TTGs suggesting thatthe siltstones were formed by detritus predominantly derived fromthe weathering of the TTGs. AcontributionfromMMGBrhyodacitesis also indicated by the close similarity between the Cr/Th ratios ofthetwolithologies(siltstones 5.94;rhyodacites 5.54). Ontheother hand, dacites from the MMGB have Cr/Th ratios which arecomparable to those of the shales suggesting that the dacites mayhave also been source rocks for the shales.In addition, the abundances of the REE have been used to infersourcesofsedimentaryrocks(McLennanetal., 1993;Asieduetal., 2000). For instance, mac rocks contain low LREE/HREE ratiosand no Eu anomalies, whereas felsic rocks usually contain higherLREE/HREEratios andnegativeEuanomalies (Taylor andMcLennan,1985; Roddaz et al., 2006). The Eu anomaly in sedimentary rocksis commonly regarded as inherited from the source rocks. There-fore, the REE patterns obtained in sedimentary rocks can help tomake inference on the nature of protolith (Taylor and McLennan,1985). In this regard, further constraints on possible source rocksfor the Ikorongo mudrocks can be made by using the REE compo-sitions.Chondrite-normalizedabundances andpatterns (Fig. 5) indicatethat, despite considerable variations in contents, most of the Iko-210 C. Kasanzu et al. / Precambrian Research 164 (2008) 201213Table 2Range of some elemental ratios for the Ikorongo mudstones in comparison with possible source rocks from the Musoma-Mara Greenstone Belt and Kilimafedha GreenstoneBeltLithology La/Sc Sc/Th Cr/Th Co/Th SourceRange Mean Range Mean Range Mean Range MeanRhyolites 0.190.31 0.28 Manya (2005)Dacites 2.3321.78 8.63 Manya (2005)Gabbro 18.34665.40 140.99 Manya (2005)K-Granites 0.060.95 48.47 Manya (2005)Na-Granitoids 0.235.68 2.01 Manya (2005)Rhyodacites 0.4029.43 5.54 Manya (2005)High-magnesium basaltic-andesite 10.55166.52 48.47 Manya (2005)Tonalite-Trondjemite-Granodiorites 2.9127.5 10.03 0.0412.05 0.67 01.80 0.12 0.202.94 0.88 Elisaimon (unpublished data)Biotite granites 7.3128 23.68 0.030.32 0.11 0.020.17 0.08 Elisaimon (unpublished data)Calcic granites 6.1230.68 13.68 0.070.54 0.23 0.040.72 0.24 Elisaimon (unpublished data)Basaltic-andesites 0.441.64 1.26 2.286.93 3.48 5.3091.61 19.4 5.4111.48 6.91 Messo (2004)Basaltic-tranchyandesites 0.641.46 1.15 2.425.27 3.23 4.715 7.9 5.129 6.2 Messo (2004)Basalts 61.221850 608.5 110.2265 160.5 Mtoro (2007)Sands from felsic rocks 2.516 0.051.2 0.57.7 0.221.5 Taylor and McLennan (1985)Sands from basic rocks 0.441.1 2025 22100 7.18.3 Taylor and McLennan (1985)Post-Archaean Australian Shale (PAAS) 2.81 1.27 7.94 1.59 Taylor and McLennan (1985)Average Proterozoic Shale (PS) 2.23 1.18 8.04 1.25 Condie (1993)Brown shales 2.025.20 3.25 0.741.58 1.08 5.0517.68 8.83 0.282.64 1.11 This studyGreen shales 2.097.06 2.92 0.751.22 1.01 6.4314.19 9.05 0.421.79 1.08 This studySiltstones 2.1013.84 4.39 0.091.11 0.66 1.3918.24 5.94 02.78 1.37 This studyAlso included are ratios for PAAS, PS and sands derived from mac and felsic protoliths.rongo mudrock samples studied have striking similarities in theirREE patterns. All samples are characterized by an enrichment ofthe LREE, negative Eu anomalies and relatively at HREE patterns.These features, particularlythe negative Euanomalies suggest a dif-ferentiated protolith, similar to granite (e.g., McLennan et al., 1993;Asiedu et al., 2000). REE patterns for samples from the IkorongoGroupshowcloserresemblancetothoseoftheTarimevolcanicand plutonic rocks and to a lesser extent with rhyolites from theSuguti (Fig. 13). On the other hand, magmatic rocks from Ikoma tothe southeast of Ikorongo do not seemto have fed sediments to theIkorongo basin since their REE patterns do not match with those ofthe Ikorongo mudrocks (not shown).Based on the REE compositions of the source candidates, mix-ing calculations of Albarede (2002) were carried out to estimatethe relative contribution of source materials required to generatethe Ikorongo mudrocks. Briey, for a system o, containing severalelements (i =1, . . ., m) hosted in phases (j =1, . . ., n), let Mj be themass of phase j and mijthe mass of element (or species) i hostedin the phase j. Then, the composition of species (or element) i inphase j can be mathematically dened asCij =mijMjForthebulkmaterial, massconservationrequiresthatM0 =

nj=1Mj.Therefore, for a given element, i, the proportion of fj of the phasej is such that: fj=Mj/M0 and Ci0 = mi0/M0 =

nj=1mij/M0 (all equa-tions adopted from Albarede, 2002).The high-magnesium-basaltic-andesites (HMBA), granitoidsand dacites comprise major part of the exposed crust in the MMGB.Based on their aerial distribution and geochemical afnity to theTable 3Representative REE compositions of possible source rocks located in the MMGB and the Ikorongo mudrocksHMBA (N=13) Dacites (N=27) Granitoids (N=33) Average shale Average siltstoneAverage STDEV Average STDEV Average STDEV Average STDEV Average STDEVLa 28.79 7.29 48.10 19.22 52.34 21.38 57.42 57.74 36.58 13.68Ce 60.75 14.88 96.93 38.01 100.96 41.76 100.19 26.06 69.02 29.77Pr 7.22 1.73 10.94 4.14 11.04 4.54 11.97 23.61 7.59 3.15Nd 29.00 6.85 41.39 15.69 39.16 15.52 39.48 14.51 26.71 10.65Sm 5.31 1.12 6.53 2.43 6.12 2.41 7.22 7.26 5.59 1.96Eu 1.44 0.25 1.58 0.58 1.11 0.36 1.46 3.45 1.24 0.32Gd 4.55 0.75 5.17 1.71 4.78 2.01 5.45 3.77 5.18 1.40Tb 0.65 0.08 0.60 0.20 0.63 0.28 0.81 2.56 0.76 0.22Dy 3.61 0.33 2.93 0.99 3.28 1.47 4.35 2.02 4.03 1.20Ho 0.73 0.07 0.54 0.18 0.64 0.30 0.80 1.59 0.75 0.22Er 1.87 0.19 1.33 0.46 1.67 0.76 2.36 1.57 2.26 0.76Tm 0.29 0.03 0.20 0.07 0.26 0.12 0.35 1.45 0.34 0.13Yb 1.93 0.19 1.33 0.47 1.83 0.75 2.28 1.42 2.19 0.91Lu 0.28 0.03 0.19 0.07 0.27 0.11 0.33 1.34 0.34 0.16Eu/Eu* 0.90 0.05 0.83 0.04 0.64 0.02 0.71 0.03 0.71 0.05(La/Yb)CN10.15 2.73 24.31 3.80 20.38 5.60 15.20 2.06 11.14 2.7(La/Sm)CN3.41 0.25 4.63 0.19 5.18 0.10 5.02 0.42 4.11 0.86(Gd/Yb)CN1.91 0.13 3.13 0.05 2.07 0.02 1.95 0.02 1.91 0.45C. Kasanzu et al. / Precambrian Research 164 (2008) 201213 211Fig. 13. Comparison of the Chondrite-normalized REE patterns of the Ikorongo mudrocks with those of possible source rocks from MMGB. Other symbols as in Fig. 3.Ikorongo mudrocks (Figs. 12 and 13), 14 HMBA, 33 granitoids and27 dacite samples were used to model the detritus that fed sedi-ments into the Ikorongo basin. The three rock types were treatedasdistinctcomponentsbecausetheyaremajorrocktypeswithdistinctive geochemistry from which the Ikorongo mudrocks arethought to be derived from.For this case, REE concentrations of shale samples were aver-aged and the resulting composite was assigned average shale. TheTable 4Results of mixing and mass balance calculations and comparison between original and calculated parametersHMBA DCT GRDProportions (%)Model shale 42 11 47Model siltstone 43 4 53Model shale Average shale %Variation Model siltstone Average siltstone %VariationLa 41.98 57.42 27 42.04 36.58 15Ce 83.63 100.19 17 83.51 69.02 21Pr 9.43 11.97 21 9.40 7.59 24Nd 35.14 39.48 11 34.88 26.71 31Sm 5.82 7.22 19 5.78 5.59 3Eu 1.30 1.46 11 1.27 1.24 3Gd 4.73 5.45 13 4.70 5.18 9Tb 0.64 0.81 21 0.64 0.76 16Dy 3.38 4.35 22 3.41 4.03 16Ho 0.67 0.80 17 0.67 0.75 10Er 1.72 2.36 27 1.74 2.26 23Tm 0.27 0.35 24 0.27 0.34 20Yb 1.81 2.28 20 1.85 2.19 16Lu 0.26 0.33 21 0.27 0.34 20(Eu/Eu*) 0.77 0.71 9 0.76 0.71 7(La/Yb)CN16.52 15.20 9 16.14 11.14 45(La/Sm)CN4.38 5.02 13 4.40 4.11 7(Gd/Yb)CN2.12 1.95 9 2.04 1.91 7DCT means dacites. GRD means granitoids.212 C. Kasanzu et al. / Precambrian Research 164 (2008) 201213Fig. 14. Comparison of Chondrite-normalized REE patterns between (a) average shale and model shale, and (b) average siltstone and model siltstone.sameprocedurewasperformedforsiltstonesamples(Table3).Conversely, for the case of siltstones, three samples (IK 19, 79 and80) were eliminated in the calculations since they are signicantlyenriched in SiO2 (81.4692wt%). The HREE contents of these sam-plesseemtobehighlydilutedbyquartzleadingtoinconsistentLREE/HREE ratios.From the average data shown in Table 3, modeling for the aver-age shale protolith was done using the ratios (La/Yb)CN, (Gd/Yb)CNas well as theEu/Eu*, andthemixingcalculations was set inamatrixform asEuEuLaYbGdYb=a b c0.9 0.83 0.6410.2 24.31 20.41.91 3.13 2.07abc=0.7115.21.95shaleameansHMBA; b=dacites; c =granitoids. TheratiosLa/YbandGd/Yb are Chondrite-normalized.The results of the mixing calculations (Table 4) showthat, gran-itoids and the HMBA from the Tarime region were the main sourcerocks (granitoids 47%: HMBA 42%), while the dacites suppliedlesser amounts of detritus (11%). This observation, therefore, con-forms to an almost 1:1 mixture between felsic and mac protolithsimilar totheupper crust composition(Taylor andMcLennan,1985). Similar resultswerealsoobtainedfor siltstonesamplesexcept that granitoids show a relatively higher proportion (53%)whereas the HMBA supplied about 43% and dacites 4%.Fromthe calculations, optimal tting of average REE concentra-tions of the Ikorongo Group with those of source candidates wereachieved by mass balance calculations.Toobtainthecalculatedvalues, thefollowingequationfromAlbarede (2002) was used:WRmix= C1+C2+C3WRmixrefers to the calculated whole rock compositions while , andstand for the proportions of the HMBA, dacites, and gran-itoids, respectively, obtained in mixing calculations. C1, C2 and C3stand for the respective species (elements) in the HMBA, dacites,and granitoids used in the mixing calculations. The results of thecalculations, whichwerebasedontheREEparameters, arepre-sented in Table 4 and Fig. 14 for comparison.6. ConclusionsSource rock weathering and provenance of the Ikorongo Grouphave beenassessedusing geochemical studies. Major element com-positions suggest that the Ikorongo mudrocks were derived frommoderately weathered protoliths. Th/U ratios coupled with Th ver-susScandLaThScplotssuggestanuppercrustalprotolithforthe Ikorongo mudrocks similar to the PAAS and PS protolith. ThefractionatedREEpatternsandthenegativeEu/Eu*anomaliesofthe Ikorongo mudrocks further attest to an upper crust provenancetypical of a craton interior. Based on palaeocurrent measurements,thesourcerocksfortheIkorongoGroupliestothenorthofthebasin suggesting that the MMGB, which comprises of older felsicand mac igneous rocks, is a possible source terrane. The REE pat-terns and elemental ratios such as La/Sc, Sc/Th, Cr/Th and Co/Thof the studied mudrocks reveal that the source rocks include mag-matic rocks from the Tarime and Suguti segments of the MMGB.Basedonmixingandmassbalancecalculations, theshaledetri-tuscanbemodeledbyamixtureof47%granitoids, 42%HMBAand 11% dacites. 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