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BULGARIAN GEOLOGICAL SOCIETY, 80-th Anniversary

KYANITE-STAUROLITE-GARNET-BEARING SCHISTS FROM OGRAZHDENMOUNTAIN, SW BULGARIA – METAPELITES OR ORTHOSCHISTS?

Lubomira Macheva, Rositsa Titorenkova, Nikola Zidarov

Central Laboratory of Mineralogy and Crystallography “Acad. I. Kostov”, BAS, 1113 Sofia, Bl.107, Acad. G. Bonchev Str.;e-mail: lu_macheva@dir.bg, rosititorenkova@dir.bg, nzidarov@interbgc.com,

Key words: Ograzhden mountain, metamorphism, metapelites, orthoshists, kyanite

IntroductionIn the vicinity of village Lebnitza (OgrazhdenMountain) kyanite-garnet schists with minor amountsof staurolite and chloritoid are observed. In a fewprevious investigations in this region (Зидаров и др.,1967; Aleksic et al., 1988) these rocks have beendescribed as metapelites. With their wealthy mineralcomposition such a kind of rocks is the most sensitiveindicator for reconstructing the pressure andtemperature conditions of the metamorphism. By thisreason we focus our attention on the metamorphicevolution of these rocks in order to provide informationon the P-T path with a special regard to the numberand the grade of metamorphic episodes. However,our field, petrological, geochemical and zircon typologyinvestigations in such a kind of rocks in Ograzhdenas well as in Belasitza Mountain, revealed somepeculiarities leading to another interpretation of theschists, namely that they are formed in a hightemperature deep crustal shear zone that transformedthe granitoid rocks into aluminous orthoschists.Displaying these features is the aim of the presentcontribution.

Geological setting and field descriptionThe studied area is situated in the most southwesternpart of Bulgaria, in Ograzhden Mountain (Fig.1, left-bottom insertion) and is assigned to belong to theSerbo-Мacedonian Massif (Kockel & Walther, 1965).The main part of the metamorphic basement in thisregion is built up of two facial varieties (equigranularand porphyritic after K-feldspar one) of meta-granitoids overprinted by upper amphibolite faciesmetamorphism, heterogeneous deformation andmigmatisation (Titorenkova et al., 2002). Nowadays,metagranitoids appear as augengneiss, augenlayeredor layered structures. Both U-Pb (TIMS) and Hfzircon data for equigranular and porphyriticmetagranitoids points to ages of 459.9±7.6 Ma and451+18/-9 Ma, respectively, interpreted to reflect thetime of the magma intrusion (Zidarov et al., 2003).

Typical MME (melanocratic microgranular enclaves)meter to decimeter sized as well as concordantlenticular amphibolite bodies and boudins of a biggersize (up to tens of meters) are scattered in meta-granitoids. In the vicinity of the village Lebnitza, athin body of schists containing large kyanite- (up to2-4 cm long) and garnet porphyroblasts (up to 4 cmin diameter) can be mapped mainly at the transitionzone between the porphyritic and equigranularmetagranitoids (Fig.1). At the contacts of metagrani-toids with these rocks, the foliation deepens steeply(50-700) to NW with a remarkable almost horizontalSE lineation. Near the schist body, metagranitoids arestrongly isoclinaly folded and mylonitized but moremicaceous in composition and containing thin kyanitestreaks elongated parallel to the foliation. At thislocality, the thickness of the schist body varies from10 to 100 m and can be traced for several hundredmeters along the strike. The schist body reveals adistinct zonality (Fig 1, top-right insertion) with

Fig. 1. Geological sketch map of the eastern slopes of theOgrazhden Mountain. 1-Quaternary; 2-Tertiary volcanites;3a-Equigranular metagranites; 3b-Porphyritic metagra-nites; 4-Kyanite-staurolite-garnet-bearing schists; 5-Am-phibolites. Inserted figure (top right)-schematic pictureof the zonal schist body across the profile line.

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variations from rocks highly resembling metagranitesto high aluminous schists.

Petrography and mineral chemistryIn the vicinity of village Lebnitza four mainmineralogical zones can be recognized within andadjacent to the schist body, including strongly foliatedmica-rich metagranites bearing scarce kyanitestreaks, noted as zone I. Zone II represents highlydeformed melanocratic two-mica schists, containingsmall garnet and tourmaline crystals. Zone III is morecoarse-grained, built up of large garnet poikiloblastsand elongated kyanite segregations both set up in aquartz-mica matrix. Zone IV (the central part of thebody) is highly differentiated and built up of alternatingkyanite- and quartz-enriched layers 10-15 cm inthickness. The coarse-grained kyanite with nodularappearance is stretched parallel to the foliation. Inthis zone, one can see finely lineated, pale-blue towhite colored, almost solely kyanite comprising thinmylonite layers of a thickness up to 10 cm. They arecomposed of finely oriented kyanite needles.

The texture and the mineral composition of gneissesin zone I resemble strongly that of metagranitoids(plagioclase + biotite + white mica +K-feldspar + quartz+ additional small kyanite and garnet grains as well asaccessory phases including rutile, ilmenite, apatite andzircon). The kyanite and garnet grains are formed onthe account respectively of sericitised mineral (mostprobably plagioclase) and biotite.

The mineralogy of zone II is substantially differentfrom that of metagranite. It is built mainly of largeflakes of biotite and white mica in addition to smallamounts of plagioclase, quartz, kyanite and two garnetgenerations (coarse-grained, synkinematic poikilo-blasts, containing white mica, kyanite and quartz

inclusions and small-grained (0.1-0.3 mm) latekinematic garnet scattered in all other minerals).Accessory tourmaline is frequently observed.

Zone III is a typical aluminous rock without anyfeldspar. The two garnet generations are wellrepresented together with large kyanite segregationswith nodular appearance set up in a quartz-micamatrix. Kyanite nodules are made up of small radialsometimes bent crystals of a clearly pseudomorphiccharacter (Fig. 2a). Scarce staurolite porphyroblastsare formed in kyanite nodules.

Quartz-rich layers in zone IV are built up of 90-95%quartz plus a minor amount of finely grained white mica,biotite, and small garnet crystals without any preferredorientation. The layers containing kyanite nodules aremost intensively sericitized and rich in mica (mostly whitemica) bands. The larger garnet crystals are often fullydecomposed, preserving sometimes only their formercontours. Differently oriented ideal prismatic stauroliteand chloritoid porphyroblasts were observed, grownobviously in a static manner, at the expense of sericitisedkyanite nodules (Fig. 2b). Parallel to the foliation, thinkyanite needles are formed wrapping the sericitisednodules. In this highly mylonitized zone, which is nearlymonomineral (being composed of 90-95 % fine kyaniteneedle crystals), one observes stripes of finely grainedgarnet, oriented strongly parallel to the foliation (Fig.2c).Quartz bands are scarce in these kyanite layers.

The chemistry of minerals is used for obtaininginformation on the conditions of metamorhism. Bothtypes of garnet are almandine rich (Alm76-86), with smallamounts of pyrope (Pyr7-16), grossular (Grs1-7) andspessartine (Sps2-7). By using the method of Droop(1987), we recalculated the content of Fe3+ to be up to4.57% from Fet. Most garnet profiles exhibit a plateau-like distribution of FeO, CaO and MnO, typical of the

Fig.2. Microscopic images a) Kyanite nodules with pseudomorphic character; b) Staurolite and chloritoid porphyroblastsformed after kyanite; c) Kyanite mylonite – fine kyanite streaks oriented parallel to the foliation. Finegrained garnetscattered in layers parallel to the lineation. All images in crossed polars. Magnification – 40.

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high-grade metamorphism (Tracy, 1982). The coexistingsmall-grained garnet is rich in Mg and poor in Mn,chemically homogenous and of the same compositionas the rims of the large-sized garnet.

Staurolite and chloritoid were locally observedand obviously formed at the expense of sericitizedkyanite. Staurolite is unzoned, with a Fet/(Fet+Mg)ratio of 0.77-0.82, Fe-rich (~13wt% FeOt), and donot contain MnO and ZnO. Chloritoid is lowmagnesian (XFe =0.84-0.88), but higher magnesianthan both staurolite and garnet, which is typical ofthe retrograde formation. Two generations of whitemica can be distinguished on the basis of texturalfeatures: 1) an early kinematic one with a relativelyhigh celadonite content (up to 3.34 p.f.u.), with a lowparagonite component (to 0.12 p.f.u.) and with arelatively high Fe+Mg sum (to 0.42 p.f.u.) mainly atthe expense of Mg.; 2) syn- to late kinematic whitemica, characterized with a moderate to low Si content(3.08-3.18 p.f.u), a small Fe+Mg sum (0.11-0.18) anda relatively low paragonite content (0.13-0.22). Wesuppose that the early-kinematic white mica is formedat high temperatures and the second generation atlower temperatures. The biotite composition is quiteuniform, with a high Fet/(Fet+Mg) ratio (0.50-0.62)and an Al content of 0.40-0.51 p.f.u.

On the basis of microstructural observations andmineral chemistry the following sequence of mineralparagenesis can be distinguished: 1) HT/MP(HP?) -garnet + kyanite + quartz + white micaI + tourmaline;2) MT/MP-LP - staurolite + chloritoid + quartz +whitemicaII ± biotite .

Metamorphic conditionsThe pressure and temperature conditions are derivedusing two different methods, namely a selected cationexchange and net transfer reactions texturallyobserved on a thin section scale, and a simultaneous

Fig.3. Zircon crystals separated from schists: a) euhedral crystal with typology identical to zircon from metagranites(S1-2); b) zircon with resorbed crystal faces; c) strongly resorbed zircon grain with morphology typical for peraluminousmetagranites (S6). Marker – 100 µm.

calculation of all possible end-member reactions withinassemblages of local equilibrium using an internallyconsistent data set. The following results wereobtained via both methods: T~6700C and Pmin = 7kbar for the peak metamorphic conditions. Theformation of kyanite porphyroblasts as well as ofgarnet rims around the biotite flakes in themetagranitoids infers higher pressure conditions thanthe calculated Pmin. Tmax= 5500C for the retrogradestaurolite and chloritoid formation are obtained. Thetime of the kyanite retrogression to sericite remains,however, unknown.

Zircon morphologyThe zircon morphology from the schists studiedresembles strongly that of zircons from metagranitoids(Titorenkova et al., 2002). Zircons with a well-preserved crystal form, identical with that of zirconsfrom metagranitoids, are presented (Fig. 3a), althoughcrystals with curved and resorbed surfaces dominate(Fig.3b). These peculiarities could be explained witha high fluid activity in the shear zone. Multi-facetedforms, aggregate crystals and weakly rounding ofsteep pyramids are typical of zircon from both rocktypes - metagranitoids and schists (Fig.3c). Isometricand irregularly shaped grains are also present. Theobtained SEM images give evidence for secondaryalteration processes. Most of the crystals aremetamict or fluid overprinted.

ConclusionsBased on complex field, petrological, geochemical andzircon typology investigations, an orthometamorphicorigin of the schists, cropping in the vicinity of Lebnitzavillage as well as in other localities of Belasitza andOgrazhden Mountains, is suggested. Peculiarities ofthe schist body in the vicinity of Lebnitza village, suchas its zonality, the total absence of pelitic lithologies

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in metagranitoids as well as the presence in the schistsof finely grained melanocratic inclusions that resemblestrongly MME in metagranitoids, the morphology andthe inner textures of zircons from schists and theirmelanocratic inclusions and metagranitoids, allowed usto consider the schist body under study as a part of adeep crustal shear zone. The schist body reveals a distinctzonation that indicates a dramatic change in the

mineralogical composition from granite at the wall rocksto garnet-kyanite schists with staurolite and chloritoid inthe zone center. All processes of such transformationsare possible only in the presence of canalized fluid flowthrough the deep crustal shear zone. Further U-Pb zirconages from the schist body can give an additional evidenceto support our hypothesis for the orthometamorphiccharacter of these rocks.

ReferencesAleksic, V., S. Dimitriadis, M. Kalenich, R. Stojanov, I.

Zagorchev. 1988. Precambrian in Serbo-MacedonianMassif. - In: Zoubek, V. (ed.) Precambrian in YoungerFold Belts. J. Wiley & Sons, 780-819.

Droop, G. T. R, 1987. A general equation for estimating Fe3+

concentrations in ferromagnesian silicates and oxidesfrom microprobe analyses using stoichiometric criteria.- Mineral. Mag,. 51, 431-435.

Kockel, F., W. Walther. 1965. Die Strimonlinie als Grenzezwischen Serbo-Mazedonischen und Rila-RhodopeMassiv in Ost Mezedonien. - Geol. Jb., 83, 573-602.

Titorenkova, R., L. Macheva, N. Zidarov. 2002. Zircontypology of metagranites from Maleshevska andOgrazhden Mountains, SW Bulgaria. - C.-R. Acad.bulg. Sci., 55, 9, 61-66.

Tracy, R. J.1982. Compositional zoning and inclusions in

metamorphic mineras. – In: Review in Mineralogy, 10,355-397.

Zidarov N., I. Peytcheva, A. von Quadt, V. Andreichev, L.Macheva, R. Titorenkova. 2003. Timing and magmasources of metagranites from the Serbo-MecedonianMassif (Ograzhden and Maleshevska Mountains, SWBulgaria): constraints from U-Pb and Hf-zircon and Srwhole rock isotope studies. – Proc. of annual scientificconf. of BGD “Geology 2003”, December 11-12, Sofia,89-91.

Зидаров, Н., Ил. Костов, П. Игнатовски, Л. Михайлова,Г. Хаджиев, В. Чешмеджиева. 1966. Доклад върхугеологията на Огражден планина и южнитеотдели на Малешевска планина (Геоложко карти-ране и търсене на полезни изкопаеми, М 1: 25000,проведено през 1966; Геофонд MOCB)

КИАНИТ-ГРАНАТ-СТАВРОЛИТОВИ ШИСТИ ОТ ОГРАЖДЕН ПЛАНИНА –МЕТАПЕЛИТИ ИЛИ ОРТОШИСТИ

Любомира Мачева, Росица Титиренкова, Никола Зидаров

В района на с. Лебница, (Огражден планина) серазкриват кианит-гранатови шисти, съдържащиставролит и хлоритоид, досега описвани катометапелити. Нашите теренни, петроложки, геохи-мични и цирконови морфоложки изследванияпоказват някои особености, които дават основаниеза друга интерпретация на протолитите на тезискали, а именно като високотемпературна дълбо-чинна зона на срязване, в която гранитоидите сатрансформирани във високоалуминиеви шисти.Тялото от шисти в изследвания район показва ясназоналност. От контакта с метагранитите къмцентралната му част са отделени четири зони,различаващи се по минерален състав. Зона Iвключва силно нашистени, богати на слюдениминерали метагранитоиди, рядко съдържащикианитови порфиробласти и дребни гранатови зърна;зона II - съдържа силно деформирани дребно-зърнести, меланократни двуслюдени шисти с дребни

гранатови и турмалинови порфиробласти; зона III еизградена от едропорфиробластен гранат и удъл-жени кианитови нодули, включени в кварц-слюденматрикс; зона IV (централната част на тялото) –изградена от редуващи се богати на кианитовинодули и богати на кварц ивици. За сметка насерицитизираните кианитови нодули се образуватставролит и хлоритоид в статична обстановка. Средскалите от тази зона се наблюдават милонитниивици, изградени почти изцяло от кианит. Получениса следните данни за Р-Т условията на мета-морфизма: Т~ 670 0C и Рmin ~ 7 kbar за темпера-турния пик и около 550 0С за образуването наставролита и хлоритоида. Въз основа на проведе-ните изследвания може да се предположи ортомета-морфен произход на шистите от района на с.Лебница чрез драстична промяна в химизма иминералогията на метагранитите в дълбочинна зонана срязване с активно флуидно участие.