Mineralogical and Petrological Characteristics of the Neoproterozoic Orthoamphibolite and Ortho
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Vo1.84 No.3 pp.563—580 ACTA GEOLOGICA SINICA(English Edition) June 2010 Mineralogical and Petrological Characteristics of the Neoproterozoic Orthoamphibolite and Orthogneisses in the Mutki Area, the Bitlis Massif,Southeast Turkey
Kerim KOCAK and F Mehmet CERAN Geology Department,Selcuk University,Campus Konya 42075,Turkey
Abstract:The rocks form as amphibolite±garnet±epidote and orthogneisses in the Pan-African basement of the Bitlis Massif.The petrochemical data of the studied metamorphic rocks suggest different igneous protoliths ranging from calcalkaline basalt to andesite in composition. Petrochemically.the rocks can be classified as group 1(1ow Zr and La)and group 2(high Zr and La), all showing variOHS enrichments in large ion lithophiles and light rare earth elements.and a depletion in high-field strength elements,suggestive of a destructive plate margin setting.The protoliths of the all samples might have formed mostly by the partial melting of an enriched source,possibly coupled with the fractional crystalization of plagioclase。apatite。and titaniferous magnetite±olivine 4- cIinopyr0xene±amphibole in relation with subduction-related magmatism neighboring the Andean- type active margins of Gondwana.The group 2 samples could,however,be generated by a relatively lower degree of the partial melting of an inhomogeneous source with a preponderance of a high-level, fractional crystallization process in comparison to group 1.The protoliths of the samples were metamorphosed up to amphibolite facies conditions,which destroys original igneous texture and mineral assemblages.Geothermobarometric caiculations show that the metamorphic rocks are finally equilibrated between 540 and 610。C and一5 kbars.foliowing a clockwise P-T.t path.
Key words:amphibolite,petrochemistry,metamorphism,Bitlis Massif,Gondwana,Turkey
1 Introduction Turkev is an essential component of the Alpine- Himalayan orogenic system.It contains a number of continental blocks separated by suture zones,formed by the closure of the different branches of the Neo.Tethyan Ocean during the Late Cretaceous-Eocene(Sengor and Yilmaz,1981)(Fig.1a).The major metamorphic massifs in Turkey,such as the Bitlis and the Menderes massifs。 could be formed during the Pan—African orogeny(Sengor et al一1984).It was suggested that during the Early Mesozoic,the Bitlis Massif rifted from the northern margin of Arabia,and was re—atnalgamated to Arabia by the Miocene(Yllmaz et a1.,1993;Robertson,1998; Robertson et a1.,1996;Robertson et a1..2007). The Bitlis Massif is an arcuate metamorphic belt, approximately 500 km long and 30 km wide in southeastern Turkey(Fig.1 b),representing fragments of a Cadomian active margin and bordering the northern Corresponding author.E—mail:kkocak@yahoo.corn margin of Gondwana after its final amalgamation fUstaomer et a1..2008).It consists of Precambrial1.to— Cretaceous rocks characterized by an”old basement”f1 e Hizan Group)and an”epimetamorphic cover”(The Mutki Group)separated by an unconformity(Boray,1 975; Yilmaz,1975;Hall,1980,Goncuoglu and Turhan,1984, oberh/insli et a1..2007).Metamorphic rocks of the Bitlis Massif overlie the Paleozoic—to.Tertiary shelf sequences of the Arabian Platform(Hal1,1976)with all intervening tectonic zone of ophiolite and Tertiary flysch(Fig.1b). The Bitlis Massif constitutes a component of the Tethyan suture zone that was assembled during Late Mesozoic- Early Cenozoic(Yflmaz et a1.,1993;Robertson,1996) (Fig.1c). The samples studied form as amphibolite and amphibole—bearing orthogneisses of the Andok gneiss (Fig.Ic),a major part of the old metamorphic basement.It provides constraints for understanding the magmatic and tectonometamorphic evolution of the north Gondwana margin during the NeoproterOzoic,but has not yet been subjected to any mineralogical or geochemical study.we V01.84N0-3 ACTA GEOLOGICA SINICA(English Edition) June 2010 565 present new petrography,m ajor and trace elemental data f0r the amphibolite and orthogneiss from the Mutld area of Bitlis massif.The Durpose of this contribution is thus to: (i characterize the different metamorphic rocks with igneous protoliths;(ii)understand the magma type,its sources,and the prevailing tectonic setting during the Proterozoic;and(iii)determine the pressure(P)一 temperature fT) and time (t) conditions of the metamorphism.
2 Geological Setting and Field Characteristics The Bitlis Massif is exposed as a widespread,flat.1ying nappe within a thick,S—vergent fold and thrust belt(the Bitlis suture zone),produced by the collision of the Arabian Platform to the south,and Anatolia to the north in the Miocene.The basement rocks also display the effects of at least three deformation episodes(Boray.1975):D1 and D2 produced a regional foliation with isoclinal and open foldings,respectively,whereas the D3 phase created only microfolding. The high—grade basement of the Bitlis Massif contains amphibolites,homblende schists, mica schists, rnigmatites,marble,eclogite(G6nctio ̄lu and Turhan, 1984;Okay et a1.,1985;Tolluo ̄lu,1988),all of which were cross—cut by metavolcanics and granitic and leucocratic rocks.Granitic rocks occur as two distinct groups(Usta6mer et a1.,2008).The first group of rocks cut the high—grade basement units.These plutons are peraluminous granites and granodiorites,displaying calcalkaline trends.The second group of granitic rocks Occurs as dykes and larger intrusions that are a few kilometers wide and 1ong fe.g.in the Mutki area);they intruded into the Devonian metacarbonates and quartzites, with a dominant metaluminous composition.The epimetamorphic cover consists of Paleozoic—to.Triassic platform sediments fOberh/insli et a1. 2007).is over】ain by extrusive and intrusive rocks of Tertiary age(basalts, andesites,and dolerites),and by sedimentary rocks(in the Mus Basin)ranging in age from Eocene to Paleocene. Paleozoic and Mesozoic rocks of the Bitlis Massif can be correlated with the units of corresponding ages in the Arabian promontory(G6nctio ̄lu and Turhan, 1984; 6nctio豆lu and Kozlu,2000;Oberh/insli et a1.,2008). The basement of the Bitlis Massif was metamorphosed prior to the Devonian age of the oldest—dated cover sediments.Yllmaz f 1 97 1)dated country rock gneisses and amphibolites at 505_+37 Ma to 920_+224 Ma,and intruding granite at 325_+3 Ma by the Rb/Sr whole—rock method.A wel1.defined Rb/Sr isochron gives 347_+52 Ma for whole— rock samples and mineral separates of another granite(the Yayla metagranite).However,Usta6mer et a1.r2008) reported a 545.5±6.1-531.4±3.6 Ma(Ediacaran-Early Cambrian1 U/ ∞Pb zircon crystallization age for granite, compared to the amphibolites and orthogneisses studied by laser inductively—coupled plasma mass spectrometry, and suggested that the Bitlis Massif was affected by Cadomian arc—type magmatism.The amphibolite and orthogneisses with igneous protoliths might then have formed and metamorphosed prior to 530 Ma,that is,they could be Neoproterozoic in age.The metamorphism of the epimetamorphic cover postdates deposition of the youngest—dated rocks in this cover sequence(Late Triassic),but predates to the Upper Cretaceous of the oldest, unconformably overlying, unmetamorphosed sediments(Upper Maastrichtian Kinzu Formation) (G6ncfio ̄lu and Turhan.1 984). The basement rocks display the effects of at least four metamorphic episodes:eclogite—facies metamorphism, amphibolite—facies metamorphism. greenschist—facies metamorphism,and cataclastic metamorphism.The age of the eclogite—facies(Okay et a1.,1985,Cagatay,1987, UstaOmer et a1., 2008) and amphibolite—facies metamorphisms is probably Precambrian in age.The 1ast metamorphic event in the area is believed to have been related to Miocene thrusting,as suggested by other researchers(Hall,l976;Mason,1975). The metamorphic rocks studied are generally very poorly exposed,and no minor structures,other than narrow shearing zones,have been observed(Hal1.1974). They consist mostly of amphibolites,garnet amphibolites, amphibole—bearing ort_hogneisses,and lesser epidote amphibolites at the old basement.The garnet amphibolite cannot be distinguished from the other amphibolites in the field.That is,the discriruination can only be made by a petrographic study.The orthogneisses and amphibolite forillS at the bottom、while the garnet and epidote amphibolites OCCUr at the upper levels of the old basement. A11 amphibolites Occur as 1enses.pods.and various sil1. 1ike bodies at several levels.aligned conformably with the foliation of the enclosing gneisses.It has green,dark green,and bluish colors,and exhibits a moderate.to—strong foliation defined by the subparallel alignment of amphibole-plagioclase crystals,with the average grain size<1 rain.The orthogneisses are medium grained. slightly foliated.and show vertical and latera1 gradations to the amphibolites.