Precambrian polycyclic metamorphism in the Krishna Province, southeastern margin of the Eastern Dharwar Craton

Kaptan, Om Prakash (2014) Precambrian polycyclic metamorphism in the Krishna Province, southeastern margin of the Eastern Dharwar Craton. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The southeastern margin of ~2:5 Ga Eastern Dharwar Craton, tonalite-trondhjemite -granodiorite, along with the easternmost ~2:7 Ga Nellore greenstone belt, is juxtaposed with Krishna Province rocks comprising of the granulite-facies Ongole Domain, amphibolite-facies Vinjamuru Domain and the greenschist-facies Udayagiri Domain, which are polydeformed. As the Paleoproterozoic Vinjamuru (meta) granitoids contain deformed older enclaves of a dismembered metasedimentary succession, the present study was carried out to identify and characterize a probable polycyclic metamorphic history undergone by these rocks. The metapelitic enclaves within the Vinjamuru granitoids occur, from south to north, at Saidapuram, Sangam and Malakonda. They are essentially kyanitegarnet- muscovite-schists with spatially varying development of staurolite porphyroblasts; these schists are polydeformed with spectacular tight folds of kyanite (and muscovite) laths in an axial planar position to a pervasively-developed crenulated muscovite-de�ned schistosity (here termed Vinjamuru Domain Encl S₂ as opposed to the earlier relict foliation preserved in kyanite porphyroblasts as Encl S₁). The Vinjamuru granitoids are in juxtaposition with the low grade metasedimentary rocks of the Udayagiri Domain along a transitional, linear N-S trending narrow zone, comprising a metamorphosed succession of rhyolite, andesite-dacite and interlayered with volcaniclastics sequence of chert and banded magnetite-baryte with minor carbonate, the age of which is constrained by the � ~1:78 Ga crystallization age of the rhyolite. These rocks preserve a single dominant foliation (termed the Transitional Zone [TZ] S₁) and undervent a major prograde metamorphic event (here termed as TZ M₁) which, with available age constraints would be younger than 1:78 Ga. This major metamorphic and deformational event is seen to a�ect the granitoids of the Vinjamuru Domain and are correlatable. From petrographic studies, the equilibrium mineral assemblage for Encl M₁ event in metapelites is found to be Grt+Ky+Ms+Ru+Qz�Chl. Inclusion trails of Qz and oxides de�ning Si [� S1]foliation and external foliation de�ned by Ms �akes, Se [�S₂] wrapping over relict Ky grains indicate atleast two distinct deformation events. The equilibrium prograde mineral assemblage for Encl M₂ event is Chl+Ms+St+Bt and it is distinct from assemblage of Encl M₁. Ky and Ms inclusions are found in St porphyroblasts indicating their Encl M₁ status. Polymorphic transformation of Ky to Sil and Grt that has completely reacted out with Chl to form St+Bt as it is evident from St+Bt growing over Chl to form Encl M₂ phases. Classical geothermobarometry and pseudosection approach has been applied to constrain the P-T conditions prevailed during Encl M₁ and Encl M₂ as well as TZM1 (temporally Encl M₂=TZM₁). Relict equilibrium mineral pairs in metapelite that could be used for better constraining Encl M₂ and TZM₁ using classical approach, could not be identi�ed. Encl M₁ was constrained using Grt-Hbl-Pl-Qz (GHPQ) assemblage in metabasic rocks (sample S2) and Grt-Ky-Pl-Qz assemblage in metapelite (sample UG39). The temperature and pressure for Encl M₁ in metabasics is found to be 546�586 �C (Grt-Hbl thermometer of Graham & Powell, 1984) and 8.4�9.4 kbar (GHPQ barometer of Kohn & Spear, 1990) respectively. Based on Garnet-Biotite and Garnet-Aluminosilicate-Silica-Plagioclase (GB-GASP) program of Ganguly et al. 1996 gives T � 609

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