Deformation and metamorphism of a schistose terrane accreted with the Proterozoic Pakhal sequence of Godavari valley near Kothagudem, Andhra Pradesh

Abstract

From the recent research by several workers it is becoming evident that the geology, particularly the structure and metamorphism of the Proterozoic sedimentary basins are not simple, which can at times be compared with foldthrust belts. In the southern part of the Pranhita-Godavari (PG) basin in its close proximity with the Eastern Ghat Mobile belt (EGMB) in the east and Eastern Dharwar craton (EDC) in the west there is a supracrustal association exposed near Kothagudem in Andhra Pradesh, which was earlier referred in published literature as Mulug and Mallampalle Groups of lower Pakhal sequence. From the field and petrological studies we have attempted to enumerate here the deformation and metamorphic characteristics and the tectonic setup of this schistose unit. It is revealed from our studies that this lithotectonic unit represents a strongly deformed and metamorphosed deeper crustal supracrustal assemblage, which is in sharp contrast with the unmetamorphosed and weakly deformed true Pakhal sequence occurring in the western part of the area. In order to explain the spatial association of these two contrasting litho-tectonostratigraphic units we propose a westerly verging thrust model, supported by convincing structural and metamorphic evidences, whereby the schistose terrane is tectonically juxtaposed with the unmetamorphosed Pakhal sequence. Imprints of polyphase deformation and metamorphism could be recognized to document the complex history of the schistose terrane. At least three generation structures (D1, D2, D3) are identifiable in quartzite-metapelite-carbonate assemblages. D1 and D2 are quite pronounced and conspicuous by the development of respective planar and linear elements, while D3 is relatively mild in intensity. Development of different generations of folds (F1, F2, F3) and associated axial planar cleavage/ schistosity (S1, S2, S3) together with different types of fold interference patterns, demonstrated mainly in carbonate rocks, bear testimony to successive events of fold formation as a result of superposed deformations. The tectonic trend defined by N-S to NNESSW (approx.) elongation of the belt results from the combined effects of all the three deformations. The structural pattern indicate a possible combination of predominantly pure shear and subordinate simple shear transpressional type of tectonism accompanied by considerable E-W (approx.) tectonic shortening across the belt. The rocks show low to medium grade metamorphism with the characteric appearances of chlorite, biotite, andalusite, chloritoid, staurolite and garnet porphyroblasts in pelitic rocks. From a study on ‘Si-Se’ tectonites in different mineral assemblages we have elucidated the history of deformation and metamorphic mineral growth in this assemblage. A variety of relationship exists between ‘Si’ and ‘Se’ demonstrating multi-stage growth of porphyroblasts, particularly of andalusite and staurolite, which took place during at least two deformational events. The important mineral assemblages include: (i) garnetbearing andalusite- biotite schist, (ii) garnet free andalusite-biotite schist, (iii) staurolite-andalusite-garnet-mica schist, (iv) andalusite-chloritoid-muscovite schist, and (v) garnet-chloritoid-chlorite-muscovite schist. The study establishes a prograde metamorphic evolution in each rock type followed by a retrograde history. But the relative timing of prograde and retrograde reactions with respect to successive phases of deformation varies widely.