Structural features of granulites from Rayagada, Eastern Ghats, India: some preliminary observations.

Abstract

A suite of granulates including metasedimentary rocks (khondalite, quartzite and calc-silicate granulite), mafic granulite, orthopyroxene granulite, and garnetiferous granite gneisses (leptynite, sillimanite granite and porphyritic granite) exposes in the Rayagada area, north-central part of Eastern Ghats Granulite Belt of Peninsular India. Among these, important marker horizons like calc-silicate granulate and quartzite have been used to establish the sequence and geometry of folds on a regional scale. Signatures of three phases of deformations (D1, D2, D3 from the earliest to the latest) are present in metasedimentary rocks and mafic granulite, whereas most of the massive orthopyroxene granulite, leptynite and sillimanite granite exhibit foliation corresponding to the second deformation event. These three deformations correspond to three folding episodes: F1 (intrafolial fold), F2 (tight to isoclinal fold) and F3 (open fold) respectively. Axial surfaces of F1 and F2 are mostly parallel, trending NNE-SSW and F3 axial plane is at right angle to that of F1 and F2 folds. Corresponding axial plane foliations are gneissic banding in khondalite and mafic granulate (S1), a pervasive foliation in granite gneiss (S2) and a sporadic fracture cleavage in all the rock-types (S3). In most cases, structures of first deformation (D1) were superimposed by the second one (D2), resulting D2 structures (both folding and foliation) as the dominantly preserved deformation structure. D1 deformation and D2 deformation have been inferred to be related to the E-W or ESE-WNW directed compression and progressive simple shear and D3 deformation might be related to the strike-slip displacement and NNE-SSW transtension.