Petrology of the granulite facies rocks with special reference to the basic granulites around Patharkhang in west Khasi hills district Meghalaya

Abstract

The present research work is the first attempt towards an understanding of metamorphic evolution of the granulite facies rocks exposed around Patharkhang, West Khasi Hills District, Meghalaya, an unknown Proterozoic terrain in India. The chief aim of the present study is to obtain a clear understanding of metamorphic evolution of the granulites on the basis of petrological and geochemical studies. Overall field studies show that the study area represents a volcano-sedimentary terrain witnessing multiple deformations of folding and magmatic intrusions. The earliest magmatic activity possibly relates to the emplacement of basic magma, which presently represent as basic granulites, dominant granulite facies rocks of the area. The basic granulites occupy a major part in association of minor intercalations of metapelites within the milieu of regionally developed granite gneisses. These rocks give clear indication of multi phase deformations characterised by F1 F2, F3 foldings respectively and polymetamorphic events. The polymetamorphic scenario of the rocks as reflected from critical mineralogical and textural features show that the prograde granulite facies leading to the crystallisation of garnet, sillimanite, biotite bearing metapelites and orthopyroxene, clinopyroxene, plagioclase, opaque phases with or without green brown hornblende in basic granulites (M1).. The earliest metamorphic episode (pre-M1) so far identified as inclusions of quartz, sillimanite, biotite and plagioclase within coarse garnet might represent as relict of early metamorphic event or the prograde evolutionary P-T path of the M1 event of the Patharkhang metapelites while in basic granulites, no petrographic evidence is obtained that indicates prograde P-T path of the granulite facies metamorphism of these rocks The retrogression took place in two stages leading to development cordierite corona (M2) at the expense of garnet and sillimanite + quartz and post - M1 melting with the development of porphyroblastic cordierites and secondary garnet...