P–T estimates from the Nellore schist belt (India) and evidence for superimposed metamorphic events

The Nellore schist belt (NSB) is one of the prominent schist belts of southern India. It is thrust over an unmetamorphosed Proterozoic sedimentary sequence (Cuddapah Basin) in the west and in turn is overthrust by the Eastern Ghats Granulite Terrain (EGGT) in the east. Metamorphic grade has been considered to be either greenschist to amphibolite facies or else to show high- and low-grade groups of rocks. Detailed mineralogical and P, T studies on representative rock types across the high- to low-grade groups (metapelite and metadacite from the high- and low-grade groups respectively) near Vinjamuru reveal that there are high (M1) and medium (M2) grade metamorphic events. While metapelite documents both M1 and M2 events, metadacite shows only the M2 event, thus indicating the presence of high- and medium-grade groups of rocks in the NSB. The stable mineral assemblage during the early prograde part of M1 was Qtz+St₁+Ms₁+Pl₁+Bt±Grt, which subsequently suffered high-grade metamorphic conditions (T∽715–765°C and P∽8·6–9·2 kbar) leading to the demise of St₁ and melting of Ms₁. In contrast, medium-grade metamorphism (M2) is characterized by the stable association of St₂+Qz. Quantitative geothermobarometry suggests T=520–570°C and P=6·1-6·8 kbar, and these results show good consistency with the average P, T estimates obtained from the THERMOCALC program. This metamorphic episode is probably coeval with the 900 Ma granulite facies metamorphic event in the EGGT. The superimposition of M2 over M1 probably has led to widespread retrogression of the high-grade rocks to medium grade, thereby giving the general impression of a greenschist to amphibolite facies gradation. © 1998 John Wiley & Sons, Ltd.