High Pressure and Ultrahigh Temperature Metamorphism at Diguva Sonaba, Eastern Ghats Mobile Belt (India): New Constraints from Phase Equilibria Modelling

The investigated area around Diguva Sonaba represents a part of granulite facies terrain of Eastern Ghats Mobile Belt (India). The Precambrian metamorphic rocks of the area consist of mafic granulite (± garnet), khondalite, leptynite (± garnet, biotite), charnockite, enderbite, calc-granulite, migmatic gneiss and sapphirine-spinel-bearing granulite, which comprise the major rock types of the area. Sapphirine-spinel-bearing granulite occurs as lenticular bodies in khondalites, leptynites and calc-granulites. Textural relations such as presence of corroded blebs of biotite within garnet and orthopyroxene, resorbed hornblende within pyroxenes, and coarse prisms of sillimanite presumably pseudomorphs after kyanite, provide evidence of either, an earlier episode of upper amphibolites facies metamorphism or represent relict of a single prograde event leading to granulites facies metamorphism. In the sapphirine-spinel granulite, osumilite and sapphirine + spinel + quartz were stable during the thermal peak of granulite facies metamorphism (early stage) but were later replaced by the (Crd-Opx-Qtz-Kfs) - symplectites and variety of reaction coronas respectively during the retrograde episode (middle stage). Variable amounts of retrograde biotite or biotite + quartz symplectite replace orthopyroxene, cordierite, (Opx-Crd-Kfs-Qtz)-symplectite (late stage). The prograde path was followed by peak metamorphism at a temperature of c. 1000 °C and a pressure of c. 12 kbar as computed by isopleths of XMg garnet and XAlorthopyroxene. The sequence of reactions as deduced from the corona and symplectites assemblages, together with petrogenetic grid and pseudosection considerations, records a clockwise P-T evolution.