A MULTISTAGE PRESSURE-TEMPERATURE RECORD IN THE CHILKA LAKE GRANULITES - THE EPITOME OF THE METAMORPHIC EVOLUTION OF EASTERN GHATS, INDIA

Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P-T record. A high-T decompression from above 10 kbar to 8 kbar around 1100 degrees C has been determined from Mg-rich metapelites (X(Mg)>0.60) with quartz-cordierite-orthopyroxene-sillimanite and cordierite-orthopyroxene-sapphirine-spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670 degrees C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene-quartz-garnet-plagioclase-cordierite assemblage, and are further supported by the garnet + quartz double right arrow orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650 degrees C al 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P-T values are obtained from metapelites with a quartz-plagioclase-garnet-sillimanite-cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700 degrees C and the rims 5 kbar, 650 degrees C, suggesting a third decompression. The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P-T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P-T path replicate parts of the P-T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.