ULTRAMETAMORPHISM IN PRECAMBRIAN GRANULITE TERRANES - EVIDENCE FROM MG-AL GRANULITES AND CALC-SILICATE GRANULITES OF THE EASTERN GHATS, INDIA

High Mg-Al granulites and calc-silicate granulites provide evidence fo r ultra-high temperatures of metamorphism (ca. 1000 degrees C) at mode rate pressures (9-10 kbar) in the Eastern Chats Belt, India. Lack of p roper geochronological data prevents the dating of this extreme metamo rphism. High Mg-Al granulites contain different subsets of mineral ass emblages involving spinel, quartz, sapphirine, cordierite, orthopyroxe ne, garnet and sillimanite coexisting with either rutile-ilmenite or t itanohaematite-ferrianilmenite. These high Mg-Al rocks are poor in Zn and Cr, as reflected primarily in the composition of spinel. Evidence of ultra-high temperature metamorphism comes from (i) textural interpr etation of the former coexistence of spinel-cordierite-quartz and sapp hirine-quartz and stabilization of the assemblages orthopyroxene-silli manite-cordierite and spinel-quartz-sapphirine-garnet and (ii) the hig h Al2O3 content of orthopyroxene coexisting with garnet and/or cordier ite. Consideration of the sequence of deduced mineral reactions in pet rogenetic grids in the system FMAS attests to an anticlockwise P-T pat h of evolution for the granulites. In calc-silicate granulites stabili zation of nearly pure meionite and of the wollastonite-plagioclase-and radite-rich garnet, wollastonite-scapolite-grandite garnet-calcite ass ociation corroborate high temperatures of metamorphism. Conventional m ineralogical geothermobarometry in all the rocks record lower temperat ures (maximum 950 degrees C) at 9-10 kbar pressures, attributed to res etting of the mineral compositions during cooling. Following peak meta morphism, the rocks firstly experienced near-isobaric cooling followed by near-isothermal decompression. On the basis of the available evide nce it appears that non-extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra-high te mperature metamorphism on an anticlockwise path in the Eastern Ghats B elt.