Ultrametamorphism in Precambrian granulite terranes: evidence from Mg-AI granulites and calc-silicate granulites of the Eastern Ghats, India

Abstract

High Mg-Al granulites and calc-silicate granulites provide evidence for ultra-high temperatures of metamorphism (ca. 1000°C) at moderate pressures (9-10 kbar) in the Eastern Ghats Belt, India. Lack of proper geochronological data prevents the dating of this extreme metamorphism. High Mg-Al granulites contain different subsets of mineral assemblages involving spinel, quartz, sapphirine, cordierite, orthopyroxene, garnet and sillimanite coexisting with either rutile-ilmenite or titanohaematite-ferrianilmenite. These high Mg-AI 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 interpretation of the former coexistence of spinel-cordierite-quartz and sapphirine-quartz and stabilization of the assemblages orthopyroxene-sillimanite-cordierite and spinel-quartz-sapphirine-garnet and (ii) the high Al₂O₃ content of orthopyroxene coexisting with garnet and/or cordierite. Consideration of the sequence of deduced mineral reactions in petrogenetic grids in the system FMAS attests to an anticlockwise P-T path of evolution for the granulites. In calc-silicate granulites stabilization of nearly pure meionite and of the wollastonite-plagioclase-andradite-rich garnet, wollastonite-scapolite-grandite garnet-calcite association corroborate high temperatures of metamorphism. Conventional mineralogical geothermobarometry in all the rocks record lower temperatures (maximum 950°C) at 9-10 kbar pressures, attributed to resetting of the mineral compositions during cooling. Following peak metamorphism, the rocks firstly experienced near-isobaric cooling followed by near-isothermal decompression. On the basis of the available evidence it appears that non-extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra-high temperature metamorphism on an anticlockwise path in the Eastern Ghats Belt.