New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh-temperature sapphirine granulites

Abstract

The sapphirine granulites from Gangaraju Madugula, Eastern Ghats Belt (EGB), India preserve a rich variety of mineral assemblages and unique isolated and composite mineral inclusions within garnet that provide robust evidence for extreme crustal metamorphism at ultrahigh temperature (UHT) conditions (>900°C). Diagnostic UHT assemblages in these rocks include sapphirine+quartz, spinel+quartz and high alumina orthopyroxene+sillimanite+quartz. The stability of spinel+quartz, sapphirine+quartz and orthopyroxene+sillimanite+quartz assemblages provides evidence for temperatures exceeding 960°C at moderate pressures. The mineral association of garnet-orthopyroxene is indicative of a subsequent high P-UHT metamorphic event as indicated by the high alumina contents of orthopyroxene (>10wt% Al2O3) coexisting with garnet. Peak P-T conditions of ∼970°C and 9.5kbars are calculated from conventional garnet-orthopyroxene geothermobarometry. Calculated isochemical sections constructed in the model system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO) for the sapphirine granulites and garnet-orthopyroxene granulites adequately predict phase relationships that are consistent with those observed in the rocks. An evaluation of the assemblages and textures and P-T estimates indicate a three-stage evolution of the sapphirine granulites and associated garnet-orthopyroxene granulites: (1) inclusion assemblages with sapphirine, spinel and quartz on the low-pressure prograde path (M1 stage); (2) a peak UHT assemblage of porphyroblastic garnet-orthopyroxene (M2 stage) and (3) a retrogression that resulted in orthopyroxene-cordierite intergrowths and biotite rims on Grt (M3 stage). The sapphirine granulites and associated garnet-orthopyroxene granulites indicate that they grew during the prograde and retrograde stage. The thermo-barometric estimates from mineral compositions and the complimentary isochemical sections approached from bulk rock compositions allow tighter constraints to be placed on the P-T evolution of this sector of the EGB. © 2010 John Wiley & Sons, Ltd..