Abstract
The Southern Granulite Terrane within the southern Dharwar Craton, South India, representing the lower continental crust, has become a crucial region for investigating late Neoarchean tectonic regimes. This study examines intermediate granulites (garnet-bearing and garnet-free two-pyroxene granulites) from the Shevaroy and Namakkal blocks, employing detailed petrography, laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon U-Pb analyses, phase equilibrium modeling, garnet-equilibrated Al-in-orthopyroxene thermometry, and rare earth element-in-two-pyroxene thermometry. Garnet-bearing two-pyroxene granulites have a peak assemblage of garnet, clinopyroxene, orthopyroxene, plagioclase, quartz, ilmenite, hematite, and melt, with retrograde K-feldspar, amphibole (amp1), and symplectitic garnet-quartz. The garnet-free two-pyroxene granulite contains peak minerals similar to those in the garnet-bearing variety, except for the absence of garnet, and develops retrograde amphibole, K-feldspar, and quartz. Phase equilibrium modeling indicates peak metamorphic conditions of 9.2−12.2 kbar and 920−1150 °C for the garnet-bearing two-pyroxene granulite, with retrograde conditions at 8.8−9.7 kbar and 870−888 °C. Comparable conditions were identified for the garnet-free two-pyroxene granulite, with peak conditions of 8.2−11.0 kbar and 925−1045 °C and retrograde conditions of 8.8−9.5 kbar and 875−896 °C. Both rock types underwent ultrahigh-temperature (ultrahigh-T) metamorphism, consistent with garnet-equilibrated Al-in-orthopyroxene and REE-in-two-pyroxene thermometries. LA-ICP-MS U-Pb analysis on metamorphic zircons from garnet-bearing and garnet-free two-pyroxene granulites yielded metamorphic ages of ca. 2510−2500 Ma, which likely correspond to the timing of peak or near-peak granulite-facies metamorphism. Both the garnet-bearing and garnet-free two-pyroxene granulites experienced ultrahigh-T metamorphism during the late Neoarchean, followed by near-isobaric cooling. The scale of late Neoarchean ultrahigh-T metamorphism in the granulite terrane of South India is much larger than previously recognized. Phase equilibrium modeling reveals that the development of garnet-bearing and garnet-free intermediate granulites is primarily controlled by the difference in bulk-rock compositions. The near-isobaric cooling path derived in this study, in conjunction with previously published data, is indicative of mantle-derived magma underplating. This underplating provided heat for the ultrahigh-T metamorphism and was associated with asthenospheric mantle upwelling as a result of extension following the aggregation of small continental blocks.