First Report of the Spinel + Quartz Assemblage from Kodaikanal in the Madurai Block, Southern India: Implications for Ultrahigh-Temperature Metamorphism

Abstract

We report the equilibrium assemblage of spinel (X Mg = Mg/(Fe+Mg) = 0.40-0.43, ZnO~0.58 wt%, Cr2O3 = 0.39-0.44 wt%) and quartz enclosed within the core of poikiloblastic garnet in pelitic gneiss from the Kodaikanal region, Madurai Block, southern India. The petrogenetic grid in the KFMASH system suggests that such Zn- and Cr-poor spinel associated with quartz is an indicator of ultrahigh-temperature (UHT) metamorphism, which is confirmed by calculated peak P-T conditions using a garnet-orthopyroxene assemblage in charnockite (860-990°C and 7.5-8.5 kbar) and antiperthitic feldspar in a leucocratic layer of pelitic migmatite (T ~1000°C). Detailed observation of retrograde textures as well as mineralogical and thermobarometric data suggest that the area underwent near isobaric cooling and then decompression after the UHT event. Comparison of available P-T trajectories of the Madurai Block and adjacent areas suggests that the Kodaikanal region underwent ~1000°C UHT metamorphism similar to other domains in the Madurai Block, although the peak pressure condition is slightly lower than other areas, possibly indicating shallower crustal levels. Results of this study confirm that the Madurai Block is an example of a regional UHT terrane associated with the Pan-African orogeny attending final amalgamation of the Gondwana supercontinent.