Ultrahigh-Temperature Metamorphism in the Palni Hills, South India: Insights from Feldspar Thermometry and Phase Equilibria

Abstract

The Southern Indian shield is a classic terrain that may be regarded as a model for Precambrian crustal evolution. The Palni Hills represent a portion of the granulite-facies terrain of the Madurai block. Determination of ultrahigh temperature (UHT) in granulites requires re-integration of antiperthite, perthite, and mesoperthite to yield original hypersolvus feldspar compositions, which then yield peak metamorphic temperatures by feldspar solvus geothermometry. Backscattered electron images were used to calculate modal proportions of host and exsolved lamellae in feldspars. These data were combined with quantitative point chemical analyses of both host and lamellae to obtain re-integrated feldspar compositions, providing consistent ultrahigh temperatures in excess of 900°C. The feldspar compositions coexisting during closure of intercrystalline Al-Si exchange were calculated by reversing the K-Na exchange through shifts of Ab and Or contents of both feldspars at constant An contents until the equilibrium tie-line and the common isotherm on the ternary feldspar solvus were found. Besides, UHT metamorphic conditions were also obtained using the convergence method in exchange thermometers, suggesting peak metamorphism conditions at > 900°C at 9 kbar for the Palni granulites. Prevalence of UHT conditions in the many parts of the world could be useful in deducing Proterozoic tectonometamorphic processes along the deep crust-mantle interface.

Geochronology and phase equilibria modelling of ultra‐high temperature sapphirine + quartz‐bearing granulite at Usilampatti, Madurai Block, Southern India

“…The scientists from all over the world involved in interpreting the maximum temperature attained by the granulites during the ultra‐high temperature metamorphism are in general consensus that regional metamorphism of these granulites requires additional heat apart from that available from a thickened crust (Ashwal et al, ; Prakash et al, ). A possible heat source, though commonly neglected, is viscous dissipation or mechanical heating.…”

The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

“…Of those, the Himalayan‐scale East African Orogen (EAO) extending from northeast Africa, through western Arabia, East Africa, Madagascar, South India, Sri Lanka, and Antarctica [ Kriegsman , ; Boger and Miller , ; Jacobs and Thomas , ; Collins and Pisarevsky , ; Prakash et al ., ; Fritz et al ., ], marks a belt of highly deformed and metamorphosed Precambrian rocks that were reworked during the final stages of Gondwana amalgamation. The tectonic style of the EAO orogen changes from predominantly juvenile accretionary tectonics of the Arabian‐Nubian Shield at the northern margin [ Johnson et al ., ; Robinson et al ., ] to continental‐style collisional tectonics toward the south [ Meert and Van Der Voo , ; Collins and Pisarevsky , ; Prakash et al ., ; Fritz et al ., ]. The timing of the metamorphism associated with the collisional‐style tectonics differs across the orogen from west to east.…”

“…The circa 650-500 Ma assembly of the continental domains that made up the supercontinent Gondwana took place along a number of dispersed Proterozoic mobile belts [Meert and Van Der Voo, 1997;Stern, 2002;Meert, 2003;Collins and Pisarevsky, 2005]. Of those, the Himalayan-scale East African Orogen (EAO) extending from northeast Africa, through western Arabia, East Africa, Madagascar, South India, Sri Lanka, and Antarctica [Kriegsman, 1995;Boger and Miller, 2004;Jacobs and Thomas, 2004;Collins and Pisarevsky, 2005;Prakash et al, 2006;Fritz et al, 2013], marks a belt of highly deformed and metamorphosed Precambrian rocks that were reworked during the final stages of Gondwana amalgamation. The tectonic style of the EAO orogen changes from predominantly juvenile accretionary tectonics of the Arabian-Nubian Shield at the northern margin [Johnson et al, 2011;Robinson et al, 2014] to continental-style collisional tectonics toward the south [Meert and Van Der Voo, 1997;Collins and Pisarevsky, 2005;Prakash et al, 2006;Fritz et al, 2013].…”

New SHRIMP U-Pb zircon ages of the metapelitic granulites from NW of Madurai, Southern India

“…The principal lithologies are metasediments (argillaceous -arenaceouscalcareous), mafic granulites, charnockites, anorthosites and discrete granite bodies. The metapelites show mineral assemblages of cordierite -garnet -spinel-sillimanitesapphirine -orthopyroxene where cordierite -garnet - sapphirine assemblages dominantly occur in the central part of the highland massifs (Raith et al 1997;Prakash et al 2006;Shimpo et al 2006;Collins et al 2007a;Tsunogae et al 2008;Santosh et al 2009) while sillimanite -garnet bearing assemblages are common in the lowland areas such as Usilampatti -Andipatti -Ganguvarpatti (Mohan and Windley, 1993;Prakash and Arima, 2003;Sajeev et al 2004). Marble and calc-silicates constitute carbonate lithologies that occasionally contain rotated blocks of gneisses, quartzites and ultramafics.…”

“…Common mineral assemblages of constituent metapelites include garnet, cordierite, sapphirine, orthopyroxene, spinel and sillimanite (Mohan and Windley, 1993;Mohan et al 1996;Prakash and Arima, 2003;Sajeev et al 2004;Tsunogae et al 2008). The metamorphic P-T conditions recorded in the Madurai block strongly indicate a decompressional history (Mohan et al 1996;Raith et al 1997;Prakash et al 2006;Santosh and Sajeev, 2006;Tsunogae and Santosh, 2006;Braun et al 2007;Prakash et al 2007a). The rocks of Madurai block…”