High-grade regional metamorphism of ultramafic and mafic rocks from the Archaean Sargur terrane, Karnataka, South India

Deformed and metamorphic ultramafic to mafic rocks emplaced into the Archaean Sargur supracrustal series (>3.0 Ga) in Karnataka, southern India, represent layered igneous bodies. The terrane has been affected by several episodes of deformation and metamorphism in the time span from 3.4 to 2.5 Ga ago. During the regional metamorphism about 2.5 Ga ago the igneous bodies re-equilibrated partly or completely at conditions of the upper amphibolite to granulite facies. The development of sagvandites with enstatite + magnesite and anthophyllite + magnesite-bearing assemblages, and of mafic garnet-pyroxene charnockites indicates the presence of CO2-rich intergranular fluids (XCO2 ≥ 0.5) in these rocks during metamorphism. The physical conditions of metamorphism have been estimated by applying methods of geothermobarometry to the recrystallized ultramafic assemblages with olivine, pyroxenes and spinel and to the charnockitic assemblages with garnet, pyroxenes, plagioclase and quartz. A best temperature estimate of 700 ± 30°C was derived with the geothermometers of Evans and Frost (Ol-Spi), Fabriès (Ol-Spi), Wells (Opx-Cpx), Powell (Opx-Cpx), Ellis and Green (Gra-Cpx), Lal and Raith (Gra-Opx), and Danckwerth and Newton (Al2O3-content in opx). A mean pressure estimate of 8.6 ± 0.8 kbar has been obtained with the models of Perkins and Newton (Gar-Opx/Cpx-Plag-Qtz). The P-T data indicate a minimum crustal thickness of about 35 km at c. 2.5 Ga in this part of the South Indian Archaean craton