Global type area charnockites in southern India revisited: Implications for Earth's oldest supercontinent

Abstract

The global ‘type area’ charnockites and those in the surrounding localities within Madras block of the Southern Granulite Terrane in India dominantly comprise felsic to intermediate, coarse to medium grained, orthopyroxene-bearing anhydrous granulite facies rocks with sporadic garnet. In several localities, the charnockitic suite contains mafic magmatic enclaves of gabbroic to dioritic composition showing calcic and peraluminous composition. The charnockite suite shows compositional range from monzonite through granodiorite to granite, and are calcic and calc-alkalic, peraluminous, including both magnesian and ferroan types. Their major and trace element variations are consistent with progressive magmatic differentiation. The charnockites show high Ba-Sr content with a trend from normal arc-related rocks to adakites. The geochemical features of the charnockites and mafic enclaves are consistent with subduction-related arc setting, and slab-derived magmas interacting with mantle wedge peridotite. The P-T conditions as estimated through mineral phase equilibria modelling and pseudosection computations of representative charnockite and mafic enclave samples show a range of ca. 7 to 9 kbar and 870 to 960 °C, suggesting high- to ultra-high temperature granulite facies conditions during the peak metamorphism.The zircon grains from the charnockites show magmatic features with oscillatory or banded zoning, and in many cases display core-rim structure indicating dissolution and metamorphic overgrowth. The magmatic grains/domains show typical steep LREE to HREE pattern, whereas the metamorphic domains show relatively flat HREE. Magmatic zircon U-Pb data indicate crystallization ages of ca. 2.53 Ga to 2.57 Ga. The identical age from magmatic zircon grains in the mafic enclaves suggests bimodal magmatism with underplated mafic magmas intruding into the felsic magma chamber. The U-Pb data from metamorphic zircon and monazite indicate that all the rocks were metamorphosed coevally at ca. 2.47 Ga to 2.49 Ga, soon after their emplacement. The close timing between magmatism and metamorphism of ca. 40 Myr also suggests the formation of the magmatic suite along an active convergent margin, followed by collisional metamorphism during the termination of subduction and ocean closure. The Lu-Hf analyses of magmatic domains in zircon show mostly positive ɛHf(t) values up to +8.7, with only a few spots showing slightly negative values up to −0.8. Zircon grains in the mafic enclaves also show mostly positive ɛHf(t) values up to +4.3. The U-Pb-Hf data are consistent with juvenile arc building during late Neoarchean, with no significant older components. Together with the Hf model ages, the data indicate that the magmas were derived from depleted mantle components of Meso- to Neoarchean age, which would suggest an active subduction regime that continued until the ocean closure during end Neoarchean-earliest Paleoproterozoic. The granulite blocks surrounding the southern margin of the Dharwar craton including the Madras block are interpreted as multiple arcs that coalesced and accreted onto the craton during the Neoarchean-Paleoproterozoic transition. These blocks, which are dominated by charnockites and ranging in age from Mesoarchean to late Neoarchean, and their equivalents in other cratonic fragments over the globe, can be correlated to the ‘expanded Ur’, in building the oldest supercontinent on Earth.