Petrogenesis and in situ U-Pb zircon dates of a suite of granitoid in the northern part of the Central Indian tectonic Zone: Implications for prolonged arc magmatism during the formation of the Columbia supercontinent

Abstract

The timing and petrogenesis of magmatic rocks in the Central Indian Tectonic zone (CITZ) have paramount importance in understanding the (a) growth of the Indian shield and (b) formation and disintegration of the Columbia and Rodinia supercontinents. In this study, the petrology, geochemistry and U-Pb in-situ zircon dating of a suite of porphyritic granitoids from the Makrohar Granulite Belt (MKGB) of the CITZ are presented. The studied rock suite develops megacrystic K-feldspar and plagioclase phenocrysts, embedded in medium to fine-grained matrix of biotite ± amphibole + quartz + K-feldspar + plagioclase + ilmenite ± garnet with widely variable phenocryst to matrix ratio. The bulk compositions of the studied rocks overlap with the compositions of the I-type granitoid, emplaced in a continental arc setting. The in-situ U-Pb zircon age fixes the timing of magmatism at 1750 Ma. Systematic geochemical trends combined with the variation in modal mineralogy suggest crystal fractionation to be the dominant process during the evolution of this arc magma. Phase equilibrium modeling in Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 system under various physicochemical conditions suggests that both post-emplacement fractionation of a felsic magma and combined fractionation-crustal assimilation of a mafic magma at shallow depths (≤5 kbar) can produce the chemical variation observed in the studied arc granites. Petrography and field evidences negate substantial granite formation from combined fractionation and crustal assimilation of arc basalt in the studied area. Plagioclase and K-feldspar occur as early crystallising phases. Variation of the modeled melt compositions simulates the variations of the major element compositions in the studied granitoid suite. The ca. 1750 Ma old arc magmatism in the MKGB is also reported from the Mahakoshal Supracrustal Belt (MSB). This feature is consistent with the view that both MKGB and MSB, now separated by the Son Narmada South Fault formed a coherent block till 1750 Ma. In combination with the published information, the present study demonstrates southward progression of the arc front in the Indian shield from ca. 1900 Ma to ca. 1600 Ma during growth of the Columbia supercontinent.