Effect of source heterogeneity, melt extraction and crystal separation on the composition of a suite of ferroan (A-type) granites from parts of the Chotanagpur Granite Gneissic Complex (CGGC), India

Abstract

Ferroan granitoids constitute a considerable proportion of the upper continental crust formed during the Proterozoic eon and their origin is usually linked to high-temperature processes (fractionation or anatexis) occurring at shallow crustal depths. However, the relative role of mantle melts vs. crustal rocks as the source of these granitoids has been largely debated. In the Chotanagpur Granite Gneissic Complex (CGGC), located in the East Indian shield, a suite of Mesoproterozoic (ca. 1450–1350 Ma) ferroan granites has intruded the Paleoproterozoic (ca. 1700–1630 Ma) arc granites and currently occurs either as migmatitic gneisses or as biotite gneisses in different parts of the terrane . Previous studies postulated that these Mesoproterozoic extensional granites were sourced from older crustal rocks with limited mantle input. In the current study, the geochemistry of the biotite gneisses is combined with that of the migmatitic gneisses and coeval ferroan granites from northern and western parts of the CGGC to comprehend the wide spatial and compositional variation observed. With the help of phase equilibrium modelling, the current study quantifies the role of the heterogeneity of the crustal source and path-dependent melting in shaping the composition of the ferroan granites. The study demonstrates that the composition of the modelled melt derived from the older (late Paleoproterozoic) granites under low-pressure high-temperature conditions (750-1000 °C; 4–8 kbar) closely resembles the compositions observed in the younger (Mesoproterozoic) ferroan granites. Compositional variation of the melt is further accentuated by high-temperature progressive melting and extraction of melt in small batches during anatexis . Phase equilibrium studies further delineate the significant role of crystal fractionation in shaping the chemical variations observed in the studied granites. • Mesoproterozoic ferroan granites of CGGC formed via anatexis of older arc granites. • Source heterogeneity produces variable composition of the granites. • Melt extraction in small batches produces chemical variation in ferroan granites. • Post-emplacement crystal separation accentuates the compositional variations.