Petrogenesis of magmatic charnockite-biotite granite suite from parts of the Chotanagpur Granite Gneissic Complex (CGGC), eastern Indian shield: Implication for the break down of the Columbia Supercontinent

A rare porphyritic charnockite that is girdled by and mineralogically grades to biotite granite occurs as a part of the Chotanagpur Granite Gneissic Complex (CGGC) in and around Massanjore, Jharkhand, India. Preservation of certain textural features, including (a) euhedral to subhedral grains of orthopyroxene, (b) low dihedral angle subtended by orthopyroxene and plagioclase grains, and (c) relict intergranular and porphyritic textures, are consistent with the view that orthopyroxene in the studied rocks has a magmatic origin. Preserved magmatic features and other petrological attributes of these rocks do not support any significant mass change beyond a few tens of microns during the overprinting high-grade metamorphism. The geochemical variation of the felsic rock suite (porphyritic charnockite and biotite granite) indicates that they are cogenetic and are derived from a ferroan A-type granitoid magma by crystal fractionation. The observed geochemical trend of the studied felsic rock suites has been simulated by phase equilibria modelling in the open system using the system components Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O₂. The observed mineralogical and geochemical attributes and the results of the modelling study are consistent with a petrogenetic process in which high magma temperature (> 900 °C), low pressure, and low water activity in the parental melt favoured the separation of an orthopyroxene bearing cumulate assemblage (orthopyroxene + plagioclase + quartz + K-feldspar + ilmenite + magnetite) in the initial part of the magmatic differentiation. Removal of this anhydrous cumulitic assemblage raised the bulk H₂O content in the residual melt. Orthopyroxene became unstable with respect to biotite in the evolved melt that eventually crystallised minerals that formed the biotite granite. An increase in magma fO₂ also restricts the orthopyroxene stability in felsic magma. Taken together all the petrological and geochemical attributes, we demonstrate that fractionation of an orthopyroxene-bearing crystal cumulate from the melt is essential to form the charnockites, and that the biotite granite forms from the evolved melt after the fractionation. The charnockite-biotite granite association of the studied area was formed in an extensional tectonic setting, presumably during the breakdown of the Columbia Supercontinent.