Petrological and geochemical characterisation of a porphyritic felsic rock from the north-eastern part of CGGC, India

Abstract

The porphyritic felsic rock, exposed near Massanjore in Dumka district, Jharkhand, has been studied with an aim to decipher the geochemical characteristics of the pristine magma. The rock is deformed and metamorphosed and occasionally holds enclaves of mafic granulites and migmatitic felsic gneiss. At places, deformed mafic dykes (now amphibolite) also intruded the porphyritic felsic gneiss. The Porphyritic Felsic Gneiss, has been characterised in the field as a Felsic Augen Gneiss from outcrop view. It majorly contains Alkali Feldspar (Kfs), Plagioclase (Pl), Quartz (Qtz), Orthopyroxene (Opx), Clinopyroxene (Cpx), Garnet (Grt) and Amphibole (Amp) as the major constituents, with presence of some other accessory phases like Zircon and Apatite. Under the microscope, the most notable reaction texture that is observed is between Cpx and Pl, leading to the formation of Grt and Qtz. Upon geochemical analysis, it was determined that the rock belonged to Granodiorite composition, highly ferroan, mostly calc alkaline to slightly calcic and majorly metaluminous to slightly peraluminous in character. The Primitive mantle normalised trace element spider diagram showed a monotonously decreasing abundance pattern ,from the most incompatible side to less incompatible side, with depletion of Cs, Ti and Sr and moderate enrichment of Rb, Ba and Ta. The Chondrite normalised REE plot shows a slightly negative slope, with flat HREE trends and moderately enriched LREE trends, with a moderately negative Eu-anomaly. The melt was more Na2O rich than K2O, which might indicate to the presence of Amp at the source, during melting, but high Ga/Al ratio, high Rb and Ba contents indicate Biotite melting at the source. The high HFSE contents, with high TaN and NbN data indicates a clear involvement of mantle input with the major continental crust, during the formation of the Porphyritic granitoid. From the plots of Whalen et al(1987) , the rock is classified as an A type granitoid , which was further classified as an A2 type granitoid by the plots of Eby et al(1990,1992), which characterises it to have formed in a post orogenic setup.In the plots of Pearce et al (1984), the tectonic affinity of the granitoid was classified as a “WPG” or within plate granite. The temperature of the magma was calculated from Zircon saturation, by the formulae of Watson et al(1983) and Boehnke et al(2007) and was found to be 820±26˚C (Watson et al) and 773±29˚C (Boehnke et al) respectively