Geochemistry and geochronology of Granitoids around Northern part of the Chitradurga greenstone belt, in the Gadag area, Dharwar craton, South India: significance to late Archean crustal evolution

Abstract

The evolution of planetary crusts is governed largely by continuous as well as episodic processes. Earth?s first crust is believed to be of mafic in composition formed by the cooling of a magma ocean soon after its accretion. The early continental crust made up of granitoid rocks are formed by repeated melting of the mafic crust ca. 4.3 Ga ago as evidenced by U-Pb isotope dating of detrital zircons from the 3 Ga old Mt Narryer quartzites in Western Australia and early depletion of light rare earth elements in the mantle sources as evidenced from Sm-Nd isotope studies and Archean komatiites. Various mechanisms have been suggested for the growth of continental crust. In the simplest terms, the continental crust grew by addition of magmas (1) directly from the underlying mantle (crustal over- and underplating involving the extrusion of volcanic rocks and intrusion of dykes and sills and also similar forms from mantle plumes), and (2) by tectonic accretion of thicker portions of oceanic crust, such as island arcs or oceanic plateaus. Models of crust formation and continental growth can be divided into two major groups, (1) models of early crustal growth and subsequent recycling and (2) models of continuous and episodic crustal growth. Although not well understood, an episodic distribution of crustal isotopic ages has been well documented. Major peaks in continental crustal formation ages are recognized at about 2.7, 1.9 and 1 Ga. Based on Nd model ages the ?average? age of the continental crust was calculated as and#8776; 2.2 ±0.1 Ga, which indicates that 50 ? 60 % of the continental crust was formed before 2.5 Ga. Hence, studies of the Archean granitoid rocks that make up and#8776; 70% of the continental crust are very important to understand the growth of the continents. The Archean granitoid rocks are spatially and temporally associated with greenstone belts in various cratons and are referred to as granite-greenstone terranes.