Multi-element geochemical modelling of crust–mantle interactions during late-Archaean crustal growth: the Closepet granite (South India)

Abstract

The Closepet granite, in the Dharwar craton of south India, is a large, late Archaean magmatic body. Its composition can be explained as a result of interactions between mantle-derived melts and pre-existing continental crust (TTG gneisses). Using geochemical modelling based upon major and trace element compositions the following petrogenetic model is proposed for the formation of the batholith: (i) an enriched mantle (garnet- and amphibole-bearing lherzolite) melts to produce a basaltic liquid. (ii) The basaltic liquid undergoes limited fractional crystallization of biotite and amphibole (F>0.9). (iii) The differentiated liquid rises into the old continental crust, and induces water-saturated anatexis of the TTG gneisses (K-Feldspar+Qtz+Plagioclase→melt). (iv) Both mantle-derived and anatectic liquid mix and give rise to magma compositions ranging from quartz-monzonite to granite. The origin of the mantle enrichment is discussed. In the context of the regional geological setting the most likely possibility seems to be metasomatism by slab melts the metasomatism of a mantle wedge by slab melts. This suggests a two-stage evolution for the Dharwar craton during the late Archaean: (i) a subduction related event with the formation of TTGs, sanukitoids, and with associated mantle metasomatism; and (ii) re-melting of the metasomatized mantle. This evolutionary history implies that at least some of the K-rich, late-Archaean granites are juvenile, rather than products of intracrustal reworking, as frequently assumed.