Geochemistry of the Archean low- to high-grade transition zone, Southern India

Nature and origin of lower crustal rocks of Dharmapuri area, Tamil Nadu, southern India-a goechemical approach.

“…Various field, petrographic and geochemical evidence, such as: (i) the presence of granulite texture in the enderbites; (ii) the absolute freshness of the constituent minerals; (iii) inci pient charnockitization of tonalitic gneisses of the transition zone and appearance of greasy lustre in southern parts of the transition zone as well as in the granulite zone; (iv) formation of hypersthene from biotite and hornblende; (v) gradational increase of hypersthene and P-T con ditions to the south; and (vi) the similarity in the compositional range of the tonalitic gneisses and enderbites in both the zones, suggest that the (Condie et al, 1982;Janardhan et al, 1982;Allen et al, 1983;Rameshwar Rao, 1985). The similarity of immobile element concen trations and differences of LIL element abun dances (Fig.…”

“…It is relevant to briefly review the models that have been proposed for the derivation of the or thogneisses of transition and granulite zones: (a) Intracrustal partial melting (Fyfe, 1973;Pride and Muecke, 1980); (b) Primary fractionation of andesite-dacite magma (Holland and Lambert, 1975;Field et al, 1980); (c) Partial melting of a mafic source rich in hornblende and/or garnet (Weaver and Tarney, 1980;Condie et al, 1982); and (d) Fractional crystallization of wet basaltic magma or diorite magma (Barker and Arth, 1976;Arth et al, 1978;Jahn et al, 1988). In the light of the data presented and discussion of the above models an attempt is made to arrive at a reasonable model.…”

“…Model (d), the fractional crystallization of a mafic magma is less preferred, due to lack of both significant volumes of intermediate igneous rocks in Archean terrains and of any recognizable cumulate material (Condie et al, 1982;Weaver and Tarney, 1984). It is however, difficult to distinguish between magmatic differentiation of mafic protoliths by partial melting or fractional crystallization of basic source.…”

“…A discussion in length of this model for the transition zone rocks of Dharmapuri area is given by Condie et al (1982). However, the par tial melting model of Hanson (1980), to produce tonalitic liquids is also attempted in this study.…”

“…lb). preceeded the granulite-facies metamorphic event, and that the enderbite formed by metamorphic conversion of gneisses and migmatites (Condie et al, 1982;Allen et al, 1983;Rameshwar Rao, 1985, 1987Rameshwar Rao et al, 1990). A continuous metamorphic gradation from amphibolite facies in the north to granulite facies in the south is observed (Fig.…”