Geochemistry and petrogenesis of amphibolites, Kolar Schist Belt, South India: evidence for komatiitic magma derived by low percentages of melting of the mantle

Abstract

The Kolar Schist Belt of the Dharwar Craton of South India is an Archean greenstone belt dominated by metavolcanic rocks. The mafic metavolcanic rocks occur as komatiitic and tholeiitic amphibolites. The komatiitic amphibolites occur along the margins of the N-S trending, synformal belt. They are much less abundant than the tholeiitic amphibolites and have 14 to 21-3 wt. per cent MgO. The komatiitic amphibolites from the west/central part of the belt have two distinctive REE patterns: (1) those enriched in the middle to light REE but depleted in Ce relative to Nd; and (2) those with patterns that are convex up, i.e. depleted in both light and heavy REE, although more depleted in the light REE. Associated tholeiites have light REE depleted to flat REE patterns. Komatiitic and tholeiitic amphibolites from the eastern part of the belt have enriched light REE patterns. The tholeiitic amphibolites from the Kolar Schist Belt are similar to the TH I and TH II types of Archean tholeiites of Condie (1981). The komatiitic amphibolites are similar to komatiites and komatiitic basalts of Barberton Mountainland, but have higher FeO and TiO₂ abundances and lower Yb/Gd ratios. The petrogenetic interpretations for these rocks are based primarily on a modification of the MgO-FeO diagram of Hanson & Langmuir (1978), and modelling of Zr, Ni and REE. All of the rocks have undergone some fractionation. While the modelling does not give accurate temperatures, pressures, compositions and extents of melting of the mantle sources for the various amphibolites, it does present an approach which can be used for estimating these parameters. For example, the komatiitic amphibolites appear to be derived from melts generated by 10 to 25 per cent melting of the mantle over a range of depths and temperatures greater than 80 km and 1575°C. The variation in the P-T conditions of magma generation is possibly due to adiabatic melting in mantle diapirs with a range of FeO/MgO ratios. If the tholeiitic amphibolites are derived from similar mantle sources (it is not clear that they are), their parent melts may have been generated by similar extents of melting, but at depths of less than 80 km. The komatiitic amphibolites from the west central part of the belt were generated from light REE depleted mantle, whereas those from the eastern part of the belt appear to have been generated from light REE enriched mantle. The sources for the komatiitic amphibolites in both areas were significantly enriched in FeO relative to pyrolite. Thus, a light REE depleted and a light REE enriched source appear to have provided mafic volcanics with similar major element chemistry to this belt during its evolution.