Fe-rich Archean tholeiites derived from melt-enriched mantle sources: evidence from the Kolar Schist Belt, South India

Abstract

Tholeiitic amphibolites in the circa 2700 Ma Kolar Schist Belt, south India, are intimately interlayered with minor komatiitic amphibolites. The tholeiitic amphibolites, which are similar to other Archean tholeiitic rocks, are Fe-enriched, with FeO_total varying from about 9 to 13 wt %. Olivine and plagioclase have similar calculated saturation surface temperatures ranging from about 1125 to 1225°C. On many trace and major element plots there is a distinct compositional gap between the komatiitic and tholeiitic amphibolites that cannot be explained by density filtering because the calculated melt densities for the komatiitic (2.73-2.79) and tholeiitic (2.61-2.72) amphibolites nearly overlap. The rare earth element (REE) patterns for the tholeiites are flatter and have lower light REE abundances than those for many of the associated komatiitic rocks. Modeling of major and trace element data suggests that the tholeiitic rocks are not related to the komatiitic rocks by: (1) differentiation of magmas of the komatiitic rocks, (2) melting of the komatiitic mantle source at the same depths at which the komatiites were derived, (3) differentiation of liquids produced by melting the komatiite source at shallow levels, or (4) shallow-level fractional crystallization (open or closed-system) of a pyrolite-derived parental melt common to both komatiite and tholeiite. The tholeiitic rocks appear to have resulted from sources melting at about 35 km depth with variable Fe/Mg ratios much greater than that of pyrolite. The most likely candidate for their source is an Archean litho-sphere previously intruded by variable amounts of mantle-derived melts.