2.0 Ga old pyroxenite-carbonatite complex of Hogenakal, Tamil Nadu, South India

Abstract

The Hogenakal carbonatites form a series of discontinuous bodies within two pyroxenite dykes. Each body forms veins and lenses emplaced in pyroxenite. Based on field relationships, the sequence of formation is inferred to be pyroxenite first, syenite later, and carbonatites last. The carbonatitic rocks are classified into three types: mica-apatite-calcite (MAC) carbonatite, mica-pyroxene-apatite-calcite (MPAC) carbonatite, and carbonate mica (CM) pyroxenite. Two whole-rock mineral Rb-Sr isochrons for MPAC carbonatites yield ages of 1.984±0.078 Ga and 1.994±0.076 Ga, respectively, which makes this carbonatite not only the oldest yet known in the Indian subcontinent but also one of the few oldest carbonatites known in the world. The carbonatites are poor in alkalies and rich in Sr and to a certain extent in Ba. REE abundances and La/Yb ratios vary in the order pyroxenite < CM pyroxenite < MPAC carbonatite < MAC carbonatite; all rocks show LREE-enriched patterns with steep slopes. It is suggested that the pyroxenites represent intrusions of crystal mush formed by separation of pyroxenes from an ijolitic magma. Increase in the concentration of CO₂ in such a magma led to the separation of a carbonatitic melt which subsequently intruded the pyroxenite. Mixing of this carbonatite melt with fragmented pyroxenite led to the formation of the MPAC carbonatite and the CM pyroxenite in which pyroxenite dominates over carbonatite. In the closing stages a second pulse of carbonatite was intruded which formed the MAC carbonatite. K-metasomatism is restricted essentially to phlogopitization of pyroxenes and K-feldspar. The association of albite/oligoclase-rich fenites with the carbonatite complex is indicative of Na-K metasomatism. The near equality of the initial Sr ratios of the three samples analysed, together with their high Sr contents, argue against any significant crustal contamination. The initial ratio of 0.70169 must therefore represent the Sr composition of the mantle source of the ijolitic magma. This ratio corresponds to an ε_Sr of -6.3 ± 0.6, which implies that the subcontinental mantle which gave rise to the Hogenakal pyroxenite/carbonatite was depleted in incompatible elements even prior to 2 Ga ago, presumably due to an earlier event of crustal formation.