2.0 GA OLD PYROXENITE-CARBONATITE COMPLEX OF HOGENAKAL, TAMIL-NADU, SOUTH-INDIA

The Hogenakal carbonatites form a series of discontinuous bodies withi n two pyroxenite dykes. Each body forms veins and lenses emplaced in p yroxenite. 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) carbon atite, and carbonate mica (CM) pyroxenite. Two whole-rock mineral Rb-S r isochrons for MPAC carbonatites yield ages of 1.984 +/- 0.078 Ga and 1.994 +/- 0.076 Ga, respectively, which makes this carbonatite not on ly 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 abundan ces and La/Yb ratios vary in the order pyroxenite < CM pyroxenite < MP AC carbonatite < MAC carbonatite; all rocks show LREE-enriched pattern s with steep slopes. It is suggested that the pyroxenites represent in trusions of crystal mush formed by separation of pyroxenes from an ijo litic magma. Increase in the concentration of CO2 in such a magma led to the separation of a carbonatitic melt which subsequently intruded t he pyroxenite. Mixing of this carbonatite melt with fragmented pyroxen ite 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 carbo natite. K-metasomatism is restricted essentially to phlogopitization o f pyroxenes and K-feldspar. The association of albite/oligodase-rich f enites with the carbonatite complex is indicative of Na-K metasomatism . The near equality of the initial Sr ratios of the three samples anal ysed, together with their high Sr contents, argue against any signific ant crustal contamination. The initial ratio of 0.70169 must therefore represent the Sr composition of the mantle source of the ijolitic mag ma. This ratio corresponds to an epsilon(Sr) of -6.3 +/- 0.6, which im plies that the subcontinental mantle which gave rise to the Hogenakal pyroxenite/carbonatite was depleted in incompatible elements even prio r to 2 Ga ago, presumably due to an earlier event of crustal formation .