SPINEL (HERCYNITE) ADCUMULATE FROM THE CHIMAKURTI GABBRO-ANORTHOSITE PLUTON, PRAKASAM DISTRICT, ANDHRA-PRADESH, INDIA - EVIDENCE FOR PLAGIOCLASE BUOYANCY AND MAGMA MIXING

A spinel (hercynite) adcumulate cuts across the leuco-gabbro unit of t he Chimakurti gabbro-anorthosite pluton. The rock is composed of cumul us hercynite (approximate to 95% by volume) and intercumulus magnetite (approximate to 5%) and it displays impeccable framework of touching mineral grains. Magnetite also occurs as exsolved linear trails and mi nute dust within hercynite. Spinel shows invariably homogeneous chemis try without any intragranular compositional zonation or intergranular variation. Al2O3 and FeO are the two major oxides but appreciable amou nts of MgO and Fe2O3 are also recorded. Chromium is significantly low in these spinels. Both intercumulus and exsolved magnetites show exact ly the same composition. It is proposed that the mixing of primitive b asaltic liquid with an earlier evolved plagioclase-supersaturated resi dual liquid has brought the composition of hybrid liquid into the spin el stabilization field. Consequent to its crystallization, extreme sor ting, separation and accumulation has produced the spinel (hercynite) adcumulate.