A spinel (hercynite) adcumulate cuts across the leucogabbro unit of the Chimakurti gabbro-anorthosite pluton. The rock is composed of cumulus hercynite (≈95% by volume) and intercumulus magnetite (≈5%) and it displays impeccable framework of touching mineral grains. Magnetite also occurs as exsolved linear trails and minute dust within hercynite. Spinel shows invariably homogeneous chemistry without any intragranular compositional zonation or intergranular variation. Al2O3 and FeO are the two major oxides but appreciable amounts of MgO and Fe2O3 are also recorded. Chromium is significantly low in these spinels. Both intercumulus and exsolved magnetites show exactly the same composition. It is proposed that the mixing of primitive basaltic liquid with an earlier evolved plagioclase-supersaturated residual liquid has brought the composition of hybrid liquid into the spinel stabilization field. Consequent to its crystallization, extreme sorting, separation and accumulation has produced the spinel (hercynite) adcumulate.
