ON THE GENESIS OF MULTILAYER CORONAE BETWEEN OLIVINE AND PLAGIOCLASE IN GABBRO DYKE FROM ELACHIPALAYAM, TAMIL NADU, SOUTH INDIA

An olivine gabbro dyke near Elachipalayam, Tamil Nadu shows multilayer coronae that separate cumulus olivine and plagioclase. The corona consists of two layers, an anhydrous layer of orthopyroxene±magnetite (OM) near the olivine and a layer of green amphibole near the plagioclase. Orthopyroxene in OM corona consists of acicular grains which mimics the shape of the precursor olivine, and grows perpendicular to it. Magnetite concentrates as a thin layer within the orthopyroxene corona and simulates the shape of the olivine. In places the olivine is completely pseudomorphed by OM symplectite. Computation of possible net reactions in NCFMASH system suggests that the minerals in corona cannot form in a 'close system'. P-T pseudosection in the same system indicates that the orthopyroxene layer was formed at a temperature >700˚C and ~7 kbar pressure. Green amphibole within the corona structure encroaches both orthopyroxene and plagioclase and contains remnant of orthopyroxene within it. This signifies replacement of olivine by OM was followed by amphibole growth at the expense of orthopyroxene and plagioclase. Besides the green amphibole, a brown amphibole is found at the interface of magmatic oxide (with ilmenite and magnetite exsolution) and plagioclase. Brown amphibole has distinctly higher TiO₂compared to the green one although both are pargasitic in composition. The compositional difference in amphiboles suggests that the Ti rich oxide near the brown amphibole was the source of the component in it. Association and restricted occurrence of the product minerals along with their composition, signals for a diffusion controlled solid-state replacement of the reactants in an open system with aqueous fluid. The green amphibole forming reaction in the corona was governed by the difference in chemical potential at the contact of reactant orthopyroxene and plagioclase. In this context, the thermodynamic incompatibility of the reactant minerals was modelled in quantitative µMgO–µCaO phase diagram. The result shows that amphibole formation started at a temperature <650˚C. The estimated P-T condition for multilayer coronae development tallies well with the metamorphic condition reported from the adjoining rocks.