Chromites from meta-anorthosites, Sittampundi layered igneous complex,Tamil Nadu, southern India

摘要

Chrome-spinels of anorthosite-hosted chromitite bodies from Sittampundi layered igneous complex, southern India, metamorphosed under eclogite-facies condition have been studied. Besides anorthosites as the dominant rock type discontinuous layers of peridotite, pyroxenite and lenses of gabbroic granulite/ eclogite occur at the bottom of the complex. Chromite grains are deformed exhibiting features of intra-crystalline deformation and exsolved needle shaped rutile inclusions. Chrome-spinels are of refractory grade with Cr_2O_3 and A1_2O_3 contents varying between 34-40 wt.% and 23-28 wt.% respectively. In the bivariate Cr# (=Cr/[Cr + Al]) vs. Mg# (=Mg/[Mg+Fe~(2+)]) diagram, Sittampundi chromitites show close affinity with the Archean Fiskenaesset type deposit. The calculated Al_2O_3 values for the parental melt of Sittampundi chromitites are consistent with the A1_2O_3 content of mid-ocean ridge basalts whereas the values for FeO/MgO ratio are higher. It is suggested that an initial basic magma, similar to that of mafic granulite composition evolved in the magma chamber through fractionation of peridotite and pyroxenite to Fe,Al-rich basaltic melt, parental to the chromitite bands.