Chromian spinel compositions from Madawara ultramafics, Bundelkhand Craton: Implications on petrogenesis and tectonic evolution of the southern part of Bundelkhand Craton, Central India

摘要

Madawara ultramafic complex (MUC) in the southern part of Bundelkhand Craton, Central India comprises peridotite, olivine pyroxenite, pyroxenite, gabbro, and diorite. Coarse-grained olivine, clinopyroxene (Cpx), amphibole (Amp), Al-chromite, Fe-chromite, and magnetite with rare orthopyroxene (Opx) are common minerals in peridotite. Chromites are usually coarse-grained euhedral found as disseminated crystals in the olivine matrix showing both homogeneous and zoned texture. Al-chromite, primarily characterizes Cr-spinels and its subsequent fluid activity and alteration can result in the formation of Fe-chromite, chrome magnetite, and magnetite. Mineral chemistry data suggest that Al-chromite is characterized by moderately high Cr2O3 (38.16-51.52 wt.%) and Fe2O3 (3.22-14.51 wt.%) and low Al2O3 (10.63-21.87 wt.%), MgO (1.71-4.92 wt.%), and TiO2 (0.22-0.67 wt.%), whereas the homogeneous Fe-chromite type is characterized by high Fe2O3 (25.54-47.60 wt.%), moderately low Cr2O3 (19.56-37.90 wt.%), and very low Al2O3 (0.06-1.53 wt.%). Subsequent alteration of Al-chromite and Fe-chromite leads to formation of Cr-magnetite and magnetite. The Cr# of Al-chromite varies from 55.12 to 76.48 and Fe-gamma(3+)# from 8 to 19, whereas the ferrian chromite has high Cr# varying from 94.27 to 99.53 while its Fe-gamma(3+)# varies from 38 to 70. As a whole, the primary Al-chromite shows low Al2O3, TiO2 contents, and high Fe#, Cr# values. Olivines have forsterite ranging from 75.96% to 77.59%. The bulk-rock geochemistry shows continental arc geochemical affinities indicated by the high concentration of large-ion lithophile elements and U, Th relative to the low concentration of high-field strength elements. These petrological and mineralogical as well as primary Al-chromite compositions plotted in different discrimination diagrams suggest an arc environment that is similar to Alaskan-type intrusion.