Chromian spinel compositions from Madawara ultramafics, Bundelkhand Craton: Implications on petrogenesis and tectonic evolution of the southern part of Bundelkhand Craton, Central India
Mohanty, N; Singh, SP; Satyanarayanan, M; Jayananda, M; Korakoppa, MM; Hiloidari, S; Chalapathi Rao, NV; ,
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.