MINERALOGIC AND ISOTOPIC STUDIES OF CHROMITE-BEARING ROCKS OF THE SUKINDA ULTRAMAFIC COMPLEX, ORISSA, INDIA

The Sukinda ultramafic complex occurs within an Archean greenstone belt located within the 2.95-3.2 Ga Iron Ore Supergroup of rocks in the southern part of the Singhbhum cratonic block,peninsular Shield,India.  The complex is nearly 25 km long with an east-northeast trend.  Principal rock types are serpentinized peridotite, orthopyroxinite and chromitite.  The high-grade chromitite bodies are present throughout the complex and are the major source of chromite ore in India. We have studied the composition of Cr-spinel in the ultramafic rocks to better constrain their petrogenetic origin.  The values of Cr/(Cr+Al) and Mg/(Fe²⁺+Mg)range between 0.76 and 0.81 and between 0.66 and 0.72,respectively, in the chromitites.  These values are refractory in character, and suggest that the chromitite has not changed compositions via later subsolidus processes.  Accessory Cr-spinel in the serpentinized peridotitic rocks has lower Mg/(Fe²⁺+Mg) values (between 0.33 and 0.41), strongly suggesting that the chromitites have experienced subsolidus and metamorphic reequilibration.  We have also analyzed orthopyroxene (En=89 to 90) and Cr-spinel (Cr/(Cr+Al)=0.79)in a separate orthopyroxenite body; these results are also consistent with a refractory character.  Cr-spinel in the serpentinized peridotitic rocks shows iron enrichment along grain boundaries and fractures, with occasional rims of magnetite.  In a bivariant plot of Cr/(Cr+Al) versus Fe²⁺/(Fe²⁺+Mg), the composition of the Cr-spinels suggests a boninitic affinity for the parental melts. Preliminary O and H isotopic analyses show d¹⁸O values between 2.6 and 2.9 per mil (VSMOW) and dD values between -51 and -95 per mil (VSMOW). The isotopic data are consistent with serpentinization being the result of interaction between the ultramafic rocks and seawater.  This interpretation,in conjuntion with the boninitic nature of the parental melt,suggests a possible origin of the rocks as a part of a supra-subduction zone ophiolite complex.