PETROLOGICAL AND STABLE-ISOTOPIC EVIDENCE FOR FLUID-ROCK INTERACTION DURING FORMATION OF BONINITE-LIKE ULTRAMAFIC CUMULATES,PANGIDI ANORTHOSITE COMPLEX,EASTERNGHATS GRANULITE BELT,INDIA

The Pangidi anorthosite complex (PAC) of Mesoproterozoic age (1739±220 Ma) is a small magmatically layered body emplaced into high-grade supracrustal rocks in the southern sector of the Eastern Ghats Granulite Belt (EGGB), south India, synchronous with a regional pervasive D2-thermotectonic event. It is dominated by coarse-grained anorthosite and leuconorite, minor leucogabbronorite and post D2 boninite-like ultramafic cumulates containing chromitites. The ultramafic cumulates are orthopyroxenites, websterites and amphibole bearing harzburgites made of orthopyroxene (Mg#=0.73-0.82), olivine (Mg#=0.53-67), potassian hornblende (Mg#=0.60-0.70), plagioclase (An84-90); and chrome rich spinels (Cr no=~80), which is the hallmark of primitive boninites. The halogen bearing minerals, hornblende and apatite in harzburgites have high Cl contents than those in orthopyroxenites. The ultramafic cumulates in the PAC, have the diagnostic geochemical signatures of boninites: high MgO Cr, Ni, and Co contents, intermediate SiO2contents, high Mg#’s (MgO/(MgO FeO*), Al2O3/TiO2, and Zr (Hf)/, middle rare earth element (REE) ratios; low TiO2, enriched LREE, and high-field-strength element contents; and U-shaped primitive mantle–normalized trace element patterns. The parent melt has trace element patterns similar to arc magmas. The crystallization phases and their compositions indicate that the cumulates formed from a H2O rich basaltic rock which intruded at lower pressure (~2 Kbar) and temperatures of 1100ºC.The boninitic nature of the PAC magma, together with the field and petrographic evidence suggest interaction with magmatic fluids. Interaction of these fluids with the calcsilicate host rocks lowered their d18 values. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the PAC are part of a Proterozoic continental magmatic system.