Petrology of spinel granulites from Araku, Eastern Ghats, India, and a petrogenetic grid for sapphirine‐free rocks in the system FMAS

Abstract

Abstract Spinel‐quartz‐cordierite and spinel‐quartz are found as relic prograde assemblages in Fe‐rich granulites from the Araku area, Eastern Ghats belt, India. Subsequent reactions produced orthopyroxene + sillimanite in the former association and garnet + sillimanite in the latter. The first reaction is univariant in the FMAS system, but is trivariant in the present case because of the presence of Zn and Fe3+ in spinel. The second reaction also has high variance because of Zn and Fe3+, but also because of the presence of Ca in garnet. Thermobarometry shows that the metamorphic conditions were approximately 950° C and 8.5 kbar and the fo2 was near the NNO buffer. In Fe‐rich bulk compositions and low‐P‐high‐T conditions of metamorphism, two of the univariant reactions around the invariant point [Sa], namely (Sa, Hy) and (Sa, Cd), change topology due to reverse partitioning of Fe‐Mg between coexisting garnet and spinel. An alternative partial petrogenetic grid in the system FMAS is constructed for such conditions and is applied satisfactorily to several sapphirine‐free spinel granulites. It is shown that bulk composition (XFe and Zn) exerts greater control on the stability of spinel + quartz than fo2. The effect of the presence of Zn and Fe3+ in spinel on the proposed grid is evaluated. Reaction textures in the Araku spinel granulites can be explained from the petrogenetic grid as due to near‐isobaric cooling.