The Gothara plagiogranite: evidence for oceanic magmatism in a non-ophiolitic association, North Khetri Copper Belt, Rajasthan, India?

The Gothara granitoid, a small pluton in the vicinity of Khetri town, is one of several granitoid bodies that intrude the Palaeo-Mesoproterozoic Delhi Supergroup of the Khetri Copper Belt (KCB) in north Rajasthan, India. Detailed petrological and geochemical investigations characterise the Gothara granitoid as a plagiogranite. It exhibits profuse granophyric intergrowth between quartz and chessboard albite, besides having the low Al2O3, extremely low K2O, Rb, Ba, and high Na2O that are characteristic of plagiogranites, and matches some typical oceanic plagiogranites of the world in totality. However, unlike the other known plagiogranites, it is mineralogically and chemically exceptionally homogeneous, almost free from alteration and occurs in a non-ophiolitic setting. The field relationships of the Gothara plagiogranite, particularly its dual and overlapping relationship with the mafic magmatic rocks, its accessory mineral assemblage, I-type characteristics and the major, trace and REE geochemistry, all point towards its fractionation-related petrogenesis from a mafic magma of mantle origin in an oceanic ridge tectonic regime. This discovery is of importance not only for the petrotectonic evolution of the Khetri Copper Belt but also for the petrogenesis of plagiogranites elsewhere in the world.