U-Pb ages for zircon and titanite from the Ramagiri area, Southern India: evidence for accretionary origin of the Eastern Dharwar Craton during the late Archean

Abstract

The north-south-trending Ramagiri Schist Belt is trident shaped, with three prongs separating three granitic terranes. Whereas the western and eastern prongs contain metatholeiites and banded ferruginous quartzite, the central prong includes felsic metavolcanics and volcanogenic metasediments. The U-Pb zircon age for the pyroclastics in the central prong of the Ramagiri Schist Belt is 2707±18 Ma, considered as the time of emplacement of the felsic volcanics. To the east of the belt, migmatitic Chenna gneisses yielded a U-Pb zircon age of 2650±7 Ma, inferred as the minimum age of their magmatic precursors. These gneisses had experienced a thermal event at least 100 m.yr. later, as evidenced from the U-Pb titanite age of 2545±1Ma. From the central Ramagiri Complex, a granodiorite has identical U-Pb zircon and titanite ages of 2613±6 and 2614±4 Ma, respectively, and a quartz diorite has a titanite age of 2595±1Ma. These intrusives with roof-pendants of pillowed basalts are inferred to have been emplaced as high-level plutons that cooled rapidly. From the western Gangam Complex, U-Pb zircon and titanite ages for a granodiorite were 2528±1 and 2516±3 Ma, respectively, and a monzodiorite has yielded a titanite age of 2510±2 Ma. A postkinematic granitic vein intrusive into the Gangam Complex yielded a titanite age of 2468±4 Ma. The three granitoid terranes separated by a <3-km-wide schist belt do not have common magmatic or metamorphic histories, as evidenced by their distinct titanite and zircon ages. Intrusion of voluminous western Gangam Complex at 2528 Ma did not alter the titanite ages of granitoids in either the central Ramagiri Complex or the eastern Chenna Gneiss just a few kilometers away. Therefore, the Ramagiri Schist Belt, consisting of highly sheared rocks, could represent a terrane boundary between three disparate granitoid terranes juxtaposed after 2516 and before 2468 Ma. Available geological and geochronological information from Kolar, Ramagiri, and adjoining areas suggests that growth of Dharwar continental crust could have been accomplished by both accretion of a series of arcs onto and collision of a large, 2530 Ma continent with an older (<3000 Ma) continent at around 2500 Ma.