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

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 centra l prong includes felsic metavolcanics and volcanogenic metasediments. The U -Pb zircon age for the pyroclastics in the central prong of the Ramagiri Sc hist Belt is 2707 +/- 18 Ma, considered as the time of emplacement of the f elsic volcanics. To the east of the belt, migmatitic Chenna gneisses yielde d 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 leas t 100 m.yr. later, as evidenced from the U-Pb titanite age of 2545 +/- 1 Ma . From the central Ramagiri Complex, a granodiorite has identical U-Pb zirc on and titanite ages of 2613 +/- 6 and 2614 +/- 4 Ma, respectively, and a q uartz diorite has a titanite age of 2595 +/- 1 Ma. These intrusives with ro of-pendants of pillowed basalts are inferred to have been emplaced as high level plutons that cooled rapidly. From the western Gangam Complex, U-Pb zi rcon 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 yielde d a titanite age of 2468 +/- 4 Ma. The three granitoid terranes separated b y a M3-km-wide schist belt do not have common magmatic or metamorphic histo ries, as evidenced by their distinct titanite and zircon ages. Intrusion of voluminous western Gangam Complex at 2528 Ma did not alter the titanite ag es of granitoids in either the central Ramagiri Complex or the eastern Chen na Gneiss just a few kilometers away. Therefore, the Ramagiri Schist Belt, consisting of highly sheared rocks, could represent a terrane boundary betw een three disparate granitoid terranes juxtaposed after 2516 and before 246 8 Ma. Available geological and geochronological information from Kolar, Ram agiri, and adjoining areas suggests that growth of Dharwar continental crus t could have been accomplished by both accretion of a series of arcs onto a nd collision of a large, 2530 Ma continent with an older (<3000 Ma) contine nt at around 2500 Ma.