Age of emplacement of massif-type anorthosites in the Eastern Ghats Belt, India: constraints from U-Pb zircon dating and structural studies

Massif-type anorthosites ia the Proterozoic Eastern Chats Belt of India occ ur as complexes of varying size within a strongly deformed, high to ultra-h igh grade assemblage of metasedimentary and metaigneous rocks. Recently dis covered dikes and sheets of exceptionally HFSE and REE-enriched ferrodiorit es at the immediate contacts of three anorthosite complexes (Chilka Lake, B olangir and Turkel) contain ubiquitous magmatic zircon suitable for U-Pb da ting. Due to the coeval and comagmatic nature of the ferrodiorites, the U-P b zircon data provide reliable constraints on the time of anorthosite empla cement. Zircons from three ferrodiorite samples were analysed applying the conventional multi-grain isotope dilution method. Zircon grains in the Bola ngir ferrodiorites (D4 and B2012) are long-prismatic and show the faint fin e-scale oscillatory zoning typical for magmatic crystallisation, with some poorly luminescent thin overgrowths of metamorphic origin. The data points obtained for abraded grain fractions are slightly discordant and yield an u pper intercept age of 933 +/- 32 Ma with a lower intercept at 515 +/- 20 Ma , defined by previously published concordant U-Pb titanite data from calc s ilicate rocks of the border zone of the anorthosite. Zircon grains in the C hilka Lake ferrodiorite (OK 76-2) are prismatic and show zoned igneous inte riors that are overgrown by irregular poorly luminescent rims, presumably o f metamorphic origin. The U-Pb ages obtained for abraded grain fractions ar e concordant and define an age of 792 +/- 2 Ma. The U-Pb zircon ages for th e Chilka Lake and Bolangir ferrodiorites disprove the often quoted Mesoprot erozoic age of anorthosite emplacement in the EGB, which was based on a fou r-point Rb-Sr whole rock 'isochron' for the Chilka Lake anorthosite complex (1404 +/- 89 Ma: Sarkar et al., 1981. Combined with field and geochemical evidence, the new U-Pb zircon ages indicate that the data array in the Rb-S r evolution diagram should be interpreted as a mixing line resulting from m arginal contamination of the ascending hot anorthosite pluton with felsic m elts generated in its thermal aureole. The U-Pb zircon ages provide new ins ights in the deformation history of the Eastern Chats Belt as structural st udies indicate intrusion of the Bolangir anorthosite while its country rock s underwent a thrust shear-dominated deformation. In contrast, emplacement of the considerably younger Chilka Lake anorthosite occurred previous to or at the beginning of a regionally intense transpressive deformation. (C) 20 01 Elsevier Science B.V. All rights reserved.