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

Abstract

Massif-type anorthosites in the Proterozoic Eastern Ghats Belt of India occur as complexes of varying size within a strongly deformed, high to ultra-high grade assemblage of metasedimentary and metaigneous rocks. Recently discovered dikes and sheets of exceptionally HFSE and REE-enriched ferrodiorites at the immediate contacts of three anorthosite complexes (Chilka Lake, Bolangir and Turkel) contain ubiquitous magmatic zircon suitable for U–Pb dating. Due to the coeval and comagmatic nature of the ferrodiorites, the U–Pb zircon data provide reliable constraints on the time of anorthosite emplacement. Zircons from three ferrodiorite samples were analysed applying the conventional multi-grain isotope dilution method. Zircon grains in the Bolangir ferrodiorites (D4 and B2012) are long-prismatic and show the faint fine-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 upper 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 silicate rocks of the border zone of the anorthosite. Zircon grains in the Chilka Lake ferrodiorite (OK 76-2) are prismatic and show zoned igneous interiors that are overgrown by irregular poorly luminescent rims, presumably of metamorphic origin. The U–Pb ages obtained for abraded grain fractions are concordant and define an age of 792±2 Ma. The U–Pb zircon ages for the Chilka Lake and Bolangir ferrodiorites disprove the often quoted Mesoproterozoic age of anorthosite emplacement in the EGB, which was based on a four-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–Sr evolution diagram should be interpreted as a mixing line resulting from marginal contamination of the ascending hot anorthosite pluton with felsic melts generated in its thermal aureole. The U–Pb zircon ages provide new insights in the deformation history of the Eastern Ghats Belt as structural studies indicate intrusion of the Bolangir anorthosite while its country rocks 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.