Zircon U-Pb-Hf constraints on the geological evolution of the Bihar Mica Belt, Central Indian Tectonic Zone, and its link to Columbia assembly

Abstract

In this study, U-Pb-Hf isotope composition and trace element chemistry of zircon from basement gneisses and quartzites of the Bihar Mica Belt (BMB), a belt of supracrustal rocks within the Central Indian Tectonic Zone, is used to constrain crustal growth and reworking. The protoliths of BMB gneisses were emplaced at 1651 ± 5 Ma. The magmas were derived from juvenile and 1747 ± 7 Ma I-type long-lived felsic precursors that survive as xenocrystic cores. Zircon grains from quartzites display two distinct morphologies. Grains with little rounding of edges have internal structures comprising 1750 ± 5 Ma inherited cores mantled by 1657 ± 4 Ma igneous zones that are identical to those from the granite gneisses. The εHf (t) and trace element composition of both the xenocrystic cores and the igneous mantles in these detrital grains are identical to those of zircon in the gneisses. They constitute molasse-like sediments derived from the nearby basement. More rounded grains furnish age clusters between 1857 and 2848 Ma, suggestive of long-distance transport and derivation from multiple sources. In-situ formed metamorphic rims around zircons in gneisses and quartzites furnish 1476–1466 Ma ages, dating the earliest metamorphic overprint. The age of sedimentation is bracketed between 1657 Ma and 1466 Ma. The 1651 ± 5 Ma gneisses and its 1747 ± 7 Ma felsic precursor were I-type, and emplaced in an arc setting. They constitute part of the expansive suite of Paleoproterozoic gneisses of the Chhota Nagpur Gneiss Complex emplaced during protracted period of arc-magmatism between 1.75 Ga and 1.65 Ga. Strongly positive εHf(t) of 2.0–2.85 Ga detrital zircons conforms to the global trend of positive εHf(t) excursions and high dT/dP gradients, and may correspond to passive margin juvenile magmatism associated with the break-up of Archean supercratons prior to Columbia assembly. The 1.86 Ga and 1.75 Ga detrital and xenocrystic zircons from the BMB have enriched εHf(t), in accordance with negative excursions seen in global zircon between 2.0 Ga and 1.70 Ga, reflecting progressive assembly of Columbia through collisional orogenesis. An excursion to strongly positive εHf(t), coinciding with low dT/dP gradients at ca. 1.65 Ga, reflects wide-spread formation of arcs linked to accretionary growth of Columbia.