Abstract

Columbia was a Proterozoic supercontinent that formed ∼2.0 b.y. ago by amalgamation of almost all the present-day continental blocks. One major controversy regarding the formation of this supercontinent is the position and tectonic setting of the north Indian continental margin during this amalgamation. One school of thought suggests that this margin remained passive throughout the continental accretion process, and another school contradicts that by suggesting subduction and an active continental arc setting. The Paleoproterozoic basement rocks of the Himalaya consist of granitic gneisses from the Lesser Himalayan Sequence that belonged to the north Indian continental margin during the formation of Columbia. In this work, we present bulk-rock geochemistry and U-Pb geochronology of zircon from augen gneisses of the Lesser Himalayan Munsiari Formation and granite gneisses of the Chiplakot Crystalline Belt of the Kumaun Himalaya, from Kali River Valley, India. Our objective was to evaluate the tectonic setting of these rocks to infer the nature of the Proterozoic north Indian continental margin. Bulk-rock geochemistry of four samples from the Munsiari augen gneiss and six samples from the granite gneisses from Chiplakot Crystalline Belt shows calc-alkaline and shoshonitic composition, respectively. Depletion in Nb, Sr, P, and Ti point toward a magmatic arc origin for both units. U-Pb chronology of zircon was carried out on two samples from the Munsiari augen gneiss and three samples from the Chiplakot Crystalline Belt. All of these samples yielded ages ranging from ca. 1970 Ma to ca. 1860 Ma, with crystallization ages in the Musiari augen gneiss varying from ca. 1970 to ca. 1950 Ma and ages in the Chiplakot Crystalline Belt at ca. 1920 Ma. Based on these results, it is inferred that both the Munsiari augen gneiss and the Chiplakot Crystalline Belt belong to the Inner Lesser Himalayan Sequence, and the magmatism took place within a span of ∼100 m.y. related to active subduction along the Proterozoic north Indian continental margin. It is envisaged that the slightly older and highly fractionated calc-alkaline Munsiari augen gneiss reflects mixing of mantle and crustal melt along with assimilation and fractional crystallization. On the other hand, the Chiplakot Crystalline Belt is less fractionated and may have intruded during a later part of continental arc magmatism into a thinner crust, associated with crustal extension probably driven by slab break-off and/or slab roll-back. Our study indicates that the north Indian continental margin was indeed an active subduction zone during the formation of Columbia.

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