Pre-Himalayan crustal evolution in the northern margin of India: Implications from zircon U-Pb, Hf and O isotope study of Chhota Shigri granitoids, Himachal Himalaya, Northwest India

A systematic geochemical, geochronological (U-Pb), and isotopic study (Lu-Hf and O) of zircons from the northern margin of India has been carried out to understand the pre-Himalayan crustal evolution along the area. The rocks were characterized as granitoid and yielded two significant age peaks: 1018-715 Ma and 516-476 Ma, with sparse inherited cores of Paleoproterozoic to Archean ages, defining two major magmatic events (Tonian and Cambro-Ordovician) in the NW Himalaya. While Tonian ages are preserved in the zircon cores, the rims and numerous whole grains record the younger Cambro-Ordovician event. The Tonian zircon cores display epsilon Hf values ranging from -5.9 to +8.3, indicating both juvenile mantle-derived and recycled crustal sources. The Tonian zircon cores yield single-stage Hf model (TDM) ages of 1.66-1.19 Ga and 518O values of 5.75-10.88%o, suggesting that Tonian granitoids formed from mixed sources comprising juvenile material, recycled ancient crust and 518O- rich supracrustal components. The Cambro-Ordovician zircons yielded epsilon Hf values from -27.4 to -4.0 and single- stage Hf model (TDM) ages of 2.16-1.21 Ga, demonstrating extensive reworking of both Neoproterozoic crust and ancient crust occurred during the Cambro-Ordovician magmatism. Their elevated 518O values (8.05-11.35%o) further support widespread crustal remelting with substantial incorporation of 518O-rich supracrustal materials. These evidence the preservation of supercontinent (Rodinia) components within the NW Himalaya.