Zircon U–Pb, Hf, and O isotopic constraints on the tectonic affinity of the basement of the Himalayan orogenic belt: Insights from metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India

Abstract

Zircon U–Pb geochronology and Hf–O isotope geochemistry of metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India, provide new insights into the tectonic affinity of the basement of the Himalayan orogenic belt. Detrital zircon U–Pb ages from metasedimentary rocks in both the Lesser Himalayan Sequence (LHS) and the overlying Main Central Thrust (MCT) zone yielded dominant Paleoproterozoic ages with the youngest age peak of 1840 Ma. Along with that, the variable ɛHf (t) values (−14.56 to 8.44) indicate that these metasedimentary rocks could have been derived from the juvenile Paleoproterozoic orthogneiss basement in the Himalayas and/or the ancient Indian craton in the south. In contrast, the metasedimentary rocks in the Ediacaran Harsil Formation are dominated by the ca. 1200–800 Ma zircons with the youngest age peak of ca. 630 Ma and variable ɛHf (t) values (−16.57 to 13.32), which belong to the underlying High Himalayan Crystalline Sequence (HHCS), rather than the overlying Tethyan Himalayan Sequence. Orthogneisses in the MCT zone record ca. 1.87–1.85 Ga felsic magmatism associated with the Nuna supercontinent. The Bhagirathi leucogranites intruding the Harsil Formation were formed at ca. 496–490 Ma. These Paleoproterozoic orthogneisses and early Paleozoic leucogranites show only positive ɛHf (t) values (0 to +15) correlatable with a juvenile mantle input. However, the zircon Hf model ages older than the U–Pb ages and the oxygen isotope values (δ18O: 7.74–8.91 ‰), significantly higher than zircons crystallized from mantle-derived magma, indicate large degrees of melting and contamination of crust materials in both the granitoids. In combination with the pre-existing geochronological data, the current study indicates that the Garhwal HHCS is characterized by bimodal Neoproterozoic zircons at ca. 1100–900 Ma and ca. 860–800 Ma with wide εHf (t) values which are similar to those in the outer LHS and the Cathaysia Block (including the Jiangnan Orogen) of the South China Craton. These observations suggest that a close relationship existed between North India and South China during the Neoproterozoic since Tonian. The provenance of the Neoproterozoic HHCS is possibly a mixed source from both southern Gondwana terranes (e.g., east Antarctica) and northern South China.