Ages, Trace-Element and Hf Isotopic Compositions of the Detrital Zircons from the Metamorphic Basements in the Eastern Himalayan Syntaxis: Implications for Tectonics and Paleogeography

Abstract:

The origin of the Greater Himalayan Sequence (GHS) of the Himalaya and the paleogeographic position of the Lhasa terrane within Gondwanaland remain controversial. In the Eastern Himalayan syntaxis, the metamorphic basements of the northern Indian plate (Namche Barwa Group) and the South Lhasa terrane (Nyingchi Complex) can be studied to explore these issues. The youngest ages of detrital zircons in the metasedimentary rocks from the Pai Formation of the Namche Barwa Group (NBG) and the Nyingchi Complex suggest their maximum depositional ages are no older than 942 Ma and 1006 Ma, respectively. A granitic gneiss intruding the Pai Formation and a metarhyolite overlying the Nyingchi Complex have protolith ages of 477 ± 3 Ma and 507 ± 4 Ma, respectively, providing upper age limits for their deposition. The detrital zircons in the Pai Formation and Nyingchi Complex define four major age peaks at ~1170 Ma, ~1350 Ma, ~1565 Ma, and ~1750 Ma. Both the ~1170 Ma and ~1350 Ma zircons have large variation of e_Hf(t) values (-19.4 to +5.6); most ~1565 Ma zircons have positive e_Hf(t) values (+1.2 - +9.0), and most ~1750 Ma zircons have negative e_Hf(t) values (-7.1 to -1.9), which are consistent with those of the Mesoproterozoic orthogneiss in the Duoxiongla migmatite of NBG. The trace-element signatures of ~1.5-1.3 Ga detrital zircons indicate they were derived from sources characterized by bimodal magmatism. The potential sources for the metasediments in the Pai Formation and Nyingchi Complex include the Rayner-Eastern Ghats Orogen and its contact zone with the Archaean Indian cratons, the Central Indian Tectonic Zone, the northeastern India craton and Dharwar craton, South Lhasa terrane and probably Eastern Antarctic. Comparisons of detrital zircon age spectra of the coeval metasedimentary rocks from the Nyingchi Complex, the Pai Formation, the GHS and upper Lesser Himalayan Sequence in Arunachal Himalayan, and the Indian cratonic successions suggest that: (1) the NBG is the northeastern extension of the GHS in Arunachal Himalaya; (2) their protolithes were deposited along the northern passive margin of the Indian craton; (3) prior to the Cambrian, the South Lhasa terrane was tectonically linked to northern India rather than northwest Australia.