Zircon U–Pb age and Sr–Nd–Hf–O isotope geochemistry of the Paleocene–Eocene igneous rocks in western Gangdese: Evidence for the timing of Neo-Tethyan slab breakoff

Abstract

Northward Neo-Tethyan oceanic lithosphere subduction beneath southern Tibet in the Mesozoic–Early Cenozoic produced continental arc magmas in the ~1600km-long Gangdese belt. The most voluminous magmatism occurred in the Paleocene–Eocene, and is characterized by extensive I-type calc-alkaline to high-K calc-alkaline Linzizong volcanic rocks, and coeval plutons. These rocks have been extensively studied in the eastern Gangdese belt (east of ∼89°E), but few data exist from the western Gangdese belt. New data for eleven samples of these rocks, combined with existing data from the literature, show that they are similar to the eastern Gangdese belt rocks, with relative depletions in Nb, Ta, P, and Ti, and enrichments in Rb, Ba, Th, U, K, Pb, Zr, and Hf on a primitive mantle-normalized trace element diagrams, typical of continental arc-related igneous rocks. However, compared to the east, western Gangdese igneous rocks range to higher K2O contents (up to 6.1wt.%), higher (87Sr/86Sr)i ratios (up to 0.7151), and lower εNdi values (down to −8.1), suggesting that an evolved crustal source was involved in arc magmatism.The Gangdese arc magmatism lasted to ~80Ma, and has a gap or quiescent period afterwards. Starting at ~69Ma, the arc magmatism initiated and shifted southward from ~30.5°N to ~29.5°N in southern Tibet with an abrupt change of India-Asia convergence rate. The magmas through the whole Gangdese belt at ~69–53Ma are characterized by intermediate εNdi values (−0.6 to +4.0), positive εHfi values (+3.8 to +7.1), intermediate δ18O values (+5.0‰ to +6.5‰), and low Th/Y and La/Yb ratios (<20). These magmas were likely derived from the mantle with crustal contamination by MASH process in response to Neo-Tethyan slab rollback as proposed by previous studies.The Sr–Nd–Hf–O isotopic compositions of Linzizong volcanic rocks and coeval intrusions are relatively homogeneous during ~69–53Ma, but show a wide range at ~53–50Ma. Zircon temperature values from these rocks are distinctively high during the latter period and whole rock Th/Y and La/Yb ratios increased significantly after ~50Ma. We suggest that the high temperature and heterogeneous geochemistry of these magmas at ~53–50Ma reflect Neo-Tethyan slab breakoff during India-Asia collision, which triggered asthenospheric mantle upwelling. Mantle derived magmas resulted in extensive heating, induced crustal melting, and generated magmas with variable isotopic compositions. Tibetan crust was significantly thickened after ~50Ma.