Late Mesozoic diorites of the middle Gangdese magmatic belt of southern Tibet: New insights from SHRIMP U-Pb dating and Sr-Nd-Hf-O isotopes

Abstract

The Gangdese magmatic belt, which formed by Mesozoic northward subduction of Neo-Tethyan oceanic lithosphere and Early Mesozoic India-Asia collision, is an excellent natural laboratory for study of subducting slabs, crustal growth, the reworking of convergent continental margins and interaction of crust-mantle in general. Although many studies have examined this belt, the petrogenesis of arc magmas and the geodynamic architecture of the subducting slab are still contentious. Discoveries of igneous rock exposures are important in reconstructing the tectonic regime and evolution of southern Tibet, and provide new constraints on unresolved issues. Here we report two newly identified intermediate rocks, shedding new light on both the formation of arc-type magmas, and the tectonic evolution of the Neo-Tethys during the Late Mesozoic. Zircon SHRIMP II U-Pb dating indicates that a Late Cretaceous gabbro diorite pluton formed at 90.3 ± 1.4 Ma, and a Late Jurassic diorite porphyry pluton crystallized at 157.9 ± 2.3 Ma. The former has positive zircon εHf(t) values of 8.03–10.86 and whole-rock εNd(t) values of 4.11–4.37, with δ18Ozircon values of 5.84‰–6.70‰, slightly above the values of mantle zircon (5.3‰ ± 0.6‰). The latter also has positive zircon εHf(t) (10.29–12.77) and whole-rock εNd(t) (5.54–5.78) values, with normal mantle-like zircon δ18O values of 5.19‰ to 6.00‰. All samples of both plutons display arc-type signatures characterized by enrichment in light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti and P). Based on multiple isotopic (Sr–Nd–Hf–O) and whole-rock geochemical characteristics, we conclude that the Late Cretaceous gabbro diorite was derived from partial melting of depleted mantle that had been metasomatized by subducted-sediment-derived melts (~5 vol%). The Late Jurassic diorite porphyry is inferred to be a product of fractional crystallization of primary andesitic magmas with limited contents of Fe-Ti oxide and amphibole, and minor plagioclase accumulation. Pelagic sediments and enriched materials were a negligible component of the precursor magmas of the diorite porphyry, relative to the gabbro diorite pluton. Considering the temporospatial distribution of igneous rocks over the whole Gangdese magmatic belt, we suggest that Late Jurassic magmatism was related to normal angle subduction of the Yarlung Tsangpo Neo-Tethyan oceanic lithosphere, and early Late Cretaceous magmatism was triggered by rollback of the Neo-Tethyan oceanic slab.