Evolution of the Tethyan Bangong-Nujiang Ocean and its SE Asian connection: Perspective from the Early Cretaceous high-Mg granitoids in SW China

Abstract

High-Mg diorites (HMDs) have important implications on crust-mantle interactions and subduction processes, and are widely used to delineate ancient convergent plate margins. In this study, we described a newly-identified high-Mg diorite-granodiorite suite in the Tengchong terrane (SE Tibetan Plateau), and presented its petrography, whole-rock geochemistry, SrNd isotopes, and zircon U-Pb-Hf-O isotopic compositions. High-precision SIMS zircon UPb dating yielded consistent Early Cretaceous ages (~120 Ma) for all lithologies in this high-Mg suite. The high-Mg diorite is characterized by high MgO (4.7–9.2 wt%), Cr (169–323 ppm) and Ni (32–138 ppm) contents, together with high (87Sr/86Sr)i (0.70867–0.71060) and negative εNd(t) (−4.07 to −6.80) values. Their zircon εHf(t) vary from −5.64 to +4.12, and their δ18O (6.0 to 6.9‰) are slightly higher than the average mantle value. The granodiorite has higher SiO2 and lower MgO contents, and varying zircon εHf(t) (−12.19 to −2.71) and δ18O (6.3 to 8.1‰). The rocks are characterized by enrichments in large ion lithophile elements (LILEs: e.g., Rb, Th, U, K) and light rare earth elements (LREEs), but depleted in high field strength elements (HFSEs), and have also high (87Sr/86Sr)i (0.71057–0.711860) and negative εNd(t) (−8.42 to −9.14). Mafic microgranular enclaves (MMEs) in the granodiorite have intermediate SiO2, MgO, and trace elements and Sr-Nd-Hf-O isotope compositions between the high-Mg diorite and the granodiorite. Based on these geochemical and isotopic features, we interpreted that the high-Mg diorite was generated from the interaction of subducted sediment-derived melt with the overlying mantle wedge, whilst the granodiorite was sourced from the ancient continental crust with minor mantle-derived input. Mixing between the mantle-derived high-Mg dioritic melt and the crustal-derived granodioritic melt may have formed the MMEs. Our new geochemical (incl. isotopic) data, combined with published ones across the region, suggest that these Early Cretaceous plutons were formed by asthenospheric upwelling, which was probably led by slab rollback of the west-dipping subduction of the Luxi-Ruili Ocean between the Baoshan and Tengchong terranes. This implies that the southeastern continuation of the Tethyan Bangong-Nujiang Ocean was located between the Baoshan and Tengchong terranes, instead of between the Tengchong and West Burma terranes as previously suggested.