Geochronological, geochemical, and Sr-Nd-Hf isotopic characteristics of granitoids in eastern Tibet and implications for tectonic correlation with southeastern Asia

Abstract

Understanding the configuration of continental blocks within the Tibetan orogen prior to Indo-Asian collision is vital to determining their accretionary history. However, complex tectonic overprinting, including the development of the eastern and western Himalayan syntaxes, has hampered the investigation of correlations between tectonic blocks in the Tibetan Plateau with those in southeastern Asia. The North Lancangjiang tectonic belt is known to connect with the Longmu Co–Shuanghu suture to the west and the Changning-Menglian suture to the south, but limited work has been undertaken in eastern Tibet, and no ophiolite suites or high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic belts have been identified within this belt. This paper presents new in situ zircon U-Pb, whole-rock major- and trace-element, and whole-rock Sr-Nd-Hf isotopic data for three granitic plutons (Chaya, Jinba, and Batang) within the North Qiangtang block. These plutons yield ages from ca. 265 Ma (Chaya) to ca. 230 Ma (Batang). The Chaya and Batang granites are chemically similar to I-type granites, whereas the Jinba granite is an S-type granite. Sr-Nd-Hf isotopic data suggest that the Chaya granite formed from magmas generated by the partial melting of subduction-modified mantle wedge material and that the Jinba granite formed from magmas generated by the partial melting of Precambrian metasedimentary rocks mixed with a small amount of juvenile mantle–derived crustal material. The Batang granite formed from magmas generated by the partial melting of mafic lower-crustal material. The convergence of the North Qiangtang block with neighboring blocks was a consequence of the closure of the Paleo-Tethys Ocean, and the plutons in this area record subduction-related or syncollisional magmatism. New data from granites within the North Lancangjiang magmatic belt and the Jinshajiang suture indicate that the North Lancangjiang Paleo-Tethyan slab subducted eastward, whereas the Jinshajiang Paleo-Tethyan slab subducted westward under the North Qiangtang block at ca. 267 Ma. The timing and style of block convergence between North Qiangtang–Simao–Indochina and neighboring blocks are similar to those recorded within central Tibet and southeastern Asia. This, combined with the presence of similar Middle Mesoproterozoic–Lower Paleozoic basement materials and Paleozoic sediments hosting Cathaysian warm-climate fossils, indicates that North Qiangtang, Simao, and Indochina represent a single block.