Cambrian granitic gneiss within the central Qiangtang terrane, Tibetan Plateau: implications for the early Palaeozoic tectonic evolution of the Gondwanan margin

The Tibetan Plateau is located in the eastern Himalayan–Alpine orogen, an area where previous research has focused on ophiolites and a high-pressure metamorphic belt, whereas comparatively little research has been undertaken on the Tibetan basement. Cambrian granitic gneiss crops out in the Duguer area of the South Qiangtang terrane in northern Tibet and yields zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages of 502–492 Ma, providing insight into the possible existence of basement rocks within the South Qiangtang terrane. The granitic gneisses are geochemically similar to high-K, calc-alkaline S-type granites, and Hf isotopic analysis of zircons within the gneisses yields negative εHf(t) values (–7.4 to – 1.1) and old zircon Hf model ages (TDMC = 1757–1406 Ma). These granitic gneisses were generated by partial melting of ancient pelitic rocks, and the resulting melts were contaminated by a small amount of mantle-derived material. Combining our new data with previous research, we conclude that these Cambrian granitic gneisses developed in a post-collisional tectonic setting after Pan-African tectonism. This suggests that the South Qiangtang terrane might have the same early Palaeozoic crystalline basement as the Lhasa, Himalaya, Baoshan, Gongshan, and Tengchong terranes.

青藏高原位于东喜马拉雅-阿尔卑斯造山带内，此前该区域的研究多聚焦于蛇绿岩（ophiolites）与高压变质带（high-pressure metamorphic belt），而针对青藏高原基底的研究相对匮乏。藏北南羌塘地体的杜格尔地区出露寒武纪花岗片麻岩，其锆石经激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）U-Pb定年获得的年龄为502~492 Ma，为探究南羌塘地体内部基底岩石的存在可能性提供了新的认识。该类花岗片麻岩的地球化学特征与高钾钙碱性S型花岗岩相似；对其中锆石的Hf同位素分析显示，其εHf(t)值为负值（-7.4~-1.1），且锆石Hf亏损模式年龄（TDMC）为1757~1406 Ma。此类花岗片麻岩由古老泥质岩的部分熔融形成，且形成过程中受到了少量幔源物质的混染。结合本次新获取的数据与前人研究成果，我们认为这些寒武纪花岗片麻岩形成于泛非构造运动（Pan-African tectonism）之后的碰撞后构造环境。这表明南羌塘地体可能与拉萨、喜马拉雅、保山、贡山及腾冲地体拥有一致的早古生代结晶基底。