羌塘中部泥盆纪-石炭纪岩浆作用对古特提斯洋构造演化的制约

Research on the active continental marginal magmatism is of great significance for better understanding the subduction of the oceanic crust and crust-mantle interaction. Previous studies have revealed that Longmuco - Shuanghu Paleo-Tethys in the central Qiangtang area and have experienced a long-term tectonic evolution during early Paleozoic - Triassic evolution. However, it remains unclear magmatic activity supplementary with tectonic evolution and its geodynamic mechanism during Late Paleozoic. Therefore, these volcanic rocks including intrusions and sadimentary record in adjacent area, which formed between Late Devonian and Early Carboniferous, in the Riwanchaka and Laxiongco areas, will be mainly focused on this proposal. Based on systematically geological field survey, petrological, geochronological, geochemical and Sr-Nd-Hf isotopic studies, we will discuss the temporal and spatial variation of the magmatism in the study area, magmatic origin and dynamic mechanism, and constrain the subducted oceanic lithosphere in magma evolution processes. Meanwhile, combining with the regional geological data, we will further illustrate the tectonic setting of the Late Devonian- Early Carboniferous volcanic rocks (intra-oceanic arc or continental margin arc), and provide new constraint for the tectonic evolution of the Paleo-Tethys Ocean in the Qiangtang area, and reconstruct the ancient geography of Qiangtang Terrane in Gondwanaland. These results will offer the geological and geochemical constraints for the geodynamic machanism of the Late Devonian-Early Carboniferous magmatism in the Longmuco-Shuanghu Tethys Ocean and tectonic evolution in the Qiangtang area.

活动大陆边缘岩浆作用在探讨大洋的俯冲消减过程和壳－幔相互作用方面具有重要指示意义。已有的研究揭示羌塘腹地龙木错－双湖特提斯洋经历了早古生代－三叠纪长期的演化过程，但与之演化相对应的晚古生代岩浆活动及其动力学机制仍不清楚。本项目拟以羌塘腹地日湾茶卡、拉雄错地区与大洋俯冲消减过程相联系的火山岩为研究重点，兼顾邻区的晚古生代侵入岩及沉积记录，通过对其进行系统的岩石学、矿物化学、元素和Sr-Nd同位素地球化学、锆石U-Pb年代学及其Hf同位素研究，约束研究区岩浆活动的时空序列、岩浆起源和动力学机制，探讨俯冲洋壳岩石圈在其形成过程中的作用。在此基础上，结合区域地质资料，进一步综合限定其形成的地球动力学背景（陆缘弧或洋内弧）及其对龙木错-双湖特提斯洋构造演化的制约，重建羌塘地块在冈瓦纳大陆中的古地理格局。项目成果将为龙木错－双湖特提斯洋晚古生代时期的构造演化过程提供重要的岩石学和地球化学约束。

青藏高原羌塘地区龙木错－双湖特提斯洋在寒武纪－三叠纪一直存在，但是二叠纪之前是否存在俯冲消减或增生事件，洋盆何时由扩张转为俯冲消减？这在很大程度上制约了原-古特提斯洋早期演化历史的理解。项目经过4年的研究，在羌塘中部厘定一套晚泥盆世－早石炭世火山岩及早志留纪侵入岩，限定了该时期岩浆活动的岩浆源区及其形成的地球动力学背景，为探讨龙木错-双湖古特提斯洋早期的俯冲消减过程提供了新的制约。日湾茶卡及拉雄错地区火山岩形成于374~354Ma，属于钙碱性火山岩系列，地球化学指示日湾茶卡玄武岩很可能起源于由俯冲流体交代影响的岩石圈地幔的部分熔融，而安山岩是玄武质岩浆分离结晶的产物；日湾茶卡和拉雄错流纹岩初始岩浆来源于初生玄武质下地壳的部分熔融，认为羌塘中部晚泥盆世－早石炭世火山岩形成于龙木错－双湖特提斯洋北向俯冲的构造环境，在晚泥盆世时期北羌塘地块是泛华夏大陆南缘的活动大陆边缘。香桃湖杂岩由闪长岩、二长花岗岩、黑云二长花岗岩组成，主要形成于429~438 Ma，地球化学指示闪长岩源于与俯冲沉积物熔融交代地幔的部分熔融，二长花岗岩和黑云母二长花岗岩主要源于年轻基性下地壳的部分熔融，他们形成于陆缘弧构造环境，代表了龙木错-双湖原特提斯洋板块向南俯冲所形成的冈瓦纳北缘岩浆活动。此外，提出了龙木错-双湖-昌宁-孟连结合带特提斯洋在早古生代至晚古生代很可能是一个连续演化的大洋，经历了早古生代原特提斯大洋扩张、早古生代中晚期－晚古生代特提斯俯冲消减与岛弧带形成、晚二叠世末－早三叠世主碰撞汇聚、晚三叠世晚碰撞造山与盆山转换等阶段。基于龙木错－双湖－昌宁－孟连结合带残留洋壳信息以及两侧生物区系、地球物理、航空重磁以及造山系时空结构演化的差异，认为龙木错－双湖－昌宁－孟连结合带构筑了青藏高原上冈瓦纳大陆与泛华夏大陆的界线。