青藏高原特提斯蛇绿岩中残留古老大陆岩石圈地幔的地球动力学背景初探

Geosciences generally considered the ophiolite as the fragments of oceanic lithosphere remaining in the plate-tectonic suture zone and as the products of the interaction of crust-mantle at the spreading center, in which it stands to reason that the mantle sequence of ophiolite shows oceanic mantle affinities in petrology and geochemistry. While more and more Re-Os isotopic data on the ophiolitic peridotites and chromitites have revealed that the ancient sub-continental lithospheric mantle (SCLM) remained in the ophiolitic mantle sequences, especially, some ophiolitic chromitites likely originated from the ancient SCLM. There are two hypotheses have been currently used to explain this phenomenon of ancient SCLM remaining in the ophiolites, which represents the relatively younger oceanic lithosphere. First, rifting of ancient continent by listric fault caused the ancient SCLM isolated in the upwelling asthenospheric mantle and then became the oceanic mantle; Secondly, ancient SCLM subducted into the asthenospheric mantle in the subduction zone followed by the subduction of oceanic plate. Then, which hypothetical mechanism majorly controlled the evolution of Tibetan Tethys ophiolites? And this scientific issue has directly been to restrict the further study on the tectonic evolution of Tibetan Tethys Ocean and the genesis of the ophiolitic chromitites. Unfortunately, there is no study on the geodynamics for discussing the ancient sub-continent lithospheric mantle remained in the Tibetan Tethys ophiolites. This project plans to restore the tectonic evolution of the Amdo Tethys ophiolite at the middle of Bangong-Nujiang suture zone in central Tibet through studying the metamorphic chronology of Amdo high pressure granulites and Nagqu ophiolite and Amdo ophiolite located at the south and north margin of Amdo micro-terrane, respectively, and distinguishing the tectonic affinities of ophiolitic peridotites in petrology and geochemistry. And it predicts to accumulate knowledge to establish geodynamics for understanding the ancient SCLM remaining in the Tibetan Tethys ophiolites.

传统地球科学认为蛇绿岩是残留在缝合带中的大洋岩石圈岩块，是壳-幔相互作用的产物，地幔部分应为海洋型地幔。但是，已有研究表明蛇绿岩的地幔橄榄岩中含有古老大陆岩石圈地幔成分，而且也是铬铁矿成矿物质重要来源之一。现有两种机制解释这种古老大陆岩石圈地幔残留的现象：（1）大陆铲状裂解，致使大陆岩石圈地幔在新生的大洋岩石圈地幔中残留；（2）古老大陆岩石圈地幔在板块汇聚带俯冲再循环。那么在青藏高原特提斯缝合带是哪种机制起主导作用？这一问题直接制约着青藏高原特提斯构造演化与铬铁矿成因联系的深入研究，但至今尚无相关研究成果。本项目选择研究基础较好的班公湖-怒江缝合带中段安多微陆块中的高压麻粒岩及微陆块南北两侧的那曲和安多蛇绿岩为研究对象，通过麻粒岩的变质年代学、蛇绿岩的地幔属性和成岩时代的研究，揭示安多特提斯洋的构造演化史，为探讨青藏高原特提斯蛇绿岩中残留古老大陆岩石圈地幔的地球动力学背景积累资料。

依据项目设计，为探讨青藏高原班公湖-怒江特提斯构造域大洋岩石圈中古老大陆岩石圈地幔残留的地球动力学背景，系统开展了纵贯班公湖-怒江缝合带不同期次俯冲成因岩浆岩研究，以出露在羌塘地块南缘的雁石坪火山岩、班-怒带内安多、那曲火山岩以及拉萨地块北缘等火山岩为主要研究对象，通过详细的野外地质调查和室内精细的岩石地球化学、同位素地球化学及精准的U-Pb同位素体系测年研究，课题组成员共发表SCI论文10篇，培养博士研究生2人，其中一人获得国家人事部首批博士后创新人才基金资助，完成预期科学目标。取得如下主要进展：（1）表明班-怒特提斯洋初始洋壳形成于早二叠世，约278Ma，标志着班-怒洋在该时期已经打开；（2）首次提出羌塘南缘雁石坪三叠纪火山岩形成于班-怒特提斯洋北向俯冲构造背景，代表该区班-怒特提斯洋北向俯冲的最早时间记录，为242～236Ma；（3）揭示安多地区确哈拉组中的夹层火山岩为洋-洋俯冲成因的岛弧火山岩，时代为228 Ma；（4）介于雁石坪火山弧和安多岛弧之间的安多弧后盆地枕状熔岩年龄小于220Ma，建立了安多地区完整的弧-盆体系，结束了班-怒带有盆无弧的研究历史；（5）位于拉萨地块中北部的早白垩世火山岩形成于同碰撞环境，而非南向俯冲构造背景；（6）晚白垩世火山岩形成于拉萨-羌塘地块碰撞后的岩石圈去根背景，标志班-怒特提斯洋于晚白垩世彻底关闭。结合已有成果安多微陆块中高压麻粒岩峰期变质时代为191Ma以及安多微陆块南侧由洋壳与微陆块断离形成的那曲蛇绿岩的堆晶辉长岩年龄为184Ma；再向南位于罗麦区的为一套具有钙碱性系列特征的火山岩，年龄为170～150Ma；最南侧为永珠嘉黎洋-陆过渡型蛇绿岩，显示俯冲作用由北向南逐渐变晚的特点。推测班公湖-怒江缝合带是新特提斯洋南撤北向俯冲造成的含微陆块和数条不同成因构造背景蛇绿岩带的增生型造山带，为探讨蛇绿岩中残存古老大陆岩石圈地幔的机制提供了新认识。