青藏高原洋-陆过渡型（OCT）蛇绿岩模型的建立及构造意义

The traditional concept of ophiolite promoted to establish the tectonic framework of the Tibetan plateau, but it is difficult to interpret the origination of the assemblage of the mafic-ultramafic rocks with different tectonic and geochemical features in the suture zone, consequently restricting the accurate understanding of the early tectonic evolution of the Tibetan Plateau. According to the theory of the plate tectonics, the same assemblage of mafic-ultramafic rocks generated not only in the period of the spreading of oceanic crust and the sea floor spreading in the supra-subduction zone, but also in the initial rifting stage of the continental in the Wilson circles, in which due to the lower degree of oceanic crust, the mantle peridotites should be the obvious nature of subcontinental lithospheric mantle, and the upper lavas in the consequence of mafic-ultramafic rocks should be showing the transitional characteristics from alkaline to tholeiitic series, known as the OCT type ophiolite likely formed in the transition area from continent to ocean, mainly preserved at the magma-poor passive margins. The recognition of this new type ophiolite, help to detailedly and accurately understand the Tibetan Plateau early tectonic evolution, especially can help solve the argument for understanding the suture zones in the central Tibetan plateau. This project focused on the ophiolitic blocks in the central Tibetan plateau, and detailedly survey the mafic-ultramafic rocks cropped out in field, based on the petrographic study, using Os-Li isotopes as the main research methods to define tectonic background of mantle peridotite and the upper lavas, comparison the characteristics of the mafic-ultramafic rocks at the passive margin in the Iberian-Newfoundland of the Atlantic coast, to establish the OCT type ophiolite model for precisely discussing the early tectonic evolution of the Tibetan plateau.

传统蛇绿岩概念促进了青藏高原板块构造格局的建立，但尚难诠释缝合带内复杂多元的镁铁-超镁铁岩组合，从而制约了青藏高原早期构造演化的准确认识。根据板块构造理论，类似的组合不仅可以在大洋扩张阶段和俯冲阶段形成，而且也可以在威尔逊旋回初期的大陆裂解阶段形成，由于该阶段洋壳化程度较低，地幔橄榄岩具明显的大陆岩石圈地幔属性，同时上部熔岩具从碱性到拉班系列过渡特点，称为洋-陆过渡型（OCT）蛇绿岩，多保存于贫岩浆的被动大陆边缘。这一新类型蛇绿岩的识别，有助于精准理解青藏高原早期构造演化，特别是可以帮助解决高原腹部缝合带认识的争论。本申请以青藏高原腹部蛇绿岩为研究对象，详细观察镁铁-超镁铁岩的野外产出，在岩相学研究基础上，以Os-Li同位素体系为主要研究手段，界定地幔橄榄岩的构造属性和上部熔岩的成因构造背景，对比大西洋东岸伊比利亚-纽芬兰被动大陆边缘镁铁-超镁铁岩特征，建立青藏高原OCT型蛇绿岩模型。

传统蛇绿岩概念促进了青藏高原板块构造格局的建立，但尚难诠释缝合带内复杂多元的镁铁-超镁铁岩组合，特别是代表蛇绿岩中地幔橄榄岩单元的构造属性的界定，制约了青藏高原早期构造演化的准确认识。本项目选择研究基础较好的班公湖-怒江缝合带中南支蛇绿岩带的永珠蛇绿岩为研究对象，重点开展了地幔橄榄岩的野外产出、矿物学、岩石学和全岩地球化学和Os-Li同位素示踪研究，建立了洋-陆过渡型（OCT）蛇绿岩地幔橄榄岩的识别标准：（1）野外产出以地幔橄榄岩为主，蛇绿岩组合具贫岩浆特点；（2）组成地幔橄榄岩的矿物组合具有多世代性，主体主要是早世代矿物组成，具有粒度粗大的等粒装变晶结构，代表古老大陆岩石圈地幔橄榄岩矿物组成，晚世代矿物组合粒度较小，呈熔融囊体集合状，占岩体比例较小，代表洋壳化部分物质组成；（3）全岩主微量元素具有二元混合特征，不受原始地幔橄榄岩部分熔融趋势控制；（4）全岩Os同位素组成极度亏损，对应的Re亏损年龄明显老于洋盆存续时代。同时开展了OCT型蛇绿岩成因动力学背景的研究，主要甄别了拉萨地块北缘、班公湖-怒江缝合带南侧钠质碱性火山岩和安山岩成因构造背景研究：（1）发现洋陆断离的岩石学证据，在班公湖-怒江缝合带南侧凯蒙蛇绿混杂岩中橄榄粗安岩为洋壳俯冲末期大陆边缘陆壳与俯冲洋壳板片断离，导致软流圈地幔上涌诱发部分熔融所致，锆石U-Pb同位素定年结果表明火山岩时代约102 Ma，推测班公湖-怒江洋在早白垩世晚期之后再无俯冲作用；（2）永珠蛇绿岩带北侧的安山岩具有陆源和幔源混合特征，记录了陆壳参与的过程，为弧-陆碰撞的产物，而非俯冲成因导致，形成时代为120 Ma。综合蛇绿岩类型以及火山岩成因构造背景，提出班公湖-怒江缝合带南侧蛇绿岩为洋-陆过渡型，较好保存于被动大陆边缘。为根据蛇绿岩准确探讨古大洋演化提供了研究范例。