中国科学院海洋研究所机构知识库

特提斯洋是位于北方劳亚古陆和南方冈瓦纳古陆间长期存在的纬向展布的古大洋。特提斯洋演化划分为原特提斯洋、古特提斯洋和新特提斯洋三个阶段。

西昆仑造山带是中央造山带的主要构造单元，可划分为西昆仑北地体、西昆仑南地体、甜水海地块等不同块体。这些微地块经历了古生代-中生代一系列重要的岩浆活动事件，记录了原-古特提斯洋从起始俯冲到湮灭的历史，是理解原-古特提斯洋演化及重要成矿事件的重要载体。本文以西昆仑不同块体古生代-中生代岩浆岩为研究对象，开展年代学、主微量元素及同位素地球化学研究，深入探讨岩浆岩时代、地球化学特征以及动力学背景。取得的主要认识和成果如下：

早寒武世岩浆岩主要出露于赞坎、老并、河可兰尔以及岔河口等地，主体为中酸性侵入岩，部分侵入岩发育闪长质-辉长质包体，对侵入岩和包体进行SHRIMP和LA-ICP-MS 锆石U-Pb定年，年龄集中在530 ~ 550 Ma 之间。早寒武世甜水海地块侵入岩主要为I型花岗岩类，为低钾玄武岩源区低压脱水熔融的产物，而闪长质-辉长质包体为幔源岩浆与花岗岩发生岩浆混合的产物。甜水海地块中的大多数花岗岩样品显示出Nd-Hf同位素解耦现象，可能是由于源区不平衡熔融造成的。早古生代花岗岩同位素特征也暗示甜水海地块存在古老和新生两种性质的下地壳。

早古生代甜水海地块近南北向带状分布岩浆活动可能受原特提斯洋南向俯冲作用影响。根据古地理重建和岩浆岩的年龄分布，南昆仑地块和甜水海地块分布在东冈瓦纳大陆北缘与塔里木块体之间。以康西瓦-苏巴士缝合带和库地蛇绿岩带代表的原特提斯洋的两个分支同时存在。根据东冈瓦纳大陆北缘微块体的年代统计，原特提斯洋的俯冲具有穿时性，起源于东冈瓦纳大陆西部区并向东逐渐年轻。

石炭纪乌鲁阿特组主要由枕状玄武岩夹少量流纹岩组成，为一套双峰式火山岩组合。乌鲁阿特组玄武岩和流纹岩LA-ICP-MS 锆石U-Pb定年结果为348 ~ 354 Ma，代表玄武岩和流纹岩的喷发年龄。玄武岩为尖晶石二辉橄榄岩5%~10%程度部分熔融形成的，源区显示为亏损地幔，与锆石ε_Hf值(+7.3~+11.8)相吻合。模拟结果显示，玄武岩经历了40% ~ 95%程度的橄榄石、单斜辉石、角闪石、斜长石以及磁铁矿结晶分异作用。乌鲁阿特组流纹岩为玄武质岩浆底侵加热从而导致下地壳物质在低压条件下发生部分熔融。综合地球化学数据与区域地质特征，本文认为乌鲁阿特组火山岩沉积环境不同与大洋盆地，为古特提斯洋北向俯冲消减相关的弧后盆地火山岩建造。

二叠纪棋盘组火山岩为一套富铌玄武岩，LA-ICP-MS锆石U-Pb年龄为 287 ± 4.6 Ma，为早二叠世。棋盘组玄武岩为尖晶石二辉橄榄岩5% ~ 7%程度动态熔融形成，并经历了0%~50%橄榄石、单斜辉石以及斜长石结晶分异作用。玄武岩锆石ε_Hf(t)值为+0.2 ~ +11.5，暗示了源区不均一，可能有富集组分的加入。西昆仑地区由石炭纪（313 Ma）受古特提斯洋俯冲影响的亏损地幔特征的玄武岩向二叠纪富集/原始地幔的玄武岩突然转变，很可能受深部地幔物质加入的影响，其源区可能存在的洋岛型 (OIB) + 岛弧拉斑型 (IAT) ± 大洋中脊型 (MORB) 端元组分的混合。推测二叠纪棋盘组玄武岩可能为俯冲带与地幔柱相互作用的结果。

三叠纪岩浆岩广泛分布在西昆仑南地块和甜水海地块。沙子沟和萨罗依岩体LA-ICP-MS锆石U-Pb年龄为213.7±2.6 ~ 237.3±2.8Ma，为准铝质至弱过铝质花岗岩。沙子沟和萨罗依花岗岩为壳源花岗质岩浆与幔源基性岩浆发生强烈混合作用的产物，并分别经历了不同程度斜长石和钾长石结晶分异。结合区域资料，三叠纪花岗岩可能处于古特提斯洋俯冲向碰撞转变背景。

综合本文分析测试结果，结合区域地质资料，认为西昆仑造山带发育古生代-中生代一系列重要的岩浆活动事件，记录了原特提斯洋初始俯冲到古特提斯洋闭合的演化历史，在复杂而漫长的洋陆转换与增生-碰撞过程，形成了海底火山-沉积变质型铁矿、沉积型富锰矿、伟晶岩型锂铍矿等一系列大型-超大型矿床。

The Tethys Ocean is a long-standing, latitudinally spreading palaeo-ocean located between the Raua Palaeo-continent in the north and the Gondwana Palaeo-continent in the south. The evolution of the Tethys Ocean is divided into three stages: the Proto-Tethys Ocean, the Paleo-Tethys Ocean and the Neo-Tethys Ocean. The West Kunlun orogenic belt is the main tectonic unit of the Central Orogenic Belt and can be divided into the Northern Kunlun Terrane, Southern Kunlun Terrane, and Tianshuihai Terrane. These micro-terranes have experienced a series of important magmatic events during the Paleozoic-Mesozoic, recording the history of the Proto-Paleo Tethys Ocean from its origin to its annihilation, and are important vehicles for understanding the evolution of the Proto-Paleo Tethys Ocean and important mineralisation events. In this paper, the major and trace element geochemistry, geochronology and isotope studies were carried out on the Paleozoic-Mesozoic magmatic rocks of different blocks in the West Kunlun, the magmatic ages, geochemical characteristics and tectonic background were explored in depth. The main insights and results obtained are as follows.

The Early Cambrian magmatic rocks are exposed in Zankan, Laobing, Hekelaner and Chahekou area, and are mainly composed of moderate-acidic intrusive rocks, with diorite-gabbro enclaves, which have been dated by zircon SHRIMP and LA-ICP-MS U-Pb age of 348 ~ 354 Ma. The Early Cambrian intrusions in the Tianshuihai Terrane are mainly I-type granites, which are the product of dehydration and melting under low-pressure conditions of a low-K basalt source, while the diorite-gabbro enclaves are the product of magmatic mixing between mantle-derived magmas and granites. Most of the granite samples in the Tianshuihai Terrane show Nd-Hf isotope decoupling, probably due to unbalanced melting in the source region. The isotopic signature of granites in the Tianshuihai Terrane implies the existence of two types of lower crust in the Tianshuihai Terrane.

The Early Palaeozoic magmatism in the Tianshuihai Terrane may have been influenced by southward subduction of the Proto-Tethys Ocean. According to the palaeogeographic reconstruction and the age distribution of magmatism, the TSHT and the SKT were assumed to have been distributed between the northern margin of the East Gondwana continent and the Tarim Block. Thus, two branches of the Proto-Tethys Ocean coexisted, which was represented by the Kangxiwa Fault and Kudi ophiolite belt in the Early Palaeozoic. Furthermore, depending on the chronological statistics of micro- blocks in the northern margin of the East Gondwana continent, the subduction of the Proto-Tethys Ocean is diachronous, which initiated in the northwestern region of the East Gondwana continent and propagated eastwards.

The Carboniferous Wuluate Formation consists mainly of pillow basalts interbedded with minor rhyolites, and is a bimodal volcanic assemblage. LA-ICP-MS zircon U-Pb dating of the basalts and rhyolites of the Wuluate Formation ranges from 348 to 354 Ma, representing the age of eruption of the basalts and rhyolites. The basalts were formed by partial melting of spinel lherzolite to a degree of 5 to 10% and the source area shows a characteristic depleted mantle, which is consistent with the zircon ε_Hf(t) values (+7.3 to +11.8). The modelling results show that the basalts have undergone crystalline differentiation of olivine, monazite, hornblende, plagioclase and magnetite to a degree of 40% to 95%. The rhyolites of the Wuluate Formation are developed by partial melting of lower crustal material under low pressure conditions. Combining geochemical data with regional geological features, this paper concludes that the volcanic rocks of the Wuluate Formation were deposited in a back-arc basin associated with the northward subduction of the PalaeoTethys Ocean.

The volcanic rocks of the Permian Qipan Formation are a suite of niobium-enriched basalts with LA-ICP-MS zircon U-Pb ages of 287 ± 4.6 Ma. The basalts of the Qipan Formation were formed by dynamic melting of spinel lherzolite to a degree of 5%-7% and underwent olivine, monazite and plagioclase crystallographic differentiation to a degree of 0% to 50%. Zircons from basalts show the ε_Hf(t) values of +0.2 to +11.5 are suggestive of a heterogeneous sourceand possible incorporation of enriched components. The abrupt transition from a depleted mantle in the Carboniferous, influenced by the subduction of the Palaeo Tethys Ocean, to a Permian enriched/primitive mantle in the West Kunlun area which was likely influenced by the incorporation of deep mantle material and the possible existing mixing of Oceanic Island Basalt (OIB) + Island Arc Tholeiite (IAT) ± Mid-Ocean Ridge Basalt (MORB) components in its source area. It is hypothesised that the basalts of the Permian Qipan Formation may be the result of interaction between the subduction zone and a mantle plume.

Triassic magmatism is widely distributed in the Southern Kunlun Terrane and the Tianshuihai Terrane. The LA-ICP-MS zircon U-Pb ages of the Shazigou and Saluoyi pluton range from 213.7 ± 2.6 to 237.3 ± 2.8 Ma. The Shazigou and Saluoyi pluton are products of strong mixing of crustal-derived granitic magmas with mantle-derived basaltic magmas, and have undergone different degrees of plagioclase and K-feldspar crystal differentiation, respectively. Combined with regional data, the Triassic granites may have been developed in a Palaeo Tethys Ocean subduction to collision background.

Combining the results of this paper with regional geological data, it is concluded that the West Kunlun Orogenic Belt developed a series of important Paleozoic-Mesozoic magmatic events, which recorded the evolutionary history from the initial subduction of the Proto-Tethys Ocean to the closure of the Paleo-Tethys Ocean, and formed a series of volcanic-sedimentary metamorphic iron ore, sedimentary manganese ore, pegmatitic lithium-beryllium ore and so on, during the complex and long ocean-land transition and accretion-collision process.