Early Silurian tholeiitic-boninitic Mailisu ophiolite, South Tianshan, Kyrgyzstan: a geochemical record of subduction initiation

Ophiolite assemblages of the South Tianshan fold-and-thrust-belt (STS) track the sea floor dynamics of the late Cambrian to Carboniferous Turkestan Ocean in the western Central Asian Orogenic Belt. We interpret new geochronological and geochemical data for the Mailisu ophiolite from western Kyrgyzstan as evidence for an early Silurian incipient subduction zone outboard the South Chatkal Arc of the Middle Tianshan. Igneous zircons from a leucogabbro yielded a U-Pb age of 440 ± 6 Ma indicating one of the oldest sea floor fragments in the STS. Metabasalt samples have N- to E-MORB trace element patterns and initial ε_Nd values of ~5 to 9 support melting of depleted asthenosphere comprising a minor OIB component. Cross-cutting diabase dykes show geochemical characteristics of high-Ca basaltic boninites. Their low REE abundances and highly fractionated middle-heavy REE patterns indicate sources that were more melt-depleted than those of the metabasalts. Concave upwards LREE patterns are compatible with a residual peridotite source, fertilised by small-degree OIB-like melts with ε_Nd of ~5 to 6. Positive Ba, Sr, and Pb anomalies support the involvement of slab-derived hydrous fluids that probably facilitated the melting of the residual mantle. The association of MORB-like metabasalts with younger dykes of boninite affinity suggests subduction initiation for the origin of the Mailisu ophiolite. Our data elucidate one of the major yet little understood episodes in the Paleozoic history of the Turkestan Ocean. Future identification of similar rock assemblages will be helpful to understand the insufficiently constrained history of ocean floor subduction in the Paleozoic Oceans of the western CAOB.

中亚造山带（Central Asian Orogenic Belt, CAOB）西部的南天山褶皱冲断带（South Tianshan fold-and-thrust-belt, STS）所产出的蛇绿岩组合，记录了晚寒武世至石炭纪突厥斯坦洋（Turkestan Ocean）的海底动力学过程。我们对吉尔吉斯斯坦西部麦里苏蛇绿岩（Mailisu ophiolite）的新年代学与地球化学数据进行解译，认为其代表了中天山南查特卡尔弧外侧早志留世初始俯冲带的证据。采自淡色辉长岩的火成锆石获得了440 ± 6 Ma的铀-铅（U-Pb）年龄，这是STS中迄今已知最古老的海底岩石碎片之一。变玄武岩样品呈现正常型至富集型洋中脊玄武岩（N- to E-MORB）的微量元素配分模式，初始ε_(Nd)值约为5至9，指示亏损软流圈发生熔融，且该软流圈含有少量洋岛玄武岩（Ocean Island Basalt, OIB）组分。穿切围岩的辉绿岩脉具有高钙玄武质玻安岩的地球化学特征，其稀土元素（Rare Earth Element, REE）总量偏低，且中重稀土配分模式高度分馏，表明其源区比变玄武岩的源区更亏损熔体。向上凹的轻稀土配分模式与被ε_(Nd)约为5至6的低程度洋岛玄武岩型熔体交代的残余橄榄岩源区相吻合。显著的钡（Ba）、锶（Sr）和铅（Pb）正异常，指示俯冲板片析出的含水流体参与了熔融过程，该流体可能促进了残余地幔的熔融。洋中脊玄武岩型变玄武岩与具有玻安岩亲缘性的年轻辉绿岩脉的共生组合，表明麦里苏蛇绿岩的成因与俯冲起始作用相关。我们的数据阐明了突厥斯坦洋古生代演化历史中一项重要却鲜为人知的地质事件。未来对同类岩石组合的识别，将有助于厘清中亚造山带西部古生代洋盆海底俯冲过程中尚未得到充分约束的演化历史。