西昆仑东段苏巴什洋向北俯冲:来自早—中二叠世火山岩的证据

西昆仑东段苏巴什蛇绿构造混杂岩带南侧卡拉勒塔什群以大面积分布的酸性和中基性火山岩为特征,本次对卡拉勒塔什群弧火山岩代表性的岩石组合进行了LA-ICP-MS锆石U-Pb年龄、地球化学及锆石Lu-Hf同位素研究。研究结果表明,LA-ICP-MS锆石U-Pb测年获得酸性晶屑凝灰岩、蚀变玄武岩²⁰⁶Pb/²³⁸U加权平均年龄为(284.2±1.6) Ma和(262.6±2.0) Ma,表明研究区卡拉勒塔什群火山岩形成于早—中二叠世。卡拉勒塔什群火山岩具有富铝、钠、铁,富集大离子亲石元素K、Rb、Ba和亏损高场强元素Sr、Ta、Nb、Ti的地球化学特征。其中,基性火山岩属钙碱性-拉斑玄武岩系列,岩石稀土元素配分模式接近大洋拉斑玄武岩,Nd/Th和La/Nb比值为8.91~13.76和0.39~2.28,Lu-Hf同位素ε_Hf(t)值为-0.15~4.95,表现为地幔物质来源,但加入了地壳组分。酸性火山岩属于钙碱性系列,相对亏损P和Zr元素,Nd/Th和La/Nb比值为1.92~4.10和2.52~3.39,Lu-Hf同位素ε_Hf(t)值分别为0.94~3.78和8.26~12.45,二阶段模式年龄分别为1.07~1.25 Ga、0.51~0.78 Ga,表明酸性火山岩物质来源为古老地壳和新生地壳物质重熔后的混合物。卡拉勒塔什群总体地球化学特征表现为岛弧环境。卡拉勒塔什群岛弧火山岩与北侧苏巴什蛇绿构造混杂岩带在形成时代、空间分布以及基性岩地球化学特征均表现成对关系,与苏巴什蛇绿构造混杂岩带内发育的硫磺达坂砂岩组深水复理石建造共同构成造山带沟-弧-盆体系,表明苏巴什洋盆由南向北的俯冲极性,说明苏巴什蛇绿构造混杂岩形成于岛弧偏向于海沟的弧前盆地构造背景。

Northward Subduction of Subashi Ocean in Eastern Part of the West Kunlun Orogen: Evidence from Early-Middle Permian Volcanic Rocks

The Karaletash Group in the southern side of the Subashi ophiolite belt is characterized by widespread distribution of felsic and intermediate-felsic volcanic rocks. In this study, we conducted LA-ICP-MS zircon U-Pb geochronology, geochemistry and zircon Lu-Hf isotope analyses on representative rock assemblages of the Karaletash Group. LA-MC-ICP zircon U-Pb dating shows that felsic cystal tuff and basalt were formed from the early-Middle Permian (284.2±1.6 Ma and 262.6±2.0 Ma). The volcanic rocks are rich in Al, Na and Fe, and are enriched in large ion lithophilic elements (LILEs, e.g., K, Rb and Ba) and depleted in high field strength elements (HFSEs, e.g., Sr, Ta, Nb and Ti). Compositions of the mafic volcanic rocks fall into the calc-alkaline-tholeiite field, with chondrite-normalized REE patterns similar to oceanic tholeiite, with Nd/Th = 8.91-13.76 and La/Nb = 0.39-2.28. The zirconε_Hf(t) value are -0.15 to 4.95, which indicates that the mafic volcanic rocks were mainly originated from the mantle with some crust material input. Compositions of the felsic volcanic rocks fall into the calc-alkaline field, with relatively negative anomalies of P and Zr, and with Nd/Th = 1.92-4.10 and La/Nb = 2.52-3.39. The zircon ε_Hf(t) value are 0.94-3.78 and 8.26-12.45, which together with two-staged Hf model ages (1.07-1.25 Ga and 0.51-0.78 Ga), indicating that the felsic volcanic rocks were mainly originated from the re-melting of ancient crust materials and juvenile crust. Geochemical characteristics of the Karaletash Group resemble rocks formed in island arc environment. The Karaletash Group and Subashi ophiolite belt were closely related in their formation ages, the spatial distribution and the geochemical characteristics of the mafic rocks. Combined with deep-water flysch of Liuhuangdaban Formation sandstone in the Subashi ophiolite belt, these rock units constitute the orogenic-trench-arc-basin system. All these indicate that the subduction of the Subashi basin was north-dipping, and that the tectonic background was probably a forearc basin.