柴北缘赛什腾中二叠世辉长岩成因及其对宗务隆洋盆俯冲时限的制约

对侵位于柴北缘构造带西段达肯大坂岩群中的赛什腾钙碱性系列辉长岩进行了地球化学、年代学研究,结果显示,该辉长岩具有低SiO₂、高Al₂O₃、富MgO及FeO^T、贫ALK的特征,且富集LILE元素、亏损HFSE元素,稀土元素配分曲线整体表现为轻稀土元素相对富集、重稀土元素平坦且呈右倾型配分模式,并且低Zr和Zr/Y,指示赛什腾辉长岩形成于活动大陆边缘岛弧环境。锆石LA-ICP-MS U-Pb结晶年龄为271±3 Ma。赛什腾辉长岩原岩岩浆是地幔尖晶石二辉橄榄岩部分熔融的产物,在演化过程中经历了弱的斜长石以及较为明显的单斜辉石的分离结晶作用,并在上升侵位过程中遭受了弱的中上地壳的同化混染作用。结合区域构造演化史以及同时代岩浆岩的年代学和地球化学特征,认为赛什腾辉长岩是中二叠世早期宗务隆洋壳向欧龙布鲁克地块南向俯冲的产物,且宗务隆构造带西段俯冲-碰撞事件明显早于东段,而东西两段俯冲-碰撞时限的差异表明宗务隆洋盆存在西早东晚的"剪刀式闭合"的可能。

Petrogenesis of Middle Permian gabbro in Saishiteng Mountain of the northern Qaidam Basin and its constraint to the time of Zongwulong Ocean subduction

The gabbros were emplaced in the Dakendaban Group in the northwest of Saishiteng Mountain, west section of the tectonic belt of northern margin of Qaidam Basin. They are characterized by low SiO₂, high Al₂O₃, rich MgO and FeO^T but poor ALK, thus belonging to subalkalic calc-alkaline series. Meanwhile, The rocks are enriched in LILE and depleted in HFSE. The chondrite-normalized REE patterns show right-inclined shape with LREE enrichment and flat HREE. The rocks are characterized by low Zr and Zr/Y ratio. The authors consider that the gabbros emplaced in the Saishiteng Mountain were formed in the island-arc environment of an active continental margin. LA-ICP-MS zircon U-Pb dating of gabbros yielded the formation time of 271±3 Ma. Comprehensive analysis shows that the original rocks of gabbros can be considered as a product of partial melting of spinel lherzolite in the lithospheric mantle, and the magma might have undergone fractional crystallization of strong clinopyroxene and weak plagioclase during its evolution, which however was slightly assimilated and contaminated by mid-upper crust materials in the emplacement process. In combination with the tectonic evolution of the tectonic belt of northern margin of Qaidam Basin and the geochronological and geochemical characteristics of contemporary intrusive rocks, it is concluded that the gabbros were produced in the process when the southward subduction of Zongwulong oceanic crust toward Oulongbuluke block in early Middle Permian. The authors hold that the subduction-collision event of the western part of Zongwulong structural belt was obviously earlier than that of its eastern segment, and the difference of the subduction-collision time shows that the Zongwulong Ocean were probably characterized by scissor-like closure early in the west and late in the east.