论文部分内容阅读
锆石的阴极发光图像观察发现,柴北缘鱼卡河两种类型超高压榴辉岩-粗粒多硅白云母榴辉岩和细粒块状榴辉岩的锆石均呈半自形柱状形态,部分出现核-边结构.核部为继承性锆石,边部及无核颗粒显示扇状、面状和斑杂状等典型变质锆石的结构特征.LA-ICP-MS锆石原位微量元素分析揭示锆石核部具有明显高的稀土总量、重稀土富集型稀土配分模式以及高的Th/U比值,为原岩锆石的残留;边部及无核颗粒测点的稀土总量降低,稀土配分曲线上重稀土富集程度、Eu的负异常程度都明显减小,部分甚至出现Eu的正异常,指示矿物组合中石榴石的存在和斜长石的缺失,表明其形成在榴辉岩相条件下.LA-ICP-MS微区定年给出两类榴辉岩近乎一致的变质年龄和原岩年龄分别为(436±3)和(431±4)Ma以及783～793和748～759Ma.取两种榴辉岩变质年龄的加权平均值(434±2)Ma代表鱼卡河超高压榴辉岩的变质年龄.该研究表明榴辉岩的原岩为形成在750～800Ma的新元古代火山岩,在早古生代时期经历超高压变质作用,其间存在300Ma的年龄差距,远大于洋壳从洋中脊处形成到海沟处俯冲消减所需要的最大年龄(200Ma),结合新进获得的该剖面两类榴辉岩直接围岩的变质年龄(431±3)Ma和(432±18Ma)以及野外地质特征,提出鱼卡河榴辉岩是先期就位地壳的新元古代火山岩在古生代随陆壳一起俯冲遭受超高压变质而成,(434±2)Ma应代表本区大陆深俯冲的时代.
The results of the cathodoluminescence images of zircon show that the zircons of two types of ultrahigh-pressure eclogite-coarse-grained dolomite eclogites and fine-grained eclogite from the Yuka River in the northern Qaidam Basin show semi-self-columnar morphology , Some of them appear nuclear-edge structure.The core features of inherited zircons, marginal and non-nuclear grains show the structural features of typical metamorphic zircons, such as fan-shaped, planar and mottled.LA-ICP-MS zircon in situ trace elements Analysis revealed that zircons had significantly higher REE, REE-rich RE patterns and high Th / U ratios, which were the original zircon residues; the total amount of rare earths at the edge and non-core particles , The degree of REE enrichment on the REE curve and the negative anomalies of Eu are obviously reduced. Some of them even show the positive anomaly of Eu, indicating the presence of garnet in the mineral assemblage and the absence of plagioclase, indicating that they are formed in the Under the pyroxene facies, the dating of LA-ICP-MS shows that the metamorphic ages and protolith ages of two types of eclogites are (436 ± 3) and (431 ± 4) Ma and 783 to 793 and 748 ~ 759Ma. Take the weighted average of the ages of the two eclogites (434 ± 2) Ma for the UHP eclogite of the Yuka River The study shows that the protolith of the eclogite is a Neoproterozoic volcanic rock formed at 750 ~ 800 Ma, experienced the process of UHP metamorphism during the Paleozoic Era, with an age difference of 300 Ma, which is much larger than that of the oceanic crust The maximum age (200Ma) required for the subduction of the seafloor at the trench, combined with the newly metamorphic age (431 ± 3) Ma and (432 ± 18Ma) of the direct surrounding rocks of the two types of eclogites in this section and the geological features in the field, The Yukahe eclogite is a prehistoric crustal Neoproterozoic volcanic rock that was subducted by the UHP metasomatism during the Paleozoic. The (434 ± 2) Ma should represent the age of the deep subduction of the mainland.