【作者】 李金平；

【导师】 梅冥相； 张建新；

【作者基本信息】 中国地质大学（北京） ， 矿物学、岩石学、矿床学， 2010， 硕士

【摘要】 在北祁连HP/LT变质带中,榴辉岩相变质作用普遍记录在变质基性岩中。已有的地质和地球化学研究认为榴辉岩是洋壳俯冲的产物。最近的研究显示榴辉岩相矿物组合不仅限定在基性岩中,在榴辉岩的围岩——变沉积岩中,也发现有榴辉岩相矿物组合。本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石年代学和Hf同位素方面的一些研究结果,这些研究结果为北祁连早古生代构造演化提供新的约束。在北祁连的百经寺地区,变沉积岩包裹有榴辉岩透镜体,局部可见榴辉岩和变沉积岩互层产出。变沉积岩主要为石榴蓝闪石（绿辉石）多硅白云母石英片岩、硬绿泥石石榴多硅白云母石英片岩和变硅质岩等,峰期温压条件为T=450-520℃,P=1.9-2.3 GPa,与相邻榴辉岩的温压条件一致。全岩地球化学分析显示这些变沉积岩的原岩是不成熟的,可能产出在大陆岛弧或大陆边缘环境中。10个变硅质岩样品在页岩标准化图解上显示：轻重稀土元素分异不明显,（La/Yb）N在0.48-0.82之间,Ce/Ce*在0.91-1.19之间,无Ce异常,无Eu异常；明显不同于太平洋的硅质岩（具有明显的Ce负异常,Ce/Ce^*<<1）以及大西洋的硅质岩（中等程度的Ce负异常）,而与产于大陆边缘的南部高纬度硅质岩的特征相似。4个变沉积岩中的碎屑锆石年龄主要集中在480-560 Ma和1800 Ma左右。Hf同位素特征显示,480-560 Ma年龄的碎屑锆石的¹⁷⁶Hf/¹⁷⁷Hf比值较高,εHf（t）值为正,部分接近亏损地幔εHf值（16.4）,U-Pb年龄与单阶段、双阶段模式年龄相近,表明这些锆石没有经历明显的再循环演化过程,与古祁连洋的开启或弧增生活动有关；年龄为1800 Ma的碎屑锆石¹⁷⁶Hf/¹⁷⁷Hf值相对较低,εHf（t）值大多在零值附近,以老的单阶段模式年龄以及双阶段模式年龄为特征,结合区域资料,这些锆石应来自相邻的前寒武纪陆块。综合地质、地球化学及年代学资料,这些榴辉岩相变沉积岩的原岩不是形成于典型的洋壳环境,而可能形成于洋壳俯冲带之上的弧前盆地,并在北祁连早古生代洋壳俯冲过程中,通过俯冲（构造）剥蚀作用被运移到俯冲带中,俯冲到60-70 km深处,发生榴辉岩相变质作用。更多还原

【Abstract】 In the HP/LT belt of North Qilian Mountains, eclogitic metamorphism is always recorded in the basaltic rocks and eclogites were previously regarded as the product of the sea floor subduction. However, present study reveals that elcogitic minereal assemblages are not only recorded in the basaltic rocks, but also in the metasedimentary rocks, which are the host rock of eclogites.In this paper, we present new Petrological, geochemical and age data for metasedimentary rocks from Baijingsi area, North Qilian Mountains, in which typical early-Paleozoic ophiolitic sequences and HP/LT metamorphic rocks have been previously recognized. The studied metasedimentary rocks contain eclogitic facies assemblages reflecting PT conditions of 450-520℃and 1.9-2.3 GPa, consistent with those of adjacent eclogites.Geochemical data indicate that the precursor rocks of the metasedimentary rocks were immature sediments formed in continental margin or island arc environments. Zircons from 4 eclogitic metasedimentary samples were separated for U-Pb and Lu-Hf dating. The data demonstrate detrital zircon ages concentrate in 480-560 Ma and around ca.1800 Ma, suggesting that they derived from the mixture of Paleoproterozoic continent basement and Late Neoproterozoic-early Paleozoic oceanic materials, which were probably accreted into active continental margin during subduction. These data implies that subduction erosion of the active continental margin occurred, as these sediments, formed in the upper plate (fore-arc basin or accretionary prism), were transported into the subduction zone to 60-70 km depth prior to their exhumation.更多还原