【作者】 陈亮；

【导师】 孙勇；

【作者基本信息】 西北大学 ， 矿物学、岩石学、矿床学， 2003， 博士

【摘要】 柴北缘沙柳河、鱼卡河、锡铁山和胜利口的榴辉岩类样品的地球化学分析显示它们的原岩的主体是与洋壳有关的火山岩，其中包括岛弧拉斑玄武岩、洋岛火山岩和MORB型火山岩，样品经历了不同程度的地壳混染作用，也不排除可能少量存在产于大陆环境的火山岩。本文首次在榴辉岩中发现了稀土元素四重效应，显示在与柴北缘榴辉岩类相关的俯冲／变质环境中普遍存在开放体系下的含挥发份流体作用，四重效应的存在得到了样品高场强元素比值异常的支持，这些比值特征明显与洋壳稳定俯冲过程榴辉岩应当获得的高于球粒陨石的比值特征不同。锡铁山地区的部分样品中存在重稀土的异常富集，可能指示了原岩部分熔融过程的存在。 已发表年代学数据的综合和对比显示，柴北缘地区的超高压变质事件系统晚于区域性的陆—陆碰撞过程，加之榴辉岩与其围岩之间不存在明显的成因联系，本研究提出了两种柴北缘超高压变质事件的模型：弧—陆碰撞模型与蛇绿岩俯冲模型。在弧—陆碰撞模型中，这些榴辉岩的原岩在陆壳进行洋内俯冲时被从洋内岛弧和／或洋岛刮削下来卷入了俯冲板片的陆壳基质中，与至少部分陆壳共同经历了峰期变质过程，由于弧—陆碰撞往往发生在陆陆碰撞之前，榴辉岩的峰期变质年龄会略小于陆—陆碰撞的时代。 如果仰冲至陆壳上的蛇绿岩随典型陆壳岩石—同参加俯冲，那么部分原岩为洋壳的榴辉岩了指示了陆壳而不是洋壳的俯冲，当榴辉岩中存在显著大于峰期变质时代的原岩年龄时，这种情况才可以被明确地鉴别出来；否则的话，对于原岩为洋壳的榴辉岩，这种蛇绿岩的俯冲成为一种难以排除的可能性。上述两种模型均与陆壳俯冲和超高压变质作用有关。 综合地质对比显示，古亚洲洋南部和相邻的秦祁昆洋区域在寒武—奥陶纪期间的演化以小洋盆，中小型陆块相间的复杂构造格局为特征，在可能与晚泛非运动有关的整体汇聚背景下普遍出现强烈的岛弧型岩浆作用和活跃的岛弧拼贴、陆壳增生过程。上述阶段和上述范围内集中形成了多条超高压变质带，这种这指示了古亚洲洋及邻区的独特构造背景与超高压变质事件之间的成因联系。 在综合古地磁和地球物理资料的基础上，本文首次提出快速俯冲过程是一种驱动陆壳进行俯冲的因素，在超高压变质带的形成过程中，快速俯冲过程可能起到了某种程度的驱动作用。在苏鲁—大别、西喜马拉雅、可能还有Kokchtav，有证据显示在陆—陆碰撞之前存在异常快速的陆块运动过程。这说明，海洋板片的拖曳不是形成超高压变质岩石的唯一驱动力。更多还原

【Abstract】 Geochemistry of eclogites from Shaliuhe, Yukahe, Shitieshan and Shenglikou area of North Qaidam, West China reveals that the majority of their protoliths are geochemically distinguished with mafic igneous rocks of oceanic affinity, including IAT, OIB and MORE. These rocks expirenced assimilation of continental crust to variant extents, and the occasional involvement of igneous rocks of continental affinity could not be excluded.This work firstly reprot tetrade effect of REE in eclogitic rocks, indicating the presence of active volatile- bearing fluid-rock interactions under open system in subductional process. Previously observed connection between tetrade effect and systemic low Nb/Ta, Zr/Hf, Y/Ho ratios also presents. However, the Nb/Ta ratios lower than chondrite value (~17) in eclogites are inconsistent with previously proposed Nb/Ta high in subducted oceanic crust to complement the mass balance of Nb and Ta in the evolution of continental crust.Anomalous enrichment of heavy REE are observed in some of samples from Xitieshan, which may resulted from partial melting with garnet as extraction residue during subduction.Comparation on published age-dating shows the ultra-high pressure maramorphism in North Qaidam occurred during 450~500Ma beyond errors and systemiclly antedated the regional continent- continent collision at 440Ma.Based on above-mentioned results and interpretations, this work hereby propose 2 different models on the early paleozoic evolution in North Qaidam, arc-continent collision and ophiolite-subduction. During arc-continent collision, protoliths of eclogites were scratched off from oceanic island-arc and oceanic islands and carried downwards by subducting continental slab. They expirence HPM or UHPM together in matle depth then exhumed buoyantly, and tectonicly extruded probably with the assistance of normal faults in crust level. The ophiolite-subduction is firstly proposed by this work, which supposes part of protoliths of present eclogites form North Qaidam was formerly ophiolite obduted onto continental edge and involved into the subduction together with continental slab. It is worth noting that only if the earlier formation ages of ophiolite are positively recognized, the ophiolite-subduction scenario could be confirmed, otherwise this may lead to ambiguity for oceanic eclogites cropped out with typical crustal rocks like grantic gneiss and marbles.The arc-continent collision and ophiolite-subduction are both related to continental suduction and probably UHP metamorphism, and neither of them has a position inMaruyama’s (1996) classification, as suggests that the UHPM are not uniquely assocated with continent-continent collision.A combination of regional geology and geochemistry indicates the Cambrian and Ordovician evolution in the south of central Asian zone is charactelized by complex collages between continental blocks with island arcs under the compressional background perhaps related to the final convergence within Gondwanaland, the so-called late Pan-african movement. This may provide an explanation for the relatively mass occurance of UHPM belts within central Asian zone and adjacent Qinling-Qilian-Kunlun zones.With the constraints from published paleopmagnetic data on motion rates of continental blocks prior to their collision, this work firstly proposed the ultra-fast subduction rate could assist the underthrust of low-density continental crust into mantle depth to some extents. The ultra-fast motion is cofirmed before collision between North China and Yantze, Eurasia and India, and may also be appliable to that related to Kokchtav UHPM event. The significance of ultra-fast motion in UHPM formation lies in that the slab pull could not be the unique mechanism to drive continental crust to subduct.更多还原