东昆仑诺木洪地区榴辉岩的发现及其对东昆仑北早古生代俯冲带演化过程的制约

As the important part of the Central Orogenic Belt, the East Kunlun Orogenic Belt has always been a hot topic. Ecolgite and Garnet-Amphibolite are recognized recently in Nuomuhong region in East Kunlun Mountains. The lenticular eclogite is distributed in metamorphic rocks of Paleoproterozoic Baishahe Formation and Neoproterozoic granite gneiss. The eclogite facies rock association is composed of Eclogite and Garnet-Amphibolite, primarily composed of garnet+omphacite and garnet+amphibole+plagioclase+quartz respectively. Previous research is reflected the existence of an early Paleozoic subduction zone in the northern part of East Kunlun Tectonic Belt, but the specific researches have been carried out rarely on the early Paleozoic subduction zone. According to this application, we will carry out a detailed study in situ U-Pb dating of zircon from this rocks and a detailed study on petrography, mineral chemistry in order to reconstruct the metamorphic P-T-t path of the eclogite by software Perple_X to determine their age, attribute and metramorphic history. The petrography, whole rock geochemistry and isotope geochemistry characteristics of the surrounding rock will also be carried out for demonstrating the geological and geophysical processes responsible for deep subduction and exhumation of continental crustal material, restrain the depth, attitude and space distribution and tectonic evolution of the northern part of East Kunlun Tectonic Belt.

东昆仑造山带作为中央造山带重要的部分，历来是地质学家研究的热点。近年来，在北昆仑地块发现的多处榴辉岩为研究昆中断裂附近俯冲带的类型、构造演化历史及造山过程提供了良好的地质条件。前人众多的研究结果均反映在东昆仑北构造带内存在有早古生代的俯冲带，但鲜见对这一早古生代俯冲带开展的针对性研究。因此，本项目拟选择诺木洪地区这一新发现的早古生代榴辉岩、退变质榴闪岩及其围岩为研究对象，开展高精度的锆石U-Pb同位素年代学和系统的岩石学、地球化学和同位素研究。结合前人在东昆北构造带东西两侧发现的榴辉岩研究基础，以期进一步确定东昆仑北构造带内早古生代俯冲带的存在，并精确限定俯冲开始、碰撞以及折返的时间；通过对上述研究对象的原岩岩石成因和构造背景的探讨，揭示该时期的构造-岩浆作用过程及动力学机制，为东昆仑北构造带早古生代的地质构造演化提供新的依据。

青藏高原北部东昆仑造山带记录了原特提斯洋北支长期的增生和碰撞历史。在这里，本项研究报告了在东昆仑造山带东部诺木洪新发现的榴辉岩的岩石学、地球化学和地球化学数据以及变质P–T条件，这为原始特提斯洋的最终闭合提供了新的限制。榴辉岩主要由石榴石、单斜辉石（绿辉石和低钠单斜辉石）、角闪石、斜长石组成，含少量斜方辉石、石英和副金红石/钛铁矿、钛矿、磷灰石和锆石。根据岩相观察和热气压计算，确定了五个变质阶段：（1）P–T条件>14.0kbar/470–506ºC的进变质阶段（M1）；（2）P–T条件为21.5–26kbar/480–570°C的峰值阶段（M2）；（3）高压麻粒岩相的早期退变质阶段（M3）；（4） 随后的中压麻粒岩相逆行阶段（M4），温度为860–900ºC；斜长角闪岩相晚期（M5），P–T条件<6.2 kbar/710–730ºC。这些P–T约束构造了一条顺时针P–T路径，其特征是从榴辉岩相的峰值P阶段到麻粒岩相的峰值T阶段减压时轻微加热。全岩地球化学资料表明，榴辉岩具有洋中脊玄武岩（MORB）样原岩，表明它们是洋壳的增生碎片。榴辉岩的SHRIMP锆石分析得出加权平均206U/238Pb年龄为418±1.8 Ma（n=31，MSWD=1.1），解释为峰值到退变质的时间。我们的新数据表明，原特提斯洋的东昆仑洋壳在约418 Ma的碰撞造山作用中经历了早期俯冲-增生到约80 km深度，然后剥露到中地壳水平。结合区域资料，约430-410 Ma的俯冲碰撞事件标志着原特提斯洋的最终