Protolith and tectonic implications of the No. Ⅳ pluton in Xiarihamu area, East Kunlun, Qinghai Province

Abstract

东昆仑造山带西段夏日哈木地区IV号岩体是镁铁-超镁铁质侵入体还是蛇绿岩单元仍存争议。本文对IV号岩体开展了野外地质特征、岩相学、地球化学及锆石U-Pb年代学的研究。IV号岩体主体为斜长角闪岩，呈透镜状产在片麻岩中。斜长角闪岩主要由角闪石、斜长石组成，含少量石英、黑云母、榍石和铁钛氧化物。具有基性岩的化学成分，属于拉斑玄武岩系列岩石，稀土配分模式中Eu显示正异常（δEu=1.3~1.6），ε_Nd（t）值为+3.5~+5.3，推测其原岩可能为堆晶辉长岩，明显不同于含铜镍矿的Ⅰ号镁铁-超镁铁岩体，与该地区出露的榴辉岩（退变榴辉岩）可以对比。斜长角闪岩中的锆石具有弱环带，采用LA-MC-ICP-MS方法得出的²⁰⁶Pb/²³⁸U加权平均年龄为421±4Ma （MSWD=2.8），Th/U比值为0.01~3.26，指示锆石为变质-深熔成因，代表了斜长角闪岩的形成时代。夏日哈木地区的斜长角闪岩和该地区出露的榴辉岩及榴闪岩的原岩可能都是早期形成于陆壳环境的基性岩（大陆边缘的基性侵入岩或喷出岩），在430Ma左右发生陆壳深俯冲作用时形成榴辉岩，420~410Ma榴辉岩发生不同程度退变形成榴闪岩和斜长角闪岩。由此推测，IV号岩体的构造属性与该地区的榴辉岩属性相同，形成于陆壳环境，其原岩不属于蛇绿岩单元。斜长角闪岩的出现指示了该地区原特提斯洋壳已俯冲消亡，深俯冲的陆壳开始折返，这对认识东昆仑早古生代的构造演化和铜镍矿形成的地质背景具有重要意义。;Whether the protolith of the No. IV pluton in Xiarihamu area in the western part of the East Kunlun Orogenic Belt belongs to a mafic-ultramafic intrusive body or a component (gabbro) of the ophiolitic unit remains debated. Here, we reported the occurrence and the petrographic, geochemical and zircon U-Pb geochronological analysis results of this pluton to clarify the above disagreement. Petrographically, this pluton is an amphibolite block occurring as lens within a set of gneisses. The amphibolite rocks mainly consist of hornblende and plagioclase, with minor quartz, biotite, titanite and Fe-Ti oxide. The samples collected from amphibolite pluton have SiO₂ contents varying from 43% to 50% with Mg^# values from 42 to 73, and chondrite-normalized REE patterns show they have a positive Eu anomaly (δEu=1.3~1.6), implying their protoliths were cumulative mafic rocks and belong to tholeiitic series. Furthermore, the amphibolite rocks have positive whole-rock ε_Nd(t) values of +3.5~+5.3, similar to those of the eclogite or retrograded-eclogite and different from those of the No. I Ni- and-Cu-bearing mafic-ultramafic rocks in Xiarihamu area. Zircon grains from the amphibolite pluton show weakly zonation on their CL images with Th/U ratios varying from 0.01 to 3.26, suggesting they have a metamorphic-anatexis origin rather than a magmatic one. The zircon grains yield a ²⁰⁶Pb/²³⁸U weight mean age of 421±4Ma (MSWD=2.8), which represent the formation time of the pluton. In other words, the formation of this pluton can be divided into two stages: the previous cumulate gabbro underwent a high-pressure metamorphism and formed eclogite in the subduction stage, then followed by a retrograde stage which led to the transformation from eclogite to amphibolite. To be more clearly, the protoliths of the amphibolite, eclogite and retrograded-eclogite in the Xiarihamu area are mafic rocks (mafic pluton or eruption) formed in a continental margin, they turned into eclogites during a deep continental subduction approximately at 430Ma, and finally became retro-eclogite and amphibolite with various retrograde metamorphism at 420~410Ma. Therefore, the protolith of the No. IV pluton is not a gabbro component of the ophiolitic unit. Our studying results indicate that the evolution of this region experienced a closing of Paleozoic Proto-Tethys oceanic basin and then an exhumation of subducted continental crust, which is important for understanding the Paleozoic tectonic evolution of East Kunlun Orogen and the geological setting of the Cu-Ni mineralization.