The Neoproterozoic metamorphism of North Qaidam UHPM belt, western China : Constrain from petrological study and monazite dating of paragneiss

同一造山带中所包含的多期造山作用信息是研究不同时代区域构造演化的重要依据,对理解不同时期造山过程中岩石组合及其地球化学演化有重要的指示意义。但由于晚期造山作用往往会部分或者完全抹除岩石中保存的早期造山作用信息,使得对记录多期造山作用的岩石中早期造山带变质作用及年代学信息的研究变得十分困难。独居石为一种副变质岩中的常见副矿物,由于其具有很高的U-Th-Pb体系封闭温度和对流体及变质温压条件的敏感性,使其可以记录多期造山过程中丰富的年代学信息。电子探针独居石原位化学定年方法使得年代学信息与岩石中矿物学信息及变质反应相联系,从而得到不同时期岩石记录的P-T-t轨迹。我们利用独居石电子探针原位U-Th-Pb定年手段与岩石学研究相结合的方法,在柴北缘早古生代加里东期超高压变质带锡铁山地区的含石榴石蓝晶石/夕线石黑云斜长片麻岩基质矿物及石榴石变斑晶的独居石中获得886±18Ma格林威尔期的年龄等时线。独居石稀土元素配分特征与新元古代变质独居石相吻合。通过传统矿物对温压计计算得到格林威尔期现存矿物组合记录了高角闪岩相变质温压条件607~727℃,6.5~10.0kbar,略高于区内记录古生代变质作用的副片麻岩。与记录古生代加里东期变质年龄的副片麻岩相比,格林威尔期副片麻岩在微量元素地球化学上具有高的稀土总量和明显的Eu的负异常特点(Eu/Eu* =0.50),并相应的亏损Ba、Sr元素,表现出活动大陆边缘沉积岩的地球化学特征。结合全球格林威尔期造山事件及罗迪尼亚超大陆的形成及裂解过程,我们认为柴北缘地区在新元古代时期应为与罗迪尼亚超大陆形成有关的活动大陆边缘地区。The poly-phase orogeny information included in one orogenic belt is the key for studying the regional tectonic evolution at different time period. It also has important significance of understanding the rock association and geochemical evolution of different orogeny. However, the later orogenesis always fully or partially erases the information about the early orogenesis, making it much harder to investigate the metamorphism and geochronology information of the poly-phase orogenesis. Monazite, as a common accessary mineral in paragneiss, has high closure temperature of the U-Th-Pb isotopic system and essential to the fluid and metamorphic P-T conditions, making it much easier to record the abundant geochronological information about the poly-phase orogenesis. In-situ electron microprobe monazite dating method makes it possible to connect the geochronological information and the mineralogical characteristics and metamorphic reactions in order to get the P-T-t path of the mineral assemblage with different ages. We have combined in-situ electron microprobe monazite dating method with petrological investigations on the garnet-bearing kyanite/sillimanite biotite gneiss from the Xitieshan terrane of North Qaidam Caledonia ultrahigh pressure metamorphic belt and yielded 886 +/- 18Ma Grenville ages. The rare earth element distribution pattern is similar with the Neoproterozoic ones in the literature. By means of traditional mineral geothermobarametry, the high amphibolite facies 607 similar to 727 degrees C, 6.5 similar to 10. Okbar conditions has been conducted, which is slightly higher than the P-T condition recorded by the Paleozoic paragneiss. Compare with the trace element geochemistry with the Paleozoic paragneiss, the Grenville paragneiss are characterized by high total rare earth element and obvious Eu negative anomaly (Eu/Eu * = 0.50) with correspondingly be depleted in Ba and Sr element, which shows the active continental margin sediment characters. By reviewing the Grenville orogenesis and the formation and decomposition of the super-continent Rodinia globally, we consider that the North Qaidam UHP belt is related to the active continental margin region during the formation of super-continent Rodinia at Grenville age.国家自然科学基金项目,中国地质科学院地质研究所基本科研业务费,中国地质调查项目SCI(E)中文核心期刊要目总览(PKU)中国科技核心期刊(ISTIC)中国科学引文数据库(CSCD)123715-37283