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东昆仑造山带三叠纪花岗岩类和流纹岩类的成因:洋壳到陆壳的转化
邵凤丽
Subtype博士
Thesis Advisor牛耀龄
2017-05-19
Degree Grantor中国科学院大学
Place of Conferral北京
Degree Discipline海洋地质专业 理学博士
Keyword东昆仑造山带 同碰撞花岗岩类 洋壳熔融 大陆地壳生长 过碱性流纹岩
Abstract东昆仑造山带位于青藏高原的北部,是青藏高原上最重要的岩浆岩带之一。通过东昆仑造山带的岩浆岩记录可以反演古特提斯洋的构造演化规律。我们从东昆仑造山带东段的都兰岩基采集了三叠纪的花岗岩类。通过这些岩石的锆石U-Pb定年结果、主量元素、微量元素和Sr-Nd-Hf同位素组成来讨论同碰撞花岗岩类的成因和流纹岩类的成因演化规律。锆石U-Pb定年结果表明都兰岩基的主体侵位于~ 240-235 Ma。东昆仑造山带东段都兰岩基的花岗岩类属于高钾-中钾钙碱性序列的准铝质-弱过铝质岩石。这些具有安山质成分特征的同碰撞花岗岩类稀土(REE)配分形式右倾,富集大离子亲石元素(LILEs)和轻稀土(LREEs),重稀土(HREE)具有平坦的分布形式,高场强元素(HFSEs)相对亏损(例如Nb、Ta)。依据东昆仑造山带都兰岩基的元素特征(如Nb/Th,Ta/U)和继承性的幔源同位素特征,我们推测其成因可以用已经俯冲下去的阿尼玛卿洋(古特提斯洋的一部分)洋壳在角闪岩相发生部分熔融产生安山质岩浆作用来解释,简单的Sr-Nd-Hf同位素质量平衡模拟计算结果表明源岩组成可能是~ 85%的古特提斯洋MORB和~ 15%的成熟陆壳物质的混合物。依据MMEs与其寄主花岗岩类具有相同的矿物组成(前者的暗色矿物含量较高,如角闪石、黑云母等)、锆石U-Pb年龄和Sr-Nd-Hf同位素组成,我们推测MMEs是同期岩浆作用早期结晶出的暗色矿物堆晶形成的。东昆仑造山带同碰撞花岗岩类的平均组成与平均大陆地壳组成十分相似,我们认为“陆-陆碰撞带为大陆地壳净增长的主要场所”的假说也适用于东昆仑造山带。通过对东昆仑与西秦岭结合部位的构造岩石特征的分析,我们认为秦岭-祁连造山带在被右行走滑的温泉断裂错断开之前可能是作为一个整体的造山带。
结合东昆仑造山带东段三叠纪的同碰撞花岗岩类的研究工作以及本人对澳大利亚昆士兰东南部Glass House Mountains地区典型过碱性流纹岩的研究工作,本人采集了东昆仑造山带东段同一区域内三叠纪的火山岩类岩石样品,发现东昆仑造山带东段香日德附近出露的A类流纹岩发育非常壮观的柱状节理,其锆石U-Pb定年结果表明岩体形成于晚三叠世(~209 Ma),其地球化学特征类似于典型的过碱性流纹岩,根据其较高的Nb和Ta含量、极低的Ba、Sr、Eu、P、Ti含量,推测其成因类似于典型的过碱性流纹岩,为幔源玄武质岩浆发生分离结晶作用形成的,该时期东昆仑造山带处于后碰撞伸展阶段,幔源岩石在伸展构造背景下发生减压熔融形成玄武质岩浆,玄武质岩浆经过长期的分离结晶作用形成了东昆仑造山带类似于碱流岩的A类流纹岩。通过与A类流纹岩对比研究,我们推测东昆仑造山带广泛出露的晚三叠世鄂拉山组陆相火山岩系中的中酸性火山岩(安山岩、英安岩和流纹岩)是下地壳在伸展构造背景下发生减压熔融形成的安山质岩浆经过一定程度的分离结晶作用形成的。
Other AbstractThe East Kunlun Orogenic Belt (EKOB), an important part of the Greater Tibetan Plateau, is an ideal region for understanding the tectonic evolution of the Anyemaqen Ocean. Here, we present zircon U-Pb ages, bulk-rock major and trace element analyses and Sr-Nd-Hf isotope compositions on representative samples of the syn-collisional Dulan batholith at the eastern end of the EKOB. The zircon U-Pb age data indicate that the bulk of the Dulan batholith was emplaced at 240-235 Ma. The granitoids have high- to medium-K and metaluminous characteristics. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in some high field strength elements (HFSEs, e.g., Nb and Ta), while having a flat heavy REE (HREEs) pattern. The mafic magmatic enclaves (MMEs) share the same age, mineralogy and indistinguishable Sr-Nd-Hf isotopes with their granitoid hosts except for the higher HREE abundances. We show that the MMEs represent cumulate formed at earlier stages of the same magmatic system. The trace element data (e.g., Nb/Th, Ta/U) and inherited mantle isotopic characteristics of the Dulan batholith are also consistent with an origin via partial melting of the last fragments of underthrusting ocean crust. Simple mass balance calculations using the Sr-Nd-Hf isotopic data show that ~ 85% Paleo-Tethys MORB and ~15% mature crustal material (the Proterozoic gneiss of the study area) contribute to the source of the granitoids. The Dulan batholith shows compositional similarities to the bulk continental curst with inherited mantle isotopic signatures. The syn-collisional felsic magmatism must have contributed to the net continental crust growth in the EKOB. We infer that the Kunlun and Qinling orogens may actually be one single orogen offset later by the Wenquan fault system.
 
Based on the research on the typical peralkaline rhyolites from the Glass House Mountains in Australia, we collected Triassic volcanic rocks from the EKOB. The rhyolites that exposed near Xiangride area have spectacular columnar joints. The zircon U-Pb dating result shows that these rhyolites are formed in the Late-Triassic (~ 209Ma). According to the high Nb and Ta content and very low Ba, Sr, Eu, P and Ti content features, we infer that the petrogenesis of the rhyolites are similar to the typical peralkaline rhyolites which are originated from mantle-drived basaltic magma. The widespread felsic volcanic rocks which belong to the Elashan formatin in the EKOB erupted during Late-Triassic (232Ma-225 Ma). Compared with the near-peralkaline rhyolites, we argue that these volcanic rocks are originated from the lower crust under the post-collision extentional tecnonic setting.
Language中文
Document Type学位论文
Identifierhttp://ir.qdio.ac.cn/handle/337002/136568
Collection海洋地质与环境重点实验室
Affiliation中国科学院海洋研究所
First Author AffilicationInstitute of Oceanology, Chinese Academy of Sciences
Recommended Citation
GB/T 7714
邵凤丽. 东昆仑造山带三叠纪花岗岩类和流纹岩类的成因:洋壳到陆壳的转化[D]. 北京. 中国科学院大学,2017.
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