| 东昆仑按纳格角闪辉长岩体地球化学特征及其对古特提斯洋演化的制约 |
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| 引用本文： | 赵旭,付乐兵,魏俊浩,赵玉京,唐洋,杨宝荣,管波,王晓云.东昆仑按纳格角闪辉长岩体地球化学特征及其对古特提斯洋演化的制约[J].地球科学,2018,43(2):354-370. |
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| 作者姓名： | 赵旭 付乐兵 魏俊浩 赵玉京 唐洋 杨宝荣 管波 王晓云 |
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| 作者单位： | 1.中国地质大学资源学院, 湖北武汉 430074 |
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| 基金项目： | 中国地质调查局项目12120114081401中国地质调查局项目12120114000701中央高校基本科研业务费专项资金CUGL170413国家自然科学基金项目41302065 |
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| 摘 要： | 目前关于东昆仑地区二叠纪-三叠纪基性岩浆岩的研究极为薄弱, 这不仅限制了对该时期东昆仑地区地幔源区性质的理解, 也在一定程度上制约了对古特提斯洋演化历史的准确认识.对东昆仑东段按纳格角闪辉长岩体进行了锆石U-Pb年代学、全岩地球化学和Sr-Nd-Hf同位素研究.结果显示:角闪辉长岩的锆石U-Pb定年结果为242±2 Ma; 岩体具有较低的SiO2(46.00%~52.40%)及(Na2O+K2O)含量(3.07%~3.79%), 碱度(δ)值为1.26~3.14;另外, 岩石具有较高的Al2O3(17.35%~20.10%), MgO(4.65%~6.53%)和FeOT(8.77%~11.07%)含量, Mg#值为68~75, 属于钙碱性系列.岩石(La/Yb)N为1.72~5.48, δEu为0.87~1.00, 具有弱的轻重稀土分异和负铕异常, 其富集大离子亲石元素, 亏损高场强元素, 具有低Nb/Ta值(3.7~8.8), 显示岛弧岩浆岩的地球化学特征.全岩(87Sr/86Sr)i值(0.708 80~0.710 36)和εNd(t)值(-4.8~-3.4)相对集中, 锆石εNd(t)值为-4.9~-0.4.综合分析表明, 岩浆起源于俯冲板片流体交代的地幔楔, 为尖晶石相金云母二辉橄榄岩的部分熔融产物, 岩浆在上升侵位过程中未遭受明显地壳混染, 但经历了少量单斜辉石和斜长石的分离结晶作用.综合区域构造演化史以及同时代岩浆岩的年代学和地球化学特征, 认为按纳格角闪辉长岩体侵位于古特提斯洋北向俯冲的晚期, 该区古特提斯洋的最终闭合时间为中-晚三叠世.
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| 关 键 词： | 角闪辉长岩 三叠纪 古特提斯洋 俯冲 东昆仑造山带 地质年代学 地球化学 |
| 收稿时间： | 2017-07-08 |
Geochemical Characteristics of An'nage Hornblende Gabbro from East Kunlun Orogenic Belt and Its Constraints on Evolution of Paleo-Tethys Ocean |
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| Abstract: | At present, the research on Permian-Triassic basic magmatic rocks in East Kunlun orogenic belt is extremely weak, which not only limits the understanding of the nature of the mantle source in the study area during this period but also restricts the accurate understanding of the evolution processes of Paleo-Tethys Ocean.The zircon LA-ICP-MS U-Pb geochronology and major and trace elements, as well as Sr-Nd-Hf isotope geochemistry of the An'nage hornblende gabbro in the East Kunlun orogenic belt are applied in this paper.LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 242±2 Ma for the intrusion.It is characterized by low SiO2(46.00%-52.40%), low(Na2O+K2O)(3.07%-3.79%) contents.In addition, it is enriched in Al2O3(17.35%-20.10%), MgO(4.65%-6.53%) and FeOT(8.77%-11.07%) with high Mg#(68-75), indicating that it belongs to the calc-alkaline series.This intrusion has low(La/Yb)N and Nb/Ta ratios, negative Eu anomalies, with enriched large-ion lithophile elements and depleted high field strength elements.All geochemical characteristics show that this intrusion is the result of typical island arc magmatic activity.Additionally, the samples have relatively uniform(87Sr/86Sr)i ratios(0.708 80-0.710 36) and εNd(t) values(-4.8 to -3.4), and relatively high εHf(t) values(-4.9 to -0.4).According to the lithology, geochemical characteristics and isotopic compositions of the intrusion, we propose that the magma has not been significantly contaminated by the crust and it was derived from the low-degree partial melting(2%-10%) of the mantle wedge, which had undergone metasomatism induced by the fluid from the subducted slab.The mantle source was characterized by the spinel phase-phlogopite lherzolite.Slight fractional crystallization of pyroxene and plagioclase might also occur during the evolution of magma.In combination with the tectonic evolution of the East Kunlun orogenic belt and the geochronological and geochemical characteristics of contemporary intrusive rocks, it is concluded that the An'nage hornblende gabbro was produced at the late stage of Paleo-Tethys Ocean subduction, and the final closure timing of Paleo-Tethys Ocean should be the Middle Triassic to Late Triassic. |
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