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拉萨地块西段尼雄地区早白垩世晚期花岗闪长岩的成因及构造意义
崔浩杰1,苟正彬2,刘函2,李俊2,杨洋1
0
(1. 成都理工大学地球科学学院, 四川 成都 610059;
2. 中国地质调查局成都地质调查中心, 四川 成都 610081)
摘要:
青藏高原拉萨地体中西部广泛发育白垩纪岩浆岩,为探讨青藏高原新生代之前的地质演化历史提供了重要线索。然而其成因及构造背景还存在不同的认识,严重阻碍了青藏高原演化历程的反演工作。本文对藏西措勤县尼雄地区的一套花岗闪长岩进行了详细的年代学和全岩地球化学分析,结果表明,尼雄花岗闪长岩的SiO2含量为65.24%~67.61%,Na2O/K2O为0.91~3.93,铝饱和指数(A/CNK)为0.94~1.14,属I型钙碱性岩石。它们富集轻稀土元素(LREEs)和大离子亲石元素(LILEs,如Rb、Th等),亏损重稀土元素(HREEs)和高场强元素(HFSEs,如Nb、Ta等),轻重稀土分馏明显[(La/Yb)N =7.6~12.2],具有弱-中等的负Eu异常(δEu为0.6~0.9)。两个花岗闪长岩样品中获得的岩浆锆石U-Pb定年结果分别为119.7 Ma和120.5 Ma,代表了尼雄地区的花岗闪长岩为早白垩世晚期岩浆活动的产物。综合研究表明,这套花岗闪长岩主要来源于古老下地壳物质的重熔,并伴有同期幔源岩浆的加入和混合。它们可能是雅鲁藏布江新特提斯洋壳岩石圈北向俯冲的产物。
关键词:  拉萨地块  早白垩世  花岗闪长岩  岩石成因  地球化学  地质年代学  特提斯
DOI:
投稿时间:2018-07-16修订日期:2018-09-10
基金项目:国家自然科学基金项目(41802071, 41773026和41303028);中国地质调查局项目(DD20190053)
The petrogenesis and tectonic significance of the late Early Cretaceous granodiorites in the Nyixung area, western Lhasa block, Xizang
CUI Haojie1, GOU Zhengbin2, LIU Han2, LI Jun2, YANG Yang1
(1. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, Sichuan, China;
2. Chengdu Center, China Geological Survey, Chengdu 610081, Sichuan, China)
Abstract:
The Cretaceous magmatic rocks widely exposed in the central and western Lhasa block, Xizang may provide important clues to the study of the pre-Cretaceous evolutionary history of the Qinghai-Xizang Plateau. However, there is still considerable debate concerning the petrogenesis and tectonic significance of the Cretaceous magmatic rocks in the restoration of the evolutionary processes of the Plateau. The emphasis in this study is placed upon the late Early Cretaceous granodiorites in the Nyixung area, western Lhasa block, Xizang. The petrographic and whole-rock geochemical studies have disclosed that these rocks have SiO2 contents from 65.24% to 67.61%, Na2O/K2O ratios of 0.91 to 3.93, and aluminium saturation index (A/CNK) values of 0.94 to 1.14%, indicative of I-type calc-alkaline granitic rocks. The Nyixung granodiorites are enriched in LREEs and LILEs (e.g. Rb and Th) and depleted in HREEs and HFSEs (e.g. Nb and Ta), with remarked LREE and HREE fractionation[(La/Yb)N=7.6-12.2] and slightly to moderately negative Eu anomalies (δEu=0.6-0.9). The measured two granodiorite samples give the LA-ICP-MS zircon U-Pb ages of 119.7 Ma and 120.5 Ma, respectively, which are dated at the late Early Cretaceous. The results of research in this study have reflected that the granodiorites were derived from the partial remelting of ancient lower crust, and associated with the mingling and mixing of the contemporaneous mantle-derived magmas. It is inferred that the granodiorites may be the products of the northward subduction of the Yarlung Zangbo Neo-tethyan oceanic crust.
Key words:  Lhasa block  Early Cretaceous  granodiorite  petrogenesis  geochemistry  geochronology  Tethys

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