Acta Scientiarum Naturalium Universitatis Pekinensis ›› 2019, Vol. 55 ›› Issue (4): 663-674.DOI: 10.13209/j.0479-8023.2019.048

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Zircon U-Pb-Hf of the Altered Basic Volcanics in Jinzhouying Formation, Lüliang Mountains and Their Significance in Geological Evolution

CAO Shutai1, LI Qiugen1,†, LIU Shuwen1, WANG Zongqi2, SUN Di1, LIU Zhengfu1, WANG Xiaoyu1   

  1. 1. Key Laboratory of Orogenic Belts and Crustal Evolution (MOE), School of Earth and Space Sciences, Peking University, Beijing 100871 2. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037
  • Received:2018-09-13 Revised:2018-12-05 Online:2019-07-20 Published:2019-07-20
  • Contact: LI Qiugen, E-mail: qgli(at)pku.edu.cn

吕梁地区近周营组蚀变火山岩U-Pb-Hf同位素特征及其地质意义

曹淑泰1, 李秋根1,†, 刘树文1, 王宗起2, 孙迪1, 柳政甫1, 王筱煜1   

  1. 1. 造山带与地壳演化教育部重点实验室, 北京大学地球与空间科学学院, 北京 100871 2. 中国地质科学院矿产资源研究所, 北京 100037
  • 通讯作者: 李秋根, E-mail: qgli(at)pku.edu.cn
  • 基金资助:
    国家自然科学基金(41372193, 41530207)资助

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Abstract:

Lüliang Group, as one of the lithostratigraphic units in the Lüliang Mountains, is composed of the original Lüliangshan Group and the low-grade metamorphic part of the original Chashang Group. This study chooses the altered volcanics belonging to the later Group in Lüliang Group to perform zircon U-Pb dating and Hf isotopic analysis in attempt to determine its formation time. The zircon U-Pb results show two groups of ages: one is the younger with concordant age of 1813±6 Ma (n=7) and the other is the older with 207Pb/206Pb weighted mean age of 2516±31 Ma (n=2). The former is likely to be interpreted as the crystallization age of the volcanic rock, whereas the latter is considered as the captured zircons. Magmatic zircons from the volcanic rock have εHf(t) values of -10.8--2.3, with TDM1 of 2308-2655 Ma, whereas captured zircons have εHf(t) values of +10.0-+13.1. The Hf isotopic signatures of ~2.5 Ga ages signify that their εHf(t) value is higher than the DMM line. U-Pb isotopic and Hf isotopic were analyzed in different domains of zircon, so the obtained Hf isotopic compositions are geologically meaningless mixtures. By contrast, the Hf isotopic signatures of ~1.8 Ga ages signify that their derivation from enriched mantle or depleted mantle contaminated by the crust. These results, together with previous data from the literatures, indicate that ~1.81 Ga magmatic event in the Lüliang area occurred at the post-orogenic setting.

Key words: altered volcanic rocks, zircon LA-ICP-MS U-Pb dating, Hf isotopes, crustal evolution, Lüliang Area

摘要：

选择吕梁群中原岔上群北部地层的蚀变火山岩进行锆石U-Pb年代学和Hf同位素研究。锆石U-Pb测试获得两组年龄结果, 较年轻的谐和年龄为1813±6 Ma (n=7), 较老的207Pb/206Pb加权平均年龄为2516±31 Ma(n=2), 前者为火山岩喷发时代, 后者代表捕获锆石年龄。年轻锆石的εHf(t)值为-10.8~-2.3, TDM1值为2308~2655 Ma; 捕获锆石的εHf(t)值为+10.0~+13.1。年龄约为2.5 Ga锆石的εHf(t)值高于亏损地幔演化线, 考虑到UPb同位素和Hf同位素测点位置不完全相同, 说明所获得的Hf同位素组成为无地质意义的混合数值; 年龄约为1.8 Ga锆石的Hf同位素特征反映其可能源于富集地幔或受地壳物质混染的亏损地幔。结合前人的研究成果, 推断岩浆作用事件发生在约1.81 Ga 的碰撞后阶段。

关键词: 蚀变火山岩, LA-ICP-MS定年, Hf同位素, 地壳演化, 吕梁地区

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