湘东北幕阜山梅仙花岗岩成因及其构造意义:锆石U- Pb- Hf同位素与全岩地球化学制约
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本文为湖南省科技厅重点研发项目(编号2019SK2261)、中国博士后科学基金(编号2022M713508)、中南大学有色金属成矿预测与地质环境监测教育部重点实验室开放基金(编号2022YSJS07)、湖南省自然资源厅科技计划项目(湘自资科20230102DZ)和湖南省地质院科研项目(编号HNGSTP202302、HNGSTP202318)联合资助的成果


Zircon U- Pb- Hf isotope and whole- rock geochemical constraints on the origin andtectonic significance of the Meixian granites in the Mufushan area, NE Hunan
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    摘要:

    梅仙花岗岩体位于幕阜山复式花岗岩基西南部,其成岩时代、岩石成因对湘东北地区新元古代花岗岩带成因研究以及其构造背景有着重要的指示意义。本次研究对梅仙岩体中粗粒、细粒两种结构的黑云母花岗岩分别开展了LA- ICP- MS锆石U- Pb定年、锆石Hf同位素和全岩地球化学测试。分析结果显示,中粗粒和细粒花岗岩年龄分别为819. 6±4. 6 Ma和810. 4±4. 7 Ma,证实梅仙岩体形成于新元古代,代表江南造山带中段新元古代构造- 岩浆活动高峰的结束。梅仙岩体的中粗粒花岗岩εHf (t)=5. 4~10. 1, tDM2=1. 31~1. 04 Ga,细粒花岗岩εHf (t)=5. 0~9. 9, tDM2=1. 33~1. 06 Ga。梅仙花岗岩具有正的εHf (t)值,且其Hf二阶段模式年龄接近冷家溪群碎屑锆石Hf二阶段模式年龄峰值(1. 4~1. 0 Ga),暗示其母岩浆有新生地壳物质的参与。梅仙花岗岩具有富硅铝、高钠、中低钾、弱过铝质,较富集Rb、Th、U、Li、LREE等元素,亏损Ba、Nb、Sr、Sm等元素的特征,以及复杂的Eu异常(δEu=0. 58~1. 61)特征,属于或近似于火山弧花岗岩。梅仙花岗岩的Mg#值(45~59)明显高于地壳熔体的Mg#值(17~38),又暗示其母岩浆可能混入一定比例幔源物质。结合前人研究,本文认为梅仙花岗岩应是幔源岩浆底侵导致新生火成岩地壳部分熔融形成的非典型I型、I- S型花岗岩,其形成于扬子- 华夏板块陆陆碰撞后期,江南造山带构造背景由挤压向伸展转换的特殊时期。

    Abstract:

    The Meixian granitic pluton is located in the southwest of Mufushan granitic batholith. Its diagenetic age and petrogenesis are of great significance to study of the genesis of Neoproterozoic granite belt and its tectonic background in northeast Hunan. In this study, LA- ICP- MS zircon U- Pb dating, zircon Hf isotope, and whole- rock geochemistry were carried out for medium- coarse- grained and fine- grained biotite granites from the Meixian pluton. The analysis results show that the ages of medium- coarse- grained and fine- grained granites are 819.6±4.6 Ma and 810.4±4.7 Ma, respectively. Thus, it is confirmed that the Meixian pluton was formed in the Neoproterozoic and represent the end of the Neoproterozoic tectono- magmatic activities peak in the middle of the Jiangnan Orogen. The Meixian medium- coarse- grained granite has εHf (t)=5.4~10.1, tDM=1.31~1.04 Ga, and the fine- grained granite has εHf (t)=5. 0~9. 9, tDM2=1.33~1.06 Ga. The Meixian granites have positive εHf (t) values, and its Hf two- stage model ages are close to the peak values of Hf two- stage model ages (1.4~1.0 Ga) of clastic zircons in the Lengjiaxi group, suggesting that its parent magma has the involvement of new crustal materials. In terms of geochemical composition, The Meixian granites are characterized by silica- alumina- rich, high sodium, medium- low potassium, weak peraluminosity, more enriched in Rb, Th, U, Li and LREE, deficient in Ba, Nb, Sr and Sm, and complex Eu anomalies (δEu=0.58~1.61), belonging to or similar to volcanic arc granites. The Mg#values of the Meixian granites (45~59) are significantly higher than those of crustal melt (17~38), suggesting that their parent magma may be mixed with a certain proportion of mantle- derived materials. Combined with previous studies, this paper suggests that the Meixian granites should be atypical I- and I- S- type granites formed by partial melting of the neogenic igneous crust caused by the underplating of mantle- derived magma. They were formed at the late stage of continental collision between the Yangtze and Cathaysia blocks, and the special period when the tectonic background of the Jiangnan Orogen was transformed from compression to extension.

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陈旭,邵拥军,孟德保,文春华,陈剑锋,曹创华.2023.湘东北幕阜山梅仙花岗岩成因及其构造意义:锆石U- Pb- Hf同位素与全岩地球化学制约[J].地质学报,97(7):2222-2240.
CHEN Xu, SHAO Yongjun, MENG Debao, WEN Chunhua, CHEN Jianfeng, CAO Chuanghua.2023. Zircon U- Pb- Hf isotope and whole- rock geochemical constraints on the origin andtectonic significance of the Meixian granites in the Mufushan area, NE Hunan[J]. Acta Geologica Sinica,97(7):2222-2240.

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  • 收稿日期:2022-06-04
  • 最后修改日期:2022-07-19
  • 录用日期:2022-07-19
  • 在线发布日期: 2023-07-24
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