安徽铜陵矿集区中生代中酸性侵入岩成因:来自锆石和磷灰石地球化学证据
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1.合肥工业大学;2.合肥工业大学资源与环境工程学院

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国家自然科学基金重点项目“陆内Cu-Au和W-Mo成矿机制:以下扬子成矿带为例”(编号:42030801),国家自然科学基金项目(面上项目,重点项目,重大项目)


Petrogenesis of Mesozoic intermediate-acid intrusive rocks in the Tongling ore concentration area of Anhui Province: Evidence from geochemistry of zircon and apatite
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1.School of Resources and Environmental Engineering, Hefei University of Technology, Hefei;2.School of Resources and Environmental Engineering, Hefei University of Technology

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    摘要:

    安徽铜陵矿集区广泛发育中生代中酸性侵入岩,且其与区内铜金多金属成矿关系密切。关于其成因已有大量研究成果发表,但仍存有极大争议。本文选择区内铜官山和冬瓜山石英闪长岩以及凤凰山和胡村花岗闪长岩为研究对象,在主微量元素和全岩Sr-Nd同位素地球化学研究基础上开展锆石Hf-O同位素以及磷灰石主微量元素地球化学研究,以进一步探讨其成因机制。综合前人研究资料和本次工作成果可以确定,区内中酸性侵入岩主微量元素和稀土元素组成特征基本一致,指示其均属高钾钙碱性岩石系列,具有弧型岩浆岩和埃达克质岩特征和富集的Sr-Nd同位素组成,结镁铁质微粒包体以及斜长石反环带结构等岩石学特征,指示其岩浆具有壳幔混合来源。本次工作测得岩石中的锆石εHf(t)值介于-20.5~-5.1之间,对应两阶段Hf模式年龄(tDM2)为1.5~2.9Ga,δ18O值介于5.93‰~8.85‰之间,且发现较多年龄分别集中于1.0Ga~0.8Ga和2.4Ga~2.0Ga的继承锆石;磷灰石具有高Cl 含量(平均为0.3wt%)和Cl/F值、较高REE含量(1714×10-6~5903×10-6)及负铕异常(δEu=0.26~0.68)。基于主微量元素和同位素地球化学特征以及上述锆石和磷灰石地球化学特征,结合区域地质背景和构造演化,本文对前人有关区内中酸性侵入岩不同成因观点进行了深入分析和质疑,进而提出自己的成因观点。作者认为,铜陵矿集区中生代中酸性侵入岩起源于壳幔岩浆混合,即富集岩石圈地幔源岩浆与新元古代新生弧地壳源岩浆的混合,上升过程中可能混入了少量古元古代―中元古代古老地壳物质。其中的富集岩石圈地幔源岩浆并非起源于古太平洋俯冲洋壳或俯冲洋壳析出流体交代上覆岩石圈地幔,而是新元古代华夏洋俯冲析出流体交代扬子板块岩石圈地幔所形成。中生代古太平洋板块俯冲之后的回卷是导致古老造山带加厚再伸展、岩石圈地幔减薄和熔融的地球动力学机制。

    Abstract:

    Mesozoic intermediate-acid intrusive rocks are widely developed in the Tongling ore concentration area of Anhui Province, and they are closely related to the polymetallic-copper mineralization. A large number of previous studies have been issued on the genesis of these intrusive rocks, but there are still some controversies. In this paper, Tongguanshan and Dongguashan quartz diorite and Fenghuangshan and Hucun granodiorite are selected as research objects to further explore their genetic mechanism. Based on the research results of major and trace elements and Sr-Nd isotope of the whole rocks, a special effort is made to investigate the geochemical characteristics of zircon and apatite from these rocks. Integrating the previous research results and the analysis data of our job, it can be concluded that the Mesozoic intermediate-acid intrusive rocks have similar major and trace element compositions, are all of high-K calc-alkaline rock series with characteristics of arc magmatic rocks and adakitic like rocks, are also enriched in Sr-Nd isotope compositions. These geochemical characteristics combining with the petrological characteristics of mafic microgranular enclaves(MMEs)and antiband structures of amphibole and plagioclase indicate that their parental magma was generated by the mixing of mafic magma and felsic magma. Our new analytical results are as follows: the U-Pb ages of the inherited zircons are mainly in two ranges of 1.0Ga~0.8Ga and 2.4Ga~2.0Ga, the εHf(t) values of zircon grains are -20.5~-5.1 corresponding with calculated Hf two-stage model ages between 1.5Ga~2.9Ga, and the δ18O values of zircon are 5.93‰~8.85‰, the apatites have high Cl contents (average 0.3wt%) with relatively high Cl/F values (0.052~0.244), high REE contents (1714×10-6~5903×10-6) and negative δEu values (0.26~0.68). Based on the above-mentioned geochemical characteristics of major and trace elements and Sr-Nd isotopes of whole rocks, and the geochemical characteristics of singer minerals zircon and apatite obtained from this job, combined with the regional geological tectonic evolution, the different previous views on the genesis of the Mesozoic intermediate-acid intrusive rocks in the area are debated and questioned, and proposed our own point of view. It is held that the Mesozoic intermediate-acid intrusive rocks in the Tongling ore concentration area were derived from a mixture of crust-derived and mantle-derived magma, that is, the mixing of enriched lithospheric mantle derived magma and Neoproterozoic juvenile arc crustal derived magma, a small amount of ancient crustal materials from Paleo-Proterozoic to Meso-Proterozoic had also been mixed in the magma ascending process. The enriched lithospheric mantle sources magma was derived from the subducted oceanic crust or the fluid metasomatic overlying lithospheric mantle when Cathaysia oceanic slab beneath the Yangtze craton in the Neoproterozoic era, but not the Paleo-Pacific slab subducted into South China plate in the Mesozoic era. The roll-back after subduction of the Mesozoic Paleo-Pacific slab was the geodynamic mechanism that led to the ancient orogenic belt extending after thickening, the lithospheric mantle thinning and melting.

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  • 收稿日期:2023-03-13
  • 最后修改日期:2023-05-09
  • 录用日期:2023-05-17
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