Geochronological and Geochemical Constraints on the Siqian Granitoids in Central Wuyishan and Their Geological Implications
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摘要:
本文对武夷山中段的司前岩体进行了系统的岩石学、地球化学、年代学及 Lu-Hf 同位素研究.结果表明, 司前岩体形成于(140±1) Ma, 主要由黑云母二长花岗岩组成, 富碱(K2O+Na2O = 6.67 wt%~8.25 wt%), 富钾(K2O/Na2O = 1.16~2.41), A/CNK值介于1.01~1.25之间.岩体具有较高的∑REE(177.73×10-6~ 427.88×10-6)、Zr+Nb+Y+Ce含量(262.6×10-6~581.5×10-6)和Zr饱和温度(平均824°C), FeOt/MgO(3.06~3.93)和10000×Ga/Al(2.64~3.28)比值均较高, 属典型的铝质-过铝质 A 型花岗岩.岩体的锆石 εHf(t)值均为负值(–18.6 ~ –7.9), 暗示其源于古老的地壳物质重熔.综合上述结果和区域背景推测, 司前岩体的源岩为新元古代麻源群变质火山-碎屑岩, 源岩可能经历早古生代和白垩纪两期熔融事件, 地幔岩浆为花岗岩的形成提供了热源, 但并未贡献物质, 岩体的形成与古太平洋板块俯冲过程中因俯冲板片后撤诱发的弧后扩张作用有关.
Abstract:This paper presents a combined petrologic, geochemical and geochronological study and Lu-Hf isotopic analysis of the Siqian granitoids in central Wuyishan. The granitoids are predominately composed of biotite monzogranite, and igneous zircons therein yielded a consistent weighted mean 206Pb/238U age of (140±1) Ma. Ten samples from the granitoids yielded the data K2O+Na2O=6.67 wt%~8.25 wt%, K2O/Na2O =1.16~2.41 and A/CNK ratios of 1.01~1.25. They have high REE content of 177.73×10-6~427.88×10-6, Zr+Nb+Y+Ce content of 262.6×10-6~581.5×10-6, FeOt/MgO ratios of 3.06~3.93, and high 10 000×Ga/Al ratios of 2.64~3.28; all these data are consistent with features of aluminous to peraluminous A-type granite. All zircons from the samples show negative εHf(t) values of –18.6 ~ –7.9, recording a predominant contribution of the melting of ancient crustal rocks to the granitoid sources. These results, coupled with regional considerations, led the authors to believe that the granitoids might have been generated by melting of supracrustal material of the Mayuan Group volcanic-sedimentary rocks. The source rocks might have undergone two melting events in the early Paleozoic and Cretaceous, and the heat that induced crustal melting might have been supplied by asthenospheric mantle upwelling. Geodynamically, the Siqian A-type granitoids may represent a product of back-arc extension induced by roll-back of the subducting Paleo-Pacific plate in the early Cretaceous.
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Key words:
- Early Cretaceous /
- A-type granite /
- crustal extension /
- Wuyishan /
- the Siqian granitoids /
- zircon U-Pb /
- LA-ICP-MS
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