Mantle Source Lithology of Late Mesozoic Mafic Dikes in Southeastern China
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摘要: 中国东南部晚中生代的岩浆活动被认为与古太平洋板块的俯冲作用密切相关,而板块的俯冲作用又势必会对地幔的性质产生重要影响.晚中生代基性岩脉在中国东南部尤其是沿海地区广泛分布,为揭示中国东南部地幔演化历史及其与古太平洋板块俯冲之间的潜在成因联系提供了理想的研究对象.因此,对湘、赣、浙、闽、粤五省基性岩脉的年代学和地球化学数据进行了总结,通过恢复它们的原始岩浆组成,厘定其地幔源区岩性,揭示了研究区地幔的岩性演化历史.研究发现,中国东南部晚中生代基性岩脉的源区岩性在地域上没有显著差异,在时间尺度上表现出明显变化.在150~110 Ma期间,中国东南部地幔源区的岩性包含富硅辉石岩和贫硅辉石岩两类;而在110~64 Ma期间,地幔源区的主体岩性转变为贫硅辉石岩,伴随部分橄榄岩.基于上述地幔岩性的演化规律,并结合前人对研究区基性玄武岩的研究工作,认为研究区晚中生代地幔的岩性转变主要受控于古太平洋板块的俯冲过程,是板块俯冲角度改变的结果.Abstract: Late Mesozoic magmatism in southeastern China was controlled by the subduction of paleo-Pacific plate, and the mantle beneath the area should also have been affected by such process.Late Mesozoic mafic dikes are widely distributed in southeastern China, especially in the coastal areas, which provide an ideal setting for studying the mantle evolution and subduction process of paleo-Pacific plate. Here it summarizes the chronological and geochemical data of mafic dikes in Hunan, Jiangxi, Zhejiang, Fujian and Guangdong provinces. Source lithologies of these rocks are identified by the compositions of fractionation-corrected primary magmas. The study indicates that the mantle lithologies of these mafic dikes in southeastern China do not show obvious differences in space, but significant changes with time. The source lithologies of mafic dikes during the period of 150-110 Ma are composed of SiO2-rich pyroxenite and SiO2-poor pyroxenite, whereas the source lithologies during 110-64 Ma are primarily composed of SiO2-poor pyroxenite, with minor peridotite. Therefore, it is suggested that the lithological variation of Late Mesozoic mantle beneath southeastern China is controlled by the subduction of paleo-Pacific plate via the variation of the subduction angle.
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Key words:
- mafic dike /
- primary magma /
- source lithology /
- paleo-Pacific plate /
- southeastern China /
- petrology
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图 1 中国东南部晚中生代基性岩脉分布
Fig. 1. Distribution of the Late Mesozoic mafic dikes in southeastern China
图 2 中国东南部晚中生代基性岩脉全岩MgO与CaO/Al2O3协变图
基性岩脉数据来源已在文中列出,黑色十字表示用“Petrolog 3”软件(Danyushevsky and Plechov, 2011)模拟的分离结晶过程,初始熔体1、2分别为Xyk4和ZHC-13的原始岩浆成分,潜在结晶矿物相为橄榄石(Ol)和单斜辉石(Cpx),“L-Ol”表示橄榄石分离结晶,“L-Ol-Cpx”表示橄榄石和单斜辉石分离结晶,“L+Ol”表示橄榄石堆晶
Fig. 2. Variations of CaO/Al2O3 versus MgO for Late Mesozoic mafic dikes in southeastern China
图 3 中国东南部晚中生代基性岩脉全岩SiO2与K2O/TiO2协变图
其中经历过单斜辉石分离结晶的样品(MgO < 7.5%,图中半透明符号所示)已被排除;中国东部大陆地壳数据引自Gao et al.(1998)
Fig. 3. Variations of K2O/TiO2 versus SiO2 for the Late Mesozoic mafic dikes in southeastern China
图 4 中国东南部晚中生代基性岩脉原始岩浆成分相图(a); MgO与CaO协变图(b); MgO与SiO2协变图(c)
图a为以摩尔比例从Diopside向CATS-Olivine-Quartz平面投影的假三元相图(Herzberg, 2011),箭头表示温度降低方向;图b中黑色分离结晶趋势线以及橄榄岩实验熔体(2~7 GPa)区域据Herzberg (2011);图c中贫硅辉石岩、富硅辉石岩及橄榄岩实验熔体区域,华南晚中生代玄武岩原始岩浆数据均据Zeng et al.(2016),缩写:Ol.橄榄石;Cpx.单斜辉石;Opx.斜方辉石、Qz.石英;Grt.石榴子石;Plag.斜长石
Fig. 4. Phase diagram of primary magmas for Late Mesozoic mafic dikes in southeastern China (a), variations of CaO versus MgO (b), variations of SiO2 versus MgO (c)
图 5 中国东南部晚中生代基性岩脉Fe/Mn与FeOT/CaO-3MgO/SiO2协变图
橄榄岩熔体推荐FC3MS值上限(0.65)、橄榄岩和辉石岩实验熔体数据引自Yang et al.(2016)
Fig. 5. Variations of FeOT/CaO-3MgO/SiO2 versus Fe/Mn for the Late Mesozoic mafic dikes in southeastern China
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