云开地块加里东期原特提斯洋俯冲作用:来自晚奧陶世镁铁质侵入岩的岩石地球化学证据  

Caledonian subduction of Proto-Tethyan Ocean beneath the Yunkai Massif:Petrological and geochemical evidence from Late Ordovician mafic intrusions

作　　者：华为诚 郭锋[1] 文雅倩 赵亮[1] HUA WeiCheng;GUO Feng;WEN YaQian;ZHAO Liang(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广州510640 [2]中国科学院大学,北京100049

出　　处：《岩石学报》2024年第8期2469-2488,共20页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42073032、42021002)资助.

摘　　要：本文选择云开地块高州地区陈六坑村的富闪侵入岩和佑尾甬村富钙铝与富镁侵入岩开展了综合的岩相学、锆石U-Pb定年和原位Hf-O同位素组成、全岩主-微量元素和Sr-Nd-Pb-Hf同位素组成研究,旨在理解其岩石成因和形成的大地构造环境。研究结果显示,两个镁铁质侵入体的锆石U-Pb年龄为447~451Ma,为晚奥陶世侵位的镁铁质岩浆。陈六坑村样品含有大量角闪石,佑尾甬村辉长岩中存在大量的富钙斜长石(An>95),表明这两个晚奧陶世侵入体的母岩浆都是富水的亚碱性玄武岩浆,形成于俯冲带环境。此外,两个镁铁质侵入体都相对富集大离子亲石元素和轻稀土元素而亏损重稀土和高场强元素,与典型镁铁质弧岩浆类似。两个侵入体都表现出富集的锆石Hf-O同位素组成和变化较大的Sr-Nd-Pb-Hf同位素组成。运用全岩Nb/La与Sr-Nd-Hf同位素的协变关系排除地壳同化混染作用的影响后,我们认为变化较大的Sr-Nd-Pb-Hf-O同位素组成反映了熔融地幔源区中不同比例的再循环地壳成分的加入。进一步的锆石Hf-O同位素模拟结果显示,陈六坑村富闪侵入岩的熔融源区包含了20%~50%的陆源沉积物组分,而佑尾甬村富钙铝与富镁辉长岩的源区中再循环陆源沉积物组分比例在35%~45%之间。根据古板块构造恢复结果,云开地块在晚奧陶世期间位于原特提斯洋和东冈瓦纳大陆之间,我们认为原特提斯洋向华南陆块俯冲作用能较好地解释当时的岛弧型镁铁质岩浆作用成因。随着俯冲作用的进行,陆源沉积物发生脱水熔融改造上覆地幔,沉积物富集的地幔楔发生含水熔融形成富水的亚碱性玄武质岩浆,它们经历了一定程度的矿物分离结晶/堆晶作用,形成了富闪侵入岩和富镁与富钙铝辉长岩。本研究结果为揭示原特提斯洋向华南云开地块的俯冲作用提供了关键岩石学和地球化学证据。This paper presents a comprehensive study including petrography,zircon U-Pb dating and in-situ Hf-O isotope compositions,whole-rock major and trace element and Sr-Nd-Pb-Hf isotope compositions on two mafic intrusions from the Gaozhou region in Yunkai Massif,South China,with aims to understand their petrogenesis and tectonic setting.Zircon U-Pb dating reveals an age range of 447~451Ma,confirming their emplacement in the Late Ordovician.Samples from the Chenliukeng Village contain a large amount of amphibole and can be regarded as appinites,whereas those from the Youweiyong Village are Ca-Al-rich and Mg-rich gabbros with the occurrence of calcic plagioclase(An>95).Both types of mineral assemblage indicate that the parental magmas were hydrous subalkaline basalts probably formed at a subduction zone.The two mafic intrusions are characterized by enrichment of large ion lithophile elements(LILEs)and light rare earth elements(LREEs)but depletion in high field strength elements(HFSEs),analogues to modern arc mafic magmas.They show highly variable and enriched zircon Hf-O isotopic compositions and bulk-rock Sr-Nd-Pb-Hf isotopic features,while correlations between the bulk-rock Nb/La and Sr-Nd-Hf isotopes argue against a significant role of crustal assimilation and/or contamination.We thus conclude that the large Sr-Nd-Pb-Hf-O isotopic variations reflect the different proportional addition of recycled crustal components in the mantle sources.Further zircon Hf-O isotope modelling results show that the melting source for the appinites likely contained 20%~50%of the subducted terrigenous sediment,while the proportion of this recycled sediment component in the source of gabbros was 35%~45%.Based on the paleogeographic reconstruction that the Yunkai Massif was ever located between the Proto-Tethys Ocean and the East Gondwana continent during the Late Ordovician,we consider the subduction of Proto-Tethys Ocean as a likely mechanism to interpret the petrogenesis of contemporaneous arc mafic magmatism in the study region.Followin

关 键 词：原特提斯洋俯冲作用 岩石地球化学 镁铁质侵入岩 晚奧陶世 云开地块 

分 类 号：P588.125[天文地球—岩石学] P597.3[天文地球—地质学]