海南岛晚新生代石山组橄榄玄武岩的地幔源区组成及动力学机制  

Mantle sources and dynamic mechanism of Late Cenozoic Shishan Formation olivine basalts in Hainan Island,China

作　　者：冯光英[1,2] 刘飞 牛晓露[1,2] 杨经绥 Feng Guangying;Liu Fei;Niu Xiaolu;Yang Jingsui(Center for Advanced Research on Mantle(CARMA),Key Laboratory of Deep-Earth Dynamics of Ministry ofNatural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou,Guangdong 511458,China;State Key Laboratory for Mineral Deposits Research,Nanjing University,Nanjing,Jiangsu 210023,China)

机构地区：[1]中国地质科学院地质研究所,自然资源部深地动力学重点实验室,地幔研究中心,北京100037 [2]南方海洋科学与工程广东省实验室(广州),广东广州511458 [3]南京大学内生金属矿床成矿机制研究国家重点实验室,江苏南京210023

出　　处：《地质学报》2022年第8期2725-2742,共18页Acta Geologica Sinica

基　　金：南方海洋科学与工程广东省实验室(广州)人才团队引进重大专项(编号GML2019ZD0201);中国地质调查局地质调查项目(编号DD20221630);国家自然科学基金项目(编号92062215、41773029、41672063、41720104009)联合资助成果。

摘　　要：南海北部边缘新生代玄武岩广泛分布,分布于海南岛北部的全新世早期石山组玄武岩,岩石组成为碱性橄榄玄武岩,其中可见较大颗粒的橄榄石斑晶,橄榄石斑晶具有较高的Fo值(82.8~83.5)和Ni含量(0.14%~0.20%)。橄榄玄武岩的微量元素及同位素组成与洋岛玄武岩(OIB)高度吻合,富集大离子亲石元素Rb、Ba和轻稀土元素,同时富集高场强元素Nb、Ta、Zr和Hf。矿物组成及主微量元素特征指示其源区为含石榴子石辉石岩源区,经历了较低程度的部分熔融(约5%)。此外,石山组橄榄玄武岩具有基本一致的Sr-Nd-Pb同位素组成,表明岩浆在上升过程中没有明显的地壳物质的混染,但是相比正常洋中脊玄武岩(MORB),石山组橄榄玄武岩具有较高的^(87)Sr/^(86)Sr比值,较低的εNd(t)值,以及较高的^(207)Pb/^(204)Pb(15.639~15.643)和^(208)Pb/^(204)Pb(38.977~38.996)比值,说明其源区为亏损地幔(DM)和富集地幔端元(EMII)的混合。结合地球物理学证据,我们认为亏损地幔来源于软流圈,而EMII端元为含再循环古洋壳及俯冲沉积物的地幔柱,随着海南地幔柱上升,再循环物质熔融并与周围软流圈地幔橄榄岩反应形成含石榴子石辉石岩,随着地幔柱的不断上升,辉石岩首先发生部分熔融,形成的岩浆经历了橄榄石和微弱单斜辉石的分离结晶作用后形成了石山组橄榄玄武岩。华南新生代玄武岩同石山组橄榄玄武岩具有相似的地球化学特征,反映二者具有相同的物质端元组成,但两者岩浆演化上升的动力学机制不同,同时不同端元混合的比例及部分熔融程度也有差异。The Cenozoic basalts are widely distributed in the northern margin of the South China Sea.The Early Holocene Shishan Formation basalts crop out in the north of the Hainan Island.These rocks are mainly consisted of olivine basalts.The olivine basalts contain large olivine phenocrysts characterized by high Fo(82.8~83.5)and Ni(0.14%~0.20%).The studied olivine basalts are enriched in large ion lithophile elements Rb and Ba,and light rare earth elements,as well as high field strength elements Nb,Ta,Zr and Hf,similar to OIB-type rocks.Their mineral compositions and signatures of major and trace elements indicate that the Shishan Formation olivine basalts were generated by low degree partial melting of garnet pyroxenites.The olivine basalts are characterized by homogeneous Sr-Nd-Pb isotopic compositions,which exclude the possibility of significant crustal contamination.Compare with the MORB,the Shishan Formation rocks have higher ^(87)Sr/^(86)Sr ratios,lowerεNd(t),and higher ^(207)Pb/^(204)Pb(15.639~15.643)and ^(208)Pb/^(204)Pb(38.977~38.996)ratios,which define a mixing trend between a depleted MORB mantle end-member and an enriched mantle component(EMII).Combined with geophysical evidences,we suggest that the depleted mantle originated from the asthenosphere,while the EMII end-member originated from the Hainan mantle plume which contains subducted oceanic crust and sediments.During the ascent of the Hannan plume,the recycled material melted and reacted with the surrounding mantle peridotites in the asthenosphere to form garnet pyroxene.The Shishan Formation olivine basalts were formed after partial melting of garnet pyroxene and crystallization of olivine and minor clinopyroxene.The similarities and differences between the Cenozoic basalts from South China and the Shishan Formation basalts indicate that they had the same end member compositions but were different in the dynamic mechanism,the ratios of different end member and the degrees of partial melting.

关 键 词：海南晚新生代玄武岩 地幔源区 再循环洋壳及沉积物 海南地幔柱 

分 类 号：P736[天文地球—海洋地质]