海南岛北部晚新生代玄武岩成分演化及成因讨论

2022,96(8):2705-2724
牛晓露1,2, 冯光英1,2, 刘飞1,2, 杨经绥1,2,3
1） 自然资源部深地动力学重点实验室，中国地质科学院地质研究所，北京，100037;2. 2） 南方海洋科学与工程广东省实验室（广州），广东广州，511458;3） 南京大学地球科学与工程学院，江苏南京，210023
摘要：板内玄武岩通常具有复杂的成分组成和成因过程。海南岛北部（简称“琼北”）自渐新世始逐渐发育了大量板内玄武岩，岩性跨度大，从石英拉斑玄武岩到橄榄拉斑玄武岩，再到碱性橄榄玄武岩和碧玄岩均有分布。对琼北晚新生代玄武岩虽已开展了广泛的研究，但“琼北晚新生代玄武岩的成分随时间演化规律”这一问题却未引起足够的关注。本文从晚更新世道堂组橄榄玄武岩入手，对其开展了细致的岩石学、矿物学和主微量及Sr- Nd- Pb同位素地球化学的研究；并结合文献中琼北其他时代（组）玄武岩的数据，阐述了琼北晚新生代玄武岩成分随时间演化的规律并对其成因进行了探讨。研究发现，自中—上新世到更新世再到全新世，琼北晚新生代玄武岩呈现从碱性玄武岩过渡到拉斑玄武岩再突变到碱性—强碱性系列的演化规律；中—上新世到更新世这一时期的碱性玄武岩和拉斑玄武岩可能形成于石榴子石二辉橄榄岩由低到高程度的部分熔融作用，而全新世碱性岩则更可能形成于石榴子石辉石岩的部分熔融。琼北晚新生代玄武岩的Sr- Nd- Pb同位素组成揭示其源区地幔以PREMA端元为主，但经历了来自古老陆壳的沉积物不同程度的改造作用。采用“海南地幔柱”模型可以较好地解释琼北晚新生代玄武岩的复杂组成和成因。
关键词：海南岛；板内玄武岩；晚新生代；拉斑玄武岩；碱性玄武岩；地幔柱

Geochemical evolution and origin of Late Cenozoic basalts from north Hainan Island (southern China)

Niu Xiaolu1,2, Feng Guangying1,2, Liu Fei1,2, Yang Jingsui1,2,3
1) Key Laboratory of Deep- Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2. 2) Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, Guangdong 511458, China;3) School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China
Abstract：Intraplate basalts usually have complex compositions and genetic processes. Since the Oligocene, voluminous intraplate basalts have developed in the north Hainan Island. These basalts show a wide spectrum of compositions from quartz tholeiites, through olivine tholeiites, to alkaline olivine basalts and basanites. Extensive studies have been carried out on those basalts; however, the problem of “geochemical evolution pattern of the basalts to their eruptive history” still needs to be paid more attention. In this study, we report newly analysed whole- rock major, trace and Sr- Nd- Pb isotopic data for the olivine basalts of the Daotang Formation. These data, along with the existing geochemical data of the basalts of other ages (formations) in north Hainan Island, reveal a sequential evolution pattern, which begins with eruptions of alkaline basalt followed by tholeiitic basalt during the Middle Pliocene to the Pleistocene, and immediately followed by outpourings of more alkaline series in Holocene. The initial alkaline basalt and the following tholeiitic basalt during the Middle Pliocene to the Pleistocene might have been formed by the low to higher degree of partial melting of garnet lherzolite, while the Holocene alkaline rocks were more likely to have been formed by the partial melting of garnet pyroxenite. The Sr, Nd and Pb isotopic compositions of the basalt in north Hainan Island document a PREMA (prevalent- mantle) reservoir in their mantle source, and it has been modified by sediments derived from ancient continental crust (EMII). The possible occurrence of “Hainan mantle plume” could explain the complex compositions and origins of the Late Cenozoic basalts in north Hainan Island well.
Key words：Hainan Island; intraplate basalt; Late Cenozoic; tholeiitic basalt; alkaline basalt; mantle plume
收稿日期：2022-06-23
基金项目：本文为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）、自然资源部深地动力学重点实验室自主研究课题(编号J1901 35)、国家自然科学基金项目（编号41672063，41773029，92062215）和中国地质调查局地质调查项目（编号DD20221630）联合资助的成果。
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