利用洋中脊玄武岩中橄榄石斑晶制约南海打开的地幔动力学机制  

Using olivine to constrain the mechanism for the opening of the South China Sea

作　　者：杨帆 黄小龙[2] 徐义刚[1,2] 贺鹏丽 于洋[2] YANG Fan;HUANG XiaoLong;XU YiGang;HE PengLi;YU Yang(Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China;State Key Laboratory of Isotope Geochemistry,CAS Center for Excellence in Deep Earth Science,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China)

机构地区：[1]南方海洋科学与工程广东省实验室(广州),广州511458 [2]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,中国科学院深地科学卓越研究中心,广州510640

出　　处：《岩石学报》2024年第7期2186-2202,共17页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41890812、42372075、42002048);南方海洋科学与工程广东省实验室(广州)人才队伍经费(GML2022006)联合资助。

摘　　要：地幔柱与板块构造是地球运行的两大基本动力学机制。南海及其周缘地区处于海南地幔柱与东南亚环形俯冲系统时空交汇的背景,而新生代地幔柱在南海形成演化过程中所扮演的角色存在颇多争议。地幔源区富含辉石岩是南海及其周缘与地幔柱活动相关的玄武岩的共有特征,基于此推测,本文利用国际大洋发现计划(IODP)367航次在南海北缘U1500B站位钻遇的南海扩张初期洋中脊玄武岩(MORB)的橄榄石成分来制约南海扩张初期的地幔岩性,以检验南海扩张初期是否存在地幔柱-洋脊叠加。U1500B MORB所含橄榄石斑晶的Fo=76.4~89.4,NiO=0.09%~0.26%,CaO=0.25%~0.34%,MnO=0.16%~0.33%。这些橄榄石斑晶具有与南海西南次海盆扩张末期U1433B钻孔的MORB以及全球其他开放大洋MORB相似的NiO、CaO、MnO含量和FeO/MnO比值,代表其源区岩性主要为橄榄岩。U1500B钻孔MORB与该区存在地幔柱叠加的基性岩浆所含橄榄石斑晶成分的差异显著,表明南海扩张初期不存在地幔柱叠加,与前人估计的U1500B洋中脊玄武岩的正常地幔潜能温度(~1380℃)相符合。由于南海扩张初期洋中脊岩浆产率高,但缺乏地幔热异常和易融组分如辉石岩等,推测其地幔中可能有较多的早前俯冲板片贡献的再循环挥发分来维持高岩浆产率。所以,俯冲板片后撤导致的岩石圈伸展可能是南海打开的主要动力学机制。Mantle plumes and plate tectonics represent two fundamental geodynamic mechanisms that play crucial roles in Earth s geological processes.The South China Sea(SCS)is located at the intersection of the Hainan mantle plume and the convergent subduction zone within Southeast Asia.The role of the Cenozoic mantle plume in the opening of the SCS remains a subject of debate.Given that basalts genetically linked to mantle plumes in the SCS and its surrounding areas share a pyroxenite mantle source,we used olivine from mid-ocean ridge basalts(MORBs)recovered from the northern SCS during the International Ocean Discovery Program(IODP)Expedition 367 to constrain the lithology of the mantle source for this nascent ocean and testify the involvement of mantle plumes in its initial opening.The olivine phenocrysts in the U1500B MORB exhibit a range of compositions of Fo=76.4~89.4,NiO=0.09%~0.26%,CaO=0.25%~0.34%and MnO=0.16%~0.33%.The concentrations of NiO,CaO and MnO and the FeO/MnO ratio of the olivine phenocrysts are similar to those observed in the MORBs recovered from Site U1433B near the fossil ridge of the southwest subbasin of the SCS and MORBs from mid-ocean ridges off mantle plumes.The peridotite source is inferred for the initial opened SCS,suggesting the absence of the Hainan mantle plume for the initial opening of the SCS.This result aligns with the previously estimated normal mantle potential temperature for the U1500B mid-ocean ridge basalts(~1380℃).The initial ridge of the SCS exhibits high magmatic productivity but shows little mantle thermal anomaly or fusible components such as pyroxenite,suggesting an abundance of recycled volatile from subducted slabs.Therefore,it is plausible that lithospheric extension resulting from the rollback of subducted slabs may serve as the primary mechanism for the opening of the SCS.

关 键 词：南海 橄榄石 源区岩性 地幔柱 洋中脊玄武岩 

分 类 号：P578.942[天文地球—矿物学] P588.145[天文地球—地质学]