海南岛三叠纪中基性岩的年代学、地球化学及其地质意义

Triassic neutral and basic rocks in Hainan Island,geochemistry and their geological signinficance

辉长岩大多为地幔岩石部分熔融的产物,辉绿岩脉及碱性正长岩的形成通常与伸展构造有关,本文对海南岛万宁辉长岩及辉绿岩脉和分界洲正长岩进行了系统的年代学和岩石地球化学研究,并以此来讨论其构造意义。LA-ICP-MS锆石U-Pb定年结果表明,万宁辉长岩及辉绿岩脉形成约在240 Ma,分界洲正长岩形成约在231 Ma,主量元素特征表明万宁辉长岩和辉绿岩分别属碱性系列和亚碱性系列,分界洲正长岩属于典型的碱性岩浆岩。万宁辉长岩及辉绿岩脉的稀土元素具有轻稀土富集的特点(LREE/HREE=7.22～8.50和8.11～11.10),微量元素具有岛弧型火山岩的特征,富集大离子亲石元素K、Rb、Ba、Th,贫高场强元素Nb、Ta和Zr、Hf;分界洲正长岩的微量元素特征显示出与A型花岗岩类似的特征。海南岛三叠纪中基性岩如分界洲正长岩和万宁辉长岩及辉绿岩脉形成的构造背景为陆内伸展环境,指示海南岛在240～230 Ma处于印支造山运动的应力松弛阶段。

Gabbro is mostly partial melting of mantle rocks,the formation of mafic dykes and alkaline syenite are usually related with the extensional tectonics,the research on Triassic gabbro,mafic dykes and alkaline syenite in Hainan Island has important implications to its tectonic background. Systematic geochronology and geochemistry of gabbro and mafic dykes in Wanning and syenite in Fenjiezhou, Hainan Island,were done in this paper, and the tectonic significance was discussed. The result of LA-ICP-MS zircon U-Pb dating shows that the age of gabbro and mafic dykes is about 240 Ma,the age of syntite is about 231 Ma. The major element shows that gabbro and mafic dykes belong to alkaline series and subalkaline series, syenite in Fenjiezhou belongs to typical alkaline rocks. The REE elements of gabbro and mafic dykes both have a feature of enrichment in LREE （ LREE/HREE = 7.22 - 8.5 and 8.11 - 11.10）, the trace element has a feature of island arc, which is enrichment in LILE （K, Rb, Ba, Th）but lack HSFE （Nb,Ta,Zr, Hf）, while the trace element of syenite in Fenjiezhou shows a feature of A-type granite. The tectonic background of gabbro, mafic dykes in Wanning and stenite in Fenjiezhou, Hainan Island,is intracontinental extension environment,shows that Hainnan Island was in the stage of stress relaxation of Indo-orogenic during 240 - 230 Ma.