浙闽沿海晚中生代伸展构造的岩石学标志:东极岛镁铁质岩墙群  

Late Mesozoic extension in the coastal area of Zhejiang and Fujian provinces:A petrologic indicator from the Dongji Island mafic dike swarms

作　　者：董传万[1] 闫强[1,2] 张登荣[1] 杜振永[1] 竺国强[1] 

机构地区：[1]浙江大学地球科学系,杭州310027 [2]中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2010年第4期1195-1203,共9页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40872051和40372035);国家"973"项目(2007CB411706)联合资助

摘　　要：在浙江舟山群岛之东极岛上,出露大规模的镁铁质岩墙群,它们沿晚中生代花岗岩和火山碎屑岩的节理侵位,产状近于直立且与围岩截然接触,指示岩墙岩浆以主动方式沿构造裂隙侵入。东极岛镁铁质岩墙群的岩性主要是角闪辉绿岩,全岩Ar-Ar年龄为93.4Ma。在地球化学上,它们属弱碱性系列,SiO₂含量为46.88%～52.55%,CaO 5.40%～7.82%,Al₂O₃16.30%～17.31%,TiO₂ 1.53%～1.18%,（Na₂O+K₂O）4.62%～6.88%,Mg^#=34～42;具有轻稀土元素和大离子亲石元素（LILE,如Sr、K、Rb、Ba等）富集、高场强元素（HFSE,如Nb、Ta等）亏损等特点,它们的I_Sr为0.7084～0.7098,ε_Nd（t）为-3.21～-8.11,与福建沿海的镁铁质岩墙群及浙闽沿海晚中生代基性岩浆岩（辉长岩/辉绿岩和玄武岩）相似。它们由起源于受消减作用影响的岩石圈地幔部分熔融而产生并受地壳混染的玄武岩浆,在伸展构造环境中形成,是浙闽沿海晚中生代伸展构造的重要岩石学标志。Large-scale mafic dike swarms occur in Dongji Island located in the Zhoushan archipelago in Zhejiang Province,and are termed as Dongji Island mafic dike swarms(DIMDS) in this paper.DIMDS emplaced along joints of the Late Mesozoic granites and pyroclastic rocks with attitudes of NNE(10°～30°),NNW(330°～350°) and nearly EW(100°～110°) trendings,which steeply contacted with the granite or pyroclastic rocks.The dike rocks are mainly hornblende diabase,and the Ar-Ar age of whole rock is 93.4Ma.Geochemically,DIMDS are weak-alkaline series of basalt compositions with SiO_2 46.88%～52.55%,CaO 5.40%～7.82%,Al_2O_3 16.30%～17.31%,TiO_2 1.53%～1.18%,(Na_2O+K_2O) 4.62%～6.88%and Mg~#=34～42.All these rocks are enriched in LREE,LILE(Sr,K,Rb,Ba),and depleted in HFSE,especially Ta and Nb.I_(Sr) values of DIMDS from 0.7084 to 0.7098,andε_(Nd)(t) values from -3.21 to -8.11.The overall elemental and Sr-Nd isotopic characteristics of DIMDS are similar to that of mafic dikes from the coastal area of Fujian Province,and in consistent with that of the Late Mesozoic basic igneous rocks (gabbros/diabase and basalt) in Zhejiang and Fujian provinces.The results suggest that DIMDS were probably derived from basaltic magma produced from partial melting of enriched iithospheric mantle and contaminated by crustal material.The DIMDS formed in the continental extension setting,and can be as a petrologic indicator of Late Mesozoic extension in the coastal area of Zhejiang and Fujian provinces.

关 键 词：镁铁质岩墙群 伸展构造 岩石学标志 晚中生代 东极岛 

分 类 号：P585[天文地球—岩石学]