柴北缘东段奥陶纪埃达克岩-富Nb玄武岩:对大陆深俯冲之前大洋俯冲及地壳增生的启示  

Ordovician adakite-Nb-enriched basalt suite in the eastern North Qaidam Mountains:Implications for oceanic subduction and crustal accretion prior to deep continental subduction

作　　者：路增龙 张建新[1] 毛小红[1] 周桂生 滕霞 武亚威 LU ZengLong;ZHANG JianXin;MAO XiaoHong;ZHOU GuiSheng;TENG Xia;WU YaWei(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]北京大学地球与空间科学学院,北京100871

出　　处：《岩石学报》2020年第10期2995-3017,共23页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41630207、41572180);中国地质调查局地质调查项目(DD20190006)联合资助。

摘　　要：在柴北缘东段识别出早古生代埃达克岩-富Nb玄武岩的火山岩组合。埃达克岩富Na_2O、贫K_2O,K_2O/Na_2O比值介于0.14~0.43之间;高Sr (614×10^(-6)~1043×10^(-6)),但亏损Y (3.26×10^(-6)~14.1×10^(-6))和Yb (0.33×10^(-6)~1.46×10^(-6)),具有高的Sr/Y比值(44~282);富集Sr、Ba等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素及Cr、Ni、Co、V等相容元素。富Nb玄武岩富Na_2O、贫K_2O、高TiO_2,其Nb含量较高,介于16.9×10^(-6)~17.9×10^(-6)之间,具有高的Nb/Ta、Nb/U、(Nb/La)N比值,同时富集高场强元素。埃达克岩锆石U-Pb定年得到453±4Ma和457±4Ma的结晶年龄。锆石ε_(Hf)(t)范围较大,介于3.40~13.23之间,对应的二阶段模式年龄t_(DM2)介于1059~566Ma之间,显示以新生物质为主的特征。综合研究表明柴北缘东段埃达克岩可能为岛弧环境下俯冲的南祁连大洋板片部分熔融的产物。板片来源的埃达克质熔体交代或与上覆地幔楔橄榄岩反应,导致被交代的地幔橄榄岩部分熔融而形成富Nb玄武质岩浆。柴北缘东段埃达克岩-富Nb玄武岩火山岩组合的厘定表明南祁连洋可能直到~455Ma之前并未完全闭合,同时表明俯冲大洋板片的部分熔融可能是柴北缘早古生代地壳增生的一种重要方式。Early-Paleozoic adakite-Nb-enriched basalt volcanic suite is recognized in the eastern North Qaidam Mountains.The adakitic rocks are characterized by high Na2O and low K2O contents,with a K2O/Na2O ratio of 0.14~0.43.They are rich in Sr(614×10-6~1043×10-6),but poor in Y(3.26×10-6~14.1×10-6)and Yb(0.33×10-6~1.46×10-6)with high Sr/Y ratios(44~282).They are also enriched in LILE(large ion lithophile elements)(e.g.Ba and Sr)and deficient in HFSE(high field strength elements)(e.g.Nb,Ta and Ti),with low contents of compatible elements(Cr,Ni,Co and V).Nb-enriched basalts are rich in Na2O and TiO2,and poor in K2O.They have high contents of Nb(16.9×10-6~17.9×10-6)and HFSE,with high Nb/Ta,Nb/U and(Nb/La)Nratios.The magmatic crystallization ages of the adakites are 453±4 Ma and 457±4 Ma obtained by zircon U-Pb dating.The values ofεHf(t)of the zircons in adakites are within the range of 3.40~13.23,with corresponding Hf isotope tDM2(two-stage model age)of 1059~566 Ma,indicating an origin mainly from juvenile materials.Comprehensive studies show that the adakites were most probably the products of partial melting of a subducted South Qilian oceanic slab.Additionally,the slab-derived adakitic melts metasomatized or interacted with the upper mantle wedge peridotites,from which the partial melting of metasomatized mantle peridotites gave rise to a Nb-enriched basaltic magma.Thus,the adakite-Nb-enriched basalt association indicates that the South Qilian Ocean has not been completely closed until^455 Ma,and partial melting of subducted oceanic slab may be an important way of Early Paleozoic crustal growth in the North Qaidam Mountains.

关 键 词：埃达克岩 富Nb玄武岩 奥陶纪 柴北缘东段 大洋俯冲 板片熔融 

分 类 号：P588.145[天文地球—岩石学] P542[天文地球—地质学]