丹凤超高压地体英云闪长岩脉成因及其对北秦岭早古生代高压-超高压岩石折返过程的制约

Genesis of Tonalitic Dike in Danfeng UHP Terrane,Eastern North Qinling:Constraints on Early Paleozoic HP-UHP Exhumation in the North Qinling

在详细野外地质调查的基础上,对秦岭岩群东段陕西丹凤大寺沟超高压变质地体中混合岩化英云闪长岩脉体开展了详细的LA-ICP-MS锆石U-Pb定年和岩石地球化学研究.结果表明:(1)英云闪长岩脉的结晶年龄为417±4Ma,内含1 832~980 Ma、506~490 Ma以及462~443 Ma等多组继承/捕获锆石年龄,其中:第一组年龄与秦岭岩群长英质片麻岩的碎屑锆石年龄一致,后两组年龄对应锆石微区Th/U比值<0.2,且分别显示了榴辉岩相和麻粒岩相变质锆石的稀土元素配分曲线特征;(2)脉体的SiO_(2)含量变化于64.94%~67.89%,显示富Al_(2)O_(3)(18.24%~20.33%)、高碱(Na_2O+K_2O=6.67%~7.54%)特征,Na_2O/K_2O>2,A/CNK>1.1,属过铝质中钾钙碱性系列,与I型花岗岩特征相似;微量元素方面富集Rb、Ba、K和轻稀土元素,具有明显Eu正异常,同时具有高Sr(815×10^(-6)~941×10^(-6)),低Y(2.66×10^(-6)~6.90×10^(-6))和Yb(0.26×10^(-6)~0.68×10^(-6))的特征,具有埃达克质岩石属性.综合脉体的岩石地球化学特征、脉体形成年龄(~420 Ma)明显晚于北秦岭超高压变质作用峰期变质时代(~500 Ma)而与区域第二期退变质作用时代(~420 Ma)相当的事实,尤其是脉体锆石中部分榴辉岩相和麻粒岩相继承锆石的存在,无疑表明大寺沟英云闪长岩脉是经历超高压变质的秦岭岩群变质岩石折返过程中部分熔融的产物,而且以榴辉岩的部分熔融为主,伸展背景下幔源岩浆上涌可能为本次部分熔融提供了部分热源.

Detailed LA-ICP-MS zircon U-Pb dating and geochemical analysis results of a tonalitic dike from the Dasigou ultrahigh-pressure metamorphic terrain in the eastern Qinling Group were carried out in this paper.The results show that the tonalitic dike was formed at 417±4 Ma.Three groups of inherited/captured zircon ages of 1 832~980 Ma,506~490 Ma and 462~443 Ma are all from the cores of zircons.Among them,the first group of ages(1 832~980 Ma) being consistent with the detrital zircon ages of the gneisses from the Qinling Group.The Th/U ratios of the latter two groups of zircons are less than 0.2,and show the characteristics of REE distribution patterns of eclogite-and granulite-facies metamorphic zircons,respectively.The SiO_(2)content of the dike varies from 64.94% to 67.89%,showing the characteristics of high Al_(2)O_(3)(18.24%~20.33%) and alkalinity contents(Na_(2)O+K_(2)O=6.67%~7.54%),Na_(2)O/K_(2)O>2,A/CNK>1.1,belonging to the peraluminous mediumpotassium calc-alkaline series,which are similar to I-type granites.The high Sr(815×10^(-6)~941×10^(-6)),low Y(2.66×10^(-6)~6.90×10^(-6)) and Yb(0.26×10^(-6)~0.68×10^(-6)) contents are similar to adakitic rocks.The rock is rich in Rb,Ba,K,Pb and LREE,with strong Eu positive anomaly in the chondrite-normalized REE pattern.Taken the formation age(~420 Ma) of the tonalitic dike is corresponds to the age of the second phase of retrograde metamorphism(~420 Ma) and obviously later than the peak metamorphic age(~500 Ma) of the UHP metamorphism in the North Qinling,especially due to the existence of zircons inherited from eclogite-and granulite-facies metamorphicrocks in the zircons from the tonalitic dike into condsideration,we believe that the Dasigou tonalitic dike is the product of partial melting during the exhumation of the Qinling Group,which experienced ultrahigh pressure metamorphism,and that the partial melting of eclogites is the main one,possibly accompanied by the upwelling of mantle-derived magma in the extensional setting.