南祁连北缘东段晚奥陶世构造演化:多藏角闪辉长岩U-Pb年代学、岩石地球化学和矿物化学证据  

The Late Ordovician tectonic evolution of the eastern section of the northern margin of the South Qilian Block:evidences from geochronology,geochemistry,and mineral chemistry of the Duozang hornblende gabbro

作　　者：师江朋 韩效忠[2] 乔树岩[2] 王致山 霍腾飞[1] 杨浩田[1] 杨德彬[1] 

机构地区：[1]吉林大学地球科学学院,吉林长春130061 [2]中国煤炭地质总局特种技术勘探中心,北京100073

出　　处：《地学前缘》2017年第6期46-59,共14页Earth Science Frontiers

基　　金：中国地质调查局地质调查工作项目(1212011221145);国家自然科学基金项目(41472052);中央高校基本科研业务费项目

摘　　要：为约束多藏角闪辉长岩的岩石成因和南祁连北缘东段晚奥陶世的构造演化,对多藏角闪辉长岩进行了LA-ICP-MS锆石U-Pb年代学、岩石地球化学和矿物化学分析。结果表明,角闪辉长岩中锆石具条痕状吸收和高的Th/U比值(0.37~1.16),暗示其岩浆成因。岩浆锆石的206Pb/238 U定年结果为(444±3)Ma,表明多藏角闪辉长岩形成于晚奥陶世,两组捕获锆石的年龄为520 Ma和484 Ma,代表早期的岩浆事件。多藏岩体具相对低的SiO_2含量(51.68%~55.66%)和高的MgO(4.48%~7.37%)、TiO_2(1.01%~1.24%)、FeO^T(7.99%~8.72%)含量以及高的Mg#值(53~63);LREEs相对富集、HREEs相对亏损,(La/Yb)N=4.03~7.89,具有弱的Eu异常(δEu=0.78~1.16),富集LILEs(Rb、Ba、K)、亏损HFSEs(Nb、Ta),具Sr、P、Ti元素的亏损。结合矿物化学分析表明,南祁连北缘东段多藏角闪辉长岩起源于受俯冲流体交代的富集岩石圈地幔物质的部分熔融,形成于洋-洋汇聚向洋-陆碰撞转换背景下俯冲大洋板片断离后的伸展构造环境。A combined study of LA-ICP-MS zircon U-Pb geochronology, whole-rock geochemistry, and mineral chemistry for the Late Ordovician Duozang hornblende gabbro was conducted to constrain the petrogenesis and tectonic evolution of the eastern section of the northern margin of the South Qilian Block. Zircons from the Late Ordovician Duozang hornblende gabbro show striped absorption with high Th/U ratios （0. 37 - 1.16）, indicative of a magmatic origin. The concordant 206 pb/238U ages yielded an emplacement age of 444± 3 Ma, indicating the hornblende gabbro was formed in the Late Ordovician, whereas the captured zircon U-Pb ages of 520 Ma and 485 Ma represent early magmatic events. The Duozang hornblende gabbro has low SiO2 （51.68%- 55.66%）, high MgO（4. 48%- 7.37%）, TiO2 （1.01%- 1.24%）, and FeOT （7.99%- 8.72%） contents, and high Mge （53 - 63）. It is enriched in light and depleted in high rare earth elements with high （La/Yb）N values （4.03 - 7.89） and weak Eu anomalies （δEu= 0. 78 - 1.16） ; in addition, it shows enrichments of large ion lithophile elements （e. g. , Rb, Ba, and K） and depletions of high field strength elements （e. g. , Nb and Ta） and Sr, P and Ti. Together with the mineral chemical analysis, this study suggests that the primary magma for the Duozang hornblende gabbro was derived from partial melting of an enriched lithospherie mantle that had been metasomatized by subduction-related fluids. In summary, the hornblende gabbro of the south Qilian block was formed in an extensional tectonic setting related to the breaking-off of subducting oceanic slab under a conversion background that was changing from ocean-ocean convergence to ocean-continent collision.

关 键 词：晚奥陶世 角闪辉长岩 年代学 多藏 南祁连 

分 类 号：P588.12[天文地球—岩石学] P597.3[天文地球—地质学]