大岔大坂MOR型和SSZ型蛇绿岩对北祁连洋演化的制约

Constraints on the evolution of the North Qilian ocean basin:MOR-type and SSZ-type ophiolites from Dachadaban

北祁连大岔大坂剖面保存了MOR型和SSZ型两类蛇绿岩,记录了两种构造环境的岩浆活动。剖面南段的蛇绿岩由蛇纹石化的超基性岩和辉长岩组成,其中辉长岩具有N-MORB地球化学特征,代表了扩张形成的洋壳。从辉长岩中分选出大量锆石,采用SHRIMP方法测得这些锆石的年龄为505±8Ma,代表了辉长岩的结晶年龄,它与熬油沟辉长岩(504Ma)和东草河辉长岩(497Ma)基本同时形成,表明寒武纪末期北祁连洋盆存在一次规模较大的海底扩张事件。剖面北段主要由枕状熔岩组成,其地球化学性质类似于玻安岩,代表了由洋壳向北俯冲形成的岛弧。枕状熔岩形成较晚(483Ma),这与其覆盖在辉长岩上的野外关系是一致的,说明从残留洋壳的形成到岛弧的出现间隔了大约20Ma。这项研究为认识北祁连洋盆的构造演化提供了重要依据。

MOR-type and SSZ-type ophiolites occur along the Dachadaban profile in North Qilian Mountains （NQM） of northwest China,which have recorded magmatic activities in different tectonic environments.Geochemical data show that the gabbro in the southern part of the profile has a pattern similar to N-MORB pattern,characterized by low SiO2 （48.04%～51.20%） and MgO （7.29%～10.81%）,Mg index varying from 0.52 to 0.69,high TiO2 （1.27%～1.47%） and Al2O3 （12.66%～14.50%）,and high （REE content ranging from 37.18×10-6 to 62.09×10-6,about 10 times higher than that of chondrite.The REE pattern indicates a flat distribution or slight LREE depletion relative to HREE,with （La/Yb）N ratios ranging from 0.66 to 0.80,and weak negative Eu anomalies （δEu from 0.83 to 0.91）.The gabbro has similar Ti/V ratio to N-MORB,but is enriched with Rb,Ba,Th and U.Geochemical characteristics similar to N-MORB imply that the gabbro is of oceanic crust origin.Zircons separated from a gabbro sample are fine-grained （50～100 μm） and mostly have long prismatic shape.CL images show that zircons contain an oscillatory zone with Th/U ratio ranging from 0.72 to 1.18,suggesting a magmatic origin.All zircons are concordant and have an age of 505±8 Ma by SHRIMP U-Pb dating,representing the formation time of gabbro.This gabbro is coeval with the Aoyougou （504 Ma） and Dongcaohe gabbro （497 Ma） in NQM,suggesting a common oceanic spreading and formation of oceanic crust at the end of Cambrian in the North Qilian Mountain.By contrast,pillow lavas from the northern part of the profile are characterized by high SiO2 （52.33%～55.62%） and MgO （10.43%～14.17%）,Mg index varying from 0.72 to 0.78,low TiO2 （0.29%～0.44%） and Al2O3 （10.10%～13.34%）,and low ΣREE content （9.93×10-6～19.04×10-6）,10 times lower than ΣREE content of chondrite.The REE pattern displays LREE depletion with （La/Yb）N ratios ranging from 0.45 to 0.69.Most pillow lava samples have negative Eu anomalies （δEu ranging from 0.58 to 1.08）,with distinct negative-Nb,Ta anomalies and a Ti/V ratio similar to that of arc tholeiite.The overall geochemical characteristics resemble those of boninite,which originated from an island arc related to the northward subduction of oceanic crust.Pillow lavas were formed slightly later （483 Ma）,consistent with the field relationship of pillow lavas overlying gabbro.These data indicate that there is about 20 Ma time difference from the formation of oceanic crust to the production of island arc.This study provides important constraints for the evolution of ocean basin in the NQM.