中祁连元古代皋兰岩群的解体:古元古代变质事件与早-中寒武世大陆裂谷作用

Disintegration of Proterozoic Gaolan Group in central Qilian:Paleoproterozoic metamorphic event and Early-Middle Cambrian continental rifting

中祁连东段元古代皋兰岩群是一套变质程度极不均一的变质岩系,本次工作根据变质程度将其重新划分为北部的低绿片岩相与南部的高绿片岩相(局部低角闪岩相)两个岩石单元。两者分别具有不同的岩相学、锆石U-Pb年代学和岩石地球化学特征,并以韧性剪切带相隔。低绿片岩相岩石单元中玄武岩和流纹岩的成岩年龄为502.5±3Ma、500.8±3Ma和507.8±4Ma,表明其形成时代为早-中寒武世;高绿片岩相(局部低角闪岩相)岩石单元中的斜长角闪岩变质锆石的U-Pb年龄为1873±10Ma,暗示其形成于古元古代中-晚期中祁连地块基底形成阶段,该岩石单元是中祁连地块对哥伦比亚超大陆聚合事件的物质记录。结合年代学、岩石组合以及区域资料,将低绿片岩相岩石单元从皋兰岩群中解体出来,划归北祁连寒武纪黑茨沟组。岩石地球化学分析显示,新解体出来的黑茨沟组双峰式火山岩形成于大陆裂谷环境,其中玄武质岩浆可能来自软流圈地幔,在上涌过程中受到轻微的地壳混染;流纹岩主要由玄武质岩浆结晶分异而来,并且有少量地壳物质的加入。综合分析祁连山地区原特提斯洋开启时限研究成果,认为新元古代晚期至早古生代早期北祁连地区的裂解作用乃至大洋化过程并非同步进行,而是呈现出西早东晚的特征。

The Gaolan Group in eastern Central Qilian block was a metamorphic rock series with distinct metamorphic features.In this study,we divide Gaolan Group into two metamorphic stratigraphic units,including lower greenschist facies in the north and upper greenschist-lower amphibolite facies in the south,and there are significant differences between them in terms of petrography,zircon U-Pb chronology and geochemical characteristics.These two metamorphic stratigraphic units are separated by a ductile shear zone.Zircon U-Pb ages of three samples of the basalts and rhyolites from the lower greenschist facies rocks are 502.5±3Ma,500.8±3Ma and 507.8±4Ma,indicating the volcanic-sedimentary sequence deposited during the Early-Middle Cambrian.On the other hand,the metamorphic original zircons from amphibolite in the upper greenschist-lower amphibolite facies rocks yield a concordant U-Pb age of 1873±10Ma,demonstrating the metamorphism of the basement of Center Qilian took place at Late Paleoproterozoic,and the metamorphic stratigraphic units of the upper greenschist-lower amphibolite facies is the material record of the convergence event of the Columbia supercontinent in the Centre Qilian block.Combined chronology,lithostratigraphy and regional data,the lower greenschist facies rock units should be disintegrated from the Gaolan Group and assigned to the Cambrian Heicigou Formation of the North Qilian orogen.The basalt and rhyolite constitute a bimodal volcanic series whose whole-rock geochemistry analysis results show that they were genetically related to continental rifting.The basalts were derived from the asthenospheric mantle with a small amount of crustal contamination during the upwelling process,while rhyolites could be derived from basaltic magmatic crystallization together with crustal contamination.Based on the research findings on the timing of the initial opening of the Proto-Tethys Ocean in the Qilian Mountains area,it is believed that the breakup and oceanization processes in the North Qilian area during the Late Neoproterozoic to Early Paleozoic were not carried out simultaneously,i.e.,the western sector had an earlier initiation compared to the eastern sector.