柴达木盆地北缘全吉群红藻山组凝灰岩锆石U-Pb年龄及其地质意义

U-Pb zircon ages of tuff beds from the Hongzaoshan Formation of the Quanji Group in the north margin of the Qaidam Basin,NW China,and their geological significances

全吉群是不整合于柴达木北缘全吉（欧龙布鲁克）地块变质基底之上的第一套盖层型沉积,目前多数研究者认为其时代大体为新元古代晚期。本文在柴北缘全吉山的全吉群红藻山组建组剖面上,对位于该组下部的两层凝灰岩开展了LA-MC-ICPMS锆石U-Pb同位素定年研究,获得其岩浆锆石的U-Pb同位素年龄分别为（1 640±15）Ma和（1 646±20）Ma,标定了红藻山组的形成年龄,为柴北缘中新元古界年代地层的重新厘定与划分,提供了新的、直接的年代学约束,具有重要的科学意义。首先,这一新进展表明,在柴北缘地区,传统的全吉群红藻山组以下各组的时代,并非一直以来的＂新元古代晚期南华纪—震旦纪（埃迪卡拉纪）＂,而应属于国内的中元古代长城纪、国际上的古元古代固结纪（Statherian,1 800～1 600Ma）晚期;其次,根据当前定年并结合新近在红藻山组与上覆地层之间发现有古风化壳及区域性不整合、全吉群上部发育有埃迪卡拉纪化石及冰碛砾岩等证据,本文进一步确认,柴北缘全吉群在红藻山组与其上覆黑土山组等地层单位之间其实存在着相当长时间的地层缺失,传统的＂全吉群＂应予解体。由此并参考已有资料,本文认为,早先代表柴达木地块形成的＂全吉运动＂并不能与华南的＂晋宁运动＂对比,而应与华北的＂吕梁运动＂相当,全吉地块乃至整个柴达木及周边地区的中、新元古代地质演化历史及其大地构造亲缘属性等,都应重新予以考虑。

Located between the North Qaidam Block and the Qilian Orogen,the Quanji Massif（i.e.Oulongbuluke microcontinent）is characterized by a cratonic continental remnant composed of a medium-to high-grade metamorphosed basement and an unmetamorphosed cover,representing the typical double-crust units.The Quanji Group,which unconformably overlies the Palaeoproterozoic Dakendaban Group and underlies the Cambrian Oulongbuluke Group,is the first sedimentary cover of the Quanji Massif.At present,with imprecise constraints,most geologists believe that the Quanji Group is the late Neoproterozoic in age.In this study,as the first time,we report the LA-MC-ICPMS zircon U-Pb ages of the tuff from the Hongzaoshan Formation of the Quanji Group, which have been divided into Mahuanggou, Kubaimu,Shiyingliang,Hongzaoshan,Heitupo,Hongtiegou,and Zhoujieshan formations,in ascending order.Two LA-MC-ICPMS zircon U-Pb ages,1640±15 Ma and 1646±20 Ma,were obtained from the tuff beds in lower part of the carbonate-dominated Hongzaoshan Formation at the Quanjishan section,where the Hongzaoshan Formation was defined as a formal lithostratigraphic unit in 1980＇s.These new ages suggest that the lower Quanji Group,including the Mahuanggou,Kubaimu,Shiyingliang,and Hongzaoshan formations,should be revised into late Paleoproterozoic（Statherian Period）time. Combined with the palaeo-weathering crust and disconformity on the top of the Hongzaoshan Formation,and the previous studies on Ediacaran fauna in the Zhoujieshan and glacial diamictite in the Hongzaoshan Formation,it may infer that there should be a huge time gap between the Hongzaoshan Formation and the overlying formations,and the Quanji Group,as a lithostratigraphic unit,should be redefined.Moreover,now it may suggest that the ＂Quanji Movement＂,represented by the unconformity between Dakendaban Group and Quanji Group and conventionally corresponding to the～850Ma Jinning Movement in Yangtze Craton,should correlate to the～1.8Ga Lüliang Movement in North China Craton（NCC）.It seems that,during the Mesoproterozoic time,the Quanji Massif should have more closely tectonic affinity with the NCC than that with the Yangtze Craton.