华北克拉通北缘化德群中碎屑锆石的LA-ICP-MS U-Pb年龄及其构造意义

化德群出露地区位于华北克拉通北缘中部,紧邻中亚造山带南缘,呈近东西向展布。在它的西边是早-中元古代的白云鄂博裂谷和渣尔泰—狼山裂谷,东南面是由长城系、蓟县系和青白口系组成的早-新元古代的燕辽裂陷槽,南边分布着1.9~1.8Ga麻粒岩相变质的丰镇群（孔兹岩系）,北边出露有代表中亚造山带的古生代岩石。化德群由一套浅变质和未变质的沉积岩组成,无火山岩夹层。地层序列包含多个沉积旋回,每个旋回自下而上为含砾砂岩、砂岩、碳酸盐岩和泥质岩。岩石组合反映了从河流—滨海—浅海相的沉积环境。化德群的地层序列可以和白云鄂博群及渣尔泰群相对比。本文对化德群四个变质砂岩样品中的碎屑锆石进行了LA-ICP-MS U-Pb年龄测定,年龄主要集中在1800±50Ma和1850±50Ma,另外还有~2500Ma和~2000Ma的次要峰值。化德群底部变质含砾云母长石石英砂岩中碎屑锆石的最小谐和年龄是1758±7Ma,限定了化德群沉积时代的下限。碎屑锆石的CL图像显示,1800±50Ma和1850±50Ma的锆石主要是变质成因,少量岩浆成因,说明化德群的源区主要是古元古代的变质岩,少量岩浆岩。~2500Ma和~2000Ma的碎屑锆石代表了更为古老的源区。碎屑锆石的U-Pb年龄限制了化德群的沉积时代为古元古代晚期—中元古代,年龄峰值对应华北克拉通的重要构造热事件,而无与中亚造山带地质事件相关的年龄信息。沉积组合特征表明化德群属于稳定的浅水—半深水沉积盆地。化德盆地、渣尔泰—狼山盆地和白云鄂博盆地共同构成华北克拉通北缘的被动陆缘裂谷系,该裂谷系的形成可能与燕辽及熊耳裂陷槽的打开是同时期的。因此,华北克拉通的北界应该置于化德群出露区域以北。基于锆石特征的详细分析及对比,我们认为化德群以南的孔兹岩系可能是化德群的主要源区。

LA-ICP-MS U-Pb geochronology of detrital zircons from the Huade Group in the northern margin of the North China Craton and its tectonic significance

The Huade Group, consisting of low-grade and un-metamorphosed sedimentary rocks without volcanic rock, is located at the northern margin of the North China Craton and adjoining the south part of the Central Asian Orogenic Belt. It is east to the Paleo-Mesoproterozoic Bayan Obo and Zhaertai-Langshan rifts and northwest to the Paleo-Neoproterozoic Yanshan aulacogen, in which the typical Changcheng, Jixian and Qingbaikou systems are developed. The Fengzhen Group (khondalite series), which were metamorphosed to granulite facies with 1.9~1.8Ga U-Pb isotopic ages, occur south to the Huade Group. The Paleozoic rocks of the Central Asian Orogenic Belt outcrop north to the Huade Group. The stratigraphic sequences show several cycles of deposition. Each of them developed pebbly sandstones, sandstones, carbonate rocks, pelites from bottom upward. The rock association indicates the sedimentary environments evolving from fluvial through littoral to shallow marine facies. The stratigraphic sequences of the Huade Group are comparable to those of the Banyan Obo Group and the Zhaertai Group. The LA-ICP-MS U-Pb ages of the detrital zircons from four samples of the Huade Group show two main age peaks at 1800±50Ma and 1850±50Ma, as well as two other minor age peaks at ~2500Ma and ~2000Ma. All the four age populations correspond to significant tectonic-thermal events of the North China Craton. The youngest concordant age of the detrital zircons from meta-pebbly arkosite at the bottom of the Huade Group is 1758±7Ma, which constrains the oldest sedimentary age of this group. According to the CL images of the zircon grains, the main peaks at 1800±50Ma and 1850±50Ma are mostly of metamorphic grains, and a few of magmatic ones. This indicates that the provenances of the Huade Group are mainly Paleoproterozoic metamorphic rocks, with minor magmatic rocks. The detrital zircons of ~2500Ma and ~2000Ma represent older provenances of the Huade Group. The U-Pb ages of detrital zircons constrain the sedimentary age of the Huade Group from Late Paleoproterozoic to Mesoproteozoic. The age populations correspond to significant tectonic-thermal events of the North China Craton and there is no Paleozoic age information related to geological events of the Central Asian Orogenic Belt.The stratigraphic characteristics indicate the Huade Group is a stable shallow-hypabyssal sedimentary basin. The Huade basin, Zhaertai-Langshan basin and Bayan Obo basin commonly constitute a passive continental margin rift system at the northern margin of the North China Craton. This rift would have initiated coeval with the Yanshan and Xionger aulacogens. It is reasonable to suggest that the boundary between the North China Craton and the Central Asian Orogenic Belt is further north to the Huade Basin. Based on detailed analyses of the characteristics of detrital zircons, we suggest that the khondalite series south to the Huade Group were likely to be the main provenance of the Huade Group.