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(kbondalite 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.