The Early Mesozoic Chronostratigraphic Framework Of The Yanshan Fold-thrust Belt, North China And Its Implications

The North China Craton?NCC?remained stable for an extended period following its final cratonization at¹.80 Ga,and underwent several stages of contraction and extension during the Mesozoic,resulting intense tectonic deformation,frequent magmatic activities,dramatic geomorphic diversity and the formation of a series intermountain basin.The Mesozoic is a key translation period for the NCC from the stable to the active phase.The Yanshan Fold-and-Thrust Belt?YFTB?is a classic intraplate deformation belt located in the northern part of the NCC that underwent an important transition from the Paleo-Asian Ocean to the Paleo-Pacific tectonic realms during the Mesozoic,and it is a classic study area for the magmatic activities,tectonic deformation,basin evolution,craton destruction and plate dynamics.The establishment of accurate chronostratigraphic framework,which is basic for understanding the tectonic events and deformation,basins research,and paleontology evolution,depends on the systemic and detailed geology survey and accurate dating work on key strata and important plutons.The previous studies focus on the late Mesozoic?Late Jurassic to Cretaceous?,and our works emphasize the early Mesozoic.We choose typical cross-sections in classic basins and collect samples for isotopic dating.Combine with the previous studies,we try to establish an accurate early Mesozoic chronostratigraphic framework in the YFTB,northern margin of the NCC.The Upper Triassic strata in the YFTB mainly consist of terrigenous clastics,and short of fossils and dateable volcanic rocks,which limits the acknowledgements of the ages of the strata.The pyroxene andesite sample collected from the base of the Upper Triassic volcanic rocks yield zircon LA–ICP–MS U–Pb age of 229±3 Ma and biotitic⁴⁰Ar–³⁹Ar plateau age of 232.6±0.6 Ma.Different methods yield the consistent ages within the error range,indicating their reliability.The volcanic rocks,which overlay the flint band dolomite of the Jixian Group,are the lowest in the stratigraphic sequence of the Xiabancheng basin.The Liujiagou,Heshanggou and Ermaying formations overlie theses volcanic rocks,whose age is²30 Ma,indicating all of them are Late Triassic in age.The realization of the volcanic rocks,which were signed to be the Jurassic Nandaling Formation,would lead to the rethinking and discussion on the stratigraphic division and correlation,basin evolution and tectonic events.The“Yanshanian movement?tectonism?”was first recognized in the Western Hills of Beijing,where is the birthplace of modern geological studies of China.The“Yanshanian movement”is defined by major unconformities in the stratigraphic record resulting from deformation,and represents a period of intensive tectonic deformation.The Yanshanian tectonism is considered to be the most intense Mesozoic tectonic event which has defined the tectonic framework of eastern China.The extensive and significant influence of the Yanshanian tectonism throughout North China and eastern China has long been acknowledged.Extensive magmatism,widespread folding and faulting,and the formation of a series of basins have all been attributed to the Yanshanian tectonism.The Yanshanian tectonism was considered to occur during the time period from the Jurassic to the Cretaceous.However,it is an ambiguous concept that does not conform to the principles of tectonic stages,and the timing for initiation of the Yanshanian tectonism is poorly constrained.The Jurassic sedimentary succession in the Western Hills of Beijing consists of two volcanic units and interbedded coal-bearing strata,which is similar to those in other regions of the YFTB.As the earliest Mesozoic volcanism in the Western Hills of Beijing,the Nandaling Formation contains the earliest volcanic and sedimentary records that are associated with the initiation of the Yanshanian tectonism.Previous researches considered the Nandaling volcanic rocks as Early Jurassic in age based on the floral fossil assemblage of Coniopteris–Phoenicopsis in the Yaopo Formation that conformably overlies the Nandaling Formation.In this study we perform a systematic geological survey and present the results of zircon U–Pb dating of the Nandaling volcanic rocks to demonstrate that both the Nandaling volcanic rocks and the overlying floral fossil assemblage are Middle Jurassic in age.LA–ICP–MS zircon U–Pb dating of the Nandaling volcanic rocks in the northern part of the basin yields eruption ages of 175±1 to 168±2 Ma.Dioritic porphyry sills are intruded concordantly into folded Mesoproterozoic–lower Paleozoic strata of the Qingbaikou anticline,and yield a zircon U–Pb age of 177±1 Ma.These ages indicate that the initiation of the Yanshanian basins,and therefore the initiation of the Yanshanian tectonism,occurred no earlier than the Middle Jurassic.Our ⁴⁰Ar–³⁹Ar dating of the Nandaling volcanic rocks yields younger ages of 138–151 Ma that show a southward–younging trend,indicating that the Nandaling volcanic rocks in the Western Hills of Beijing were overprinted by a tectono–thermal event that peaked during the Early Cretaceous.The HaifanggouFormationconsistsofsedimentaryclasticrocksand intermediate–acid volcanic rocks,and overlies the Beipiao coal-bearing Formation or older strata with angular unconformity.The angular unconformity beneath the Haifanggou Formation was regard as the production of the main episode of the“Yanshanian tectonism”and the discovery of Schmeissneria sinensis,Xingxueanthus sinensis,Euanthus Panii in the Haifanggou Formation provides key evidence for the origin and evolution of angiosperm.Therefore,the dating of Haifanggou Formation is significant to understand the Mesozoic tectonic events and angiosperm evolution.We chose two cross–sections and collected samples for zircon LA–ICP–MS and SHRIMP U–Pb dating,and obtained ages ranging 169–164 Ma.Combine with the previous studies,the ages of Haifanggou and Beipiao formations are limited to be 169–161 Ma and 177–169 Ma,respectively.Therefore,the ages of Beipiao Formation are older than Yaopo Formation,and the origin of angiosperm is no later than the Middle Jurassic.A series of intermountain basins were formed during Mesozoic in the YFTB,where widely distributed of volcanic and sedimentary rocks.The dating of strata mainly depends on either the plant fossils or isotopic ages of volcanic rocks.In this paper,we combine two methods and establish an accurate chronostratigraphic framework of the YFTB.We subdivid the early Mesozoic deformation into four phases:The YFTB underwent the NS–trending contraction during Middle Triassic?D₁?,which could result from the closure of the Paleo-Asian Ocean to the north or the amalgamated of the Yangtze and the NCC to the south.The tectonic environment of the YFTB during Late Triassic to Early Jurassic?D₂?is still puzzling.The different opinions mainly focus on the contact relationship of Lower Jurassic conglomerates and the older strata.Some researchers considered that the Xingshikou Formation was syntectonic conglomerates and unconformably overlay upon the older strata,indicating a contractile environment.On the other hand,some other geologists denied the existence of the angular unconformity and argued that the Upper Triassic to Lower Jurassic conglomerates,such as Xingshikou and Dengzhagnzi formations,deposited in an extensional environment.The Triassic to Early Jurassic deformational events?D₁ and D₂?were intense in western Liaoning and northern Hebei while had little influence in the Western Hills of Beijing.Distribution of the deformation suggests that the YFTB was partly influenced and the spatial migration of deformation weakened from the margin to the interior of the NCC.The angular unconformities under the Haifanggou and Tiaojishan/Lianqi formations were considered to represent the main episode of the Yanshanian movement.However,it has been demonstrated the Haifanggou Formation is much older than the Tiaojishan/Lanqi Formation.Thus the D₃ and D₄deformational events are distinguished in this study.The discrepancy of the ages of angular unconformities beneath Tiaojishan and Haifanggou formations maybe represent the temporal and spatial variation of a tectonic event or respond to different tectonic events.No matter which speculate,more detailed field survey and stronger evidence are necessary.Jurassic coal-bearing strata are widely distributed in the NCC and other areas of northern China.These coal-bearing strata were previously considered to be Early–Middle Jurassic in age based on plant fossils,particularly the fossil assemblage of Coniopteris–Phoenicopsis.Since coal-bearing strata are interbedded with volcanic units in the basins of the YFTB,northern NCC,isotopic dating of the volcanic units can therefore provide age constraints on the coal-bearing strata and the Coniopteris?Phoenicopsis assemblage.In this paper,we performed a systematic geological survey and present the results of zircon U–Pb dating of the volcanic units and a pluton in typical basins of the YFTB.These data,combined with the results of previous studies,indicate that the ages of the Nandaling/Xinglonggou,Haifanggou,Jiulongshan,and Tiaojishan/Lanqi formations are 180–168,169–161,161–157,and 161–153 Ma,respectively.The ages of the interbedded coal-bearing Yaopo and Beipiao formations are constrained to be 169–161 and177–169 Ma,respectively.Our results demonstrate that both the coal-bearing strata and the Coniopteris?Phoenicopsis assemblage are Middle Jurassic in age,which is younger than that previously considered.This fossil assemblage plays a critical role in age constraints on the Jurassic coal-bearing strata.The refinement of its age gives a more precise dating of the coal-bearing strata,especially in northwestern China,where datable interbedded volcanic units are lacking.