Late Paleozoic Tectonic Evolution Of The Eastern Segment Of The Northern Margin Of The North China Craton

The tectonic melange belts developed in the eastern segment of the northern margin of the North China Craton(NCC)provide valuable constraints for studying the closure of the Paleo Asian Ocean and subsequent formation and evolution of Central Asian Orogenic Belt.Due to the lack of Devonian and Carboniferous geological records in the eastern segment of the northern margin of the NCC,the study of tectonic evolution during the early Late Paleozoic is relatively weak.As an important part of the tectonic melange belt,the Late Paleozoic Xia'ertai Group has recorded a lot of Late Paleozoic geological information,however,its structural attributes and genesis still need to be studied in detail.Therefore,this paper focuses on the material composition,and rock association of the Xia'ertai Group,by through the systematic geological survey,mineralogy and petrography.Meanwhile,the petrogenesis,source nature,material source,formation age and tectonic environment of the geologic bodies will be determined,based on the geochemistry,zircon U-Pb isotopic chronology.Finally,the tectonic attributes and genesis of the Xia'ertai Group will be discussed.Combined with the regional published geological achievement,this research aims to summarize the Late Paleozoic tectonic-magmatic events in the eastern part of the northern margin of the NCC,and to establish the late Paleozoic tectonic evolution model.Together they may provide an important scientific basis for exploring the tectonic evolution of the eastern segment of the northern margin of the NCC.The material composition of the Xia'ertai Group,in order from southeast to northwest,is Middle Devonian metamorphic terrigenous clastic rocks,Early Carboniferous metamorphic volcanic rocks,Late Carboniferous metamorphic carbonate rocks with clastic rocks layers and Early-Middle Permian metamorphic volcanoclastic rocks,all of which are oriented in a northeast direction.The Middle Devonian metamorphic terrigenous clastic rocks are mainly mica quartz schist and monzonitic granulite.Their protoliths are a set of sandstones from the felsic igneous source area.The clastic rocks are relatively enriched in light rare earth elements(LREEs)and depleted in heavy rare earth elements(HREEs),which suggests that fractional distillation of rare earth elements(REEs)is obvious,and the Eu negative anomaly is not.The zircon LA-ICP-MS U-Pb ages of the metamorphic terrigenous clastic rocks range from 384 Ma to 443 Ma,and the age spectrum shows three age peaks,namely 392 Ma,423 Ma and 443 Ma,indicating three stages of magmatic thermal events.With the lower age-limit of 384 Ma(Middle Devonian),the metamorphic terrigenous clastic rocks are thought to be formed in a relatively stable environment at the active continental margin.The rock types of the Early Carboniferous metamorphic volcanic rocks are mainly metamorphic rhyolite,metamorphic dacite,mica(monzonitic)microcrystalline schist,mica quartz schist,(epidote)amphibole plagioclase schist,plagioclase(epidote)amphibole schist,plagioclase leptite,amphibole plagioclase granulite,amphibole plagioclase gneiss.Their protoliths are a set of intermediate-acid volcanics and pyroclastic rocks,belonging to calc-alkaline series,the set of volcanics is metaluminous-weak peraluminous in its composition.According to petrographic and geochemical characteristics,they can be divided into metamorphic acid volcanic rocks and metamorphic neutral volcanic rocks,which are both relatively rich in LREEs and depleted in HREEs,indicating an obvious fractionation,while the Eu negative anomaly is not so distinct.The total content of REEs in metamorphic neutral volcanic rocks is less than that of metamorphic acid volcanic rocks,with the latter obviously deficient in Sr and P.With their exposed area and HFSEs correlation characteristics taken into consideration,the two are thought not to differentiate from a single basic magma.The metamorphic volcanics contain zircons with the LA-ICP-MS U-Pb age of 341?348 Ma,which indicates that the crystallization ages of their protoliths are Early Carboniferous.The original magma of metamorphic acid volcanic rocks comes from partial melting of crustal materials,while the metamorphic neutral volcanic rocks are driven from the lithospheric mantle near the subduction zone and mixed with crustal materials.Both two types of rocks are formed in a volcanic arc environment at the active continental margin.The Late Carboniferous metamorphic rocks are mainly calcite marble and graphite marble,interbedded with mica plagioclase granulite,plagioclase leptite,mica microcrystalline schist.Their protoliths are a set of marine carbonate rocks interbedded with clastic rocks.The main elements of metamorphic carbonate rocks are mainly CaO and MgO,and the content of REEs is very low.The LREEs are relatively deficient,while the HREEs are relatively enriched.The fractional distillation of the REEs is relatively weak and the negative anomaly of Eu is obvious.The conodontic fossils are found in the metamorphic carbonate rocks,indicating their protoliths formation ages are Late Carboniferous.The zircon LA-ICP-MS U-Pb ages of the carbonaceous biotite microcrystalline schist interbeddings in the marbles indicate formation ages between 304 Ma and 342 Ma,which determining the lower limit of its deposition to be 304 Ma(Late Carboniferous).The formation of metamorphic carbonate rocks is influenced by hydrothermal processes and they are formed in an anoxic environment between the ocean floor and the continental margin.The Early-Middle Permian metamorphic volcano-clastic rocks are composed mainly of metamorphic clastic rocks and interbedded with metamorphic volcanic rocks,and the two mingled in the field.The metamorphic clastic rocks are mainly mica quartz schist,epidote quartz schist and garnet monzonitic granulite,the protoliths of the metamorphic rocks are argillo-arenaceous and arenopelitic.In meta-clastic rocks,LREEs are relatively deficient,while HREEs are relatively enriched,indicating a strong fractional distillation,however,the Eu anomaly is not obvious.The zircon LA-ICP-MS U-Pb ages of the two samples are mainly between 267 Ma and 332 Ma,with their lower limits of deposition ages 267 Ma and 269 Ma respectively,both belonging to the Middle Permian.The argillo-arenaceous likely comes from recirculating sedimentary rocks with parent rocks of felsic rocks,while the arenopelitic is driven from recirculating sedimentary rocks with parent rocks of both felsic and mafic rocks,both of which are formed in the Continental Island Arc(CIA)and Oceanic Island Arc(OIA)environments,respectively.The rock types of metamorphic volcanic rocks are mainly mica schist,feldspar two-mica schist,plagioclase amphibole schist,and epidote propylite(meta-andesite),they are a set of calc-alkaline volcanic rocks that are metaluminous-weak peraluminous.According to petrographic and geochemical characteristics,they are divided into metamorphic acid volcanic rocks and metamorphic intermediate-basic volcanic rocks.Both of them were relatively enriched in LREEs and depleted in HREEs,indicating an obvious fractional distillation of light and heavy rare earth elements,while the Eu anomaly was not obvious.However,they have different contents of rare earth elements,with the metamorphic acid volcanic rocks obviously deficient in P and Ti.Combined with the correlation characteristics of high field strength elements,it is considered that they are not the products of the same magma differentiation.The LA-ICP-MS U-Pb ages of zircons from the meta-volcanic rocks range from 272 Ma to 288 Ma,representing their crystallization ages of Early Permian.The original magma of metamorphic acid volcanic rocks came from the partial melting of crustal materials,while the original magma of metamorphic intermediate-basic volcanic rocks came from the lithospheric mantle near the subduction zone and suffered from the mixing of crustal materials.Both of them formed in a volcanic arc environment at the active continental margin.The material composition of the Xia'ertai Group has various characteristics of formation ages and different tectonic environments.During the process of oceanic crust subduction,various geologic bodies with different ages and environments were,in turn,coll aged laterally,and then transformed by later tectonic events.Therefore,the"Xia'ertai Group" is composed of a series of tectonic slices,it is more reasonable to define it as "Xia'ertai" tectonic complex.The "Xia'ertai" tectonic complex indicates that the eastern part of the northern margin of the NCC was in an active continental margin during the Late Paleozoic,and its tectonic evolution can be divided into six stages,that is,arc-continent collision and post-orogenic stage in Devonian,active continental margin arc forming stage in Early Carboniferous,back-arc spreading stage in which fore-arc and back-arc basins formed in Late Devonian,arc-continent collisional and new continental arc forming stage in Early Permian,and pre-continental collision stage in Late Permian.