Tectonic Evolution In The Central Segment Of The Northern Margin Of The North China Plate From Early Paleozoic To Dcvonian

The central segment of the northern margin of the North China Plate as a corepart of the Central Asian Orogenic Belt, is a key part of the study of geologicalevolution in the Central Asian Orogenic Belt, which experience and records theformation, development and demise of the Palao Asian tectonic domain. In this paper,based on field geological survey, explore the tectonic evolution of the northern marginof the North China Plate from Early Paleozoic to Devonian using geochronology andgeochemistry research, which is guided by the theory of plate tectonics andcontinental dynamics.1．Early Paleozoic subduction-accretion complex beltThe subduction-accretion complex consisting of the Ondor Sum ophiolite,ultrabasic rock, Deyanqimiao amphibolite and trondhjemite formed in intra-oceanicsubduction in the the central segment of the northern margin of the North China Plate,which is the product of the residual Paleoasian Ocean Plate and arc magmatism, itsformation, evolution and tectonic emplacement from the Cambrian until the middleSilurian. The Ondor Sum ophiolite is a typic SSZ(subduction belt) ophiolite suite,which formed forearc in the subduction belt. Deyanqimiao amphibolite original rockis mafic igneous rocks, which is affected by the hydrothermal fluid, experienced ahigh temperature amphibolite facies metamorphism in the late Ordovician. The lateOrdovician trondhjemite belonged low potassium tholeiitic sreies, positive anomaliesin Sr, Eu and negative anomalies in Nb, which is a transitional type between oceanplagiogranite and adakitic magmatic rock, formed in arc tectonic environment. TheB-type blue schist in Ondor Sum group high-pressure metamorphic belt is formed in the environment of the oceanic subduction-collision zone.The Paleoasian Ocean Plate subducted from north to south in early Palaeozoic, inOndor Sum group formed three fold and a ductile deformation zone. The early interlaminar overturned-recumbent fords formed in the plastic flow defermation of rocks.The middle fords are huge and diverse, trace nearly east-west axis, which are theproducts of north-south compressional deformation. The late fords’ maxumumprincipal stress orientation changes to east-west compressional deformation. There arelots of L-type and L>S type tectonites in the ductile deformation zones which islocated in the southern of the fords belt, lineation statistical analysis showed asoutheast-east stretch which is the tectonic deformation in the deeper levels of thefords belt.2．Early Paleozoic arc magmatic active beltEarly Palaezoic volcanic and intrusive rocks in Bainaimiao arc magmatic activebelt are commonly. The acidic volcanic rocks are in the lower of Bainaimiao Group,but the basic volcanic rocks of the upper, its nature of the source regions are thesubduction fluid metasomatism of the mantle source region, at the same time to acertain extent crustal material contamination, similar to Anden-type active continentalmargin volcanic rocks. The Late Cambrian garnet muscovite granite which issubjected to the occurrence of hydrothermal alteration of REE tetrad effect, has a flatREE pattern and strong negative Eu anomaly, similar to the “M” type distributionpattern of the tetrad effect, is experiencing a strong hydrothermal alteration of I-typegranites, which is an early produce of subduction of oceanic crust. The lateOrdovician quartz diorite has the characteristics of rich Na, poor K, LREE enrichmentand HREE loss, there is no obvious Eu anomaly and strongly depleted in high fieldstrength elements Nb, P, Ti in the quartz diorite, which is similar to arc subductionzone and continental margin arc volcanic rocks, is a produce of Bainaimiao arcmagmatism.3．Early Paleozoic back-arc basinThe Mid-Upper Silurian Xuniwusu Group is the lower coarse flysch formation inthe south of Bainaimiao arc magmatic rocks, its provenance comes from Bainaimiao arc magmatic rocks. The Xuniwusu Group has obvious rhythmic, which can bedevided into six major depositional cycles and each cycle is from the bottom to thetop of granularity tapering, belong to the lower of the coarse flysch formation,represents a strong hydrodynamic conditions sedimentary environment, but theShaiwusu Group which stay away from Bainaimiao arc reflect relatively stableshallow marine depositional environment. The Upper Silurian Xibiehe andSidaozhangpeng Group which is a set of molasse formation, have obvious rhythmic,they are devided into many sedimentary cycles and each cycle begins withconglomerate, tapering upward granularity, reflecting the large number of terrigenousmaterial and frequent fluctuatuons in the crust sedimentary environment. XibieheGroup belongs to orogen foreland basin molasse formation, which is formed in theextrusion of tectonic environment. It shows that Bainaimiao island arc and the NorthChina Plate happened arc-continent collision in the Late Silurian, which marks thetrench-arc-basin tectonic evolution fanished in early Paleozoic in the middle of thenorthern margin of the North China Plate.4．Devonian post-collision extensionAfter Bainaimiao island arc-continent collision in the late Silurian, begin to largescale regional extension, which formed early-middle Devonian magmatism in theGuyang, Shangdu, Zhangjiakou and Chifeng area. The middle Devonian A-typegranites in the Chifeng Jiefangyingzi and its southern regions are belong toA2-type(post-collision or post-orogenesis), which show that the northern margin ofthe North China Plate entered post-collision extension stage in the Devonian, andgeochrononlogy data show that the extension is likely to continue throughout thewhole Devonian.