Intracontinental Deformation And Magmatism Of The North China Craton Induced By Subduction Of The Paleo-pacific Plate

The east Asian continent borders the western Pacific,it has been a consensus that the evolution of East Asian continent was controlled by the subduction of the westernPacific plate since Early Jurassic.The subduction of the western Pacific plate has played an important role in the major geological events in eastern China during the Mesozoic?Cenozoic,such as the destruction of the North China Craton(NCC),the Yanshanian movement,the reconstruction of the South China Continent,the oil-gas basins and the formation of large-scale metal mineral resources.However,there are still divergence and deficiencies in the understanding of the detailed subduction process of the paleoPacific plate,and the relationship with the Yanshanian movement and the destruction of the NCC.Northeast Asia is close to the western Pacific Ocean,and there are extensive magma activities and accretive complex belts related to the subduction of the paleo-Pacific plate,which is an excellent place to study the detailed subduction process of the paleo-Pacific plate.Through statistical analysis of Mesozoic magmatism and porphyry-skarn deposit in the north margin of NCC,Shandong peninsula,Northeast China,Korean peninsula and Japanese islands,we finds that: the Jurassic magma and ore deposits presented NNW trend,and migrated from NE to SW from Early Jurassic to Late Jurassic,indicating that paleo-pacific subducted toward SW during the Jurassic;the Cretaceous magma and ore deposits presented NE trend,and migrated from NW to SE from Early Cretaceous to Late Cretaceous,indicating that paleo-pacific subducted toward NW and retreated toward SE during the Cretaceous,and the subduction direction changed from SW to NW during Late Jurassic.During Early Jurassic,the Tan-lu fault zone was partially resurrected,and strong dextral ductile shear occurred in the Honam shear zone in the Korean peninsula and the Funatsu shear zone in Japan.During Early Cretaceous,the Tan-lu fault zone showed a strong left-lateral strike-slip movement,the Honam and Funatsu shear zone showed a strong left-lateral brittle shear,also indicating that the paleo-Pacific plate subducted toward SW during Early Jurassic and subducted toward NW during Early Cretaceous.During Late Mesozoic,the Qinling-Dabie orogenic belt experienced intense intracontinental orogeny,accompanied by extensive magmatism and Mo-Cu-Au mineralization.During Early-Middle Jurassic(190?160 Ma),Dabashan in South Qinling thrusted into SW to form an arc-type tectonic belt,significant uplift and denudation occurred in the Dabie-Sulu orogenic belt in this period.The North China Craton and South China plate collision was completed during Late Triassic and transformed into post-collision extension,characterized by extensive a-type granite,rapakiwi granite and lamprophyre.Therefore,the intra-continental orogeny of the Qinling-Dabie belt in the Early-Middle Jurassic period is not related to the continuous convergence between NCC with South China plate,but is closely related to the far effect of SW subduction of the paleo-Pacific plate.From ~160 Ma,large-scale magmatic activities and porphyry-skarn-type Mo?Cu?Au deposits appeared in the Qinling-Dabie orogeny,those rocks show high oxygen fugidity.Previous studies have shown that porphyry deposits are closely related to the subduction of the paleo-Pacific plate.In addition,some high magnesium adakites related to porphyry copper deposits are exposed in this area,and geochemical data indicate that the rocks originated from partial melting of oceanic crust related to the subduction of the paleo-Pacific Ocean.160 Ma may indicate that the Qinling-Dabie orogen began to extend from compression,which may be closely related to the change of the paleo-Pacific plate subduction direction from SW to NW.The spatial and temporal scope,and dynamic background of the Yanshanian movement have always been controversial.During Late Mesozoic,the YinshanYanshan tectonic belt on the northern margin of the NCC shows the double features of dextral striking-slip and thrust faults,the Qinling-Dabie orogenic belt on the southern margin developed a series of sinistral striking-slip and thrust faults,the centrally located Tai-hang Mountains experienced remarkable uplift due to thrust faults and folds,and the east of the TLF was rejuvenated.Through detailed tectonic analysis,these tectonic deformations could be plausibly interpreted as a unitary geodynamic regime governed by the westward indentation of the wedge-shaped NCC,driven by subduction of the paleo-Pacific plate.This view has been well verified by the sand simulation experiment.The mechanism of lithospheric removal and destruction of the North China Craton(NCC)has been hotly debated for decades.It is now generally accepted that the subduction of the(Paleo)-Pacific plate played an important role in this process.However,how the plate subduction contributed to the craton destruction remains unclear.In this paper,we found that the Yunmengshan granite and Fangshan granodiorite in the north margin of NCC contains very high oxygen fugacity characteristics.Statistics show that high oxygen fugacity magma is very widespread in NCC.It is shown that the characteristics of high oxygen fugacity are neither derived from the basement rocks nor gradually accumulated by magmatic evolution,but are caused by the input of oxidizing materials.Since the Early Jurassic,the North China Craton has been at the edge of the active continent.A large number of fluids and melts released by the subduction plates entered the mantle wedge and changed the mantle wedge,the mantle wedge beneath NCC gradually become oxidizing.During this process,the strength of the mantle wedge gradually decreased,which may be the prelude to the destruction of NCC.Lithospheric thinning occurred in the NCC during Late Mesozoic,and the mixing of oxidized mafic magma with felsic magma may be the main way for the formation of high oxygen fugacity magma.The relationship between the Yanshanian movement,the destruction of NCC and the subduction of the paleo-Pacific plate has always been confused.In this paper,a model is presented to explain the relationship between the three.The Yanshanian movement was mainly characterized by two stages extrusion of the Mid-late Jurassic(170?165 Ma)and Early Cretaceous(140?135 Ma).After 135 Ma,lithospheric thinning and destruction occurred in the NCC,which was characterized by strong extensional deformation and extensive magmatic activity,corresponding to the process of the retreat of the paleo-Pacific plate.The Yanshanian movement and the destruction of NCC are closely related to the subduction of the paleo-Pacific plate,but they correspond to different subduction stages.The Fangshan intrusive suite is a composite pluton in the North China Craton that has been suggested to result from the incremental assembly of small magma batches.The pluton consists of four intrusive units with abundant mafic enclaves.In this contribution,the history of crystallisation is detailed through zircon LA-ICP-MS U-Pb dating,which records the prolonged crystallisation of each intrusive unit at uppercrustal levels between 132.5–128.7 Ma.The magmas were episodically extracted from a deep storage area and ascended to the final intrusion level at a palaeo-depth of 10-16 km.Zircon trace element compositions,Ti-in-zircon temperatures and Hf isotopic compositions from the four intrusive units and mafic enclaves show significant differences and suggest that their crystallisation occurred in isotopically and chemically diverse magma batches.These magma batches formed in the lower crust from the mingling and mixing of various proportions of residual melts,derived from the fractional crystallisation of mafic magmas,with crustal partial melts at high temperatures.Four type of zircons were observed in mafic enclaves based on a simple textural classification,including antecrysts and autocrysts(type 1),xenocrysts(type 2 and type 3),and recrystallised zircon(type 4),recording the entire evolutionary history of mafic magmas.Part of type 1 zircons were not formed at the emplacement level and are “antecrystic”,having formed at a deeper,hotter level and been entrained into the ascending melts.Most type 2 zircons were captured from coarse-grained monzonite,and a very small amount of grains were sourced from porphyritic granodiorite.Type 3 zircons display a core-rim texture,illustrating that xenocrysts may successively grow in mafic melts.Type 4 zircons display patchy zoning that represents a disequilibrium texture,manifested by the replacement of U–Th–REE-rich zircon by U–Th–REE-poor zircon,which occurred in response to magma mixing between mafic and felsic melts.This study shows that zircon chemistry coupled with detailed textural analyses can provide a powerful tool to elucidate the complex evolution of a magmatic system.