Granitic Magmatism And Constraints On Crustal Evolution In The Southern Margin Of The North China Craton

The southern margin of the North China Craton is mainly located in the Yu-Shaan Rift Valley along the southern part of the North China Craton.It has a large number of Paleoproterozoic mafic rocks,volcanic rocks and granites,which is an important geological unit of the North China Craton during Precambrian.In addition,some alkaline granites are also distributed in the area.However,the research on these granites is relatively rare,and the record of the geological evolution history of the North China Craton is still unclear.In this paper,we carry out Longwangchuang granite,which is the largeset Paleoproterozoic alkaline rock intrusion in the area.The geochronology and geochemistry of Paleoproterozoic Longwangchuang granite are also reported,which supply a better understanding for the Paleoproterozoic acidic magmatism in the southern margin of the North China Craton.The Longwangchuang granite pluton represents a typical rare example of the Paleoproterozoic intrusions of A-type granite in the southern margin of this craton.This pluton consists of arfvedsonite syenogranite and syenogranite,with similar high silica(Si02=71.1 to 73.4 wt.%),high alkaline(Na2O+K2O=8.10 to 9.26 wt.%,K2O/Na2O>1)and low Al2O3(11.8 to 12.8 wt.%)contents.Syenogranite rocks display metaluminous to weakly peraluminous bulk chemistry,with ASI[molar Al2O3/(Na2O+K2O+CaO)]values between 0.93 and 0.98.Geochemically,they show typical characteristics of A-type granites,such as high 10000Ga/Al ratios(4.10-7.28),high FeOtot/(FeOtot+MgO)ratios,high Zr(484-1082 ppm),Ce(201-560 ppm)and Y(78-156 ppm)contents.Analyzed samples are significantly enriched in large ion lithospheric(LILE)and rare earth elements with strong negative Eu anomalies.Trace element spider diagrams show enrichment in Rb,Th and Pb whereas strong negative anomalies of Ba,Sr,Eu and Ti,indicating crystal fractionation process.Zircon grains from arfVedsonite syenogranite and syenogranite yield U-Pb ages of 1601 ± 10 Ma(MSWD=0.93)and 1609±18 Ma(MSWD=2.0),respectively.Their ?Nd(t)values show variation from-0.2 to-5.2.Based on geochemistry and enrichment in Nd isotopes,the Longwangchuang A-type granite is interpreted as partial melting product of basement rocks like gneisses of the Taihua Group.Together with previously published data obtained from other A-type granite plutons of?1.80 Ga to 1.53 Ga,exposed in the southern margin of the craton,it is observable that these A-type granites originated from a similar magma source.But,geochemically they can be divided into two groups:A2-type forming earlier during?1.8-1.6 Ga and A1-type later during?1.6-1.5 Ga.In combination with the variation in whole-rock Nd and zircon Hf isotopic composition at ca.1.6 Ga,it is proposed that a tectonic transformation event had happened in the Columbia supercontinent at that time.Since the Phanerozoic,the southern margin of the North China block has also become an important part of the Qinling Orogenic Belt.The Qinling Orogenic Belt presents a complex tectonic evolution history.Multidisciplinary researches show that the current tectonic units can be divided into three evolution periods,the formation of the orogenic belt basement during Late Archean-Paleozoic,the plate tectonic evolution stage during Neoproterozoic to Middle Triassic,and the intro-continent evolution during Mesozoic and Cenozoic.The late Mesozoic granites are widely distributed in the southern margin of the North China Craton.In addition,they have close genetic association with the largest Mo-(Au-Ag)polymetallic mineralization belt in China.The study of the petrogenesis of the Late Mesozoic granites is of great significance for understanding the growth and differentiation of the continent,which can provide constraints for crust evolution in the Qinling orogenic belt.In the present study,zircon U-Pb ages and Sr-Nd-Pb isotopic geochemistry are reported for four representative granite plutons(Heyu in Waifangshan area;Lantian,Laoniushan and Huashan in Xiaoqinling area)exposed in the eastern Qinling terrane.We obtain the emplacement timing of typical granites and debate the characteristic of magma source in order to constrain the temporal and spatial distribution and petrogenesis of the Late Mesozoic magmatism.The study of the petrogenesis of granites is significant for understanding the growth and differentiation of the continental crust,as well as the tectonic evolution of the orogenic belt.Heyu granite formed during 145?135 Ma with two periods of magma activities.Heyu pluton mainly consists of biotite monzogranite with K-feldspar phenocryst well developed.It is also characterized by adakite-like affinity without significant Eu anomalies.Heyu pluton displays relatively high initial 87Sr/86Sr ratios ranging between 0.7063 and 0.7093,and variable ?Nd(t)values of-29.2 to-4.8.Two stage Nd model age TDM2=-2.7?-1.8 Ga.These feautures indicate genesis via reworking of the lower crust beneath North China and partial involvement of juvenile crustal material to different extents.The crystallization age of Lantian granite in Xiaoqinling area is 152?148Ma.The rock type is dominated by biotite monzogranite with K-feldspar phenocryst,which is a weak aluminous I-type granite with adakite characteristics.Lantian pluton displays high initial 87Sr/86Sr ratios ranging between 0.7071 and 0.7109,and variable ?Nd(t)values of-21.2 to-7.9.Two stage Nd model age TDM2=-2.6?-1.6 Ga.The crystallization age of Laoniushan granite is ca.148 Ma.The rock type is dominated by biotite monzogranite,which is a weak aluminous I-type granite with adakitic characteristics.Laoniushan pluton displays high initial 87Sr/86Sr ratios ranging between 0.7068 and 0.7092,and variable ?Nd(t)values of-28.7 to-14.3.Two stage Nd model age TDM2=-2.7?-2.1 Ga.The crystallization age of Huasuan granite is 136?132 Ma.The rock type is dominated by biotite monzogranite,which is a weak aluminous I-type granite.Huashan pluton shows high Sr/Y=58-87 and weak Eu negative anomaly(Eu*=0.83=0.92).It also presents high initial 87Sr/86Sr ratios ranging between 0.7087 and 0.7099,and variable ?Nd(t)values of-17.5 to-12.3.Two stage Nd model age TDM2=-2.4?-2.0 Ga.It is thought to be the product by reworking of the lower crust beneath North China.In addition,the ancient inherited zircons of 207Pb/206Pb=1.8 Ga-2.4 Ga are found in the four granites,which are consistent with the two-stage Hf model age reported.The inherited zircon U-Pb ages are also coincide with the diagenetic age of the Taihua complex in the area,suggesting that the Taihua complex is a potential source of late Mesozoic granites on the southern margin of the North China Craton.According to those reported geochemical characteristics and emplacement time of granitoids,Mesozoic granites can be further divided into two peroids:Late Jurassic to Early Cretaceous(t=150-125 Ma)and Early Early White(t<125 Ma).There is a marked variation between granitoids aged pre-and post-125 Ma in terms of spatial distribution,petrology,geochemistry and isotopic featuResearch The early period(t=160?125 Ma)magmatism occurred intensively in a transitional setting from compression to extension and was likely induced by lithosphere thinning and asthenosphere upwelling beneath eastern China in the Cretaceous.However,the late period(t<125 Ma)granite suitepresents obvious negative Eu anomaly and low Sr/Y ratios,which is formed in a much shallower source.The early period(t=160?125 Ma)granite consist of monzogranite,where as the younger suite(t<125 Ma)occurs only in the easternmost part of the orogen,which is mainly compsed of granite and syenogranite.After the post-orogenic extension collapse in the eastern Qinling-Dabie orogen at late Early Cretaceous,the late period granites belonging to intraplate alkaline A-type granites formed by partial melting of lower crust induced by lithospheric thinning and asthenospheric upwelling in the eastern China.Moreover,late period granites shows more depleted Nd and Hf isotope compositions,indicating the addation of much more newly crust materials than the earlier granites.The tectonic environment of S-NCC may complete the transition at 125 Ma.The above features indicate that the Late Mesozoic tectonic regime of the Qinling Orogen was an integrated effect of post-collisional compression-extension transition and the back-arc extension related to paleo-Pacific subduction.