Geochemical Characteristics Of Ultrabasic Rock And Its Tectonic Significance In Qinglongcun Area,Jilin Province

The study area is located in Helong City,Yanbian Korean Autonomous Prefecture,Jilin Province.Tectonically it belongs to the north side of the eastern part of the northern margin of the North China plate and the western margin of the Yanbian Hercynian period fold belt.Its north and east sides are Jiamusi massif and Khanka massif respectively.Dunhua-Mishan fault and Liangjiang-Tianqiaoling faults pass through on the west side.On the south it is separated with North China Craton by Gudonghe fault.The subject of the paper is the ultrabasic rocks in the Qinglongcun group of the Permian that develops from Wolong to Changren in Toudaozhen of the Helong city.We discuss the geological features,petrogenesis,age and tectonic environment of the ultramafic rocks by field geological investigation,petrography,geochemistry and geochronology of the ultramafic rocks in the Late Paleozoic.The ultramafic rocks are distributed in the Qilongcun metamorphic complex.They mainly composed of pyroxenite,pyroxene peridotite and peridotite.The rocks occur mainly as lenses,veins and kidney-shaped;and occur as lenses,irregular-basin,funnels in the profile.Most of the rocks are small in size: the lengths range from 100 m to 300m(the longest is 1100m),the widths range from ten to a few tens meters(the widest is 600m),the thicknesses range from a few tens to one hundred meters.The ultramafic rocks extend range from 100 m to 1400 m.The exposed areas range from 0.002 to 0.5 km2.The ultrabasic rocks contain low SiO2(39.46%~44.97%)and TiO2(0.373%~0.744%),but have high Al2O3(4.26%~9.00%),CaO(2.73%~8.19%),MgO(21.53%~30.18%)and FeO(7.52%~9.48%)as well as average Na2O+K2O(0.79%~3.51%),displaying the rocks belong to calc-alkaline series.The MF is 24.58~32.91,Mg# is from 78.96 to 84.79,m/f is 3.07~5.48.These characteristics suggest the rocks were ferruginous.The REE of the rocks changes greatly(?REE=34.405×10-6~120.054×10-6,LREE=27.865×10-6~105.690×10-6,HREE=6.372×10-6~14.364×10-6).The REE distribution pattern appears a negative slope to the right-wing in overall,showing a feature that LREE is enrichment,and HREE is deficition and exist an obvious fractionation between the two kinds REE(LREE/HREE=3.074~7.358,?Eu=0.820~0.953).Among all the trace elements K,Rb,Th are concentrated and Ta,Nb,P,Zr,Hf,Ti and Ba are endured.Geochemistry characters suggest that,the source of the ultrabasic rocks was mantle-derived magma.REE,trace and major elements display that the ultramafic rocks have a characteristic of E-MORB,and the source of the ultramafic rocks were affected by enrich-fluid and crustal or lithospheric contamination.LA-ICP-MS Zircon U-Pb age shows that the surface ages concentrate upon 210~220Ma.The surface ages form a inconcordant curve in the 206Pb/238U-207Pb/235 U concordia plot and the age on U-Pb concordia upper intercept is 251±22Ma,indicating that age of the ultrabasic rocks in the Qinglongcun Group is on the end of late Permian.The surface ages of the zircon(210~220Ma)represent the metamorphic time.It suggests that the orogenesis of Yanbian fold belt,beginning at the end of late Paleozoic may continue to the late Triassic.