Geochemical Characteristics And Forming Tectonic Environment Of Dolerite Dykes Group In Gubaoquan Area,Gansu Beishan Orogenic Belt

Beishan Orogenic Belt is located in the junction of Gansu-Xinjiang-Inner Mongolia Provinces(Autonomous Regions),which is belongs to the southern margin of the Central Asian Orogenic Belt in its tectonic region.A large number of mafic-ultramafic rocks have developed in Beishan Orogenic Belt,Gansu Province(BOBG),in addition to the Permian Liuyuan basalt,there are also many mafic-ultramafic intrusions such as Luotuoshan,Shuangjingzi,Niujuanzi,Sangejing,and plenty of dolerite dykes.BOBG is also one of the important copper polymetallic,copper-nickel,lead-zinc metallogenic belts in China,in which has discovered Heishan large copper-nickel deposits and Dashantou,Guaishishan,Luotuoshan,Xinanshan copper-nickel mineralized spots and so on.In recent years,the formation background of mafic-ultramafic rocks/dykes still remains controversial;some scholars believe that the mafic-ultramafic rocks/dykes were formed by the magmatism in continental rift environments,and the origin of magma is related in the lithospheric debasement and asthenosphere upwelling;It is also believed by other scholars that those maficultramafic rocks/dykes were form by the decompression and melting of the upwelling asthenosphere caused by the separation of the subduction plate fragments,after the plate subduction;but there are some scholars take a view that BOB mafic-ultramafic rocks/dykes are related to Tarim LIPs,which is a part of the Tarim LIPs.The mafic dykes in the Beishan area are mainly consisted by delerite dykes,which were derived from asthenosphere or lithosphere mantle,suggesting extension setting.In this paper,by researching at mineralogy,petrology,geochemistry and chronology in Gubaoquan dolerite dykes,the main achievements are as follows:1.The youngest zircon age is 285.0±4.7 Ma,indicating that the upper limit of dyke emplacement might be the early Permian.SHRIMP U-Pb age of xenoliths zircon in Gubaoquan dolerite supports that the ancient crust in Beishan area was formed before paleoproterozoic and crustal growth events occurred in the mesoproterozoic.2.Gubaoquan dolerite is generally rich in Fe but poor in Mg.It is a Fe-rich mafic rock,which type is medium-low kalium mid-ocean ridge tholeiite basalt.The trace element ratio in Gubaoquan dolerite dykes is flat,and the fractionation of LREE and HREE elements is not obvious,with a slight Eu negative anomaly.Nb and Ta are obviously deficient in trace elements,Gubaoquan dolerite magmatic source is asthenosphere plume,which is slightly endured the crustal contamination during the upwelling and invasion process,indicating that Gubaoquan dolerite dykes have different magma source regions and magmatic evolutionary processes with Tarim dolerite and basalt at same.period.3.Combining previous research results,and the magma source region and assimilation hybrid research,the results show that the Gubaoquan dolerite dykes formation background is that: in late Devonian-late Carboniferous perid,Beishan ocean basin was closed,and became into extension environment of continental rift,the depleted asthenospheric interchanged whit subducting slabs fluid,then upwelled by the influence of lithospheric devolution,and slightly endured the crustal contamination during the upwelling and invasion process,finally invaded into Gubaoquan intrusion.