Middle Paleoproterozoic Magmatism And Its Tectonic Setting In Jingqishan Area,Jiaobei

The Jiao-Liao-Ji belt is a representative Paleoproterozoic orogenic belt of the North China Craton and an important window for studying the process of aggregation and dispersal of the Colombian supercontinent and revealing the evolution of the North China Craton and Paleoproterozoic crust.At present,the Paleoproterozoic tectonic evolution model of the Jiao-Liao-Ji belt is still enigmatic,which produced by model(1)intracontinental rift opening and closing mode,(2)continental-arc-continent collision model,and(3)intracontinental Valley opening-initial ocean formation-ocean subduction-collisional orogenic model.Obviously,the systematic study for the petrogenesis and tectonic environment of metamorphic igneous rocks in the middle Paleoproterozoic(2200-2100 Ma)has important research value,it is further explored the Paleoproterozoic tectonic evolution process of the Jiao-Liao-Ji belt.The research objects include ca.2.1Ga metabasic rock and granitic rock in Jingqishan area of Jiaobei terrane.In this article,field geological survey,petrographic observations,whole-rock geochemistry and zircon U-Pb chronology data were provided for granitic rocks and metabasic rock,resulting for these(1)Combined the zircon cathodoluminescence(CL)image with LA-ICP-MS zircon dating method,the average age of the magmatic zircon of two metabasic rocks is 2110±8Ma(MSWD=0.83),2097±13Ma(MSWD=0.34),the average age of magmatic zircons of granitic rocks is 2097±22Ma(MSWD=1.8)and 2103±11Ma(MSWD=1.4),indicating that a period of 2.1Ga Paleoproterozoic magmatic activity happend in the region.(2)The whole-rock geochemistry data indicates the Silica dioxide content in granitic rock and metabasic rock respectively 69.91?72.03%and45.7?54.52%,showing that the obvious discontinuities of the two.Besides,there are obvious differences in the ratio of Na₂O/K₂O,the content of MgO and TiO₂.Combined with the 1:50,000 regional geological survey,the granitic rock is generally intruded by the basic rock in Jiaobei Jingqi mountain,and its exposed area is much wider than the metabasic rock.Therefore,the metabasic rock and granitic rock in this area form the heterogeneous bimodal rock combination.(3)Metabasic rocks show the features of iron-rich,aluminum-rich,low-potassium and low-titanium.Rocks enriched with large ion lithophile elements Rb,K,Ba and U,low Nd,depleted Ta,Nb,Zr,Hf,Th,P and other high field strength elements,showing a weak Eu anomaly(mean value of?Eu is 0.99).These reveal that it was formed in the early post-arc extensional tectonic environment,derived from the mantle original magma with weak crystallization differentiation and crustal contamination during the magma ascent.(4)The granite rock has high silicon,iron,high potassium,low calcium,and low magnesium.It attributes to the high-potassium calcium-alkaline partial aluminum A₂type granite,forming in the extensional environment of mantle magma underplating.It is related to the partial melting of the Neoarchean TTG gneiss in the overlying young crust during the expansion of the continental crust.In summary,combined all datas with the latest progress in the mid-Paleoproterozoic magmatism and sedimentation in other areas of the Jiao-Liao-Ji belt,the magmatic rocks in the Jingqishan area may be formed in back-arc extension,revealing that the Jiao-Liao-Ji belt in the middle and late Paleoproterozoic successively experienced the orogenic process of back-arc extension-collision-extrusion orogenesis-after orogenic rapid exhumation-extension.