Early Paleoproterozoic Magmatism And Tectonic Evolution Of The Southern North China Craton

The period of 2.45–2.20 Ga in the early Paleoproterozoic era is a special time,which is called as ?Quiet Interval‘ because of the decreased significantly magmatic activities and the significant environmental changes in the world.According to the traditional view,this may be caused by the stagnation of global plate tectonic.The stagnation of subduction leads to the suspension of volcanism and continental crust growth.However,recent studies have found that the global tectonic magmatism of2.45-2.20 Ga was not completely ceased,which had resulted in the converging of plates and the production of juvenile crust in some regions,as well as the subsequent formation of accretionary orogenic belts.The central belt of the North China Craton is one of the regions where magmatic rocks are widely distributed in the world during the quiet interval,which records the crustal accretion process under the setting of the active continental margin.In this research,we present new studies of geology and petrology,as well as new zircon U-Pb ages and Hf-O isotopes,whole-rock geochemistry and Sr-Nd isotopes for the Paleoproterozoic diabase,dioritic gneiss,TTG gneiss and potassic gneiss in the southern margin of this craton.These data provide new constraints on the formation ages,petrogenesis,tectonic background,and the temporal and spatial distributions of these early Paleoproterozoic magmatic rocks,which provide insights into the early Paleoproterozoic tectonic evolution of the central belt of the North China Craton.The main achievements of this research are listed below:(1)The early Paleoproterozoic magmatic rocks in the southern margin of the North China Craton mainly include diabases,dioritic gneisses,TTG gneisses and potassic granites,which have recorded two tectonic thermal events of ~2.3 Ga and~2.2 Ga.The peak age of magmatic activities is 2313 Ma.The influence range of the early Paleoproterozoic tectonic thermal events covers the whole southern margin of the North China Craton.(2)The mafic magmatic records of the early Paleoproterozoic in the southern margin of the North China Craton are mainly distributed in the Dengfeng area.The Songshan diabase yields zircon U–Pb age of 2301 ± 16 Ma and has low Si O2,K2 O,total alkali contents,K2O/Na2 O ratios,high Mg O contents and Mg#,belonging to low potassic tholeiite series.The diabase samples show enrichments in LREE and large ion lithophile elements(Sr,Ba and Pb)and depletions in Nb,Zr and Ti,indicating the geochemical characteristics of island arc magmas.The diabase rocks have experienced crystallization fractionation from clinopyroxene to orthopyroxene,then plagioclase,suggesting a hydrous fractionation order.The rocks have relatively similar εNd(t)values(-0.2-+0.8)and variable zircon εHf(t)values(-3.0-+6.6).These geochemical features indicate that the mafic intrusions were mainly generated by the high degree partial melting of mantle wedge metasomatized by subducted slab-derived fluids in an active continental margin arc setting.(3)The Paleoproterozoic Dioritic gneisses in the southern margin of the North China Craton are mainly distributed in the Xiaoqinling and Xiong‘ershan areas,recording two important magmatic events of ~2.2 Ga and ~2.3 Ga.The ~2.2 Ga diorites are characterized by low Si O2,Na enrichment,high contents of Mg O,Cr,Ni,Ba,Sr and Mg#,showing the characteristics of low titanium sanukitoids.The high La/Sm and Th/Yb ratios of the samples reflect the addition of sediments.The zirconεHf(t)values vary from-3.6 to-1.7,and the corresponding Hf isotopic two-stage model ages(TDM2)are of 2.73-2.97 Ga.The rocks have εNd(t)values from-3.2 to+2.0.These characteristics suggest that the ~2.2 Ga high-Mg diorites are products of partial melting of mantle wedge peridotite metasomatized by subducted sediments-derived melts.The ~2.3 Ga diorites are characterized by high Si O2,Al2O3,Na2 O and low Mg O,with high Na2O/K2 O ratios,belonging to mataluminous and calc alkaline series.The samples are generally of negative εNd(t)values(-4.0 to-3.5)andεHf(t)values(-9.6 to +3.9).Their Hf isotopic two-stage model ages(TDM2)(2.62-3.44Ga)and whole rock Nd isotopic two-stage model ages(TDM2)(2.86-3.04 Ga)are much older than the emplacement age.Thus,the ~2.3 Ga low-Mg diorites were produced by partial melting of the Taihua basement rocks with or without significant input of mantle components.(4)The Paleoproterozoic TTG gneisses in the southern margin of the North China Craton are mainly distributed in the Xiaoqinling,Xiong‘ershan and Lushan areas,displaying two important magmatic events of ~2.2 Ga and ~2.3 Ga.The ~2.2Ga TTG gneisses are characterized by high Si O2,low Al2O3,Mg O and Fe Ot,and belong to low-Al trondhjemite.They are characterized by the enrichment of light rare earth elements and the relative flat heavy rare earth elements,with relatively high(La/Yb)N ratios and obvious negative Eu anomalies.The ~2.3 Ga TTG gneisses are characterized by low Si O2,Mg O,Fe Ot,and high Al2O3,and belong to high-Al tonalite.The rocks have low light rare earth elements and Sr/Y ratios,obvious positive anomalies of Rb and Th,and depletions of Ba,Nb and Sr.The TTG gneisses of ~2.2 Ga and ~2.3 Ga have negative whole-rock εNd(t)values and old Hf isotopic two-stage model ages.Therefore,the ~2.3 Ga high-Al tonalites and the ~2.2 Ga low-Al trondhjemites were formed by partial melting of ancient lower crust at medium pressure and low pressure,respectively.(5)The Paleoproterozoic potassic granites in the southern margin of the North China Craton are mainly formed at ~ 2.3 Ga and ~ 2.27 Ga,of which the potassic granites at ~ 2.3 Ga are mainly I-type granites,and the potassic granites at ~ 2.27 Ga are coexistence of I-type and A-type granites.The ~2.27 Ga A-type granites are characterized by high Si O2,K2 O,Fe Ot,(Na2O + K2O),low Mg O,high Ga/Al ratios and high zirconium saturation temperatures.They have obvious ―seagull–type‖pattern of REE fractionation between LREE and HREE,showing obvious negative Eu anomalies.The I-type granites are characterized by high Si O2,K2 O and low Mg O,and belong to high-K calc-alkaline to shoshonite series,showing light rare earth element enrichments and heavy rare earth depletions,with significant positive Eu anomalies.These potassic granites generally have low Cr and Ni contents,Mg# values,and enriched isotopic characteristics,indicating that they were generated by partial melting of ancient basement.The I-type and A-type granites were formed at medium-high pressure and low pressure,respectively.(6)The central belt of the North China Craton was in a subduction setting at ~2.3Ga.In the subduction process,the crust is continuously thickened and a large number of magmatic rocks are formed,recording a lateral accretion process of subduction.At~2.2 Ga,the stress state of the crust was changed significantly,from subduction and compression in the early stage to extension and tension.The thickness of the crust decreased and the contribution of the mantle increased.The ~2.2 Ga magmatic activities of in the central belt of the North China Craton were mainly formed in a continental margin arc setting.Under the condition of plate retreat or plate breakoff,the upwelling of deep asthenosphere material occurred and caused the decompression melting,resulting in the lithosphere thinning.(7)Extensive early Paleoproterozoic magmatism was recorded in the southern margin of the North China Craton.The alkaline and related elements(such as Rb,Th,Ba and Nb),K2O/Na2 O ratio and the zircon δ18O values of the Paleoproterozoic magmatic rocks increased significantly,indicating that the plate subduction has started prior to ~2.3 Ga.