The Paleoproterozoic Tectonic Evolution Of The Central Segment Of The North China Craton

In recent years,the North China Craton(NCC)has been extensively studied in terms of the subdivision of crystalline basement and major tectonic,magmatic,metamorphic events.However,tectonic transformation from late Neoarchean to early Paleoproterozoic,tectonic nature during middle Paleoproterozoic(a long–term island arc or have at least a intracontinental rift stage)and detail amalgamation process at late Paleoproterozoic of the NCC are still controversial and need to be resolved.The Lüliang Complex,outcropped in the central part of Trans-North China Orogen(TNCO),consists of multi–stage Paleoproterozoic magmatic rocks and meta–supracrustal rocks,which is an ideal area to probe the geological evolution of NCC in the Paleoproterozoic.The integrated study on petrogenesis and tectonic setting of Gaijiazhuang granite,Lüliang Group meta–basic rocks,late–Paleoproterozoic granitoids,and metamorphic P–T–t paths of Jiehekou group pelitic migmatites have been researched to provide pivotal constraints on Paleoproterozoic geological evolution of the NCC.The early Paleoproterozoic Gaijiazhuang syenogranite obtained the LA–ICP–MS zircon U–Pb ages of 2398~2408 Ma.Geochemically,they are characterized by high Si,K,alkaline and Fe OT/Mg O,low Ca,Mg,enrichment of Rb,Th,U,K,Pb,as well as depletion of Ba,Nb,Ta,Sr,P,Ti,and “seagull–type” pattern of slight REE fractionation between LREE and HREE with strong negative Eu anomaly,showing the typical features of typical A2 type granite.Their positive ?Nd(t)of 2.01~2.30,TDM2(Nd)of 2606~2629 Ma,positive zircon ?Hf(t)of 2.25~7.10,and TDM2(Hf)of 2494~2791 Ma,slightly higher or close to its formation age,all demonstrate that Gaijiazhuang syenogranite is mainly derived from the Neoarchean continental crust materials,likely with minor mantle–derived component involved.Combined with regional geology and the widespread coeval K–rich granitiods,the Gaijiazhuang syenogranite together with the K–rich granitiods represent a tectonothermal event occurred at ~2.4 Ga due to partial melting of juvenile continental crust materials triggered by the mantle upwelling,which resulted from the crustal extension after the ~2.5 Ga blocks amalgamation of NCC,suggesting that the central sement of the NCC has been entered into an extensional tectonic environment during the early Paleoproterozoic of ~2.4 Ga.The middle Paleoproterozoic meta–basic rocks from Jinzhouying Formation of Lüliang Group yields a LA–ICP–MS zircon U–Pb age of 2180±19 Ma.Geochemically,they belong to calc–alkaline series,with low Ti O2 and high Al2O3,K2 O.They are enrichment in large ion lithophile element(Rb,Ba,Th,U,Sr),depletion in high field strength element(Nb,Ta,Ti),high in Pb and slightly right inclined chondrite-normalized REE patterns.All above are similar to the typical arc volcanic rocks in the subduction zone.The positive ?Nd(t)(1.0~2.6)and ?Hf(t)(1.8~9.2)imply that they are derived from depleted mantle.The low Ti/Y(246~385)and Ce/Y(0.99~1.77)further reveal that the mantle wedge is commonly modified by materials from fluids and subducted sediments.Thus,the rocks are originated from part melting of depleted mantle of spinel and garnet lherzolites transition zone and then experience fractional crystallization of olivine,clinopyroxene,and so on.Combined with the close spatial and temporal association of adakitic rocks,Nb–enriched basalts,andesites,magnesian andesites and arc–related granitoids,we suggest a 2.2 Ga active continental marginal system in the central sement of the NCC.The late Paleoproterozoic granitoids from Lüliang Complex give LA–ICP–MS zircon ages of 1854±20 Ma for the Huijiazhuang granite,1830±21 Ma for the Xiyupi granite vein and 1760±20 Ma for the Dacaoping porphyritic granite,respectively.The Huijiazhuang and Xiyupi granites have variable Si O2,high K2O+Na2O,low Al2O3 and P2O5,highly differentiated REE patterns with medium negative Eu anomalies and enrichment of Rb,Th,U,K,Pb,as well as depletion of Nb,Ta,Sr,P,Ti,similar to I–type granite.The negative ?Nd(t)(–5.7 to –4.1)and zircon ?Hf(t)(–5.5 to 2.3,–9.6 to –3.1)suggest that they are mainly derived from Paleoproterozoic to Archean continental crust rocks.The Dacaoping porphyritic granites are characterized by high Si O2,K2 O,Fe OT/(Fe OT+Mg O),“seagull–type” REE patterns with strong negative Eu anomaly(?Eu=0.16~0.35)and higher 10000*Ga/Al(2.99~3.36),Zr+Nb+Ce+Y(378~583 ppm),showing an affinity of typical A2–type granite.The negative ?Nd(t)(–6.4 to –6.1)and zircon ?Hf(t)(–7.9 to –5.2)of Dacaoping porphyritic granites suggest that they are mainly derived from Archean continental crust rocks.All the granites belong to high–K calc–alkaline to shoshonite series and show enrichment of Rb,Th,U,K,Pb,depletion in Sr,P,Ti,which is consistent with high–K calc–alkaline granitoids formed under the post–collision extension setting.Combined with previous studies,the tectonic regime of late–Paleoproterozoic granitoids change obviously around 1.82 Ga.Before 1.82 Ga,the granites are produced by partial melting of a slightly thickened continental crust rocks,whereas after 1.82 Ga,the A2–type granites are generated from the thinned lower crust.Thus,it has been the post–collision setting at 1.89~1.76 Ga and continental crust is thinned intensively after 1.82 Ga.A combined study of petrography and phase equilibria modeling is carried out on two mesosome sample of pelitic migmatites from the Jiehekou Group in the Lüliang Complex.The K–feldspar gneiss sample preserves the thermal peak assemblage of garnet,biotite,K–feldspar,sillimanite and quartz,without plagioclase.It records two near–isothermal decompressional indicators that rutile is moated by ilmenite and garnet is replaced by cordierite.Also small plagioclases occurre in the rim of K–feldspar and garnet.Four stages(M1–M4)of metamorphic evolution have been identified based on the mineral assemblages and P–T pseudosection.It is interpreted to record pre–Tmax decompression from rutile–stability field(M1,represented by rutile relicts in matrix and biotite + sillimanite + quartz inclusions in garnet porphyroblasts)at ~8.9 kbar to the Tmax stage(M2,characterized by ilmenite,garnet rim and biotite + K–feldspar + sillimanite + quartz in matrix)of 815~838 °C.The following suprasolidus post–Tmax cooling comprises M3 stage(represented by appearance of cordierite in the assemblage of garnet + biotite + K–feldspar + sillimanite + ilmenite + quartz)of 780–838 °C/5.0~6.3 kbar and M4 stage(defined by the final mineral assemblage of garnet + biotite + K–feldspar + plagioclase + sillimanite + cordierite + ilmenite + quartz)of ~775 °C/~5.0 kbar.Two–feldspar gneiss sample preserves the thermal peak assemblage of garnet,biotite,plagioclase,K–feldspar,sillimanite,quartz.It also records pre–Tmax decompression from rutile–stability field at 9.0 kbar to the Tmax stage of 796–821 °C/6.4–7.1 kbar,followed by suprasolidus post–Tmax cooling stage of 778–803 °C/5.0~5.7 kbar.The clockwise P–T paths are consistent with those of orogenic belt.Furthermore,the zircon U–Pb dating on mesosome and leucosome of pelitic migmatite yield metamorphic ages of 1.95 Ga and 1.91 Ga,respectively,in which,the former represents the early stage of cooling after Tmax,nearly close to the time of pressure peak metamorphism.Combined with those granulites in the TNCO,a common long–lived continent–continent collision processes is proposed,which may culminate at ~1.95 Ga.The integrated study on magmatism and metamorphism indicates that after the ~2.5 Ga blocks amalgamation of NCC,it has been an extensional tectonic environment at ~2.4 Ga,active continental marginal system at 2.2 Ga,main collision with peak metamorphic stage at 1.95 Ga and post–collision setting at 1.89~1.76 Ga,during which continental crust is thinned intensively after 1.82 Ga.