Paleoproterozoic Metamorphism And Partial Melting In The Wuhe Complex At The Southeastern Margin Of The North China Craton:Insights Into The Tectonic Relationship With The Jiao-Liao-Ji Belt

The Wuhe Complex at the southeastern margin of the North China Craton(NCC),belonging to the Precambrian metamorphic basement,consists of a variety of rocks types,mainly including TTG gneisses,garnet amphibolites,garnet granulites,granitic gneisses,micaschists,marbles and meta-sandstones,associated with the Paleoproterozoic gneissic K-feldspar granites and Mesozoic granitoids.This master thesis focuses on 2.1 Ga granitic rocks including granitic gneisses and K-feldspar granites and 1.85-1.90 Ga basic granulites based on an integrated investigation of petrology,geochemistry and zirconology.The obtained main advances are as follows:(1)identifying three metamorphic stages for the basic granulites and defining their corresponding P-T conditions and ages,and reconstructing a metamorphic P-T-t path;(2)clarifying the ages of emplacement,metamorphism and anataxis of Paleopro-terozoic granitic rocks and deciphering their petrogenesis and tectonic background;(3)providing detailed petrographic evidences of partial melting of meta-basic rocks.Based on the above results,this study explores the Paleoproterozoic crustal evolutional processes in the Wuhe Complex,and thus provide for the first time insights into their tectonic relationship with the Jiao-Liao-Ji Belt.Three stages of metamorphic assemblages are identified in the high-pressure(HP)basic granulite and the P-T conditions of each stage have been evaluated with various geothermobarometers.The derived metamorphic P-T path shows a clockwise pattern characteristed by isothermal decompression and decompression-cooling following peak stage.The peak HP granulite-facies metamorphism(M1)is characterized by high-Ca cores in granet,high-Al cores in clinopyrexene and plagioclase,quartz and rutile as matrix minerals,yielding P-T conditions of 850-900? and 1.5 GPa.The medium-pressure(MP)granulite facies assemblage(M2)mainly consists of garnet +orthopyrexene + plagioclase ± clinopyrexene and garnet + clinopyrexene +plagioclase surrounding the amphibole porphyroblasts,recording P-T conditions of900?/1.1-1.2 GPa and 800?/1.1 GPa respectively.Symplectites or coronas of hornblende + plagioclase(M3)surrounding the garnet porphyroblasts indicate garnet decompressional reactions occurring at 600-680? and 0.65-0.75 GPa.The LA-ICP-MS zircon ages of garnet amphibolite and garnet granulite can be categorized into three groups of?1.90 Ga,?1.85 Ga and?1.78 Ga,corresponding to the time of HP granulite facies,MP granulite facies and amphibolite facies metamorphism.The Wuhe Complex experienced 2.1 Ga cruatal anataxis,forming two types of granitoids,i.e.,K-feldspar/albite granites and granitic gneisses.They share the identical 207Pb/206Pb ages of 2096 ± 9 Ma and 2096 ± 8 Ma defined by zircon SHRIMP U-Pb dating.These granitic rocks are commonly enriched in HREE,Zr,Nb,Ga and Y,and depleted in V,Cr and Ni,most of which have high TFeO/MgO ratios and Zr-saturation temperatures over 850 ?,suggestive of an affinity with A-type granite.In addition,both have consistent whole-rock sNd(t)values(-3.0 to-0.9)and zircon Hf isotopic compositions(sHf(t)=-3.3 to +1.9).The latter are similar with ones of the Neoarchean TTG gneisses in the Eastern Block of the NCC.Thus these 2.1 Ga granitoids in the region were most likely derived from the partial melting of?2.5 Ga TTG gneisses.The zircon grains from the granitic gneisses can be categorized into core,mantle and rim domains with distinct CL images and trace-element characteristics.The grey mantles exhibit steep REE patterns,low LREE,Th,Pb contents and remarkably low Th/U ratios(<0.1),and define mean U/Pb ages of?1.85 Ga.The dark rims display relatively higher LREE,U,Th,Pb contents and Th/U values(0.1-0.3)with slightly younger ages of 1.81-1.83 Ga than mantles.Accordingly,the zircon mantles and rims are interpreted as metamorphic zircons formed during granulite facies metamorphism and anatectic zircons crystallized from limited melts,respectively.The meta-basic rocks also experienced anataxis during granulite-facies metamorphism,producing felsic multiphase mineral inclusions composed of K-feldspar + quartz + albite in garnet.The 2.1 Ga magmatism,-1.90 Ga HP granulite-facies and?1.85 Ga MP granulite-facies metamorphism as well as related anataxis occurred in the Wuhe Complex and the Jiao-Liao-Ji Belt(JLJB),suggesting that they share the same tectonic evolutional processes and constitute a Paleoproterozoic collisional orogen in the Eastern Block of the NCC.