Geochemical And Geochronological Insights Into The Tectonic Evolution Of The Paleoproterozoic Jiao-Liao-Ji Belt,Sino-Korean Craton

The Sino-Korean Craton,one of the oldest cratons in the world,was originally formed by the amalgamation of the Liaonan-Nangrim,Longgang,Ordos and Yinshan blocks along three orogenic belts(i.e.Jiao-Liao-Ji Belt,Trans North-Central Orogen and Khondalite Belt)at ~1.9 Ga and thus records important information on the assemblage and breakup of the Paleoproterozoic supercontinent Columbia.Among these belts,the Jiao-Liao-Ji Belt(JLJB)is the most important Paleoproterozoic orogenic belt.It is the key to investigating the process of the assemblage and breakup of the supercontinent Columbia,and the early tectonic-magmatic evolution relations between the North China Craton(NCC)and the Korean Peninsula.However,due to late polyphase tectonothermal events(e.g.,subduction of the paleo-Pacific Plate),considerable and continuing controversy has surrounded how this orogenic belt formed,with models including(1)opening and closing of an intracontinental rift,(2)collision of a continent–arc–continent system,and(3)a rifting–initial ocean formation–oceanic subduction–collision cycle.Here,we focus on Paleoproterozoic magmatic rocks with accurate formation ages and long time span,and report new zircon U–Pb data,whole-rock geochemistry,and Sm–Nd isotope analysis of meta-mafic rocks and felsic volcanic rocks in the Liaodong Peninsula.Combining with previous studies,and our research on U–Pb age and trace elements of detrital zircons from the Langzishan Formation,the lowest sedimentary sequence of the Liaohe Group,this study firstly reconstruct the synthetic tectonic evolution of the JLJB,and provide a new starting point for future study on Precambrian geology of the Sino-Korean Craton.Sedimentary and volcanic successions of the JLJB can be divided into two zones based on stratigraphy and metamorphic history(e.g.,South and North Liaohe groups in the Liaodong Peninsula).The meta-mafic rocks in the South Liaohe Group are(garnet-)amphibolite;whereas the mafic rocks in the North Liaohe Group are dominantly composed of meta-diabase and meta-gabbro,with minor amphibolite.LA–ICP–MS zircon U–Pb age analyses yield the same crystallization and metamorphic ages of ca.2130 Ma and ca.1900 Ma,respectively.In addition,they have similar whole-rock geochemical and isotopic compositions,suggesting that they were derived from the same mantle source in the same tectonic environment,and underwent late Paleoproterozoic metamorphic event together.We thus infer that the South and North Liaohe groups(zones)should deposit in the same tectonic setting.All these meta-mafic rocks were derived from partial melting of a depleted asthenospheric mantle in the spinel stability field,with extensive fractional crystallization and crustal assimilation,and limited metasomatism from subduction-related fluids and/or melts.The geochemical features of the mafic rocks in the Liaohe Group contrast with those of typical intra-continental rifts and volcanic arcs,suggesting that they cannot form in a tectonic setting related to intra-continental rift or volcanic arc.Therefore,these mafic rocks with the geochemical features of both mid-oceanic ridge basalt(MORB)and volcanic arcs should form in a back-arc extension environment.The volcanic rocks with an area of approximately 300km~2 are well exposed and distributed in an E-W trending belt.They are composed of dominant felsic tuffs and mainly interlayered with meta-sedimentary rocks of the Li'eryu Formation.The zircon SHRIMP U–Pb ages suggest formation of the tuffs crystallized at ca.2.17 Ga,which are early than the crystallization of meta-mafic rocks related to back-arc extension.They show calc?alkaline features of dacite and rhyolite,similar to the features of continental arc andesites.Major and trace elements,and isotopic compositions suggest that these felsic tuffs were derived from mixing of mafic(e.g.,those in the Liaohe suite)and felsic(e.g.,the Liaoji granitoids)magmas,followed by fractional crystallization.We thus propose that they were formed in a continental arc prior to the back-arc extension.This study synthesizes the geochronological,geochemical,and isotopic data on the Paleoproterozoic igneous rocks in the JLJB.The available data suggest that the Paleoproterozoic magmatism in the JLJB lasted from ca.2200 Ma to ca.1800 Ma,with five magmatic flare-ups at ca.2190–2160 Ma,ca.2160–2110 Ma,ca.2110–2080 Ma,ca.2010–1895 Ma and ca.1875–1850 Ma.(1)During the early stage of Paleoproterozoic oceanic plate northwestward subduction,strong slab rollback resulted in trench retreat and extension of the overriding plate(i.e.,the Longgang-Liaonan-Nangrim Block)and induced upwelling and decompression melting of asthenospheric mantle to produce basaltic magma.The overriding Archean continental crust was heated by the underlying basaltic magma and melted to produce ~2190–2160 Ma aluminous A2-type granites,and minor basaltic magma mixed with this crustal melt to form ~2180–2160 Ma calc-alkaline,andesitic-rhyolitic tuffs.(2)With ongoing extension,the overriding plate thinned,and a back-arc basin(or rift)opened and widened.Asthenospheric mantle melted in the spinel stability field to produce ~2160–2110 Ma tholeiitic mafic rocks with the geochemical features of both mid-oceanic ridge basalt(MORB)and volcanic arcs.(3)Decreased back-arc mafic magmatism suggests that subduction ceased due to collision between eastern North Australian Craton(NAC)and the active subduction zone during ~2110–2080 Ma.The continuous collision and compression further induced new subduction initiation in the back-arc basin(or rift),and this subduction resulted in forearc extension.Decompression melting of the subarc mantle produced basaltic magma,which was metasomatized by subducted slaband ancient sediment-derived melts to form ~2110–2080 Ma mafic rocks with both calc-alkaline and tholeiitic features.Simultaneously,the overriding continental crust was heated by the basaltic magma and melted to form ~2110–2080 Ma aluminous A2-type granites.(4)Subsequently,the orogeny following the collision between the NAC,the continental arc(Nangrim-Liaonan Block + Gyeonggi massif?)and the Longgang Block began.This process was accompanied by prograde or peak amphibolite-to granulite-facies metamorphism and minor magmatism,which produced the ~2000–1895 Ma adakitic granites derived from partial melting of thickened lower crust.(5)During the late Paleoproterozoic,termination of the collisional orogenic event occurred,as evidenced by regional retrograde metamorphism(i.e.,exhumation and cooling)and strong postcollisional extension,which generated widespread ~1875–1850 Ma igneous rocks in the JLJB.Langzishan Formation,the lowest sedimentary sequence of the Liaohe Group,is adjacent to Archean basement material and has only been identified within northern part of the North Liaohe Group.703 U–Pb age analyses on detrital zircons from eight samples show a strong age peak of ca.2529 Ma,and three minor age peaks at ca.2529,ca.2700,ca.2347,and ca.2198 Ma,respectively.Such an age distribution is totally different from those of other stratigraphic units with a major age peak at ca.2.2–2.1 Ga in the Liaohe Group.Provenance analysis indicates that the Langzishan Formation was derived from basement rocks in the Longgang Block with minor contributions from early magmatic rocks within the JLJB.The rock assemblages and detrital zircon age contribution further suggests that the deposition of the Langzishan Formation was in a passive continental margin during ca.2205–2190 Ma.We thus propose that the Paleoproterozoic oceanic plate subduction commenced no earlier than 2205 Ma.In summary,the five stages of magmatism in the JLJB record a complete tectonic cycle in the Eastern Block,including subduction initiation,back-arc extension,closure of the back-arc basin(or rift),collisional orogeny and postcollisional extension.