Petrogenesis Of The Late Paleozoic-early Mesozoic Intrusive Rocks In The Xing'an Terrane,and Its Geological Implication

This thesis conducts systematic studies on the petrology,zircon LA-ICP-MS U-Pb geochronology,whole-rock geochemistry,and zircon in-situ Hf isotope of the late Paleozoic-early Mesozoic intrusive rocks in the Xing'an terrane,NE China,with the aims of 1)ascertaining their magmatism stages,rock associations and spatial-temporal distribution,2)discussing petrogenesis of these rocks and their tectonic settings,3)further providing constraints on the closing time of the Hegenshan Ocean,and 4)finally reconstructing the late Paleozoic-early Mesozoic tectonic-magmatic evolution history of the Xing'an terrane.Zircons from the late Paleozoic-early Mesozoic intrusive rocks in the Xing'an terrane,are euhedral to subhedral in shape,and display fine-scale oscillatory growth zoning or striped absorption,which,together with their high Th/U ratios,implies a magmatic origin.Zircon LA-ICP-MS U-Pb dating result indicates that the late Paleozoic-early Mesozoic magmatic events in the Xing'an terrane could be divided into two stages,i.e.,early Carboniferous to middle Permian?356²60 Ma?and middle Triassic to late Triassic?244²06 Ma?.The late Paleozoic intrusive rocks of the Xing'an terrane were emplaced at 356²60Ma,which are products of the early Carboniferous to middle Permian magmatism.They are distributed in a NE-SW orientation along the Dongwuqi-Aershan-Wuerqihan-Nenjiang belt,which is nearly subparallel to the Hegenshan-Heihe suture.The represented plutons of this stage include the Meitian gabbro-diorite,Wuqi granite,and Shiligou granite from the Wuerqihan area,the Dulaer complex and Sangduoer syenogranite from the Aershan area,and the Huolongmen syenogranite-monzogranite from the Nenjiang area.Among them,the early Carboniferous Meitian gabbros and diorites have relatively low SiO₂contents and high Mg^#values.They are enriched in LREEs and LILEs,and depleted in HFSEs?e.g.,Nb,Ta,and Ti?with positive?_Hf?t?values,similar to the geochemical features of island arc-related igneous rocks,suggesting that the gabbros and diorites might be derived from the partial melting of an enriched lithospheric mantle that had been metasomatized by subducted slab-derived fluids.The late Carboniferous Dulaer complex is composed mainly of quartz diorite,monzodiorite,and monzogranite,as well as the coeval fine-grained dioritic enclaves within the host monzogranites.They contain high Sr contents and Mg^#values?59⁶4?,and low Y and Yb components,similar to those of typical adakites.A magma mingling/mixing model has been proposed to account for the formation of the Dulaer complex,i.e.,they were derived from hybridization of the enriched mantle-derived basaltic melts and juvenile crust-derived felsic melts,which then underwent fractional crystallization.The rest of the late Carboniferous to early Permian granites along the Dongwuqi-Nenjiang belt belong to high-K calc-alkaline series.They are enriched in LREEs and LILEs,and depleted in HFSEs with positive?_Hf?t?values,showing affinities to rocks formed in an active continental margin setting.It implies that their primary magma originated from the partial melting of juvenile lower crust materials.The geochemical features,rock associations,and their NE-SW trend distribution demonstrate that the late Paleozoic intrusive rocks in the Xing'an terrane were most likely emplaced in an active continental margin setting,indicating that the northwestward subduction of the Hegenshan oceanic plate beneath the Xing'an terrane occurred during the late Carboniferous to middle Permian.The early Mesozoic intrusive rocks of the Xing'an terrane were emplaced at 244²06Ma,which are products of the middle to late Triassic magmatism.They are distributed along the Kutihe-Chabaqi-Taerqi belt,which elongates in a NE-SW orientation nearly subparallel to the Hegenshan-Heihe suture.They consist primarily of monzogranites and syenogranites,with minor quartz diorites.Geochemical characteristics indicate that the granitic rocks of this stage have different origins,although they all belong to highly fractionated I-type granites.Among them,the²44 Ma Taerqi granites,the 230²06 Ma Kutihe granites,and the 242²32 Ma Wuerqihan granites have relatively high SiO₂contents,low MgO,Cr,and Ni concentrations,and positive?_Hf?t?values,and are enriched in LILEs and depleted in HFSEs,suggesting an origin of partial melting of pre-existing juvenile arc-type rocks under low pressures in a relatively shallow crustal level.The²10Ma Chabaqi quartz diorites contain relatively high SiO₂ contents and Mg^#values,and positive?_Hf?t?values,implying that they likely originated from the hybridization between a depleted mantle component and juvenile crustal materials.The²12 Ma Kutihe granites have high Sr and Sr/Y values,and low Y,Yb and Mg^#values,similar to those of adakites,suggesting an origin of partial melting of a thickened lower crust.The existence of the middle Triassic to late Triassic high potassium calc-alkaline granites show a post-orogenic extension setting after the closure of the Hegenshan oceanic basin.In summary,one model has been established to reconstruct the late Paleozoic to early Mesozoic tectonic evolution of the Xing'an terrane.In the early Carboniferous,a north dipping subduction zone induced the generation of the arc-related igneous rocks in the central Xing'an terrane.During the period of late Carboniferous to early Permian,the subduction zone migrated laterally and moved from the central part to the southeastern margin of the Xing'an terrane,thus a wide active continental margin type intrusive rock belt were formed.Spatially,this belt extends as a long strip in a NE direction,subparallel to the Hegenshan-Heihe suture,which totally corresponds to the subducted mechanism of the Hegenshan oceanic plate.In the middle-late Triassic,the emplacement of high potassium calc-alkaline highly fractionated I-type granites and A-type granites,together with the sedimentary strata recording continental deposit,manifests that the Xing'an terrane had transformed to a post-orogenic extension environment.Based on the rock spatial-temporal distribution,their petrogenesis,and transformation of tectonic regim,together with the previous study results of the regional strata and the Hegenshan ophiolite,we propose that the final amalgamation of the Xing'an terrane and the Songnen terrane took place between the late Permian and early Triassic,which led to the closure of the Hegenshan Ocean.During the late Paleozoic to early Mesozoic,a large scale continental crustal growth had occurred in the Xing'an terrane as well,which includes the lateral accretion mechanism related to the subduction and the vertical one related to underplating of mantle-derived magma.