Petrogenesis And Geodynamic Mechanism Of The Mesozoic Volcanic Rocks In The Central Great Xing'an Range

The Great Xing'an Range?GXR?lies along the eastern edge of the Central Asian Orogenic Belt?CAOB?,which lay at the junction of Paleo-Asian Ocean?PAO?,Mongol-Okhotsk Ocean?MOO?and Paleo-Pacific Ocean?PPO?tectonic regimes.The CAOB records the generation of a continental mass from numerous micro–continental blocks.Following docking of the micro-continental blocks,abundant magmatic activities and mineralization occurred during the Mesozoic,which became a unique volcanic belt in China.Owing to its special geographic location,researchers have strongly debated the petrogenesis of volcanic rocks,the geodynamic evolution of this region during the Mesozoic in the GXR.Mesozoic magmatic rocks outcrops in the central GXR are relatively complete.Due to this accessibility,the central GXR is ideal for studying Mesozoic magmatism,petrogenesis and geodynamic mechanism in the GXR.Here,we present new zircon U–Pb ages,whole-rock geochemical data,and zircon Hf isotopic compositions for the Mesozoic volcanics in the central GXR.Our data,together with regional geological data,provide evidence that further explores the timing of the Mesozoic volcanism,petrogenesis and geodynamic mechanism in the central GXR.In addition,we summarised the spatial-temporal variation in Hf-Nd isotopic compositions of Mesozoic felsic igneous rocks in order to determine the newly accreted continental crust in the GXR.Mesozoic volcanism in the central GXR can be subdivided into three episodes:Triassic?235–205 Ma?,Late Jurassic–Early Cretaceous?155–115 Ma?episode with no magmatic hiatus,that reaching its peak at ca.125 Ma.The main volcanic sequences include Hadataolegai,Manketouebo,Manitu,Baiyingaolao and Meiletu formations in the central GXR.The volcanic rocks of the Hadataolegai Formation erupted during the Late Triassic?235–225 Ma?,not during the Early Triassic as previously suggested.The volcanic rocks of the Manketouebo and Manitu formations are Late Jurassic to Early Cretaceous in age,with age ranges from155–140 Ma and 150–125 Ma,respectively.The volcanic rocks of both the Meiletu and Baiyingaolao formations are Early Cretaceous in age?135–120 Ma and 130–120 Ma,respectively?.Numerous new zircon U-Pb dates obtained from Late Jurassic–Early Cretaceous volcanic rocks in the central GXR suggest that previously defined timing of the volcanic sequences are diachronous and problematic.We consider the use of age data and lithology from the volcanic rocks to be more appropriate for establishing the timing of volcanism,as opposed to relying solely on stratigraphic relationships.The andesitic-dacitic volcanic rocks of the Hadataolegai Formation have high Sr contents,low Y and Yb contents,slight Eu anomalies,and some of the volcanics are relatively high in MgO contents,similar to“adakite-like”rocks.The dacites were formed by fractional crystallization of coeval andesitic magmas.All samples have highly fractionated REE patterns,they are characterized by enrichment in the light rare earth elements?LREEs?and large ion lithophile elements and depletions in high field strength elements?HFSEs?.The zircons within the volcanic rocks yield higher positive?_Hf?t?values?+6.3 to+12.7?and model ages(T_DM2)between 858 Ma and 452 Ma,which indicates that the magmas were generated by a newly accreted continental crustal source during Neoproterozoic-Early Paleozoic.These characteristics indicate that the volcanic magmas originated due to the partial melting of the delaminated lower crust and interaction with surrounding mantle material as the magma ascended.Late Jurassic-Early Cretaceous andesitic-dacitic volcanic rocks are enriched in LILEs and LREEs and are depleted in HFSEs,with positive?_Hf?t?values?+6.2 to+13.4?and Hf single-stage model ages(T_DM1=556–270 Ma).This indicates that they were derived from the partial melting of lithospheric mantle that was modified by previously subducted slab-derived fluid-related metasomatism and introduced with limited crustal contamination during the ascent of magmas.Late Jurassic-Early Cretaceous rhyolitic volcanic rocks are characterized by enrichment in LILEs and LREEs and more depletion in HFSEs and Eu than andesitic-dacitic volcanic rocks.The zircons within the volcanic rocks yield positive?_Hf?t?values?+4.3 to+10.2?and Hf two-stage model ages(T_DM2=910–544 Ma),with more enrichment of LILEs and LREEs and more depletion of HFSEs and negative Eu anomalies,implying that they likely originated from partial melting of Neoproterozoic-Early Paleozoic accreted medium-high K basaltic crust?0.7 Gpa?and has undergone a differentiated during evolution.Significant newly accreted continental crust in the central GXR,northern Xing'an block,and Erguna block predominantly occurred during Neoproterozoic-Early Paleozoic,Neoproterozoic and the Mesoproterozoic-Neoproterozoic,respectively.The mechanism and history for newly accreted continental crust in the central GXR is different from the northern Xing'an block and Erguna block,implying the GXR has history of spatial and temporal heterogeneity of crustal accretion.Combined with the regional structural characteristics,paleomagnetic data,and the distribution of polymetallic depositsls in northeast China,we propose that?1?Early-Middle Triassic?250-240 Ma?,the PAO slab continued to subduct northward,forming an east-west collisional granite belt and a subduction-related porphyry Cu-Mo deposit,which dominated by the collage of microcontinents associated with the closure of the Paleo-Asian oceanic plate.?2?Late Triassic?235-205 Ma?,there are abundant post-orogenic granitoids and moraine construction and the distribution of porphyry Cu-Mo ore deposits with adakite-like rocks in the central-southern GXR,which is a reflection of extension after the final closure of the PAO.?3?Early-Middle Jurassic?180-165 Ma?,I-type granitoids outcrop in the central GXR and eastern Jilin-Heilongjiang Province,Erguna block has been identified cropping out mainly arc-type igneous rocks,“bimodal igneous rocks”and N-S–trending porphyry Cu-Mo ore deposits outcrop in the eastern Songnen block and in the western Jiamusi block,respectively.These suggest that Jiamusi block and eastern Songnen block Early-Middle Jurassic magmatism-mineralization was mainly controlled by the subduction of the Izanagi plate beneath the Eurasian continent.However,Early-Middle Jurassic magmatism in the central GXR was associated with the closure of the MOO.?4?Late Jurassic–initial Early Cretaceous?155-138 Ma?,the north-dipping subduction of the PP oceanic plate but the MOO gradually closed in a scissor-like style from the west to the east.The magmatism in the central GXR was dominantly controlled by upwelling of asthenospheric material related to the Mongolia-Okhotsk oceanic slab on the western Erguna block breakoff and delamination of the lithosphere.?5?late Early Cretaceous?137–115 Ma?,large-area highly fractionated I-type granitoids,A-type granitoids and polymetallic deposits were distributed in the GXR and the timing of the magmatism gradually getting younger towards the north,suggesting that the magmatism-mineralization in the central GXR formed within an extensional environment induced by the closure of the MOO on the northern Erguna block,while the upwelling of the asthenosphere material was mainly caused by the rollback of the Izanagi plate in the Songliao Basin and its east region.The two tectonic regimes together contributed to the large-scale magmatism-mineralization in northeast China.