Tectonic Evolution Of Xar-Moron Suture Zone In Late Paleozoic-Early Mesozoic

Balinyou-Ar Horqin Banner in southeastern Inner Mongolia is located on the southern margin of the Xingmeng Orogenic Belt,eastern segment of the Central Asian Orogenic Belt,spanning Solonker-Xar-Moron-Changchun-Yanji Suture Zone and the northern margin of North China Plate.It is a key area for studying the evolution history of Paleo-Asian Ocean and Xar-Moron Suture Zone.Due to the serious cover of sand in this area,the study was weak,relatively.There was no systematic report on the geologica records from Late Paleozoic to Early Mesozoic,which lead to the debates on the tectonic evolution and restrict the study on the position and time for the final closure of the Paleo-Asian Ocean along the Xar-Moron Suture Zone.In this study,the Late Paleozoic-Early Mesozoic magmatic rocks,subduction-accretion complex and sedimentary strata in the study area are taken as the study objects,and detailed field geological survey was carried out.Combined with the field contact relationship and petrography,this study used the LA-ICP-MS zircon U-Pb dating to determine the formation age of rocks and analyze the provenance of sedimentary rocks,and then established the chronological framework of magmatic rocks in the study area.A comprehensive analysis of rock assemblages,petrogeochemistry and in-situ zircon Hf isotope characteristics were used to discuss the petrogenesis and source properties of magmatic rocks.Based on the previous regional magmatic rocks,ophiolites,stratigraphic paleontology and other data,this study analyzed the petrogenesis and tectonic setting of the magmatic rocks and subduction-accretion complex,and then discussed the Late Paleozoic-Early Mesozoic tectonic evolution history of the study area,as well as the location and time for the final closure of the Paleo-Asian Ocean along the Xar-Moron Suture Zone.1.Based on the newly obtained LA-ICP-MS zircon U-Pb chronological data and the previous chronological data in the region,Late Paleozoic-Early Mesozoic magmatic activity in the Balinyou-Ar Horqin Banner region is divided into four periods:Late Carboniferous（314～305Ma）,Early Permian（298～276Ma）,Middle Permian（271～261Ma）and Triassic（242～232Ma）.The Late Carboniferous magmatic activity（314～305Ma）is represented by the Wudahada volcanic rocks,which are newly recognized Late Carboniferous volcanic rocks in the northern margin of North China Plate.Wudahada volcanic rocks are enriched in light rare earth elements（LREEs）and large-ion lithophile elements（LILEs）,and depleted in heavy rare earth elements（HREEs）and high-fieldstrength elements（HFSEs,e.g.,Nb,Ta,Ti）.The positive zirconεHf（t）values（+6.64 to+9.49）of the volcanic rocks and the high Mg^#values（47.05～51.47）of some samples indicate the contribution of mantle materials to the sources.Integrated with geological,geochemical,and Lu–Hf isotopic studies,it is considered that Wudahada volcanic rocks formed in a subduction-related continental margin arc setting.Combined with Late Carboniferous magmatic events in the northern margin of North China Plate,it is suggested that the southward subduction of Palaeo-Asian Ocean beneath the northern margin of North China Plate was still occurring in Late Carboniferous.Early Permian magmatic activity（298～276Ma）is represented by gabbros,monzogranites and quartz diorite porphyries.Banlashan gabbros are Early Permian Nb-enriched gabbros first discovered in the study area.The gabbros are tholeiitic-calc-alkaline series.The gabbros are slightly enriched in LREEs,LILEs（Rb,K,Ba）and Th,and depleted in HREEs,HFSEs（Ta,Hf,Ti）and Sr,and derived from the partial melting of the mantle wedge metasomatized by adaktic melt released from the subducted slabs.Hatetaiaobao monzogranites are Early Permian high-silicon adakites first found in the study area.They are high-K calc-alkaline and peraluminous I-type granites,They are enriched in LREEs,LILEs（Rb,K,Ba）and Th,and depleted in HREEs,HFSEs（Nb,Ta,Ti）and P,derived from partial melting of subducted oceanic slabs.Luotuojingzi monzogranites and Banlashanlinchang quartz diorite porphyry are calc-alkaline-high-K calc-alkaline,metaluminous-weak peraluminous I-type granites,which are enriched in LREEs,LILEs（Rb,K,Ba）and Th,and relatively depleted in HREEs,HFSEs（Nb,Ta,Ti）and P,Sr,Hf,reflecting they were formed in a subduction-related setting.Combined with the study of Early Permian reginal magmatic events,the Early Permian magmatic activity in the study area was formed in the background of the active continental margin,suggesting that southward subduction of the Paleo-Asian Ocean was still occurring duing Early Permian.Middle Permian magmatic activity（271～261Ma）is represented by Baoridaobu granodiorites,quartz diorites and Xishangjiujingzi quartz diorites.They belong to calc-alkaline-high-K calc-alkaline,metaluminous I-type granites,which are enriched in LREEs,LILEs（Rb,K,Ba）,and relatively depleted in HREEs,HFSEs（Nb,Ta,Ti）and Sr,P.The magmatic sources show the characteristics of crust source,which formed in a continental margin arc setting affected by subducted slab melts and/or fluids.Combined with the study of the Middle Permian magmatic events in the region,the Middle Permian magmatic activity in the study area formed in the background of the active continental margin,suggesting that southward subduction of the Paleo-Asian Ocean was still occurring duing Middle Permian.Triassic magmatic activity（242～232Ma）is represented by Bayanchagan two-mica monzogranites.The two-mica monzogranites are high in Si O₂,Al₂O₃ and low in Mg O,Fe₂O₃,belonging to strongly peraluminous series（A/CNK=1.10～1.25）,enriched in light rare earth elements and large ion lithophile elements,and depleted in heavy rare earth elements and high field strength elements（Nb,Ta）,showing features of S-type granite.Hf isotopic analysis results showε_Hf（t）values of-1.8 to+4.01,two-stage model ages ranging from 1006Ma to 1380Ma,respectively,indicating that the primary magma was derived from the partial melting of a thickened crust.Based on the study of rock geochemistry,zircon Hf isotope of Bayanchagan two-mica monzogranites and Triassic collision-related magmatic events,we suggest that it was at the period of continental collision during Triassic.2.Based on the characteristics of field geology,petrology,geochemistry and zircon chronology in the study area,combined with the previous study on ophiolites in the region,two periods of subduction-accretion complexes were identified,including Banlashan subduction-accreation complex in the early Late Paleozoic and Xishangjiujingzi subduction-accreation complex in the late Late Paleozoic.Banlashan complex is a set of subduction-accretion complex rich in SSZ type ophiolites in the early Late Paleozoic.The petrogeochemical analysis shows that the amphibolites（block）belong to the tholeiitic series,showing MORB-like characteristics,and formed in the initial intra-oceanic subduction setting.Dacites（block）belong to calc-alkaline series,showing I-type granite characteristics and formed in subduction-related continental margin arc setting.LA-ICP-MS zircon U-Pb dating shows that the magmatic crystallization age of amphibolites is 416.3±3.4Ma,and that of dacites is410.5±1.7Ma,both of which belong to Early Devonian.Youngest detrital zircon age of the silty slate（matrix）is 355Ma,indicating that the subduction-accretion emplacement age of Banlashan complex is late Early Carboniferous to early Late Carboniferous.Xishangjiujingzi complex is a set of subduction-accretion complex rich in SSZ type ophiolites in the late Late Paleozoic.The petrogeochemical analysis shows that gabbros（block）belong to tholeiitic series with MORB-like characteristics and formed in the initial intra-oceanic subduction setting.The HMA and rhyolites（blocks）belong to tholeiitic-calc-alkaline series and formed in a relatively mature intra-oceanic subduction setting.LA-ICP-MS zircon U-Pb dating shows that the crystallization age of gabbros is283.5±1.8Ma,which belongs to Early Permian.The crystallization age of the HMA is262.5±1.9Ma,and that of rhyolites are 267.2±1.3Ma and 262.8±2.1Ma,both of which belong to Middle Permian.Detrital zircons of the sedimentary rocks（matrix）show relatively simple provenance,and youngest detrital zircons are 259～249Ma,indicating that the accretion emplacement of the Xishangjiujingzi complex occurred in late Late Permian to early Early Triassic.3.Comparing the detrital zircon ages of the Middle Permian Kedehe Conglomerate and Yujiabeigou Formation in the study area,their peak ages overlap at～2500Ma,～1800Ma,～480Ma,～440Ma and～280Ma,showing the similar provenance and affinity with North China Plate,suggesting that the final suture zone of the Paleo-Asian Ocean is probably located to the north of them.4.Based on the study of two-stage subduction-accreation complex in Late Paleozoic,arc magmatic rocks Late Carboniferous to Middle Permian and the provenance of the Middle Permian sedimentary rocks in study area,combined with the previous chronology results of ophiolites in the region,we believe that the final combination of the North China Plate and the Siberian Plate in central-eastern Inner Mongolia probably occured along Solonker,Mandula,Xingshuwa,Wudaoshimen and Xishangjiujingzi.Combined with the study of sedimentary environment,paleontology and paleomagnetism in the southeast of Inner Mongolia,it is suggested that the final closure of the Paleo-Asian Ocean along the Xar-Moron Suture Zone occured in late Late Permian to early Early Triassic.