Studies Of Geochronology And Geochemistry Of Paleozoic Magmatism In XiLinhot Area, Inner Mongolia

The Xilinhot is located in the eastern section of Central Asian Orogenic Belt (CAOB) that separates the Siberia Craton to the North from the Tarim and North China Cratons to the South. This region is adjacent to Siberia Plate by Mongolia-Okhotsk suture belt to the west and north, adjacent to North China Plate by Xar Moron River suture belt to the south. Subduction of oceanic lithosphere and accretion of different paleo-microcontinents resulted in closure of the Paleo-Asian Ocean between the Siberia and North China Cratons in the area. CAOB is one of the regions that underwent significant Proterozoic-Phanerozoic continental crust growth in the world. With multi-stage geological evolution and strong reconstruction, the crust has complicated substance composition, texture and structure, and which is the ideal region for the research on dynamic evolution of continental lithosphere.Magmatism is the product of the movement of upper mantle of deep crust on the surface orshallow crust of the earth, which recorded abundant information of the evolution of crust and mantle and their interaction in geological history. Igneous petrotectonic assemblage or series is one of the important research contents to retrieve paleo-tectonic environment and discuss geodynamic processes of lithosphere. Based on analysis lots of field and test data from1:250000and1:50000geological survey programs in Xilinhot area, and researching on Paleozoic-Early Mesozoic igneous petrotectonic assemblage combined with previous research achievements in regional stratigraphy paleontology and tectonic geology, we attempt to reconstruct the tectonic framework of Xilinhot area and unravel the process of the closure of the Paleo-Asian Ocean.The accurate geochronology frameworks of the magmatic rocks are the basic for the further researches on the evolution of regional tectonic environment. In this paper, we obtain14zircon U-Pb ages of9intrusive rock as Tabudaimaimai granodiorite formation (231±7Ma), Wendurodele granodiorite formation (252±2Ma), Xilinhot reservoir diorite formation (275±2Ma), BoRen aobao granodiorite formation (288.2±1.8Ma), Dituojika mountain granodiorite formation (318±1Ma), Tongxunlian field gabbro formation (320±2Ma), Yintougou biotite adamellite formation (323±2Ma), Aligatuwula moyite formation (330±2Ma), Xiretu gneissose tourmaline two-mica granite formation (458±1Ma); and2zircon U-Pb ages of volcanic rock for Hala formation (411±6Ma) in Paleozoic Baoerhantu group and Mesozoic Harhada formation (241±14Ma).During the late stage of early carboniferous, the S.I.A type granite assemblage formed with features of postorogenic granite after continent-continent collision; Including the trace elements of the moyite shows the geochemical characteristics of A type granite. As the invasion sequence of moyite-monzonitic granite-gabbro-granodiorite, the A type moyite and the later gabbro are formed in the extensional environment of collision post-orogenesis. A process of extension-(6Ma)-compression-(5Ma)-extension-(1Ma)-compression during orogenesis, with a long interval at early and shorter at later, which made the later granodiorite have the largest emergence area as the later extensive collision.As the environment of collision post-orogenesis during late stage of early carboniferous to late carboniferous changed into the emplacement of granodiorite during early Permian, the small oceanic basin disappeared and uplift by step. The geochemical characteristics of granodiorite emplaced in Permian is similar to the composition of I-type granite。The volcanic rock from Dashizhai group, which has been well researched, might formed in an extensional environment from back-arc.Combined with the geochronology and geochemistry information published recently, we divided the tectonic-magmatic activities during Paleozoic and early Mesozoic into two major periods:1. Stage of the island arc junction in Caledonian (from Ordovician to Silurian);2. Stage of the development of regenerated oceanic basin during Hercynian and Indosinian (from Carboniferous to Triassic). Based this division, the tectonic evolution since Paleozoic in Xilinhot area would be discussed.During middle-late Cambrian (stage of Caledonian), the Paleo-Asian ocean reached main extension between Siberia antient land and North China antient land. As the expansion and subduction is coexistence during the early Paleozoic (from Ordovician to Silurian), the Paleo-Asian ocean once underwent the subduction in two directions:northward subduction to the Xilinhot micro blocks and southward subduction to the Shuangjing micro blocks. The northern ocean take the subduction to the Xilinhot micro blocks and the Mongolia plate. Continued subduction of the oceanic crust finally resulted in disappearance of the ocean at the southern of the Xilinhot micro blocks and collision between the accretion zones of the Kedan Mountain and the southern margin of the Xilinhot micro blocks. The complex collision suture zone is was found from west to east, which is consisted of Ordovician volcanic rocks belongs to island arc calc-alkaline series, associated with Caledonian basic-ultrabasic rock and granite. the associated tourmaline two-mica granite shows lots of zicon ages during the Neoproterozoic-Cambrian period. The back arc basin deposited Xuniwusu group of flysch sediments and Xibiehe group of Molasse formation as sedimentary cover during the Silurian in southern margin of the volcanic island arc.Permian-Triassic represents the evolution of revived oceanic basin. Caledonian movement caused the uplifting, which then be eroded and lead to a hiatus in early D-C1sedimentary and magmatism (C2sediments are lost in Xilinhot). In the Wusinihei ophiolites, north of Xilinhot, the late Carboniferous continental clastic rocks of Gegenaobao formation overlay on the ophiolites unconformably. Meanwhile, Sonid Left Banner rifted again and began to deposit. Proto Xilinhot arc then show the characteristics of underwater uplifting due to rifting, and have some constrains on the sedimentary:in early-stage (C2), north of the swelling sink and accept the C2sediments; while in late-stage (P-T2) the south of the swelling depressed, which is the strongest and deepest rupture in this region, it accepted the P-T sediments and the sedimentary thickness is quite great. Hercynian and Indosinian magmatisms in this region are strong and especially focused along the Xilamulun fault and core of Xilinhot anticlinorium. They are consists of early-Carboniferous post-collisional orogenic igneous rocks, early-Permian granites with continental arc characteristics and early-Triassic peraluminous granites. The early-Carboniferous A-type granites might be produced in the extension background during orogenic process, e.g. spilite with the transitional characters between arc and mid-oceanic ridge tholeiites have been found in the Dazhai bimodal lavas, indicating that the oceanic basin open again and even produced the oceanic crust in Xilinhot since early-Permian. The Permian diabase dyke found in the Huoziliang-Jiuzhuanyao Sihe farm also give evidence to the Hercynian-Indosinian depression of revive oceanic basin. Late-Permian to early-Triassic, I type>S type granites suggest the closure of oceanic basin and the termination of this stage.The Central Asian Orogenic Belt (CAOB) is a complex accretionary orgrnic belt composed of Precambrian micro-continental blocks, ancient island arcs and ophiolites, which grow by lateral accretion of arc complex and vertical mantle materials underplating. The CAOB experienced complex crustal growth and celebrated for its massive juvenile crustal production in the Phanerozoic. In this paper, we studied the relationship between the Paleozoic magmatisms in Xilinhot and tectonic evolution, and found that the Paleozoic is the important period of crustal growth in CAOB. The accretionary mechanisms are different during different tectonic-magmatic evolutionary stage: lateral accretion of arc complex in Caledonian, vertical mantle materials underplating in Carboniferous to early-Permian, and the lateral accretion is dominantly in early-Triassic. Even at the same times, the acctrtionary ways in different tectonic background have some differences:during early-Permian period, it is dominated by vertical accretion in southern Linxi region, while the lateral accretion is dominantly in Xilinhot. Crustal growth in Xilinhot is a multi-stage and multi-cycle process, which is throughout the whole stage of CAOB evolution.