Late Paleozoic Tectonic Evolution Of The Central-northern Margin Of The North China Craton: Constraints From Zircon U-Pb Ages And Geochemistry Of Igneous Rocks In Ondor Sum-jining Area

Ondor Sum-Jining is an important area to research the Paleo-Asian Ocean andthe evolution of Xingmeng Orogenic Belt. It across the North China Craton (NCC)and the southern edge of Xingmeng Orogenic Belt, specifically it locates in the southof Solon Suture Zone in the central Inner Monglia. In study area, there are threeeast-west trending Faults including Ulanhada Fault, Xuniwusu Fault and Ondor SumFault. These faults divide the study area into four tectonic units of which are thePrecambrian NCC, Mesoproterozoic-Neoproterozoic Rifted Continental Margin,Bainaimiao Island Arc Rock Belt and Ondor Sum Subduction-Accretionary ComplexBelt. Based on the detailed geological survey and petrographic study, then with thezircon LA-ICP-MS U-Pb and zircon SHRIMP U-Pb dating can confirmed theemplacement age of igneous and also established the chronology framework ofregional magmatism; the rock assemblages, petrographic and geochemicalcharacteristics were used to discuss the igneous magma source and their tectonicsetting. Finally, combined the distribution feature of the igneous rocks andinformation of Late Paleozoic igneous in adjacent areas to explore the Late Palezozoictectonic evolution history of the study area. 1. D·evonian extension settingMarine molasse of Upper Silurian Xibiehe Formation marked the end ofPaleozoic trench-arc-basin system in the northern margin of the North China Plate andthe collision between Bainaimiao Island Arc and the NCC. The Devonian igneousrocks mainly composed of the syenite, monzonite-diorite and syenite granite, theyemplaced in Chahayouhouqi area in the northern NCC. The major alkaline darkminerals in the syenite are aegirine, aegirine-augite and arfvedsonite. The igneousrocks are metaluminous and its magma source was alkaline-basaltic magma fromelithosphere mantle and of lower earth crust. According to zircon LA-ICP-MS U-Pb dating result of410.9±1.2Ma, the previously reported381.5±4.1Ma ofmonzonite-diorite in the study area and Devonian igneous rocks in the Baotou-Jining-Zhangjiakou-Chifeng area, these datas indicate that the middle-western part of thenorthern North China Plate is extension setting and the combination of intrusive rocksare syenite, monzonite and A2-type granite, the volcanic rock mainly is rhyotaxitictuff.2. Ca·rboniferous-Early Permian an active continental margin arc settingThe Early Carboniferous magmatism is represented by monzonite granites, whichemplaced at342.5±4.9Ma reproted in the previous study. They are metaluminous-weakly peraluminous magma series, highly fractionated I-type and high K calc-alkaline granite. The volcanism is represented by andesite with the zircon SHRIMPU-Pb age333±4Ma. The volcanic rock is named Chagannuoer volcanic rock that ismainly composed of andesite, dacite and rhyolite.The Early Permian magmatism is represented by hornblende gabbro and have thefeatrues of the Bowen Reflect Series that the plagioklases typies are oligoclase,andesine, labradorite and bytownite, some amphibole aroud clinopyroxene. The rockemplaced into the northern NCC at297.2±1.7Ma resulted by the zircon LA-ICP-MSU-Pb dating.The volvanism is represented by the andesite and crystal tuff in the ElituFormation and magmatic eruption at nearly277.59±0.43Ma and275.28±0.96Ma.In Carboniferous-Early Permian, the intrusive rocks are mainly distributed near inthe Chahayouhouqi in the northern NCC, volcanic rocks are distribuded in the Mesoproterozoic-Neoproterozoic Continental Margin Rift and Bainaimiao IslandArc. Intrusive rocks are composed of monzonite granite and hornblende gabbro andvocanic rocks are composed of andesite, dacite crystal tuff, rhyolite and litter basalt,belong to calc-alkaline series-high K calc-alkaline series-tephrite series, theirmagma sourced from the lithosphere mantle-lower earth crust, enrichments in largeion lithophile elements and depletions in high field-strength elements, indicatemagmatisms formed in active continental margin arc setting related to the subductionevent of the Paleo-Asian Ocean beneath the northern margin of North China Plate.3. Early-Middle Permian continental margin arc transform to the syncollisional settingThe intrusive rocks are represented by quartz-diorite, granodiorite, biotitemonzonite granite. They have a large number of magic microgranular enclaves andhave the zircon LA-ICP-MS U-Pb dating age is272.8~268.7Ma. Rocks belongto high K calc-alkaline series and have high contents of∑REE, enrichments in lightrare elements and depletions in hight rare elements, monzonite granite have highercontents of∑REE than others. Their magma derived from upper-middle crust.Quartz-diorite formed in the continental margin arc setting, granodiorite formed in thesetting that continental margin arc trasform to the syn collision, biotite monzonitegranite formed in the syn collision. Their tectonic settings indicate that the closuretime of Paleo-Asian Ocean is earlier than the formation of the biotite monzonitegranite and Xuniwusu overthrust structure formed near the arc-continental collagezone during the syn collision.4. Middle Permian post-collisional tectonic settingThe Middle Permian magmatism is represented by monzonite granite, syenitegranite and alkali feldspar granite. Rocks primarily output as a large batholith and theage is267.2~265.1Ma by zircon LA-ICP-MS U-Pb dating. They belong to stronglyperalminous rocks, calc-alkaline-high K calc-alkaline-shoshonite series and calc-alkaline to alkaline series evolutiong trend, magma mainly sourced from crustmaterial and mixed with some mantle material and emplaced in the post-collisional extentional setting.