Geochronological Framework, Hf Isotope Characteristics Of Phanerozoic Granitoids In Southern Mongolia-Bohai Bay Geological Corridor And Tectonic Implications

The Southern Mongolia-Bohai Bay geological corridor stretches across Central Asian Orogenic Belt and North C hina Craton. In this study, detailed petrogenesis, zircon U-Pb geochronology, geochemistry and zircon Hf isotope studies have been carried out on granitic rocks from the Ulaanbadrah area, Southern Mongolia and the Huanggangliang area, Mid-eastern Inner Mongolia. Then, published zircon U-Pb geochronology and zircon Hf isotopic data for Phanerozoic granite from the Southern Mongolia-Bohai Bay area have been collected. Phanerozoic granitoids maps showing spatial- temporal framework and zircon Hf isotopic characteristics have been compiled and the section maps have been made to disclose the deep crustal prpperties. Finally, implications of the zircon Hf isotopic characteristics of the granitoids have been discussed to provide additionan evidences for the identification of plate boundary and paleo-continental block.The Ulaanbadrah area is located in Dornogovi Province from South Mongolia, adjacent to Erenhot-Hegenshan ophiolite belt in the south. The syenogranite and syenogranite dyke from the Ulaanbadrah area yield LA-ICP-MS zircon U-Pb ages of 337±4Ma and 332±2M, respectively. They almostly have the similar emplacement time and belong to products of the late early Carboniferous. The granites of Ulannbadrah are weakly peraluminous and belong to the high-K calc-alkaline series, showing petrological and geochemical features with high- fractionated I-type affinity. Zircon εHf(t) values and two staged Hf model age of the syenogranite is +10.0-+14.9, 712-393 Ma. Zircon εHf(t) values and two staged Hf model age of the syenogranite dyke is +9.7-+14.3, 721-425 Ma. The testing results of Hf isotope indicate that they were probably derived from juvenile source, with rarely old crustal materials materials. Combined with the regional geology and the homochromous magmatic evolution, there exist one important magmatic events of the early carboniferous epoch in the East Ujumqin Banner fold belt including Ulaanbadrah area, the pluton shows a banding distribution in N E, which is consistent with the extension direction of regional tectonic belt, these magmtisms maybe formed in a regional extensional setting related to the post-orogenic background, the Palaeo-Asian Ocean may have been closed in early Carboniferous.Huanggangliang area is located in Linxi-Keshiketen banner from Mid-eastern Inner Mongolia, north of Xar Moron suture zone. The porphyroclastic lava and syenogranite from the Huanggangliang volcanic- intrusive complexes yield a consistent LA-ICP-MS zircon U-Pb age of 138±1Ma, which is consistent with the most widespread magmatism in the southern Great Xing?an Range. Both porphyroclastic lava and syenogranite are metaluminous-weakly peraluminous and belong to the high-K calc-alkaline to alkaline series, showing petrological and geochemical features with high- fractionated I-type affinity. Zircon εHf(t) values and two staged Hf model age of the porphyroclastic lava is +2.4-+7.4, 1039-717 Ma. Zircon εHf(t) values and two staged Hf model age of the syenogranite is +2.1-+13.5, 1061-330 Ma. The testing results of Hf isotope indicate that they were probably derived from juvenile crust, with minor old crustal materials. Combined with the regional geology studies, these Early Cretaceous magmtisms formed in a regional extensional setting related to the post-orogenic extension under the Mogol-Okhotsk tectonic regime. The degree of magama evolution is higher in Cretaceous than in Jurassic. It implies the characteristics of magama evolution in the process of Orogenic belt gradually collapse and continental crust gradually thin from Jurassic to Cretaceous.Statistics of the zircon U-Pb data(299 ages) indicates the magmatism of Phanerozoic granitoids in the Southern Mongolia-Bohai Bay area can be divided into six major episodes, namely Cambrian-Devonian(499-359Ma), Carboniferous(359-99Ma), Permian(299-252Ma), Triassic(252-201Ma), Jurassic(201-145Ma), and Cretaceous(145-109Ma). Among them, the Carboniferous-Permian, Jurassic-Cretaceous magma activities are the most intensive.The Hf isotope data of the Phanerozoic granitoids shows a gradually northward increasing εHf(t) values from the North C hina Craton to East Ujimqin Banner fold belt, but the TDM2 values show a opposite trend. In the south and north flanks of the Xar Moron suture zone, granitoids show contrast different Hf isotopic characteristics, which implies that the Xar Moron suture zone could be the boundary between Central Asian orogenic Belt and North China Craton. The granitoids to the north of the Xar Moron suture zone are generally characteristized by positive εHf(t) values and young model ages, showing a typical juvenile source nature of Central Asian orogenic Belt. The old materials exist sporadically, which provide a powerful evidence of the existence of some micro continents, such as Xilinhot micro continent. The granitoids to the south of the Xar Moron suture zone are generally characteristized by negative εHf(t) values and old model ages, which is consistant with the nature of ancient bacement in North C hina Craton. In some palces, relatively juvenile material exists in the crust, which may be caused by subduction of Paleo-Asian ocean plate in the Paleozoic and destruction of North China Craton in the Mesozoic. The North China Craton can be divided into continental marginal orogenic belt and relatively stable North C hina Craton based on the degree of alteration. The boundary of intracontinental orogenic belt and the North C hina Craton generally believed to locate in C hifeng area, Hf isotopic characteristics further confirm that the deep fault located about 50 kilometers north of C hifeng city should be the boundary between the North China Craton and continental marginal orogenic belt.