SHRIMP U-Pb zircon geochronology and its implications on the Xilin Gol Complex，Inner Mongolia,China
Abstract The Xilin Gol Complex, consisting of deformed and metamorphosed rocks, was exposed as a large geological unit within the Central Asian Orogenic Belt, but its forming and subsequent deformed and metamorphic time has been an issue of little consensus. Petrographic analyses and SHRIMP dating on biotite-plagioclase gneiss, one of the major rocks within the Xilin Gol Complex, in southeast Xilinhot City, Inner Mongolia, China, where the Xilin Gol Complex was identified and named, yield its lower limit age of 437 ± 3 Ma (2s ) by its magmatic zircon SHRIMP U-Pb dating, and an upper limit age of 316 ± 3Ma (2s ), which was constrained by SHRIMP dating of magmatic zircons from adjacent undeformed garnet-bearing granite which intruded the Complex.The Complex was thus determined to be formed and subsequently deformed/metamorphosed from the late Ordovician-early Silurian to the mid-Carboniferous. Consequently, it is not the Precambrian terrane as previously considered by most geologists. More or less, the major rock —— biotiteplagioclase gneiss within the Complex is more likely to be Paleozoic fore-arc turbidite formation before metamorphism and intensive deformation, in which the detrital zircons gave sporadic Precambrian ages as old as up to 3.1 Ga. The source of the turbidite formation is multiple, which may be derived either from the North China Craton, or from the South-Mongolia Micro-continent, or probably came from a potential and undiscovered in situ terranes aged 600—800 Ma or even up to ca 3.1 Ga near the Complex.
Emplacement age and tectonic implications of the Xilinhot A-type granite in Inner Mongolia,China
Abstract A new rock type of granite, approximate 45km2 in area and located about 10 km south of Xilinhot, Inner Mongolia, was found in the Sunitezuoqi (or called Suzuoqi)-Xilinhot tectonic belt and identified as an A-type miarolitic intrusion. The pluton has miarolitic structure and is composed chiefly of perthite, quartz, euhedral albite and potassium feldspar. Various types of textures occur in the pluton, such as perthitie, graphic and myrmekite textures.Only quartz is found in miarolitic cavity. This A-type granite with seagull-shaped REE patterns and obvious negative Eu anomaly (d Eu = 0.24—0.28) is high in SiO2 (76%—77%), K and Na (Na2O + K2O = 7.75%—8.15%) and low in Ca (CaO= 0.20%—0.22%), Fe and Mg. Both petrographical observations and chemical compositions indicate that it is an A-type granite. Zircon SHRIMP U-Pb analyses indicate that this A-type granite was emplaced at 276 ± 2 Ma and coeval with the same type of granites in the adjacent areas. Therefore, it suggests that this pluton was likely formed in a post-orogenic extensional setting and probably related to break-off of subducted slabs in Central Asian Orogenic Belt (CAOB), which indicate that the Sunitezuoqi-Xilinhot belt was tectonically evolved into post-orogenic stage since early Permian.
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