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 (2σ) by its magmatic zircon SHRIMP U-Pb dating, and an upper limit age of 316 ± 3Ma (2σ), 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 undiscoveredin situ terranes aged 600–800 Ma or even up to ca 3.1 Ga near the Complex.