Isotope Chronology and Geochemistry of the Lower Carboniferous Granite in Xilinhot, Inner Mongolia, China

Geochronological and geochemical analyses were performed on K-feldspar granites and monzonitic granites from the Xilinhot area, Inner Mongolia, China. Zircon U/Pb ages indicate that the two types granites were emplaced during the Lower Carboniferous. The K-feldspar granites (332 Ma) have the typical A-type granite characteristics of a post-collision setting. The monzonitic granites have an emplacement age of 323 Ma. Zircon εHf values of the former range from +12.8 to +14.2, with an average T_DM2 of 453 Ma. The latter have lower zircon ε_Hf values, ranging from +5.4 to +10.7, with an average T_DM2 of 798 Ma. The strong, positive ε_Hf values of the zircon indicate that both sets of samples are from a juvenile crust formed in an oceanic crust subduction stage, although the monzonitic granite may have undergone a hybridization of crustal materials. These results indicate a younger post orogenic event. The Paleo-Asian Ocean had closed before the Early Carboniferous and the Xilinhot area started its post-orogenic evolution with an extensional tectonic environment during the Early Carboniferous.