LA-ICP-MS zircon U-Pb dating and geochemistry of granitoids in Tamusu, Alxa Right Banner, Inner Mongolia

Acid and intermediate-acid intrusive rocks are widely distributed in Tamusu, northern Alxa massif. These rocks are mainly biotite granite, granodiorite, adamellite and tonalite, with the last three forming the main body of the intrusive rocks in Tamsag area. The massif, unconformably covered by Cretaceous and Jurassic strata, was intruded into Late Carboniferous-Early Permian Amushan Formation, and its regional distribution shows sub-latitudinal feature. The LA-ICP-MS zircon U-Pb ages of the monzonitic granite and tonalite in this region are respectively 264.2±3.3Ma and 273.0±3.3Ma. The granodiorite, monzonitic granite and tonalite are of calc-alkalic to high potassium calc-alkalic suite, characterized by A/CNK ratios of 0.874~1.01, 1.06~1.54, and 0.91~1.03, and ∑REE values of 156.99×10-6~212.87×10-6, 126.92×10-6~212.98×10-6 and 128.13×10-6~214.5×10-6, respectively. These intrusive rocks show a close affinity to island-arc granites in Rb-(Yb+Nb) and Rb-(Yb+Ta) diagrams, with enrichment of Ba, Rb, Th, K and other large ion lithophile elements, and depletion of Nb, Ta, P, Ti and other high field strength elements. It is considered that the northern margin of Alxa might still have had some arc-related magmatic activities during the Permian period.