ZIRCON U-Pb DATING AND GEOCHEMISTRY OF THE FANGJIAGOU PLUTON IN ZHALANTUN AREA, INNER MONGOLIA

The Fangjiagou monzogranite pluton shows a typical magmatic origin with its clear crystal interior structures of zircon, developed oscillatory growth zonation and high Th/U ratio of 0.99 -3.10. The LA-ICP-MS zircon U-Pb dating is (152.9±2.2) Ma. The rock is peraluminous, belonging to high-K calc-alkali series, with SiO₂ content of 73.06%-76.25% and Rittman index (σ) of 2.51-2.63. The ratios of ΣLREE/ΣHREE and (La/Yb)_N are respectively 6.21-10.43 and 4.98-12.18, with enrichment in LREEs and depletion in HREEs. The δEu values range from 0.52 to 0.72. The features above indicate that the Fangjiagou pluton belongs to peraluminous highly fractionated I-type granite, which may be formed due to the crust melting by temperature rising and pressure reducing in post-orogenic extensional environment, with crust thinning, materials from asthenosphere upwelling and mantle-derived magma underplating.