LA-MC-ICPMS zircon U-Pb dating of Wulanwuzhuer porphyritic syenite granite in the Qimantag Mountain of Qinghai and its geological significance

Wulanwuzhuer porphyritic syenite granite in Qimantag region is characterized by high silica, high alkali (AR 3.02-5.88,NK/A 1.04-1.17,and A/CNK 0.96-1.07), high TFeO/MgO ratio, poor CaO and MgO, relatively high ∑REE,slight enrichment of LREE,indistinct light and heavy REE fractionation,negative Eu anomalies,somewhat right-oblique “V”-shaped REE patterns,significant enrichment of Rb, K, Th and other large ion lithophile elements, relative enrichment of Zr, Hf and other high field strength elements, and relative depletion of such elements as Ba, Sr, P, Ti, and Nb. These characteristics suggest an aluminous A2-type granite, whose formation might have been associated with the young crust formed by oceanic crust and island arc. Using LA-MC-ICPMS zircon U-Pb dating, the authors obtained the 206Pb/238U age of Wulanwuzhuer granite of 388.9±3.7 Ma(MSWD=3.1),which represents the formation age of the rock. In combination with geochemical and structural features and regional geological setting, the authors hold that the intrusive body was formed in a relative extension environment during post-orogenic or late orogenic evolution.