U-Pb zircon geochronology and geochemistry of volcanic rocks of the Yixian Formation in the Lingyuan area, western Liaoning

Zircon U-Pb dating of a rhyolite sample from the Yixian Formation in the Lingyuan area, western Liaoning, yields an age of 124.4±1.4 Ma, indicating an Early Cretaceous age. The age agrees with the previous results obtained using K-Ar, Ar-Ar and U-Pb methods for the Yixian volcanic rocks in the Beipiao and Lingyuan areas. Geochemical compositions of 7 samples of typical Yixian volcanic rocks from the Lingyuan area show that they consist of trachyte and rhyolite, with SiO2＝65.6-73.4％, MgO＝0.35-1.23％, Mg#＝16-41, Al2O3=13.40-16.79% and Na2O＝3.54-4.26％. In addition, the volcanic rocks are characterized by enrichment in LREE and depletion in HREE, with high Sr (445-774)×10-6 and Sr/Y (49-66) and low Y (7-14)×10-6 and Yb (0.51-0.97)×10-6 and LaN/YbN＝33-48, showing the geochemical characteristics of adakite. However, they are low in Mg#. The authors suggest that they may have originated by partial melting of the thickened mafic lower crust.