Zircon U-Pb geochronology and geochemistry of Mesozoic volcanic rocks from Dasijiazi area at Zhangwu, west Liaoning Province

Zircon U-Pb dating by LA-ICPMS reveals that the age of Mesozoic volcanic rocks from the Zhangwu area in west Liaoning Province is 122.4±0.4Ma, which belongs to Early Cretaceous. Geochemical compositions of 23 typical volcanic samples are studied. The results show that most samples except three rhyolites have similar geochemical characteristics of high-Mg adakite (SiO2=56.46%-65.14%, Al2O3=14.60%-17.19%, Mg#=50-59, Sr=501-700 μg/g, Yb=1.04-1.54 μg/g, Y=12.0-17.5 μg/g, Eu/Eu*=0.85-0.97, Sr/Y=29-46, LaN/YbN=13-28). They also have high initial 87Sr/86Sr (0.70639-0.70647) and low Nd (122 Ma) values (-6.43 to -12.26) that are inconsistent with the original ones from slab melting. Pyroxene phenocrysts are characterized by reversed compositional zoning. Rare earth element concentrations decrease from pyroxene core to rim and also show negative Eu anomalies (Eu/Eu*=0.64-0.76). Combined with the previous studies on volcanic rocks from the low part of Yixian Formation in Zhangwu area, we propose that our samples from Zhangwu area resulted from delamination and magma mixing. A crustally derived felsic magma resulted from partial melting of the low crust, heated by asthenosphere upwelling and subsequently mixed with a mantle-derived high Mg adakitic melt during magma storage or ascent to surface to form the high-Mg adakites in the Zhangwu area.