LA-ICP-MS zircon U-Pb dating and geochemical characteristics of the volcanic rocks in Deren Formation, northern Liaoning Province

This paper presents detailed petrologic, geochemical and geochronologic data of the volcanic rocks in Deren Formation of Pinggang Basin, northern Liaoning Province. The authors investigated their petrogenesis, formation age and tectonic settings and discussed the thinning time and mechanism of North China Craton. The LA-ICP-MS zircon U-Pb dating was conducted on dacitic breccia-crystal-lithic tuff in Deren Formation and the upper rhyolite. The former yielded ²⁰⁶Pb/²³⁸U ages of 246.8±3.5Ma and 169.6±4.6Ma which are supposed to represent two stages of magmatism, reflecting the Early Triassic and Middle Cretaceous magma activities in the basement of Pinggang Basin. The latter yielded ²⁰⁶Pb/²³⁸U age of 117.0±0.9Ma, which indicates that the rhyolites were formed in the Early Cretaceous. Volcanic rocks in Deren Formation are enriched in large ion lithophile elements (e.g., K, Rb, Ba) but are depleted in high field strength elements (e.g., Nb, Ta, Zr, Hf, Y, Yb). Besides, due to enrichment of LREEs and depletion of HREEs, the REE patterns exhibit right-inclined feature. These geochemical characteristics show that the source of the primitive magma was derived from the crust, and they were probably formed at the setting of volcanic arc along the active continental margins, resulting from remelting of the basement magma rocks. Major changes from positive to negative landform in the study area occurred during 117~135Ma. In combination with wide distribution of extensional basins and metamorphic core complexes, the authors hold that the magma might have been affected by mantle contamination during remelting of these basement rocks, and the thinning of lithosphere might have occurred during Early Cretaceous (earlier than 117±1.2Ma), which resulted in the delamination.