Zircon U-Pb Chronology Geochemical Characteristics and Geological Significance of Alkaline Maficionado Dikes and Intermediate Volcanic Rocks in the Ziyang-Zhenping Area of South Qinling

An alkaline magmatic belt, consisting of widespread ultrabasic-basic dikes and alkaline volcanic rocks, occurs along the Hongchunba-Zengjiaba fault in the northern Daba shan aera of South Qinling. Zircon U-Pb dating of trachytic volcanic rocks in the Ziyang-Zhenping area shows that the trachytic rocks erupted in early Silurian (442 Ma～439 Ma), coeval to the basic dikes in the area. Whole-rock compositions of the trachytic rocks and adjacent diabase show that the trachytic rocks have relatively high SiO₂ (59.1%～65.4%) and alkali content (K₂O+Na₂O, 8.40%～11.4%), but low CaO (0.35%～1.89%), MgO (0.64%～3.31%), and TiO₂ (0.74%～1.67%) contents. The trachytic rocks show significant enrichment of large ion lithophile and light rare earth elements, indicating that they underwent a high-degree evolution. The trachytic rocks have high Nb/U ratios (=39.9～112, average 58.77) and Nb/Ta ratios (=15.7～43.9, average 19.9) close to the values of the primitive mantle, revealing that both the trachytic rocks and the basic dikes are derived from mantle. The average Nb/Th, Th/Yb, Nb/Ta, Th/U ratios of the trachytic volcanic rocks (10.5, 3.9, 20.0, and 5.7, respectively) are similar to those of the diabase (11.7, 1.6, 19.6, and 5.8, respectively), suggesting that they evolved from cogenetic magma with different degrees of fractional crystallization. Combined with the regional chronological data, geochemical and tectonic characteristics, it is proposed that the early Paleozoic trachytic rocks and alkaline basic rocks in the Ziyang-Zhenping area were formed in a rift environment under the continental extensional setting. The trachytic and basic rocks originated and evolved from the initial basaltic magma, which triggered by the partial melting of the upper mantle, and evolved through different degrees of fractional crystallization.