Geochemical characteristics and LA-ICP-MS zircon U-Pb age of the trachytic volcanic rocks in Zhushan area of Southern Qinling Mountains and their significance

Basic rock-trachyte belts, trending in NW-SE direction, are widely distributed in Zhushan area of Southern Qinling Mountains, intruding in Silurian strata. Previous researchers conducted detailed studies focused on basic rocks. Compared with the ba-sic rocks, the research on trachytic volcanic rocks seems very insufficient, and hence accurate data forming chronologic data are espe-cially lacking. The dating of trachytic volcanic rocks from Zhushan area in Southern Qinling Mountains shows that the trachytic vol-canic rocks are alkaline series rocks with relatively high Na₂O+K₂O. Lithophile elements(Ba, Th, U)and high field strength elements (Nb, Ta)are relatively abundant, but the elements such as Sr, Ti and Yb are relatively poor. Rare earth elements(ΣREE, which are rich in light REE and deficient in heavy REE)are relatively high. Combined with the trace element characteristics, it is shown that the trachytic volcanic rocks were formed in a continental rifting environment. The crystallization of trachytic volcanic rocks took place at 430.6±2.7Ma, as shown by the LA-ICP-MS zircon U-Pb dating of trachytic pyroclastic rocks in Zhushan area. The studies show that a strong cleavage activity occurred in the period of early Silurian and formed a deep water basin with rich siliceous and car-bonaceous rock combination in the Zhushan area of Southern Qinling Mountains. Combined with the discovery of ocean island ba-salts as well as island arc and oceanic basalts, the authors hold that this region might have developed a small finite ocean basin after Si-lurian, and the ocean basin was probably the eastward extension of the Mianlue Ocean in Southern Qinling Mountains. The study of the geochemical characteristics and zircon U-Pb age of the trachytic volcanic rocks provides an important basis for early Paleozoic tectonic evolution in Zhushan area of Southern Qinling Mountains.