ZIRCON SHRIMP U-Pb AGE AND GEOCHEMISTRY OF THE TRIASSIC DIXIONGSHAN ROCK MASS IN LIAODONG PENINSULA:Geological Significance

The zircon SHRIMP U-Pb dating result indicates that the Dixiongshan rock mass in Liaodong Peninsula, as a part of the Triassic granites found in the east of North China, was emplaced in the Triassic epoch (205.2±2.1 Ma). The whole rock chemical analysis shows that the Dixiongshan granites are characterized by high SiO 2, Al₂O₃ and K₂O and low TiO₂, Na₂, MnO and CaO, with K₂O+Na₂ ranging from 7.88% to 9.28%, K₂O/Na₂≥1.16-1.46, CaO/Na₂=0.08-0.23, A/CNK=0.95-1.10. Furthermore, muscovite appears in the mineral assemblage, which means the rock mass belongs to metaluminous-peraluminous granite. In the SiO₂-Zr diagram, all the sample points fall in the S-type granite area. All the characteristics above suggest that the Dixiongshan granites be of metaluminous-peraluminous S-type. The REE curves and parameters show intense fractionation between LREE and HREE and distinct negative anomaly of Eu, suggesting that a fractional crystallization happened to plagioclase or other Ca-rich minerals after the formation of magma in this area, which are typical REE characteristics for metaluminous-peraluminous granites. In the primitive mantle-normalized trace element spider diagram, all the granites are enriched in Rb and Th, but distinctly depleted in Nb, Ta, Sr and Ti. For all samples, Rb=133×10^-6-360×10^-6, most of which are higher than the average value of granites(200×10^-6). Sr(25×10^-6-135×10^-6) and Ba (48×10^-6-507×10^-6) are obviously lower than the average value of granites (Sr=300×10^-6, Ba=830×10^-6). The depletion of Ba and Sr reflects the magma experienced a relatively complete fractional crystallization. The enrichment of large ion lithophile elements (LILEs, such as Rb and Th) and the depletion of Nb and Ta indicate that the continental crust is the source rock of magma. The characteristics above show that the magma substance is from the terrigenous clastic rock. Combined with the history of regional geodynamical evolution, it is thought that the Dixiongshan rock mass is a product of the subduction of the Kula-Pacific plate towards Eurasia and response of weak extensional setting of North China lithosphere in the Late Indosinian epoch.