Petrogenesis Of High-Ti Andesites At The Bottom Of Dabeigou Foundation In Chengde Basin And Its Dynamic Implications To North China

Based on Wang's previous studies, this paper focuses on the studies of high Ti content andesites of Dabeigou Group from the Chengde Basin in North China which is first found. To acquire more geochemical data, we collected samples from the Chengde Basin again (08CD21, 08CD22, 08CD23, XYY), and analyzed age of zircons by LA-MC-ICP MS (08CD21, 08CD22, 08CD23), concentrations of major elements and trace elements (CD-13,DBG-30,DBG-31,DBG-32,08CD21,08CD22,08CD23,XYY (only for reference)), Sr-Nd-Pb isotope (CD-13,DBG-30,DBG-31,DBG-32) as well.Based on CL (cathodoluminescence) features and Th/U ratios, the very few zircon ages obtained from high Ti content andesites (08CD22: sp3) is suggested to inherited from wall rocks. The youngest age of 120±1Ma constrain the maximum age of high Ti andesites. The weighted mean age of another andesite sample (08CD21) is 113.8±1.2 Ma (n=26), which is similar to the age of 113.6±0.87Ma obtained by Wang. It indicates that rocks from Dabeigou Group are all formed during the same period.According to the correlations of different major element concentrations, trace element distribution models and High Field Strength Elements (HFSE), especially the distinctive Ti and P abundances, it is inferred that the basalts and the top andesites share a common source, but is different from the high Ti andesites. The latter one is evolved from the former one. The high Ti andesites may have originated from the upwelling asthenospheric mantle which reacted with lower crust, while the basalts may be caused by further melting of crust with some contamination from highly evolved asthenospheric mantle.Though the thinning mechanism of North China Craton is still controversial, the interaction between lower crust and asthenospheric mantle may have played an important role.