Mineral chemistry, geochemistry and U-Pb SHRIMP zircon data of the Yangxin monzonitic intrusive in the foreland of the Dabie orogen

摘要

The Yangxin composite intrusive consists of three generations of plutons, among which 90% are the second generation intrusions. The second generation intrusion has U-Pb SHRIMP zircon age of 134±2 Ma, substantially younger than the early stage of the Yanshanian. Amphibole and biotite, major mafic minerals in the Yangxin plutons, are rich in magnesium, suggesting a deep source of their parental magma, possibly derived from the upper mantle. Whole rock geochemical data on these rocks show that they have high Al_2O_3 (15.92%—16.38%), relatively high alkali (Na_2O+K_2O = 6.95%-7.37%) and higher Na_2O contents than K_2O (Na_2O/K_2O = 1.47-1.94). They also have high Sr contents (816-897 μg/g) are enriched in strong incompatible elements such as Rb, U, Th, K and LREE, but depleted in HREE and Y. Strong fractionation of LREE from HREE as indicated by high (La/Yb)_N ratios ranging from 17.83 to 20.86. Although with little difference, the whole characteristics of the Yangxin intrusive body are greatly similar to the adakite, referring that the formation of the parental magma may relate with the partial melting of the basalt underplating from the mantle after delamina-tion of the mountain root of the Dabie orogen. The body has been uplifted about 5 km since cooling concretion, the average rate was only about 0.04 mm/a, much slower than that of the Dabie orogen in late Mesozoic, but the rates of denudation in two area are similar in Cenozoic.