GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL IMPLICATION OF THE ACIDIC-INTERMEDIATE DYKE AND LAVA OF THE MAJOR CYCLE OF YIXIAN FORMATION

The geochemistry and petrogenesis of the acidic-intermediate dyke and lava of the second cycle of Yixian formation are studied. The result shows that, the dykes have an average Mg^# value of 36.68, with average Na₂O/K₂O=1.07. In trace element spider diagram, the dykes are characterized by enriched Rb, Ba and K and depleted Nb, Ta, P, Ti and Sr. In addition, the rocks present a pronounced negative Eu anomaly, overlapped LREE pattern with that of lava and a strong fractionation in HREE in chondrite-normalized REE pattern, with an average Y/Yb ratio of 10.1 and (Ho/Yb)_N=1.13. For the intermediate and intermediate-basic lava, the average Mg^# is more than 55. Except for absence of significant Sr and Eu anomalies, the rocks have a close affinity to the dykes with respect to trace element spider diagram. Especially for the following characteristics:parallel LREE patterns and apparently negative correlation with their SiO₂ contents in the chondrite-normalized REE patterns, which can be explained by the mixing of two heterogeneous end-members. Moreover, the intermediate and intermediate-basic lava show a typical adakitic feature, with average Y/Yb ratios of 11.27 and 11.98, and (Ho/Yb)_N of 1.25 and 1.32, respectively, as well as Sr > 400×10^-6, Sr/Y > 40. On a whole, the geochemical features of the dykes and lava suggest that the former are most likely generated from the melting of thickened amphibole-granulitefacies continental crust; whereas the latter, derived from the melting of thickened garnet-granulite-facies lower continental crust and contaminated subsequently by the mantle-derived basic magmas. Combined with the genesis for the high Mg^# basalt from the bottom of Yixian formation, the magmatic mechanism for the dykes and lava may signify a dynamic magmatic interaction process between the lower continental crust and mantle in North China Craton during Early Cretaceous.