义县组主期中酸性岩墙与熔岩地球化学特征及其地质意义

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

对义县组主期中酸性岩墙与熔岩的地球化学、成因岩石学对比分析显示:中酸性岩墙Mg#平均36.68%,Na2O/K2O平均为1.07;微量元素标准化配分图表现出富集Rb、Ba、K大离子亲石元素,亏损Sr及高场强元素Nb、Ta、P、Ti;稀土元素标准化配分图上表现出负Eu异常,HREE强烈分异,Y/Yb平均10.10,(Ho/Yb)N平均1.13,LREE配分曲线与熔岩LREE配分曲线重合.中性、中基性熔岩Mg#平均大于55%,除无Sr负异常外,微量元素标准化配分图与岩墙相似;中性、中基性熔岩稀土元素标准化配分曲线相互平行,并且配分高低与SiO2含量呈反相关关系,说明二者是部分熔融的非同源岩浆发生混合作用的产物;中性、中基性熔岩Y/Yb分别平均为11.27、11.98;(Ho/Yb)N分别平均为1.25、1.32,Sr大于400×10-6,Sr/Y均大于40,显示出了典型埃达克岩地球化学特征.地球化学特征表明中酸性岩墙与火山熔岩来自不同源区,前者来源于斜长石稳定的加厚角闪石麻粒岩地壳部分熔融,而后者来源于受幔源岩浆底侵并且混染过的加厚石榴石麻粒岩相下地壳部分熔融,并且岩墙母岩浆、熔岩岩浆与幔源底侵岩浆在形成过程中可能发生过不同比例的混合作用.结合义县组最底部高Mg#幔源玄武质岩浆成因机制,主期中酸性岩墙与熔岩岩浆形成机制揭示了早白垩世期间华北板块壳-幔之间岩浆动力学过程.

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-granulite- facies 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 anti lava may signify a dynamic magmatic interaction process between the lower continental crust and mantle in North China Craton during Early Cretaceous.