北京南口中生代双峰式岩墙群的组成及其特征

在北京南口地区发现了一套NNW走向、颇具规模的、由基性钾玄岩和酸性花岗质岩石组成的双峰式岩墙群，其K－Ar同位素年龄为119-114Ma。本文重点讨论了岩墙群的野外地质特征及其岩石学特征。研究表明酸性岩墙和基性岩墙的稀土配分模式非常接近，并且无明显的铕异常，推测它们的岩浆来自两个有成因联系的岩浆源，而且两套岩浆之间不存在的结晶分异作用的关系，我们称之为双峰式岩墙群。研究还表明，两种岩浆最初占位时共呈液相，彼此接触部位成分发生一定程度的混染作用，但总体是互相独立的，岩墙产状和地球化学特征反映岩墙群是在伸展构造背景下侵位的,具有深源浅侵位的特征，反映了晚中生代研究区存在的伸展构造这对于全面认识华北“燕山运动”是有价值的。

Composition and Features of the Mesozoic Bimodal Dike Swarm in Nankou Area, Beijing

A significant NNW striking bimodal dike swarm was discovered in Nankou area of Beijing, which is composed of basic shoshonite and acidic granitic rocks with K-Ar isotopic ages of 119-114Ma. The geological and petrological features of the dike swarm are investigated in this work. The acidic and basic dikes in the swarm have very similar REE patterns. And both of them do not have obvious Eu abnormality, indicating that no correlation of crystal fractionation exists between the magmas forming the acidic and basic dikes. Thus it is suggested that they came from different magma sources with genetic relationship and such a dike swarm may be referred to as bimodal. Certain degree of hybridism on the borders shows that both kinds of magma intruded almost at the same time when they were both in liquid state but still independent from each other in the whole. The attitude and geochemical features of the deep-source shallow-intrusion dike swarm reflect a Late-Mesozoic extensional tectonic background for the magma intrusion in the concerned area, which sheds some light on completely understanding the Yanshanian Movement in North China.