北京西山大灰厂东狼沟组钾质火山岩形成的构造背景探讨

Early Cretaceous Donglanggou Potassic Volcanics from Dahuichang, Western Hills, Beijing: Tectonic Setting and Geological Significance

北京西山大灰厂早白垩世东狼沟组火山岩为钾质粗面玄武岩、钾玄岩和安粗岩，总体属于严格意义的钾玄岩系列（橄榄安粗岩系列）。该套火山岩富集钾等大离子亲石元素（LILE）和轻稀土元素（LREE），是由于底侵幔源母岩浆在高压（1．2～1．5GPa，相当于42～50km深度）的岩浆房内经历结晶分离作用所致，母岩浆相对富钾也是原因之一。该火山岩形成于造山带开始发生崩塌的后碰撞阶段起始时期，在大地构造位置上处于中生代东亚大陆活动陆缘宽广的内带。深俯冲到250km或更深深度的大洋板片的脱水作用是诱发该火山岩地幔源区熔融的主要因素。东狼沟组钾玄岩的发育表明当时燕山存在约50km的厚地壳，古地形高度可达海拔3．9km。燕山中生代高原（高山）地貌和相应的气候条件可能为原始鸟类的形成演化提供了非常适宜的环境。

The Early Cretaceous Donglanggou volcanic rocks from Dahuichang area, western Hills of Beijing, were composed of potassie traehybasalt, shoshonite and latite, and belong to shoshonite series in petrochemical characteristics. They enrich K, LILE and LREE. The high potassium of rocks were mainly due to the highly K-enrichment process associated with crystal fraction of underplating magma under high pressure （1.2 - 1.5 GPa or 42 - 50 km）, and in less extent due to the relative high K content of parental magma. The voleanies formed in the initial period of post-collision stage, and represented the beginning of orogenic collapse in Yanshan. The tectonic setting of the rocks was inferred as the inboard portion of active margin resulted by Paleo-Pacific plate subduction beneath Eastern Asia. The dehydration of deep-subducted （ to depth of 250 km or more） slab triggered the partial melting of lithospheric mantle, and resulted in the intensive Jura-Cretaceous magmatism in Yanshan （ include Western Hills of Beijing）. The occurrence of Donglangou shosho- nite implies the existence of 50 km thick crust in Yanshan during Early Cretaceous. According to the thermal equilibrium and buoyancy principle, a 3.9 km a. s. 1. paleo-topography was estimated. The highland topography and related climate might be very suitable for the origin and evo- lution of primordial Ayes.