北京云蒙山地区花岗岩穹隆及伸展构造的探讨

GRANITE DOME AND EXTENSIONAL TECTONICS IN THE YUNMENG MOUNTAINS, BEUING

北京云蒙山花岗岩为一中生代侵入的花岗岩穹隆,花岗岩穹隆的叶理普遍发育,叶理轨迹基本平行于穹隆的外部边界,并显示出从核部到边部逐渐增强,东南侧明显强于西北侧的特点。变形构造研究显示,花岗岩穹隆的边部及围岩中普遍存在不同层次及不同运动方向的伸展构造。东南侧以具河防口-水峪伸展型韧性剪切带为特征,剪切运动标志显示为从NW-SE的正剪切运动,有限应变分析估算其剪切位移量在10km以上,剪切带上部被河防口正断层所切,微构造特征及石英组构研究,发现在垂直于剪切带走向方向,其变形温度从下到上逐渐降低,可能与花岗岩的冷却历史有关。而花岗岩穹隆的西北侧则主要表现为较浅层次的正断层及平移正断层。

Granite in the Yunmeng mountains of Beijing is intrusive as a Mesozoic granite dome. Within the granite dome foliation is frequently observed and its traces are parallel with the boundary between granite and host rocks. The foliation development progressively strengthens from the core to edges, and the foliation on the SE side is stronger than that on the NW side. Deformation structures indicate that extensional tectonics at different crustal levels and in different directions are prevalent on the edges of the granite dome and its host rocks. The Hefangkou-Shuiyu 'extension type' ductile shear zone characterizes its SE side. The indication of the shear shows a NW-SE normal shear movement. From a finite strain analysis, it is estimated that the shear displacement is over 10 km. The upper part of the shear zone is cut by the Hefangkou normal fault. Microstructures and quartz fabrics indicate that the deformation temperatures decrease from below upward in a direction perpendicular to the strike of the shear zone, which might be related to the cooling history of granite. On the other hand, the NW side of the granite dome is mainly manifested by a shallower-level normal fault and normal-strike slip fault.