北京西山房山岩体西北部强变形带的成因新解

北京西山房山岩体西北缘不对称发育一条弧形塑性强变形带,前人根据岩体东南部组构缓倾和岩体内部强应变分布特征提出岩浆斜向底辟侵位模式(王人镜等,1990;张吉顺和李志中,1990)。本文在前人研究的基础上,对房山岩体的组构进行系统观测,研究了西北缘固态塑性强变形带地质特征,着重探讨了岩体侵位时围岩热结构状态。文章认为岩浆同侵位固态塑性强变形带的形成需要早期岩浆进入准固态-固态和后期岩浆膨胀动力这两个必要条件。房山岩体西北部的强变形带是由于岩体侵位时西部围岩的温度较低,岩浆较早进入准固态-固态,后期岩浆侵位时膨胀动力双重因素造成的,提出了围岩热结构状态是房山岩体不对称的固态塑性强变形带的控制因素,而不是岩浆斜向底辟侵位的结果。这一研究成果不仅较合理地解释了房山岩体强变形带不对称发育特征,而且对研究同类花岗质岩体的定位机制有一定启示意义。

SUBSOLIDUS DEFORMATION ZONE IN THE NORTHWESTERN MARGIN OF THE FANGSHAN PLUTON AND ITS NEW INTERPRETATION

An arcute subsolidus plastic deformation zone is developed in the west of the Fangshan pluton,western hills of Beijing,which was called as ductile shear zone and proposed that it was formed by oblique diapiric emplacement of magma by previous workers.Based on the study of fabrics in the Fangshan pluton,this paper presents geological features of this deformation zone and mainly studies thermal conditions of country rocks during emplacement of the pluton.It is suggested that formation of plastic deformation zone in plutons needs both early magma cooling and late magma ballooning,and thermal structure of country rocks is one of requirements to form subsolidus plastic deformation zone.Our detailed data show that temperature of country rock in the west of the Fangshan pluton is lower than other parts during the pluton emplacement,and magmas in the west of the pluton cooled earlier into subsolidus,and then an arcute subsolidus deformation zone in the west of the pluton was formed by late magma ballooning.It is thus proposed that asymmetrical thermal structure of country rocks in the pluton is key to form this arcute subsolidus deformation zone,and the model of oblique diapiric emplacement of magma indicated by previous workers is wrong.This study not only reasonably explains asymmetry of the deformation zone,but also implicates for the emplacement of other granitic plutons.