东大别条带状片麻岩的构造变形特征及对大别杂岩流变机制的指示

Structural Deformation of the Banded Gneiss from the Eastern Dabie Orogen and its Implication for Rheological Mechanism of the Dabie Complex

大别造山带是由华北板块和扬子板块经过长期构造演化、最终碰撞拼合的产物。东大别造山带作为大别造山带和郯庐断裂带的过渡部位,对认识大别造山带的构造演化及其与郯庐断裂带的构造关系有重要的意义。本文对东大别造山带南北向霍山‒马垅剖面进行了野外构造观察和室内显微构造分析。东大别造山带发育地壳深部构造层次上的条带状片麻岩,通过角闪石‒斜长石温压计得到其形成条件为T=737±40℃,P=0.94±0.15 GPa,属于高角闪岩相‒低压麻粒岩相变质,形成深度约28km左右,处于中下地壳流变层。对条带状片麻岩及糜棱岩的应力进行恢复,结果显示东大别主要受到NE-SW向和NW-SE向两个方向上的挤压以及顺造山带的拉伸,可能代表受特提斯和太平洋构造体制共同作用的结果。大别造山带受特提斯构造体制影响的应力方向是NE-SW向,此应力可以分解成垂直于造山带挤压和平行于造山带向东牵引的力,并且南北向的空间缩短已经形成,应力已经作用到大别杂岩内部,大别杂岩需要在此应力的作用下由西向东发生塑性流动,进行应力释放,这也是塑性流动得以持续进行的动力。大别杂岩由西向东的塑性流动边界是晓天‒磨子潭剪切带与殷店‒马垅剪切带,开始于晚三叠世210±10 Ma,至少持续到早白垩世130±5 Ma。

The Dabie orogenic belt is the product of the long-term tectonic evolution and collision between the North China and the Yangtze blocks.As the transitional zone between the Dabie orogenic belt and the Tanlu fault zone,the eastern Dabie orogen has important scientific significance for understanding the tectonic evolution of the Dabie orogenic belt and its structural relationship with the Tanlu fault zone.Field structural observation and microstructural analysis were conducted on the Huoshan-Malong section of the eastern Dabie orogen.Amphibole-plagioclase thermobarometer calculation indicates that the banded gneiss was formed under temperature and pressure conditions of T=737±40℃,P=0.94±0.15 GPa,which belong to high amphibolite to low pressure granulite facies metamorphism,suggesting that banded gneiss was developed at the deep tectonic level of the crust in the eastern Dabie orogen.The formation depth of the banded gneiss is estimated at about 28 km,which is located in the rheological layer of the middle and lower crust.Based on the restored stress field of the banded gneiss and the mylonite,it can be inferred that the eastern Dabie orogen experienced mainly NE-SW and NW-SE compression,which may be the results of tectonic juxtaposition of the Tethyan tectonic system and Pacific tectonic system.The stress direction of the Dabie orogenic belt affected by the Tethyan tectonic system is NE-SW direction,which can be decomposed into compressive force perpendicular to the orogenic belt and eastward pulling force parallel to the orogenic belt.The North-South spatial shortening is significant and even the interior of the Dabie complex was affected.To release the stress of N-S compression,the Dabie complex underwent a West to East plastic flow,which resulted in a continuous plastic flow that began at 210±10 Ma in the Late Triassic and lasted to ca.130±5 Ma in the Early Cretaceous,whereas the boundary of the plastic flow is likely the Xiaotian-Mozitan shear zone and the Yindian-Malong shear zone.