Microstructural Characteristics and Deformation Mechanism of the Shear Zone of Fangshan Pluton, North China

Abstract:

Using the research of microtectonics, this paper studied in detail the ductile shear zone of southeastern Fangshan pluton, which is the key to understand its structural style and origin. We collected 38 directional samples in Liugou and Longmenkou. The microtectonic features in this zone mainly consist of sub solidus plastic deformation and high temperature ductile deformation, the reason why they co-exist can be explained by multiple pulse of magma and the earlier cooling into sub solidus of magmas.

In the sampling route parallel to the diameter of Fangshan Rock, the microtectonic feature mainly consists of high-temperature ductile deformation, such as myrmekite and GBM. In particular, the high-temperature ductile deformation becomes more obvious when closer to the center, indicating the condition of 500-700℃. In the sampling route which meets diameter at a right angle, the microteconic feature mainly consists of lower-temperature ductile deformation, including more BLG, indicating the condition of 300-500℃. We assume the superposition of microtectonic features in different period and different temperature condition is caused by multiple pulses of magmas.

Fangshan intrusive body mainly consist of Yanshan granite, and its quartz is an appropriate finite strain gauge. Study of finite strain shows the X/Z ratio of 1.74~2.59 and X/Y ratio of 1.59 ~ 2.52. The S (indication of length ration of shear zone’s thickness before and after deformation) of 96% samples are less than 1, revealing that the shear zone has being thinned and quartz has experienced shearing dominated by pure shearing. 80% samples’ Kinematic vorticity is less than 0.71, which also shows that pure shear is dominate. Samples whose Kinematic vorticity is more than 0.71 is close to the center of the pluton, indicating possible spiral emplacement of pluton.

EBSD analysis of quartz C-axis shows a complex preferred orientation, which indicates that the sample have experienced at least two periods of deformation. The more evident pattern is prism <a> slip,which also indicates the degormation condition of 500-600℃. Besides the projection points close to Z-axis indicate another lower temperature deformation, which supports the results of study in microstuctural features.