Study of Magnetic Fabrics and Deformation across Meta-granite along Heping River, NE Taiwan

Abstract:

Heping area of Hualien in the NE Taiwan is located at the region of subduction flip of oblique convergence between the Philippine Sea and Eurasian Plates. The ductile deformation in the region is consisted of the development of N65E-striking foliation and N60W-trending stretching lineation with top-to-southeastern shear. Distinguished mylonitic gneissosity is observed near the lithology contact between the marble and meta-granite but the flow occurrence of granite is still found in the downstream area of meta-granite core. To investigate the deformation pattern and the development of mylonization of meta-granite, study of magnetic fabrics across the meta-granite body is conducted via anisotropy of magnetic susceptibility (AMS) to evaluate the strain path of gneissosity development.

AMS results show that the attitude of magnetic foliation and lineation is consistent with that of genissosity and stretching lineation. From the core to the lithology contact of meta-granite, generally anisotropy is increasing and susceptibility ellipsoids change from prolate to oblate. However due to different shearing on gneissosity, the anisotropy and magnetic ellipsoid vary pretty much even in the same site. Based on current analyses among meta-granitic and mylonitic samples, it suggested that strain path of mylonitization is evolved from prolate shape with low-anisotropy in the meta-granitic core through oblate shape with low-anisotropy in weakly gneissic samples to various ellipsoids from oblate to prolate with high-anisotropy in mylonitic samples. Our findings provide insights into understanding the deformation pattern across the meta-granite body and further establishing the strain path of mylonitic gneissosity development. Further studies of identifying magnetic carrier(s) and domain size to evaluate influences of magnetic minerals to the AMS pattern are needed.