Microstructures of Deformed Grains and Kinematic Analysis in the Gneiss Rocks of Qazzaz Metamorphic Core Complex, Northwestern Arabian Shield, Saudi Arabia

The granitic mylonites of the Qazzaz metamorphic core complex in the northwest region of the Arabian Shield, Saudi Arabia were subjected to a through fabric, kinematic and microstructural analyses. The deformed grains (gneisses rocks) enclose a blasto-mylonitic texture that is distinguished by enormous retort-shaped porphyroclasts grains, which are deflected by the ductile, fine to medium grains matrix of biotite, muscovite, feldspar, and quartz. Feldspars exhibit both brittle and ductile behavior, due to reduction processes such as grain boundary migration, subgrain rotation, and fracture. The microstructural of the feldspars, quartz, and mica are generally compatible with fabric formation in greenschist to amphibolite-facies conditions in the mylonitic gneisses for the Qazzaz metamorphic core complex. The kinematic vorticity is measured by using the RGN and the Passchier techniques. Furthermore, in mylonitic gneisses, the kinematic vorticity number (Wm) ranges from 0.68 to 0.86 for felsic minerals and between 0.69 and 0.85 for hornblende. For metavolcano-sedimentary and schist rocks, the range is between 0.68 and 0.89 for felsic minerals and between 0.66 and 0.90 for hornblende. The present data for mean kinematic vorticity number (Wm) are the same for different type of rocks. The long axes (X axis) direction shows a WNW trend with minimal dipping. For finite strain data, the short axes (Z axes) direction is represented as vertical with a corresponding horizontal foliation. The findings of the strain investigations and kinematic vorticity are illustrated by simple shear with various deformations.