MICROSTRUCTURAL AND KINEMATIC ANALYSIS OF L-S TECTONITES FROM THE ST. CYR AREA, YUKON-TANANA TERRANE, CANADIAN CORDILLERA: A CLASS PROJECT

A suite of quartz-rich mylonites from the St. Cyr area, Yukon-Tanana terrane (YTT), Canadian Cordillera, was studied by a graduate class in Microstructures at the University of Iowa. Nine students each worked with an oriented sample. The St. Cyr area experienced polyphase metamorphism and deformation from the late Devonian to the Cretaceous, including a period of high-pressure (HP) metamorphism in the late Permian. Unraveling deformation patterns within the St. Cyr area is key to understanding the emplacement of the HP rocks within the YTT. The St. Cyr area was previously mapped as a tectonic mélange, yet field relationships suggest a more coherent geometry, e.g. foliations of the host rock and mafic bodies are subparallel to contacts between these lithologies. Samples are composed of ≥ 70% quartz, ± white mica, biotite, garnet and sillimanite. Dynamic recrystallization of quartz was accommodated grain boundary migration recrystallization, placing deformation at ≈500 °C. Foliations strike NW and dip 30° to 78° to the SW or 48° to 76° to the NE. The majority of lineations trend NW. Three quartzites were analyzed with electron backscattered diffraction (EBSD) analysis to obtain quartz crystallographic preferred orientations. Two samples display asymmetric c-axis patterns indicative of non-coaxial deformation, while the third displays a type II cross-girdle with a component of constrictional flow. Kinematic indicators vary, three display top-up geometries and three display top-down geometries. Top-up geometries show N-verging, NW-verging and SE-verging shear sense, while top-down geometries show NW-verging, W-verging and SE-verging shear sense. All samples have a moderate to significant strike-slip component. The complex kinematics displayed by L-S tectonites of the in the St. Cyr area support the hypothesis that the region of HP rocks represents a tectonic mélange. However, since the deformation occurred in the amphibolite facies after an eclogite facies event, the mixed kinematics could also indicate complex exhumation paths in a subduction channel, thrusting onto the continental margin, followed by normal faulting during the collapse of the orogen.