USING QUARTZ-HOSTED RUTILE NEEDLES TO ASSESS STRAIN IN THE MYLONITIC WILDHORSE DETACHMENT, PIONEER MOUNTAINS, IDAHO

Quartz-hosted rutile needles have the potential to record strain in mylonitic shear zones. Rutile needles in these settings behave as material lines due to their high aspect ratio and tensile strength relative to ductilely deforming quartz. The small size and pervasive distribution of rutile needles allow for detailed strain assessment continuously through a shear zone where macroscopic strain indicators may not be present. This study assesses the usefulness of rutile needles as strain indicators by analyzing needle alignment and orientation in 2D and 3D and by describing microstructural and geochemical relationships between rutile needles and mylonitic host quartz. Samples are taken from the Kinnikinic Quartzite in the footwall of the Wildhorse Detachment, the brittle-ductile fault that bounds the Pioneer Metamorphic Core Complex (PMCC; Idaho, USA) to the north and west. Top-to-the-NW extension along the detachment resulted in early-Eocene exhumation of the PMCC, and quartzite in the detachment displays a moderately NW-dipping foliation and a consistent NW-trending lineation defined by stretched quartz. Microstructures indicating pervasive mylonitic shearing include stretched quartz, recrystallized grains, and an S-C-C’ fabric. The quartzite is >90% quartz with minor plagioclase and mica. Samples are pervasively rutilated with needles measuring ~1 µm in diameter and up to ~70 µm in length. Titanium depletion in quartz surrounding aligned and boudinaged rutile needles suggests needles exsolved from quartz during deformation (Nachlas et al. 2018). 2D analyses of sections parallel and perpendicular to quartz lineation show rutile needles strongly oriented parallel to the shear direction. 3D analyses of rutile needle orientation and distribution were performed using X-ray computed tomography (XRCT). Measurements of boudinaged needles indicate up to 37% elongation. Where quartz defines an S foliation, rutile needles are inclined by 14-20° with host quartz grains such that the needles parallel C bands. This independent alignment parallel to shear, along with observations of needles crossing quartz grain and subgrain boundaries, suggests needles behave independent of surrounding quartz, and thus can provide an independent record of strain in plastically deformed rocks.