Strain-Induced Rutilation of Quartz in Mylonites and Timescales of Ductility in an Extensional Shear Zone
William Nachlas, University of Minnesota
Donna Whitney, University of Minnesota
Christian Teyssier, University of Minnesota
Rory McFadden, Salem State University
We document rutile needles that have exsolved from quartz during retrograde shearing and exhumation in an extensional shear zone. The Pioneer Core Complex (PCC) of north-central Idaho, USA, is a domal exposure of gneissic basement rocks in the hinterland of the Sevier orogenic belt. Eocene exhumation of the PCC was accommodated by the brittle-ductile Wildhorse detachment system (WDS), which localized in a zone of ductilely-sheared metasediments and juxtaposes lower crustal migmatitic gneisses with upper crustal Paleozoic sediments. Deformation in the WDS was partially localized within a continuous sequence (~200 m) of quartzite mylonites, wherein quartz grains are densely rutilated with microscopic rutile needles that are strongly oriented into the axis of flow. Quantitative analysis of the Ti content of rutilated quartz using in-situ techniques (EMPA, SIMS) is hampered by the very small size and close spacing of the needles and the trace concentration levels of Ti substitution in quartz. Instead, we apply cathodoluminescence (CL) intensity as a proxy for Ti content to semi-quantitatively map the Ti concentration field surrounding rutile needles. A depletion halo in CL intensity surrounding rutile indicates an exsolution (unmixing) origin, and we use the length and geometry of the CL intensity profile as a snapshot into the kinetics of Ti diffusion during exsolution. Three samples spaced throughout the quartzite section were analyzed and reveal unique diffusion distances and profile geometries recorded at different levels of the shear zone. By constraining temperature estimates from classical thermometers in nearby rocks and from applying Ti-in-quartz thermobarometry to recrystallized quartz in the same samples, we are able to bracket the duration of ductile shearing in the WDS. Applying the experimentally-determined Arrhenius relationship for Ti diffusion in quartz, results of our diffusion analysis show that if the shear zone was deforming at T=500°C, then the predicted timescales for exsolution range from 0.63 – 3.1 Myr; if the shear zone was deforming at T=600°C, then the predicted timescales for exsolution range from 0.05 – 0.23 Myr. If we posit that crystal plastic deformation of quartz is an effective mechanism to facilitate Ti exchange in response to changing solubility conditions, then our approach introduces a new technique to constrain the duration of ductile deformation in shear zones.