FIELD, GEOCHEMICAL, AND PETROGRAPHIC CHARACTER OF MYLONITIZED PSEUDOTACHYLYTE IN THE GRIZZLY CREEK SHEAR ZONE, GLENWOOD CANYON, COLORADO

The Grizzly Creek shear zone (GCSZ) is a brittle-plastic thrust located near Glenwood Springs, Colorado. This Proterozoic shear zone records top-to-south displacement along mylonites and hundreds of north-dipping pseudotachylyte (pst) veins hosted by a fine-grained granodiorite and gneiss. Pst veins consist of an opaque, fine-grained matrix with relict lithic clasts composed mostly of quartz, plagioclase, k-spar, and accessory zircon and allanite. Brittle ruptures that formed pst have been locally overprinted by viscous flow, generating mylonitized pseudotachylyte (mpt) during post-pst aseismic creep. Mpt displays a new foliation and consists of dynamically recrystalized quartz ribbons with brittley fractured alpha-shaped porphyroclasts of plagioclase feldspar. Both micro- and meso-structures within the GCSZ confirm top-to-south displacement during both pst and mpt forming events. Field relations observed within the fine-grained granodiorite of the upper shear zone show that mpt overprint on pst is localized, and that some of these overprinted mpt veins have been cut by latter pst, indicating coeval seismic-aseismic deformation. This mutual cross-cutting of mpt and pst is not widely reported in shear zone literature.

Whole-rock chemistry from XRF spectroscopy shows little variance of major-element oxide trends within the bulk mpt/pst and host rocks, which indicates melts were locally derived. Element maps, covering areas up to 100 cm²using a micro-XRF analytical microscope with 10 and 100 µm beams, show that the matrix of both mpt and pst veins are depleted in silica in comparison with the wall rock; a result of quartz forming the majority of relict lithic clasts. Element maps show distinct chemical homogenization of Al, Fe, Ca and K within pst and mpt, and allows us to recognize multiple rupture events within single samples. This homogenization may be the cause of localized mylonitization on pre-exisiting pst veins. Elongated Si bands allow variably mylonitized domains to be distinguished in mpt. Further studies hope to provide insight into the coeval production of pst and mpt and how these multiple-generation events fit into models of shear zone evolution within the middle crust.