METAMORPHOSED AND MYLONITIC PSEUDOTACHYLYTES FROM THE HOMESTAKE AND SLIDE LAKE SHEAR ZONES, COLORADO, USA

Mylonitic pseudotachylyte, the disguised and atypical ductile overprint of brittle fault rocks, remains the least described type of pseudotachylyte. This presentation will compare the petrology and microstructure of mylonitic and metamorphosed pseudotachylytes from a system of subvertical and low-angle, mid-crustal shear zones in order to discuss models of their probable origin and preservation. We also report the discovery of pseudotachylyte as a component of the Slide Lake shear zone (SLSZ).

The Homestake shear zone (HSZ), Colorado, is host to a ~1.4 Ga, 25-km-long system of exhumed pseudotachylyte veins, mylonite, and ultramylonite. These tectonites are distributed across a zone up to 4–5-km-wide and strike to the E–NE following a composite subvertical to steeply NW-dipping foliation in the host rock – a biotite-rich, semi-pelitic gneiss. The HSZ hosts a large system of relatively pristine pseudotachylyte – dark-gray to black aphanitic veins with visible lithic-survivor clasts and a cryptocrystalline, isotropic matrix likely devitrified from an original melt. A 25-m-wide mylonite and ultramylonite zone is localized on the NW boundary of the shear zone. In contrast, the Slide Lake shear zone, 4 km S of the HSZ, consists of 1 to 10-m-thick, subhorizontal to low-angle mylonites.

In both shear zones, cryptic exposures of recrystallized, locally mylonitic pseudotachylytes are found within and near the mylonites. In outcrop, they are light-brown to rust-red in color, with stretched lithic clasts and a relatively coarse matrix of mm-scale biotite. Injection veins are locally transposed subparallel to foliation in the host gneiss. In thin section, the overprinted pseudotachylyte shows a new foliation subparallel to the vein walls consisting of biotite, muscovite, plagioclase, and stretched quartz grains revealing multiple regimes of recrystallization. These crystals are interpreted to have nucleated and grown from the original matrix as a result of dynamic metamorphism associated with the mylonitic rocks. The pseudotachylyte has thus been metamorphosed to form thin bands of biotite schist. The presence of metamorphosed pseudotachylyte in both shear zones indicates ductile deformation was coeval with brittle faulting and provides additional evidence for a temporal linkage between the SLSZ and HSZ.