FIELD RELATIONS AND TITANIUM-IN-QUARTZ THERMOMETRY OF MYLONITIC PSEUDOTACHYLYTES FROM THE SLIDE LAKE AND HOMESTAKE SHEAR ZONES, SAWATCH RANGE, COLORADO

The Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) both contain a similar suite of tectonites including mutually overprinting mylonite and mylonitic pseudotachylyte (mpt). However, the age relations and pressure–temperature conditions between the two shear zones is unclear. Field mapping and titanium-in-quartz thermometry (TitaniQ) were utilized in order to further characterize these relations. Both shear zones show mid-crustal fabrics overprinting supracrustal, amphibolite-facies migmatitic gneisses. At the HSZ, we mapped a 25-m-wide, NE-striking fault zone consisting of at least 8 concentrated bands of subvertical mpt exposed along strike for 100 m. The zone is 1 km SE of, and subparallel to, a distinct 25-m-wide mylonite-ultramylonite zone. Overall, the mpt zone closely resembles the geometry of a multi-km system of pristine pseudotachylyte zones 2–4 km SE of the mylonite. At the SLSZ, cryptic mpt’s are both subvertical and subhorizontal and crop out in and near m-scale mylonite zones. Unlike the HSZ, individual fault veins are difficult to trace for more than a few decimeters, are locally folded, and are both concordant and discordant to the local foliation. Most mpt is dispersed across a 300-m-wide, E-NE-striking belt where foliation is folded and partially transposed into a shallowly-dipping domain.

TitaniQ analysis was completed using 6 WDS on an ARL-SEMQ electron microprobe with a detection limit of 8 ppm. We analyzed survivor quartz clasts within the metamorphosed, biotite-rich matrix of mylonitic pseudotachylytes from both shear zones, and from unmetamorphosed pristine pseudotachylytes from the HSZ. The data sets from all three sample suites are indistinguishable with most point analyses showing Ti concentrations from BD to 30 ppm. Using an average Ti concentration of 15 ppm with the 5 kbar TitaniQ calibration curve yields T~400 ˚C. This is consistent with that inferred from quartz deformation fabrics in mylonites (280–500 ˚C for HSZ, and 280–600 ˚C for SLSZ). Since this is below the regional metamorphic peak (T>650 ˚C) this suggests that Ti was reset during frictional melting and that both pristine and mylonitic pseudotachylytes from the HSZ and SLSZ may have formed at similar crustal levels. Ongoing work will compare these data sets to Ti concentrations in the regional metamorphic host.