PALEOMAGNETISM OF PSEUDOTACHYLYTE FROM THE MESOPROTEROZOIC HOMESTAKE SHEAR ZONE, SAWATCH RANGE, COLORADO

Paleomagnetic data from pseudotachylyte from the Homestake Shear Zone (HSZ) of Central Colorado shows a well-preserved characteristic remnant magnetization (ChRM) that is interpreted as primary. Two distinct groupings of magnetization direction: A well defined northeast declination and moderate negative inclination, and a moderately well defined zone with NW declination and steep negative inclination. Preliminary data from the host migmatitic gneiss exhibits a well-defined north declination and steep negative inclination, which is similar to the steep NW pseudotachylyte mean. This similarity as well as the lack of definition in the two pseudotachylyte ChRM directions is likely due to large amounts of wall rock in the sample cores. Paleomagnetic data from the pseudotachylytes and host rocks were obtained to help resolve an apparent conflict between geochronologic and field evidence for the age of the shear zone. The HSZ is a series of brittle-plastic fault and shear zones that are interpreted to have formed near the mid-crustal brittle-ductile transition. Ductile deformation occurred ~1400 Ma, and field and microstructural relationships suggest the pseudotachylyte formed concurrently with the ductile shear zone implying a Mesoproterozoic age. However, ⁴⁰Ar/³⁹Ar whole rock age determination of a pseudotachylyte vein yielded a date of ~137 Ma. By refining the NE pole we will be able to create an accurate virtual geomagnetic pole (VGP) that will be used to constrain the age of the pseudotachylyte-forming events, as well as provide information on the timing of magnetization/remagnetization of the host rock. We hope that these results will contribute to resolving the apparent discrepancy between field and geochronologic evidence for the age of the pseudotachylyte-forming event.