ENIGMATIC AGES OF TITANITE IN DENALI FAULT ZONE MYLONITES - DO THEY RECORD THE INCEPTION OF NORTHERN CORDILLERAN ACCRETION?

Little is known about the absolute age of Denali fault zone inception and its relation to earliest accretionary tectonics in the Northern Cordillera orogen (NCO). Rare bedrock exposures of the NW-SE-striking eastern Denali fault zone (EDFZ) in the Kluane Ranges of SW Yukon, Canada show the juxtaposition of Wrangellian, Permian to Mississippian Hasen Creek metapelites and late Triassic Bear Creek overlap assemblage metavolcanics.

Strain localization in the metapelites SW of the juxtaposition is manifest as ultramylonite (UM) overprinted by a 30- 60-meter-wide principal slip zone (PSZ) composed of clay-rich carbonaceous (CM) cataclasite. The UM is strongly foliated and tightly folded with fold axes parallel to fault strike, consistent with a component of subhorizontal NE-directed shortening. The UM contains Ab, Qz, Ms, Chl, Ep, Cal, Ttn, Ap, and CM. Ttn grains are anhedral and blocky, but a few are rounded with disaggregated stringers aligned with foliation consistent with a dextral slip component. Chl, Ep, and Cal form a retrograde assemblage and the Ttn is internally altered showing complex textural diversity. EBSD on representative UM samples indicates intracrystalline plastic deformation in Pl, Qz, and Ttn also consistent with fault-parallel dextral slip. Raman spectroscopy of CM indicates the UM reached a maximum temperature of ~400°C consistent with EBSD results. LA-ICP-MS U-Pb analyses of ~150 Ttn grains suggest ~148 Ma mineral growth. Ttn from this sample is characterized by low U concentration (0.2-118 ppm, with an average of 7 ppm) and relatively high, but variable, common Pb. Future elemental analyses of Ttn will help elucidate a metamorphic or igneous origin.

This Late Jurassic age is significant as it is younger than the Permian Hasen Creek metapelites, but older than the earliest fault movement indicated by cooling dates adjacent to the EDFZ (~95 Ma). Our results are the only absolute ages from PSZ UM along the EDFZ thus far. Although there is >400 km of dextral separation documented along the EDFZ at this site, the enigmatic ages and map relations suggest that the UM may be part of a “xenolithic”, but not far-travelled fault sliver. If true, and the Ttn formed in a dynamothermal setting, these rocks may record the dawn of the highly localized Denali fault zone associated with earliest terrane accretion in the NCO.