RHYOLITE PORPHYRY DIKES IN THE LITTLE COTTONWOOD STOCK, CENTRAL WASATCH RANGE, UTAH

The Little Cottonwood Stock (LCS) and associated intrusions can provide information on the uplift and exhumation of the Wasatch Range along the Wasatch Fault, Utah. The Wasatch Fault is a 300 km N-S trending normal fault with 10-12 km total displacement. The intrusion depth and cooling history of the LCS have been studied, yielding some information on the exhumation of the Wasatch Front. Rhyolite porphyry dikes (RPD) within the LCS had not been studied and are likely to be several million years younger than the LCS. Through five days of field mapping we documented the existence of these dikes in upper Bells Canyon, compared their orientations to those of other dikes and fractures cross-cutting the LCS, and collected samples for petrographic analysis, whole-rock geochemistry, and ⁴⁰Ar/³⁹Ar thermochronology.

Aplite dikes and coarser (locally pegmatitic) felsic dikes have diffuse contacts with the LCS and are in some cases commingled with it. We interpret the aplite and felsic dikes to be co-magmatic with the LCS. In contrast, sharp contacts of the RPD with the LCS and chilled rhyolite porphyry dike margins indicate these dikes were intruded after the LCS had cooled. Epidote-mineralized fractures also appear to have formed after the LCS had cooled. RPD are near-vertical and have an ENE strike. They are parallel to the aplite dikes and epidote-mineralized fractures. The strike of the RPD is parallel to the trend of the Wasatch Intrusive Belt. If the RPD are several mill. yrs. younger than the LCS, then the same stress field persisted for several mill. yrs. or recurred.

Pyrite crystals up to 4mm across, found only in the RPD, indicates a possible correlation with the White Pine Phase, an intrusion 7 km to the ENE that is 5-6 mill. yrs. younger than the LCS. We are generating XRF analyses of the RPD to compare to the LCS and the White Pine Phase and are have sent samples to UC Santa Barbara for ⁴⁰Ar/³⁹Ar dating. If the dike is significantly younger than the LCS, it could provide additional constraints on the history of movement on the Wasatch Fault.