REVISITING THE QUATERNARY RHYOLITES OF THE MINERAL MOUNTAINS: NEW 40AR/39AR GEOCHRONOLOGY, PALEOMAGNETIC, AND GEOCHEMICAL DATA

The Mineral Mountains in central Utah consist of Miocene granites through which Pleistocene (0.8 to 0.5 Ma) high-silica rhyolites have erupted. These rhyolites commonly take the form of effusive flows and domes, and explosive products are relatively rare. The western flows are crystal poor, whereas the rhyolites at higher elevations have greater crystal abundance. Previous studies showed that eruption of these rhyolitic lavas bracket the last major global geomagnetic polarity reversal, and that the rhyolites become more chemically evolved with increasing elevation. Previous K/Ar dating of these rhyolites limited the temporal resolution of chemical variability. To build upon previous studies, we employed modern state-of-the-art techniques to obtain new geochemical, geochronological, and paleomagnetic analyses. Here we present new whole rock (XRF/ICPMS) and feldspar (EMP) geochemistry, along with paleomagnetic directions and ⁴⁰Ar/³⁹Ar ages for several Pleistocene rhyolites. Sanidine and biotite phenocrysts, and in some cases glassy groundmass, were separated from the rhyolites for ⁴⁰Ar/³⁹Ar single crystal incremental heating or fusion analyses to improve upon and evaluate the accuracy of the published K/Ar ages. Several flows were cored and remanent paleomagnetic directions were measured using a >15-step alternating field demagnetization procedure. Our preliminary results indicate that three major groups can be identified within the mountain range by chemical similarities. The ages of the flows have a resolvable difference, allowing us to develop a more comprehensive eruptive stratigraphy, model petrogenetic processes that relate chemical groups, and provide additional age constraints for the geomagnetic polarity time scale.