GEOCHEMISTRY, GEOCHRONOLOGY, AND PETROLOGY OF A VOLCANIC ASH LAYER ASSOCIATED WITH THE TUFF OF SAGUACHE CREEK, EASTERN SAN JUAN VOLCANIC FIELD, SOUTHERN COLORADO

The eastern San Juan Volcanic Field is comprised of a complex sequence of early Oligocene intermediate composition lavas and breccias, mid-Oligocene silicic ash-flow tuffs and associated calderas, and Miocene-Pliocene basalts and rhyolites. One of the earliest manifestations of siliceous magmatism in the area, informally named the tuff of Saguache Creek (TSC), has been shown to be aerially extensive, yet its source caldera remains unidentified (Turner et al., 2002). Where the TSC is well exposed an overlying poorly consolidated ash layer, up to 45 meters thick, may be preserved. The relationship of this overlying ash unit to the TSC is unclear. Inclusion of this ash layer with the TSC will more than double the volume of erupted material. More accurate estimates of this eruptive volume can better constrain the size of the currently unrecognized source caldera for the tuff of Saguache Creek, and thus, help locate the source.

The location with the largest known vertical exposure of the ash layer, north of Trickle Mountain, was selected for close interval sampling, detailed geochemical analysis, and geochronology. This location is believed to represent the complete ash layer because of the preservation of a highly welded capping tuff, approximately 3.5 meters thick, also of unknown relationship. The ash unit and capping tuff have been described in detail using Ar⁴⁰/Ar³⁹ dating, XRF analysis of pumice inclusions, electron microprobe analysis of minerals, and petrographic descriptions to allow for comparison to the tuff of Saguache Creek. XRF, microprobe, and petrographic analyses have been compared to the detailed study of the tuff of Saguache Creek completed by Simon (2000). Ar⁴⁰/Ar³⁹ analysis was also completed for the TSC itself, and the ages presented for the three units represent original work.