TANDEM U-PB GEOCHRONOLOGY AND TREE ANALYSIS OF DETRITAL ZIRCON IN THE CAMBRIAN BOLSA QUARTZITE, SOUTHERN ARIZONA, USA

The Bolsa Quartzite, a Middle Cambrian quartz arenite that mantles 1.65–1.7 Ga basement in southeastern Arizona, USA, represents sedimentation linked primarily to transitional marine environments formed during the Laurentian Sauk II transgression. Improved maximum depositional ages (MDA), determined using detrital zircon geochronology from basal Cambrian sandstones found across the southwest USA, can place constraints on the rate of shoreline regression. We present tandem LA-ICPMS and CA-ID-TIMS detrital zircon U-Pb geochronology from the Bolsa Quartzite in French Joe Canyon, Arizona. The geochemical dataset includes trace- and rare-earth-element (TREE) concentrations for each zircon grain. Two samples from the lower Bolsa Quartzite, collected 8 meters and 25 meters suprajacent to the nonconformity with the Pinal Schist, display similar age populations, with modes indicating subequal contributions from the Llano-Grenville, Granite-Rhyolite, and Mazatzal Provinces. Preliminary data also record an abundant population of Cambrian detrital zircons, centered at 501–504 Ma, that provide an MDA for the unit. Use of CA-ID-TIMS will further improve the precision and accuracy of these results and, when paired with TREE geochemical ratios, allow for comparison to potential igneous sources. The provenance of ~500–505 Ma Cambrian detrital zircons is not well constrained for southwestern Laurentia. Magmatism at 510 Ma in the nearby Florida Mountains intrusive complex, known to be a source of detritus for the Bolsa Quartzite, is notably older. Younger plutons in this region may yet be undiscovered. Transport from the ~520 and 430 Ma Notre Dame arc and Lushs Bight oceanic tract in eastern Laurentia would require an unsupported continental-scale fluvial system. Continued work to build a dataset of tandem U-Pb dates with paired TREE for detrital zircons from basal Cambrian sandstones is necessary not only to determine rates of transgression, but to resolve the provenance of Middle Cambrian grains and determine the tectonic processes that led to their introduction into the Laurentian sedimentary system.