SPATIAL VARIATION OF GEOCHEMISTRY ALONG THE MARGINS OF THE COLORADO PLATEAU DURING THE LAST 5 MA

We are investigating if there are changes in petrology/geochemistry across the Colorado plateau. In order to achieve this objective, we are using a data set collected from the NavDat database (www.navdat.org). The data set covers volcanism in Arizona, Colorado, Nevada, and New Mexico over the last 5 million years. Basalt comprises the majority of volcanism across the region. However, bimodal volcanism is indicated by the presence of both rhyolites and basalts, and is most pronounced in that last 2.5 million years. Spatially, basalts dominate the west margin of the Colorado plateau and more silica rich compositions are restricted to the southeastern margin of the Colorado Plateau within the Jemez lineament. On chondrite normalized REE spider diagrams, the majority of samples have shallow slopes, consistent with garnet free source regions. Samples with steeper slopes indicative of the presence of garnet in the magma source region (La/Yb ratios greater than ~30) are restricted to the Jemez lineament and the basin and range of Nevada, and are generally absent from the western margin of the Colorado Plateau. These spatial differences in geochemistry in the last 5 Ma indicate that Jemez lineament magmas have distinct sources when compared to other regions of the basin and range and Colorado plateau. We are continuing to analyze the spatial variations in magma composition and are beginning to investigate if these changes in chemistry correlate with changes in deformation or eruptive style.