Geochemical and geochronological characterization of Grand Mesa Volcanic Field, western Colorado
— R. Cole, A. Stork, W. Hood, and M. Heizler

The Grand Mesa Volcanic Field (GMVF) in western Colorado covers about 166 km² and ranges in elevation from 3452 (east) to 3000 m (west). Miocene (?) and Eocene strata underlie the basaltic lavas. The field can be subdivided topographically and geochemically into three areas: Western Tableland (WT), Crag Crest-Crag Crest Bulge (CC-CCB), and Ridge and Peak (RAP). Data for this study are based on 29 whole-rock ⁴⁰Ar/³⁹Ar dates, 81 ICP-MS analyses, and 404 ED-XRF analyses of samples collected at 46 locations. Dates range from 10.92±0.24 to 9.63±0.16 Ma, with a possible bimodal distribution. Age values for the dike and flow samples in the RAP area (N=9) range from 10.49±0.06 to 9.99±0.01 Ma, whereas the dike and flow samples in the CC-CCB area (N=4) are between 10.74±0.05 to 10.52±0.05 Ma. The flow samples from the WT area (N=16) range from 10.92±0.24 to 9.63±0.16 Ma. Major-element-oxide values from both ICP-MS and ED-XRF analyses show geographic partitioning. Samples from the CC-CCB area are noticeably enriched in silica, potassium, and phosphorous compared to those from the WT and RAP areas. Using a TAS classification, the CC-CCB samples are mostly shoshonite with minor latite, whereas the WT and RAP samples are mostly basaltic andesite and basalt. Chemo-stratigraphic variations documented in a drill core in the WT area, suggest that both magmatic differentiation and injection of new magma occurred during eruption of the flow sequence. The CC-CCB area has the only well-defined vent complex, consisting of a large discontinuously exposed dike and associated pyroclastics. The chronological and geochemical correlation of this complex to flows in the WT and RAP is not well understood. Additional undiscovered vent areas are likely present.