PETROLOGIC AND GEOCHEMICAL INVESTIGATION OF THE FLAT TOPS VOLCANIC FIELD NEAR YAMPA, COLORADO

The Miocene Flat Tops volcanic field, located in north-central Colorado, is comprised of basaltic lava flows that represent the northern most extent of the Rio Grande Rift system. In order to understand the origin and evolution of the Flat Top volcanics we described a 300 meter section at the Devils Causeway east of Yampa, CO. We collected 33 representative samples for petrographic analyses and later selected a subset of 18 samples for geochemical analyses. The section contains flows that range from 4 to 11 meters in thickness and many show similar flow morphologies. At the top of flows the rocks are highly vesicular, mostly red in color, and in some cases have well-defined pahoehoe flow textures. The center of some flows display poorly to moderately developed columnar joints. The petrographic analyses revealed that all samples contained olivine phenocrysts, with an approximate average mode of 8%. Plagioclase phenocrysts are less prevalent, occurring in 55% of the samples with a mode of <2%. Clinopyroxene phenocrysts are less abundant than plagioclase, occurring in 21% of the samples with a mode of <1%. The geochemical analyses indicate that the rocks are medium to high potassium basalts. Profiles of compatible and incompatible trace elements imply that there are three groups of geochemically distinct flows. Covariation of Sr versus Zr show an enrichment of Sr with increasing Zr suggesting that there was little to no plagioclase fractionation. Covariation of major elements versus MgO show minor enrichment of SiO₂, FeO_total, and constant CaO content with decreasing MgO. These trends are consistent with the fractionation of olivine followed by olivine plus clinopyroxene. Fractionation of clinopyroxene is also supported by Sc concentrations that remain nearly constant with increasing Zr. We conclude that the fractionation of olivine followed by olivine + clinopyroxene is the result of high pressure crystallization (>0.5 GPa) and that plagioclase was a late-stage phase with little affect on the differentiation history of the lavas' parental magmas.