PETROLOGY AND GEOCHEMISTRY OF THE RED MOUNTAIN AND FLAT TOP BASALTS, GUNNISON COUNTY, COLORADO

Red Mountain and Flat Top, in central Gunnison County, are adjacent mesas capped by ~10 Ma basaltic lavas. These lavas were both erupted into the paleo-Gunnison River drainage and have undergone a topographic inversion. Red Mountain consists of up to fourteen stacked aa flows with an aggregate thickness of up to 65 m. The erosional remnant of this field covers ~1.0 km². Flat Top consists of interlayered thick and thin pahoehoe flows, is up to 40 m thick and currently covers ~3.7 km². Both show no evidence of significant time gaps between the flows.

Red Mountain lavas are olivine phyric shoshonites and potassic trachybasalts. Flat Top lavas are olivine-plagioclase phyric medium-K basalts and basaltic andesites. Red Mountain and Flat Top flows are chemically distinct with Red Mountain having a greater range of MgO (5.0-7.3%), and more K₂O (2.0-2.5%), TiO₂ (1.7-1.9%), and P₂O₅ (0.6-0.7%) and less Al₂O₃ (15-16%) than lavas from Flat Top at comparable MgO concentrations. Vertical variations show that at least three separate magma batches were tapped on Red Mountain. Flat Top lavas are more chemically uniform. Potentially transitional lavas occur at the top of the Red Mountain and bottom of the Flat Top sections.

The more Mg and Ni-rich Red Mountain lavas are LREE enriched (La_n/Yb_n ~27) and show distinct Ta and Nb depletions (La/Ta 31-36). The less Mg and Ni-rich Flat Top lavas have lower overall abundances of incompatible elements, slightly lower LREE enrichments (La_n/Yb_n ~17) and more distinct Ta and Nb depletions (La/Ta 39-47). Red Mountain and Flat Top lavas need to come from compositionally and/or mineralogically distinct mantles that have characteristics typical of subduction modification.