ABSTRACT:

A sequence of mafic-to-intermediate composition volcanic flows crops out in a paleochannel along the Arkansas River at Salida in central Colorado. The flows uncomfortably overlie the Wall Mountain Tuff (36.69 Ma) and Precambrian basement and may have developed either at late-stage intermediate volcanism associated with the San Juan or Thirty-Nine-Mile volcanics or with the early development of the northern Rio Grande Rift.

Thirty-six samples were obtained from the Tenderfoot Hill region and several canyons to the south for analysis. Four major units were identified: 1) Clinopyroxene-bearing alkaline olivine basalt (AOB) is a 1,000 meter thick unit interbedded with thin layers of ash. The unit includes plag (4.5mm), pyroxene and olivine phenocrysts in an olivine, cpx, plag-rich groundmass. 2) Hypersthene-bearing trachyandesite (HBTA) found in the northern section of the study area, has phenocrysts of flow-oriented plag with clino- and orthopyroxene. 3) Hornblende-bearing trachyandesite (HTA) occurs in a small outcrop on the west side of Tenderfoot Hill and contains oxyhornblende, ortho- and clinopyroxene. 4) Pyroxene-bearing trachyandesite (PBTA) occurs as thick flows and laharic breccias. Rock types are diverse, but contain plag, ortho- and clinopyroxene.

Silica contents of the four units range from 51.9% (AOB) to 59.1% (PBTA). Based on geochemical graphs, AOB is a basaltic trachyandesite and the other units are trachyandesites. Except for the AOB, they are tholeiitic. They are all very high in K₂0 (shoshonitic field) and Zr. Mg#s are highly evolved (~12-23). Ti and to a lesser degree Fe and Mg serve as excellent discriminators among the four flows. These flows all show similar REE patterns, ~300x enriched in LREE’s with a slight negative Eu anomaly. Incompatible element plots show strong enrichment in K, Rb, Ba relative to MORB and a pronounced Nb-Ta trough. They are a good match for OIB or UCC. Tectonic discriminate diagrams are ambiguous; they plot as both rift and arc basalts.

The geochemical data suggest the flows are late Thirty-Nine Mile Volcanics that show crustal contamination (high K, Zr) but are derived from a mantle source. The Ta/Nb trough might be inherited from magmatism that occurred 1.8 Ga in the Pre-Cambrian.