MAGMATIC EVOLUTION AND ERUPTION PROCESSES IN A TUFF CONE DEPOSIT, CON SHEA VOLCANO, WESTERN SNAKE RIVER PLAIN, IDAHO

Con Shea volcano is one of more than 30 phreatomagmatic edifices identified in the Snake River Plain, Idaho. It is a tuff cone that formed during an episode of basaltic magmatism that occurred from 2.2 to 0.1 Ma in the Western Snake River Plain, concomitant with the drainage and filling of Lake Idaho approximately 4.5 to 2 Ma. At Con Shea, nearly 15 meters of massive, planar, and cross-bedded volcanic tuffs are exposed. These deposits are composed of silt to pebble sized particles of both accidental and juvenile material. Juvenile clasts consist of olivine and plagioclase crystals in addition to matrix glass. The matrix glass ranges in character from dense, glassy blobs to highly vesiculated fragments to varying degrees of altered and palagonitized masses.

Matrix glass fragments from five units were analyzed using an electron microprobe. The samples were all low in silica (46.9 wt% to 47.8%), with the slightly lower values occurring in later deposits. The most notable compositional changes occurred in the FeO (11.7 wt% to 16.8%) and TiO2 (2.5 wt% to 4.6%) values, both increasing with time. Slight increases were also noted in the P2O5 and MnO contents, in addition to decreases Al2O3 and Na2O. Fourier transform infrared spectroscopy (FTIR) was used to analyze the volatile (H2O and CO2) content of melt inclusions found within olivine crystals and fragments of the matrix glass. Melt inclusions from two units of different stratigraphic height were analyzed. A notable decrease in water content (0.39 wt% to 0.11%) in the inclusions occurred as the eruption progressed; whereas, only a minor decrease in carbon dioxide (527 ppm to 360 ppm) was observed. Pristine matrix glass from six units was analyzed, revealing a decreasing amount of dissolved water (0.43 wt% to 0.04wt%), corresponding to an increase in stratigraphic height. This decrease in water content correlates with petrographic observations of increasing vesicularity and microcrystalinity of the matrix glass with increasing stratigraphic height. Through petrographic, electron microprobe, and FTIR analysis, a tholeiitic compositional trend and drying out of the system have been identified and correlated with the increased presence of vesicles and olivine and plagioclase microlites found in the matrix glass.