HR: 0830h
AN: V51G-0365    [PDF]
TI: Origin, Age, and Geochemistry of the Tuff of Saguache Creek, Southwestern Colorado
AU: * Turner, K J
EM: kjturner@mines.edu
AF: Colorado School of Mines, 1500 Illinois St, Golden, CO 80401 United States
AU: Young, M D
EM: mdyoung@mines.edu
AF: Colorado School of Mines, 1500 Illinois St, Golden, CO 80401 United States
AU: Wendlandt, R F
EM: rwendlan@mines.edu
AF: Colorado School of Mines, 1500 Illinois St, Golden, CO 80401 United States
AB: A crystal-poor, rhyolitic, ash-flow tuff in the Eastern San Juan Volcanic Field currently known as the tuff of Saguache Creek (TSC) was reinterpreted by Simon and Wendlandt (1999) as not being distal Sapinero Mesa Tuff. Currently there is no source caldera established for this tuff thereby leaving uncertainties in the volcanic history. Regional mapping of the TSC has subsequently been completed to constrain a possible source caldera. Petrographic studies, and EPMA and LA-ICPMS analyses of the mineral assemblage have been performed to characterize further the tuff, enable correlation of mapped TSC, and constrain petrogenetic models. Petrographic and chemical identification of the TSC builds on characteristics set forth by Simon (2000). These characteristics include abundant feldspar clusters, alkali feldspar mantled plagioclase, no modal quartz, Fe-Ti oxides (often with apatites and zircons), and sparse biotite and pyroxene. Pyroxenes show curiously high MnO (avg 2.70 wt%) and Mg\# (avg 79.2) and might be xenocrystic. In comparison, biotites have much lower MnO (avg 0.82 wt%). Average sanidine (Or50) and plagioclase (Ab71) compositions fall within the documented range (Simon, 2000) and show little trace element substitution including Eu $<$3 ppm and Ba $<$1200 ppm. Titanites are enriched in Ce (17,000-20,000 ppm) and Pr (2,200-2700 ppm). Apatites show similar enrichment in Ce (2200-7600 ppm) and Pr (330-350 ppm). Confirmed outcrops of TSC are best preserved just north of Trickle Mtn where it is approximately 30 m thick and overlain by a welded airfall ash deposit preliminarily dated at 29-30 Ma. Outcrop localities south of Colorado Highway 114 show thinning and are interpreted as erosional remnants plastered on older Conejos age volcanic deposits. Similarly, an andesitic topographic high just west of Trickle Mtn has TSC vitrophyre on the east-facing slope suggesting strong north-south channeling of the flow unit by paleo-topographic lows. Outcrop elevations also decrease from north to south supporting a possible source caldera to the north. This source could be explained by a distinct circular gravity low on the southeastern slope of the Continental Divide. The low is bordered to the north by a circular arrangement of peaks as well as debris breccias, andesitic flow breccias, and lahars.
DE: 3620 Crystal chemistry
DE: 3670 Minor and trace element composition
DE: 8499 General or miscellaneous
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting