PETROLOGY, GEOCHEMISTRY, AND AERIAL DISTRIBUTION OF THE TUFF OF SAGUACHE CREEK, EASTERN SAN JUAN VOLCANIC FIELD, COLORADO

The tuff of Saguache Creek (TSC), a crystal-poor rhyolitic ignimbrite located in Saguache Co., south-central Colorado, is one of the earliest manifestations of siliceous magmatism in the eastern San Juan Volcanic Field. Accordingly, recognition of its aerial extent, compositional attributes, petrology, and caldera of origin is an important constraint on the volcanic history of this poorly characterized region. Previously mapped as a distal outlier of the Sapinero Mesa Tuff, the TSC is now known to be not correlative with that tuff.

The TSC is 35-40m thick near Trickle Mtn, typically devitrified, strongly welded, homogeneous, and a prominent cliff former. Discontinuous vitrophyres are common near the base of the flow. The TSC contains 8.2 ± 2.4 vol% phenocrysts of alkali feldspar, plagioclase, and sparse biotite, augite, apatite, titanite, and Fe-Ti oxides. The tuff is identified and distinguished from the Sapinero Mesa Tuff by abundant feldspar clusters in thin section, plagioclase phenocrysts rimmed by alkali feldspar, distinctive alkali feldspar (Or_47-51) and plagioclase (An_16-38) compositions, magnesium-rich biotite (mg#=58-61), Zr/Ba>0.5, Ce/Sr>2, and Rb/Sr=2-3.5.

The TSC is slightly younger than (approximately 32.3 Ma; unpublished, P. Lipman), and possibly genetically related to, the early crystal-rich dacite of the Bonanza Tuff. A thin fluvial gravel deposit and the Fish Canyon Tuff most commonly overlie the TSC. The tuff can be mapped over 25 km in a NE-SW trend near Trickle Mtn. Outcrop thicknesses increase in this area, related to a paleochannel controlling the path of the flow. In depositional lows, white airfall and water-laid tuffs up to 90m thick may be preserved between the TSC and overlying gravel and Fish Canyon Tuff. A topographic high, located 1 km west of Trickle Mtn, redirected the flow south. Relicts of TSC vitrophyre are observed plastered onto the east-facing slope of this structure whereas to the west of the structure, the Fish Canyon Tuff lies directly on the Conejos Fm. The overlying Fish Canyon Tuff ultimately obscures the extent of the TSC to the SW. Erosional relicts of TSC can be traced NE to within 8 km of the Bonanza Caldera, suggesting an origin in that vicinity.