Middle Tertiary Explosive Volcanism in the Southwestern US: The Atascosa-Tumacacori Volcanic Complex:

Many years ago, just after starting my first teaching job at Colgate University, I cast about for a mapping project in explosive felsic volcanic rocks in a warm climate.  Gordon Haxel of the USGS, one of my master’s thesis advisors, suggested that the Atascosa and Tumacacori Mountains of southern Arizona could provide many seasons of mapping joy.  These rocks have provided two topical studies, one on flow banding in the Aliso lava dome resulting from stretching of mingled blobs of magma (Seaman et al., 1995) and one on the origin of crystal clusters, enclaves, and xenocrysts with Sr isotopic zoning, all in the capping lava flow of the Atascosa Range (Seaman, 2000).  Based on Ar-Ar work done in Bill McIntosh’s lab at NM Tech, the volcanic rocks of the complex erupted over a 4.5 million year time period (23.5-28 Ma).   They are slightly alkalic and range in composition from earlier trachyandesites through later dacites and trachyrhyolites.  The southern of the two ranges is that Atascosa Mountains.  They are dominated by pyroclastic rocks.  The Tumacacori Mountains, to the north, are dominated by felsic lava flows.  The Bartolo Mountain flow, on the western margin of the Atascosas, is the subject of a paper in review now (Seaman et al., Bulletin of Volcanology) about the influence of heterogeneity in water concentration in rhyolitic lava leading the flow banded texture in instances where magma mingling is not a factor.