The Mammoth Mountain and Wason Park Tuffs: Magmatic evolution in the central San Juan Volcanic Field, southwestern Colorado
Webber, Karen Louise
Stormer, John C., Jr.
Doctor of Philosophy
The Mammoth Mountain Tuff (MMT) and Wason Park Tuff (WPT) were the third and fourth, respectively, caldera-forming ash-flow tuffs in the central San Juan Volcanic Field (SJVF) in southwestern Colorado. In this region, 7 large volume ash-flow tuffs were erupted from a common central area, resulting in the formation of a set of nested calderas in less than 2 Ma. The temporal and spatial association of the MMT and WPT with the older and extensively studied Fish Canyon Tuff (FCT) and Carpenter Ridge Tuff (CRT) offers an excellent opportunity to document magmatic evolution of a large volume silicic system through time and to examine the processes involved in the petrogenesis of the individual ash flows. The MMT, previously described as a zoned ash flow from early-erupted, phenocryst-poor rhyolite to later-erupted phenocryst-rich dacite, consists in fact of two unrelated ash flows, referred to in this work as the FMT and PMT. Based on geochemical and mineralogical similarities, the early-erupted rhyolites (FMT) are believed to be CRT equivalents, as supported by stratigraphic relationships (Lippman, GSA, 87). However, later-erupted dacites (PMT) were generated from a separate, deeper and hotter source. Mineralogical relationships and evidence for disequilibrium requires that the PMT resided in a shallow crustal chamber prior to eruption. The FMT and CRT can be derived from the FCT primary magma by crystal-fractionation processes. Trachytic magma intruded the FMT-CRT chamber, and partially quenched forming fiamme found in both ash flows. Petrographic evidence indicates magma mixing was an important process in the WPT, whereas mineralogical evidence indicates the trachytic magma which formed fiamme in the CRT and FMT, was a mixing component in the WPT as well. Therefore, the WPT represents a hybrid between magma of the zoned fractionation sequence FCT-CRT-FMT and the trachytic magma, establishing a genetic relationship between the central SJVF ash flows FCT, CRT, FMT, and WPT.