ABSTRACT The Mogollon-Datil volcanic field is a 40–24 Ma cluster of calderas that formed during ignimbrite flare-up eruptions in southern New Mexico associated with sub-duction, and possible delamination, of the Farallon plate beneath the North American plate. This study uses magmatic zircon sampled from four ignimbrites from a nested caldera system and an additional ignimbrite located outside of the nested system to compare the processes and timing of magma accumulation in southern New Mexico. These ignimbrites include: the Whitewater Tuff, the Cooney Canyon Tuff, the Davis Canyon Tuff, and the Shelley Peak Tuff from the Mogollon Mountains and the Bell Top 4 Tuff from the Uvas volcanic field. The ignimbrites range from crystal-poor, high-silica rhyolite to crystal-rich, low-silica rhyolite. We compare previous 40 Ar/ 39 Ar sanidine eruption ages to new U-Pb crystallization ages and trace-element compositions of zircon. Weighted mean zircon ages define two magmatic groups. Group one includes the Bell Top Tuff (34.5 ± 0.5 Ma), the Cooney Canyon Tuff (34.8 ± 0.8 Ma), and the Whitewater Creek Tuff (36.2 ± 0.4 Ma). The second group includes the Davis Canyon Tuff (28.7 ± 0.5 Ma) and the Shelley Peak Tuff (29.6 ± 0.5 Ma). Weighted mean zircon ages are within published 40 Ar/ 39 Ar ages, with the exception of the Shelley Peak Tuff, which is ~1 m.y. older. Hafnium contents and Th/U and Yb/Gd ratios suggest the dominant mechanism that produced eruptible melt was rejuvenation or remobilization of a crystal mush accompanied by minimal partial melting of the continental crust.

The southeastern margin of the Colorado Plateau (CP) lies in southwestern New Mexico and southeastern Arizona. It is defined as the boundary between the CP, the Rio Grande rift, and the Basin and Range (BRP) provinces. Along its western and southern margins, the CP is physically distinguishable from the BRP by major escarpments. Because of a thick cover of volcanic material associated with the Mogollon-Datil volcanic field (MDVF), no such demarcation exists in the study area. To accurately determine lithospheric structure in this area, two seismic refraction lines that intersect within the MDVF were studied. In addition, gravity data along these lines were simultaneously analyzed. Based on these data, velocity and density profiles were created via an iterative forward modeling processes. Major crustal boundaries for each paired gravity/refraction profile were constrained to agree at the point of intersection and with other geological and geophysical control. Structurally, the CP margin appears as an abrupt northward deepening of the Moho just south of Datil, New Mexico. This lies beneath a major low density-low velocity upper crustal body, interpreted to be a plutonic complex approximately 230 by 150 by 6 km. It is very likely that this complex provided the source material for the MDVF. Stoping appears to be the vehicle for emplacement of this body, implying a major crustal reorganization in middle Tertiary time.

The Taylor Creek Rhyolite, a group of Tertiary high-silica-rhyolite lava domes and flows in southwestern New Mexico, contains cassiterite-bearing veins whose tin was derived from the host rhyolite as it degassed, cooled, and devitrified immediately after emplacement. Theoretical considerations and studies of fumarolic deposits at many volcanoes worldwide indicate that tin is highly mobile in a vapor phase, probably as halogen complexes, thus favoring the occurrence of such auto-mineralization in a cooling-lava environment. Mass-balance calculations for the New Mexico situation indicate that much of the tin evolved during devitrification of the rhyolite cannot be accounted for in the mineralized deposits. Some of this “missing” tin almost certainly was dispersed into alluvium during erosion of mineralized parts of the lavas, and some may have been transferred to the atmosphere around fumaroles rooted in the cooling lavas. In addition, tin may have been lost to the atmosphere from Taylor Creek Rhyolite magma that was erupted in fountains. The recent recognition of fountain-fed fallback in the New Mexico rhyolite field suggests that this third means of moving tin out of erupting magma may indeed have contributed to the overall tin history in the Taylor Creek Rhyolite magma system. Fountain-fed flows of silicic lavas are not well known, whereas mafic counterparts are known to be common as a result of observations of many eruptions of basaltic magma. Characteristic properties of silicic magmas that collectively tend to result in relatively high viscosity inhibit the occurrence of eruption columns whose fallback is hot enough to thoroughly weld and perhaps totally rehomogenize into a melt that subsequently feeds lava flows. However, high volumetric rates of eruption, high magmatic temperature (relative to solidus temperature), and any other conditions that help to reduce viscosity, in concert with factors that result in relatively brief periods of trajectory for lava clots, favor the formation of fountain-fed silicic lava flows. Fluorine-rich magma of about 830°C produced such silicic lavas in the Taylor Creek Rhyolite, and rocks with a similar mode of emplacement in other volcanic areas are herein hypothesized to be far more abundant than presently recognized. Possible examples elsewhere include large-volume sheets of silicic lavas in southwestern Idaho (eruption temperature of 950° to 1,100°C) and in Trans-Pecos Texas, where lava-flow and welded pyroclastic textures intermixed within individual eruptive units have led to confusion and difficulty in interpreting the mechanism of emplacement. Documentation of fountain-fed silicic lavas is rare; eruptions of silicic magmas are infrequent relative to the average human lifespan, and very few have occurred during historic time. Moreover, evidence of a lava-fountain origin may be only weakly preserved in the rocks so formed; the evidence also may be entirely lacking, as is commonly the case for the closely observed mafic examples.