The Late Jurassic (157–150 Ma) Morrison Formation of the Western Interior of the United States contains abundant altered volcanic ash. On the Colorado Plateau, this formation accumulated behind and downwind of a subduction-related volcanic arc along the western margin of North America. The ash in these distal fallout tuffs probably drifted eastward from coignimbrite ash clouds related to collapse calderas. Altered volcanic ash is particularly abundant in the Brushy Basin Member of the upper part of the Morrison Formation. In one 110-m-thick section in eastern Utah, 35 separate beds were deposited in a 2.2 m.y. period. Alteration occurred when glassy volcanic ash fell into fluvial and lacustrine environments, where it was diagenetically altered to various mineral assemblages but most commonly to smectitic clay. Periodically, ash fell into saline, alkaline lakes, and diagenetic alteration of the glassy ash produced a crudely zoned deposit on the Colorado Plateau. Altered volcanic ash beds in the outermost part of the lacustrine deposits are argillic (with smectitic clay), whereas zeolitic (clinoptilolite, analcime) and feldspathic (K-feldspar and albite) alteration dominates the interior zones. Feldspathic ash layers contain secondary silica, and consequently immobile element (e.g., Al, Ti, and high field strength elements) abundances were strongly diluted in these rocks. In contrast, the argillic ash beds experienced strong SiO 2 depletion, and, as a result, they are enriched in the relatively immobile elements. The compositions of the zeolitic ash beds are intermediate between these two extremes and experienced the least alteration. As a result of these changes, immobile element concentrations are less reliable than ratios for determining the original magmatic composition of the ash. Most of the altered ash (regardless of type) was also depleted in water-soluble elements like the alkalies, U, and V. The latter two elements were oxidized during diagenesis of the ash, became soluble, and were partially leached away by groundwater. Locally, U and V in groundwater were reduced upon contact with organic materials and formed important ore deposits. Several aspects of the mineralogy and geochemistry of the altered volcanic ash beds yield information about their original magmatic compositions. The volcanic ash beds typically have small phenoclasts of quartz, sanidine, plagioclase, biotite, zircon, apatite, and Fe-Ti oxides. Titanite is present in ∼40% of the ash beds; pyroxene and amphibole were found in less than 5%. Phenocryst assemblages, mineral compositions, inferred high f O 2 , rare earth element patterns, and immobile element ratios all suggest the parent magmas for the altered tuffs were subduction-related dacites and rhyolites. Small numbers of tuffs have Fe-rich biotite, amphibole, and/or clinopyroxene; both pyroxene and amphibole are alkali rich. These tuffs lack titanite, but some contain anorthoclase and F-rich apatite. Combined with enrichments in Nb and Y, these features show some tuffs had an A-type character and were related to some type of within-arc extension. Paleowind directions, and distribution, radiometric ages, and compositions of the volcanic ash beds and of plutons in the western United States suggest that the most likely eruption sites were in the subduction-related Jurassic magmatic arc, which extended across western Utah and central Nevada and southward into the Mojave of California and southern Arizona (present-day coordinates). Pb isotopic compositions show that at least some of the ash was erupted from magma systems (now exposed as plutons) in the Mojave Desert. We conclude that a brief ignimbrite flare-up from 157 to 150 Ma, but focused on the time period from 152 to 150 Ma, in this region may have been driven by slab steepening and conversion to a strike-slip boundary after a preceding phase of folding and thrusting. The presence of ash beds with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was geologically a transitional time in North America when subduction was changing to transtensional movement along the western plate boundary.

Volcanic clasts incorporated in the lower portion of the Tertiary Santa Fe Group sedimentary rocks of the Culebra graben, San Luis Basin, Colorado, provide constraints on the timing of regional tectonic events by provenance determination. Based on currently exposed volcanic terrains, possible clast sources include Spanish Peaks and Mount Mestas to the east, the San Juan volcanic field to the west, and the Thirtynine Mile volcanic field, a remnant of the Central Colorado volcanic field, to the north and east of the San Luis Basin. Provenance was determined by a variety of geochemical, mineral chemical, and geochronologic data. Large porphyritic Santa Fe Group volcanic clasts are potassic with a wide compositional range from potassic trachybasalt to rhyolite. The whole-rock chemistry of the Culebra graben clasts is similar to that of the Thirtynine Mile and San Juan volcanic fields. Culebra graben amphibole and biotite chemistry is generally consistent with that of rocks of the San Juan volcanic field, but not with Spanish Peaks samples. Trace-element data of Culebra graben volcanic clasts overlap with those of the San Juan and Thirtynine Mile volcanic fields, but differ from those of the Mount Mestas. Thermobarometric calculations using mineral chemistry suggest that many Culebra graben rocks underwent a three-stage crystallization history: ~1120 °C at 7–10 kbar, ~1100 °C at 2.3–4.6 kbar, and hornblende formation ~800 °C at 3 kbar. Within the Culebra graben clasts, zircon rim U-Pb geochronologic systematics as well as amphibole and biotite 40 Ar/ 39 Ar plateau data yield ages ranging from 36 to 29 Ma. These ages are consistent with ages of the Thirtynine Mile volcanic field (36–27 Ma) and the Conejos Formation of the San Juan volcanic field (35–29 Ma), but predate Spanish Peaks (ca. 27–21 Ma) and Mount Mestas (ca. 25 Ma). Based on these data, Spanish Peaks and Mount Mestas are excluded as potential source areas for the Santa Fe Group volcanic clasts in the Culebra graben. The San Juan volcanic field is also an unlikely source due to the distance from the depositional site, the inconsistent paleo-current directions, and the pressure-temperature conditions of the rocks. The most likely scenario is that the Central Colorado volcanic field originally extended proximal to the current location of the Culebra graben and local delivery of volcanic clasts was from the north and northeast prior to the uplift of the Culebra Range and Sangre de Cristo Mountains.

Abstract The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km 3 ) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.

Early Proterozoic supracrustal rocks near Gunnison and Salida, Colorado, include sequences of tholeiitic metabasalt, metarhyolite to metadacite, and interbedded volcaniclastic turbidite. These rocks were intruded by synchronous gabbroic sheets, complexly folded and metamorphosed in upper greenschist to upper amphibolite facies, and intruded by plutons ranging from quartz diorite to granite. U-Pb ages of zircons show that an early period of volcanism in the Gunnison area occurred from 1,770 to 1,760 Ma and was followed by emplacement of plutons from 1,755 to 1,750 Ma. A younger sequence of volcanic rocks was formed in both the Gunnison and Salida areas between 1,740 and 1,730 Ma. In the Gunnison area these rocks were intruded by major plutons from 1,725 to 1,714 Ma. Near Howard, Colorado, southeast of Salida, metarhyolite yielding ages of 1,713 and 1,668 Ma is believed to be part of the younger sequence. Late, post-tectonic granite plutons were emplaced in both areas from 1,700 to 1,670 Ma. The age data, petrography, and geochemistry of these rocks indicates that they are part of a broad belt of juvenile, arc-related terranes, exposed from southern California across Arizona, New Mexico, Colorado, and known in the subsurface as far east as western Missouri, that was accreted to the southern edge of the continent during the Early Proterozoic.