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.

Abstract Recent examination of volcanic rocks near the Bingham Canyon Cu-Au-Mo deposit, Utah, suggests that primitive alkaline magmas are an important factor in the formation of this world-class porphyry copper deposit. The Bingham deposit is spatially associated with a monzonitic intrusive complex emplaced at 39 to 37 Ma into Paleozoic sedimentary rocks. Coeval igneous rocks vented to the surface and formed a volcanic pile, part of which is preserved on the eastern flank of the Oquirrh Mountains. Bingham volcanic rocks are divided into three chrono-lithologic suites: an older volcanic suite, a nepheline minette-shoshonite suite, and a chemically distinct younger volcanic suite. Petrographic and geochemical data indicate that the intrusive complex and older volcanic suite are largely comagmatic. This relationship is substantiated by similarities in ages and proximity of the older volcanic rocks to the intrusive suite. No significant chemical differences occur between these two suites, except where hydrothermal alteration has increased the concentration of Cu and K in the intrusions at the expense of Na. The nepheline minette lava flows extruded at ~38 Ma are primitive. They are characterized by Mg # >65, high concentrations of volatiles, LILE, and LREE, and strongly compatible elements. They contain relatively low concentrations of Ti, Nb, and Zr on a mantle-normalized basis, 15 to 18 percent normative nepheline, and 1 to 15 percent normative leucite. The nepheline minette magmas may be primary melts that are products of small degrees of partial melting of metasomatized, lithospheric mantle. Associated shoshonites lack normative nepheline and normative leucite. At comparable SiO 2 contents, the younger volcanic rocks emplaced at ~32 Ma are generally more aluminous and sodic, and less magnesian and potassic than the older volcanic and intrusive suites. All magmas except the nepheline minette were sulfide-saturated when erupted and contain magmatic sulfides as 1 to 100 micron-diameter elliptical inclusions in mafic mineral phenocrysts. Analytical data suggest that sulfides accommodate most of the Cu and Ag that are present in the magmas. Resorption and oxidation of the sulfides may have made these metals available to the hydrothermal system. The sulfide-undersaturated character of the primitive, alkaline magmas may have allowed them to rise through the crust with almost no loss of S, Cu, and other chalcophile metals. Fractional crystallization, magma mixing, and assimilation all played roles in determining the composition of the magmas in the Bingham system. Trace-element modeling shows that the minette flows are not related to the other volcanic suites or to the intrusive suite by fractional crystallization. However, mixing of the minette magma with latitic magma could have created the shoshonites and the magma of the older suite. Trace-element modeling also indicates that late mineralized dikes may be formed by mixing of about 10 percent minette magma and 90 percent calc-alkaline magma. Degassing of mixed minette magma underlying the calc-alkaline magma may have contributed even larger proportions of sulfur, volatiles, and metals to the ore-forming system.