David G. Tingey

The Oligocene Lund Tuff, Great Basin, USA: a very large volume monotonous intermediate

Abstract Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km 3 ) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km 3 . It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO 2 ) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.

The Great Basin Altiplano during the middle Cenozoic ignimbrite flareup: insights from volcanic rocks

Uncertainty surrounds the fate of the orogenic plateau in what is now the Great Basin in western Utah and Nevada, which resulted from the Mesozoic and earliest Cenozoic contractile deformations and crustal thickening. Although there is some consensus regarding the gravitational collapse of the plateau by extensional faulting and consequent crustal thinning, whether or not the plateau existed during the middle Cenozoic Great Basin ignimbrite flareup – one of the grandest expressions of continental volcanism in the geologic record – had remained in doubt. We use compositions of contemporaneous calc-alkaline lava flows as well as configurations of the ignimbrite sheets to show that the Great Basin area during the middle Cenozoic was a relatively smooth plateau underlain by unusually thick crust. We compare analyses of 376 intermediate-composition lava flows in the Great Basin that were extruded at 42–17 Ma with compositions of >6000 analyses of the late Cenozoic lava flows in continental volcanic arcs that correlate roughly with known crustal thickness. This comparison indicates that the middle Cenozoic Great Basin crust was much thicker than the present ca. 30 km thickness, likely as much as 60–70 km. If isostatic equilibrium prevailed, this unusually thick continental crust must have supported high topography. This high terrain in SE Nevada and SW Utah was progressively smoothed as successive ignimbrite outflow sheets were emplaced over areas currently as much as tens of thousands of square kilometres to aggregate thicknesses of as much as hundreds of metres. The generally small between-site variations in the palaeomagnetic directions of individual sheets lend further support for a relatively smooth landscape over which the sheets were draped. We conclude that during the middle Cenozoic, especially towards the close of the ignimbrite flareup, this Great Basin area was a relatively flat plateau, and because it was also high in elevation, we refer to it as an Altiplano. It was not unlike the present-day Altiplano-Puna in the tectonically similar central Andes, where an ignimbrite flareup comparable to that in the Great Basin occurred at ca. 9–3 Ma. Outflow ignimbrite sheets that were deposited from 35 to 23 Ma on the progressively smoothed Altiplano in south-eastern Nevada were derived from source calderas to the west. Of the 12 major sheets from seven sources, nine are distributed unevenly east of their sources while the remaining three sheets are spread about as far east as west of their sources. This eccentricity of sources near the western margin of 75% of the sheets indicates the existence of a NS-trending topographic barrier in central Nevada that restricted westward dispersal of ash flows. In a symmetric manner, eastward dispersal of ash flows from sources farther west seemed to have been impeded by this same topographic barrier. The westward dispersal was controlled in part by westward-draining stream valleys incised in the sloping flank of the Great Basin Altiplano in western Nevada and adjacent California; at least one of these ash flows travelled as far west as the western foothills of the Sierra Nevada. The nature and origin of the implied topographic barrier are uncertain. It is possible that heavy orographic precipitation on the western slope of the Altiplano and consequent focused denudation and isostatic uplift created a NS-trending topographic high at the crest of the western slope and facing the smoothed Altiplano to the east. The barrier also lies near and essentially parallel to the buried western edge of the Precambrian basement and to a zone of thermal-diapiric domes that were spawned in thickened crust as the basement edge was overrun by late Palaeozoic–Mesozoic thrust sheets.

Active and inactive groundwater flow systems: Evidence from a stratified, mountainous terrain

We present a new conceptual model of groundwater flow that describes active and inactive groundwater flow regimes. The model is based on an analysis of interactions between surface water and shallow and deep groundwater in the 240-km-long Wasatch Range and Book Cliffs, Utah, USA. Active zone groundwater flow paths are continuous, responsive to annual recharge and climatic variability, and have groundwater resident times “ages” that become progressively older from recharge to discharge area. Active zone groundwater systems discharge at thousands of springs that issue from the 700+-m-thick, gently dipping, clastic bedrock formations. Springs waters contain appreciable 3 H and anthropogenic 14 C. In contrast, inactive zone groundwater has extremely limited or no communication with annual recharge and has groundwater mean residence times that do not progressively lengthen along the flow path. Groundwater in the inactive zone may be partitioned, occur as discrete bodies, and may occur in hydraulically isolated regions that do not have hydraulic communication with each other. Inactive zone groundwater is encountered in-mines (coal-mines 300–700 m below ground surface) where groundwater discharge rates decline rapidly and the waters have δ 2 H and δ 18 O compositions that are distinguishable from near surface groundwater. In general, deep waters have no 3 H and have mean 14 C residence times of 500 to 20,000 yr (45.9 to 4.9 pmc). Chemical evolution modeling, porosity-permeability core plug analysis, and in-mine hydrographs also indicate hydraulic partitioning.

Enhanced fracture permeability and accompanying fluid flow in the footwall of a normal fault: The Hurricane fault at Pah Tempe hot springs, Washington County, Utah

The Pah Tempe hot springs discharge ~260 L/s of water at ~40 °C into the Virgin River in the footwall damage zone of the Hurricane fault at Timpoweap Canyon, near Hurricane, Utah, USA. Although these are Na-Cl waters, they actively discharge CO2 gas and contain signifi cant quantities of CO2 (~34.6 mmol/kg), predominantly as H2CO3 and HCO3 –. Because of excellent exposures, Pah Tempe provides an exceptional opportunity to observe the effects of enhanced fracture permeability in an active extensional fault. Pah Tempe waters have been deeply circulated (>5 km; >150 °C) into basement rock as illustrated by the clear water-rock exchange of oxygen isotopes. Waters were probably recharged under colder climate conditions than present and therefore have a prolonged subsurface residence. Discharge of both water and gas in the springs correlates to the density of fractures in carbonate rocks above stream level. This observation suggests that clusters of high fracture density in the faultdamage zone act as pathways from the likely regional aquifer, the eolian Queantoweap Sandstone, through the overlying confining unit, the gypsiferous silty Seligman Member of the Kaibab Formation. Mass-balance modeling suggests that the majority of CO2 discharge is the product of the quantitative dissolution of CO2 gas at depth within the fault zone. Upon discharge, most of the carbon is released to the surface as dissolved species. It appears that the subsurface production rate of CO2 is relatively low because Pah Tempe waters are grossly undersaturated in CO2 at inferred minimum circulation depths and temperatures. Geological and geochemical data also suggest that the CO2 is dominated by a crustal component complemented by minor mantle contributions.

The 36–18 Ma Indian Peak–Caliente ignimbrite field and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions

The Indian Peak–Caliente caldera complex and its surrounding ignimbrite field were a major focus of explosive silicic activity in the eastern sector of the subduction-related southern Great Basin ignimbrite province during the middle Cenozoic (36–18 Ma) ignimbrite flareup. Caldera-forming activity migrated southward through time in response to rollback of the subducting lithosphere. Nine partly exposed, separate to partly overlapping source calderas and an equal number of concealed sources compose the Indian Peak–Caliente caldera complex. Calderas have diameters to as much as 60 km and are filled with as much as 5000 m of intracaldera tuff and wall-collapse breccias. More than 50 ignimbrite cooling units, including 22 of regional (>100 km 3 ) extent, are distinguished on the basis of stratigraphic position, chemical and modal composition, 40 Ar/ 39 Ar age, and paleomagnetic direction. The most voluminous ash flows spread as far as 150 km from the caldera complex across a high plateau of limited relief—the Great Basin altiplano, which was created by late Paleozoic through Mesozoic orogenic deformation and crustal thickening. The resulting ignimbrite field covers a present area of ∼60,000 km 2 in east-central Nevada and southwestern Utah. Before post-volcanic extension, ignimbrites had an estimated aggregate volume of ∼33,000 km 3 . At least seven of the largest cooling units were produced by super-eruptions of more than 1000 km 3 . The largest, at 5900 km 3 , originally covered an area of 32,000 km 2 to outflow depths of hundreds of meters. Outflow ignimbrite sequences comprise as many as several cooling units from different sources with an aggregate thickness locally reaching a kilometer; sequences are almost everywhere conformable and lack substantial intervening erosional debris and angular discordances, thus manifesting a lack of synvolcanic crustal extension. Fallout ash in the mid-continent is associated with two of the super-eruptions. Ignimbrites are mostly calc-alkalic and high-K, a reflection of the unusually thick crust in which the magmas were created. They have a typical arc chemical signature and define a spectrum of compositions that ranges from high-silica (78 wt%) rhyolite to andesite (61 wt% silica). Rhyolite magmas were erupted in relatively small volumes more or less throughout the history of activity, but in a much larger volume after 24 Ma in the southern part of the caldera complex, creating ∼10,000 km 3 of ignimbrite. The field has some rhyolite ignimbrites, the largest of which are in the south and were emplaced after 24 Ma. But the most distinctive attributes of the Indian Peak–Caliente field are two distinct classes of ignimbrite: 1. Super-eruptive monotonous intermediates. More or less uniform and unzoned deposits of dacitic ignimbrite that are phenocryst rich (to as much as ∼50%) with plagioclase > biotite ≈ quartz ≈ hornblende > Fe-Ti oxides ± sanidine, pyroxene, and titanite; apatite and zircon are ubiquitous accessory phases. These tuffs were deposited at 31.13, 30.06, and 29.20 Ma in volumes of 2000, 5900, and 4400 km 3 , respectively, from overlapping, multicyclic calderas. A unique, and possibly kindred, phenocryst-rich latite-andesite ignimbrite with an outflow volume of 1100 km 3 was erupted at 22.56 Ma from a concealed source caldera to the south. 2. Trachydacitic Isom-type tuffs. Also relatively uniform but phenocryst poor ( > clinopyroxene ≈ orthopyroxene ≈ Fe-Ti oxides >> apatite. These alkali-calcic tuffs are enriched in TiO 2 , K 2 O, P 2 O 5 , Ba, Nb, and Zr and depleted in CaO, MgO, Ni, and Cr, and have an arc chemical signature. Magmas were erupted from a concealed source immediately after and just to the southeast of the multicyclic monotonous intermediates. Most of their aggregate outflow volume of 1800 km 3 was erupted from 27.90 to 27.25 Ma. Nothing like this couplet of distinct ignimbrites, in such volumes, have been documented in other middle Cenozoic volcanic fields in the southwestern U.S. where the ignimbrite flareup is manifest. Magmas were created in unusually thick crust (as thick as 70 km) where large-scale inputs of mantle-derived basaltic magma powered partial melting, assimilation, mixing, and differentiation processes. Dacite and some rhyolite ignimbrites were derived from relatively low-temperature (700–800 °C), water-rich magmas that were a couple of log units more oxidized than the quartz-fayalite-magnetite (QFM) oxygen buffer at depths of ∼8–12 km. In contrast to these “main-trend” magmas, trachydacitic Isom-type magmas were derived from drier and hotter (∼950 °C) magmas originating deeper in the crust (to as deep as 30 km) by fractionation processes in andesitic differentiates of the mantle magma. “Off-trend” rhyolitic magmas that are both younger and older than the Isom type but possessed some of their same chemical characteristics possibly reflect an ancestry involving Isom-type magmas as well as main-trend rhyolitic magmas. Andesitic lavas extruded during the flare up but mostly after 25 Ma constitute a roughly estimated 12% of the volume of silicic ignimbrite, in contrast to major volcanic fields to the east, e.g., the Southern Rocky Mountain field, where the volume of intermediate-composition lavas exceeds that of silicic ignimbrites.

Paleohydrologic record of spring deposits in and around Pleistocene pluvial Lake Tecopa, southeastern California

Tufa (spring) deposits in the Tecopa basin, California, reflect the response of arid groundwater regimes to wet climate episodes. Two types of tufa are represented, informally defined as (1) an easily disaggregated, fine-grained mixture of calcite and quartz (friable tufa) in the southwest Tecopa Valley, and (2) hard, vuggy micrite, laminated carbonate, and carbonate-cemented sands and gravels (indurated tufa) along the eastern margin of Lake Tecopa. High δ18OVSMOW (Vienna standard mean ocean water) water values, field relations, and the texture of friable tufa suggest rapid nucleation of calcite as subaqueous, fault- controlled groundwater discharge mixed with high-pH, hypersaline lake water. Variations between δ18OVSMOW and δ13CPDB (Peedee belemnite) values relative to other closed basin lakes such as the Great Salt Lake and Lake Lahontan suggest similarities in climatic and hydrologic settings. Indurated tufa, also fault controlled, formed mounds and associated feeder systems as well as stratabound carbonate-cemented ledges. Both deposits represent discharge of deeply circulated, high total dissolved solids, and high p CO2 regional groundwater with kinetic enrichments of as much as several per mil for δ18OVSMOW values. Field relations show that indurated tufa represents episodic discharge, and U-series ages imply that discharge was correlated with cold, wet climate episodes. In response to both the breaching of the Tecopa basin and a modern arid climate, most discharge has changed from fault-controlled locations near basin margins to topographic lows of the Amargosa River drainage at elevations 30–130 m lower. Because of episodic climate change, spring flows may have relocated from basin margin to basin center multiple times.

The 36–18 Ma Central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions

One of the greatest global manifestations of explosive silicic volcanism in the terrestrial rock record occurred during the middle Cenozoic over a large part of southwestern North America, from the Great Basin of Nevada and western Utah into Colorado, Arizona, New Mexico, and Mexico. This subduction-related ignimbrite flareup is the only one known in the world of its magnitude and of Mesozoic or Cenozoic age that is not related to continental breakup. The southern Great Basin ignimbrite province was a major product of the flareup . Its central and eastern sectors developed on the Great Basin altiplano, a high orogenic plateau of limited relief dating from pulses of late Paleozoic through Mesozoic orogenic contractile deformation. Caldera-forming activity migrated southwestward through time in response to rollback of a once-flat slab of subducting lithosphere. In the central sector of the southern Great Basin ignimbrite province, 11 partly exposed, mostly overlapping source calderas and one concealed source comprise the 36–18 Ma Central Nevada caldera complex. Calderas have diameters as much as 50 km, to possibly 80 km. Intracaldera tuff and intercalated wall-collapse breccia are at least 2000 m thick. Surrounding outflow ignimbrites consist of 17 regional cooling units (>200 km 3 ) that have been correlated over two or more mountain ranges on the basis of stratigraphic position, paleomagnetic direction, chemical and modal composition, and 40 Ar/ 39 Ar age. Many additional smaller cooling units have been recognized. Possibly as many as eight of the ignimbrites resulted from super-eruptions of 1000 km 3 to as much as 4800 km 3 . This Central Nevada ignimbrite field is presently exposed over an area of ∼65,000 km 2 in south-central Nevada and had a volume of 25,000 km 3 corrected for post-volcanic crustal extension. Six of the largest eruptions broadcast ash flows over an extension-corrected area of greater than 16,000 km 2 and as much as 160 km from their caldera sources. Individual sections of outflow tuff include as many as 14 ignimbrite cooling units; aggregate thicknesses locally reach a kilometer, and stacks a few hundred meters thick are common. Sequences are almost everywhere conformable and lack substantial intervening erosional debris and angular discordances that would testify to synvolcanic crustal extension. Beds of fallout ash a few meters thick associated with the largest eruption have been recognized in the mid-continent of the U.S. Six caldera-forming eruptive episodes are separated by five lulls in activity, each lasting from 1.7 to 4.4 m.y., during which time little ( 3 ) or no ignimbrite was deposited. Some of the longer lulls that preceded the most voluminous eruptions likely reflected the time for accumulation of magma in huge shallow chambers before eruption was triggered. Other long lulls preceded the last two, single eruptions as the arc magma-generating system was waning prior to the transition to non-arc magma production to the south in the Southwestern Nevada volcanic field. Central Nevada ignimbrites are mostly calc-alkalic and high-K with trace element patterns typical of subduction-related arcs; they range from high-silica (78 wt%) rhyolite to low-silica (63 wt%) dacite. Most ignimbrites are rhyolite, from the earliest to the latest eruptions in the field, and most of these are phenocryst rich. The largest ignimbrite (4800 km 3 ), emplaced at 31.69 Ma, is a phenocryst-rich, normally zoned rhyolite-dacite. Three monotonous intermediate cooling units of relatively uniform phenocryst-rich dacite were erupted in rapid succession at 27.57 Ma; they have an estimated aggregate volume of 4500 km 3 . These “main-trend” rhyolite and dacite ignimbrites were derived from relatively low-temperature (700–800 °C), water-rich magmas that equilibrated a couple of log units more oxidized than the QFM (quartz-fayalite-magnetite) oxygen buffer with an assemblage of plagioclase, sanidine, quartz, biotite, Fe-Ti oxides, zircon, and apatite with or without hornblende, pyroxene, and titanite at depths of ∼8–12 km. Magmas were created in unusually thick crust (∼60 km) as large-scale inputs of mantle-derived basaltic magma powered partial melting, assimilation, mixing, and differentiation processes. “Off-trend” ignimbrites include cooling units of the 600 km 3 trachydacitic Isom-type tuffs that contain sparse phenocrysts of plagioclase, clino- and ortho-pyroxene, and Fe-Ti oxides derived from drier and hotter magmas. These magmas erupted immediately after the monotonous intermediates, from ca. 27 to 23 Ma, and were derived by fractionation from andesitic differentiates of the mantle-derived magmas in the deeper crust. Younger, off-trend rhyolitic magmas possessed some of the same unusually high TiO 2 , K 2 O, Zr, and Ba contents as those of the Isom type and may be rhyolitic differentiates of Isom-type trachydacites or rhyolitic melts contaminated with Isom-type magma. The distinctive couplet of monotonous intermediates and trachydacitic Isom-type tuffs in the Central Nevada field is found in much greater volume in the coeval Indian Peak–Caliente field to the east, where monotonous intermediates have an extension corrected volume of 12,300 km 3 and Isom-type tuffs have a volume of 4200 km 3 . However, in the rhyolite-dominant Western Nevada field to the west, monotonous intermediates have not been recognized and trachydacitic Isom-type tuffs occur in only very small volumes, probably no more than 50 km 3 total. These composition-volume contrasts appear to be related to the crustal thickness that diminished westward during the middle Cenozoic ignimbrite flareup. The distinctive couplet of ignimbrites has not been recognized elsewhere, to our knowledge, in the flareup fields in southwestern North America. Extrusion of intermediate-composition lavas at the inception of the ignimbrite flare up in the northeastern part of the Central Nevada field created large lava piles. Later extrusions from 33 to 24 Ma were virtually absent but modest activity resumed thereafter and persisted until the end of the ignimbrite flareup. All together, the volume of andesitic lava is less than one-tenth the volume of contemporaneous silicic ignimbrite; like proportions occur in the ignimbrite fields to the west and east in the southern Great Basin ignimbrite province. This small proportion, together with the absence of basalt lavas, reflects the unusually thick crust in which silicic magmas were being generated during the ignimbrite flareup. In sharp contrast, flareups in volcanic fields elsewhere in the southwestern U.S. resulted in subordinate ignimbrite relative to lavas.

Time-transgressive and extension-related basaltic volcanism in southwest Utah and vicinity

Late Cenozoic (<15 Ma) basaltic rocks define two temporally and spatially distinct magmatic belts in southwest Utah that are located in the Basin and Range Province, the transition zone, and into the Colorado Plateau interior. The belts share certain time-space-composition characteristics and are dissimilar in others. The north-south Black Rock–Grand Canyon belt (generally 2.5–0 Ma) exhibits no province-wide time-space patterns, but shows distinct space-composition variations involving both lithospheric and asthenospheric magma sources. Magmatism is inferred to have occurred in response to east-west extension, and the composition of the basalts is compatible with maximum thinning and extension by pure shear of the mantle lithosphere occurs near the transition zone. In contrast, the Pahranagat–San Rafael belt (15–3.5 Ma) exhibits a northeastward time-space progression of magmatism. The magmatic migration vector is similar to the Yellowstone trend, suggesting the ultimate influence of a fixed heat or magma source in the asthenosphere, although the basalts record apparent lithospheric signatures. Basalts from this belt are progressively more isotopically primitive and more alkaline into the Colorado Plateau. These chemical variations can be interpreted as a change in the mantle source, depth or degree of partial melting, or decreasing crustal contamination to the northeast.

Tertiary Minette and Melanephelinite Dikes, Wasatch Plateau, Utah - Records of Mantle Heterogeneities and Changing Tectonics

A swarm of minette and melanephelinite dikes is exposed over 2500 km2 in and near the Wasatch Plateau, central Utah, along the western margin of the Colorado Plateaus in the transition zone with the Basin and Range province. To date, 110 vertical dikes in 25 dike sets have been recognized. Strikes shift from about N80°W for 24 Ma dikes, to about N60°W for 18 Ma, to due north for 8–7 m.y. These orientations are consistent with a shift from east-west Oligocene compression associated with subduction to east-west late Miocene crustal extension. Minettes are the most common rock type; mica-rich minette and mica-bearing melanephelinite occurs in 24 Ma dikes, whereas more ordinary minette is found in 8–7 Ma dikes. One melanephelinite dike is 18 Ma. These mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions; they appear to have crystallized from primitive magmas. Major, trace element, and Nd-Sr isotopic data indicate that melanephelinite, which has similarities to ocean island basalt, was derived from small degree melts of mantle with a chondritic Sm/Nd ratio probably located in the asthenosphere, but it is difficult to rule out a lithospheric source. In contrast, mica-bearing rocks (mica melanephelinite and both types of minette) are more potassic and have trace element patterns with strong Nb-Ta depletions and Sr-Nd isotopic compositions caused by involvement with a component from heterogeneously enriched lithospheric mantle with long-term enrichment of Rb or light rare earth elements (REE) (epsilon Nd as low as −15 in minette). Light REE enrichment must have occurred anciently in the mid-Proterozoic when the lithosphere was formed and is not a result of Cenozoic subduction processes. After about 25 Ma, foundering of the subducting Farallon plate may have triggered upwelling of warm asthenospheric mantle to the base of the lithosphere. Melanephelinite magma may have separated from the asthenosphere and, while rising through the lithosphere, provided heat for lithospheric magma generation. Varying degrees of interaction between melanephelinite and small potassic melt fractions derived from the lithospheric mantle can explain the gradational character of the melanephelinite to minette suite.

Age and petrogenesis of volcanic and intrusive rocks in the Sulphur Spring Range, central Nevada: Comparisons with ore-associated Eocene magma systems in the Great Basin

Widespread base- and precious-metal anomalies, oxidized sulfi de veins, silicifi ed calcareous shales and carbonates, and altered porphyry intrusions occur in the northeastern Sulphur Spring Range, Nevada, 80 km south of important gold deposits in the Carlin trend. The small historic mines and prospects in the area are spatially and perhaps genetically related to a suite of variably altered dikes, small lava fl ows, silicic domes, and related pyroclastic rocks. New major- and trace-element data and U-Pb zircon ages show that the East Sulphur Spring volcanic suite is Eo-Oligocene in age (36‐31 Ma) and ranges in composition from high MgO- basaltic andesite to peraluminous rhyolite. The major- and trace-element compositions of the volcanic rocks are characteristic of continental margin subduction zone magmas and form a high-K, calc-alkaline suite with low Fe/Mg ratios. In addition, the rocks have negative Nb and Ti anomalies and elevated Ba, K, and Pb on normalized trace-element diagrams. Crustal melting is indicated by the eruption of a peraluminous garnet-bearing ignimbrite and as a component in hybridized andesite. The nature of this suite and its potential for mineralization is elucidated via comparisons to other Eocene age volcanic rocks associated with much larger gold and copper deposits in the Great Basin. The East Sulphur Spring suite is more similar to Eocene igneous rocks found along and near the Carlin trend than it is to those erupted while the Bingham porphyry copper deposit developed 300 km farther to the east. For example, the East Sulphur Spring suite and the Eocene magmatic rocks along the Carlin trend are less alkaline than the Bingham suite and lack its unusual enrichment of Cr, Ni, and Ba in intermediate composition rocks (58‐68 wt% silica). Nonetheless, the Bingham and East Sulphur Spring volcanic suites both preserve evidence of mixing that created intermediate compositions. For example, an andesite has obvious mineral disequilibrium with plagioclase, biotite, clinopyroxene, orthopyroxene, olivine, and amphibole coexisting with extensively resorbed megacrysts of quartz, K-feldspar, and garnet—indicative of mixing basaltic andesite or andesite and largely crystallized garnet-bearing rhyolite. On the other hand, we found no evidence for mixing with a mafi c alkaline magma like that in the Bingham Canyon magma-ore system. We conclude that: (1) an unusual tectonic setting prevailed during the Eocene and Oligocene of the western United States that promoted the production of oxidized mafi c magma in an arclike setting, but far inland as a result of the rollback of the Farallon slab; (2) the mafi c magmas intruded or erupted separately, or mixed with more silicic magma generated by fractional crystallization and assimilation of crustal materials; and (3) these mafi c magmas may have delivered signifi cant amounts of sulfur and chalcophile metals to upper crustal magma chambers and eventually to Paleogene ore deposits in the eastern Great Basin.