ALBERT

Description: 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 (〈200 km 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 flareup 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.