The 36–18 Ma Southern Great Basin, USA, Ignimbrite Province and Flareup themed issue The 36–18 Ma Indian Peak–Caliente ignimbrite fi eld and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions Myron G. Best1, Eric H. Christiansen1, Alan L. Deino2, Sherman Gromme3, Garret L. Hart4, and David G. Tingey1 1Department of Geological Sciences, Brigham Young University, Provo, Utah 84602-4606, USA 2Berkeley Geochronology Center, Berkeley, California 94709, USA 3420 Chaucer Street, Palo Alto, California 94301, USA 4Pacifi c Northwest National Laboratory, Richland, Washington 99352, USA Dedicated to J. Hoover Mackin, who initiated study of the Indian Peak–Caliente ignimbrite fi eld with his rec- ognition in the 1950s that the “lava fl ows” near Cedar City are actually widespread ignimbrites, including the unusual trachydacitic Isom-type tuffs and the colossal Needles Range monotonous intermediates. ABSTRACT of meters. Outfl ow ignimbrite sequences com- sibly kindred, phenocryst-rich latite- prise as many as several cooling units from andesite ignimbrite with an outfl ow volume The Indian Peak–Caliente caldera complex different sources with an aggregate thickness of 1100 km3 was erupted at 22.56 Ma from a and its surrounding ignimbrite fi eld were locally reaching a kilome ter; sequences are concealed source caldera to the south. a major focus of explosive silicic activity in almost everywhere conformable and lack 2. Trachydacitic Isom-type tuffs. Also rela- the eastern sector of the subduction-related substantial intervening erosional debris and tively uniform but phenocryst poor (<20%) southern Great Basin ignimbrite province angular discordances, thus manifesting a with plagioclase >> clinopyroxene ≈ ortho- during the middle Cenozoic (36–18 Ma) lack of synvolcanic crustal extension. Fallout pyroxene ≈ Fe-Ti oxides >> apatite. These ignimbrite fl areup. Caldera-forming activity ash in the mid-continent is associated with alkali-calcic tuffs are enriched in TiO2, K2O, migrated southward through time in response two of the super-eruptions. P2O5, Ba, Nb, and Zr and depleted in CaO, to rollback of the subducting lithosphere. Ignimbrites are mostly calc-alkalic and MgO, Ni, and Cr, and have an arc chemical Nine partly exposed, separate to partly over- high-K, a refl ection of the unusually thick signature. Magmas were erupted from a con- lapping source calderas and an equal num- crust in which the magmas were created. cealed source immediately after and just to ber of concealed sources compose the Indian They have a typical arc chemical signature the southeast of the multicyclic monotonous Peak–Caliente caldera complex. Calderas and defi ne a spectrum of compositions that intermediates. Most of their aggregate out- have diameters to as much as 60 km and are ranges from high-silica (78 wt%) rhyolite to fl ow volume of 1800 km3 was erupted from fi lled with as much as 5000 m of intracaldera andesite (61 wt% silica). Rhyolite magmas 27.90 to 27.25 Ma. Nothing like this couplet tuff and wall-collapse breccias. were erupted in relatively small volumes of distinct ignimbrites, in such volumes, have More than 50 ignimbrite cooling units, more or less throughout the history of activ- been documented in other middle Ceno- including 22 of regional (>100 km3) extent, ity, but in a much larger volume after 24 Ma zoic volcanic fi elds in the southwestern U.S. are distinguished on the basis of stratigraphic in the southern part of the caldera complex, where the ignimbrite fl areup is manifest. position, chemical and modal composition, creating ~10,000 km3 of ignimbrite. Magmas were created in unusually thick 40Ar/39Ar age, and paleomagnetic direction. The fi eld has some rhyolite ignimbrites, crust (as thick as 70 km) where large-scale The most voluminous ash fl ows spread as far the largest of which are in the south and were inputs of mantle-derived basaltic magma as 150 km from the caldera complex across a emplaced after 24 Ma. But the most distinc- powered partial melting, assimilation, mix- high plateau of limited relief—the Great Basin tive attributes of the Indian Peak–Caliente ing, and differentiation processes. Dacite and altiplano, which was created by late Paleozoic fi eld are two distinct classes of ignimbrite: some rhyolite ignimbrites were derived from through Mesozoic orogenic deformation and 1. Super-eruptive monotonous intermediates. relatively low-temperature (700–800 °C), crustal thickening. The resulting ignimbrite More or less uniform and unzoned deposits water-rich magmas that were a couple of log fi eld covers a present area of ~60,000 km2 of dacitic ignimbrite that are pheno cryst rich units more oxidized than the quartz-fayalite- in east-central Nevada and southwestern (to as much as ~50%) with plagioclase > bio- magnetite (QFM) oxygen buffer at depths of Utah. Before post-volcanic extension, ignim- tite ≈ quartz ≈ hornblende > Fe-Ti oxides ± ~8–12 km. In contrast to these “main-trend” brites had an estimated aggregate volume sanidine, pyroxene, and titanite; apatite magmas, trachydacitic Isom-type magmas of ~33,000 km3. At least seven of the largest and zircon are ubiquitous accessory phases. were derived from drier and hotter (~950 °C) cooling units were produced by super-erup- These tuffs were deposited at 31.13, 30.06, magmas originating deeper in the crust (to as tions of more than 1000 km3. The largest, and 29.20 Ma in volumes of 2000, 5900, and deep as 30 km) by fractionation processes in at 5900 km3, originally covered an area of 4400 km3, respectively, from overlapping, andesitic differentiates of the mantle magma. 32,000 km2 to outfl ow depths of hundreds multicyclic calderas. A unique, and pos- “Off-trend” rhyolitic magmas that are both Geosphere; August 2013; v. 9; no. 4; p. 864–950; doi:10.1130/GES00902.1; 78 fi gures; 8 tables; 6 supplemental fi les. Received 16 January 2013 ♦ Revision received 13 February 2013 ♦ Accepted 16 February 2013 ♦ Published online 13 June 2013 864 For permission to copy, contact [email protected] © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/864/3346014/864.pdf by guest on 29 September 2021 Indian Peak–Caliente ignimbrite fi eld younger and older than the Isom type but of the explosive activity in the Great Basin constitute the Indian Peak–Caliente ignimbrite possessed some of their same chemical char- was the Indian Peak–Caliente caldera com- fi eld, the subject of this article. acteristics possibly refl ect an ancestry involv- plex from which over 50 eruptions (Table 1) As is true for volcanism generally through- ing Isom-type magmas as well as main-trend broadcast ash fl ows to at least 150 km distant out the Great Basin, caldera-forming eruptions rhyolitic magmas. in southwestern Utah and southeastern Nevada in the Indian Peak–Caliente ignimbrite fi eld Andesitic lavas extruded during the to cover a present area of ~60,000 km2. The migrated southward through time (Best and fl are up but mostly after 25 Ma constitute aggregate volume of the eruptions is estimated Christiansen, 1991). (In our usage, the Great a roughly estimated 12% of the volume of to be 32,600 km3; at least seven had volumes Basin [Fig. 1] encompasses western Utah and silicic ignimbrite, in contrast to major vol- of more than 1000 km3 (Table 2) and, thus, nearly all of Nevada, rather than just the smaller canic fi elds to the east, e.g., the Southern qualify as super-eruptions (Miller and Wark, hydrographic basin.) It is generally agreed that Rocky Mountain fi eld, where the volume of 2008; de Silva, 2008; volumes are corrected this southward sweep in volcanism through- intermediate-composition lavas exceeds that for an assumed uniform 50% east-west crustal out the Great Basin was a result of southward of silicic ignimbrites. extension post-dating volcanism; see below). steepening in dip, or rollback, of a formerly The eruptions created six partly exposed, “fl at” subducting oceanic lithosphere that had INTRODUCTION mostly overlapping source calderas to the extended far inland from the continental margin north and three to the south (Fig. 2). Some during the early Cenozoic. Volcanic rocks older The southern Great Basin ignimbrite prov- calderas are nested, or multicyclic. Several than ca. 18 Ma in the Great Basin bear an arc ince (Fig. 1) comprises on the order of 250 additional source calderas have not been accu- chemical signature indicative of their subduc- cooling units of silicic ash-fl ow tuff and 43 rately located because of concealment beneath tion-related origin. Younger volcanic rocks lack at least partly exposed calderas formed dur- younger deposits; some were engulfed in this characterizing signature and were formed ing the middle Cenozoic ignimbrite fl areup in younger calderas. The calderas and their sur- during a subsequent extensional tectonic regime southwestern North America. A major focus rounding related outfl ow ignimbrite sheets supplanting the earlier subduction regime. Figure 1. Middle Cenozoic, southern Great Basin ignim- brite province in Nevada and Indian Peak- Caliente southwestern Utah resulting Silicic Ignimbrite Andesite Lava Western NV Central NV from the 36–18 Ma ignimbrite caldera caldera caldera fl areup (modifi ed from Stewart 42° and Carlson, 1976). The prov- California ince is divided into three parts: Nevada Utah the Western Nevada (NV) fi eld and calderas (blue), the Central 86 Sri = 0.706 Nevada fi eld and calderas (red), Sr/ 87 Salt Lake City and the Indian Peak–Caliente GREAT Elko fi eld and calderas (green). The western edge of the Precam- SIERRA BASIN brian continental basement is 40° indicated by the dashed initial 87 86 Sr/ Sr = 0.706 line (modifi ed Reno Austin from Wooden et al., 1999). Just Ely to the east, the yellow band Marysvale marks the approximate posi- tion of the western lip of the middle Cenozoic Great Basin altiplano (Best et al., 2009).