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Home , Fish Canyon Tuff, Great Basin, Ignimbrite

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120°W 117°W 114°W 111°W 109°W

50 0.706 Line 40 41°N

Utah

t l

Salt e

Lake b t

City s u

e r

n Reno 50 h Figure 4. Major fold-and-thrust belts in i l t the Great Basin of Nevada and western

e d

50 g n (Oldow et al., 1989; McQuarrie and n a Austin i Chase, 2000; DeCelles, 2004; Long et al.,

h d 40 l 2014) and hypothetical contours (in km) Ely o 39°N Figure 4 is interactive. You can view different f of early Cenozoic (Paleogene) crustal h r items in the legend by moving the cursor over c e thickness (Coney and Harms, 1984). The t i a v them or you can toggle the symbols on and s e western edge of the Precambrian conti- off with the Layers panel in Adobe Acrobat or a S nental basement near 117°W is indicated W f o 87 86 Adobe Reader. t by the black dashed Sr/ Sr0 = 0.706 line n (modified from Wooden et al., 1999). The ro Cedar F Wasatch hinge line passes approximately Tonopah City Caliente through and Cedar City, Utah. Figure is modified from Best et al. (2009, their fig. 1).

37°N 60

Las Vegas Thrust faults and folds

50 Paleogene crustal thickness (km)

0 50 100 150 200 km

compositions took their place (Christiansen et al., 2007a, their fig. 11). The ex- Absence of Significant Regional Tectonic Extension during the tension-related bimodal suite included both aluminous and peralkaline rhyo- Flareup lites (e.g., Farmer et al., 1991). During the ignimbrite flareup, extrusion of inter­ mediate-composition, chiefly andesitic, was an order of magnitude smaller Controversy surrounds the time when the orogenically thickened crust in volume than silicic explosive eruptions (Fig. 2; Best et al., 2013b, 2013c). in the Great Basin was subjected to significant extensional faulting In western Utah and eastern Nevada, southward-sweeping magmatism and thinning to its current thickness of ~30 km (Allmendinger et al., 1987). (ca. 45–18 Ma) is expressed by more or less separate, subparallel, roughly east- The timing of extensional thinning of the crust—before, during, or after the west belts of volcanic rocks and minor granitic intrusions (e.g., Stewart and ignimbrite flareup—has a critical bearing on the role of crustal thickness in the Carlson, 1976). The greatest volume of mid-Cenozoic volcanic rocks occurs in ignimbrite flareup. a swath of mostly mountain-range exposures of silicic ignimbrite and lesser Prevolcanic extension has been advocated by, for example, DeCelles (2004, andesitic­ extending from the southwestern corner of Utah westward p. 149), who concluded that, in view of the excess gravitational potential en- across the southern Great Basin and beyond into the (Fig. 2). ergy residing in a thick orogenic plateau, “Within limits of available tempo- This is the 36–18 Ma southern Great Basin ignimbrite province. ral resolution, hinterland extension and frontal thrusting were coeval in the

GEOSPHERE | Volume 12 | Number 4 Best et al. | Slab-rollback ignimbrite flareups 1101 Research Paper

Rock Compositions 11 A I Indian Peak-Caliente n = 466 C Central Nevada ignimbrites n = 433 ignimbrites of all ages (36–18 Ma) occur throughout the province I I and are accompanied by temporally more restricted and trachydacite 10 CI I I Trachydacite CI I I I I ignimbrites, especially in the eastern sector (Fig. 7). Most ignimbrites contain I I I I CII I II IIIIII I I I I Isom Fm. C I I II I I I III I I IIII C variable proportions of plagioclase, sanidine, , biotite, hornblende, Fe-Ti I II I II CI II I I I II C I IC I I I ICI I C ) n=69 I C C I I oxides, , and trace amounts of zircon, apatite, and, rarely, titanite, 9 I I CI I I C I C CIII II C I I C C I CI C C C C C that equilibrated in relatively wet at shallow crustal depths of 7–9 km. C IC I C I C I CCCC C C CI I C I I II CI I C CC CCCIC C w t % I C I CICC CC C CI I C ICCCC C CC C ( I C C CCC I CC CCC ­ show -rich compositions, upward of 50% of the on dense I CC I CCC I C CICCCCI IC I CC O I I CI I I II C CCIC CCCI CICCIC I CCICI

2 C rock equivalent basis (Best et al., 2013b); nearly all formed by super-eruptions, 8 I I I CII I II I C CI C CI C ICCC CCCCC C C C I CCCC I ICC CCCCCCC K I I I I C I C CC C C C I C I I I CI C CI I CI CCI CC I CI CC C CCCC C and their relatively unzoned, uniform nature qualifies them as monotonous + C I CI CI CC Latite II I II CI II ICI I C C C C CCCC C CC C O I II I I IC CC IC I IC CCC C C 2 I I I I C C CC CC I I C C I intermediates in the sense of Hildreth (1981). These dacite magmas are inter- II IICI C I I IIIC IC I CI I CCCCCC CC I I I I II I C CI I I C C CC C CCIC N a I I I I I I I I C I C preted to have originated by mixing of andesitic and rhyolitic magmas in the 7 C I III I I I CCICI IC I I I II IIIC CI C C C C I I I C I I II II II I I ICIIIIC III I I C C C C I I CI IIIIIII II III C I CC I C C C C CC C deeper crust (Best et al., 2013b). Unusual trachydacites (Best et al., 2013b) have I I II I II I II II I IIII I I II I IC I CI C CC C I I I I IIII III III IIII I I II I CI C C C sparse (<15%) phenocrysts of plagioclase, two pyroxenes, and Fe-Ti oxides that IIII I I II IIC I II I I III I CC I C I IIIII IIII I II III I II I I I C I I C C II II II I I C were derived from drier, hotter magmas equilibrated at greater crustal depth. 6 I CII I I I III CI I II C C Stone Cabin I I I II I I CI C C C Andesite I III I I C C n=52 These Isom-type tuffs (Figs. 7–9) have >300 ppm Zr and high TiO2 /CaO ratios. I II I I C I I I Dacite Ignimbrites are alkalic to calcic (Fig. 8); their high-K to shoshonitic nature I C Rhyolite (Fig. 9) is consistent with the thick crust in which the magmas originated. 5 Intermediate-composition lavas in the southern Great Basin province are 60 62 64 66 68 70 72 74 76 78 80 mostly high-K andesite (Best et al., 2009). Notably, their volume is an order SiO2 (wt %) Figure 7 is interactive. You can view different of magnitude less than that of silicic ignimbrite (Table 1; Stewart and Carlson, items in the legend by moving the cursor over 1976; Best et al., 2013b, 2013c). As previously indicated, is absent until 11 B them or you can toggle the symbols on and after ca. 20 Ma. off with the Layers panel in Adobe Acrobat or ignimbrites Lavas and ignimbrites have an arc geochemical signature—wet, oxidized, w undivided n = 219 Adobe Reader. 10 Campbell Creek with low Fe/Mg ratios, enrichments of fluid-soluble elements, and depletions Isom-type n = 13 Nine Hill n = 79 n = 36 w w of high field strength elements, producing high Ba/Nb ratios (Best et al., 2013b, w www w www w 2013c; Henry and John, 2013). Sr and O isotopic ratios are high and consistent ) Trachydacite w wwwww w 9 w w w w w www ww with the assimilation of large proportions of felsic crust in the eastern sector w w w w ww ww www www w w w wI w w w I w wwwwwwwww w w t % Iww w ww w ww ww

of the Great Basin province; Sr isotope ratios are much lower in the western ( www ww w w ww w I w wwwwww w ww wwwwww w O ww w ww ww wwwwww ww sector, where the crust is younger and more (Figs. 5 and 10). 2 w w w w ww www ww 8 w ww w www w ww K w w w w w w I w ww ww w w w www www ww

+ w w w ww wwwww w w O w w 2 ww Three Sectors w w w w wwww

N a 7 w w w Caetano and The southern Great Basin province is conveniently divided into three Cove Mine n = 120 contrasting sectors, each corresponding to discrete clusters of sur- rounded by more or less separate ignimbrite outflow fields (Fig. 6; Table 1). In 6 Rhyolite the eastern sector, the Indian Peak–Caliente field (Best et al., 2013b) surrounds Andesite Dacite the nested calderas of the Indian Peak–Caliente complex astride the 5 Utah-Nevada state line. In the central sector of the southern Great Basin prov- ince, the Central Nevada caldera complex of nested sources is surrounded 60 62 64 66 68 70 72 74 76 78 80 by the Central Nevada field, the defining ignimbrite outflow sheets of which SiO2 (wt %) partly overlap those of the Indian Peak–Caliente field. All but one ash flow from Figure 7. International Union of Geological Sciences (IUGS) classification (Le Maitre, 1989) for Central Nevada caldera sources lie entirely to the east of a north-south topo- southern Great Basin ignimbrites. Red line separates alkalic (above) from calc-alkaline (below) graphic barrier, or drainage divide, near the western edge of the Precambrian . (A) Central Nevada and Indian Peak–Caliente fields (central and eastern sectors). Nearly all the trachydacite ignimbrites are Isom type (yellow shade). (B) Western Nevada field (western basement (Figs. 2, 3, and 6; Best et al., 2009, 2013c). In the western sector of sector). For clarity, fields for four much-analyzed ignimbrites are shaded. Many trachydacites in the Great Basin province, the Western Nevada ignimbrite field surrounds 23 this sector are not Isom type, and dacites and calc-alkaline rhyolites are less common than in A.

GEOSPHERE | Volume 12 | Number 4 Best et al. | Slab-rollback ignimbrite flareups 1105 Research Paper

10 known source calderas that are more widely scattered and apparently smaller A than those to the east. C In the western sector, dacites are rare, and rhyolites dominate (Figs. 7, I I I I II C I I I III II I II C C CCI 11, and 12); subordinate trachydacites have mineral assemblages that equili­ 8 CI I II I I I I II III CI II IC CC III I I I ICIIIIC IC CCCCCC CC CI C Isom Fm. III IIII I CI I C C CCCC CIC ICCICC C brated—like rhyolites—in relatively wet magmas. Notably, ignimbrites are I III CI C CC CCICC C CICCCCCCCI CCCC I I I I C C I C II CCCCICICICCIICCIC CICCCCCCCCC ) II CI CI I C IC CI C CC CCCC CCCCCCCC I relatively more alkalic overall (Figs. 8 and 9) because of greater concentra- I C CIC CIC I III C II CICCCCCCI CI I CCCCCCI I C I I C C IICCCCC C CCCCC CCI CCCCCCCC C CC II I I I C CCI I I CC CIC CCC C C C tions of Na2O. In contrast, all trachydacites to the east are Isom type. These 6 I I CCI C I CI CI C CC CI I CC ICCC C C w t % I I I I C ICCII I I I IC CIIIC CCICICC CCCC C I tuffs are especially voluminous in the eastern sector, where immediately older ( Alkaline I C C C CC IC C CCI I C C C ICI I CCC C C C C CC C O C ICI ICIIIC C CC C C C phenocryst-rich dacites, or monotonous intermediates, are the dominant ig- I I C CI CC I C ICI IICC CCC C C I I II ICC I CIIICICCICCC IC C C CC C a CC I ICI C I I I C II C I CC C C C C nimbrite. In the central sector, both Isom-type tuffs and monotonous interme- - I I C CCII II I C 4 II I I I C CIC CI CIII ICIIC C CC I C CC C O I C C I I IIICI CCI CCI I CI CI I CC C C C diate ignimbrites are less voluminous than to the east. 2 Alkali- C I I II I CICIICI C C C I C I I I I CIICIIC IC ICI C CI C C C I I I II II CI IIIICICCCI I ICI I I C I C C Stone Figure 13A reveals further details of the ignimbrites in the eastern sector. calcic I C I II I ICIII I II CCI C I I C C + K I III II III CI II I I I I III I IICCI C C Cabin After a few million years of small, precursory rhyolite eruptions, three super-­ O I II I I III IIII I CI ICI CCI I 2 2 III I II II I CI IIIIII II I I I ICI I I C eruptions totaling 12,300 km3 of monotonous intermediate magmas occurred CI I III IIIIIIIIIIIII CI III IIC ICI N a Calc- I I I I III II II II I II C CC I from 31.1 to 29.2 Ma (all 40Ar/39Ar ages are based on an age of 28.20 Ma for I III II I IIII I alkalineI I ICIII I C I C Central Nevada I CI I I I the ). The source calderas of these three eruptions overlap 0 I C I ignimbrites n = 433 I (Fig. 14), indicating a sustained and narrowly focused supply of a large volume C I Indian Peak-Caliente I I of crystal-rich dacite to the shallow crust in just a few million years. Calcic I ignimbrites n = 466 Following this burst of activity, at least four Isom-type trachy­dacite ignimbrites –2 totaling 3600 km3 were erupted from a concealed source just to the south- 59 61 63 65 67 69 71 73 75 77 east at 27.9 to 27.3 Ma (Fig. 6). After a hiatus in explosive activity of ~4 m.y.,

Figure 8 is interactive. You can view different SiO2 (wt %) voluminous­ eruptions from nested calderas in the Caliente complex (Fig. 6) items in the legend by moving the cursor over Campbell Creek n = 36 ­occurred episodically until ca. 18 Ma; rhyolite dominated, but a super-­eruption them or you can toggle the symbols on and 10 of 2200 km3 at 22.6 Ma created an unusual phenocryst-rich, ande­site-latite off with the Layers panel in Adobe Acrobat or B Nine Hill Adobe Reader. ignimbrite­ that may be a monotonous intermediate unit. n = 79 w ww w ww w wwwwwww ww The central sector of the province (Fig. 13B) shares aspects with the adjacent ww wwwwwwwwww w www 8 w wwwwwww w wwww sectors to the west and east. To the west, most eruptions were of rhyolite, but, w wwwwwwwwwwwwwwwwww w ww w w w wwww ww in the central sector, three super-eruptions totaling 4500 km3 of monotonous

) w w w w w wwww w w w w ww w wwwwwwwwww 3 Alkaline ww www w w w wwww intermediate dacite occurred in rapid succession at 27.6 Ma. Only about 600 km 6 w w w wI www w w ww w t % w wwww w ww of slightly younger Isom-type trachydacite is recognized. In lieu of an earlier ( w w www w w w O w w w Caetano & monotonous intermediate ignimbrite paralleling the brief burst of this activity w w w w w in the eastern sector, 4800 km3 of zoned rhyolite-dacite erupted at 31.7 Ma. C a 4 Cove Mine

- Alkali- n = 120 As previously noted, during the mid-Cenozoic ignimbrite flareup, the O calcic ww 2 crust in the western sector of the southern Great Basin province was thinner

+ K 2 Calc- (~50 km), younger, and more mafic (accreted Phanerozoic oceanic terranes) O

2 than the two eastern sectors, which were founded on thicker Precambrian alkaline Western Nevada ignimbrites N a metamorphic-granitoid­ basement covered by a thick wedge of sedimentary 0 w undivided n = 219 rock (Fig. 3). How these east-west contrasts in the crust influenced the con- Isom-type n = 13 Calcic trasting nature of the ignimbrites in the province will be considered after a –2 review of other Cenozoic volcanic fields. 59 61 63 65 67 69 71 73 75 77 SiO2 (wt %) COMPARISONS WITH OTHER CENOZOIC VOLCANIC FIELDS Figure 8. Modified alkali-lime index (Frost et al., 2001) for southern Great Basin ignimbrites. (A) Central Nevada and Indian Peak–Caliente fields. Nearly all alkaline and alkali-calcic samples Mid-Cenozoic fields in southwestern (Fig. 1; Table 2) devel- are trachydacitic Isom-type ignimbrites (yellow shade). (B) Western Nevada field. Four ignimbrite oped on continental crust above subducting oceanic lithosphere (e.g., Sever- units are shaded, as in Figure 7. Calcic ignimbrites are absent; compare with A. inghaus and Atwater, 1990; DeCelles, 2004; Dickinson, 2006; Humphreys, 2009).

GEOSPHERE | Volume 12 | Number 4 Best et al. | Slab-rollback ignimbrite flareups 1106 Research Paper

13 A 8 C Central Nevada ignimbrites n = 433 I Indian Peak-Caliente ignimbrites n = 466 C 12 7 I I 11 C I II I I C Central Nevada I I I C I il) I III II I II C 6 I I II I I CII C C m Isom Fm. I IIII IIIII I III I I III I I IC I I C CCCCCCI C Western I I I I IIII I C II CII I I CC CCCC er 10 Indian Peak I II II I I I I ICICI CICCCCCCCCCICCCIC CCC Nevada I I I C I ICCCI CI CICC C CICC CC Grizzly Pk. Altiplano I IC I I CCICCI CCICCI CCCCCICICCCCCCCICCICC I I CCCI C CCCI CCI I CCCC CCCCICCI CCCCCCCCCIC O (p 5 I I I ICCCIC C CCIC I I CCCCCCCICIC CCC CICICCCCCCICCICCCCCCI C 9 CeC rro Galan -Puna ) C I III C C C Shoshonitic I C CIII C I CI C I CIC CICCICCCI CICCCCCICICC CCCCCC C C I CI I II I C I CI CCCC C C C a 18 % I II C CCC IC I CCI ICCCCC I CCC CCCC CCCIC I I II CIC III CII ICICI ICII CCICCCI CC CCCC C C Latir t I II IICCCIIICIICIIIICIC IC C C C I I C I IICCCIICCI I CCIICII IIICICI ICCIC IC I CCI CC C Amalia T. w I I C I I CI I C 4 CI I I III I ICI ICCIC C C C C delt 8 ( I I I II I II I I IICI CCI CC San Juan I II I II I IIIICI I II CCCII C I C C C II IIII IIIII IIIII CCCICCCCCICC IC IIC CC C I tz I I II IICIII II C II CI I I C O C I IIIIII III IIIII IIII IICICI I I CCCCC I C 2 I ICIIIIIIIII III ICCI CC I I I

I I I uar II I IIIIIIIII II I II I II I I C I I CIC IC I 7 K I I I I I C I C C I I I Q 3 I I I IIICI CI I C I C High-K I I I I I C I II I I I I I I 6 I I 2 I I 5 Medium-K 1 4 Plain 0 3 59 61 63 65 67 69 71 73 75 77 0.7040.706 0.7080.710 0.712 0.7140.716 SiO 2 (wt %) 87 Sr/ 86 Sr i Figure 9 is interactive. You can view different items in the legend by moving the cursor over Figure 10. Sr-O isotope relations for quartz-bearing rhyolites and dacites from flareup volcanic them or you can toggle the symbols on and B 8 fields. Peraluminous tuffs from the Altiplano-Puna are shown as light blue ellipses. Most of the off with the Layers panel in Adobe Acrobat or Western Nevada ignimbrites rocks from the Southern Rocky Mountain volcanic field fall in the boxes for the San Juan and Adobe Reader. w undivided n = 219 Latir centers, but the Grizzly Peak Tuff from the northern part of the field and the peralkaline 7 Isom-type n = 13 Nine Hill Amelia Tuff from southern have distinctly higher initial Sr isotope ratio. Data are from: Indian n = 79 Peak and central Nevada—Best et al. (2013a, 2013b), Hart (1997), and Larson and Taylor (1986); 6 w w w for western Nevada—Henry et al. (2013) and Colgan et al. (2013); San Juan focus—Larson and w w ww Taylor (1986) and Lipman et al. (1978); Grizzly Peak—Johnson and Fridrich (1990); Latir—John- ww w w 18 w wwwwwwwww ww w w son et al. (1990); and the Altiplano-Puna—Kay et al. (2010, 2011) and Folkes et al. (2013). Low O w w wwww wwwwwww w rhyolites from the Central are shown for comparison (Boroughs et al., 2005; 5 w wwwww w wwwwwwwwwwwwww w wwwww wwwwwwwwwwww w Ellis et al., 2013). ) ww Iww ww w wwww w w www Shoshonitic w w w wwww www w wwwww wwwwww ww w w ww wwwww w w w ww www w w w t % 4 w w w w ( w w O w ww 2 w Campbell The mid-Cenozoic volcanic rocks in these fields possess arc chemical signatures K High-K w Creek n = 36 3 Caetano and consistent with a subduction heritage: Marysvale (Cunningham et al., 1997); Cove Mine Southern Rocky Mountain (Lipman et al., 1978; Askren et al., 1997; Bachmann 2 n = 120 et al., 2002; Parat et al., 2005); Mogollon-Datil (Bornhorst, 1980; Davis et al., Medium-K 1993); (Ferrari et al., 2007). The flareups, which totaled 1 perhaps 400,000 km3 of ignimbrite in these volcanic fields, were nearly synchro- nous from ca. 36 to 18 Ma, with the greatest eruptive volumes ca. 32 to 23 Ma. 0 59 61 63 65 67 69 71 73 75 77 and Plateau Laccoliths: No Ignimbrite Flareup SiO 2 (wt %)

Figure 9. K2O-SiO2 classification of Ewart (1982) and Le Maitre (1989). (A) Central Nevada and This modest-sized field, which contains a cluster of small calderas, lies Indian Peak–Caliente fields. Nearly all of the shoshonitic samples with <73 wt% SiO2 are trachy­ just east of the southern Great Basin ignimbrite province on the northwestern dacitic Isom-type ignimbrites. (B) Western Nevada field. Four ignimbrite units are shaded. margin of the ; it was active ca. 28–19 Ma (Table 2; Figs. 1

GEOSPHERE | Volume 12 | Number 4 Best et al. | Slab-rollback ignimbrite flareups 1107 Research Paper

A 12 B 6 Trachydacite Isom Fm. Rhyolite ○ ○ ○ ○ ○ ○ ○ 10 ○ ○ ○ ○ ○ ○ ○○ ○ 5 ○ ○ ○ ○ ○ ○ ○○○○ ○ ○○ ○○○ ○ ○ ○ ○○○ ○○ ○○ ○○○○ ○○○ ○○○○ ○ ○○○○○ ○ ○ ○ ○○ ○ ○ Latite ○○ ○ ○○○○○○○○○○ ○○○○○ ○○○ ○○○○○○○○○○○ ○○ ○○○○○ ○ ○ ○ ○○○ ○○ ○ ○ ○ ○○○○○○○○ ○ ○○○○ ○○○○○ ○ ○ ○○○○○ ○ ○○ ○○ ○ ○○ ○○○○ ○ ○○○○ ○○○○○○ ○○○○○○○ ○○○○○ ○ ○ ○ ○ ○ ○○○○ ○ ○ ○ ○○○○ ○○○○○○○○○○○○○○○○ ○○○ ○ ○○ ○ ○ ○ ) ○○ ○ ○○○○○○○○○ ○ ○ ○ ○ ○○ ○ ○F○○ fff f○ff ○○○○○ ○○○○ ○○○○ ○ ○ ○ ○○○ ○ ○ ○○○○○F○F○○○f ○f FFf○f○fffffff○f ○○ ○ ○ ○ ○ ○○ ○ ○ ○○○ ○ ○ % ○ ○ ○ ○○ F○○○fF f ○ F ○F F ○ ○ ○ ○ ○ ○ ○ 8 ○○○○○○○ F○FfFFF fF Fff F ○ ○ ○○ ○○○ ○○ 4 ○○ ○○○ t ○ ○ ○○○○ ○○○○○○○F○○F○○ f○f ○ ○○○ ○○ ○○ ○ ○○ ○ ○ ○ ○○ ○○○○○F○F F ○ ○ ○ ○ F○ ○○ ○○○○ ○ w ○ ○ F ○ ○ F ○ ○ ○○ ○ ○ ) ○ F ○ ○ ( ○ ○○○ ○ ○ ○ ○○ ○ ○ ○ ○ ○ ○ ○ ○○○ ○○○○ ○ ○ ○ F F F ○ ○ ○ ○ ○ ○ % FFFfFFF ○ ○

○ ○ O ○ ○ ○ Ff○○○○ ○○ ○ ○ t f F ○ ○ F F f fF○○○ ○ ○ ○ ○ 2 ○ f f ○ ○ ○ F fFF○ FF○F ○ ○

K ○ ( w fFfff○ ○ ○ ○ ○○ 6 ○ 3 f f fff ff ○ fff ○○ ○ ○ ○ + f ○ F ○ O f ○ f f f f fff ○○ ○ a ○ ○○○ ○ ○ O ○ m. C ○ 2 ○ ○○ ○ ○ ○○○ F a ○○ ○ Southern Rocky Mountain ○○ ○ ○○ ○○ ○ om N ○ ○ ○ ○○ ○ ○ ○ ○ ○○ ○ ○○ ○ Is 4 volcanic eld ignimbrites 2 ○○ ○ ○○○ ○ ○ ○ ○ ○ ○○ ○○○○○ ○ ○○ ○ ○ ○ ○ ○○ ○ ○ Dacite ○ ○○○ ○○ ○ ○ ○ Bonanza center ○○○○○○ ○○○ ○ ○ ○○ ○○ ○○○○○○ ○ ○ ○ ○○○○○○○○○○○○ ○○ ○○ ○ ○ ○ Andesite F Fish Canyon Tu ○○○ ○○○○○○○○ ○○ ○ ○ 2 ○ ○○○○ ○ ○ ○ ○ San Juan locus 1 ○ ○○ ○○○○○○○○○ ○ ○ ○○○○○○○○○○○○○ ○ ○○ ○○○ ○ ○○○○○○○○ ○ ○ Latir volcanic locus ○○○○○ ○○ ○○○ ○ ○○○○○○ ○○○○ ○○ ○ ○○○○○○○ ○○○ ○○○ ○ ○○○○○○○ ○○ 0 0 ○○ ○○○○○○ ○ ○ 56 60 64 68 72 76 80 0.0 0.2 0.4 0.6 0.8 1.0 1.2 SiO2 (wt %) TiO2 (wt %) Figure 16 is interactive. You can view different items in the legend by moving the cursor over C 700 them or you can toggle the symbols on and off with the Layers panel in Adobe Acrobat or Adobe Isom Fm. Reader. 600 ○ ○○ ○ ○ ○ ○ ○ ○ ○ ○ ○ 500 ○ ○ ○○ ○ ○○ ○ ○ ○ ○ ○ ○ ○○ ○ ○ ○ ○ ○ ○ 400 ○ ○ ○ ) ○○ ○ ○ ○ ○ ○ ○ ○ ○ m ○ ○ ○ ○ p ○ ○ ○ ○ ○ ○ ○ ○○ p ○ ○ ○ ○○ ○ ○ ( ○ ○ ○ ○ ○○○ ○○○○○○○○ ○ ○ 300 ○○○○ ○○ ○ ○○ ○ ○○○○○○○○ ○ ○ ○ ○ Z r ○○○ ○ ○ ○○ ○○ ○ ○ ○ ○○○○○ ○○○○○○○○ ○ ○○ ○○ ○ ○ ○○○○ ○○○○○ ○○○○○○ ○ ○ ○ ○ ○ ○ ○ ○○○○ ○ ○○○○○ ○○○○ ○ ○○○ ○○ ○ ○ ○○ ○○ ○ ○○○ ○○○○○○○○ ○ ○ ○ ○ ○ ○ ○○○○○○○ ○ ○○ ○ ○○○ ○○ ○○○○○ ○ ○ ○○ F○○○FF○○f○fF○Ff○F○○ ○○ ○ ○○ ○ ○○○○○ ○○○ ○ ○ 200 ○ ○ ○FfF f○fF f○fFfFfFfFff○ff○ff○f○f○○ff○f ○ ○ ○○ ○○○○ ○ ○○○○○○○○ ○○○Fff fffff ○○○○○ ○ ○○○○ ○ ○ ○○ ○ ○ f ○ ○ ○ ○○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○○ ○ ○ ○ ○ ○ 100 ○ ○ ○ ○ ○

0 2 3 4 5 6 7 K2O (wt %)

Figure 16. Chemical compositions of ignimbrites in the Southern Rocky Mountain volcanic field. Because of alkali (mainly Na2O) enrichment, some tuffs designated as dacite on the basis of silica content by Lipman (2007) plot in the trachydacite field. Isom Fm. (yellow shade) refers to Isom-type trachydacite in the eastern sector of the Great Basin province (Fig. 7A). (A) International Union of Geological

Sciences (IUGS) classification. Green line divides alkalic rhyolite (above) from calc-alkaline (below). (B) CaO-TiO2. (C) Zr-K2O.

GEOSPHERE | Volume 12 | Number 4 Best et al. | Slab-rollback ignimbrite flareups 1113 Research Paper

TABLE 3. OUTCROP AREAS FOR VOLCANIC ROCKS A 12 IN THE MOGOLLON-DATIL FIELD Silicic tuff Andesitic lava Silicic tuff and andesitic lava (km2) (km2) (km2) Trachydacite 10 29–22 Ma 3300 5100 4700 Latite 36–31 Ma 2200 1300 2600 Total550064007300 8 Note: Measured from the geologic map of New ( Bureau of O (wt%) 2 Geology and Mineral Resources, 2003, scale 1:500,000). 6 Rhyolite O + K 2

Na Dacite The Central Andean Neogene ignimbrite province produced by the flareup 4 Mogollon-Datil volcanic eld covers the length and breadth of the Central (de Silva and Francis, Andesite Kneeling Nun Tu 1991; de Silva et al., 2006; Freymuth et al., 2015) over an area of ~250,000 2 Apache Spring Tu km2, comparable to that of the flareup in the (Fig. 19). Like the Other ignimbrites mid-Cenozoic North American flareup, several distinct fields and associated Lavas calderas developed (Fig. 19). What little is known about the entire province 0 suggests that it was time transgressive, with the age of volcanism decreas- 56 60 64 68 72 76 80 ing from north to south (de Silva, 1989; Freymuth et al., 2015). Available SiO (wt%) 2 isotopic data indicate that the older ca. 20 Ma ignimbrites in the north are less “crustal” than the younger <10 Ma ignimbrites south of 22°S (Freymuth

Figure 18B is interactive. You can view different et al., 2015). Seismic and petrologic evidence in the Central Andes indicates items in the legend by moving the cursor over B 12 the presence of delaminated slabs of lower continental crust, which are be- them or you can toggle the symbols on and off lieved to include dense garnetiferous residua from melting, as well as of with the Layers panel in Adobe Acrobat or Adobe Trachydacite Rhyolite mantle lithosphere sinking into the underlying hotter mantle (Kay, 2014); Reader. II 10 CI III I I I CI I I II CII I II some of the uplift might have been the result of delamination of dense IIIII III I I I I I III I C CI II I II CII II I I II II I II II IC I I ICC I I I I ICII I C I I I CI I I CC I I C CIIIC ICI C crustal material. C ICCI C CI CI C I CCCCCCC CC CI Latite I IC I II CC I CII C CI I CCCCICCCCCIC C I I CCCC CI CCCCI I I C CCCI CCCII CCICCCCCCCC I I I I I I CI I II I CII CC CCICC CCCI CCICICCIC CI CICICI 8 I I C C I IC I CI C CI CCCCI CC CICCCCCCCCCC I C II C I I I CI CI II I CI CCI C I CC CI CICCCCC C CCCC CC I II I II I CI II ICIIII CCC C IC CCCCICCCI C CCCCCC I I I I CI I I I CII I C II CCIC IC CC IICI CCCC C C IC I II I IICII II I CII I CI IC I CI C C I CCCCCI C CCICC Ignimbrite Flareup in the Altiplano-Puna Volcanic Complex C I IIII I I I I ICII IIC I I I I II I III C I IC C C C C I I I I CI CI I I III III II ICI ICIIIIC I II I I C I C C CCCCC CC 2 I II II IIIII III C C II C C C C I I IIII IIII III II I I IIIII I IIIII I I IC I CI CCI C CC CC I IIIIIIII IIII IIIIC I I I II I II I IICI I CC II C C I I II I I III I III IIII I II I I I I C C 6 I CIII II III I III CI I II C C A distinct flareup within the Neogene Central Andean ignimbrite prov- I I I I I I II I I CI CI C CC C I I II I I I I I C IC ince produced the most voluminous ignimbrite plateau in the southern half

2 Dacite of the province from 22°S to 24°S, the Altiplano-Puna volcanic complex (Fig. Na O + K (wt%) 4 Sierra Madre Occidental volcanic rocks 19A; de Silva, 1989). Here, the crust is now 58–76 km thick (Yuan et al., 2002; McGlashan et al., 2008). Ignimbrite eruptions began at ca. 10 Ma and contin- Eastern Great Basin 2 Andesite ued until ca. 1 Ma, with a distinct pattern of waxing, climax, and waning in I Indian Peak-Caliente ignimbrites three main pulses: >2400 km3 at 8.41–8.33 Ma, >3000 km3 at 5.65–5.45 Ma, C Central Nevada ignimbrites and >5400 km3 at 4.09–2.89 Ma (Fig. 20; Salisbury et al., 2011). This ignimbrite 0 ­flareup created at least six major calderas as much as 60 km in diameter and 56 60 64 68 72 76 80 several smaller ignimbrite shields (de Silva and Gosnold, 2007; Salisbury et al., 2011). The volume-time pattern of explosive activity is like that of the central SiO2 (wt %) and eastern sectors of the southern Great Basin (Fig. 13). Figure 18. Tuffs and lavas of the (A) Mogollon-Datil volcanic field (Bornhorst, 1980; Davis et al. Ignimbrites in the Altiplano-Puna volcanic complex include dacite and 1993; Davis and Hawkesworth, 1994) and the (B) Sierra Madre Occidental province (Ferrari et al., rhyolite with minor trachydacite, andesite, and latite (Fig. 21). The compo- 2007; Cameron and Hanson, 1982) on the International Union of Geological Sciences (IUGS) classification diagram (Le Maitre, 1989). Green line separates calc-alkaline rhyolite (below) from sitional pattern of these ignimbrites resembles those of the eastern sec- alkalic (above). of the southern Great Basin ignimbrite province more closely than the dominantly rhyolitic and more alkalic fields previously discussed (Fig. 7A).

GEOSPHERE | Volume 12 | Number 4 Best et al. | Slab-rollback ignimbrite flareups 1115 Research Paper

33,000 Indian Peak-Caliente VF Central Nevada VF 30,000 Southern Rocky Mountain VF Mogollon Datil VF Indian Peak-Caliente VF 27,000 Altiplano Puna Volcanic Complex (APVC)

24,000 Central Nevada VF Figure 20. Volume-time relations of silicic ignimbrites in the Altiplano-Puna volcanic )

21,000 3 complex in the Central Andean plateau

m (Kay et al., 2010; Salisbury et al., 2011; de Silva and Gosnold, 2007). Vertical red lines 18,000 indicate volumes of individual ignimbrite eruptions. Punctuated activity of several

Figure 20 is interactive. You can view volcanic olume ( k thousand cubic kilometers occurred at fields in the legend by moving the cursor over Southern Rocky Mountain VF e v 8.4, 5.6, and 4.0 Ma. Blue area shows them or you can toggle the symbols on and off 15,000 ti v cumulative ignimbrite volume. Labeled with the Layers panel in Adobe Acrobat or Adobe sloping line shows average rate of erupted Reader. ignimbrite volume for the indicated time umul a a C period. Volume of intermediate-composi- M

) 2,400 12,000

3 APVC (age relative) tion lavas of the same age is small. Cumu­

m lative volumes for other volcanic fields k

( (VF) from the United States that experi- Mogollon Datil VF y 8.4-2.9 e 1,800 m 3 9,000 enced ignimbrite flareups are compared at m /

u m the same scale. l k o v

e 1,200 t ~2,700 6,000 i

r 5.6 Ma 4.0 Ma b 8. 4 Ma 2.9 Ma m i

n 600 g Intermediate 3,000 lavas

APVC I 0 0 36 34 32 30 28 26 24 22 20 18 Age (Ma) for USA volcanic elds

Summaries in de Silva et al. (2006), de Silva and Gosnold (2007), Lipman and of the Great Basin. Detailed information on specific monotonous intermedi- McIntosh (2008, their table 4), and Kay et al. (2010) underscore the similarities ate ignimbrites in the Central Andes is available from Lindsay et al. (2001) for between the dacite ignimbrites in the Altiplano-Puna volcanic complex and the 3.9 Ma, 2500 km3 Atana ignimbrite, and Schmitt et al. (2001) for the 1 Ma, eastern sectors of the Great Basin and the Southern Rocky Mountain volcanic 100 km3 Purico ignimbrite and 3.69 Ma, >800 km3 Tara ignimbrite. Rhyolites field. These are phenocryst-rich, calc-alkaline, high-K dacitic to rhyodacitic ig- are less common, the largest being the ~300 km3 Toconao ignimbrite, which is nimbrites of the “monotonous intermediate” type of Hildreth (1981). The 40% interpreted to be from the upper zone of the Atana-Toconao magma reservoir to 50% are a low-pressure assemblage of quartz, plagioclase, (Lindsay et al., 2001). biotite, and Fe-Ti oxides with accessory apatite and titanite. Amphibole is A suite of peraluminous ignimbrites in the eastern Altiplano-Puna volcanic common in some northern Puna ignimbrites near the arc, minor amounts of complex (Figs. 10 and 19) has the most “crustal” Sr and Nd isotopic compo-

clinopyroxene and orthopyroxene can occur in ignimbrites with <67% SiO2, sitions, attesting to the influence of a metapelitic basement in the east (Caffe and sanidine is found in some dacite and rhyolite units. Pre-eruptive tempera- et al., 2012). These are also monotonous intermediates and testify to hyper­ tures are generally near 700 °C to 850 °C, with pressures before eruption cor- solidus temperatures in the orogenically thickened crust, in a similar manner responding to depths of 5 to 10 km, while magmatic oxygen fugacities (~2 log as the Late Cretaceous peraluminous just north of the caldera com- units above Quartz-Fayalite-Magnetite [QFM]) are similar to many silicic tuffs plex in the eastern sector of the Great Basin province.

GEOSPHERE | Volume 12 | Number 4 Best et al. | Slab-rollback ignimbrite flareups 1117