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Slab-Rollback Ignimbrite Flareups in The Research Paper 120°W 117°W 114°W 111°W 109°W Nevada 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 Utah (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 Salt Lake City and Cedar City, Utah. Figure is modified from Best et al. (2009, their fig. 1). 37°N 60 Arizona Las Vegas California 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- Ignimbrite Flareup lites (e.g., Farmer et al., 1991). During the ignimbrite flareup, extrusion of inter- mediate-composition, chiefly andesitic, lavas 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 altiplano 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 lava 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 Sierra Nevada (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 ignimbrites n = 466 C Central Nevada ignimbrites n = 433 Rhyolite ignimbrites of all ages (36–18 Ma) occur throughout the province I I and are accompanied by temporally more restricted dacite 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, quartz, biotite, hornblende, Fe-Ti I II I II CI II I I I II C I IC I I I ICI I C ) 9 n=69 I I C CC I I I oxides, pyroxene, and trace amounts of zircon, apatite, and, rarely, titanite, I CI I I I C CIII CI C % I I C C C C C I CI C CI I CC C that equilibrated in relatively wet magmas at shallow crustal depths of 7–9 km. t IC C I C C C C CI I C I I II CI I C CC CCCIC C w I I CC C C C ICC ( CICC CI I CC C CC C I C C CCC I CC CCC Dacites show phenocryst-rich compositions, upward of 50% of the tuff 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 I CII I II I C C C CI C C C rock equivalent basis (Best et al., 2013b); nearly all formed by super-eruptions, 8 I I C CI CI CCCICC CICC C CCC K C I CCC CCCC C I C C II I I C C II I I CI IC C CI CCCC C C C + C I I C I C C C I 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 C C CC C I I intermediates in the sense of Hildreth (1981). These dacite magmas are inter- I I C I IIIC IC CI C I CCCCCC C CC a II ICI I I I C C I I I I II I C CI I I C C CC C CCIC N 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, basalt is absent until 11 B Western Nevada 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 ) w w 9 Trachydacite w w w www w w % w w w w ww w w ww w with the assimilation of large proportions of felsic crust in the eastern sector t w w w w w ww ww w w wI w w w I w wwwwwwwww w w Iww w ww w ww w ( w 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 2 w w w ww w sector, where the crust is younger and more mafic (Figs. 5 and 10). 8 w w w www ww www K w w ww wwww ww ww wwwww wwww ww + I w w w w w ww wwwww w w O w w Three Sectors 2 w ww w w a w w www N 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 calderas sur- rounded by more or less separate ignimbrite outflow fields (Fig.
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