...Christopher J. Fridrich; Gail A. Mahood Abstract The 34 Ma Grizzly Peak Tuff in west-central Colorado preserves evidence for distinct compositional layers in a high-level magma chamber. Intracaldera tuff consists of a single cooling unit, at least 2.7 km thick, zoned from a high-silica rhyolite...

... of hydrothermally altered rocks, and lava flows formed along a circular swarm of cone-sheet fractures around the site of the future caldera. Early, largely rhyolitic uppercrustal magmatism culminated in the caldera-forming eruption of the Grizzly Peak Tuff. Intracaldera tuff is zoned from high-silica rhyolite...

...CHRISTOPHER J. FRIDRICH; GAIL A. MAHOOD Abstract The deeply eroded Grizzly Peak cauldron, 17 by 23 km and source of the 34-m.y.-old Grizzly Peak Tuff, is located in the Sawatch Range in west-central Colorado. The Lincoln Gulch composite stock is exposed in the core of a resurgent dome...

... ( Jacob et al. 2015 ). The Grizzly Peak caldera complex has the lowest εNd i values (<–12), and some of the post-caldera intrusions overlap ancient felsic country rocks ( Fig. 11a ). Calcium isotope data corroborate the incorporation of significant ancient felsic material into the Grizzly Peak Tuff...

...—increasing cumulative eruptive volume (right axis). Inset—slopes corresponding to different cumulative eruption rates. Abbreviations: AT—Amalia Tuff; B—Bonanza Tuff; BC—Badger Creek Tuff; CP—Chiquito Peak Tuff; CR—Carpenter Ridge Tuff; FC—Fish Canyon Tuff; GP—Grizzly Peak Tuff; LJ—La Jara Canyon Tuff; BCr...

... the strongly compositionally zoned Grizzly Peak Tuff at 34.3 Ma ( Fridrich and Mahood, 1987 ; Fridrich et al., 1991 ). Two additional ignimbrite-caldera centers (Marshall Creek, Bonanza) that are 2–4 m.y. younger than the Wall Mountain and Badger Creek Tuffs and lie along the north-south Sawatch Range uplift...

... Costa Group includes a bimodal volcanic sequence, dominated by two mappable mafic volcanic formations ( Figure 3 ), as well as less extensive felsic tuffs and thin lenticular mafic volcanic flows. The two principal occurrences are the Grizzly Peak Basalt (also called the Moraga Formation...

... Caldera, Lipman 1983). Asymmetric, trapdoor-type subsidence of the caldera floor, followed by asymmetric uplift, greatest in the area of thickest tuff accumulation, is seen in the Platoro Caldera (Lipman 1975) and the Grizzly Peak Caldera (Fridrich 1983). Walker (1984) has pointed out that many subaerial...

... of 371 ± 9 Ma to 353 ± 3 Ma; felsic tuff in the Sierra Buttes Formation yielded an age of 363 ± 7 Ma; and three felsic hypabyssal bodies intruded into the Sierra City mélange yielded ages of 369 ± 4 Ma to 358 ± 3 Ma. These data provide a younger age limit for assembly of the Shoo Fly Complex and indicate...

... brown lines delineate Hat Creek fault. Red line labeled A–A′—location of profile modeled in Figure 7 . CB—Cinder Butte; GPA—Grizzly Peak anticline; HCF—Hat Creek fault; WPP—West Prospect Peak. Towns: BiB—Big Bend; Bu—Burney; FRM—Fall River Mills; McC—McCloud; P—Pittville. CORRESPONDENCE...

... centers, but the Grizzly Peak Tuff from the northern part of the field and the peralkaline Amelia Tuff from southern have distinctly higher initial Sr isotope ratio. Data are from: Indian Peak and central Nevada— Best et al. (2013a , 2013b ), Hart (1997) , and Larson and Taylor (1986) ; for western...

...), Siesta Formation (lacustrine claystone, siltstone, sandstone, tuffs, and limestone), and the Bald Peak Formation (terrestrial basalt flows). Each formation within the Contra Costa Group is deposited conformably over each other ( Fig. 2 ), with the Bald Peak Formation being the uppermost exposed unit...

... intrusions but not to have removed intracaldera tuff (e.g., Grizzly Peak, Fridrich et al., 1991 ). The Middle Fork caldera ( Bacon and Lanphere, 1996 ), named for the Middle Fork of the North Fork of the Fortymile River, is such a resurgent caldera structure that encompasses a 10 km × 20 km area of rhyolite...