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Middle Cretaceous ash-flow tuff and caldera-collapse deposit in the Minarets Caldera, east-central Sierra Nevada, California R. S. FISKE Department of Mineral Sciences, Smithsonian Institution, NHB-119, Washington, D.C. 20560 O. T. TOBISCH Earth Science Board, University of California, Santa Cruz, California 95064 ABSTRACT areas in the Ritter Range Pendant, east-central Sierra Nevada, is un- derlain by a thick and massive sequence of metavolcanic rocks no- A 2.3-km section of ash-flow tuff and associated caldera-collapse ticeably less deformed than those immediately to the east. Fiske and deposit, representing the extrusive facies of part of the Sierra Nevada Tobisch (1978) interpreted these rocks to be remnants of a caldera-fill batholith, is totally exposed from its floor to its top in the Minarets complex deposited in what they called the Minarets Caldera. Later Caldera, east-central Sierra Nevada. Rapid burial and subsequent work by us, reported here, clarifies basic stratigraphic relations in this hornblende hornfels facies metamorphism resulted in remarkable pres- complex. ervation of primary textures and structures, despite the development of The rocks of the Minarets Caldera are noteworthy because they cleavage domains in parts of the caldera fill; late Tertiary uplift and provide a superbly exposed record of large-scale caldera formation Quaternary erosion have produced a rugged terrain where every meter coeval with mid-Cretaceous volcanism in the east-central Sierra Ne- of section is available for study. Large-scale caldera-filling eruption of vada. The caldera rocks have yielded mid-Cretaceous radiometric ash-flow tuff was interrupted by emplacement of a wedge-shaped mass ages (Table 1, samples 1-3), suggesting that they may be genetically of caldera-collapse deposit as much as 2 km thick, whose volume ex- related to nearby granitoids of the Shaver, Buena Vista, and Merced ceeded 70 km3. Individual clasts in the caldera-collapse deposit range to Peak intrusive sequences (Stern and others, 1981; Bateman, 1992). as much as 1.8 km across and include a wide variety of andesitic to rhyolitic To date, only a few localities of mid-Cretaceous volcanic rocks lavas and related volcaniclastic rocks, remnants of a precaldera volcanic have been recognized in the Sierra Nevada. Kistler and Swanson field that was probably much more extensive than the caldera itself. The (1981) reported a 100 Ma volcanic neck 8 km northeast of the Minarets caldera-fill sequence rests with angular unconformity on a rugged sur- Caldera; Saleeby and others (1990) described extensive exposures of face eroded into older volcanic rocks; the sequence is capped by bedded 110-100 Ma silicic volcanic rocks in the Kings Canyon area, 60 km to volcaniclastic rocks, including delicately laminated tuffs of probable the south; and Saleeby and Busby-Spera (1986) noted similar rocks in caldera lake origin. The total aerial extent of the Minarets Caldera is not the Lake Isabella area near the southern end of the Sierra Nevada. It known, but the area studied, plus scattered pendants of ash-flow tuff is probable that much of the Sierra Nevada batholith was mantled by and associated volcaniclastic rocks to the west, defines a 30- x 22-km coeval volcanic rocks in the mid-Cretaceous, but only scattered rem- elliptical area that may approximate its original shape. The caldera fill nants of this volcanic field remain. is invaded by a body of quartz monzonite porphyry, locally miarolitic, Of all of these remnants, however, those contained in the Min- that was probably emplaced during an episode of caldera resurgence. arets Caldera are unique for their superb exposures and remarkable preservation of original volcanic features. Initially altered by caldera- INTRODUCTION related fluids (Hanson and others, 1993), these rocks were rapidly buried to depths of several kilometers and heated by emplacement of Huber and Rinehart (1965) recognized that the rugged terrain nearby granitoids, producing fine-grained groundmass textures that comprising Mount Ritter, Banner Peak, the Minarets, and nearby for the most part faithfully preserve original volcanic features. Late TABLE 1. PRELIMINARY ZIRCON U/Pb AGES FROM THE MINARETS CALDERA AND ASSOCIATED ROCKS Sample* Concentration (ppm) Atomic ratios Ages (Ma) »ft 204pb ™pb ^Pb mPb ^Pb ^Pb OTPb 2«Pb »spb mPb ^Ih 1 275.5 5.93 153.9 0.00485 0.12022 0.35497 98.3 99.5 129 98.6 2 782.1 13.53 337.0 0.00100 0.06635 0.18151 101.1 108.4 104.1 3 781.3 13.13 339.3 0.00093 0.06268 0.17505 99.2 101.1 98.6 4 526.8 14.40 369.0 0.00174 0.07450 0.28380 144.0 143.7 140 140.6 5 413.6 10.82 247.2 0.00292 0.09181 0.29740 131.8 132.2 131.6 Note: sample locations are shown in Figure 2. •Sample 1, ash-flow tuff, from massive unit 1.5 km west of westernmost Davis Lakes; Sample 2, clast of rhyolite tuff in caldera-collapse deposit, 3.4 km southwest of Mount Ritter summit; Sample 3, quartz monzonite porphyry of Shellenbarger Lake, 0.4 km east of Shellenbarger Lake; Samples 4 and 5, thick-bedded lapilli tuff near western margin of Bench Canyon shear zone, 0.7 and 0.8 km, respectively, southwest of Hemlock Crossing. Analyses obtained in 1974-1977 by T. W. Stem, U.S. Geological Survey, on bulk zircon separates from 50- to 100-kg rock samples. In the text, samples 1-3 and 4-5 are referred to as mid-Cretaceous and Lower Cretaceous, respectively, because the uncertainties in the radiometric ages are largely indeterminate. The fact that the ^VbP^V ages are within -1-2 m.y. of the 206Ph/2BU ages in four out of the five samples, however, suggests that the radiometric ages are likely to fall within the broad system/period designations given above (compare Stem and others, 1981, p. 5). Geological Society of America Bulletin, v. 106, p. 582-593, 9 figs., 1 table, May 1994. 582 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/5/582/3381902/i0016-7606-106-5-582.pdf by guest on 28 September 2021 MINARETS CALDERA, SIERRA NEVADA, CALIFORNIA rocks are bounded on the east and northeast by a near-vertical fault zone that changes strike through about 80° of arc. We interpret this curved fault zone to mark the original caldera boundary, complicated by later normal faulting and postcaldera intrusions. A large mass of diabase porphyry postdates faulting and obscures the fault zone south of Ediza Lake (Fig. 2); the elongate outcrop of this mass suggests that its configuration may have been influenced by the pre-existing fault zone. Rocks of the caldera fill are truncated on the west by the Bench Canyon shear zone, where high-angle reverse motion has brought lower Cretaceous volcaniclastic rocks into contact with the caldera fill. Still farther to the west, small pendants of mid-Cretaceous ash- flow tuff and associated volcaniclastic rocks are associated with co- eval granitoids (Stern and others, 1981, Table 1). Peck (1983, p. 4) suggested that these intrusive and extrusive rocks "may represent the preserved lower part of a caldera." We agree with this possibility and have drawn a heavy dotted and dashed line around a 30- x 22-km elliptical area that encloses all known pendants in the Ritter Range area containing rocks likely deposited in the caldera environment (Fig. 1), but it is uncertain whether this line encloses a single caldera or a complex of several such structures. STRUCTURAL ASPECTS OF THE CALDERA FILL The caldera fill is tilted to the southwest, best shown by the 8° dip of its base near Ediza Lake and the 25°-60° dips of the overlying volcaniclastic rocks near Twin Island Lakes (Fig. 2). Tectonic cleav- age in these rocks is generally absent, but isolated domains of weakly Figure 1. Geologic map of the Minarets Caldera and nearby areas: to moderately developed cleavage are present (Tobisch and Fiske, ccd, caldera-collapse deposit; af, ash-flow tuff and associated rocks in- 1982, and unpub. observations). In the field, the cleavage is defined by terpreted to have been deposited in a caldera environment; c, mid- preferred alignment of the long axes of lithic lapilli (and/or breccia Cretaceous granitoids approximately coeval with the caldera-fill rocks; fragments), which becomes discernible at relatively low values of and ovr, older volcaniclastic rocks. Undesignated areas are chiefly strain (that is, ~15%-20% shortening, Tobisch, 1984, Fig. 4). At still younger granitoids of the Sierra Nevada batholith. The Bench Canyon lower values (<15% shortening), however, field observation of pre- shear zone (heavy wavy lines) separates caldera-fill deposits described in ferred alignment of lapilli becomes problematic. The average com- this report from areas to the west and southwest where similar rock paction strain normal to bedding is about 10% in undeformed lithic types (af) are preserved in scattered pendants. Heavy dashes mark ap- lapilli tuffs from magmatic arc environments (Tobisch and others, proximate boundary fault along the northeast caldera margin; heavy 1977, Table 1), and it is possible that a substantial part of the caldera dots depict speculative outline of entire caldera complex. Geology is fill may have absorbed a small additional component of tectonic strain simplified from Huber and others (1989), Huber and Rinehart (1965), but not enough to alter primary textural relationships. and this study. In addition, many exposures of nonwelded ash-flow tuff display a foliation defined by flattened pumice lapilli, but it is very difficult to establish whether this planar structure is (1) a primary compaction Tertiary uplift and Quaternary erosion have produced a rugged, highly foliation (collapse of glassy pyroclasts during cooling) that was sub- dissected terrain having more than 1.2 km of local relief; glacially sequently tilted to high dips, (2) a primary flow foliation that devel- polished surfaces reveal original volcanic textures and structures in oped varying dips as the ash-flow deposit compacted upon a topo- intricate detail.

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