Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs Peter W. Lipman U.S. Geological Survey, M.S. 910, 345 Middlefield Road, Menlo Park, California 94025 Michael A. Dungan Départment de Minéralogie, Université de Genève, 13 rue des Maraîchers, CH-1211 Genève 4, Switzerland Laurie L. Brown Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003 Alan Deino Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 ABSTRACT tive subsidence of at least 10 km. The rapid re- the same tuff sheet in widely separated parts of generation of silicic magmas requires the sus- the volcanic field; (2) lithologic similarities Reinterpretation of a voluminous regional tained presence of an andesitic subcaldera among some discrete ash-flow sheets at the same ash-flow sheet (Masonic Park Tuff) as two sep- magma reservoir, or its rapid replenishment, stratigraphic level, (3) miscorrelations between arate tuff sheets of similar phenocryst-rich during the 1 m.y. life span of the Platoro com- outflow tuff sheets and their intracaldera equiva- dacite erupted from separate source calderas plex. Either case implies large-scale stoping lents; (4) failure to locate and correctly interpret has important implications for evolution of the and assimilative recycling of the Tertiary sec- unconformities along topographic caldera walls, multicyclic Platoro caldera complex and for tion, including intracaldera tuffs. which may lie many kilometres beyond the struc- caldera-forming processes generally. Masonic tural boundary of a caldera; (5) misinterpretation Park Tuff in central parts of the San Juan field, INTRODUCTION of large coherent slide blocks and lithologically including the type area, was erupted from a mixed caldera-collapse breccias, leading to errors concealed source at 28.6 Ma, but widespread “You don’t know anything about a volcano in the inferred stratigraphic sequence; and tuff previously mapped as Masonic Park Tuff until you have the stratigraphy straight.” (6) even failure to recognize entire calderas, espe- in the southeastern San Juan Mountains is the (Tom Steven, ca. 1969, oral commun.) cially nonresurgent structures buried by later vol- product of the youngest large-volume eruption canic deposits. A persistent problem in this region of the Platoro caldera complex at 28.4 Ma. This Developing a reliable stratigraphic and struc- has been establishing reliable criteria for distin- large unit, newly named the “Chiquito Peak tural framework for large, multisource silicic guishing among lithologically similar crystal-rich Tuff,” is the last-erupted tuff of the Treasure volcanic fields, and determining structural evo- dacitic tuffs that occur in close stratigraphic prox- Mountain Group, which consists of at least 20 lution of caldera sources, depends critically on imity. Four such tuff sheets in the eastern San separate ash-flow sheets of dacite to low-silica potentially problematic correlations among Juan field are deceptively similar in field appear- rhyolite erupted from the Platoro complex dur- widely distributed voluminous ash-flow sheets ance and phenocrysts, all consisting dominantly ing a 1 m.y. interval (29.5–28.4 Ma). Two Trea- (Hildreth and Mahood, 1985). A broad geologic of plagioclase, biotite, and augite (La Jara Can- sure Mountain tuff sheets have volumes in framework for the San Juan volcanic field was yon, Chiquito Peak, and Masonic Park Tuffs, and excess of 1000 km3 each, and five more have developed in the 1960s (Steven et al., 1974), but the tuff of Blue Creek; see Table 1). These dacites volumes of 50–150 km3. The total volume of recent work disclosed the need for restudy of lack conspicuous compositional zonations (“mo- ash-flow tuff exceeds 2500 km3, and caldera-re- some regional ash-flow sheets and interleaved notonous intermediates” of Hildreth, 1981), al- lated lavas of dominantly andesitic composition lava sequences (Lipman et al., 1989; Dungan et though some are “cryptically zoned,” as is evident make up 250–500 km3 more. A much greater al., 1989a). An integrated field, petrologic, pale- only from detailed minor-element or mineral- volume of intermediate-composition magma omagnetic, and geochronologic study has now composition data (Dungan and Lipman, unpub. must have solidified in subcaldera magma substantially modified our understanding of the data). Although compositionally uniform, these chambers. Most preserved features of the Plat- eruptive history of the recurrently subsided Pla- tuffs vary in appearance vertically and laterally oro complex—including postcollapse asym- toro caldera complex in the southeastern San because of complex welding and devitrification metrical trap-door resurgent uplift of the Juan volcanic field. This revised caldera history zonations: densely welded zones forming rugged ponded intracaldera tuff and concurrent infill- places important constraints on the chemical cliffs alternate with weakly welded intervals ex- ing by andesitic lava flows—postdate eruption evolution of subcaldera magmas in the upper pressed as vegetation- and talus-covered benches. of the Chiquito Peak Tuff. The numerous large- crust and the importance of large-scale assimi- As a result, compound cooling boundaries within volume pre–Chiquito Peak ash-flow tuffs docu- lation accompanying caldera subsidence. a single tuff sheet can be difficult to distinguish ment multiple eruptions accompanied by re- Critical difficulties and impediments to deter- from contacts between multiple discrete sheets of current subsidence; early-formed caldera walls mining valid regional stratigraphic correlations similar lithology. In addition, distal facies of the nearly coincide with margins of the later Chiq- for the San Juan field have included (1) the enor- dacitic tuffs can have sizable local thickness vari- uito Peak collapse. Repeated syneruptive col- mous areal extent of individual ash-flow sheets, at ations and abrupt flow-front terminations, appar- lapse at the Platoro complex requires cumula- times resulting in different names being applied to ently reflecting sluggish emplacement energetics. GSA Bulletin; August 1996; v. 108; no. 8; p. 1039–1055; 8 figures; 5 tables. 1039 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/8/1039/3382516/i0016-7606-108-8-1039.pdf by guest on 02 October 2021 LIPMAN ET AL. TABLE 1.TUFFS, SOURCES, AND 40Ar/39Ar AGES, SAN JUAN VOLCANIC FIELD, COLORADO Ash-flow sheets Composition Caldera source Volume Age Southeast complex Central cluster West cluster (est. km3) (Ma) Sunshine Peak Si rhyolite–dacite Lake City 200-500 23.1 Snowshoe Mountain Mafic dacite Creede >500 26.7 Nelson Mountain Low-Si rhyolite–dacite San Luis >500 26.8?* Cebolla Creek Mafic dacite San Luis 250 26.9?* Rat Creek Low Si rhyolite–dacite San Luis 250 27.0?* Wason Park Rhyolite South River >500 27.1 Blue Creek Dacite Lake Humphrey 250 27.2 Carpenter Ridge Low-Si rhyolite–dacite Bachelor >1000 27.3 Fish Canyon Dacite La Garita >4000 27.8 Crystal Lake† Low-Si rhyolite Silverton 50–100 Chiquito Peak Dacite Platoro 1000 28.41 Masonic Park Dacite Mount Hope? 500 28.6 South Fork Low-Si rhyolite Platoro (Summitville?) 50 28.76 Ra Jadero Si dacite Platoro (Summitville?) 150 28.77 Sapinero Mesa† Low-Si rhyolite San Juan >1000 Dillon Mesa† Low-Si rhyolite Uncompahgre? 25–100 Blue Mesa† Low—Si rhyolite Lost Lakes 100–500 Ute Ridge† Dacite Ute Creek >500 29.1 Ojito Creek Si dacite Platoro 100 29 Middle tuff Dacite Platoro 100 29 La Jara Canyon Dacite Platoro 1000 29.3 Black Mountain Si dacite Platoro 100 29.5? Lower rhyolite tuff Low-Si rhyolite Platoro 75 29.5? Rock Creek Andesite “Platoro” 25 30–31 Note: bold type indicates tuff units discussed in this paper. *Age and stratigraphic relationship to Snowshoe Mountain is uncertain. †Stratigraphic relationship to southeast area is uncertain. A long-standing problem has been the origin ate-composition lava flows erupted from scat- on the eruptive sequence (Tables 2, 31). Litholo- and emplacement of the widespread and volu- tered central volcanoes (>35–29.5 Ma) to over- gies are described more thoroughly by Lipman minous dacitic ash-flow tuff that overlies Trea- lying voluminous ash-flow sheets erupted from (1975a); recent petrologic studies document com- sure Mountain units and underlies the Fish multiple caldera sources that were associated positional and isotopic characteristics in relation Canyon Tuff (Table 1). Steven and Lipman with continuing lava eruptions (29.5–26 Ma). to changing magma-chamber processes and (1973, 1976) named this assemblage the Ma- Caldera sources (at least 17) have been identified crustal interactions (Dungan et al., 1989b, 1995; sonic Park Tuff and included within it a large re- for most large-volume San Juan tuff sheets Dungan and Lipman, unpub. data). gional ash-flow sheet and a second more local (Table 1), and caldera subsidences associated tuff, both inferred to have erupted from the with other tuffs are likely but difficult to docu- Precaldera Volcanism: Mount Hope caldera (Fig. 1). In contrast, we ment because of burial by younger volcanic The Conejos Formation have found that the tuff originally mapped as a units. The Oligocene sequence is locally over- single regional sheet consists of two lithologi- lain by thin flows of Miocene basalt and minor All intermediate-composition lavas and brec- cally similar but distinct ash-flow sheets, with rhyolite. The volcanic rocks now occupy an area cias in the southeastern