Recurrent Eruption and Subsidence at the Platoro Caldera Complex, Southeastern San Juan Volcanic Field, Colorado: New Tales from Old Tuffs

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 (29.5Ð28.4 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 50Ð150 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 250Ð500 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 preÐChiquito 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. 1039Ð1055; 8 figures; 5 tables.

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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 rhyoliteÐdacite Lake City 200-500 23.1 Snowshoe Mountain Mafic dacite Creede >500 26.7 Nelson Mountain Low-Si rhyoliteÐdacite San Luis >500 26.8?* Cebolla Creek Mafic dacite San Luis 250 26.9?* Rat Creek Low Si rhyoliteÐdacite 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 rhyoliteÐdacite Bachelor >1000 27.3 Fish Canyon Dacite La Garita >4000 27.8 Crystal Lake† Low-Si rhyolite Silverton 50Ð100 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? 25Ð100 Blue Mesa† Low—Si rhyolite Lost Lakes 100Ð500 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 30Ð31 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 (>35Ð29.5 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 (29.5Ð26 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 San Juan Mountains that separate sources and spectacularly antithetic of more than 25 000 km2 and have a volume of predate ash-flow eruptions have long been in- distributions. We retain the name “Masonic about 40 000 km3. Early intermediate-composi- cluded in the Conejos Formation (Cross and Park” for the tuff of the type area, which is tion lavas constitute the volumetric bulk of the Larsen, 1935; Lipman, 1975a; Dungan et al., widespread in the east-central part of the vol- volcanic field; the more intensively studied ash- 1989a). Clustering of Conejos-age stratovolca- canic field and which was erupted from a con- flow tuffs are less voluminous, even though at noes in the area that became the Platoro caldera cealed, centrally located source. The younger least five individual tuff sheets have volumes complex has been interpreted as recording the ini- Chiquito Peak Tuff (newly named) is the most greater than 1000 km3 each (Table 1). tial rise of intermediate-composition magma bod- widespread ash-flow sheet in the southeastern ies to shallow crustal levels, preparatory to ash- part of the San Juan field and represents the last ERUPTIVE SEQUENCE flow volcanism (Lipman et al., 1978). Conejos major eruptive product from the multicyclic lavas in the vicinity of the Platoro caldera com- Platoro complex. The Oligocene eruptive sequence in the eastern plex have been subdivided into the Horseshoe San Juan region is summarized only briefly, with Mountain, Rock Creek, and Willow Mountain REGIONAL FRAMEWORK emphasis on new stratigraphic and age data in re- members on the basis of differences in composi- lation to volcanic evolution of the Platoro caldera tion and phenocryst contents (Colucci et al., The Oligocene San Juan volcanic field is complex. This caldera system (Fig. 1) is an ex- 1991). K-Ar and 40Ar/39Ar ages indicate activity among the largest erosional remnants of wide- ceptional example of focused recurrent subsi- from before 33 Ma to about 29.5 Ma. A local ash- spread calc-alkalic magmatism that character- dence, from which seven major tuff sheets with flow sheet on the northeast flank of the Platoro ized the North American Cordillera during mid- individual volumes of 75Ð1000 km3 erupted with- Tertiary time (Larsen and Cross, 1956; Lipman in a period of about 1 m.y., along with roughly 1 et al., 1989). Volcanism in the San Juan field twice as many smaller tuff sheets (5Ð10 km3 GSA Data Repository item 9643, complete analytical data and additional notes on methods, is avail- 40 39 progressed regionally (Lipman et al., 1970; each). New laser Ar/ Ar determinations on able on request from Documents Secretary, GSA, Steven et al., 1974) from dominantly intermedi- sanidine phenocrysts provide critical constraints P.O. Box 9140, Boulder, CO 80301.

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Fig. 2

Figure 1. Generalized map of the eastern San Juan region, showing distribution of the Chiquito Peak and Masonic Park Tuffs in relation to inferred sources from the Platoro (P) and concealed Mount Hope (MH?) calderas, respectively. Some units younger than the Chiquito Peak are not shown in certain locations or are highly generalized, in order to clarify the distributions of the Masonic Park and Chiquito Peak Tuffs. Other calderas: B—Bachelor; C—Creede; LG—La Garita, including its newly recognized southern extension (LGS); S— Summitville; SR—South River.

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TABLE 2. ASH-FLOW SHEETS OF THE TREASURE MOUNTAIN GROUP, AND caldera complex, the tuff of Rock Creek (Lipman, COMPARISONS WITH PREVIOUSLY USED GEOLOGIC NAMES 1975a), is petrologically similar to directly under- Lipman (1975a) Dungan et al. (1989a) This report lying dacitic lavas of the Rock Creek member and Masonic Park Tuff Upper dacite of Masonic Park Tuff Chiquito Peak Tuff Masonic Park Tuff Lower dacite of Masonic Park Tuff Masonic Park Tuff is considered part of the Conejos Formation. This tuff can also be regarded as the precursor to more Upper tuff Rhyolite unit of Masonic Park Tuff South Fork Tuff Ra Jadero Member Ra Jadero Member Ra Jadero Tuff voluminous and silicic ash-flow magmatism be- Ojito Creek Member Ojito Creek Member Ojito Creek Tuff cause its eruptive source lay within the subse- Middle tuff Middle member Middle tuff quent caldera complex. Fox Creek unit La Manga unit La Jara Canyon Member La Jara Canyon Member La Jara Canyon Tuff Platoro Caldera Complex and the Lower tuff Lower member Treasure Mountain Group upper unit Black Mountain unit Black Mountain Tuff lower unit Lower unit Lower rhyolite tuff Note: Italicized unit names not included within Treasure Mountain; all other units Multiple ash-flow sheets erupted from the Pla- considered part of Treasure Mountain, including Chiquito Peak Tuff of this report. toro caldera complex and deposited widely across the southeastern San Juan region were previously designated as members of the Trea- sure Mountain Tuff (Lipman, 1975a). This ap- proach subdivided the tuff, but only slightly modified the prior nomenclature (Treasure Mountain TABLE 3. SUMMARY OF 40AR/39AR AGES, SOUTHEAST SAN JUAN Rhyolite of Larsen and Cross, 1956), in order to ASH-FLOW SHEETS AND ASSOCIATED LAVAS emphasize the pyroclastic nature of these large- Unit Sample Ar-Ar age Mineral Data volume distinctive units, as well as their eruption (revised name) location (Ma ± 1σ) sources* from a common caldera source. Several infor- Dike, dacite of Park Schinzel Flats 26.15 ± 0.07† Sanidine 4 mally designated ash-flow members of the Trea- Creek sure Mountain are now recognized to be as wide- Fish Canyon Tuff Fun Valley 27.84 ± ?? Sanidine 1 spread and voluminous as the formally named (South Fork) 27.79 ± 0.07 Sanidine, biotite 2 27.50 ± 0.15 Sanidine 3 members (Table 1; Dungan et al., 1989a). In addition, several ash-flow sheets of the Treasure Dacite of Fisher Gulch Fisher Gulch 28.38 ± 0.09† Sanidine 4 Mountain are as voluminous as most tuff sheets Chiquito Peak Tuff, Platoro Reservoir 28.43 ± 0.07† Sanidine 4 designated as separate formations elsewhere in intracaldera the San Juan field (Table 1). Accordingly, we now raise the rank of each major named tuff Chiquito Peak Tuff, Bishop Rock 28.39 ± 0.07† Sanidine 4 outflow sheet to formational status within the revised Treasure Mountain Group (Table 2). Although all ¤ Masonic Park Tuff Masonic Park 28.60± 0.23 Biotite 3 seven regional units of the Treasure Mountain South Fork Tuff Bennett Peak 28.76 ± 0.07 Sanidine 4 Group are unequivocally tied to eruptive sources within the Platoro caldera complex, intracaldera Ra Jadero Tuff Bennett Peak 28.76 ± 0.07† Sanidine 4 SE Del Norte 28.79 ± 0.07† Sanidine 4 facies of only the two largest eruptions, the La Jara Canyon and Chiquito Peak, are exposed. Ojito Creek Tuff Alamosa River 29.1 ± 0.3 Biotite 5

La Jara Canyon Tuff, Three localities 29.3 ± 0.3# Biotite 5 Initial Caldera(?) Eruptions: The Lower outflow Rhyolite Tuff and Black Mountain Tuff

Black Mountain Tuff Black Mountain 29.4 ± 0.4 Biotite 5 *Data sources: (1) Cebula et al. (1986), Sampson and Alexander (1987). (2) Kunk et al. (1985). The lower rhyolite tuff is a widespread but (3) Lanphere (1988) and unpublished data, incremental-heating plateau ages. (4) Laser fusion discontinuously preserved tuff sheet of crystal- ages, this paper. Sanidine grains were hand-picked from crushed whole-rock samples for analysis poor low-silica rhyolite. Two lithologically sim- by methods similar to those reported by Deino and Potts (1990) and Deino et al. (1990). In any large suite of analyses of individual sanidine phenocrysts from ash-flow tuffs or silicic lavas, most ilar rhyolite cooling units are present locally, for crystals are of excellent analytical quality, but a few exhibit anomalous atmospheric contamination example, near the geographic Treasure Moun- or have anomalous ages (complete anlytical data and additional notes on methods are presented tain. The Black Mountain Tuff is a densely in the Data Repository table; see footnote 1 in text). To define the most reliable geologic age from the available analyses, suspect or lesser-quality runs have been culled from the data set by using welded silicic dacite, widely characterized by empirical criteria. A cutoff of 98% radiogenic 40Ar was used to identify suspect analyses on the ba- lithophysal cavities and large pumice lenses and sis of their radiogenic to atmospheric 40Ar proportions, a level that eliminated about 12% of the analyses. None of these omissions changed the reported ages beyond reported analytical preci- commonly containing both dacitic and horn- sion. Precisions are reported and 1σ errors. Sanidine from dike KF-132 contained less radiogenic blende-bearing andesitic pumice in an upper vit- sanidine than the effusive rocks, so the less stringent criterion of 97% radiogenic 40Ar was used. rophyre zone. Although exposed only discontin- One run was rejected because ages vary more than two standard deviations from the sample mean. (5) Balsley et al. (1988) and Balsley (1994), incremental-heating plateau ages. uously because of cover by younger units, this †Weighted mean of two replicate analyses; see Data Repository (see footnote 1 in text). tuff sheet is almost as widespread as the largest ¤For valid comparison, reported Ar-Ar biotite plateau age (28.3 Ma) was increased by 0.3 Ma overlying tuffs. In accord with the informal to adjust for differing standard calibrations (about 1%), as reflected in reported ages of Fish Canyon Tuff. nomenclature of Dungan et al. (1989a), this unit #Average of biotite plateau ages for three outflow localities (29.0, 29.4, 29.7). is here named the Black Mountain Tuff, for cliff exposures above the Conejos River canyon

1042 Geological Society of America Bulletin, August 1996

along the southwest side of Black Mountain, verse fault). In addition, interflow sedimentary sheet, the La Jara Canyon Tuff is inferred to have which is designated as the type locality (Lipman, units west of the contact contain angular blocks ponded within the Platoro complex to a thick- 1975b; Dungan et al., 1989a, Fig. 2-3). of Ojito Creek Tuff, derived from the slopes to ness of several kilometres, comparable to the in- Areal distributions, thickness changes, and the east. Along the northern extension of this tracaldera accumulations documented by ex- distributions of proximal vs. distal facies of the topographic wall, in the South Fork of Rock posed Chiquito Peak Tuff and by the fills of lower rhyolite tuff and the Black Mountain Tuff Creek, andesite flows and sedimentary units lap many other calderas in the San Juan field and demonstrate that they were erupted from the Pla- onto the South Fork Tuff and underlying ash- elsewhere. toro caldera complex and deposited to maximum flow units. The concealed wall of the Chiquito thickness in broad valleys between Conejos vol- Peak caldera passes south of Bennett Peak, per- Later Caldera-related Ash-Flow Deposits, canic edifices (Lipman, 1975a, Fig. 9). Both mitting preservation of Treasure Mountain units Lacking Exposed Collapse Sources these tuff sheets have sufficiently large outflow within another remnant of the La Jara Canyon volumes (75Ð100 km3) that associated caldera subsidence (Fig. 3). The inner wall must have After eruption of the La Jara Canyon Tuff, its formation seems virtually certain. formed during Chiquito Peak eruptions, as indi- caldera was filled by Summitville Andesite and cated by the presence of Chiquito Peak Tuff several interlayered ash-flow tuffs erupted from La Jara Canyon Tuff and Evidence for within the caldera, as well as ponded upper- within the Platoro complex. Caldera Collapse member lavas of the Summitville Andesite and Middle Tuff. This unit consists of 10Ð15 sep- the Fish Canyon Tuff (erupted from the central arate ash-flow sheets, each with volumes that are The La Jara Canyon Tuff is an exceptionally San Juan caldera cluster). smaller (5Ð10 km3 each?) than other tuffs of the widespread and voluminous ash-flow sheet of West Caldera Remnant and Intracaldera Treasure Mountain Group. The overall distribu- phenocryst-rich dacite (plagioclase + biotite Tuff. Small remnants of the La Jara Canyon cal- tion of the middle tuff documents sources from + augite, no sanidine), with an estimated original dera fill are also preserved along the west mar- within the Platoro complex (Lipman, 1975a; Bal- volume in excess of 1000 km3. The La Jara gin of the Platoro complex: in Gold Creek, sley, 1994). The middle tuff is important to the Canyon is the first major tuff sheet for which ev- along the upper Alamosa River, and near Pros- interpretations developed here as further evi- idence of collapse is preserved at the Platoro pect Mountain (Fig. 4). The apparent top of in- dence for extraordinarily recurrent explosive vol- complex. Previously, most exposed features tracaldera La Jara Canyon Tuff is preserved canism from a single caldera complex. within the caldera complex were interpreted as along the Alamosa River and against the east Ojito Creek and Ra Jadero Tuffs. These tuffs related to the La Jara Canyon eruption, including slope of Prospect Mountain, although interpre- are thin but widespread ash-flow sheets of densely much of the preserved caldera wall and the resur- tations are somewhat hindered by severe alter- welded silicic dacite, containing sparse andesitic gently uplifted block of thick intracaldera tuff ation. On Prospect Mountain and nearby in pumice lenses near their tops. Their distributions (Lipman, 1975a). Our recent work demonstrates, Schinzel Flats, La Jara Canyon Tuff is exposed are similar to those of underlying tuffs, except that however, that the tuff on the resurgent block is continuously from nearly flat lying outflow tuff the Ojito Creek is absent on the north flank of the the younger Chiquito Peak Tuff and that most eastward into tuff that dips 40°Ð50° into the cal- Platoro caldera complex (Lipman, 1975a, Figs. 24 exposed features of the caldera complex resulted dera, marking the lip of the caldera wall (Lip- and 25). The Ra Jadero Tuff is the earliest Treasure from this eruption. Remnants of the La Jara man, 1975a, Fig. 64). Similar dips into the cal- Mountain unit to contain sanidine phenocrysts. Al- Canyon caldera that have escaped later burial are dera characterize the mapped La Jara Canyon though both the Ojito Creek and Ra Jadero Tuffs a lenticular scallop along the northeast caldera Tuff along caldera-wall slopes south of the are ponded within the outer topographic wall of wall and three small tuff remnants deposited Alamosa River. The tuff is least altered in this the Platoro complex, indicating proximal sources, against the southwest wall (Fig. 2). area, and stained thin sections failed to disclose neither is associated with exposed intracaldera tuff Northeast Caldera Remnant. The northeast the sparse sanidine phenocrysts that are diag- or other evidence of collapse. Lipman (1975a) in- scallop, about 12 km long and 1 km wide, is nostic of Chiquito Peak Tuff. ferred that both these tuffs were erupted from the marked by intracaldera lava flows and overlying Several variably welded tuff units that lap Summitville caldera, a paleodepression within the tuff units (Ojito Creek, Ra Jadero, and South against the caldera wall near the head of Gold northwest Platoro caldera complex. As reinter- Fork Tuffs) banked unconformably against an ir- Creek, previously interpreted as the Ra Jadero preted here, most of the exposed Platoro complex regular southwest-concave slope that truncates and overlying tuff units of the Treasure Moun- formed during eruption of the Chiquito Peak Tuff the flanks of several Conejos-age volcanoes tain (Lipman, 1974), now seem more likely to be and later resurgence. Although some apparently (Fig. 3). No intracaldera La Jara Canyon Tuff is units of the middle tuff. By either correlation, the reactivated ring faults may reflect concealed cal- exposed, but the presence of the Ojito Creek and caldera wall against which these tuffs bank must dera structures related to eruption of the Ojito younger tuffs that are exceptionally thick and have formed during an eruption predating the Creek and Ra Jadero Tuffs, no topographic or di- densely welded provide an upper limit on the Chiquito Peak Tuff. The geometric continuity of rect stratigraphic evidence remains for the inferred time of subsidence. this wall segment with caldera walls just to the Summitville caldera. The lenticular remnant of lavas and tuffs that north, which are tied to eruption of the La Jara South Fork Tuff. A petrologically distinctive filled the La Jara Canyon caldera is truncated Canyon Tuff, suggests a similar time of origin. rhyolitic tuff, which overlies the Ra Jadero Tuff along a inner, westward-dipping unconformity, Thus, surviving remnants of the east and west on the north and northwest flanks of the Platoro now interpreted as the east topographic wall of topographic walls demonstrate that the La Jara caldera complex, is here named the “South Fork the Chiquito Peak caldera, against which are Canyon caldera was roughly similar in size to Tuff” (Table 2) and is included in the Treasure banked thick flows of Summitville Andesite the overall Platoro complex. The small expo- Mountain Group because (1) its distribution is (Fig. 3). In the Alamosa River Valley, this contact sures of intracaldera La Jara Canyon Tuff along similar to other Treasure Mountain units on the was previously mapped as a fault (Lipman, the Alamosa River indicate that caldera subsi- north and west flanks of the Platoro complex; (2) 1974), but it is highly irregular and dips steeply dence accompanied eruption of this tuff sheet. proximal-distal phenocryst size and abundance westward (which would anomalously imply a re- From the large extent and volume of its outflow variations in outflow South Fork Tuff indicate

Geological Society of America Bulletin, August 1996 1043

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 Figure 2. Generalized geologic map of the Platoro complex, showing preserved remnants of successive topographic walls related to eruptions of the La Jara Canyon and the Chiquito Peak Tuffs. Surficial deposits and most faults omitted. Revised from Lipman (1974).

1044 Geological Society of America Bulletin, August 1996

Figure 3. Geologic map of northeast margin of the Platoro complex, showing preserved remnants of successive topographic walls related to eruption of the La Jara Canyon and the Chiquito Peak Tuffs. Generalized and revised from Lipman (1974).

that this unit was erupted from the Platoro area; is thicker, more densely welded, and more phe- Juan volcanic field. In early regional studies, and (3) new 40Ar/39Ar determinations indicate nocryst rich closer to the caldera complex. Bulk- Cross and Larsen (1935) included the currently that the age of this tuff (28.76 ± 0.07 Ma, sample phenocryst contents of only about 5% recognized Masonic Park and Chiquito Peak Table 3; see footnote 1) is indistinguishable near the most distal exposures north of South tuff sheets within their undivided Treasure within analytical uncertainty from the underly- Fork (Fig. 1) are interpreted as the result of crys- Mountain Rhyolite. During 2° quadrangle map- ing Ra Jadero Tuff (28.77 ± 0.05). The South tal sorting during emplacement. In contrast, ping in the 1960s, the uppermost identified ash- Fork Tuff overlies Ra Jadero Tuff and is over- proximal South Fork Tuff at Bennett Peak flow unit in the Treasure Mountain was sepa- lain by Chiquito Peak Tuff at its designated type (Fig. 3) is phenocryst rich (25%). rated as the Masonic Park Tuff (Steven and locality east of the mouth of Willow Creek, 3 km Lipman, 1973), because it was found to extend east of South Fork (Fig. 2). The South Fork Tuff Eruption of the Chiquito Peak Tuff and much farther west and north than the earlier ash-

consists of low-silica rhyolite (72% SiO2), typi- Formation of the Main Platoro Caldera flow sheets of the Treasure Mountain. Although cally containing 10%Ð15% phenocrysts (plagio- concern was expressed about the great south- clase, sanidine, biotite, augite). Exposures in the The newly identified Chiquito Peak Tuff easterly extent of the Masonic Park Tuff, this South Fork area are a distal facies; this tuff sheet (Table 2) is a major ash-flow sheet of the San distribution was erroneously rationalized on the

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Figure 4. Geologic map of southwest margin of the Platoro complex, showing preserved remnants of successive topographic walls related to eruption of the La Jara Canyon and the Chiquito Peak Tuffs. Generalized and revised from Lipman (1974).

basis that underlying tuff sheets had provided a feldspar). In most areas, there is only a single benches along the contact demonstrate a break low-relief surface for Masonic Park ash flows lithology, and the two tuff units are present to- between two separate ash-flow sheets, rather (Lipman, 1975a, p. 41Ð45). gether only in a narrow zone along the South than compound cooling within a single, compo- A Chiquito Peak lithology subtly distinct Fork of the Rio Grande and Wolf Creek (Fig. 1). sitionally zoned sheet. from the Masonic Park was initially recognized The Chiquito Peak lithology is thus closely co- Outflow Tuff Sheet. The Chiquito Peak Tuff from small differences in biotite chemistry extensive with other large tuff sheets of the is a petrologically uniform crystal-rich dacite (Dungan and Lipman, 1988), paralleled by the Treasure Mountain Group. In the overlap corri- (plagioclase, biotite, augite, sanidine). The out- presence of sparse sanidine (5%Ð10% of total dor, local float of bedded tuff and sandstone on flow tuff sheet is typically only partly welded

1046 Geological Society of America Bulletin, August 1996

and vapor-phase crystallized; it contains ubiqui- Petrology. Despite the diagnostic presence of dera source areas. Six new sites for the Chiquito tous 1Ð2 cm lithic fragments, mainly brownish sanidine, bulk compositions of the Chiquito Peak, combined with published data, show con- andesite. The Chiquito Peak Tuff is named for Peak are similar to the La Jara Canyon Tuff, ex- siderable scatter in paleopole positions and some excellent palisade exposures on the southwest cept for slightly lower Y and heavy REE con- overlap with better-clustered data for the Masonic flank of Chiquito Peak (Fig. 2), where it over- tents in the Chiquito Peak (Dungan and Lip- Park Tuff (Fig. 5). Although the two data sets have lies the South Fork, Ra Jadero, and older tuff man, unpub. data). Other than this subtle overlapping circles of confidence, they represent sheets of the Treasure Mountain Group and is distinction, compositions vary more within each distinct populations at the 95% confidence level. overlain by distal Fish Canyon Tuff (Dungan et tuff sheet, mainly as a function of crystal-ash The Chiquito Peak data are less well clustered al., 1989a, Fig. 1-9). At Chiquito Peak, this tuff separation during emplacement, than between than directions for other San Juan welded tuffs, displays a weak compound cooling zonation but the tuff units. Also indistinguishable are phe- even though the within-site scatter is small after is only partly welded despite a thickness of nocryst compositions (plagioclase, biotite, progressive demagnetization. The scatter among about 100 m (near its outflow maximum). To augite) from these two units despite their having the Chiquito Peak sites may be related to the the north near South Fork, as many as three erupted nearly 1 m.y. apart, supporting interpre- widespread weak welding and vapor-phase crys- partly welded ledges are separated by weakly tation of eruption of both from the Platoro cal- tallization of this tuff, especially in distal areas welded zones of vapor-phase crystallization. It dera complex. Sanidine phenocrysts in the where all the paleomagnetic samples were ob- is commonly light tan to gray-tan, in contrast to Chiquito Peak are notably more potassic than tained. Similar problems have been encountered a more gray-green color of much of the Ma- those in other late tuffs of the Treasure Moun- with paleomagnetic results for vapor-phase zones

sonic Park Tuff. tain Group (Or64Ð67; in contrast to Or54 for Ra from some other welded tuffs (Hagstrum et al., The outflow Chiquito Peak Tuff is nowhere Jadero Tuff, and Or48Ð51 for South Fork Tuff). 1982; Hagstrum and Lipman, 1991). preserved within 5 km of the caldera margin. Such relatively potassic sanidine compositions The AMS data for Chiquito Peak Tuff show Around the south flank of the Platoro caldera, are typical of later tuffs from the central San well-clustered maximum and minimum direc- the closest Chiquito Peak is 10Ð20 km distant Juan caldera cluster (Lipman, 1975a; Lipman tions, with maxima near horizontal and minima from the caldera rim. In contrast, the La Jara and Weston, 1996), suggesting inception at Pla- close to vertical. The mean maximum direc- Canyon Tuff is present as a densely welded dark toro of a petrologic progression toward later- tions, assumed to represent flow directions or tuff unit along both east and west margins of the erupted magma types. syncompaction downslope creep, point toward Platoro caldera complex. This contrast in proxi- All the new analytical data are bulk samples the Platoro complex as the source, except for mal distribution probably reflects preferential except for a single pumice block from weakly two distal sites from north of South Fork, where erosion of the stratigraphically higher and less welded Chiquito Peak Tuff (Table 4, no. 7); pum- local paleotopography may have influenced welded Chiquito Peak Tuff, although both tuff ice lenses are difficult to separate and are com- flow directions. sheets preserve evidence of local depositional monly compositionally modified by alkali ex- wedge-outs against Conejos volcanic highlands change and other secondary processes. Samples Lava Flows Associated with the toward the caldera margins. from low and high in sections of both Chiquito Platoro Caldera Complex Intracaldera Tuff. The intracaldera tuff is Peak and Masonic Park Tuffs have yielded no in- densely welded, dark gray, and intensely propy- dication of chemical zoning or multiple magma Areally associated with the Platoro caldera litically altered; its appearance differs strikingly compositions, in contrast to those documented complex, and intertongued between many of the from any outflow tuff in the Treasure Mountain for the Ojito Creek and Ra Jadero Tuffs (Dungan ash-flow sheets, are thick sequences of locally Group (Lipman, 1975a, p. 27Ð29). Pumice and and Lipman, unpub. data). derived lavas. These lavas have previously been other pyroclastic textures have been widely Age. Intracaldera and outflow samples of the divided into the Summitville Andesite, the obliterated, and phenocrysts are variably altered Chiquito Peak Tuff yielded analytically indistin- Sheep Mountain Andesite, and the andesite of to epidote, calcite, chlorite, and other secondary guishable ages by the laser-fusion 40Ar/39Ar tech- Summit Peak (Steven et al., 1974), but the rein- minerals. On Cornwall Mountain, the intra- nique (Table 3; see footnote 1). The weighted terpreted relations among the ash-flow sheets caldera tuff is exposed over 800 m vertically means of two replicate analyses of each sample presented in this paper also have implications without reaching its base. are 28.43 ± 0.07 and 28.39 ± 0.07 Ma; the pre- for correlation of the associated lavas, espe- A key locality is a locally exposed marginal ferred age is the weighted mean of all four analy- cially among members of intracaldera Sum- vitrophyre along the southwest caldera wall, near ses, 28.41 ± 0.04 Ma. This result is supported by a mitville Andesite. The Summitville Andesite, Platoro Reservoir (Fig. 4). The vitrophyre and single 40Ar/39Ar age of 28.38 ± 0.09 Ma on the im- which is preserved largely as fill within the Pla- adjacent devitrified tuff preserve unaltered sani- mediately overlying dacite of Fisher Gulch. toro complex, contains two members: (1) a dine phenocrysts, in contrast to the absence of Paleomagnetic Evidence. Simple polarity widespread lower member of lavas, and associ- this phenocryst phase in all outflow La Jara data were early used to test correlations among ated sedimentary units, emplaced after eruption Canyon Tuff. This vitrophyre was originally in- tuff sheets from the Platoro complex (Lipman of the La Jara Canyon Tuff and (2) more areally terpreted by Dungan et al. (1989a) as an evolved and Steven, 1970), but only sparse data have restricted upper-member lavas interpreted as phase of the La Jara Canyon that was confined to been published for paleopole directions (Tanaka emplaced after eruption of later tuff sheets of the subsiding caldera, but sanidine has since been and Kono, 1973; Diehl et al., 1974) or aniso- the Treasure Mountain from the Summitville identified in the least altered tuff from several tropy of magnetic susceptibility (AMS) indica- caldera (Lipman, 1975a). other intracaldera sites. Accordingly, we interpret tions of flow directions (Ellwood, 1982). Both Reinterpretation of most of the exposed Pla- the entire main mass of exposed intracaldera tuff the Chiquito Peak and Masonic Park Tuffs, as toro caldera complex, as related to eruption of as Chiquito Peak, excluding only the small expo- reinterpreted here, were previously known to be the Chiquito Peak Tuff, requires that the bulk of sures along the western margin that appear to reversely polarized. exposed intracaldera lavas belongs to the upper represent a remnant of intracaldera La Jara Can- New paleomagnetic results (Table 5) further member of the Summitville Andesite. The only yon Tuff (Fig. 4). constrain correlations of these units and their cal- confirmed intracaldera lavas of lower-member

Geological Society of America Bulletin, August 1996 1047

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TABLE 4. CHEMICAL ANALYSES OF CHIQUITO PEAK AND MASONIC PARK TUFFS Column 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Chiquito Peak Tuff—outflow sheet Chiquito Peak—intracaldera tuff Masonic Park Tuff—outflow sheet Field 93L- MAD- 93L- 93L- 93L- 93L- MAD- 65L- 93L- MAD- 93L- 93L- 93L- MAD- 93L- 93L- MAD- MAD- 93L- 93L- no. 10B 128 11B 10A 9A 5B 192 127 14 8 15 16B 16A 241 11A 9B 240A 240 13B 5A

SiO2 63.76 64.2 64.36 64.66 65.15 66.52 66.9 68.2 63.99 64.3 64.38 65.58 66.01 61.62 61.84 62.10 63.51 63.52 64.88 68.23 Al2O3 17.36 17.45 16.67 17.13 16.78 16.58 16.25 16.4 17.24 17.94 17.47 17.14 17.06 16.45 16.33 16.20 16.10 16.08 15.51 13.56 Fe2O3T 4.62 4.70 4.35 4.24 4.18 3.59 3.43 3.3 4.61 4.29 4.31 3.96 3.88 6.51 6.37 6.58 5.63 5.63 5.59 5.00 MgO 1.28 1.47 1.38 1.42 1.36 0.95 1.02 0.76 1.06 1.42 1.26 0.64 0.74 2.47 2.29 2.30 1.87 1.82 2.01 1.45 CaO 3.76 3.99 4.06 3.54 3.31 2.93 3.35 2.8 4.04 3.83 3.46 3.38 3.06 4.65 4.85 4.42 4.61 4.69 5.03 4.34 Na2O 4.17 3.90 3.69 3.93 3.94 4.01 3.40 3.4 4.34 4.80 4.25 4.20 4.13 3.53 3.44 3.37 3.53 3.53 2.65 2.64 K2O 4.04 3.27 4.53 4.12 4.35 4.53 4.90 4.7 3.73 2.61 3.94 4.21 4.22 3.52 3.61 3.78 3.53 3.52 3.17 3.70 TiO2 0.63 0.63 0.59 0.59 0.59 0.55 0.48 0.49 0.61 0.60 0.59 0.58 0.57 0.84 0.81 0.81 0.78 0.78 0.75 0.68 P2O5 0.27 0.24 0.26 0.27 0.27 0.26 0.20 0.00 0.26 0.22 0.25 0.26 0.24 0.34 0.36 0.34 0.33 0.33 0.34 0.33 MnO 0.10 0.10 0.12 0.09 0.07 0.09 0.09 0.10 0.10 0.11 0.08 0.06 0.10 0.08 0.10 0.10 0.09 0.09 0.07 0.06 Total* 100.00 99.9 100.00 100.00 100.00 100.00 100.0 100.1 100.00 100.1 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

LOI† 2.16 N.D. 3.43 2.58 1.88 1.89 N.D. 1.40 3.01 1.23 1.60 1.13 1.54 2.07 2.20 0.60 0.41 3.70 2.16

Nb¤ 11 11 12 10 10 12 N.D. N.D. 10 11 10 10 10 10 13 10 14 14 14 11 Rb¤ 87 83 93 86 85 100 99 N.D. 79 132 85 88 94 84 85 90 80 72 74 50 Sr¤ 650 805 570 630 660 550 544 N.D. 700 784 660 620 620 580 580 540 600 590 650 470 Zr¤ 215 267 215 225 210 215 200 N.D. 225 247 210 215 225 160 170 160 176 170 166 138 Y¤ 27 22 24 27 21 30 22 N.D. 22 21 21 26 22 26 27 25 26 26 24 22 Ba¤ 1250 1050 1000 1100 1050 1100 1079 N.D. 1050 1022 1050 1050 1100 910 900 900 930 850 850 720 Ce¤ 75 76 65 70 70 73 N.D. N.D. 64 N.D. 64 63 73 64 76 78 62 66 74 50 La¤ 42 38 42 47 40 43 N.D. N.D. 31 N.D. 36 37 37 30 35 43 30 30 38 30 Notes: N.D.—not determined. New major oxide analyses are by X-ray fluorescence (XRF) methods by J. S. Mee and D. F. Siems; minor-element analyses by energy dispersive XRF (KEVEX) methods by J. Kent and B. W. King. Major oxides recalculated to 100%, volatile free. The following sample notes correspond to the numbers at the head of each column.Chiquito Peak Tuff: (1) Wolf Creek ski area, ridge crest; upper densely welded zone, lat 37¡27′22″N, long 106¡42′43″W. (2) Ridge crest 2 km east of Del Norte Peak; partly welded tuff. (3) Beaver Creek Youth Camp; lower welded zone, above andesitic lava flow, lat 37¡36′48″N, long 106¡40′47″W. (4) Wolf Creek ski area, ridge crest; lowermost densely welded zone, lat 37¡27′49″N, long 106¡43′23″W. (5) U.S. Highway 160 road cut, at Highway Springs Forest Service Campground; partly welded lower tuff, lat 37¡37′34″N, long 106¡40′55″W. (6) Ridge between West Bear and Alder Creeks, north of South Fork; lower cooling unit of partly welded tuff, above float of bedded tuff, lat 37¡41′11″N, long 106¡37′33″W. (7) Ridge 1.6 km north of Bishop Rock; nonwelded pumice block from base of tuff sheet (see Dungan et al., 1989, Fig. 1-8; from Dungan and Lipman, unpub. data).(8) East of Lake Creek (tributary of Wolf Creek), at elevation 3230 m (10 600 ft); lower densely welded tuff (from Lipman, 1975a, Table 6, no. 1).(9) Platoro Reservoir road; densely welded devitrified tuff with conspicuous pumice lenses and identifiable sanidine phenocrysts, lat 37¡19′50″N, long 106¡34′55″W. (10) Platoro Reservoir west bank; basal vitrophyre, lat 37¡19′10″N, long 106¡35′33″W (from Dungan and Lipman, unpub. data). (11) Platoro Reservoir west bank; devitrified tuff 5 m above vitrophyre (sample 7), lat 37¡19′10″N, long 106¡35′33″W. (12) Platoro Reservoir east side; densely welded devitrified tuff from base of cliff, lat 37¡19′22″N, long 106¡34′34″W. (13) Platoro Reservoir east side; densely welded devitrified tuff from top of cliff, lat 37¡19′18″N, long 106¡34′38″W. Masonic Park Tuff: (14) Nose of ridge, between East and Mid- dle Alder Creeks; upper cooling unit, lat 37¡42′46″N, long 106¡38′35″W. (15) Beaver Creek Youth Camp; upper partly welded zone, below andesitic lava flow, lat 37¡37′03″N, long 106¡40′59″W. (16) Highway Springs Forest Service Campground, partly welded upper tuff, lat 37¡37′22″N, long 106¡40′58″W. (17) Alder Creek Guard Station; weakly layered densely welded glassy tuff in middle of cooling unit, large- phenocryst phase; lat 37¡42′20″N, long 106¡38′42″W. (18) Alder Creek Guard Station; weakly layered densely welded glassy tuff, small-phenocryst phase; lat 37¡42′24″N, long 106¡38′44″W. (19) Mouth of Alder Creek; basal partly welded tuff, lapping onto andesite lava flow (Conejos Formation), lat 37¡44′46″N, long 106¡38′25″W. (20) Ridge between West Bear and Alder Creeks, north of South Fork; upper cooling unit of partly welded winnowed tuff, below float of bedded tuff, lat 37¡41′09″N, long 106¡37′32″W. *Total, recalculated. †LOI—loss on ignition. ¤Parts per million.

Summitville Andesite are along the eastern cal- Masonic Park Tuff (Table 2) has only a limited with no base exposed; it is still 175 m thick just dera margin (Fig. 3). Some lower-member an- extent in the southeastern San Juan volcanic north of South Fork, but within 2 km to the desite may also overlie intracaldera La Jara Can- field: along the Rio Grande and its tributaries southeast it wedges out to zero thickness. Al- yon Tuff in the western remnant of the La Jara north and west of South Fork (includes the type though some erosion may be involved in these Canyon caldera (Fig. 4), but criteria are currently locality), from Wolf Creek Pass west to Saddle abrupt thickness changes, most are due to em- inadequate to identify such rocks reliably. Mountain, and northward to the headwaters of placement processes, because cooling subunits Goose Creek (Fig. 1). Masonic Park Tuff is also thin and merge to the southeast. Masonic Park Tuff and Its Caldera Source exposed locally beneath younger formations as Contacts between Masonic Park and overly- far west as Williams Creek south of the Conti- ing Chiquito Peak Tuff are difficult to locate The Masonic Park Tuff is a phenocryst-rich nental Divide, and as far north as Bristol Head confidently at some sites, especially where distal dacite, differing only subtly from the Chiquito west of the Creede caldera (Steven et al., 1974). Masonic Park has fewer and smaller phenocrysts Peak Tuff. In addition to the absence of sanidine, Only along the South ForkÐWolf Creek corridor and a tan color. At most localities where the Ma- phenocrysts tend to be slightly larger and more is the Masonic Park Tuff overlain by the Chiq- sonic Park Tuff underlies the Chiquito Peak near abundant in the Masonic Park Tuff, and this tuff uito Peak Tuff. The Masonic Park Tuff widely South Fork, the contact is obscure and marked typically has a more greenish-gray or greenish- rests directly on lavas and associated rocks of only by a debris-covered bench between more brown color. The Masonic Park Tuff is character- the Conejos Formation; it overlies tuffs of the welded tuffs, little different in appearance from ized by exceptionally developed compound cool- Treasure Mountain Group only in the Turkey benches between cooling subunits within each ing zonations and well-defined flow-unit and Wolf Creek drainages. of these ash-flow sheets. On one ridge north of partings. The tops of cooling units are marked by Along its southeastern depositional margin, South Fork (point “Baxter” on the South Fork distinctive large-scale platy joints that weather to the thickness of the Masonic Park Tuff de- West topographic quadrangle map), a ranch road form slabby grus-covered slopes. creases abruptly over short distances. At its type exposes about 1 m of finely bedded white ash- Distribution and Thickness. The redefined locality (Fig. 1), this tuff is about 350 m thick fall tuff and gray silty tuffaceous sandstone

1048 Geological Society of America Bulletin, August 1996

TABLE 5. PALEOMAGNETIC AND ACCELERATOR MASS SPECTROMETRY DIRECTIONS α Site Location N/N Inc. Dec. K 95 Kmax-Inc. Kmax-Dec. KÐA95 An (¡) (¡) (%) (%) Chiquito Peak Tuff TT-7 Southeast of Chiquito Peak 5/7 Ð64.4 166.9 141 6.4 7.0 275.3 18.8 5.0 TT-24 Bishop Rock 7/8 Ð41.8 166.9 97 6.4 0.6 68.6 6.4 3.5 TT-63 Fox Creek 7/7 Ð45.6 154.4 600 12.4 13.3 321.0 16.4 3.4 MP-6 North of South Fork 6/8 Ð47.5 150.4 30 2.9 16.3 268.8 15.3 5.2 MP-7 Beaver Creek 8/8 Ð63.7 176.4 107 5.4 6.9 342.7 11.3 8.3 MP-9 Northwest of Chiquito Peak 7/8 Ð57.8 198.9 90 6.4 22.3 293.6 15.8 3.0 MP-1E* Upper Beaver Creek 6/6 Ð54.8 202.4 25 11.4 9.1 123.6 N.R. 0.9 MP-3E* Northeast of South Fork 6/6 Ð44.9 144.6 41 8.9 17.9 43.2 N.R. 2.7

Mean 8/8 Ð54.3 168.2 27 10.8

Masonic Park Tuff MP-1 Lower Alder Creek 7/7 Ð52.2 192.3 205 4.2 6.1 252.9 30.9 2.6 MP-2 Upper Alder Creek 8/8 Ð42.0 192.8 106 5.4 8.4 235.4 36.0 4.4 MP-3 Lower Alder Creek 8/8 Ð61.2 190.2 22 12.0 15.3 282.3 18.1 2.5 MP-5 North of South Fork 8/8 Ð62.3 178.2 155 4.5 5.6 279.5 29.3 5.9 MP-8 Beaver Creek 8/8 Ð57.1 193.0 181 3.9 18.1 2.5 37.7 3.4 MP-D† Masonic Park 10/10 Ð70.4 206.8 231 3.2 MP-T¤ Masonic Park 8 Ð61.0 179.2 618 2.2 MP-2E* Beaver Creek 6/6 Ð52.0 195.2 304 3.3 12.9 316.4 N.R. 0.7 MP-4E* Masonic Park 6/6 Ð55.7 185.5 583 2.4 Ð3.6 45.0 N.R. 0.8

Mean 8/9 Ð55.6 188.9 113 5.2 Notes: N/N: number of samples used in calculations compared to the number of samples measured; Inc.: mean inclination, positive downward; Dec.: mean declination, east of north; K: the precision parameter;α95: radius of cone of 95% confidence around the mean direction; Kmax-Inc: mean inclination of the maximum susceptibility direction; Kmax-Dec: mean inclination of the maximum susceptibility direction; KÐA95: the 95% cone of confidence about the mean direction of the maximum susceptibility; An: percent anisotropy, calculated from [(maximum susceptibility Ð minimum susceptibility)/mean susceptibility] × 100. N.R.—data not reported. *Ellwood (1992). †Diehl et al. (1974). Omitted from mean. ¤Tanaka and Kono (1973),

along the contact. At another South Fork locality Creek (Steven and Lipman, 1973). Here, the unit and Lipman (1976), on the basis of multiple fea- near the mouth of Beaver Creek, a local flow of previously mapped as lower Masonic Park (and tures that remain partly valid as indicators of Sheep Mountain Andesite along the contact be- correlated with the real Masonic Park south of eruptive proximity, but a Mount Hope source tween the two tuff sheets provides clear evidence the divide) is an Oligocene caldera-related land- now appears much less certain than previously of a depositional break; this section was previ- slide deposit. The net effect of these mapping inferred. No compelling evidence remains for ously interpreted as containing a local near- problems was to generate an internally consistent exposure of any segment of the Mount Hope cal- source lower sheet of Masonic Park Tuff (Lip- (but erroneous) picture of a large, regional upper- dera wall. Thick Fish Canyon rocks in the man and Steven, 1976). Southwest of Wolf tuff sheet, underlain near Mount Hope by a local Mount Hope area, previously interpreted as mas- Creek Pass, both Masonic Park and overlying lower unit. The current interpretation specifies a sive tuff that ponded within a preexisting Mount Chiquito Peak Tuffs are well exposed on cliff much less extensive single sheet of Masonic Park Hope caldera (Steven and Lipman, 1976), are faces near their respective wedge edges, where Tuff. now recognized as a large lava-dome complex of they are separated by interlayered flows of Petrology. In addition to the lack of sanidine, structureless crystal-rich dacite that is petrologi- Sheep Mountain Andesite. In this area, both tuff the Masonic Park Tuff contains sparse resorbed cally similar to overlying Fish Canyon Tuff. Cal- sheets were previously included in the Masonic quartz crystals that can be reliably identified dera subsidence in the Mount Hope area possi- Park Tuff (Steven et al., 1969, 1974). only in thin section. Plagioclase is less zoned bly localized subsequent eruption of the “Fish Mistaken identification of multiple sheets of than in the Chiquito Peak or La Jara Canyon Canyon” dome complex, premonitory to explo- Masonic Park Tuff in Turkey Creek and upper Tuffs. New bulk-rock chemical data (Table 4) sive eruption of Fish Canyon Tuff. An alternative Goose Creek (Steven and Lipman, 1973) con- show consistent differences between the Ma- caldera source for the Masonic Park Tuff, con- tributed greatly to confusion about the distribu- sonic Park and Chiquito Peak Tuffs (Fig. 6). The sistent with its revised distribution, may lie to the tion of this unit. In Turkey Creek, a lower unit Masonic Park Tuff is less silicic than the Chiq- northwest of Mount Hope, completely buried originally mapped as Masonic Park is distal La uito Peak (except for one winnowed distal sam- beneath the Creede or South River calderas

Jara Canyon Tuff. The upper unit in Turkey ple (Table 4, no. 20). At any SiO2 content, the (Fig. 1). Such an alternative location is hinted at Creek (true Masonic Park) was miscorrelated Masonic Park Tuff is lower in Na, K, Ba, La, Sr, by the AMS data (Fig. 5), although confident in- with the upper unit at Wolf Creek Pass (Chiquito and Zr and higher in total Fe, Mg, Ca, P, and Ti. terpretation of the AMS results is limited by Peak Tuff), erroneously implying that a regional The single published chemical analysis ascribed scatter in the maximum susceptibility directions upper unit was underlain by a local lower unit to the Masonic Park (Lipman, 1975a, Table 6, and also by location of all sample sites in the present only near margins of the Mount Hope no. 1) is actually from winnowed distal Chiquito northeast sector of the known Masonic Park caldera. Across the continental divide to the north Peak Tuff (Table 4, no. 8). Tuff. Wherever the source of the Masonic Park in Goose Creek, two units were also mapped as Caldera Source of the Masonic Park Tuff. Tuff, whether from the Mount Hope area or a Masonic Park, separated by andesitic lava flows The Mount Hope area was interpreted as a cal- concealed source farther northwest, this tuff and breccias designated the volcanics of Leopard dera source for the Masonic Park Tuff by Steven sheet documents overlapping times of eruptive

Geological Society of America Bulletin, August 1996 1049

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Figure 5. Paleomagnetic data for Chiquito Peak and Masonic Park Tuffs. (A) Paleomagnetic directions, upper-hemisphere plot (data listed in Table 5). Patterned squares indicate mean directions; ellipses show the 95% confidence limits. One published Ma- sonic Park site (Diehl et al., 1974) has a direction that differs substantially from two other nearby sites (MP-4E, MP-T); this site (MP-D) is omitted from the plot. (B) Directions of maximum anisotropy of magnetic susceptibility (AMS), assumed to represent emplacement flow directions, pointing away from source (arrows). Inferred generalized depositional extent of each tuff sheet shown by fine-dot pattern; older volcanic rocks, by vertical-line pattern; younger volcanic rocks, by diagonal-line pattern. Calderas: C— Creede, LG—La Garita, MH—Mount Hope, P— Platoro, S—Summitville, SRP—South River.

activity between the calderas in the southeastern tral San Juan region, commencing with eruption redefined La Garita caldera has dimensions of 30 and the central San Juan regions, comparable to of the Masonic Park Tuff and then the enormous ´ 70 km (Fig. 1). Distal outflow Fish Canyon the overlap long recognized between the western Fish Canyon Tuff from the La Garita caldera at Tuff ponded to a thickness of as much as 200 m and central regions (Steven and Lipman, 1976). 27.8 Ma. In conjunction with recognition of the within the Platoro caldera complex, where it is “Fish Canyon” dome complex in the south-cen- interleaved between upper Summitville Andesite Younger Volcanic Rocks of the tral San Juan Mountains, the southern topo- and younger dacitic lavas (volcanic rocks of Central San Juan Caldera Cluster graphic margin of a much enlarged La Garita cal- Green Ridge) beneath Silver Mountain (Fig. 3). Following early ash-flow volcanism from the dera has been newly identified 30 km south of Subsequent central San Juan calderas (Bachelor, Platoro and western caldera complexes (Table 1), where it was previously inferred to lie buried be- South River, San Luis, Creede), which were explosive volcanic activity converged on the cen- neath younger calderas in the Creede area. The sources of smaller-volume and more areally re-

1050 Geological Society of America Bulletin, August 1996

by lateral variations in the distal facies of both units, at least 5Ð10 km3, is comparable to deposits sheets, in which welding and crystallization associated with small historical calderas such as zonations of cooling subunits thin and merge to- Mount Pinatubo in 1991 (Wolfe, 1992). ward their depositional wedge-outs. The most Emplacement of the Ojito Creek Tuff was plausible volcanologic interpretation is that both blocked even more widely in the northeast to crystal-rich dacitic tuffs were characterized by northwest sector, apparently overtopping the low-energy emplacement in their distal regions, Platoro wall only in a narrow sector along the where they were too sluggish to override even present Park Creek (Lipman, 1975a, Fig. 23). In low topographic barriers: the ash-flows of each contrast, the next tuff sheet, the Ra Jadero, tuff sheet flowed down the gentle constructional spread freely to the north as well as in all other aprons surrounding their source regions, but directions, a change in eruptive behavior in- could not cross the Oligocene valley between the ferred to reflect breaking down of the northern two volcanic loci. The valley system that marks barrier by caldera collapse (Summitville?) dur- the boundary between the two tuff sheets has per- ing Ojito Creek eruptions. The South Fork Tuff sisted to the present, now approximated by the spread farther to the north than had any prior South Fork of the Rio Grande and by Wolf Creek Treasure Mountain tuff sheet, yet was blocked (Fig. 1). Similar pronounced effects of small from deposition to the south, probably owing to topographic obstacles have been described for further subsidence in the northern part of the Figure 6. Chemical plots, illustrating con- the 1912 Katmai eruption, where the distal pyro- Platoro complex in response to the Ra Jadero trasts between Chiquito Peak and Masonic clastic flow had neither the momentum nor the eruptions. These contrasting depositional pat- Park Tuffs. A: SiO2 vs. TiO2. B: SiO2 vs. Zr. degree of inflation necessary to surmount a terns, seemingly in response to changing cal- glacial moraine, even though the moraine was dera-wall geometries, imply low eruption en- only 5 m higher than the resulting ash-flow de- ergy and sluggish emplacement even for the posit (Hildreth, 1983). proximal ash-flow tuffs at Platoro, in contrast The persistence of Oligocene primary volcanic with the mobility of highly inflated ash flows paleotopography has long been recognized in the (Aramaki and Ui, 1966; Miller and Smith, San Juan region (Steven, 1968), but the influ- 1977; Wilson and Walker, 1985). stricted tuff sheets (Table 1), can be visualized as ences of such paleotopography on subtle deposi- Finally, the large Chiquito Peak eruption, large volcanoes clustered within the La Garita tional features of the tuff sheets is just becoming which generated virtually all exposed features caldera. The general character of the central San clear. Other aspects of tuff deposition and distri- of the caldera complex, produced ash flows that Juan ash-flow activity is different from that at bution, brought into focus by reinterpretation of spread in all directions farther than earlier tuff Platoro: ash-flow eruptions were less frequent the Platoro complex, include influences of pre- sheets as well as ponding to great thickness but more voluminous, sanidine-rich rhyolites are caldera topography and changing patterns of cal- within the caldera. Intracaldera Chiquito Peak volumetrically important, compositional zona- dera subsidence on extracaldera distribution as Tuff was resurgently uplifted after deposition, tions to later-erupted dacites are common, post- well as ponding of tuff within subsiding areas. but as an asymmetrical trap-door block rather caldera lavas tend to be more evolved, and asso- The discontinuous distributions and loci of max- than a simple structural dome. ciated caldera subsidences are less confocal. imum thickness of the lower rhyolite tuff and Black Mountain Tuff, in basins at mid distances Recurrent Caldera Subsidence ASH-FLOW EMPLACEMENT AND from the caldera complex, reflect the dominant CALDERA EVOLUTION influence of constructional topography on the The Platoro complex is an extreme example, flanks of the clustered Conejos volcanoes in the for the San Juan field and elsewhere, in terms of Reinterpretation of the Masonic Park Tuff as area that became the Platoro complex (Lipman, successive voluminous ash-flow eruptions that al- two ash-flow sheets of similar phenocryst-rich 1975a). Major caldera collapse within the Platoro ternated with more quiescent lavas from an are- dacite erupted from separate source calderas has area almost certainly accompanied the early tuff ally restricted source. Geologically recent multi- important implications for ash-flow emplace- eruptions, based on their considerable volumes, cyclic caldera complexes are relatively few, and ment mechanisms, recurrent caldera subsi- but no evidence concerning the size or location of none records as many successive eruptions from a dence, and magma generation in subcaldera subsided areas is preserved. single source. Younger counterparts include Aso continental crust. Because of smoothing of the topography by the caldera in Japan, from which four major ash-flow early tuff sheets, the large-volume La Jara Canyon sheets erupted between about 300 and 29 ka in- Ash-Flow Emplacement Tuff was able to spread widely with less local terspersed with andesitic lavas (Ono et al., 1981; variation in thickness. Eruption of the La Jara Machida et al., 1985); the Valles caldera in New The nearly antithetic distributions of Chiquito Canyon Tuff caused large-scale caldera subsi- Mexico, where both ash-flow members of the Peak and Masonic Park Tuffs in the South dence, involving essentially the entire area of the Bandelier Tuff (1.45Ð1.1 Ma) are now recognized ForkÐWolf Creek corridor document impressive Platoro caldera complex. La Jara Canyon caldera to have erupted from a common subsidence area influences of even gentle depositional slopes on subsidence generated a northeast barrier, most (Nielson and Hulen, 1984; Self et al., 1986); and emplacement of sluggish ash flows such as these likely a high caldera wall, which impeded spread- Santorini caldera in Greece where at least four crystal-rich dacites. The limited overlap between ing of the middle tuff sheets in this direction. caldera subsidences since about 100 ka have ac- these two lithologically similar tuff sheets is a Even the relatively small multiple eruptions of the companied ash-flow eruptions with volumes of as primary depositional feature, only slightly modi- middle tuff may have been associated with minor much as 25 km3 (Druitt and Francaviglia, 1992). fied by subsequent erosion. This fact is indicated subsidence events. The typical size of these tuff Although the evidence is clear for major sub-

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sidence accompanying the Chiquito Peak eruption, as indicated by the nearly 1 km exposed thickness of intracaldera tuff on the uplifted Cornwall Mountain block, the subsidence history during earlier eruptions is obscure. The wide areal extent and large outflow volume of the La Jara Canyon Tuff, the large east-west dimension of its caldera, and its fragmentary intracaldera exposure in the upper Alamosa drainage all suggest syneruptive caldera subsidence of a volume comparable to that of the Chiquito Peak. On the basis of their outflow volumes, eruptions of the Black Mountain, Ojito Creek, Ra Jadero, and South Fork Tuffs were also likely to have been accompanied by substantial caldera collapses, but the volumes of tuff ponded within their associated calderas can be inferred only by analogy with better-exposed systems. For large calderas, on average, about half the total eruptive volume is deposited as the outflow tuff sheet, and about half ponds within the concurrently subsiding caldera (Lipman, 1984). Recognition of a Platoro caldera source for the Chiquito Peak Tuff increases the estimated volume of volcanic products explosively erupted from this magmatic system by more than 50%, to at least 2600 km3 (Table 1). Emplacement of the voluminous tuffs of the Treasure Mountain Figure 7. Cumulative time vs. volume plots for ash-flow eruptions from the Platoro caldera Group in the relatively brief interval between complex (solid line) and the central San Juan caldera cluster (dashed line). Platoro tuff units: about 29.5 and 28.4 Ma (Table 3; see footnote 1) LR—lower rhyolite, BM—Black Mountain, JC—La Jara Canyon, MT—middle tuff, OC— indicates an integrated eruption rate of about Ojito Creek, RJ—Ra Jadero, SF—South Fork, CP—Chiquito Peak. Central caldera cluster 2300 km3/m.y. (Fig. 7). Such a rate is among the tuff units: MP—Masonic Park, FC—Fish Canyon, CR—Carpenter Ridge, BC—Blue Creek, highest documented for sustained activity from a WP—Wason Park, RC—Rat Creek, CC—Cebolla Creek, NM—Nelson Mountain, SM— single continental volcanic center. A higher rate Snowshoe Mountain. Volume and age data from Table 1. of about 4000 km3/m.y. characterizes the nine major ash-flow sheets of the central San Juan caldera cluster, but this value is dominated by the at Platoro would perpetuate a thermally weak ing a relatively coherent platelike floor (Lip- unique Fish Canyon Tuff (4000+ km3) and the crustal locus, retarding solidification of the sub- man, 1984). Other caldera interiors that contain caldera sources are less nearly confocal. The av- caldera reservoir, and focusing subsequent rise complexly fragmented floors have been inter- erage magma supply for the early intermediate- and accumulation of additional magma. An- preted as having been formed by piecemeal col- composition lava sequence of the San Juan field other factor may have been the limited genera- lapse within downsagged margins, without (Conejos Formation and equivalents) is also tion of silicic differentiates in the evolving sub- bounding ring faults (Branney and Kokelaar, high, about 5000 km3/m.y., but these rocks were caldera magma, as documented by the recurrent 1994). Such structural complexities in some erupted from scattered central volcanoes distrib- eruption of andesitic lavas between more silicic calderas likely result from recurrent eruptions uted irregularly over much of the associated vol- explosive deposits, and by the absence of a post- and accompanying subsidence, as exemplified canic field. In contrast, the Platoro complex magmatic Bouguer gravity low comparable to by Platoro. erupted a volume that, if withdrawn solely from that associated with the central and western San beneath the caldera area (about 300 km2), would Juan caldera clusters (Plouff and Pakiser, 1972; Magma-Generation Processes be equivalent to a vertical crustal section of Steven and Lipman, 1976) and other caldera 8Ð9 km. These facts do not reveal the “unroofed systems characterized by silicic eruptive activ- Petrologic studies (Dungan and Lipman, un- batholith” envisioned by Daly (1933) and Hamil- ity. Such a dominantly intermediate-composi- pub. data) further constrain the evolution of the ton and Myers (1967), so much as an eviscerated tion system would tend to be hotter and less vis- Platoro magmatic system. Elemental and iso- batholith. cous, further augmenting a thermally weakened topic compositions document broad petrologic Why other caldera complexes, such as that of upper-crustal magmatic locus. continuities among products of the Platoro com- the central San Juan field, develop overlapping Recurrent subsidence at varying loci within a plex, including similar mineral chemistry, in- but less confocal subsidence structures is un- caldera complex also provides a mechanism for volvement of multiple magma components in clear, but must ultimately be related to structural generating a structurally disrupted caldera floor, some tuff sheets, and plausible generation of the controls on upper-crustal accumulation of even if individual collapse events are accommo- silicic tuffs by crystal fractionation from an- magma in subvolcanic chambers. The excep- dated along ring faults. Many deeply eroded desitic parental magmas similar to those erupted tional frequency of sizable pyroclastic eruptions calderas expose well-defined ring faults, bound- from Conejos volcanoes prior to ash-flow mag-

1052 Geological Society of America Bulletin, August 1996

matism. The continued availability of such derived basaltic melts and lower continental percentage of erupted magma in such caldera magma throughout evolution of the caldera crust to produce the dominant characteristics of systems is estimated at 10%Ð25% (Smith, 1979; complex—but without generation of a silicic the erupted volcanic deposits throughout the Crisp, 1984). The subcaldera magma chamber at batholith in the upper crust—is indicated by the San Juan field (Lipman et al., 1978; Dungan et Platoro must have developed in the upper crust sparse presence of hornblende-andesite pumice al., 1989b, 1995; Riciputi and Johnson, 1990; at an early stage, perhaps initially during the in late-erupted upper parts of some dacite tuff Riciputi et al., 1995). growth of the clustered Conejos volcanoes, be- sheets, andesitic lavas interleaved at six hori- In addition to the large volume of erupted cause a large shallow magma reservoir must have zons between the tuff sheets, and the absence of magma at caldera systems such as Platoro, even existed to accommodate caldera subsidence dur- an associated Bouguer gravity low. Isotopic greater volumes of magma must have remained ing eruption of the La Jara Canyon Tuff. Contin- studies confirm major assimilation-crystalliza- in the upper-crustal magma chamber into which uing presence of a shallow magma reservoir is tion-fractionation interactions between mantle- the caldera complex subsided recurrently; the documented by the Chiquito Peak collapse and

Figure 8. Schematic northwest-southeast section through the Platoro caldera complex, illustrating inferred cumulative thickness of ponded intracaldera fills associated with successive ash-flow and lava eruptions. Location of section shown in Figure 3. Thickness estimates for individual tuff units are based on volumes of their outflow sheet and analogies with more deeply exposed caldera systems in the San Juan field and elsewhere. No coeruptive subsidence is included for the multiple small eruptions recorded by the middle tuff, even though these tuffs are individually similar in size (5Ð10 km3) to historical eruptions associated with small caldera collapses. Initial caldera subsidences (lower rhyolite tuff, Black Mountain Tuff) are arbitrarily assumed to have involved entire caldera complex. Intracaldera lava flows are inferred only for horizons where such flows are exposed within the caldera or exist in proximal outflow sections; additional interleaved flows are likely present at other horizons in the intracaldera section. Enlargement of topographic caldera wall by landslide slumping (which augments the volume and thickness of caldera fill accumulating within the ring fault) is shown schematically, and only for the last (Chiquito Peak) eruption. For simplicity, present-day topography is omitted, postÐChiquito Peak resurgence of the Cornwall Mountain block is disregarded, and ring faults are arbitrarily shown as vertical. The former existence of large volcanic constructs above the Alamosa River and Jasper stocks (dot-dash line) is required to provide a roof for these high-level plutons, which are currently exposed at elevations approximating or exceeding the regional Oligocene land surface at the time of volcanic activity from the Platoro complex. The inferred size and shape of the overall subcaldera batholith in its final position (dashed line) are highly schematic, drawn to underlie the entire caldera complex, to connect with the exposed Alamos River, Jasper, and Cornwall Mountain intrusions (adjacent to the line of section), and to indicate that much of the caldera-fill sequence must have been removed, probably by massive stoping, during upward rise of the batholith.

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subsequent resurgent uplift of the Cornwall We also thank Balsley, K. Hon, Steven, and Hagstrum, J. T., Lipman, P. W., and Elston, D. P., 1982, Paleomagnetic evidence bearing on the structural development of the Latir vol- Mountain block. Several covolcanic intrusions S. Self for helpful comments on earlier drafts canic field near Questa, New Mexico: Journal of Geophysical Re- (including the Alamosa River and Jasper stocks: of this paper. search, v. 87, p. 7833Ð7842. Hamilton, W., and Myers, W. B., 1967, The nature of batholiths: U.S. Fig. 2) reached high structural levels, near the el- Geological Survey Professional Paper 554-C, p. C1-C30. REFERENCES CITED Hildreth, W., 1981, Gradients in silicic magma chambers: Implications evation of the regional land surface, and must for lithospheric magmatism: Journal of Geophysical Research, have been capped by high-standing volcanoes. Aramaki, S., and Ui, T., 1966, Aira and Ata pyroclastic flows and related v. 86, p. 10153Ð10192. caldera depressions in southern Kyushu, Japan: Bulletin of Vol- Hildreth, W., 1983, The compositionally zoned eruption of 1912 in the The overall effect of such magma-generation canology, v. 29, p. 29Ð48. Valley of Ten Thousand Smokes, Katmai National Park, Alaska: processes would have been to profoundly recon- Balsley, S. D., 1994, A combined stratigraphic, chronologic, and petro- Journal of Volcanology and Geothermal Research, v. 18, p. 1Ð56. logic study of an Oligocene post-collapse pyroclastic sequence, Hildreth, W., and Mahood, G., 1985, Correlation of ash-flow tuffs: Ge- struct the crustal column beneath the caldera sys- southeastern San Juan Mountains, Colorado: The middle tuff ological Society of America Bulletin, v. 96, p. 968Ð974. tem and associated upper-crustal magma cham- member of the Treasure Mountain Tuff [Ph.D. dissert.]: Dallas, John, D. A., 1995, Tilted middle Tertiary ash-flow calderas and subja- Texas, Southern Methodist University, 197 p. cent granitic plutons, southern Stillwater Range, Nevada: Cross ber, as large volumes of mantle-derived basalt Balsley, S. D., Brown, L. L., Lux, D. R., Ferguson, K. F., Colucci, M. T., sections of an Oligocene igneous center: Geological Society of Dungan, M. A., and Lipman, P.W., 1988, Combined 40Ar/39Ar, pa- America Bulletin, v. 107, p. 180Ð200. crystallized and hybridized with crustal rocks leomagnetic, and stratigraphic constraints on the eruptive history Johnson, C. M., 1991, Large-scale crust formation and lithosphere mod- (Hildreth, 1981; Lipman, 1988; Johnson, 1991). of the Platoro-Summitville caldera complex, southeast San Juan ification beneath middle to late Cenozoic calderas and volcanic volcanic field, Colorado [abs.]: Geological Society of America fields, western North America: Journal of Geophysical Research, A final complexity in the magmatic evolution Abstracts with Programs, v. 20, p. A368ÐA369. v. 96, p. 13485Ð13508. of a multicyclic caldera system such as Platoro is Best, M. G., Christiansen, E. H., Deino, A. L., Grommé, C. S., and Kunk, M. J., Sutter, J. F., and Naeser, C. W., 1985, High-precision Tingey, G., 1995, Correlation and emplacement of a large, zoned, 40Ar/39Ar ages of sanidine, biotite, hornblende, and plagioclase the fate of the caldera floor and early-emplaced discontinuously exposed, rhyolitic ash-flow tuff, Nevada: from the Fish Canyon Tuff, San Juan volcanic field, south-central caldera fill, as recurrent subsidence events cu- 40Ar/39Ar chronology, paleomagnetism, and petrology of the Colorado: Geological Society of America Abstracts with Pro- Pahranagat Formation: Journal of Geophysical Research, v. 100, grams, v. 17, p. 636. mulatively displace such material downward. At p. 24593Ð24609. Lanphere, M. A., 1988, High-resolution 40Ar/39Ar chronology of Branney, M. J., and Kokelaar, P., 1994, Volcanotectonic faulting, soft- Oligocene volcanic rocks, San Juan Mountains, Colorado: Platoro, the cumulative thickness of intracaldera state deformation, and rheomorphism of tuffs during develop- Geochimica et Cosmochimica Acta, v. 52, p. 1425Ð1434. fill likely reached 10 km or more (Fig. 8), on the ment of a piecemeal caldera, English Lake District: Geological Larsen, E. S., Jr., and Cross, W., 1956, Geology and petrology of the San Society of America Bulletin, v. 106, p. 507Ð530. Juan region, southwestern Colorado: U.S. Geological Survey Pro- basis of the exposed thicknesses of intracaldera Cebula, G. T., Kunk, M. J., Mehnert, H. H., Naeser, C. W., Obradovich, fessional Paper 258, 303 p. Chiquito Peak Tuff in the Cornwall Mountain J. D., and Sutter, J. F., 1986, The Fish Canyon Tuff, a potential Lipman, P. W., 1974, Geologic map of the Platoro caldera area, south- standard for the 40Ar/39Ar and fission-track methods, in Abstracts, eastern San Juan Mountains, Colorado: U.S. Geological Survey block, analogy with observed thickness of cal- Sixth International Conference, Geochronology, Cosmochronol- Miscellaneous Investigations Series Map I-828. dera-fill assemblages in other San Juan calderas ogy and Isotope Geology, Cambridge, 1986: Terra Cognita, v. 6, Lipman, P. W., 1975a, Evolution of the Platoro caldera complex and re- no. 2, p. 139Ð140. lated volcanic rocks, southeastern San Juan Mountains, Colorado: and elsewhere, and prorated thicknesses for Crisp, J., 1984, Rates of magma emplacement and volcanic output: U.S. Geological Survey Professional Paper 852, 128 p. Journal of Volcanology and Geothermal Research, v. 20, Lipman, P. W., 1975b, Geologic map of the lower Conejos River horizons of Summitville Andesite (assumptions p. 177Ð211. Canyon area, southeastern San Juan Mountains, Colorado: U.S. discussed more fully in the caption for Fig. 8). Colucci, M. T., Dungan, M. T., Ferguson, K. M., Lipman, P. W., and Geological Survey Miscellaneous Investigations Series Map Moorbath, S., 1991, Precaldera lavas of the southeast San Juan I-901. Complete sections through caldera fills, in- volcanic field: Parent magmas and crustal interactions: Journal of Lipman, P. W., 1984, The roots of ash-flow calderas in North America: cluding caldera floor, are rare, but syncollapse Geophysical Research, v. 96, p. 13412Ð13434. Windows into the tops of granitic batholiths: Journal of Geo- Cross, W., and Larsen, E. S., Jr., 1935, A brief review of the geology of physical Research, v. 89, p. 8801Ð8841. sections identified recently in New Mexico, Ari- the San Juan region of southwestern Colorado: U.S. Geological Lipman, P. W., 1988, Evolution of silicic magma in the upper crust: The zona, and Nevada (Lipman et al., 1989; Lipman, Survey Bulletin 843, 138 p. mid-Tertiary Latir volcanic field and its cogenetic granitic Daly, R. A., 1933, Igneous rocks and the depths of the earth: New York, batholith, northern New Mexico, USA: Royal Society of Edin- 1993; John, 1995) are as thick as 5 km for depos- McGraw Hill Book Co., 598 p. burgh, Transactions, v. 79, p. 265Ð288. Deino, A. L., and Potts, R., 1990, Single-crystal 40Ar/39Ar dating of the Lipman, P. W., 1993, Geologic map of the Tucson Mountains caldera, its from individual ash-flow eruptions that are Olorgesailie Formation, southern Kenya rift: Journal of Geophys- southern Arizona: U.S. Geological Survey Miscellaneous Investi- comparable in size to larger events at the Platoro ical Research, v. 95, p. 8453Ð8470. gations Map I-2205. Deino, A., Tauxe, L., Monaghan, M., and Drake, R., 1990, Single-crys- Lipman, P. W., and Steven, T. A., 1970, Reconnaissance geology and complex. Comparably thick intracaldera tuffs tal 40Ar/39Ar ages and the litho- and paleomagnetic stratigraphies economic significance of the Platoro caldera, southeastern San likely accumulated repeatedly within the Platoro of the Ngorora Formation, Kenya: Journal of Geology, v. 98, Juan Mountains, Colorado: U.S. Geological Survey Professional p. 567Ð587. Paper 700-C, p. C19-C29. complex during its multiple eruptive cycles, re- Diehl, J. F., Beck, M. E., and Lipman, P. W., 1974, Palaeomagnetism Lipman, P.W., and Steven, T. A., 1976, Geologic map of the South Fork quiring much of the intracaldera section to have and magnetic-polarity zonation in some Oligocene volcanic rocks area, southeastern San Juan Mountains, Colorado: U.S. Geologi- of the San Juan Mountains, southwestern Colorado: Royal Astro- cal Survey Miscellaneous Investigations Series Map I-966. been recurrently stoped and assimilated, in order nomical Society Geophysical Journal, v. 37, p. 323Ð332. Lipman, P.W., and Weston, P. E., 1996, Phenocryst compositions of late Druitt, T., and Francaviglia, V., 1992, Caldera formation on Santorini ash-flow tuffs from the central San Juan caldera cluster—Results to permit the subcaldera magma chamber to re- and the physiography of the islands in the late Bronze Age: Bul- from Creede drill-hole samples and implications for regional main in the upper crust. Such a process provides a letin of Volcanology, v. 54, p. 484Ð493. stratigraphy: U.S. Geological Survey Open-File Report, 16 p. (in Dungan, M. A., and Lipman, P. W., 1988, The Masonic Park Tuff and press). geologically rapid and efficient mechanism for Mt Hope caldera, Oligocene San Juan volcanic field (SJVF), Lipman, P.W., Steven, T. A., and Mehnert, H. A., 1970, Volcanic history cannibalizing and recycling upper-crustal vol- southwestern Colorado [abs.]: EOS (Transactions, American of the San Juan Mountains, Colorado, as indicated by potassium- Geophysical Union), v. 69, p. 1487. argon dating: Geological Society of America Bulletin, v. 81, canic products during the life span of a multi- Dungan, M. A., Lipman, P. W., Colucci, M. T., Ferguson, K. M., and p. 2329Ð2352. cyclic caldera system. Large-scale upper-crustal Balsley, S. D., 1989a, Southeastern (Platoro) caldera complex,in Lipman, P. W., Doe, B. R., Hedge, C. E., and Steven, T. A., 1978, Petro- Lipman, P. W., ed., IAVCEI fieldtrip guide: Oligocene-Miocene logic evolution of the San Juan volcanic field, southwestern Col- recycling of earlier-erupted volcanic material San Juan volcanic field, Colorado: New Mexico Bureau of Mines orado—Lead and strontium isotopic evidence: Geological Soci- and Mineral Resources Memoir 46, p. 305Ð329. ety of America Bulletin, v. 89, p. 59Ð82. adds yet another complexity to deciphering the Dungan, M. A., Colucci, M. T., Ferguson, K. M., Balsley, S. D., Moor- Lipman, P. W., Sawyer, D. A., and Hon, K., 1989, Central San Juan cal- magmatic sources of ash-flow magmas. bath, S., and Lipman, P. W., 1989b, A comparison of dominantly dera cluster: Field guide 3, South Fork to Lake City, in Lipman, andesitic pre-rift volcanism to dominantly basaltic rift volcanism, P. W., ed., IAVCEI fieldtrip guide: Oligocene-Miocene San Juan northern Rio Grande rift area: Insights into the relationships volcanic field, Colorado: New Mexico Bureau of Mines and Min- ACKNOWLEDGMENTS among tectonic setting, crust-magma interaction, and magmatic eral Resources Memoir 46, p. 330Ð350. differentiation [abs.]: New Mexico Bureau of Mines and Mineral Machida, H., Arai, F., and Momose, M., 1985, Aso-4 ash: A widespread Resources Bulletin 131, p. 78. tephra and its implications to the events of late Pleistocene in and In addition to our work reported here, impor- Dungan, M. A., Moorbath, S., Colucci, M. T., Balsley, S. D., and Fer- around Japan: Volcanological Society of Japan Bulletin, v. 30, guson, K. M., 1995, The origin and differentiation of magmas of p. 49Ð70. tant data bearing on our interpretations have the Oligocene SE San Juan volcanic field (Platoro caldera com- Miller, T. P., and Smith, R. L., 1977, Spectacular mobility of ash flows plex), Colorado, USA [abs.]: International Union of Geology and around Aniakchak and Fisher calderas, Alaska: Geology, v. 5, been generated during recent San Juan studies Geophysics (IUGG), Abstract volume, p. A444. p. 173Ð176. collaboratively with T. Steven, S. Balsley, M. Ellwood, B. B., 1982, Estimates of flow direction for calc-alkaline Nielson, D. L., and Hulen, J. B., 1984, Internal geology and evolution of welded tuffs and paleomagnetic data reliability from anisotropy the Redondo Dome, Valles caldera, New Mexico: Journal of Geo- Colucci, K. Fergusen, M. Lanphere, S. Moor- of magnetic susceptibility measurements, central San Juan Moun- physical Research, v. 89, p. 8695Ð8712. bath, and P. Weston. We especially thank Lan- tains, southwest Colorado: Earth and Planetary Science Letters, Ono, K., Kubotera, A., and Ota, K., 1981, Aso Volcano,in Kubotera, A., 40 39 v. 59, p. 303Ð314. ed., Field excursion guide to Sakurajima, Kirishima, and Aso Vol- phere for providing an unpublished Ar/ Ar Hagstrum, J. T., and Lipman, P. W., 1991, Late Cretaceous paleomag- canoes: Tokyo, Volcanological Society of Japan, p. 33Ð52. age for the Masonic Park Tuff. E. Lougee skill- netism of the Tucson Mountains: Implications for vertical axis ro- Plouff, D., and Pakiser, L. C., 1972, Gravity study of the San Juan tations in south central Arizona: Journal of Geophysical Re- Mountains, Colorado: U.S. Geological Survey Professional Paper fully drafted most of the figures for the paper. search, v. 96, p. 16069Ð16081. 800-B, p. 183Ð190.

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Riciputi, L. R., and Johnson, C. M., 1990, Nd and Pb isotope variations Smith, R. L., 1979, Ash-flow magmatism: Geological Society of Amer- western Colorado: U.S. Geological Survey Miscellaneous Geo- in the multicyclic central caldera cluster of the San Juan volcanic ica Special Paper 180, p. 5Ð27. logic Investigations Map I-764. field, Colorado, and implications for crustal hybridization: Geol- Steven, T. A., 1968, Critical review of the San Juan peneplain, south- Tanaka, H., and Kono, M., 1973, Paleomagnetism of the San Juan vol- ogy, v. 18, p. 975Ð978. western Colorado: U.S. Geological Survey Professional Paper canic field, Colorado, U.S.A.: Rock Magnetism and Paleogeo- Riciputi, L. R., Johnson, C. M., Sawyer, D. A., and Lipman, P.W., 1995, 594-I, 19 p. physics, v. 1, p. 71Ð76. Crustal and magmatic evolution in a large multicyclic caldera Steven, T. A., and Lipman, P. W., 1973, Geologic map of the Spar City Taylor, J. R., 1982, An introduction to error analysis: Mill Valley, Cali- complex: Isotopic evidence from the central San Juan volcanic quadrangle, Mineral County, Colorado: U.S. Geological Survey fornia, University Science Books, 270 p. field: Journal of Volcanology and Geothermal Research, v. 67, Map GQ-1052. Wilson, C. J. N., and Walker, G. P. L., 1985, The Taupo eruption, New p. 1Ð28. Steven, T. A., and Lipman, P. W., 1976, Calderas of the San Juan vol- Zealand. I. General aspects: Royal Society of London Philosoph- Samson, S. D., and Alexander, E. C., Jr., 1987, Calibration of the inter- canic field, southwestern Colorado: U.S. Geological Survey Pro- ical Transactions, v. A314, p. 199Ð228. laboratory 40Ar/39Ar dating standard, MMhb-1: Chemical Geol- fessional Paper 958, 35 p. Wolfe, E. W., 1992, The 1991 eruptions of Mount Pinatubo, Philippines: ogy, Isotope Geoscience, v. 66, p. 27Ð34. Steven, T. A., Schmitt, L. J., Jr., Sheridan, M. J., and Williams, F. E., Earthquakes & Volcanoes, v. 23, p. 5Ð37. Self, S., Goff, F., Gardner, J. N., Wright, J. V., and Kite, W. M., 1986, 1969, Mineral resources of the San Juan Primitive Area, Col-

Explosive rhyolitic volcanism in the Jemez Mountains: Vent lo- orado: U.S. Geological Survey Bulletin 1261-F, 187 p. MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 26, 1995 cations, caldera development, and relations to regional structure: Steven, T. A., Lipman, P. W., Hail, W. J., Jr., Barker, F., and Luedke, REVISED MANUSCRIPT RECEIVED DECEMBER 19, 1995 Journal of Geophysical Research, v. 91, 1779Ð1798. R. G., 1974, Geologic map of the Durango quadrangle, south- MANUSCRIPT ACCEPTED JANUARY 25, 1996

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