Structural, eruptive, and intrusive evolution of the Grizzly Peak caldera, Sawatch Range,

C. J. FRIDRICH* Department of Geology, Stanford University, Stanford, California 94305 R. P. SMITH Idaho National Engineering Laboratory, EG&G Idaho, Inc., P.O. Box 1625, M.S. 2107, Idaho Falls, Idaho 83415 ED DEWITT U.S. Geological Survey, M.S. 905, Box 25046, Federal Center, , Colorado 80225 E. H. McKEE U.S. Geological Survey, M.S. 941, 345 Middlefleld Road, Menlo Park, California 94025

ABSTRACT partly from emplacement of a composite SETTING granodiorite laccolith now exposed in the Volcanic and shallow intrusive features of eroded core of the dome. A belt of mafic latite The Grizzly Peak caldera extends southward the deeply dissected 34 Ma Grizzly Peak cal- to rhyolite porphyry dikes and small stocks along the crest of the Sawatch Range in west- dera in west- record evolu- formed across the center of the domed cal- central Colorado, from Independence Pass to tion of the magmatic center from pre-caldera dera during the waning of the magmatic cen- northern Taylor Park (Figs. 1 and 2). It lies in through post-resurgent stages. Pre-caldera ter. Latite intrusions in this suite represent the the western part of the , an intra- dikes, zones of hydrothermially altered rocks, penetration of relatively mafic magma to the cratonic arch that rose during Mississippian and and lava flows formed along a circular swarm surface following solidification of the felsic Pennsylvanian time (DeVoto, 1973). This late of cone-sheet fractures around the site of the subcaldera batholith. Paleozoic structure was reactivated by doming future caldera. Early, largely rhyolitic upper- and thrust faulting in the Late Cretaceous to crustal magmatism culminated in the caldera- INTRODUCTION early Tertiary Laramide orogeny (Tweto, 1979). forming eruption of the Grizzly Peak Tuff. Paleocene to early Oligocene erosion in the Intracaldera tuff is zoned from high-silica This paper describes the evolution and three- southern beveled off much of rhyolite at the base to low-silica rhyolite at dimensional configuration of structures and vol- the Sawatch and other Laramide-age uplifts, the eroded top and, further, contains dacite to canic and intrusive features in and around the forming a low-relief surface with few remnant mafic latite pumice lumps in two heteroge- Grizzly Peak caldera. The Grizzly Peak caldera highlands (Epis and Chapin, 1973). The caldera neous tuff layers in the upper third of the is an excellent site to analyze the structure and formed on a remnant highland near the Lara- preserved section. stratigraphy of a resurgent ash-flow caldera be- mide apex of the Sawatch anticline. Half of the erupted tuff ponded in the 17- cause of the nature of its exposure. Deep ero- Extensional faulting and renewed uplift of the by 23-km, >600-km3 caldera, filling the sion, culminating in recent glaciation, developed from early Miocene asymmetric depression to a compacted thick- an area-wide relief of 1.3 km and local relief of time to the present formed the current moun- ness locally >2.7 km, including intercalated 0.5 to 1.0 km in numerous steep-sided valleys tainous terrain and the Rio Grande rift. The axis rock-avalanche megabreccias shed from ring- that dissect the volcanic center. Stratigraphic of the rift in central Colorado is the Arkansas fault scarps. The asymmetric caldera has an levels exposed within the volcanic and shallow- Valley graben, which bisects the ancient Sa- inner ring-fracture zone that separates two intrusive system range through about 4 km, be- watch Uplift into the western Sawatch Range structural segments that collapsed to different cause the same high topographic relief cuts and the eastern (Fig. 1) depths. Caldera fill buried collapse structures across both resurged and unresurged parts of the (Tweto, 1979). Associated extension in the as they formed; the inner ring-fracture zone caldera. Part of the structure is eroded to a level Grizzly Peak area formed the Taylor Park basin, is a growth (or syn-depositional) fault in the where it resembles a ring complex rather than a a half-graben that cuts across the southern part single-cooling-unit tuff. Welded-tuff ring caldera. In other areas, most of the thick caldera of the caldera (Figs. 1, 2, and 3). dikes are locally exposed at erosion levels fill is preserved. The Grizzly Peak caldera is part of an Oligo- below the caldera fill. These dikes are rem- Field work for this study was completed from cene volcanic field that once covered much of nants of fissure vents in the outer ring- 1979-1981, 1980-1984, and 1983-1984 by the southern Rocky Mountains (Stevens and fracture zone. Smith, Fridrich, and DeWitt, respectively. For- Epis, 1968; Steven, 1975). The San Juan vol- Following collapse, the Grizzly Peak cal- mal naming of the Grizzly Peak Tuff, explana- canic field, 75 km south of Grizzy Peak, is the dera was uplifted, forming a complexly tion of petrologic nomenclature used, and raw largest erosional remnant of the Southern Rocky faulted resurgent dome. Resurgence resulted K-Ar data for dated units in the Grizzly Peak Mountain volcanic field, in which andesite lava caldera are contained in the Appendix. A field and breccia, the principal products in most of guide to the Grizzly Peak caldera (Johnson and the field, are overlain by rhyolite to dacite ash- *Present address: U.S. Department of Energy, Yucca Mountain Project Office, P.O. Box 98518, others, 1989) is recommended to readers as a flow tuffs erupted from large calderas (Steven M.S. 523, Las Vegas, Nevada 89193-8518. companion reference. and Lipman, 1976). Grizzly Peak is the north-

Geological Society of America Bulletin, v. 103, p. 1160-1177, 12 figs., 1 table, September 1991.

1160

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ernmost in a north-trending line of older cal- the Grizzly Peak caldera. Rhyolite was the pre- character and pattern of emplacement. The deras (including Mount Aetna and Bonanza) dominant magma to reach the upper crust. No dikes are older than the caldera because they are that lies to the northeast of the main group of mafic volcanism preceded formation of the cal- sources for clasts in collapse breccias in the in- San Juan calderas (Fig. 1) (Varga and Smith, dera, and the volume of dacite and andesite tracaldera Grizzly Peak Tuff (Ludington and 1984; Shannon, 1988). precursors emplaced at the current subvolcanic Yeoman, 1980). These porphyro-aphanitic Evolution of the Grizzly Peak caldera is level of exposure is very small. dikes are phenocryst poor (most have <5% phe- generally consistent with the resurgent cauldron An arcuate swarm of cone-sheet fractures, nocrysts) relative to Grizzly Peak Tuff. No sim- cycle of Smith and Bailey (1968). Four stages of many containing dikes, partially surrounds the ilar intrusions cut the tuff. Biotite from one of development can be discerned: (1) concentric Grizzly Peak caldera and extends 20 km to the the intrusions (the Middle Mountain stock, Fig. fracturing and shallow magmatism in and east (Figs. 1, 4, and 5) (Cruson, 1973, p. 77). 4) yielded K-Ar dates slightly older than that of around the site of the future caldera, related to The dike swarm extends around the southern the tuff (Fig. 6), but so close that these intrusions tumescence over a batholithic magma chamber, margin of the caldera but was not mapped there probably are early products of the same cycle of (2) eruption of the Grizzly Peak Tuff and con- owing to poor exposure. With few exceptions, igneous activity. The concentric arrangement of current collapse of the caldera, (3) resurgent the dikes and fractures in this system dip steeply the dikes around the caldera supports this doming, resulting partly from emplacement of a inward toward the caldera. Zones of altered hypothesis. large laccolith in the caldera fill, (4) post- rocks, which locally host greisen or porphyry- Approximately 10% of the dikes in the extra- resurgent emplacement of an east-west-trending type Mo mineralization or gold-bearing quartz- caldera swarm are unrelated to the Grizzly Peak belt of dikes and small stocks across the center of pyrite veins (Cruson, 1973; Ranta, 1974), were system. These include (1) dikes associated with the domed caldera. formed by hydrothermal activity along many of the 66 Ma Twin Lakes batholith, which are dis- the fractures and around several of the rhyolitic tinguished by megacrysts of quartz and ortho- PRE-CALDERA STAGE AND dikes and small stocks. The magmatism, altera- clase, and (2) dikes emplaced in faults related to EXTRACALDERA EXPRESSION OF tion, and mineralization indicate that the cone- the Rio Grande rift, which are petrologically MAGMA EMPLACEMENT sheet fracture system developed in association similar to Miocene intrusions in the region. The with, and tapped, a large volatile-rich magma map of extracaldera dikes (Fig. 4) includes all Field evidence of pre-caldera magmatism, chamber. three groups of dikes. cone-sheet fracture and dike formation, and hy- Most of the dikes in the extracaldera swarm The circular form of the pre-caldera cone- drothermal circulation is present in and around are cogenetic, based on their similar petrologic sheet swarm around the Grizzly Peak caldera is

108' 107' 106 107° 106*30 106*

Figure 1. Location map of the Grizzly Peak caldera. Initials indicate calderas: GP, Grizzly Peak; MA, Mount Aetna; B, Bonanza. Numbers are ages of calderas and major ash-flow sheets in millions of years. TPB, Taylor Park basin. (A) In relation to the Oligocene southern Rocky Mountain volcanic field, and (B) in the vicinity of the Sawatch Range. X is the extracaldera dike swarm; Y and Z are known remnants of the outflow sheet of the Grizzly Peak Tuff. (Map A modified from Lipman and others, 1970, and Lipman, 1976.)

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Independence Pass

welded

57 30 -

Figure 2. Generalized geologic map of the Grizzly Peak caldera. ABC is the line of cross section shown in Figure 3. In the explanation (see opposite page), "vent contacts" are the outlines of features inferred to be pyroclastic vents of the Grizzly Peak Tuff. Pre-caldera altered rocks are located at East Red Mountain, and pre-caldera lava and stock in the west-central part of the caldera.

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marker beds (breccias and heterogeneous-tuff layers) are present in the single-cooling-unit tuff, CALDERA MARGIN FAULT but no unit-wide markers and no sedimentary or Z^.-. TOPOGRAPHIC WALL, erosional breaks have been recognized in the VENT CONTACTS caldera fill. All partial cooling breaks in the in- tracaldera tuff are located around intercalated OTHER FAULTS breccias that acted as heat sinks, as described below. No clasts of other tuff units have been CONTACTS found in breccias of the Grizzly Peak Tuff, and INCLINATION OF COMPACTIONAL rare autoclasts commonly show evidence of hav- 10 FOLIATION IN TUFF EXPLANATION ing been hot on incoporation. QUATERNARY Numerous intercalated layers of caldera- FOR FIGURE 2 collapse breccia and two heterogeneous-tuff ho- ALLUVIUM rizons can be correlated between adjacent fault blocks in the caldera (Figs. 3 and 7). The OLIGOCENE heterogeneous tuffs contain collapsed pumice lumps that are larger and more mafic than those LATE• OR POST-RESURGENT found in the rest of the tuff. The thickness of the INTRUSIONS v A upper layer is about 40 m, and the lower one is Nv j -i ¿. £ 1 A A v RESURGENT INTRUSIONS at least 50 m thick. Nonrhyolitic pumice lumps in the heterogeneous-tuff layers range in compo- GRIZZLY PEAK TUFF sition from dacite to mafic latite (66% to 57% CALDERA-COLLAPSE BRECCIAS Si02). Although these more-mafic pumices compose about 10% of the layers, they contrib- M PRECALDERA LAVA AND STOCK .!•;;-,-/.'.-.-J uted little to the ashy matrices, which have rhy- ~7 PRECALDERA ALTERED ROCKS / ' olitic compositions similar to those of under- lying and overlying tuffs. PRE - TERTIARY Both heterogeneous-tuff layers directly under- UNDIVIDED lie caldera-collapse breccia in the tuff, suggesting that they represent exceptionally deep tapping of the density-stratified, compositionally zoned magma chamber in association with caldera- consistent with the stress pattern that would more abundant near the base of the tuff than at collapse events. Deeper tapping could have oc- occur over the perimeter of a buoyant batho- the top, and, in several areas, the basal part of curred because of a change in the rate or the lith-sized sill, about 25 km in diameter and per- the caldera fill is a monolithologic tuff breccia. mechanics of tapping of the magma chamber haps 10 km below the surface (Anderson, 1951; The lithic fragments represent several cubic (Spera and others, 1986) during, or just before, Pollard and Johnson, 1973). kilometers of pre-caldera rhyolite (Fridrich, major incidents of subsidence along ring frac- 1987). These lithic fragments have little varia- tures. The subsidence events then gave rise to the Pre-caldera Features within the Caldera tion in pétrographie characteristics and trace- breccias, owing to formation of high, nearly ver- element abundances and are similar both to the tical ring-fault scarps and to earthquakes that Evidence of pre-caldera magmatism within pre-caldera lava described above and to high- attended these faulting events. the caldera itself consists of (1) a high-silica silica rhyolite pumice lumps in the basal part of Mafic-upward zoning in the Grizzly Peak rhyolite lava flow and its subvolcanic roots ex- the Grizzly Peak Tuff (Fridrich, 1987). All three Tuff is taken to represent progressive downward posed along an inner ring-fracture zone in the could be the products of a single magma tapping of a compositionally layered magma col- southwestern part of the caldera (A in Fig. 4), chamber tapped within a short time interval. umn in which the top was most silicic, as has (2) a zone of advanced-argillic altered rocks as- The high-silica rhyolite lithic fragments may been inferred for other tuffs (Smith, 1979). sociated with a group of northeast-trending have been derived from a large volume of lava Zonation from the base of the tuff upward to quartz porphyry dikes along the eastern caldera flows, like the one noted above, which were the highest preserved stratigraphie level is margin (C in Fig. 4), and (3) a large volume of extruded along pre-caldera ring-fracture vents. shown by the following changes in bulk-tuff high-silica rhyolite lithic fragments in the intra- Onset of explosive pyroclastic eruption from composition. caldera tuff. Both the flow and the alteration vents in the same areas during the initial stages (1) Phenocryst modes change from (qtz > san zone are truncated by faults formed during col- of caldera formation would have preferentially = plag » bio > mt > > ap > zr > monazite) to lapse of the caldera, indicating a pre-caldera age. incorporated this one lithic type in the first- (plag > qtz > san > bio > mt > ap > zr » Both features formed at sites of future ring- erupted intracaldera tuff. monazite). fracture zones of the caldera, which suggests that (2) The volume percentage of phenocrysts in- development of the ring fractures in the roof of CALDERA-FORMING ERUPTION creases from 25% to 40%. the magma chamber preceded its caldera- (3) Silica content decreases from 77% to 70%, forming eruption. Stratigraphy of the Intracaldera Tuff accompanied by trends in other major and trace High-silica rhyolite lithic fragments are present elements similar to that of other quartz- throughout the intracaldera Grizzly Peak Tuff. The Grizzly Peak Tuff and caldera are the normative metaluminous suites (for example, These angular to subrounded inclusions are far products of a single climactic eruption. Local San Juan volcanic field, Lipman and others,

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megabreccia

km r 6

4

2

Taylor Park Fault sea level

Figure 3. Cross section of the Grizzly Peak caldera, constructed along line ABC as shown in Figure 2. Top section is a reconstruction of the caldera as it was immediately after collapse. Vertical bars are the three most complete sections of caldera fill, shown in Figure 7. Bottom section is as it appears today after resurgence, extensional faulting, and erosion. Symbols: R, late-resurgent Pine Creek intrusive body; S and T, 1 resurgent intrusions one and two; Z through U, the six subunits of the tuff as shown in Figure 7. No vertical exaggeration. , ,

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Mountain stock; RFZ, ring-fracture zone.

1978) with this range of SiC>2 contents (Fig. 7; magma in the density-stratified magma cham- element abundances over vertical distances of and Fridrich, 1987). ber; the pre-eruptive chamber did not have a 1.2 and 1.6 km, respectively (Fig. 7). The max- The intracaldera tuff is arbitrarily divided into continuous compositional gradient. A single ima in degree of welding and coarseness of 6 subunits based on variations in composition petrographic and compositional type of pumice, groundmass crystallization occur in the same

and on the positions of two especially thick and high-silica (77% Si02) rhyolite, is found in the stratigraphie position in all three sections; how- widespread breccias in the northeastern quad- basal tuff, whereas the rest of the tuff consists of ever, the coarseness of groundmass crystalliza- rant of the caldera (breccias shown by gravel various mixtures of high-silica rhyolite and six tion increases to the northeast (Fig. 8), consistent pattern, Fig. 7). The basal subunit of the Grizzly other pumice types (Fridrich and Mahood, with the increase in stratigraphie thickness (re- Peak Tuff is unzoned through a thickness that 1987). sulting in slower cooling). varies from 250 m in the least collapsed south- In the southeastern part of the caldera, the The zonal pattern of welding in the intracal- eastern quadrant of the caldera to >550 m in the preserved section of intracaldera tuff is about 0.7 dera Grizzly Peak Tuff differs from that typical most deeply collapsed northeastern quadrant km thick. StratigTaphically equivalent sections of outflow tuffs (Smith, 1960), owing mainly to (Fig. 7). The monotonous chemistry of the basal of caldera fill in the northwestern and northeast- chilling of the tuff in envelopes around breccias subunit is evidence that there was at least one ern quadrants of the caldera show the same zo- that were cold on incorporation in the tuff (Fig. voluminous layer of virtually homogeneous nation in phenocryst assemblage and trace- 8). Where tongues of breccia pinch out, the

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FAULT PHOTOLINEAMENT CALDERA-COLLAPSE FAULT TOPOGRAPHIC WALL FT'sAi, / ALTERATION ZONES - 38 52 30

Figure 5. Map of the cone-sheet fracture system around the Grizzly Peak caldera.

cooling breaks that developed against them end sink that caused it, and the tuff is most densely Sopris (Fig. 6). That tuff is contaminated with abruptly. Intracaldera tuff also chilled against welded and most coarsely crystallized at the cen- small lithic fragments of Precambrian biotite wall rocks at scarps of caldera-bounding ring ter of the caldera fill (Fig. 8). granite, and the sample dated was bulk tuff be- faults. The internal stratigraphic position of a cause fiamme are too small to separate. Fission- Outflow Tuffs locally developed vitrophyre (Fig. 8) and of a track dating on zircons from the tuff near Mount subjacent minor welding reversal (not shown) in Only two remnants of the outflow sheet of the Sopris yielded dates of 30.2 ± 2.1 and 30.1 ± 3.0 the lower part of the section is controlled by Grizzly Peak Tuff were found in this study. One Ma (Marvin and Dobson, 1979), which are not variations in the lithic content in the tuff. If is located about 50 km west-northwest of the quite within analytical error of the K-Ar dates of complicating factors such as the chilling against caldera, on the north flank of , and the intracaldera Grizzly Peak Tuff. Correlation the breccias are taken into account, the pattern the other is about 17 km east of the caldera, in to the Grizzly Peak Tuff is considered probable of welding and groundmass crystallization of the the Arkansas Valley graben (Fig. 1). K-Ar dates nonetheless because of the close similarity in intracaldera Grizzly Peak Tuff is that of a single for these two tuffs are considered compatible percentages, habits, and species of phenocrysts, cooling unit. None of the cooling breaks in the with the correlation in spite of the older date including the presence of euhedral monazite, as intracaldera tuff extends beyond the local heat (36.3 ± 0.9 Ma) on the remnant near Mount well as the composition of lithic fragments.

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best guess rocks of the caldera. The morphology of the age of caldera megabreccia bodies shows a progressive change inward from the caldera margin. The breccia Post-Resurgent bodies vary in general from wedges, where Sawmill Stock (SA WM-1) banked up against ring faults, to sheets, to tongues, and to groups of boulders strewn along Late-Resurgent horizons in the tuff, farther within the caldera Pine Ck Stock (PC-1) (Fig. 3). These morphologic changes are accom- Resurgent panied by inward increase in the proportion of Laccolith (NY-S) matrix and decrease in average clast size in the breccias. Some clasts in wedges near caldera Outflow Tuffs: Near Mt Sopris (D)-A walls are >0.5 km long, whereas clasts in the In Arkansas Valley (AVG-1) boulder horizons rarely exceed 10 m and are Intracaldera Tuff (fiamme) typically 0.2 to 2 m in diameter. Tops and, to a Biotite (MA-1) lesser extent, bases of breccia layers commonly Hornblende (ZB-2) are hummocky, owing to the protrusion of giant clasts into the overlying and underlying tuff. Precaldera Middle Mtn Stock (MM-1) Locally, exposures of caldera fill as thick as 800 m consist entirely of megabreccia. Low- angle internal contacts are commonly present in I I i -I— —I these huge thicknesses, separating layers that 30 32 34 36 38 40 differ in clast size, matrix, and clast lithology. AGE (m.y.) On close examination, sparse, discontinuous partings of tuff can in many cases be found at Figure 6. K-Ar dates of the Grizzly Peak Tuff and related intrusions, arranged with the these contacts. Thick sections of massive breccia youngest unit at the top, based on stratigraphic relations. If the five best data points (KjO locally grade laterally into sections of inter- contents indicating no alteration) are used, 34 Ma is the most likely age of the caldera. Raw bedded tuffs and breccias, indicating that some data shown in Appendix. of the thickest massive sections are the result of multiple emplacement events. Some single brec- cia emplacement units are, however, at least 300 Breccias in the Caldera Fill 1976), occurs as well-defined bodies in the m thick because they contain single clasts that Grizzly Peak Tuff throughout the caldera. The thick. Extremely coarse breccia, formed by rock clasts in these megabreccias are derived predom- Matrices of megabreccias consist of either avalanching from ring-fault scarps during cal- inantly from the Precambrian granites, gneisses, crushed rock or tuff. The interiors of some of the dera collapse (caldera-collapse breccia, Lipman, and migmatites that compose most of the wall larger breccia sheets are composed almost en- tirely of giant clasts molded into interlocking

UPPER r 3 HETEROGENEOUS TUFF HORIZON

Figure 7. Caldera-wide LOWER HETEROGENEOUS -2 stratigraphic correlation of TUFF HORIZON the Grizzly Peak Tuff, based on compositional zoning. Fault blocks A through F in the inset map are the areas where sections A through F -1 were measured. Gravel pat- tern shows two major meg- abreccias in the northeastern quadrant of the caldera. MONOTONOUS BASE

o 50 100 150 200 50 100 150 200 50 100 150 200 km SOUTHWEST NORTHWEST NORTHEAST

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shapes by internal brittle deformation. Many larger than 1 m across are internally shattered, Origin of the Breccias other breccia sheets have only crushed rock ma- causing the breccias to appear less coarse than trix in their cores. Tuff matrix is locally abun- they really are. True clast size in the breccias can The predominant lithologies of clasts in meg- dant, however, and varies from masses and seams commonly be discerned only from a distance of abreccias in the caldera fill change around the constituting <1% of total volume to a continu- tens to hundreds of meters, or by detailed map- Grizzly Peak caldera to match the distribution of ous, nearly supporting matrix that encloses giant ping, where the continuity of original structures rock types in the adjacent caldera walls (Fig. 9). boulders. such as dikes, compositional banding, and meta- Single emplacement units of breccia rarely in- Tuff matrix in the breccias shows evidence of morphic foliation can be discerned. Original clude different rock types that are not found in having been fluidized at the time of emplace- structures are offset and rotated to varying contact with each other in the walls. In at least ment. This evidence consists of the common degrees across sheared surfaces and narrow two exposures, vertical variations in lithology of dike-like form of tuff matrix seams, as well as a crushed-rock zones that riddle giant clasts, breccia clasts in the caldera fill appear to be an systematic decrease in grain size of tuff matrix resulting in a jigsaw-puzzle appearance. The ex- inversion of the stratigraphy in the wall rocks toward contacts with breccia clasts. Decrease in tent of internal disruption changes laterally (Fridrich, 1987). These relations as well as the grain size toward contacts results from shear across some breccia exposures, resulting in an changing morphology of the breccias and de- across the boundary layer of a fluidized suspen- apparent gradient in the degree of brecciation. crease in their clast size away from the walls sion (Bagnold, 1954). The tuff matrix seams Many single breccia units are heterogeneous indicate that the breccias originated by repeated formed from hot, fluidized tuff that continued to overall but are composed of numerous large slumping of the walls of the caldera. move upward through the breccias after they lenses or bands, each of which is internally The correspondence between the rock types came to rest. Large masses of tuff matrix in some homogeneous in rock type. These monolitho- of breccia clasts and that of nearby wall rocks of of the breccias show an increase in welding logic zones may have been single giant clasts the caldera is in some instances incongruous, toward their centers. that were shattered and brittly deformed into apparently owing to the current deep level of Within the megabreccias, nearly all clasts lens-shaped masses during emplacement. erosion. For instance, the Late Cretaceous Twin

SOUTHWEST NORTHEAST

BRECCIA

GRANOPHYRIC vitrophyre WELDING < DENSE

MODERATE TO UNWELDED

Figure 8. Illustrative cross section showing the pattern of welding zonation of the intracaldera Grizzly Peak Tuff. Vertical exaggeration is about 5 times. T-HSR and T-R are the tops of the high-silica rhyolite and rhyolite subunits of the tuff, respectively. IT-T is the inferred top of the tuff before erosion.

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Figure 9. Map of the Grizzly Peak caldera and surrounding rocks, show- ing correlations between the predominant lithology of clasts in caldera-col- lapse breccias and that of neighboring wall rocks and caldera-floor rocks. Heavy line surrounds in- tracaldera tuff and intru- sions. Small numbers de- note lithology of clasts in the breccias that correlate to large numbers for lithol- ogy of rocks in place. Wall-rock geology is mod- ified from DeWitt (unpub. mapping, 1984) and in- cludes compilation of mapping by Brock and Barker (1966), Ludington and Yeoman (1980), and Cunningham (1976).

QUATERNARY Twin L. Border Diorite EARLY PROTEROZOIC Alluvium PALEOZOIC 6 pctcp] Kroenke Granodiorite

OLIGOCENE Sedimentary Rocks 7 Granite of Sayers Pk. Tuff/lntruslons « «- ni Denny Ck. Gd. LATE PROTEROZOIC 8/9 (I m (r ut fl foliated/unfollated Megabreccia 4 p * St. Kevin Granite 10 Gd. of Henry Mtn. LATE CRETACEOUS 5 k'V ' 1 Granite of Taylor R. 11 ""s. Migmatite /. ' I Twin Lakes Granodiorlte

Lakes Granodiorite, which makes up a large caldera wall, makes up nearly half of the ex- explained if the roof of the Twin Lakes batholith portion of the northeastern wall of the caldera, is posed intracaldera breccia (Fig. 9). Detailed was largely intact at the time of caldera collapse. underrepresented in the breccias relative to its mapping around the caldera has shown that the Most features observed in megabreccias of the proportion in the wall rocks, whereas the Early Denny Creek Granodiorite occurs largely as Grizzly Peak caldera have been documented in Proterozoic Denny Creek Granodiorite, which roof pendants within and above the Twin Lakes studies of Quaternary rock-avalanche breccias: constitutes only a small portion of the eastern Granodiorite. Hence, the incongruity may be internal shattering of clasts without disaggrega-

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INNER RING FAULT

HIGH-SILICA RHYOLITE LOWER

Figure 10. Map of the intracaldera Grizzly Peak Tuff by subunit, with three cross sections: A-A', through fissure vent in center of caldera; B-B', through part of the inner ring-fracture zone, where the combined effects of stratigraphic growth during collapse and reactivation during resurgence result in a situation where the net throw reverses sense of offset with stratigraphic height; C-C', through the southwestern caldera margin, showing bowl-like configuration of caldera floor and upward change in dip of the tuff. Cross sections have no vertical exaggeration, are on a scale twice that of the map, and have the same symbols as does the map except in A-A', in which dashes indicate the attitude of compactional foliation in the tuff.

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tion, resulting in a jigsaw-puzzle appearance; km in diameter, near the northeastern caldera layer of surge-like tuff, typically about 20 cm local lithologic homogeneity manifested as lam- margin (Fig. 2) and (2) a probable 3-km-long thick. There is no cooling break at this surge-like inated monolithologic lenses or bands; inversion fissure vent in the center of the caldera (cross base, nor is there one at the base of the lower of stratigraphy; hummocky tops; and proximal section A-A', Fig. 10). The possible pipe is a heterogeneous tuff layer outside the vent; hence to distal changes in breccia morphology similar zone, approximately cylindrical in the high-relief the fissure vent was active during the caldera- to that documented here (compare with Voight, exposure, in which the rock type differs from forming eruption. Fiamme in the tuff within the 1978). The single feature that distinguishes the that of adjacent areas. Breccia in the pipe con- fissure vent are locally stretched and folded breccias at Grizzly Peak from typical rock- trasts with other breccias in the caldera in having owing to rheomorphic flow of the tuff down the avalanche breccias is their fluidal-textured tuff well-developed size sorting and rounding of the sides of the keel-like vent opening. matrix. This feature indicates that the Grizzly matrix-supported 2- to 10-m clasts, as well as Peak breccias formed near the source of, and intimate mixing of clasts of different rock types Collapse Structure concurrent with, a pyroclastic eruption. Similar not found in contact with each other in the cal- breccias associated with intracaldera tuffs in the dera walls. For example, St. Kevin Granite does Caldera subsidence at Grizzly Peak formed a of Colorado were inter- not contact Twin Lakes Granodiorite or unfo- fault-bounded depression having the shape of a preted by Lipman (1976) as landslide deposits liated Denny Creek Granodiorite in the walls segmented asymmetric bowl. The form of the formed by slumping at ring-fault scarps during (Fig. 9). The tuff matrix in this breccia shows caldera is as much like a bowl as it is like a caldera collapse. evidence of boundary-layer shear along the cylinder because (1) the caldera floor typically is bases and sides of large clasts but not along their downfaulted in steps across wide ring-fracture Vents for the Grizzly Peak Tuff tops, suggesting that the clasts were sinking in zones, (2) the scarps of ring faults of the caldera fluidized tuff or that the tuff was rising around slumped during collapse, and (3) the caldera Four dikes of ash-flow tuff with vertical com- them. The observed sorting and rounding of the floor sagged during deposition of the caldera fill. pactional foliation are present in the outermost clasts in this breccia suggest that the clasts were The bowl form is evident from the roughly an- ring fault of the Grizzly Peak caldera at East Red transported by turbulent pyroclastic flow rather nular distribution of the tuff subunits (Fig. 10), Mountain (Fig. 2). The caldera fill has largely than by gravity-driven plug flow, as is inferred especially in the western and central parts of the been eroded in this area, and, as presently ex- for all of the other breccias in the caldera fill. caldera. In some exposures immediately down- posed, the dikes are emplaced in the fault zone The upper size limit of clasts found in proximal thrown on ring faults (for example, along the between the wall and the subsided cauldron lag breccias in ignimbrites is 5 to 10 m (Druitt southwestern caldera margin), the dip of the block. The tuff dikes vary in width from about and Sparks, 1982; Bacon, 1983; Walker, 1985), compactional foliation of tuff changes upsection, 10 to 100 m. Each dike is distinct in bulk com- suggesting that this is the upper size limit for indicating that caldera fill thickens inward (Fig. position, and, together, the four cover most of pyroclastic transport of average-density lithic 2, and cross section C-C, Fig. 10). fragments. the compositional range of the Grizzly Peak Tuff The caldera is bounded by an outer ring- (Fridrich, 1987). The most mafic dike is only A probable fissure vent near the center of the fracture zone and is divided into two principal slightly sheared and has vitrophyric margins and Grizzly Peak caldera is marked by a vertically structural segments by an inner ring-fracture a densely welded, devitrified core. The three extensive, moderately to steeply west-dipping, zone (Fig. 4). Thin wedges of caldera collapse more-silicic dikes are extensively sheared owing irregular zone within the caldera fill. As exposed breccias are intercalated near the base of the tuff to displacement along the enclosing ring fault. on the present topography, this feature has the adjacent to both inner and outer ring-fracture Vertical compactional foliation in the tuff appearance in map view of two irregular-shaped zones, indicating that caldera collapse along dikes indicates that a horizontal compressive klippen discordantly overlying a truncated sec- these two fault systems began at the same time, stress was applied before the dikes cooled or, tion of subhorizonta! tuff and breccia layers near early during eruption of the Grizzly Peak Tuff. alternatively, that erupting tuff agglutinated the center of the caldera (Fig. 2, and cross sec- The double ring-fracture system along which against the sidewalls of the fissure vents, forming tion A-A', Fig. 10). The western side of this the Grizzly Peak caldera collapsed may be a a vertically foliated dike by progressive accre- feature is eroded but is inferred to have been a consequence of the geometry of the subcaldera tion. Progressive inward accretion is supported mirror image of the preserved eastern side. On magma chamber. The inner ring-fracture zone by locally developed vertical bedding in the tuff the basis of that inference, the three-dimensional formed in response to the stress regime around dikes. Horizontal compression and shear could form of the fissure vent before erosion was that the magma body rather than to a pre-existing result from displacement along the outer ring of the upward-flaring keel of a boat. weakness because it does not follow any con- fracture of the caldera during collapse events. Rocks in the fissure vent consist of the lower tacts in the pre-caldera rocks (Fig. 9). A magma Such events may be responsible for both open- heterogeneous tuff. At the ridgetop immediately chamber with a flat roof and vertical sides ing and closing of ring-fracture vents. Most east of the vent area, this subunit of the Grizzly would probably form a single ring-fracture zone caldera-forming eruptions involve numerous Peak Tuff is stratigraphically in place and flat in its roof in response to magma underpressure collapse events along the ring fractures, as well lying. Within the vent, the tuff is welded parallel or overpressure because shear stress would be as movements along other fractures in the cal- to the overhanging contact of the vent, which concentrated only over the periphery of the dera floor, and thus provide numerous oppor- cuts discordantly downward through at least body (Pollard and Johnson, 1973). Two zones tunities for vents to be created and destroyed 700 m of the layered tuffc and breccias of the of stress concentration in the chamber's roof, during an eruption. earlier-erupted rhyolite subunit (Figs. 2 and 10). resulting in the two ring-fracture zones, suggest Two other features mapped in the Grizzly The base of the heterogeneous tuff along the that the chamber had a split-level roof, which Peak caldera may be remnants of pyroclastic overhanging contact of the fissure vent is a fine- could be viewed as either a batholith with a vents: (1) a possible tuff-breccia pipe, about 1 grained, planar-bedded and densely welded large cupola or a pluton with a deep shoulder.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/9/1160/3381274/i0016-7606-103-9-1160.pdf by guest on 30 September 2021 Figure 11. Collapse and resurgence structures of the Grizzly Peak caldera. (A) Ring faults with estimated throws in meters. Diagonal pattern shows areas of preserved topographic walls along collapse faults. (B) Contour map of the collapse structure, showing kilometers of minimum subsidence. (C) Resurgence faults with estimated throws in meters. (D) Contour map of the resurgence structure; height of uplift above caldera moat in kilometers.

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The ring-fracture zones of the Grizzly Peak at least 5,000 m above sea level relative to pres- The apex of the resurgent dome of the caldera caldera vary widely in their surface expression in ent datums, based in structural relations ex- is a central fault-block uplift (Figs. 11 and 12), different parts of the caldera. Where exposed in posed along the eastern caldera margin where which is cored by a resurgent "central pluton" the walls of deep valleys, the structural features the outermost ring fault crosses East Red Moun- with only a fragment of its roof of intracaldera of the zones change markedly with elevation. tain at an elevation of 4,100 m (Fig. 2). The tuff preserved at the current level of erosion. The On the basis of relations in these areas, most throw of this outermost ring fault is >900 m; composite resurgent intrusion is surrounded by a variations in the surface expression of the ring- noncorrelative rocks are juxtaposed by this fault vertical piston-like fault formed during em- fracture zones can be related to variations in the over a 900-m change in elevation (Fig. 9). placement of at least the first two of three intru- structural level of exposure. At the shallowest Displacements in ring faults in different parts sions. The piston fault consists of a zone of preserved structural levels, such as along the of the caldera were collected as a basis for a map strongly foliated and subvertically lineated cata- southernmost margin of the caldera, the outer interpretation of the collapse structure (Figs. clastite present along most of the eastern contact ring-fracture zone is totally buried by tuff and 11A and 1 IB). The volume of the caldera is of the intrusions and along the contact between the caldera margin is a moderately inward- about 600 km3, which is a minimum because it the erosional remnant of the roof of the intru- dipping topographic wall (Figs. 2 and 5). The is based on fault throws that are minima. The sions and downthrown rocks to the north. Evi- only surficial evidence of a ring fault is a zone of minimum depth of subsidence in the northeast- dence for the piston fault is much less clear on strongly altered rocks about 0.5 to 1 km inward ern quadrant of the caldera is 3.4 km. Most of the western and southern sides of the intrusion, from this wall. the caldera-collapse breccia intercalated with the but zones of sheared rock and fault gouge are A topographic wall is much better preserved intracaldera tuff was emplaced near the north- present at intrusive contacts near ridge tops on along the inner ring-fracture zone (Fig. 5) owing eastern margin of the caldera, where the throw those two sides. The common roof of the resur- to subsidence along the outer ring-fracture zone. on the outer ring fault is greatest (Figs. 9 and gent intrusions was uplifted about 1,300 m The structural level of exposure within the inner 11). The depth of the Grizzly Peak caldera is on along the piston fault, based on stratigraphic ring-fracture zone becomes deeper to the east, as the same order of magnitude as that established offset on its north side (Figs. 10 and 11). The indicated by a downward stratigraphic progres- geophysically for younger calderas (Hill, 1976; piston fault coincides in many areas with the sion in exposed intracaldera tuff within and ad- Self and others, 1986) and as that documented intrusive contact of the first two resurgent intru- jacent to the zone, culminating in exposure of in the mapping of other deeply eroded calderas sions. These intrusions are finely quenched caldera-floor rocks between ring-fault traces (Lipman, 1984; Lipman and Fridrich, 1990). against this vertical contact, have concentric compositional zoning developed inward from it, (Figs. 2 and 10). On the far west side, the inner On the basis of the model of the caldera and, locally, have parallel flow foliation defined ring-fracture zone is buried by the intracaldera shown in cross section (Fig. 3), we estimate a by alignment of phenocrysts and inclusions. tuff, and its mapped location is an extrapolation. pre-erosion volume of intracaldera tuff of about Eastward toward the center of the caldera, the 300 km3, recalculated as equivalent magma Displacement along the piston fault indicates inner ring-fracture zone is most clearly expressed volume. This estimate is a minimum because the that the resurgent intrusion was emplaced by by its topographic wall. The traces of ring faults top of the tuff is eroded. If it is assumed that the forcible uplift of its roof. The radial pattern of inward from the wall are difficult to discern in volume of the caldera represents the volume of faulting and tilting of volcanic rocks around the the monotonous-appearing, granophyrically crys- magma withdrawn, approximately half of the resurgent intrusion is additional evidence for tallized tuff near the center of the caldera. Lo- erupted magma ponded in the caldera. forcible emplacement (Fig. 2). Normal faults in cally the fault traces are shown by zones of the remnant of the roof of the intrusion along its fracturing in the tuff, and by truncations of inter- CALDERA RESURGENCE north side (Fig. 11C) resemble patterns of exten- calated caldera-collapse breccias. Where thick- sional faulting of other resurgent domes, as de- nesses of subunits in the caldera fill can be Resurgence Structure scribed by Lipman (1984). In cross section, the measured on both sides of faults in the inner roof of the resurgent intrusion is a simple piston ring-fracture zone, downthrown sections are dis- Following collapse, the Grizzly Peak caldera uplift (Fig. 3), because too little of it remains to tinctly thicker (for example, cross section B-B', was uplifted to form a complexly faulted resur- resolve its structure. The intracaldera tuff is tilted Fig. 10). gent dome centered in the northern part of the away from the resurgent intrusions on the north The structurally deepest exposure of a ring- caldera. Several ring faults, formed during col- and west sides, but not on the south or southeast fracture zone is along the east to southeast mar- lapse of the caldera, were reactivated during sides (Fig. 2). Lack of outward tilt in the latter gin of the caldera, where the erosion level is formation of this dome. Resurgent reactivation areas may be due to a trapdoor style of resurgent below the base of the caldera fill. The outer is indicated (1) where intracaldera tuff is brittly uplift south and east of the piston uplift and to ring-fracture zone in this area consists of a deformed in ring faults, because the tuff re- superposition of resurgent doming on the bowl- number of subparallel arcuate faults in a zone as mained hot enough to weld across collapse like form of the collapse structure. much as 2.5 km wide (Figs. 2 and 9). The pre- faults after subsidence ceased, and (2) where About 2 km to the east and south of the apical caldera rocks exposed on both sides and within prominent ring faults have (net) throws that are fault block of the resurgent structure is the mar- the zone are cut by abundant concentric frac- negligible, or where the throw is down on the gin of a major structural block that was uplifted tures with no apparent offset. side outward from the caldera center (for exam- in a trapdoor fashion. The bounding fault of this Only minimum estimates can be made of the ple, cross section B-B', Fig. 10). In addition to uplift is concentric with the eastern and southern depth and volume of caldera subsidence at being zones of brittle deformation in the intra- margins of the apical piston uplift (Figs. 11C Grizzly Peak. Neither the caldera rim nor the caldera tuff, faults active during resurgence were and 1 ID). The bounding fault has its maximum uncollapsed pre-volcanic surface around the pathways for hydrothermal circulation and for throw, 1,300 m, at the southeast corner of the caldera is anywhere preserved in the Sawatch emplacement of resurgent- and late-resurgent- uplifted block and diminishes in throw to both Range. The rim of the Grizzly Peak caldera was stage intrusions in the caldera fill. the north and the west (Fig. 11C).

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, , OUGOCENE OTHER COLLAPSE FAULTS H^H POST-RESURGENT INTRUSIONS

RESURGENCE FAULTS

INTRUSIONS

\ N \ \ \ V \ \ X \ \ \ \ \ \ s S N \ \ \

\ \

3 THREE ik 2 RESURGENT v/ < 4 1 INTRUSIONS .1 V. jl

TUFF & BRECCIA r

PRECALDERA UNDIVIDED 0 km

Figure 12. Map of intrusive rocks in the Grizzly Peak caldera. SS, Sawmill stock.

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South of the trapdoor uplift, the caldera is the re-entrant along the south margin may be its are within analytical error of the ages of the domed. Assuming simplistically that the caldera floor. The intrusion is roofed along its north side resurgent intrusion and the Grizzly Peak Tuff floor was flat before resurgence, we estimate a by about 500 m of intracaldera tuff. A large (Fig. 6), suggesting that all of these rocks are minimum of 500 m of doming in that part of the upward-flaring dike-like apophysis extends up- products of a single resurgent cauldron cycle. caldera south of the trapdoor uplift. Pre-volcanic ward from the intrusion into its roof rocks. The The late-resurgent intrusions consist of four rocks that were under the caldera floor extend to second resurgent intrusion is almost wholly con- small felsic stocks. Two were emplaced in sec- 4,100 m elevation in the center of the caldera, tained within the first one and consists of three tions of the inner ring-fracture zone that were whereas the caldera floor is preserved in both sill-like lobes with roughly planar, gently north- reactivated during resurgence; the other two the eastern and western moats of the caldera dipping roofs (Fig. 3). This body has a complex were emplaced near the outer ring-fracture zone (Fig. 2) at elevations as low as 3,600 m. Because intersection with the rugged topography, creat- along the southwestern margin of the caldera resurgent doming was superposed on a caldera ing an irregular map pattern (Fig. 12). Over- (Fig. 12). All four stocks are spatially associated floor that sagged into a bowl-like form during hanging contacts at the bases of the eastern and with alteration and weak mineralization resem- collapse, the amount of doming south of the western lobes of the second intrusion converge bling that of porphyry molybdenum deposits. trapdoor uplift may be significantly greater than inward and steepen with depth. The apparent These stocks are the last gasp from the magma 500 m. keel-like form of the lower contacts of these chamber that produced the Grizzly Peak Tuff On the basis of the throws of faults formed lobes suggests that they were fed by underlying and the main resurgent intrusions because they during caldera resurgence, and the estimated north-trending dikes (Fig. 3). The third intrusion are more fractionated than the resurgent lacco- height of doming in the southern part of the is a small funnel-shaped stock near the south- lith and because the associated alteration sug- caldera (Fig. 11C), a contour map of cumulative eastern margin of the composite intrusion. gests that they formed at the top of a cooling uplift (Fig. 1 ID) was constructed, which shows An additional resurgent-stage intrusion is a magma body that was exsolving volatiles. a composite resurgent dome consisting of a ring dike of granodiorite porphyry emplaced in The youngest intrusions in the caldera were mosaic of fault blocks. The moat of the caldera the outer ring fault of the caldera, to the south of emplaced after resurgence. Dikes in this group was taken to be unaffected by resurgence, except the welded-tuff dikes at East Red Mountain form a radial pattern around a petrologically where clear evidence exists for resurgent-stage (Fig. 2). The porphyry ring dike is widest in similar stock (the Sawmill stock) in the center of reactivation of faults in the outer ring-fracture valley bottoms and pinches out below the ridge the caldera (Fig. 12). These intrusions range in zone. The maximum uplift of the resurgent tops, resulting in discontinuous exposure (Figs. 2 composition from latite to rhyolite; most are dome is 2.5 km at its apex. The estimated vol- and 10). This ring dike was emplaced during phenocryst-poor latite that is more primitive ume of the resurgent uplift is 170 km3. These resurgence, based on its petrologic similarity to (higher Mg number) than any preceding rocks in estimates are conservative because there may the resurgent laccolith of the caldera. The up- the caldera (Fridrich, 1987). Dikes of this final have been more reactivation of collapse faults ward narrowing of the porphyry ring dike is group locally cut both the resurgent laccolith during resurgence than was recognized, and be- consistent with the model of Smith and others and the late-resurgent stocks. The post-resurgent cause the estimated extent of doming in the (1961), in which room is created for ring dikes latites are phenocryst-poor relative to latite pum- southern caldera segment is a minimum. The owing to a stress regime in which relative radial ice of the Grizzly Peak Tuff and are distinct in resurgent laccolith of the caldera has an esti- tension increases with depth along the caldera- trace-element abundances. Hence, they repre- mated (pre-erosion) volume of about 40 km3. margin fault as a result of resurgent doming of sent a new batch of magma rather than a deeper Hence, forcible emplacement of this intrusion the cauldron block. compositional level of the pre-existing subcal- accounts for about 25% of the total uplift during dera magma chamber. Solidification of the high- resurgence. LATE- AND POST- level magma chamber evidently allowed these RESURGENT STAGE melts to penetrate the upper crust in the final Resurgent Intrusions stage of the thermal anomaly. An east-west-trending belt of dikes and small Several crystal-poor latite dikes contain xeno- The composite intrusion at the core of the stocks cuts the Grizzly Peak Tuff and older rocks liths as much as 2 m in diameter of a medium- resurgent dome consists of three nested intru- across the center of the caldera (Fig. 12). These grained unfoliated granite that is texturaily and sions. Contacts between these intrusions are intrusions belong to three age groups: (1) dikes compositionally distinct from any of the pre- sharp near the margins of the composite body related to the composite resurgent laccolith Oligocene granites in the central Sawatch but become increasingly gradational toward its based on similarity in phenocryst assemblages Range. Trace- and major-element abundances in center, indicating that each successive intrusion (resurgent intrusions), (2) a group of stocks that this granite are similar to those of the medium- was emplaced before the previous one had fully postdate faults formed during resurgence and silica rhyolite of the Grizzly Peak Tuff (Fridrich, crystallized. Each of the three intrusions is con- that coincide with the period of most intense 1987). These xenoliths may be blocks of the centrically zoned from silicic granodiorite at its hydrothermal activity in the caldera fill (late- solidified subcaldera magma chamber that were margin to mafic granodiorite or quartz monzo- resurgent intrusions), and (3) a final group of ripped up during post-resurgent emplacement of diorite at its core. Zoning is equally well devel- dikes and stocks that postdate most of the hy- relatively mafic magma. oped inward from roof, side, and overhanging drothermal alteration that affected all older in- contacts (Fridrich and Mahood, 1984). tracaldera rocks and, by inference, postdate CONCLUSIONS The first (oldest) resurgent intrusion is the solidification of the high-level subcaldera mag- largest (Fig. 12); it has vertical walls and a roof ma chamber (post-resurgent intrusions). Two Formation of the Grizzly Peak caldera was that is inclined at a low angle to the north- stocks, one from each of the late-resurgent and preceded by tumescence of the upper crust, and northeast. A low-angle overhanging contact in post-resurgent groups, yielded K-Ar dates that development of a circular zone of concentric

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cone-sheet fractures. This fracture swarm ex- cerned at the highest structural levels. With APPENDIX tends as much as 20 km east of the caldera and depth, the dome is superposed on an increas- covers an area at least five times that of the ingly complex and bowl-like collapse structure. Formal Naming of the Grizzly Peak Tuff caldera. More than 100 cone-sheet dikes were Evidence of doming also becomes obscure with emplaced along the fracture swarm. Vigorous structural depth because the resurgent intrusion The Grizzly Peak Tuff was first called the "Grizzly Peak Rhyolite" by Howell (1919), and then the "rhyo- hydrothermal systems developed along some of is a laccolith; some evidence of doming is lost at lite of the Independence Pass district" (Vanderwilt and the fractures and intrusions. The pattern of ex- erosional levels below the roof of the intrusion. Koschmann, 1932; Burbank and Goddard, 1935) or tracaldera cone-sheet fractures and dikes proba- If all of the Grizzly Peak caldera were eroded to the "Grizzly Mountain Rhyolite" (Stark and Barnes, bly represents early structural response of the the ring-complex level, the principal remaining 1935). Modern studies were begun by Cruson (1973), crust to emplacement of a magma body that evidence of resurgent uplift probably would be who recognized the tuff as fill in a deeply eroded cal- would further evolve, rise, and erupt to produce dera structure, and Candee (1971), who studied the the intracaldera intrusions. resurgent intrusion of the caldera, which he called the the Grizzly Peak Tuff. "Lincoln Gulch stock." Cruson's reference to the Eruption of the Grizzly Peak Tuff resulted in ACKNOWLEDGMENTS Grizzly Peak caldera as a "complex" is now aban- subsidence of a caldera that is now deeply doned, as more recent studies have shown that it is the product of a single resurgent cauldron cycle (Smith eroded and dissected. The current high strati- Fridrich thanks Gail Mahood, Elizabeth and Bailey, 1968). Fridrich and Mahood (1984,1987) graphic relief through the system provides Miller, Tom Steven, Charles Bacon, Alan Wal- described the compositional zoning in the composite exposure of the transition between a caldera and lace, John Pallister, Jamie Gardner, David resurgent intrusion and in the Grizzly Peak Tuff. John- an underlying ring complex. In little-eroded Boden, and Anita Grunder for thoughtful re- son and Fridrich (1990) documented nonmonotonic chemical and isotopic trends in the zonation of the tuff, parts of the Grizzly Peak caldera, it appears to views; Patrick Dobson, Jay Wilson, and Eric and Johnson and others (1989) wrote a field guide be a simple piston-like basin, as do most little- Miller for assistance in the field; and Robert that includes the Grizzly Peak caldera. The name eroded calderas. With depth, structural features, Christiansen and Peter Lipman for suggesting "Grizzly Peak Tuff' was first adopted by Cruson and which are progressively buried upsection, show the Grizzly Peak caldera as the area for this has become the recognized name for this unit. This it to be an asymmetric bowl with two coeval, study. Funding was provided by National stratigraphie name is formally adopted here for the rhyolitic ash-flow tuff in the vicinity of Grizzly Peak in complex ring-fracture zones. Science Foundation Grant EAR83-06860 to T. 12 S., R. 82 W., Chaffee County, Colorado, and all The deepest structural level of exposure is Mahood, by two Geological Society of America previous names for this unit are hereby abandoned. below the base of the caldera fill. At this level, Penrose grants, and by Stanford McGee and the caldera is defined predominantly by large Shell Fund grants. The U.S. Geological Survey Stratigraphie Nomenclature subvertical ring faults cutting through the pre- provided monetary, logistical, and analytical volcanic rocks, and by ring dikes of welded tuff support to this project as part of the Collegiate Petrologic nomenclature follows Streckheisen and porphyry emplaced in these faults. The Peaks Wilderness Study. (1973) for intrusive rocks. Best's (1981, p. 64) classifi- cation of calc-alkaline volcanic rocks is amended here structural changes that occur in the transition Smith thanks Will White and Art Bookstrom by use of modifiers high-, medium-, and low-silica for from caldera to underlying ring complex reflect of Climax Molybdenum Company for com- fresh rhyolites with 75%-77.5%, 72.5%-75%, and the evolution of the structure during the caldera- mitting funds and logistical support to the 70%-72.5% SiOj, calculated anhydrous, respectively. forming eruption, as well as the manner in northern Sawatch Range mapping project and This distinction was extended to unanalyzed and al- which resurgent doming is superposed on the tered rhyolites by comparing their phenociyst assem- for endorsing publication of the results. The blages and relatively immobile trace-element abun- collapse structure. capable field assistance of Ross Bryant is grate- dances with those of analyzed fresh rhyolites of the Early during eruption of the Grizzly Peak fully acknowledged. same suite. Tuff, subsidence of the caldera was dominated by sagging of the caldera floor, and by displace- TABLE Al. POTASSIUM-ARGON DATA ments along a complex system of ring faults in wide fracture zones enclosing the two structural Sample Mineral K2O Ar* mole/g "«Ar- Age + sigma Reference (%) (x 10"10) TS) (Ma) segments of the caldera. As collapse continued, movement became increasingly concentrated SAWM-1 Biotite 8.64 4.1353 55.1 32.9 ± 1.1 This study PC-1 Biotite 7.449 4.65 59.5 35.7 ± 1.2 Kennecott, unpub. data along the outermost fault of the outer ring- PC-1 Biotite 4.606 2.975 45.9 36.8 t 1.4 Kennecott, unpub. data fracture zone. Ring faults within the caldera are NY-5 Biotite 7.29 3.68 83.2 34.8 ± 1.1 Obradovich and others, 1969 (D>A Biotite 8.81 4.655 94.0 36.3 ± 0.9 Marvin and Dobson, 1979 growth faults in the intracaldera tuff, and the tuff AVG-1 Biotite 9.16 4.4265 60.4 33.3 ± 1.0 This study MA-1 Biotite 8.61 4.1814 62.4 33.4 - 1.0 This study is both offset and welded across these collapse- ZB-2 Hornblende 0.914 4.4898 26.9 33.8 ± 1.1 This study stage faults. In an uneroded state, the topograph- MM-1 Biotite 7.247 4.80 59.6 37.8 ± 1.3 Kenneoott, unpub. data MM-1 Biotite 6.760 4.55 — 38.9 ± 1.3 Ranta, 1974 ic walls of the caldera must have extended far beyond the currently exposed fault trace of the Note: analyst for this study, E. H. McKee, U.S. Geological Survey, Menlo Park, California. Analyst for data from Kennecott (1970), Geochron laboratories Inc. (see Rama, 1974). Samples arranged by stratigraphically determined age with the youngest at the top. Pte-1979 dates recalculated by the method of Dalrymple outermost ring fault, based on the large volume (1979). of caldera-collapse breccia that is intercalated with the intracaldera tuff. Sample Unit Latitude Longitude

Resurgent uplift of the Grizzly Peak caldera SAWM-1 Post-iesuigent Sawmill stock 39°I'39*N 106°36'54-W PC-1 Late-resurgent Pine Creek stock, core 39°2'N I06°4I'W closely followed collapse and formed a complex PC-1 Late-resurgent Pine Creek stock, surface sample 39°2'N I06°4rw structural dome consisting of a mosaic of fault NY-5 Resurgent Lincoln Gulch stock 39°5'20"N 106D38'29*W (D)-A Outflow Grizzly Peak Tuff near Mount Sopris 39°21'N 107°4'W blocks. Because many of the structures asso- AVG-1 Outflow Grizzly Peak Tuff near Twin Lakes dam 39°3'48"N 106°m6-W MA-1 Intracaldera Grizzly Peak Tuff, rhyolite fiamme 39°5'5"N 106°34'58*W ciated with the early phase of caldera collapse ZB-2 Intracaldera Grizzly Peak Tuff, latite fiamme 39°4'46'N 106°34'35*W are progressively buried upsection in the caldera MM-1 Pre-caldera Middle Mountain stock, hole MM-1 38°58'N 106°26'W MM-1 Middle Mountain Porphyry, surface sample 38-58'N 106°26'W fill, the resurgent structure is most easily dis-

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MANUSCRIPT ACCEPTED JANUARY 28,1991

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