Journal of Volcanology and Geothermal Research
Rhyodacites of Kulshan caldera, North Cascades of Washington: Postcaldera lavas that span the Jaramillo
Introduction
Compared to other circum-Pacific volcanic arcs, the scarcity of Quaternary calderas in the Cascade Range is a remarkable anomaly (Hildreth, 1996). Between Meager Mountain in British Columbia and Lassen volcanic center in California, the 1100-km-long modern Cascade arc contains >1800 Quaternary volcanoes, among which only three Quaternary calderas have been identified:
(1) the Holocene caldera at Crater Lake, Oregon (Bacon, 1983, Bacon et al., 2002); (2) the buried source of the middle Pleistocene Rockland ash and ignimbrite, now concealed by postcaldera silicic lavas of the Lassen volcanic center (Clynne, 1990, Lanphere et al., 1999); and (3) Kulshan caldera in the North Cascades (Fig. 1), described geologically by Hildreth (1996) and the subject of this report. Here we present 40Ar/39Ar and K–Ar ages and paleomagnetic and compositional data (Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7) for precaldera and postcaldera silicic lavas and dikes (1.29–0.99 Ma) as well as for the voluminous tuff (1.15 Ma) released during the caldera-forming eruption.
The 4.5×8 km caldera lies 15 km south of the international border (Fig. 1), between Mount Baker (an active andesitic stratocone) and Mount Shuksan (a spire of Mesozoic metabasalt). The area is accessible from the Fraser and Puget Lowlands by Washington Highway 542, which winds through the Heather Meadows ski area to trailheads at Artist Point and Table Mountain (Fig. 2, Fig. 3), scenic overlooks on the caldera margin. The topography of Kulshan caldera is complicated by sharp relief glacially eroded on postcaldera lavas that overlie the intracaldera tuff and by reduction of the circumcaldera landscape by Pleistocene advances of the Cordilleran Ice Sheet. The ice that repeatedly spread southward out of British Columbia blanketed all but the highest peaks, lowered the caldera rim by hundreds of meters, removed nearly all precaldera lavas, and stripped every vestige of outflow tuff.
Most lavas and tuffs associated with Kulshan caldera (Table 1) are rhyodacitic (68–73% SiO2), allowing us to define a Kulshan eruptive interval (1.29–0.99 Ma) during which less silicic eruptions were rare. In contrast, the many lavas and dikes emplaced in and around the caldera since 0.99 Ma have been predominantly andesitic (Hildreth et al., 2003), rhyodacite virtually lacking. The Kulshan rhyodacites erupted largely from vents within and peripheral to the caldera, but contemporaneous rhyodacitic lavas and dikes were also emplaced in a zone extending as far as 4 km west of the caldera (Fig. 2). This zone of extracaldera vents has been called the Chowder Ridge focus (Hildreth et al., 2003). Although the Chowder Ridge and Kulshan caldera foci were continuous during the interval of rhyodacite eruptions (and were probably fed by a common silicic magma reservoir), after 0.99 Ma the two became discrete centers of concentrated andesite–dacite magmatism, both of which remained active into the late Pleistocene (Hildreth et al., 2003).
Pre-Quaternary basement around Kulshan caldera consists of Mesozoic and Paleozoic rocks belonging to several tectonostratigraphic terranes that were assembled in the Cretaceous as a series of imbricate nappes and were redistributed during the Paleogene by extensional and strike-slip faulting (Brown, 1987, Haugerud et al., 1994, Tabor et al., 2003). Quaternary vents and dikes of the Chowder Ridge and Kulshan foci emerge principally through the Nooksack Formation, which belongs to the structurally lowest terrane exposed and consists mostly of well-stratified Late Jurassic and Early Cretaceous marine argillite and sandstone. In addition, the eastern part of the volcanic field cuts and overlies rocks of the Chilliwack Group, which here belongs to the nappe next above the Nooksack and consists of Paleozoic metavolcanic and clastic metasedimentary rocks that enclose bodies of fossiliferous marble of Devonian through Permian age. Although Tertiary sedimentary rocks crop out to the east and west, none are preserved within the volcanic field. The structure of the pre-volcanic basement is illustrated on the geologic maps and sections of Brown et al. (1987), Wheeler and McFeeley (1991), and Tabor et al. (2003).
Most of the Chilliwack composite batholith (Fig. 1), which consists of about 55 plutons of Oligocene to Pliocene age (Tepper et al., 1993, Tabor et al., 2003), lies well east of the Quaternary volcanic field. One of the youngest plutons, however, is cut by Kulshan caldera, which is itself presumably the surface expression of a still younger Chilliwack pluton, as yet unexposed.
Kulshan caldera is one element of a stepwise southwestward progression of magmatic foci (Fig. 1) underway during the last 4 Myr (Hildreth, 1996, Hildreth et al., 2003). Hannegan caldera, just NE of Mount Shuksan, is cut in basement rocks, filled with rhyolitic ignimbrite, wall-collapse breccias, and silicic dikes and lavas, and intruded by small granitoid plutons (Tabor et al., 2003, Tucker, 2000). Beneath and southwest of Mount Shuksan is the granodioritic Lake Ann stock (James, 1980), for which no volcanic counterpart has been recognized. The west end of the stock is cut by Kulshan caldera (Fig. 2). Plagioclase from glassy samples of intracaldera ignimbrite yielded 40Ar/39Ar ages of 3.772±0.020 Ma and 1.149±0.010 Ma, respectively, for the caldera-forming eruptions at Hannegan and Kulshan calderas (Table 2). Euhedral biotite from the felsic roof zone of the Lake Ann stock gave a laser-fusion age of 2.75±0.13 Ma (Table 2), roughly midway between the ages of the flanking calderas. Following collapse of Kulshan caldera, the latest southwestward shift of magmatic focus was underway by about 0.75 Ma, culminating after 0.5 Ma in growth of an andesitic stratovolcano cluster that includes the active Mount Baker cone (Hildreth et al., 2003).
Section snippets
Precaldera magmatism
Remnants of 10 units peripheral to Kulshan caldera (Fig. 2, Fig. 3) have been defined and mapped, most or all of which were emplaced prior to caldera collapse at 1.15 Ma. Still others could well have existed within the area that became the caldera, but no evidence has been found for such units, neither beneath intracaldera ignimbrite nor as lithic fragments in it. Absence of such evidence suggests that no great edifice preceded the caldera, perhaps instead just a scattering of dike-fed lava
Geochronology
The sequence of eruptive units distinguished during our mapping is laid out in Table 1, and 40Ar/39Ar ages determined for 16 samples, mostly Kulshan rhyodacites, are presented in Table 2. Except for one biotite separate, all were plagioclase separates from medium-to-high-K silicic rocks. The 40Ar/39Ar dating procedures in the Menlo Park laboratory are described by Lanphere (2000). The fluence monitor was 85G003 sanidine from the Taylor Creek Rhyolite (27.92 Ma). Age spectra for some key samples
Paleomagnetism of the rhyodacites
Lake Tapps tephra, distal fallout from the caldera-forming eruption, was reported by Westgate et al. (1987) to be enclosed by laminated lacustrine silts that are reversely magnetized at several locations. At 1.15 Ma, the great eruption would thus have taken place during the reversed interval between the Cobb Mountain Event (∼1.18 Ma) and the Jaramillo Normal Polarity Subchron (1.07–0.99 Ma). Evidence (by means of a field fluxgate magnetometer) that postcaldera rhyodacite lavas variously had
Gravity profile across Kulshan caldera
In 1995, the authors and H.W. Oliver carried out a 46-station gravity survey, traversing the caldera along the Ptarmigan Ridge trail (the only feasible route across the ruggedly incised caldera fill), backpacking a theodolite, rod, and LaCoste-Romberg gravimeter. The leveling survey was tied into benchmarks northeast of the caldera, and the gravity measurements were tied to previously measured nearby stations (Finn et al., 1984). Small adjustments (A–B corrections) were made to compensate for
Phenocryst contents (Table 5)
All Kulshan rhyodacites have abundant 0.5–3 mm plagioclase, ranging from 5 to 15%, but they lack phenocrystic alkali feldspar. The mafic silicates, hornblende, orthopyroxene, and biotite, are all present in most samples but in varied orders of abundance. Few are larger than 1 mm, though sparse hornblende grains can be 2 mm long, and the three species together seldom total as much as 5% of the rock. The sparsity of biotite and orthopyroxene in many samples suggests that their absence in a few
Discussion
The Kulshan rhyodacite episode lasted for at least 190 kyr (1.18–0.99 Ma), terminated conclusively, and (on the evidence of one dated dike) might have begun 110 kyr still earlier (∼1.29 Ma). The rhyodacite episode included emplacement of at least 12 lavas and no fewer than 20 shallow dikes and pods, as well as the phreatoplinian caldera-forming eruption at 1.15 Ma. Because the thick silicic dikes cut basement rocks or caldera fill at elevations close to those of extrusive rhyodacites nearby, we
Acknowledgements
Stalwart assistance under sometimes trying field conditions was provided by Dave Tucker, Patty Weston, Kari Cooper, and Kyle Champion. Howard Oliver, nearly 70, carried the gravimeter across the caldera and back, humming all the way. Reviews by Don Swanson, Joe Liddicoat, Willie Scott, and Wendell Duffield improved the manuscript in form and substance. We are grateful to them all, and to Dave Siems, James Saburomaru, Forrest McFarland, and Lanny Adami for essential laboratory support, and to
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