Rhyodacites of Kulshan caldera, North Cascades of Washington: Postcaldera lavas that span the Jaramillo

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Abstract

Kulshan caldera (4.5×8 km), at the northeast foot of Mount Baker, is filled with rhyodacite ignimbrite (1.15 Ma) and postcaldera lavas and is only the third Quaternary caldera identified in the Cascade arc. A gravity traverse across the caldera yields a steep-sided, symmetrical, complete Bouguer anomaly of −16 mGal centered over the caldera. Density considerations suggest that the caldera fill, which is incised to an observed thickness of 1 km, may be about 1.5 km thick and is flat-floored, overlying a cylindrical piston of subsided metamorphic rocks. Outflow sheets have been stripped by advances of the Cordilleran Ice Sheet, but the climactic fallout (Lake Tapps tephra) is as thick as 30 cm some 200 km south of the caldera. Ten precaldera units, which range in 40Ar/39Ar age from 1.29 to 1.15 Ma, are dikes and erosional scraps that probably never amounted to a large edifice. A dozen postcaldera rhyodacite lavas and dikes range in age from 1.15 to 0.99 Ma; rhyodacites have subsequently been absent, the silicic reservoir having finally crystallized. At least 60 early Pleistocene intermediate dikes next intruded the caldera fill, helping energize an acid–sulfate hydrothermal system and constituting the main surviving record of an early postcaldera andesite–dacite pile presumed to have been large. Most of the pre- and postcaldera rhyodacites were dated by 40Ar/39Ar or K–Ar methods, and 13 were drilled for remanent magnetic directions. In agreement with the radiometric ages, the paleomagnetic data indicate that eruptions took place before, during, and after the Jaramillo Normal Polarity Subchron, and that one rhyodacite with transitional polarity may represent the termination of the Jaramillo. Most of the biotite–hornblende–orthopyroxene–plagioclase rhyodacite lavas, dikes, and tuffs are in the range 68–73% SiO2, but there were large compositional fluctuations during the 300-kyr duration of the rhyodacite episode. The rhyodacitic magma reservoir was wider (11 km) than the caldera that collapsed into it (8 km).

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

References (40)

  • C.R. Bacon et al.

    Morphology, volcanism, and mass wasting in Crater Lake, Oregon

    Geol. Soc. Am. Bull.

    (2002)
  • P.A. Baedecker

    Methods for geochemical analysis

    U.S. Geol. Surv. Bull.

    (1987)
  • E.H. Brown

    Structural geology and accretionary history of the Northwest Cascades System of Washington and British Columbia

    Geol. Soc. Am. Bull.

    (1987)
  • Brown, E.H., et al., 1987. Geologic Map of the Northwest Cascades, Washington. Geol. Soc. Am., Map and Chart Series...
  • Butler, R.F., 1992. Paleomagnetism. Blackwell Sci. Publ., Boston, MA, 319...
  • M.A. Clynne

    Stratigraphic, lithologic, and major-element geochemical constraints on magmatic evolution at Lassen volcanic center, California

    J. Geophys. Res.

    (1990)
  • Ewart, A., 1979. A review of the mineralogy and chemistry of Tertiary-Recent dacitic, rhyolitic, and related salic...
  • Finn, C.A., Phillips, W.M., Williams, D.L., 1984. Gravity Map of the State of Washington and Adjacent Areas. U.S. Geol....
  • A.L. Grunder

    Low 18O silicic volcanic rocks at the Calabozos volcanic complex, Southern Andes - Evidence for upper crustal contamination

    Contrib. Mineral. Petrol.

    (1987)
  • Haugerud, R.A., Brown, E.H., Tabor, R.W., Kriens, B.J., McGroder, M.F., 1994. Late Cretaceous and early Tertiary...
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