Volcanic history and magmatic evolution of Seguam Island ...raman/papers2/JichaSingerGSA06.pdf · Seguam Island is an ~200 km2 volcanic com-plex in the central Aleutian Island arc

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805

GSA Bulletin; July/August 2006; v. 118; no. 7/8; p. 805–822; doi: 10.1130/B25861.1; 12 fi gures; 5 tables, Data Repository item 2006126.

ABSTRACT

New 40Ar/39Ar dating coupled with detailed fi eld mapping, stratigraphy, and chemical analyses have established an eruptive chro-nology that reveals and constrains the compo-sitional and volumetric evolution of Seguam Island in the Aleutian Island arc, Alaska. Sixty new 40Ar/39Ar ages from lavas, domes, and pyroclastic deposits were obtained using furnace incremental-heating techniques on replicate samples of whole-rock and ground-mass separates, and they constrain the dura-tion of Pleistocene to Holocene subaerial volcanism to 318 k.y. The 40Ar/39Ar plateau ages indicate that over 85% of the complex, ~68 km3 of material, was erupted almost continuously between 318 ka and 9 ka. At ca. 9 ka, a stratocone on the eastern half of the island partially collapsed producing a 4-km-wide crater. Rhyolitic dome-forming eruptions followed from vents in the newly created crater, and were likely contempora-neous with 8.0 km3 of basaltic and basaltic andesitic effusions from Pyre Peak, and a 1.4 km3 basaltic eruption from a monogenetic cone on the far eastern end of the island. Geo-chemical changes over the last 318 k.y. are subtle. Most notably, the earliest eruptions from 318 to 142 ka produced no andesite, and basalt from this period has larger ranges in Zr/Rb and La/Yb than younger basalts. Small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal-liquid fractionation process that varied only slightly in successive eruptive phases over 318 k.y. We identifi ed minor geochemical changes in magma composition during each of the three main stages of vol-canism, but overall the monotonic variations in major- and trace-element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed- system

fractional crystallization of basalt. Using the 40Ar/39Ar geochronology and estimates of individual fl ow volumes, we calculated a time-averaged eruptive rate at Seguam that is similar to growth rates of other well-dated arc volcanoes in the Cascades and Chilean Southern volcanic zone but less than that of Mount Katmai and Mount Mageik, which are located on the Alaska Peninsula. The eruptive fl ux at Seguam has been highly vari-able, fl uctuating more than an order of mag-nitude, from 0.07 km3/k.y. during the early history of bimodal volcanism to 1.18 km3/k.y. over the past 9 k.y.

Keywords: Aleutian island arc, 40Ar/39Ar geo-chronology, magmatic evolution, Seguam Is-land, eruptive rates.

INTRODUCTION

For many an active volcano, the historical record, as well as the isotopic and geochemi-cal composition of its lavas and tephras, is well established. Yet, high-resolution eruptive histories that extend back several hundreds of thousands of years have only been determined for very few long-lived arc volcanoes. These include Mount Adams (Hildreth and Lanphere, 1994), Tatara–San Pedro (Singer et al., 1997), Santorini (Druitt et al., 1999), Montserrat (Har-ford et al., 2002), Mount Baker (Hildreth et al., 2003a), Katmai (Hildreth et al., 2003b), and Ceboruco–San Pedro (Frey et al., 2004). Estab-lishing a chronology and quantifying the long-term growth of a volcanic complex requires detailed geologic mapping supported by K-Ar or 40Ar/39Ar age determinations, which document major events in a volcano’s eruptive history. Determining the growth rate and compositional evolution of an arc volcano over a protracted period of a few hundred thousand years is criti-cal to understanding issues such as periodicity of activity and frequency of explosive episodes. When coupled with petrologic and isotopic

studies, these can provide insight into magma chamber longevity and rates of magma differ-entiation, all of which may have important roles in volcanic hazard mitigation strategies (e.g., Harford et al., 2002).

Despite the fact that there are well-docu-mented eruptive chronologies for Aleutian con-tinental arc volcanoes (Hildreth et al., 2003b; Bacon et al., 2003), very few 40Ar/39Ar data are published from the historically active volcanoes in the Aleutian Island arc (Jicha et al., 2004). Seguam Island, located in the central Aleutian Island arc, is a 79 km3, low- to medium-K, tho-leiitic complex with multiple eruptive centers. It is unusual relative to other Aleutian Island arc centers in that it has erupted a signifi cant volume (~30% of the total volume erupted) of evolved lavas with compositions ranging from 63% to 71% SiO

2. Previous K-Ar dating of 11

whole-rock samples from Seguam indicated a 1.07 m.y. eruptive history (Singer et al., 1992a), although the precision of these K-Ar ages is poor (1σ errors are ±6%–70%). The goals of this study were to use new fi eld observations along with high-precision 40Ar/39Ar geochronol-ogy to: (1) build upon the prior work of Singer et al. (1992a, 1992b, 1992c), (2) quantify and constrain the compositional and volumetric evo-lution of the 79 km3 complex, (3) estimate the rates of magma output over the lifetime of the volcano, and compare them to other arc volca-noes worldwide, and (4) establish the precise geochronologic control required to interpret U-Th isotope disequilibrium dates in terms of time scales of crystallization and magma storage over the past 200 k.y. (Jicha et al., 2005).

Despite major advances in 40Ar/39Ar dat-ing of late Pleistocene and Holocene lava and tephra during the last decade (e.g., Renne et al., 1997; Singer et al., 2000, 2004), obtaining precise 40Ar/39Ar ages from low-K, tholeiitic lavas younger than 500 ka that are susceptible to alteration in a humid environment is a chal-lenge, due to extremely low radiogenic 40Ar* contents in the presence of large amounts of

Volcanic history and magmatic evolution of Seguam Island, Aleutian Island arc, Alaska

Brian R. Jicha†

Brad S. SingerDepartment of Geology and Geophysics, University of Wisconsin–Madison, 1215 West Dayton Street, Madison, Wisconsin 53706, USA

†Email: [email protected].

Jicha and Singer

806 Geological Society of America Bulletin, July/August 2006

atmospheric 40Ar. Additionally, estimating erup-tive rates at Seguam is complicated by glacial and marine erosion and by vegetation and ash cover. In spite of these pitfalls, we determined the time-averaged eruptive rate based on mini-mum estimates of individual fl ow volumes and 40Ar/39Ar age determinations to be 0.3 km3/k.y. Like other arc volcanoes, the eruptive fl ux has been highly variable throughout the lifetime of the volcano.

TECTONIC AND GEOLOGIC SETTING

Seguam Island is an ~200 km2 volcanic com-plex in the central Aleutian Island arc (Fig. 1). Seismic-refl ection and seismic-refraction pro-fi les from Seguam Pass between Seguam and Amlia Islands indicate that the volcanic com-plex sits atop 25–30 km of strongly extended arc crust (Geist et al., 1988; Holbrook et al., 1999; Fliedner and Klemperer, 1999). The crust maintains a similar thickness for at least 100 km behind the arc front. P-wave velocity structures have been interpreted to refl ect an overall mafi c composition of the crust, which is believed to be porous or fractured extrusive and intrusive igneous rocks and volcaniclastic sediments in the upper 7 km, a mid-ocean-ridge basalt (MORB)–like ~6-km-thick layer in the middle crust, and ~10–20 km of gabbroic residua at the base (Holbrook et al., 1999). Forty percent of the upper crust is inferred to be silicic in com-position (Fliedner and Klemperer, 1999). How-ever, the Aleutian arc lacks seismic evidence for a silicic middle crust (Holbrook et al., 1999).

Approximately 170 km south of Seguam is the ~7000-m-deep Aleutian trench, where the Pacifi c plate is subducting obliquely beneath the

North American plate at a convergence rate of 6.6–6.8 cm/yr (DeMets et al., 1994). The Eocene seafl oor dips 10° for 100 km, and then steepens to ~50° beneath the volcanic front (Holbrook et al., 1999). A P-wave velocity model suggests that the top of the slab is ~60 km beneath the active volcanoes (Holbrook et al., 1999), which is signifi cantly shallower than previous esti-mates of slab depth based on the locations of earthquake hypocenters (Engdahl, 1977).

VOLCANOLOGICAL OVERVIEW

Seguam Island comprises seven vents aligned in an east-west orientation, including those of Pyre Peak, Wilcox volcano, and Moundhill volcano. At least 79 km3 of basaltic to rhyolitic lava, tephra, scoria, and ash fl ows, along with several rhyolite fl ows and domes are preserved on the 11 × 21 km island. The volcanic complex was severely eroded by an extensive glacial ice cap that covered most of the Aleutian Islands during late Wisconsin time. Although glacial ice was also likely present on the island during each of the major Pleistocene glaciations, little evi-dence for earlier glaciations has been preserved in the Aleutians (Black, 1983). All of the lavas and tephras that have reached the shoreline have been subject to marine erosion also. Glaciated late Pleistocene fl ows and tuffs are capped by Holocene lavas, domes, and ash deposits con-sisting of 1.2 km3 of rhyolitic domes in the east and 8.0 km3 of basalt fl ows and scoria beds in the west. Moundhill volcano, a 1.4 km3 mono-genetic spatter cone, was constructed on the far eastern end of the island sometime after the retreat of glacial ice at ca. 12–10 ka. The obser-vations of geologists or other scientists attached

to exploration expeditions in the Aleutians have indicated that Seguam has been historically active since the late eighteenth century, with eight eruptions over the past 200 yr, presumably from Pyre Peak (Miller et al., 1998). In 1977 and 1992–1993 basalt and basaltic andesite erupted from a 1.0-km-long fi ssure ~2 km south of Pyre Peak.

Reconnaissance mapping and sampling facili-tated the geochemical, petrologic, and isotopic studies of Singer et al. (1992a, 1992b, 1992c), who identifi ed a four-phase geologic evolution of the island on the basis of fi eld observations and 11 whole-rock K-Ar ages. Whole-rock Sr isotope and δ18O

plag data from Seguam lavas combined

with a comparison of the modal mineralogy to published low-pressure cotectic conditions sug-gest that basaltic parental magmas crystallized at 3–5 kbar, or ~10–15 km depth, followed by closed-system differentiation to dacite and rhyo-lite between 1 and 2 kbar (i.e., crustal depths of ~3–6 km) (Singer et al., 1992a, 1992c).

Using detailed mapping and stratigraphy, 40Ar/39Ar age determinations, and geochemi-cal data, we have identifi ed a subaerial eruptive history that is virtually continuous from 318 ± 30 ka to the present, but signifi cantly shorter than inferred by Singer et al. (1992a). We have subdivided the volcanic evolution into three stages that consist of: (1) older, deeply eroded lavas and domes (318–142 ka); (2) island-wide activity (138–9 ka), which includes shallowly dipping lavas in the central and western section of the island, and the construction of a 20 km3 basaltic to rhyolitic stratocone on the eastern half of the island (98–9 ka) that partially col-lapsed and produced a 0.5 km3 dacitic ignimbrite at 9 ka; and (3) postcollapse rhyolitic activity in

500 100 200Km

6-7 cm/year

North American plate

Pacific plate

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Amchitka

Adak

Okmok

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54°N

52°N

50°N

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Aleutian Trench

Seguam▲

▲

▲

▲

▲

▲

▲

▲

▲

▲▲

▲▲

▲▲▲▲▲

▲

▲

▲

Figure 1. Bathymetric map of the Aleutian Island arc showing the location of Seguam Island and several other well-known islands and volcanic centers. Contour interval is 400 m. The Aleutian trench lies ~170 km seaward of the volcanic axis. Subduction of the Pacifi c plate becomes increasingly oblique in the western part of the arc.

Volcanic and magmatic evolution of Seguam island

Geological Society of America Bulletin, July/August 2006 807

the crater, and subsequent formation of a western caldera accompanied by mafi c tuffs and basaltic effusions (Holocene to present) (Fig. 2).

PETROGRAPHIC AND COMPOSITIONAL OVERVIEW

Seguam lavas are characterized by an anhy-drous phenocryst assemblage dominated by plagioclase (up to 42%), with lesser amounts of olivine, clinopyroxene, orthopyroxene, and titanomagnetite (Singer et al., 1992a, 1992b). Basaltic to basaltic andesitic fl ows and dikes are porphyritic, containing 22–58 modal per-cent phenocrysts, whereas andesites are mildly phyric, containing fewer total phenocrysts (5%–31%). Conversely, crystal poor (4%–10% crystals) dacitic to rhyolitic lavas are often fl ow-banded and glassy with groundmass tex-tures ranging from pilotaxitic to vitrophyric. The dacitic ignimbrite that accompanied crater formation at 9 ka (plag + cpx + opx + mt = 15 modal %) is unusual in that it contains abun-dant apatite phenocrysts within partially welded glass shards and trace amounts (<0.5 modal %) of anhedral, strongly resorbed biotite.

In Figure 3, we show the compositional range of lavas and tephras erupted during the 318 k.y. subaerial history of Seguam Island. There are no primitive lavas (<49 wt% SiO

2, >8 wt%

MgO) or high-silica rhyolites (>72 wt% SiO2)

on Seguam. The elevated FeO*/MgO ratios of Seguam lavas from basalt to rhyolite are char-acteristic of the tholeiitic series lavas as defi ned by Miyashiro (1974) and Gill (1981). Each of the three eruptive stages contains both low-K basalts and medium-K rhyolites, but the major-ity of the exposed rhyolites belongs to the post-collapse suite. Interestingly, the early history of volcanism from 318 to 142 ka is devoid of inter-mediate-composition lavas.

METHODS

Field Studies

Access to most of the island was achieved on foot. However, several shoreline exposures were only approachable via infl atable boat due to the steep sea cliffs. Field relationships were mapped using a compilation of National Oceanic and Atmospheric Administration (NOAA) maps T-10322 to T-10325 (1:20,000) as a topographic base. The volcanic units illustrated in the geo-logic map include lava fl ows, domes, ignim-brites, and ash cover inferred to have erupted from multiple vents (Fig. 4). Individual fl ows and domes were delineated on the map where possible, but several of the map units are stacks of lava fl ows that likely shared a common vent

318-142 ka

138-11 ka

9 ka-present

199236318

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Figure 2. Digital elevation models (DEMs) of Seguam Island with shaded areas indicating the three main stages of eruptive activity over the past 318 k.y. Dots represent sample loca-tions and are shown with corresponding 40Ar/39Ar ages reported in ka. The ages of 6.0 and 1.7 ka shown in the bottom panel represent U-Th mineral isochron ages from Jicha et al. (2005). The lower panel is further subdivided according to wt% SiO2 content.

Jicha and Singer


and are mineralogically and chemically similar. Most units have a narrow compositional range, although a few include lavas that span a large range in SiO

2 content. Ash cover and extensive

erosion caused by repeated glacial advances have resulted in complicated stratigraphic rela-tions. Individual map units were often correlated based on a combination of 40Ar/39Ar age, major-element geochemistry, and petrography.

40Ar/39Ar Geochronology

Holocrystalline groundmass separates were prepared from porphyritic lava samples by crushing, sieving, magnetic sorting, and hand-picking under a binocular microscope to remove olivine, pyroxene, and plagioclase phenocrysts and minimize the potential for xenocrystic con-tamination. Most groundmass separates were 250–500 μm. For a few samples, this size frac-tion contained numerous microphenocrysts of olivine and pyroxene, therefore the 180–250 μm fraction was used. Groundmass plagioclase var-ied between 20 and 90 μm. Whole-rock mini-cores, which were 5 mm in diameter and ranged from 160 to 450 mg in weight, were drilled from aphyric lava fl ows. Groundmass separates (~100–375 mg) were weighed and then wrapped in 99.99% copper foil packets and, along with the mini-cores, were placed into 2.5-cm-diam-eter Al disks with 1.194 Ma sanidine from the Alder Creek rhyolite (Renne et al., 1998) as a neutron fl uence monitor. The Al disks were irra-diated for 20–90 min at the Oregon State Uni-versity reactor in the Cadmium-Lined In-Core

Irradiation Tube (CLICIT), where they received fast neutron doses of 1.2–5.4 × 1015 n/cm2. J values for the samples were determined by interpolating laterally across the Al disks. The precision of the J values was between ±0.19 and 0.50% (1σ), which was based on the weighted average of 12–15 laser fusion analyses of single sanidine crystals from each Al disk. Based on previous experiments, corrections for undesir-able nucleogenic reactions on 40K and 40Ca are: [40Ar/39Ar]

K = 0.00086; [36Ar/37Ar]

Ca = 0.000264;

[39Ar/37Ar]Ca

= 0.000673.At the University of Wisconsin Rare Gas

Geochronology Laboratory, the groundmass packets and mini-cores were incrementally heated in a double-vacuum resistance furnace attached to a 300 cm3 gas clean-up line. Prior to each incremental-heating experiment, sam-ples were heated to 500–675 °C and pumped to remove potentially large amounts of atmo-spheric argon and water (Baksi, 1974). Fully automated experiments consisted of 4–10 steps from 675–1450 °C; each step included a 2 min increase to the desired temperature that was maintained for 10–15 min. During the heating time, and for an additional 5–8 min afterward, the sample gas was exposed to three SAES C50 Zr-Al getters. Isotopic measurements and data reduction followed the procedures of Singer et al. (2000, 2004). These measurements were critically dependent on characterizing the blank levels in the analytical system and the mass dis-crimination of the mass spectrometer. Blanks were measured over a range of temperatures between 700 and 1350 °C prior to and follow-

ing each sample. All blanks were atmospheric in composition and one to two orders of magnitude smaller than the sample signals; thus, the impact of blanks on age uncertainty was minimal. Mass discrimination was monitored using an auto-mated air pipette and varied between 1.0022 ± 0.002 and 1.0043 ± 0.002 per atomic mass unit (a.m.u.) during the analytical periods.

Even though single incremental-heating experiments often yielded precise ages, most samples required one to four replicate experi-ments on subsamples to achieve 2σ analytical precisions of ±10%–15% and ±2%–3%, for basalts and rhyolites, respectively (Fig. 5). For four samples, incremental-heating experiments were performed on whole-rock mini-cores and groundmass separates from the same rock and yielded nearly identical results (Table 1; GSA Data Repository Table DR11). Because iso-chron regressions (York, 1969) agreed with plateau ages and did not reveal evidence that excess argon was present in any of the lavas (i.e., 40Ar/36Ar intercepts that are indistinguishable from 295.5), we consider the plateau ages to give the best estimate of the time elapsed since erup-tion (Table 1). All ages were calculated using the decay constants of Steiger and Jäger (1977).

1GSA Data Repository item 2006126, Table DR1, the complete 40Ar/ 39Ar incremental heating results from Seguam Island; and Table DR2, whole-rock major and trace element compositions of 40Ar/ 39Ar dated samples, is available on the Web at http://www.geosociety.org/pubs/ft2006.htm. Requests may also be sent to [email protected].

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Older eroded volcanics (318-142 ka)

Shoreline lavas (138-11 ka)

Postcollapse lavas (Holocene-historic)

Wilcox volcano lavas (98-9 ka)

48 52 56 60 64 68 72

SiO2

Figure 3. FeO*/MgO and K2O variation versus SiO2 (wt%) for 141 samples from Seguam Island. Analyses by inductively coupled plasma–mass spectrometry (ICP-MS) at Actlabs in Ontario, Canada (n = 51), X-ray fl uorescence (XRF) at the Universität Göttingen, Germany (n = 40), and XRF from Singer et al. (1992a, 1992b) (n = 50). Samples have been subdivided according to fi eld relations and age constraints (Table 1). Compositional subdivisions of fi eld boundaries are from Miyashiro (1974).

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Cumulative % 39Ar released 0 20 40 60 80 100 0.0 0.2 0.4

39Ar/40Ar0.6 0.8 1.0 1.2 1.4

0.0040

0.0035

0.0030

0.0025

0.0020

36A

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SEG 04 35 andesite whole rockweighted mean plateau 82.9 ± 3.6 ka (2σ)

2 separate experiments

SEG 03 34 basaltic andesite groundmassweighted mean plateau 174.3 ± 4.8 ka (2σ)


SEG 04 42 basaltic andesite groundmassweighted mean plateau 202.3 ± 8.7 ka (2σ)


SEG 04 31 dacite whole rockweighted mean plateau 318.3 ± 30.1 ka (2σ)

1 experiment

SEG 04 24 andesite groundmassweighted mean plateau 41.9 ± 4.1 ka (2σ)


SEG 03 32 rhyolite whole rockweighted mean plateau 7.5 ± 2.0 ka (2σ)


SEG 03 326.1 ± 3.5 ka (2σ)40Ar/36Ari = 296.5 ± 2.4MSWD = 0.05n = 15 of 16

SEG 04 35 80.7 ± 5.5 ka (2σ)

40Ar/36Ari = 296.8 ± 2.45MSWD = 0.08

n = 13 of 13

SEG 03 34173.0 ± 14.0 ka (2σ)

40Ar/36Ari = 295.8 ± 2.8MSWD = 0.67

n = 16 of 16

SEG 04 31306.4 ± 54.6 ka (2σ)

40Ar/39Ari = 296.1 ± 2.3MSWD = 0.33

n = 6 of 7

60

0

240

180

120

60

0

240

180

120

SEG 04 42207.0 ± 12.4 ka (2σ)

40Ar/36Ari = 294.5 ± 1.8MSWD = 0.89n = 14 of 15

SEG 04 2438.8 ± 8.5 ka (2σ)

40Ar/36Ari = 296.9 ± 3.4MSWD = 0.32

n = 15 of 15

Figure 5. Representative 40Ar/39Ar age spectrum and inverse isochron diagrams for fi ve Seguam lavas and one ignimbrite. The preferred ages and ±2σ errors in bold are given by the plateau steps in the age spectra.



TABLE 1. SUMMARY OF 40Ar/39Ar INCREMENTAL-HEATING EXPERIMENTS

Sample Map SiO2 Latitude Longitude Material No. of Age spectrum Inverse isochron analysis

unit (%) (N) (W) expts. Plateau age(ka)† ±2σ

Incrementsused(°C)

39Ar%

N‡ MSWD Total fusionage (ka) ±2σ

40Ar/36Ari±2σ

Age (ka)±2σ

MSWD

Postcollapse activity (younger than 9 ka)

SEG 03 32§ rcv 70.8 52°21.07′ 172°25.02′ gm 3 7.5 ± 2.0 725–1450 99.8 15 of 16 0.04 8.4 ± 3.2 296.5 ± 2.4 6.1 ± 3.5 0.05SEG 03 44§ dig 64.5 52°21.07′ 172°25.02′ wr/gm 4 8.4 ± 1.5 725–1275 100.0 19 of 19 0.14 8.9 ± 2.1 298.2 ± 3.7 7.2 ± 3.2 0.21

Island-wide activity (138–9 ka)

SEG 04 34 nsl 58.4 52°21.61′ 172°31.45′ wr 1 11.1 ± 3.5 750–1240 100.0 7 of 7 0.39 12.7 ± 6.7 296.7 ± 3.1 9.7 ± 4.7 0.34SEG 03 67 ssb 55.2 52°17.20′ 172°24.00′ gm 2 12.1 ± 5.1 840–1300 99.2 10 of 11 0.07 12.4 ± 6.4 296.0 ± 2.5 10.3 ± 9.4 0.23SEG 03 02 fcf 62.0 52°22.66′ 172°23.50′ gm 2 22.8 ± 5.1 850–1300 100.0 9 of 9 0.75 21.6 ± 6.3 295.0 ± 4.3 27.0 ± 14.0 0.22SEG 03 66§ lpd 62.8 52°17.00′ 172°24.20′ wr 2 23.5 ± 5.8 875–1230 100.0 9 of 9 0.32 24.6 ± 8.5 287.0 ± 69.0 36.0 ± 34.0 0.55SEG 04 05 wvl 66.9 52°18.68′ 172°19.61′ wr 1 27.5 ± 1.7 865–1275 100.0 7 of 7 0.14 27.4 ± 2.1 295.6 ± 8.4 27.5 ± 2.0 0.17SEG 04 07 wvl 64.4 52°18.68′ 172°19.61′ gm 1 27.9 ± 7.7 800–1310 100.0 10 of 10 0.27 33.3 ± 17.7 296.7 ± 3.1 24.2 ± 11.8 0.24SEG 03 03§ rdf 67.9 52°22.50′ 172°23.35′ wr 2 31.7 ± 1.2 900–1375 100.0 10 of 10 0.72 31.8 ± 1.4 295.6 ± 3.6 31.8 ± 2.1 0.80SB88–3 rdf 67.1 52°22.35′ 172°25.90′ wr 2 31.7 ± 2.0 850–1375 100.0 11 of 11 0.84 31.6 ± 2.5 294.7 ± 8.7 32.1 ± 4.1 0.01SB87–56# ssl 70.1 52°16.01′ 172°31.33′ wr/gm 4 33.2 ± 0.9 900–1300 96.6 25 of 27 0.34 31.9 ± 1.1 295.0 ± 4.2 34.0 ± 1.9 0.80SEG 04 24 nsl 60.8 52°18.11′ 172°37.13′ gm 2 41.9 ± 4.1 780–1260 100.0 15 of 15 0.22 43.3 ± 4.9 296.9 ± 3.4 38.8 ± 8.5 0.32SEG 03 40 aml 61.8 52°15.95′ 172°35.18′ wr/gm 3 43.3 ± 5.7 675–1300 100.0 15 of 15 0.01 43.4 ± 6.2 293.5 ± 3.5 52.0 ± 16.0 0.32SB87–9# ssl 53.5 52°15.90′ 172°31.34′ gm 2 48.9 ± 7.9 800–1260 100.0 12 of 12 0.49 54.0 ± 11.0 296.7 ± 2.4 40.7 ± 16.2 0.01SB88–23# wvl 54.1 52°21.35′ 172°20.80′ gm 4 49.2 ± 7.6 825–1200 96.3 17 of 20 0.56 50.1 ± 8.6 295.9 ± 2.1 47.1 ± 13.3 0.26SB87–63# ssl 53.3 52°15.95′ 172°35.18′ wr/gm 3 52.9 ± 13.7 850–1100 81.7 13 of 18 0.82 55.0 ± 16.0 294.2 ± 2.0 82.7 ± 37.7 0.75SEG 03 45§ wvl 62.4 52°20.91′ 172°22.51′ wr 2 53.0 ± 1.3 850–1300 100.0 11 of 11 1.15 53.1 ± 2.3 294.9 ± 4.1 53.2 ± 2.1 0.42SEG 03 25 ssl 60.5 52°17.20′ 172°27.02′ wr 2 53.4 ± 3.8 875–1300 100.0 10 of 10 0.02 53.3 ± 4.3 294.8 ± 3.6 54.1 ± 5.2 0.13SB88–25 wvl 63.2 52°21.35′ 172°20.80′ wr 2 56.2 ± 1.4 850–1320 98.5 9 of 10 0.62 58.7 ± 2.5 296.1 ± 4.7 55.5 ± 3.9 0.87SB88–18# afp 63.4 52°22.70′ 172°24.80′ gm 2 57.7 ± 5.3 900–1250 96.9 8 of 9 0.82 63.5 ± 8.9 295.4 ± 1.2 58.6 ± 9.5 0.15SB88–16# awf 55.6 52°23.40′ 172°26.80′ gm 4 61.4 ± 5.9 920–1240 91.3 18 of 23 1.53 58.1 ± 5.1 295.2 ± 1.4 63.4 ± 10.4 1.40SEG 03 04 afp 57.6 52°23.20′ 172°25.60′ wr 2 62.0 ± 6.5 875–1300 100.0 11 of 11 0.05 61.2 ± 8.2 294.8 ± 3.1 66.0 ± 17.0 0.02SEG 03 64 ssl 58.8 52°16.71′ 172°28.27′ wr 2 64.4 ± 2.2 875–1330 97.2 11 of 12 0.86 65.4 ± 2.4 294.0 ± 10.7 65.3 ± 4.2 0.35SEG 03 48§ wvl 52.2 52°20.54′ 172°21.68′ gm 3 65.6 ± 13.7 850–1275 100.0 16 of 16 0.06 72.6 ± 16.6 296.7 ± 3.1 40.0 ± 29.0 1.16SEG 03 68 ssl 52.1 52°16.98′ 172°27.91′ gm 3 67.0 ± 12.6 900–1300 99.9 12 of 13 0.16 67.9 ± 13.7 296.3 ± 9.1 63.6 ± 34.9 0.44SEG 03 01§ rfc 70.4 52°22.83′ 172°23.81′ wr 2 76.8 ± 1.1 875–1375 100.0 10 of 10 1.20 76.8 ± 1.2 295.4 ± 5.5 76.8 ± 1.5 0.55SEG 04 36 nsl 56.4 52°21.44′ 172°32.83′ wr 2 78.2 ± 9.6 820–1150 94.0 14 of 16 0.05 90.5 ± 16.5 296.0 ± 1.4 73.6 ± 16.3 0.57SEG 03 12 ssl 54.7 52°15.69′ 172°31.08′ gm 3 79.7 ± 6.8 725–1300 100.0 16 of 16 0.05 79.7 ± 8.3 294.1 ± 3.2 86.3 ± 17.1 0.22SB87–4 ssl 58.5 52°16.55′ 172°29.60′ wr 2 82.8 ± 2.4 900–1350 99.7 10 of 11 0.02 83.1 ± 5.1 295.3 ± 3.1 82.9 ± 3.6 0.40SEG 04 35 nsl 52.9 52°21.59′ 172°32.27′ wr 2 82.9 ± 3.6 835–1270 100.0 13 of 13 0.65 84.8 ± 5.2 296.8 ± 2.5 80.7 ± 5.5 0.08SEG 03 21 ssl 64.1 52°17.33′ 172°26.04′ wr 2 83.6 ± 1.6 825–1275 100.0 10 of 10 0.99 83.6 ± 2.3 295.3 ± 14.3 84.0 ± 2.4 1.40SEG 03 49§ wvl 52.4 52°20.58′ 172°21.78′ gm 4 84.6 ± 14.2 875–1250 92.6 16 of 17 0.35 79.1 ± 17.2 295.3 ± 3.3 82.0 ± 45.0 0.23SEG 03 23 ssl 64.0 52°17.30′ 172°26.26′ gm 2 84.7 ± 3.6 850–1325 100.0 12 of 12 0.33 85.0 ± 4.8 294.7 ± 4.6 85.9 ± 9.3 0.21SJ87–47# ssl 68.6 52°17.80′ 172°25.75′ gm 2 92.8 ± 2.8 875–1325 100.0 10 of 10 0.85 93.3 ± 3.6 296.1 ± 1.8 90.4 ± 7.3 0.16SB87–49# aml 56.8 52°15.95′ 172°35.18′ gm 3 93.1 ± 9.5 880–1125 71.7 16 of 24 1.03 74.7 ± 12.4 296.3 ± 1.6 75.6 ± 27.0 0.70SEG 03 50§ wvl 52.5 52°20.04′ 172°21.55′ gm 3 98.1 ± 18.5 940–1250 100.0 12 of 12 0.24 105.1 ± 24.1 296.7 ± 4.9 84.0 ± 71.0 0.03SEG 04 38 nsl 62.8 52°21.26′ 172°33.53′ wr 1 98.8 ± 3.5 750–1250 100.0 8 of 8 0.55 99.4 ± 4.0 295.7 ± 3.7 98.4 ± 6.9 0.64SEG 04 20 ssl 54.8 52°16.65′ 172°37.75′ wr 2 112.9 ± 15.9 785–1200 90.2 14 of 15 0.06 102.3 ± 22.1 295.7 ± 2.3 111.0 ± 37.0 0.22SEG 03 35§ ssl 54.7 52°15.43′ 172°33.10′ gm 2 116.5 ± 13.6 875–1325 100.0 11 of 11 0.26 115.3 ± 16.0 296.3 ± 18.2 111.7 ± 84.3 0.30SEG 04 17 nsl - 52°29.16′ 172°33.10′ gm 1 120.1 ± 9.8 840–1220 87.9 9 of 10 1.18 106.5 ± 9.5 296.7 ± 5.2 115.7 ± 21.5 1.31SEG 04 15 nsl - 52°22.07′ 172°29.39′ wr 1 121.5 ± 6.1 800–1250 100.0 6 of 6 0.23 120.4 ± 9.9 294.8 ± 4.3 123.3 ± 12.6 0.26SEG 03 16 ssl 67.5 52°16.24′ 172°30.26′ wr 2 121.6 ± 1.3 875–1225 97.0 10 of 12 0.94 121.1 ± 1.6 284.0 ± 26.0 122.9 ± 2.6 0.00SEG 04 21 ssl 61.4 52°16.65′ 172°37.75′ gm 2 121.8 ± 12.7 825–1320 100.0 17 of 17 0.01 125.5 ± 29.2 296.1 ± 1.3 114.8 ± 20.3 0.90SEG 04 16 nsl 58.6 52°22.10′ 172°29.42′ wr 1 123.2 ± 2.7 800–1250 100.0 7 of 7 0.09 123.2 ± 3.3 294.4 ± 12.2 123.5 ± 4.4 0.10SEG 04 33 nsl 61.7 52°20.11′ 172°34.49′ wr 1 132.5 ± 3.7 790–1280 100.0 8 of 8 0.15 132.1 ± 4.9 294.9 ± 6.1 133.0 ± 6.5 0.17SEG 04 18 nsl 54.2 52°22.16′ 172°29.83′ gm 1 133.3 ± 8.4 790–1170 87.7 7 of 8 0.25 145.4 ± 11.5 296.7 ± 2.5 127.9 ± 13.7 0.09SEG 03 36 ssl 67.9 52°15.43′ 172°33.10′ wr 2 138.4 ± 1.3 875–1325 100.0 12 of 12 0.37 138.2 ± 1.5 296.5 ± 8.4 138.3 ± 2.7 1.30

Older, eroded volcanics (318–142 ka)

SEG 03 43§ tpr 68.7 52°15.43′ 172°33.10′ wr 2 141.9 ± 2.2 950–1250 96.2 10 of 12 0.00 144.1 ± 2.7 293.2 ± 9.5 143.3 ± 6.2 0.36SEG 04 27 nsr 67.3 52°18.36′ 172°36.32′ wr 1 159.2 ± 2.2 780–1280 100.0 8 of 8 0.91 160.5 ± 2.9 297.6 ± 2.3 158.0 ± 2.6 0.55SEG 03 34 eld 55.1 52°15.37′ 172°33.11′ gm 3 174.3 ± 4.8 715–1320 100.0 16 of 16 1.08 173.0 ± 5.9 295.8 ± 2.8 173.0 ± 14.0 0.67SEG 04 41 eld 54.4 52°15.73′ 172°37.78′ gm 2 175.7 ± 9.2 775–1245 100.0 19 of 19 0.11 190.6 ± 31.6 295.7 ± 1.4 173.3 ± 15.4 0.44SEG 04 43 eld 51.7 52°15.16′ 172°36.40′ gm 2 188.5 ± 11.6 760–1235 100.0 14 of 14 0.41 188.6 ± 15.5 295.8 ± 2.1 186.8 ± 17.6 0.17SEG 03 74 eld 50.5 52°18.25′ 172°22.50′ gm 5 190.6 ± 38.5 825–1340 91.4 23 of 27 0.45 176.0 ± 40.0 294.7 ± 2.3 224.0 ± 100.0 0.49SEG 04 29 eld 66.7 52°19.19′ 172°35.38′ wr 1 199.1 ± 1.8 790–1300 100.0 8 of 8 0.15 199.3 ± 2.3 297.0 ± 8.0 198.7 ± 2.6 0.15SEG 04 42 eld 53.6 52°15.16′ 172°36.40′ gm 2 202.3 ± 8.7 760–1250 99.8 14 of 15 0.18 197.7 ± 11.8 294.5 ± 1.8 207.0 ± 12.4 0.89SB87–59 eld 50.5 52°15.95′ 172°33.97′ gm 3 209.5 ± 55.7 975–1250 77.8 11 of 13 1.50 174.9 ± 51.2 296.0 ± 2.0 161.0 ± 84.0 0.70SB87–39 eld 54.9 52°18.45′ 172°22.25′ gm 2 210.6 ± 42.4 940–1270 96.6 9 of 11 0.00 198.8 ± 51.9 293.1 ± 9.1 344.0 ± 290.0 1.30SEG 04 30 eld 52.8 52°19.19′ 172°35.38′ wr 1 235.8 ± 3.8 785–1285 100.0 8 of 8 0.43 234.4 ± 4.8 292.3 ± 5.6 238.3 ± 5.8 0.29SEG 03 41 eld 52.6 52°16.13′ 172°34.55′ wr 4 281.1 ± 33.0 725–1250 100.0 19 of 19 0.46 299.5 ± 34.5 299.0 ± 10.0 168.0 ± 160.0 0.05SEG 04 31 eld 66.5 52°19.19′ 172°35.38′ wr 1 318.3 ± 30.1 800–1150 95.8 6 of 7 0.32 295.9 ± 101.2 296.1 ± 2.3 306.4 ± 54.6 0.33

Note: Abbreviations: gm—groundmass; wr—whole rock.†Ages calculated relative to 1.194 Ma Alder Creek Rhyolite sanidine (Renne et al., 1998); uncertainties reported at 2σ precision.‡N = number of plateau/isochron steps used in regression.§Data from Jicha et al. (2005).#Data from Jicha et al. (2004).

Jicha and Singer


TABLE 2. REPRESENTATIVE WHOLE-ROCK MAJOR- AND TRACE-ELEMENT COMPOSITIONS OF SEGUAM ISLAND LAVAS

Older Dacitic Postcollapse Moundhilleroded lavas Shoreline lavas Wilcox volcano ignimbrite evolved lavas Pyre Peak lavas volcano

Sample 03 74 04 42 04 35 03 64 04 38 03 50 03 47 03 45 03 44 03 30 03 31 03 29 03 19 04 32 04 02 04 09

(wt%)SiO2 50.54 53.60 52.90 58.81 62.80 52.50 57.21 62.36 64.51 69.83 71.39 51.51 53.54 59.80 51.80 51.60TiO2 0.63 0.86 0.77 1.12 1.04 0.69 1.11 0.90 0.75 0.53 0.48 0.65 0.79 0.82 0.75 0.73Al2O3 20.19 18.30 18.40 15.51 15.40 20.89 15.69 15.16 15.54 14.77 13.73 18.66 19.52 16.60 20.40 20.40FeO* 7.87 9.28 9.39 9.50 8.39 7.12 9.79 6.66 5.53 3.90 3.48 8.84 8.44 7.07 8.68 8.55MnO 0.13 0.16 0.16 0.15 0.16 0.13 0.16 0.14 0.12 0.11 0.10 0.15 0.14 0.14 0.14 0.14MgO 6.48 4.74 5.68 3.16 1.75 3.62 3.30 2.08 1.40 0.66 0.52 7.04 4.06 3.06 5.26 5.33CaO 11.71 9.32 9.97 6.89 4.71 10.48 7.53 4.73 3.94 2.52 2.12 10.42 10.51 6.07 10.42 10.35Na2O 2.29 3.18 2.71 3.51 4.69 2.79 3.50 4.33 4.59 5.17 4.97 2.42 2.92 4.14 2.64 2.60K2O 0.30 0.54 0.53 1.15 1.33 0.65 0.87 1.53 1.80 2.13 2.13 0.43 0.62 1.37 0.47 0.47P2O5 0.08 0.13 0.10 0.15 0.21 0.12 0.14 0.19 0.17 0.12 0.10 0.08 0.09 0.18 0.09 0.10Total 100.08 100.11 100.61 99.65 100.48 100.22 99.18 98.60 99.18 99.91 99.16 99.73 100.23 99.24 100.65 100.28(ppm)Cs 0.54 0.29 0.35 0.87 2.09 0.45 2.05 1.62 2.58 4.21 4.00 0.85 1.13 2.40 0.24 0.77Sc 33 39 32 35 24 27 35 23 19 14 13 33 36 24 31 30V 220 270 274 313 99 281 334 91 71 12 12 229 277 133 245 236Cr – 34 66 – 24 29 – – – – – – – 31 38 35Co 26 28 30 19 10 17 20 10 8 3 2 30 19 12 32 29Ni 61 33 53 5 85 15 6 8 4 4 3 57 22 30 41 33Cu 83 72 110 103 24 66 122 42 28 21 20 98 80 45 49 65Zn 38 79 73 66 91 45 64 68 64 67 68 47 51 78 66 67Ga 17 20 17 19 19 18 19 18 18 18 17 17 18 17 22 19Rb 7 10 13 30 32 14 20 38 42 55 51 11 14 33 9 10Sr 315 291 298 252 257 376 303 223 224 165 132 306 323 265 358 354Zr 26 57 52 95 121 45 67 134 144 177 179 33 43 119 46 48Nb 1.10 1.00 1.00 2.19 4.00 1.42 1.85 2.97 3.26 3.62 3.47 0.95 1.07 5.00 – –Ba 159 238 202 412 465 258 324 551 646 738 727 175 219 459 214 206Y 11 19 17 23 33 14 19 32 28 35 34 11 14 31 13 14La 3.17 5.41 4.00 7.77 9.46 5.40 7.06 12.14 11.14 14.90 14.38 3.56 4.41 9.40 4.24 4.12Ce 7.37 11.80 9.42 17.73 22.10 12.14 15.89 27.25 24.73 33.22 32.60 8.16 10.15 21.70 9.65 9.51Pr 1.05 1.85 1.33 2.47 3.12 1.63 2.16 3.69 3.16 4.28 4.17 1.14 1.39 2.98 1.35 1.32Nd 5.35 9.27 6.61 11.79 15.20 7.94 10.64 17.51 14.72 19.75 18.79 5.72 6.84 13.90 6.41 6.29Sm 1.64 2.69 1.97 3.52 4.25 2.25 3.01 4.94 4.03 5.23 5.14 1.72 2.06 3.79 1.79 1.77Eu 0.69 0.97 0.74 1.12 1.31 0.86 1.07 1.37 1.37 1.32 1.27 0.69 0.82 1.13 0.73 0.71Gd 1.88 3.23 2.38 3.83 4.87 2.37 3.41 5.19 4.30 5.47 5.38 1.98 2.34 4.31 2.08 2.06Tb 0.35 0.53 0.39 0.70 0.79 0.42 0.60 0.95 0.82 1.02 1.03 0.35 0.43 0.69 0.34 0.33Dy 2.17 3.62 2.74 4.29 5.45 2.53 3.66 5.73 4.99 6.14 6.15 2.20 2.69 4.74 2.34 2.28Ho 0.45 0.76 0.57 0.91 1.14 0.53 0.76 1.19 1.05 1.31 1.30 0.48 0.57 0.99 0.48 0.48Er 1.38 2.19 1.67 2.79 3.35 1.59 2.36 3.70 3.33 4.12 4.06 1.46 1.75 2.91 1.44 1.39Tm 0.21 0.31 0.25 0.43 0.49 0.24 0.36 0.57 0.52 0.63 0.61 0.22 0.26 0.43 0.20 0.20Yb 1.33 2.10 1.64 2.71 3.25 1.57 2.23 3.66 3.50 4.13 4.09 1.35 1.62 2.87 1.35 1.35Lu 0.19 0.32 0.25 0.38 0.51 0.22 0.32 0.53 0.50 0.61 0.61 0.20 0.24 0.45 0.21 0.21Hf 1.09 1.53 1.48 3.02 3.46 1.31 2.20 3.91 4.12 4.97 5.07 1.18 1.40 3.38 1.23 1.24Ta – 0.07 0.09 0.09 0.09 0.05 0.08 0.17 0.20 0.25 0.23 0.02 0.04 0.24 0.10 0.11Tl – 0.04 0.05 0.15 0.17 – 0.13 0.25 0.41 0.43 0.49 0.07 0.09 0.19 0.04 0.04Pb – 5.51 4.79 5.26 12.30 9.08 3.87 3.14 9.22 12.74 13.00 3.29 – 11.00 4.02 4.51Th 0.65 0.88 1.10 2.52 2.72 1.14 1.63 3.12 3.49 4.23 4.16 0.80 1.03 2.63 0.96 0.96U 0.34 0.50 0.63 1.46 1.58 0.62 0.90 1.80 2.01 2.42 2.33 0.46 0.59 1.51 0.50 0.51

Note: All samples have “SEG” prefi x. Major- and trace-element concentrations were determined on SEG 03 samples following the procedures described in Jicha et al. (2004); analyses of SEG 04 samples were performed at the Universität Göttingen, Germany, following the procedures of Hinners et al. (1998). Precision of the inductively coupled plasma–mass spectrometry (ICP-MS) data is described in Jicha et al. (2004). (–) denotes analysis was performed at or below detection limit.



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Jicha and Singer


ERUPTIVE CHRONOLOGY

The 34 map units shown in Figure 4 are grouped into three main stages of volcanism based on 40Ar/39Ar age, inferred eruptive vent, and clearly defi ned superposition relationships. The 40Ar/39Ar geochronology agrees with stra-tigraphy at 16 different locations on the island; nowhere do the age determinations violate the stratigraphy. These relationships as well as a detailed description of each eruptive stage are discussed below. Note that the younger than 9 ka postcollapse period of activity includes potentially coeval eruptions from several vents on the island, and therefore is subdivided into three phases based on geographic location.

Older, Eroded Volcanics (318–142 ka)

The oldest record of subaerial volcanism is preserved in three distinct locations on the island. It includes deeply eroded lavas and dikes (eld), the north shore rhyodacite dome (nsr), and the Turf Point rhyodacite dome (tpr). Two of the three localities (e.g., at the western end and along the southeastern shore) were recognized as pre-serving relatively old sequences by Singer et al. (1992a). The preserved volume at the western end of the island is composed of tens of 1–2-m-thick basaltic lavas that fl owed northward from a pre-existing center likely located offshore between the southwest corner of the island and Turf Point. The north-dipping fl ows are cut by several 2-m-wide basaltic to andesitic dikes. Near Turf Point, a 0.2 km3 rhyodacite dome (tpr) intrudes nota-bly thicker (15–30 m) basaltic lavas, which also dip gently northward. The 40Ar/39Ar plateau age from the rhyodacite dome is 142 ± 2 ka, whereas the basaltic lavas from several different locali-ties within this eroded section yield ages ranging from 202 ± 9 to 174 ± 5 ka. The dikes that cross-

cut these lavas gave ages of 281, 210, and 189 ka; the 281 and 210 ka dikes crosscut older, undated lavas in this sequence, whereas the 189 ka dike cuts through the basaltic fl ow dated at 202 ± 9 ka (Figs. 2 and 4; Table 1).

Deeply eroded, subhorizontal lavas and inter-bedded pyroclastic rocks are exposed along 2 km of the southeastern shore northeast of Lava Point (Table 2; Figs. 4 and 6). This sequence consists of moderately altered basaltic and basaltic andesitic lavas and dikes and several small dacitic fl ows. A basalt and basaltic andes-ite from this section were dated at 191 ± 39 and 211 ± 42 ka, respectively.

A recently discovered sequence of older lavas and domes crops out 2 km southwest of Saddleridge Point (Fig. 4). Several 50-m-thick basaltic and dacitic fl ows are capped by several ~10-m-thick basalt fl ows. A clast from the brecci-ated base of one of the 50-m-thick fl ows yielded a 40Ar/39Ar age of 318 ± 30 ka, the oldest material we found on the island. The other fl ow lobes gave ages of 199 ± 2 and 236 ± 4 ka (Table 1). This entire sequence of lavas was intruded by a mas-sive, 60-m-thick rhyodacite dome (nsr), which was characterized by columnar jointing that radi-ated from the core of the dome. The hexagonal columns averaged 15 cm in diameter, and were likely formed due to emplacement of the dome into ice. A single experiment from the dacite dome gave a 40Ar/39Ar plateau age of 159 ± 2 ka.

Island-Wide Activity (138–9 ka)

Shoreline Lavas (138–11 ka)These lavas represent 46% of the total vol-

ume erupted and cover more than half the island, ~110 km2, including the entire area between the eroded volcanics on the western end and the southeastern shore. The majority of these gently dipping basaltic to rhyolitic lavas crop out along

the northern and southern shorelines, although a few deeply glaciated remnants have been found within the interior of the island. Lavas along both shorelines dip gently away from the center of the island, but their vent areas are unknown. Individual fl ow lobes average 15–20 m in thick-ness, and the thickness of the entire sequence of lavas is ~300 m. North shore lavas (nsl) vary from basaltic to dacitic compositions and give 40Ar/39Ar plateau ages from 133 ± 8 to 11 ± 3 ka, whereas the south shore lavas (ssl) range from basaltic to rhyolitic and give ages from 138 ± 1 to 33 ± 1 ka (Figs. 2 and 4; Tables 1 and 2). Figure 6A illustrates the agreement between the 40Ar/39Ar geochronology and the stratigraphic relationships among the south shore lavas. Approximately 3 km northwest of Turf Point, the north and south shore lavas are overlain by a 200-m-thick sequence of nearly horizontal andesites and dacites. The remnants of these lavas (aml) have been deeply eroded into the shape of an amphitheater. The lowermost lava in this section gave a 40Ar/39Ar age of 93 ± 9 ka.

Wilcox Volcano (98–9 ka)After 200 k.y. of primarily effusive eruptions

of sheet-like subhorizontal lavas, the eruptive fl ux at Seguam increased and the focus of volca-nism shifted to the east, and a 20 km3 stratocone was constructed. Wilcox volcano consists of tens of basaltic to rhyolitic fl ows (wvl) of variable thickness that dip 30–40° radially away from the former central vent now occupied by a 0.6 km3 rhyolitic cone (Table 2). Several of the andesitic to rhyolitic lavas making up the northern fl ank (rdf, awf, afp, rfc) spread out laterally as they approached the shoreline (Fig. 4). It is unclear how far the lavas extended to the south because intense wave action has eroded away much of the southern fl ank of the volcano.

The lowest exposures of the stratocone consist of basalt fl ows intercalated with thick pyroclas-tic breccias and subvertical dikes. Several of the fl ows in this 200-m-thick section are moderately oxidized, contain chlorite and serpentinized olivine, and are weakly hydrothermally altered. Nonetheless, 40Ar/39Ar ages of 98 ± 18, 85 ± 14, and 66 ± 14 ka were obtained from stratigraphi-cally successive basaltic lavas (Table 1). These basal units are capped by numerous basaltic andesitic to rhyolitic fl ows that erupted from 62 to 27 ka (Table 2). At 23 ± 5 ka, the eruption of a ≥0.02 km3 andesitic (62.0 wt% SiO

2) ignim-

brite fi lled a valley between the Finch Cove rhy-olite (rfc) and Finch Cove rhyodacite (rdf). The ~25-m-thick ignimbrite is moderately welded throughout and contains ~20% phenocrysts (12% plagioclase; 4% clinopyroxene; 2% oliv-ine; 2% oxides), abundant lithics (up to 20 cm), and pumice (15–40 cm). Some of the last activ-

TABLE 3. COMPARISON OF K-Ar AND 40Ar/39Ar AGES OF SEGUAM ISLAND LAVASDescription K-Ar data† 40Ar/39Ar data (this study)

Sample Age(ka) ±2σ

% 40Ar* Sample Plateau age(ka) ±2σ

% 40Ar*‡ Total fusion age(ka) ±2σ

Basaltic andesite fl ow B87–63 1070 ± 320 2.7 SB87–63 52.9 ± 13.7 0.6 55.0 ± 16.0Basalt fl ow B87–32 1050 ± 180 4.7 SEG 03 74 190.6 ± 38.5 1.5 176.0 ± 40.0Basalt fl ow B87–34 220 ± 100 1.8 SB87–34 § § §

Rhyodacite dome J87–61 200 ± 40 6.8 SEG 03 43 141.9 ± 2.2 41.1 144.1 ± 2.7Dacite fl ow B87–67 180 ± 20 11.6 SEG 03 16 121.6 ± 1.3 80.0 121.1 ± 1.6Basaltic andesite fl ow J87–43 170 ± 180 0.5 SEG 03 73 # # #

Basaltic andesite fl ow B87–59 100 ± 140 1.6 SB87–59 209.5 ± 55.7 1.5 174.9 ± 51.2Dacite fl ow J87–48 80 ± 20 8.0 SEG 03 21 83.6 ± 1.6 68.1 83.6 ± 2.3Basaltic andesite fl ow J87–59 70 ± 80 1.0 SEG 03 12 79.7 ± 46.8 5.3 79.7 ± 8.3Basaltic andesite fl ow B88–23 50 ± 40 5.0 SB88–23 49.2 ± 7.6 2.6 50.1 ± 8.6Andesite fl ow B88–16 30 ± 20 0.9 SB88–16 61.4 ± 5.9 2.8 58.1 ± 5.1

†Data from Singer et al. (1992a).‡% 40Ar* was calculated from all steps comprising the plateau; % 40Ar* of each step was weighted by the

fraction of 39Ar released.§40Ar/39Ar analysis not attempted on this sample.#Sample did not yield measurable radiogenic 40Ar.



ity at Wilcox volcano prior to the sector collapse at ca. 9 ka included the eruption of dacitic to rhyolitic lavas, which formed Moundhill Point and the adjacent region to the south.

At 9 ka, Wilcox volcano partially collapsed forming a 4-km-wide crater. The sector col-lapse was accompanied by an explosive lat-eral blast, which deposited a 0.45 km3 dacitic ignimbrite (dig) to the northwest of the edi-fi ce. This sequence of events was likely simi-lar to those during the May 1980 eruption of Mount St. Helens (Christiansen and Peterson, 1981) and the October 1902 eruption of Santa María volcano in Guatemala (Williams and Self, 1983). The dacitic ignimbrite retains a relatively uniform thickness of 80–100 m for 4 km until it reaches the northern shoreline. Because offshore bathymetry is not available, the distal extent of the ignimbrite is unknown, but it is clearly recognizable 10 km north of the coastline in U.S. Geological Survey (USGS) GLORIA sidescan images. A small outcrop is also preserved along the southeast-ern rim (Fig. 4). The deposit is characterized by 10–25 cm, dark-gray dacitic pumice clasts or fi amme (64.0 wt% SiO

2) hosted in a lighter-

gray, fi ne ash supported matrix of similar com-position (64.9 wt% SiO

2). The upper 50–70 m

of the ignimbrite is nonwelded and poorly consolidated, but the base is strongly welded and contains fl attened fi amme (Fig. 6). Four incremental-heating experiments on whole-rock samples and a phenocryst-free separate of the poorly consolidated facies of the ignim-brite gave a weighted mean 40Ar/39Ar age of 8.4 ± 1.5 ka. A U-Th mineral isochron age for this sample (defi ned by cpx + mt + plag + glass + wr) gave an age of 10.1 ± 1.3 ka (Jicha et al., 2005). Thus, we infer that emplacement of this ignimbrite and collapse of the stratovolcano occurred at 9 ± 1 ka.

Postcollapse Activity (Younger Than 9 ka)

Dacitic to Rhyolitic Domes and Flows (since 7.5 ka)

Two rhyolitic fl ows, a small dome, and a rhy-olitic cone were erupted from the crater, which was created in response to partial sector collapse of the stratovolcano. In addition, dacitic and rhyodacitic lavas erupted from fl ank vents to the south of Wilcox volcano at 335 m above sea level (Fig. 4). The 0.6 km3 cone (rcc) is com-posed of vesicular lavas of similar composition, and is mantled by coarse, silicic pumice blocks up to 50 cm in diameter. An older, tundra-cov-ered 0.08 km3 rhyolite fl ow (rcv) crops out to the west of the rcc cone. This plagioclase-phyric lava dips 20° to the northwest and is strongly fl ow banded with alternating layers of red rub-

Sam

ple

/Ch

on

dri

te

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Dy Tm Yb Lu


D

10

100

Sample SiO2

SEG 04 27 67.3SEG 04 28 67.1

SEG 03 74 50.6

SJ-87-79 50.6

Sam

ple

/Ch

on

dri

te


C

10

100

Sample SiO2

SB-87-56 70.0

SEG 03 40 61.8

Sam

ple

/Ch

on

dri

te


B

10

100

Sample SiO2

SEG 03 03 67.9

SEG 03 47 57.2

SJ-88-6 51.7

Sam

ple

/Ch

on

dri

te


A

10

100Sample SiO2

SEG 03 32 70.8

SEG 03 55 63.2

SEG 03 19 53.5

SEG 03 64 58.8

SEG 03 12 54.7

SEG 04 35 52.9

SEG 03 50 52.5

SEG 03 11 58.6

SEG 03 08 51.2

Figure 7. Chondrite-normalized rare earth element (REE) patterns for (A) postcollapse lavas, (B) Wilcox volcano lavas, (C) shoreline lavas, and (D) older, eroded volcanics. Chondrite val-ues are from Anders and Grevesse (1989).

Jicha and Singer


ble and black, glassy rhyolite. Three incremen-tal-heating experiments from the glassy rhyolite yielded a weighted mean plateau age of 7.5 ± 2.0 ka (Table 1; Fig. 5). The western fl ank of the cone and southern margin of the rcv fl ow are overlain by a small, gray, vitrophyric lava fl ow (srf), the most evolved on the island (71.4 wt% SiO

2) (Table 2; Fig. 4). The 40Ar/39Ar geochro-

nology was unsuccessful on this material, but whole-rock, plagioclase, clinopyroxene, mag-netite, and glass yielded a U-Th isochron age of 6 ± 4 ka (Jicha et al., 2005). The rcv and srf fl ows likely erupted from the same vent located near the base of the cone. The 1710 dome (str), the westernmost outcrop of postcollapse rhyo-lites, abuts the southern margin of the crater rim. Glass + magnetite + orthopyroxene + whole rock

from this dome gave a U-Th mineral isochron age of 1.7 ± 0.5 ka (Jicha et al., 2005). To the south of the dome lies a small, undated dacite fl ow (lcd), which fi lled a long, narrow canyon and an adjacent valley. East of the dacite is the ~50-m-thick, Lava Point rhyodacite (lpr), which erupted along a 3-km-wide, east-west–trending fi ssure and fi lled three fl uvial valleys until the fl ows merged together to form the headlands at Lava Point (Fig. 4). The 40Ar/39Ar geochronol-ogy experiments did not yield any radiogenic 40Ar* in this lava fl ow because it is likely late Holocene in age.

Moundhill Volcano (Late Holocene)Moundhill volcano is a postglacial monoge-

netic cone located at the eastern end of the island

(Fig. 4). Its southeastern fl ank is built upon 15–20 m of andesitic lavas of unknown age, but the remainder of the cone rises from sea level up to 590 m. The entire cone is composed of numerous 1–3-m-thick, sheet-like, chemically monotonous basalt fl ows (mvl) distinguished by unusually large (up to 0.7 cm) and abundant plagioclase (40 modal %), clinopyroxene (9%), and olivine (6%) phenocrysts in a glassy matrix (Table 2). Each fl ow exhibits pahoehoe struc-ture and extends from near the summit crater all the way to the coast. Levees on several fl ows descending from the west side of the crater sug-gest that the initial fl ow direction was westward, but then shifted to the north or the south around the fl anks of the stratovolcano (Fig. 4). It is pos-sible that the entire cone formed during a single, long-lasting eruption. Several attempts to obtain 40Ar/39Ar ages from Moundhill volcano lavas were unsuccessful due to very low radiogenic 40Ar* yields.

Pyre Peak Lavas and Tephras (Holocene to Present)

The initial phase of this late Holocene activ-ity was explosive. A vent-clearing eruption resulted in the formation of an explosion caldera 3 × 4 km wide and produced a 3.2 km3 ande sitic (58.62 wt% SiO

2) ignimbrite that blankets the

Pleistocene lavas over more than 30% of the island (Fig. 4). The majority of this pyroclastic deposit has been covered by subsequent erup-tions and effusions of basalt and basaltic andes-ite from Pyre Peak, a 0.25 km3 basaltic scoria cone, which is the highest point on the island (1054 m). Pyre Peak lavas, which partially infi lled the caldera, fl owed primarily to the south and west and cascaded over the eroded cliffs of Pleistocene lavas along the southern shoreline (Figs. 4 and 6). Because the Pyre Peak lavas are presumably only hundreds to a few thousands of years old, they cannot be dated by 40Ar/39Ar methods. However, the relative stratigraphic order of several lavas has been determined based on fi eld observations. From oldest to youngest, they are the mossy basalt (msb), basalt west of base camp (bwb), western fan basalt (wfb), and brown basalt (brb). The msb, bwb, and brb units are individual lava fl ows with a volume of <1 km3. The western fan basalt (wfb), the most voluminous of these units, consists of 2.6 km3 of basaltic andesitic lavas fl ows (52.9–53.1 wt% SiO

2) that likely erupted from a vent near Pyre

Peak over a relatively short period of time. The most recent activity includes the eruption of basalt and basaltic andesite in 1977 and 1992–1993 (Miller et al., 1998; Masterlark and Lu, 2004; Price, 2004; Jicha et al., 2004), respec-tively, from a vent located 2 km to the southwest of Pyre Peak (Figs. 4 and 6).

0

2

4

6

8

10

6010 200 30 40 50Rb (ppm)

Zr/

Rb



Post collapse lavas (Holocene-historic)


Fractional crystallization

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2

3

4

5

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Figure 8. (A) Plot of Zr/Rb versus Rb (ppm). Basalts, mainly from the earliest eruptive phase, with <12 ppm Rb display a signifi cant variability in Zr/Rb ratios, which likely refl ects small variations in the degrees of partial melting or extraction and an origin from weakly het-erogeneous mantle-wedge sources with different Zr/Rb ratios. In contrast, the Zr/Rb ratios of lavas with >12 ppm Rb are remarkably constant, suggesting a fractional crystallization–controlled origin. Signifi cant crustal contamination is unlikely to have occurred. (B) Plot of chondrite-normalized La/Yb versus La. Seguam basalts and basaltic andesites with <6 ppm La defi ne an inclined array, which, similar to the Zr/Rb ratios, likely refl ects various degrees of partial melting, or melting of a heterogeneous source. The nearly constant linear trend of (La/Yb)n at ~3 displayed by all lavas with >6 ppm La implies that crystal-liquid fractionation has been a common process creating rhyolite at Seguam for at least 320 k.y.



DISCUSSION

K-Ar versus 40Ar/39Ar Ages

The ages of ten of the samples determined by 40Ar/39Ar analyses were originally determined using the conventional K-Ar method on large (11.6–25.7 g) whole-rock samples by Singer et al. (1992a). Nine of these samples yielded repro-ducible 40Ar/39Ar plateau ages ranging from 210 to 49 ka, but one basaltic andesite lava fl ow (sam-ple J87-43) failed to yield measurable radiogenic 40Ar. Incremental-heating experiments on hand-picked groundmass separates from the basaltic lavas, which gave K-Ar ages of 1.07 ± 0.32 and 1.05 ± 0.18 Ma, yielded mean 40Ar/39Ar plateau ages of 53 ± 14 and 191 ± 39 ka, respectively (Table 3). Two dacitic to rhyodacitic lavas gave 40Ar/39Ar ages that were slightly younger than the K-Ar ages, whereas 40Ar/39Ar ages from four lavas were indistinguishable at 2σ from the K-Ar ages (Table 3). One possible explanation for the younger 40Ar/39Ar ages of the two evolved lavas is that these glassy samples were affected by 39Ar recoil.

The % 40Ar* determined from 40Ar/39Ar incre-mental-heating experiments on six of the nine lavas was equal to or greater than those from the K-Ar experiments (Table 3). Interestingly, the % 40Ar* yields of three basaltic to basaltic andes-itic lavas dated by 40Ar/39Ar methods were lower than those reported by Singer et al. (1992a). We suspect that the older K-Ar ages refl ect either inaccurate measurement of 40Ar* or K content of the extremely large, coarse (500–1000 μm) whole-rock samples melted for the K-Ar analy-ses. Moreover, K-Ar ages were determined in duplicate on samples J87-43, B88-23, and B88-16 by Singer et al. (1992a), but the resulting % 40Ar* yields and ages were not equivalent in each experiment. This poor level of reproduc-ibility most likely refl ected the fact that these samples, which were 50–100 times larger than the samples used for 40Ar/39Ar analyses, were internally heterogeneous with respect to K

2O

concentration and 40Ar content. The internal reproducibility and stratigraphic consistency of the 60 new 40Ar/39Ar incremental-heating ages presented here imply that the subaerial eruptive history of Seguam Island is one-third as old as previously inferred on the basis of the eleven K-Ar ages of Singer et al. (1992a).

Petrologic Evolution

Singer et al. (1992a, 1992c) suggested that the monotonic variation in major- and trace-element abundances and narrow range of Sr, Nd, Pb, and O isotope compositions of the diverse suite of magmas (49.7–71.4 wt% SiO

2)

400 300 200 100 0

Age (ka)

80

60

40

20

0

Cumulative volumeErupted (km3)

Tota

l

Andesite

Rhyolite

Dacite

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Basaltic Andesie

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0.9

1.0

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0 20 40 60 80 100 120 140t = ka

1.0 1.5 2.0 2.5 3.0 3.5

eλt

19771993

Crater formation

0.6

Undisturbed magma storage

Figure 9. Evolution of (230Th/232Th)0 ratios measured in Seguam Island lavas as a function of eruptive age (expressed as eλt) (after Jicha et al., 2005). These ratios were obtained from the intercepts of mineral-glass whole-rock isochrons with the equiline. The lavas that erupted between 142 and 9 ka display a monotonic increase in (230Th/232Th)0 ratios, which has been interpreted to refl ect a derivation from a thermally buffered, basaltic melt lens that had been stored in the lower crust for 130 k.y. The abrupt change in magma composition at 9 ka most likely refl ects the introduction of new basaltic magma, which had a lower (230Th/232Th)0 ratio, into the plumbing system.

Figure 10. Cumulative eruptive volume versus time for the Seguam Island volcanic com-plex showing the total volume of each magma type erupted over the past 318 k.y. The total volume of erupted material is denoted by the black line. The oldest volcanism is primarily basaltic; however, >30% of the total eruptive volume has been dacitic to rhyolitic. If a small portion of rhyolitic fl ows at 318 and 200 ka are ignored, eruptions have become more com-positionally diverse over the past 200 k.y.

Jicha and Singer


erupted at Seguam are consistent with an ori-gin via closed-system fractional crystallization of basalt. Model calculations suggest that ~80 wt% crystallization of an anhydrous mineral assemblage from basalt can produce a 70 wt% SiO

2 rhyolite (Singer et al., 1992a). However,

the observations of Singer et al. (1992a, 1992b, 1992c) were based primarily on the geochemical and isotopic compositions of the older, eroded lavas and the 138–33 ka lavas along the south-ern shoreline. In light of the 60 new 40Ar/39Ar ages, and detailed fi eld mapping and sampling, plus new inductively coupled plasma–mass spectrometry (ICP-MS) major- and trace-ele-ment analyses (Table 2; electronic Table DR2 [see footnote 1]), we are now able to investigate geochemical and isotopic variations throughout the entire suite of erupted lavas.

Rare earth element (REE) patterns of basal-tic to rhyolitic lavas are distinctly different dur-ing each of the main stages of volcanism over the past 320 k.y. (Fig. 7). From 318 to 142 ka, basalts have nearly fl at REE patterns and strong positive Eu anomalies, whereas dacites and

rhyolites have slightly light (L) REE-enriched patterns and large, negative Eu anomalies, refl ecting plagioclase fractionation. In con-trast, lavas erupted from 138 to 11 ka, includ-ing those from Wilcox volcano and those from vents in the center of the island, display slight LREE-enriched patterns and only weak Eu anomalies. Postcollapse lavas have the great-est LREE enrichment of all Seguam lavas and show strong positive and negative Eu anomalies similar to those of the older eroded volcanics. The parallel nature of the REE patterns within each group of lavas provides compelling evi-dence for crystallization-dominated evolution (Singer et al., 1992a). The subtle changes in the slope of the REE patterns for each of the three stages of volcanism suggest that a new basaltic parent magma became available as a parent to the crystal-liquid fractionation processes at the beginning of each of these periods.

Incompatible trace-element ratios (i.e., Zr/Rb and La/Yb) of Seguam basaltic andes-ites to rhyolites are remarkably constant over large ranges in SiO

2, La, and Rb contents,

further supporting a fractional crystalliza-tion–controlled origin for 320 k.y. (Fig. 8). U-Th isotope data from minerals, glass, and whole rocks, interpreted in light of 40Ar/39Ar ages, and limited Sr isotope variability sug-gest rapid decompression-driven crystalliza-tion and differentiation of ascending magmas following isolated storage of the parent basalt in the lower crust (Fig. 9; Jicha et al., 2005). Conversely, Seguam basalts, particularly those of the earliest stage of volcanism from 318 to 142 ka, display signifi cant variability in Zr/Rb and La/Yb ratios, which likely refl ects either a slightly greater range in degrees of partial melting, or derivation from a heterogeneous mantle-wedge source at this time.

Compositional Trends through Time

Basalts and basaltic andesites (49–56 wt% SiO

2) make up >50% of the cumulative volume

estimated and account for >90% of the erupted products over the initial 220 k.y. of subaerial activity. From 98 to 66 ka, basaltic eruptions formed the base of Wilcox volcano and emanated from vents in the center of the island. Holocene basaltic activity has been restricted to eruptions from Pyre Peak and Moundhill volcano.

Andesites (56–62 wt% SiO2) are far less

voluminous than the basalts and basaltic andes-ites and represent only ~16% of the total vol-ume erupted (Fig. 10). Nearly all of these low- to medium-K lavas erupted between 133 and 22 ka. Examples include the 10–20-m-thick fl ow lobes exposed along the northern and southern shorelines (nsl and ssl), and those intercalated with stacks of basaltic and dacitic lavas on the fl anks of the eastern stratovolcano. Only three andesitic eruptions have occurred over the past 22 k.y., with the most notable of those being the explosive 3.2 km3 vent-clearing eruption (58.6 wt% SiO

2) associated with caldera formation on

the western half of the island.Dacites (62–69 wt% SiO

2) are the sec-

ond most abundant lava type found on the island (23% of the total eruptive volume). The 40Ar/39Ar geochronology suggests that dacitic activity extended as far back as 159 ka, when a dacitic dome intruded older, basaltic lavas and pyroclastic rocks. Interestingly, most dacites, like the andesites, erupted between 133 and 20 ka; a period in which all eruptions were tap-ping a basaltic melt lens located in the lower crust (Figs. 10 and 11; Jicha et al., 2005). How-ever, several km3 of dacite were generated over the past 20 k.y., including eruptions from the stratovolcano that fl owed to the southwest of Moundhill Point, the crater-forming eruption at 9 ka, and the postcollapse fl ank eruption of the Long Canyon dacite (lcd).

45

50

55

60

65

70

75

400 350 300 250 150 100 50 0

Age (ka)

SiO

2 (w

t%)





Cra

ter

form

atio

n

Older, eroded volcanics (318-142 ka)Island-wide activity

(138-11 ka)

200

Figure 11. SiO2 versus age and compositional evolution of Seguam lavas and tuffs. Error bars represent 2σ uncertainties. Although there is no overall apparent correlation between age and SiO2 silica content, several important patterns emerge. Older lavas between 318 and 142 ka include virtually no andesite, whereas andesite is voluminous in the shoreline and Wilcox stratovolcano lavas of 138–9 ka. With exception of two andesitic lavas dated at 12 and 11 ka, lavas comprising the shoreline phase and Wilcox stratovolcano lavas tend to become more SiO2-rich with time. Compositional diversity is most extreme, 51%–71% SiO2, in the postcollapse phase. Gray bars represent the boundaries between the three main erup-tive phrases discussed in the text. Formation of the 4-km-wide crater is noted at ca. 9 ka. Symbols are the same as Figure 3.



Rhyolites (>69 wt% SiO2) amount to 6 km3,

<8% of the total volume of products erupted over the past 318 k.y. From 133 to 33 ka, several small fl ows of rhyolite crop out as part of the thick sequence of widely scattered lavas along the southern shoreline, and the 0.4 km3 Finch Cove rhyolite erupted from the Wilcox strato-volcano at 77 ka. Rhyolitic output increased at ca. 30 ka as eruptions occurred near Turf Point and rhyolites descended from the stratocone forming Moundhill Point (Fig. 4). Over the past 9 k.y., a 0.6 km3 rhyolitic cone was constructed, and several rhyolitic lavas erupted from inside the crater and along the southern fl anks of the volcano. Most of the rhyolitic activity has occurred over the past 30 k.y., and ignoring a pair a small volume fl ows at 318 and 200 ka, eruptions have become more compositionally diverse over the past 200 k.y. (Fig. 11). Lavas erupted following creation of the crater span a larger compositional range, 51%–71% SiO

2,

than any previous phase of activity (Fig. 11).

Time-Volume Relationships

Eruptive rates over periods of thousands of years can be estimated using a detailed chronol-ogy combined with volumetric estimates based on the thickness and lateral extent of the erupted units (Table 4). The preserved subaerial volume at Seguam represents only a fraction of the total extruded volume because a signifi cant percent-age of the erupted products has been deposited in the sea or been removed via marine erosion, which peaked during interglacial highstands at ca. 10, 120, 200, and 330 ka (Shackleton et al., 1990). Moreover, glacial ice and explo-sive events have carried additional material away from the island. Therefore, the calculated magma output rates for Seguam discussed next represent minimum estimates.

The time-averaged eruptive rate for the 79 km3 complex is 0.25 km3/k.y., although the eruptive fl ux has been highly variable throughout the lifetime of the volcano. Basalts and basaltic andesites and a few dacitic domes erupted from 318 to 142 ka comprise 15% of the total volume of the complex, which corresponds to an erup-tive rate of 0.07 km3/k.y. This sequence of lavas, dikes, and domes is deeply eroded; therefore, the actual eruptive rate during this time period may have been signifi cantly higher. From 100 to 25 ka, the eruptive rate increased to ~0.6 km3/k.y., corresponding to stratovolcano growth coupled with the continued eruptions of basaltic to rhyolitic lavas from vents in the center of the island. For 15 k.y. prior to collapse of the east-ern stratovolcano, activity waned slightly, which may represent a period of quiescence. However, this likely refl ects a lack of preservation due to

the removal of volcanic material during The Last Glacial Maximum in the Aleutians from ca. 25 to 12 ka (Fig. 10; Black, 1983; Shackle-ton et al., 1990).

Postcollapse (younger than 9 ka) activity is fairly well preserved, allowing for more accurate volume estimates, but several of the lavas are covered by recent eruptions of basaltic ash and scoria. The total volume for all of the evolved (e.g., dacitic and rhyolitic) lavas and domes plus all of the basalts and basaltic andesites erupted from Pyre Peak and Moundhill volcano is 10.6 km3, or 13% of the total volume erupted.

The time-averaged eruptive rate for this period is 1.2 km3/k.y., which represents the highest magma output rate during the 318 k.y. subaerial history of the volcano. Given that most of the basaltic activity from Pyre Peak is younger than the silicic lavas and domes that erupted from 9 to 1.7 ka, the eruptive rate for Pyre Peak lavas could be as high as 4.7 km3/k.y.

Comparison to Other Arc Volcanoes

The long-term eruptive rates at Seguam can be compared to those of other well-dated arc

TABLE 4. VOLUME ESTIMATES FOR SEGUAM LAVAS AND TEPHRAS

Stage/eruptive unit Map label SiO2 range of products

Age range of activity

Present volume(km3)

Total volume(%)

Moundhill volcano

Moundhill volcano lavas mvl 51.6–51.8 Holocene 1.4 2

Pyre Peak lavas and tephras

Basaltic ash and scoria bas 51.7–53.6 Holocene-historic 0.7Bowling ball basalt bbb 51.5–51.7 1977 eruption 0.06Dacitic pumice air fall dpa 64.0–64.2 Holocene-historic 0.06Andesitic ignimbrite aig 58.6–59.8 Holocene-historic 3.2Right-angle basalt rab 51.9–52.1 Holocene-historic 0.01Brown basalt brb 52.4–53.6 Holocene-historic 0.01Western fan basalt wfb 52.9–53.1 Holocene-historic 2.6Basalt west of base camp bwb 51.7–51.9 Holocene-historic 0.6Mossy basalt msb 51.6–52.7 Holocene-historic 0.7Andesite in amphitheater aam 59.0–59.3 Holocene 0.01

8.0 10Postcollapse dacites and rhyolites

Long Canyon dacite lcd 62.9–66.4 <7.5 <0.01Lava Point rhyodacite lpr 68.5–69.5 <7.5 0.41710 rhyolite dome str 69.8–69.9 1.7 ± 0.5 0.06Spiny rhyolite srf 71.4–71.6 6 ± 4 0.01Rhyolitic cone in caldera rcc 69.8–69.9 <7.5 0.6Rhyolite fl ow in caldera valley rcv 69.9–70.8 7.5 ± 2.0 0.08

1.2 <1Sector collapse/crater formation

Dacitic ignimbrite dig 64.5–67.9 8.4 ± 1.5 0.45 <1Wilcox volcanoSouth shore basaltic andesite ssb 55.2–55.5 12.1 ± 5.1 0.2Finch Cove ash-fl ow tuff faf 62.0–66.6 22.8 ± 5.1 0.02Lava Point dacite lpd 62.7–62.8 23.5 ± 5.8 0.2Finch Cove rhyodacite rdf 67.9–68.3 31.7 ± 2.0 0.6Finch Point andesite afp 57.6–58.2 57.7 ± 5.3 0.9Andesite west of Finch Point awf 55.6–57.7 61.4 ± 5.9 0.6Finch Cove rhyolite rfc 70.2–70.4 76.8 ± 1.1 0.4Wilcox volcano lavas wvl 52.1–69.9 98.1–27.5 17.4

20.3 26Shoreline lavas

Amphitheater lavas aml 54.1–65.9 <93.1 0.2South shore lavas ssl 52.1–70.1 138.4–33.2 19.6North shore lavas nsl 52.9–65.2 133.3–11.1 16.0

35.8 46Older eroded volcanics

Turf Point rhyodacite dome tpr 68.6–68.7 141.9 ± 2.2 0.2North shore rhyodacite dome nsr 66.7–67.3 159.2 ± 2.2 0.1Deeply eroded lavas and dikes eld 51.7–55.1 318.3–174.3 11.8

12.1 15Total volume 79.2

Jicha and Singer


volcanoes (i.e., Mount Adams, Tatara–San Pedro, Santorini, Montserrat, Mount Baker, Kat-mai, and Ceboruco–San Pedro volcanic fi eld) (Fig. 12). Because several of the volcanic com-plexes in these detailed studies cover large areas (400–1600 km2) encompassing a stratocone(s) and peripheral lavas, we can make comparisons to the eruptive rates of the individual stratocones as well as to the entire volcanic fi eld.

The time-averaged eruptive rate at Seguam (0.25 km3/k.y.) is similar to 90–20 ka Volcán Tatara in the Tatara–San Pedro complex in the Chilean Southern volcanic zone (Singer et al., 1997) and the 940 ka Mount Adams volca-nic fi eld (Hildreth and Lanphere, 1994) in the Cascades, but slower than those estimated for Santorini in the South Aegean arc (Druitt et al., 1999), Mount Baker and Mount Adams strato-cones in the Cascades (Hildreth et al., 2003a; Hildreth and Lanphere, 1994), and Mount Kat-mai and Mount Mageik on the Alaska Peninsula (Hildreth et al., 2003b) (Table 5). The reasons for these differences are not well understood. Several factors may contribute to the contrast-ing eruptive rates, including the variable crustal thicknesses beneath each of the volcanoes or differences in the magma production rate for each arc. The average eruptive rate at Seguam over the past 9 k.y. was 1.2 km3/k.y., with pos-sible rates as high as 4.7 km3/k.y. for the period of basaltic volcanism from Pyre Peak. Although not representative of the entire 318 k.y. sub-aerial record, these more rapid rates of magma output in the Holocene at Seguam are compa-rable to (1) the average Aleutian eruptive rates estimated by Crisp (1984) (2.1 km3/k.y.) and Marsh (1982) (1.6 km3/k.y.), (2) the main pulses of stratocone growth at Mount Adams (2–5 km3/k.y.), and (3) the construction of 79 km3 at Mount St. Helens, which has only been active for ~40 k.y. (~2 km3/k.y.; Sherrod and Smith, 1990). Even though the magma output rate from Pyre Peak is comparable to that of several other arc volcanoes, it is almost an order of magnitude less than the eruptive fl ux at mid-ocean-ridge spreading centers (24 km3/k.y. per 400 km of ridge segment) (Crisp, 1984), and the average eruptive rate of the 8–10 ka Klyuchevskoy vol-cano (32–22 km3/k.y.), which is often cited as the most active island arc volcano in the world (Fedotov et al., 1987).

A striking feature of Figure 12 is the infl ection of the growth rate curves for several volcanoes corresponding with the penultimate (oxygen iso-tope stage 6) and last (oxygen isotope stage 2) major global glaciation maxima. For example, Seguam, Katmai, and Mount Adams all show comparable large increases in growth rate fol-lowing the penultimate glaciation ca. 125 ka, whereas these three volcanoes, plus Mount

Mt. Adams

Mt. Baker

Katmai

Seguam

Tatara-San Pedro

400 300 200 100 0

Age (ka)

100

80

60

40

20

0

Cu

mu

lati

ve v

olu

me

(km

3 )

Ceboruco

Las

t g

laci

al m

axim

um

Sta

ge

6 gl

acia

tion

Sta

ge

8 gl

acia

tion

Sta

ge

10 g

laci

atio

n

Figure 12. Minimum cumulative volume versus time for Seguam Island and other well-dated Pleistocene to Holocene arc volcanoes. Data are restricted to the volume erupted at each volcano over the last 400 k.y. Data from Katmai volcanic cluster encompass seven fron-tal arc volcanoes and fi ve rear-arc monogenetic cones along 100 km of the Alaska Peninsula (Hildreth et al., 2003a). Other volcanoes include Mount Adams (Hildreth and Lanphere, 1994), Tatara–San Pedro (Singer et al., 1997), Mount Baker (Hildreth et al., 2003b), and Ceboruco–San Pedro (Frey et al., 2004). Gray bars indicate the timing and duration of major glaciations that have occurred over the past 400 k.y. (Shackleton et al., 1990).

TABLE 5. SUMMARY OF TIME-AVERAGED ERUPTIVE RATES AT LONG-LIVED, WELL-DATED ARC VOLCANOES

Volcano Arc Crustal thickness

(km)

Volume(km3)

Duration(k.y.)

Average eruptive rate

(km3/k.y.)

Reference

Seguam Aleutians 25–30 79 318 0.25 This studyMt. Katmai Aleutians 30–36 70 89 0.79 Hildreth et al. (2003b)Mt. Mageik Aleutians 30–36 30 93 0.32 Hildreth et al. (2003b)Entire Katmai cluster Aleutians 30–36 179 292 0.61 Hildreth et al. (2003b)Tatara–San Pedro SVZ, Andes 30–35 55 930 0.06 Singer et al. (1997)Volcán Tatara SVZ, Andes 30–35 22 91 0.24 Singer et al. (1997)Mt. Adams volcanic fi eld Cascades 40–45 231 940 0.25 Hildreth and Lanphere

(1994)Mt. Adams stratocone Cascades 40–45 200 520 0.38 Hildreth and Lanphere

(1994)Mt. Baker volcanic fi eld Cascades 40–45 105 1300 0.08 Hildreth et al. (2003a)Mt. Baker stratocone Cascades 40–45 15 43 0.35 Hildreth et al. (2003a)Santorini South Aegean 20–32 300 650 0.46 Druitt et al. (1999)Montserrat Lesser Antilles 30–40 26 174 0.15 Harford et al. (2002)Ceboruco–San Pedro Trans-Mexican 35–40 81 819 0.10 Frey et al. (2004)Average arc output: Aleutians 50–70 7350 3500 2.10 Crisp (1984)

Aleutians 50–70 4700 3000 1.57 Marsh (1982)

Lesser Antilles 30–40 285 100 2.85 Crisp (1984)

Note: Eruptive volumes are minimum estimates. Average eruptive rates were calculated by dividing the erupted volume by the duration of volcanism. SVZ—Southern volcanic zone.



Baker, Ceboruco, and Tatara–San Pedro have pronounced infl ections either corresponding to, or immediately following, the Last Glacial Maximum ca. 25 ka. These data suggest that the repeated infl uence of highly erosive glaciers on large, long-lived arc stratovolcanoes (Singer et al., 1997) may be widespread. If true, one must be extremely cautious in using the preserved volumes of the portions of these volcanoes older than ca. 125 ka to constrain long-term eruptive rates in subduction zones.

CONCLUSIONS

The new chronostratigraphy of Seguam Island demonstrates that the 40Ar/39Ar furnace incre-mental-heating technique can provide precise ages for latest Pleistocene to Holocene, low-K, tholeiitic volcanic rocks. Replicate analyses of subsamples of each lava or tuff yielded 2σ ana-lytical precisions of as low as ±2%–3%. Sixty new 40Ar/39Ar ages, including four age determi-nations of 12 ka or younger, indicate that sub-aerial volcanic activity began as early as 318 ka, not 1.07 Ma, as suggested by the K-Ar ages of Singer et al. (1992a). Detailed mapping, strati-graphic relationships, and geochemical correla-tions along with the new age determinations pro-vide insights into the spatial, compositional, and volumetric evolution of Seguam Island. Volca-nism was focused beneath multiple vents aligned in an east-west orientation, but shifted across the island in a nonsystematic way over time. Erup-tions of andesite through rhyolite were coeval with basaltic activity, often emanating from vents on opposites ends of the island. Petrologic and isotopic evidence suggests that the compo-sitional spectrum of lavas erupted at Seguam is the result of rapid, closed-system crystal-liquid fractionation of magma following isolated stor-age of basaltic melt in the lower to middle crust for tens of thousands of years, and this has prob-ably persisted for more than 320 k.y. Variations in the proportion of fractionating minerals and the extent of fractionation were subtle over the past 320 k.y., but REE patterns and U-Th iso-topes indicate that new basaltic parent magma replenished the crustal plumbing system at least four times during the past 318 k.y.

Like other arc volcanoes, the eruptive fl ux at Seguam is highly variable through time, with periods of elevated activity occurring during the construction of a stratovolcano from 100 to 30 ka. Glacial ice and coastal erosion removed a signifi cant percentage of the erupted prod-ucts, allowing for minimum estimates of the cumulative eruptive volume. Despite this short-coming, the calculated time-averaged eruptive rate at Seguam is similar to that of the 940 ka Mount Adams volcanic fi eld, and greater than

that of other well-dated arc volcanoes in the Lesser Antilles arc, Chilean Southern volcanic zone, and Trans-Mexican volcanic belt. More-over, the average growth rate of Seguam Island is 40% less than that of Mount Katmai and Mount. Mageik, which are located on the Alaska Peninsula, suggesting that the eruptive fl ux may be higher in the continental sector of the Aleu-tian arc. This along-arc variation was originally observed by Marsh (1982), who noted that erup-tive output decreased systematically from east to west along the Aleutian arc owing to increas-ingly oblique subduction of the Pacifi c plate toward the west (Fig. 1). Further work is needed to quantify magma output rates at several other Aleutian arc volcanoes in order to quantify the eruptive fl ux in the oceanic and continental sec-tors of the arc.

ACKNOWLEDGMENTS

Our most sincere thanks go to Captain Kevin Bell and the seasoned crew of the U.S. Fish and Wildlife Service M/V Tiglax for support during two of our fi eld campaigns, Lee Powell and Xifan Zhang for their expertise and quality-control assistance in the Rare Gas Laboratory, and staff at the Oregon State Univer-sity reactor for performing the irradiations. We also would like to thank Charlie Bacon and an anonymous reviewer for their careful reviews and many thought-ful comments that helped us improve the paper. Our work was supported by U.S. National Science Founda-tion (NSF) grants EAR-0114055 and EAR-0337667, a Geological Society of America Bruce L. “Biff” Reed grant to Jicha, and a University of Wisconsin–Madi-son Graduate School Research Award to Singer. With his permission, we have named the prominent volcano dominating the eastern half of Seguam after Ray E. Wilcox, a University of Wisconsin alumnus and U.S. Geological Survey scientist, known best for his pio-neering studies of Paricutín volcano in Mexico, and the Near Islands in the Aleutians.

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Volcanic history and magmatic evolution of Seguam Island ...raman/papers2/JichaSingerGSA06.pdf · Seguam Island is an ~200 km2 volcanic com-plex in the central Aleutian Island arc