Volcanism in the earliest stage of back-arc rifting in the Izu-Bonin arc revealed by laser-heating 40Ar/39Ar dating

Volcanism in the earliest stage of back-arc rifting inthe Izu-Bonin arc revealed by laser-heating 40Ar/39Ar dating

Osamu Ishizuka a;�, Kozo Uto a, Makoto Yuasa b, Alfred G. Hochstaedter c;1

a Institute of Geoscience, Geological Survey of Japan/AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japanb Geoinformation Division, Geological Survey of Japan/AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

c Earth Science Board, University of California, Santa Cruz, CA 95064, USA

Received 10 December 2001; accepted 13 May 2002

Abstract

The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due torepeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high densitysampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation ofvolcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated ataround 2.8 Ma in the middle part of the Izu-Bonin arc (30‡30PN^32‡30PN). Volcanism at the earliest stage of rifting ischaracterized by the basaltic volcanism from north^south-trending fissures and/or lines of vents. (2) Following thisearliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in thewide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone.(3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and riftingwere confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Boninarc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearlyindicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.< 2002 Elsevier Science B.V. All rights reserved.

Keywords: Izu-Bonin arc; back-arc rifting; Ar-40/Ar-39 dating; back-arc volcanism

1. Introduction

The oceanic island arc accommodates di¡erenttypes of volcanism depending on the exact loca-tion within the subduction system, for example,volcanism at the volcanic front, behind the front,

and volcanism associated with back-arc riftingand spreading. Among these, back-arc riftinghas primary importance because it often accom-panies dynamic input of new mantle material andformation of new oceanic crust. The mechanismwhich initiates back-arc rifting is still rather un-clear. Hence, the identi¢cation of volcanism at theearliest stage of rifting and understanding thetemporal variation of volcanism can provide val-uable information to elucidate mantle and crustalprocesses at the initiation of rifting. Generally, it

0377-0273 / 02 / $ ^ see front matter < 2002 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 3 6 5 - 7

1 Present address: Earth Sciences Department, MontereyPeninsula College, Monterey, CA 93940, USA.* Corresponding author. Fax: +81-298-56-8725.E-mail address: [email protected] (O. Ishizuka).

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is di⁄cult to determine which volcanism repre-sents the initial stage of rifting due to the paucityof reliable age data on volcanic rocks and burialby subsequent volcanism and tectonic movement.In terms of the Izu-Bonin arc, Hochstaedter et al.(1990a,b) suggested that MORB-like basalterupted from the earliest stage of rifting at 1.1^1.4 Ma, because even the oldest lava from theSumisu back-arc rift shows chemical characteris-tics recognized as back-arc basin basalt (e.g. Sin-ton and Fryer, 1987). But the data were con¢nedto the currently active rift zone. Morita (1994)and Ishizuka et al. (1998) showed that the areawest of the active rift zone was also a¡ected byrifting. Based on the preliminary KÂr dating re-sults, Ishizuka et al. (1998) suggested that volca-nism related to rifting in this region preceded thevolcanism in the currently active rift zone. Fur-thermore, the chemical characteristics of the vol-canism changed before and after the initiation ofrifting and were also di¡erent between the earlyand late stages of rifting. This suggests that itshould be possible to understand the temporalvariation and characteristics of volcanism in theinitial stage of rifting by systematically obtainingmore reliable and precise age data. This paperpresents the laser-heating 40Ar/39Ar dating resultson samples systematically collected from the re-gion west of the currently active rift zone andde¢nes the temporal variation of characteristicsof volcanism at the initial stage of back-arc rift-ing.

2. Geological background

The Izu-Bonin arc is one of the oceanic islandarcs located in the northeastern margin of thePhilippine Sea plate. This arc has a broad vol-canic zone (up to 400 km wide) extending in anorth^south direction and bounded by the Izu-Bonin Trench on the east and the Shikoku Basinon the west (Fig. 1A). Around Sumisu Jima andAoga Shima, situated in the middle of this arc(30‡30PN^32‡30PN), the arc is characterized bythe existence of volcanic chains both in frontalarc region (i.e. volcanic front) and back-arc region(back-arc seamount chains). Between the volcanic

front and back-arc seamount chains is a north^south-trending rift zone (Fig. 1B). The rift zone isdivided into two parts, the active rift zone and theback-arc knolls zone (Fig. 1B). The active riftzone occupies the eastern part of the rift zone,and the back-arc knolls zone occupies the westernpart.The volcanic front includes stratovolcanoes

(e.g. Aoga Shima) and large submarine calderas

Fig. 1. (A) Schematic map of the back-arc region of the Izu-Bonin arc. The area where the studied samples came from isdesignated as a rectangular. The Izu-Bonin arc is dividedinto four bathymetric features, i.e. volcanic front area, activerift zone, back-arc knolls zone, and back-arc seamountchains area. (B) Bathymetric map of the middle part of theIzu-Bonin arc. Sample locations dated in this study are alsoshown. Four chains of back-arc seamounts extend obliquelyin ENE^WSW direction from the back-arc knolls zone tothe Shikoku Basin. Active rift zone, including the Aogashimaand the Sumisu Rift, and back-arc knolls zone (Honza andTamaki, 1985) run parallel to the Quaternary volcanic frontline. Two dotted lines crossing the map are the survey linesfor single-channel seismic re£ection pro¢ling shown in Fig. 4.

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(e.g. Myojin Knoll), and is characterized by pro-duction of both basaltic and rhyolitic tu¡ (Fujio-ka et al., 1992; Gill et al., 1992).On the other hand, the back-arc seamount

chains extend into the Shikoku Basin from thewestern part of the back-arc knolls zone in an

east-northeast^west-southwest direction, obliqueto the trend of the volcanic front and rifting-re-lated features. The seamounts in the back-arc sea-mount chains are much larger than the knolls inthe back-arc knolls zone, more than 2000 m highwith basal diameters of up to 20 km (Figs. 1B and

Fig. 1 (Continued).

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2). Volcanism in the back-arc seamount chains ismainly andesitic to basaltic, and acidic lavas arefound only in limited number of the seamounts.Andesite lavas are generally porphyritic andpoorly to moderately vesiculated and contain pla-gioclase, clinopyroxene and orthopyroxene asphenocrysts. Some andesite contains minoramount of hornblende instead of pyroxene as aphenocryst.The active rift zone is located within 10^15 km

behind the volcanic front and includes the Sumisuand the Aogashima Rift. Holocene basalt lavas(6 0.1 Ma) and hydrothermal activity occuralong the central axis of the Sumisu Rift, whereasolder lavas (V1.4 Ma) crop out along the riftwalls (Hochstaedter et al., 1990a; Urabe and Ku-sakabe, 1990). Lavas from the active rifts arecompositionally bimodal. Taylor (1992) suggestedthat rifting of the Sumisu Rift initiated at 2.35^2.9Ma based on an unconformity observed in thedrilling core from the eastern £ank of the SumisuRift.The back-arc knolls zone lies west of the active

rift zone. Originally de¢ned by Honza and Tama-ki (1985) on the basis of bathymetry and single-channel seismic pro¢les, the back-arc knolls zoneconsists of north^south-trending ridges and smallvolcanoes. In the back-arc knolls zone, north^south-trending ridges occur in both northernand southern parts. These ridges extend about15^25 km in a north^south direction and theirelevation from the surrounding sea£oor is about300 m. Fig. 3 shows a detailed bathymetric mapof the northern part of the back-arc knolls zoneproduced in this study. A north^south-trendingridge extends about 17 km (Fig. 3). This ridge isoverlain by small knolls. The knolls in the back-arc knolls zone are relatively small, 500^1000 mhigh in most cases (Figs. 2 and 3). Fig. 4 showsschematic cross sections of the back-arc knollszone based on the single-channel seismic re£ectionpro¢les (Honza and Tamaki, 1985; Nakao et al.,1985). In the northern part of the back-arc knollszone in the studied area (pro¢le along 31‡38PN),normal faults and small knolls are common, andsubsidence of the basement was limited and didnot form a basin structure related to faulting. Onthe other hand, in the southern part of the studied

area (pro¢le along 30‡45PN), large subsidence ofthe basement occurred and a fault-bounded basin¢lled with thick sediment (s 750 m) was formed.Using detailed bathymetry and side-scan sonardata, Morita (1994) has shown that the back-arcknolls zone is a region where seamount chain vol-canoes have been dissected by north^south-trend-ing lineations and ¢ssure ridges. These character-istics of the back-arc knolls zone strongly suggestthat extension a¡ected a 50 km width of the back-arc knolls zone. Volcanism in this area is bimodalin composition (Hochstaedter et al., 2000), andvesiculated basalt lavas with clinopyroxene andolivine phenocrysts and dacite to rhyolite lavaspredominate. Andesitic to dacitic lavas from theback-arc knolls zone comprise both hornblende-bearing and hornblende-de¢cient lavas. Horn-blende-bearing lavas contain plagioclase andhornblende as phenocrysts, and rarely biotiteand quartz.In terms of the age and chemical characteristics

of the volcanics, Ishizuka et al. (1998) reportedpreliminary KÂr ages for the volcanics fromthe back-arc seamount chains, and showed thatvolcanism in the back-arc seamount chains hadbeen active during the period between 12.5 and2.9 Ma, i.e. after the cessation of the spreading

Fig. 2. Comparison of sizes of the seamounts between theback-arc seamount chains and back-arc knolls zone. Sizes ofthe seamounts were estimated by the procedure described byYuasa et al. (1991).

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of the Shikoku Basin. On the other hand, volca-nism in the back-arc knolls zone had been activeafter 2.8 Ma to 1 Ma. Thus the volcanism in thisregion had been mainly active after the volcanismceased in the back-arc seamount chains.Ishizuka et al. (1998) suggested that volcanism

in the back-arc knolls zone was related to back-arc rifting and chemical characteristics of back-arc volcanism changed at the onset of rifting.Lavas from the back-arc knolls zone show tho-leiitic trend, while those from the back-arc sea-mounts have more calc-alkaline trend (Fig. 5).The basalts from the back-arc knolls zone showlower SiO2, lower Na2O and higher CaO contentcompared to the back-arc seamount chains. Ishi-zuka et al. (1998) also showed that volcanism inthe back-arc knolls zone is di¡erent in chemistry

from that in the active rift zone, implying thatchemical characteristics of volcanism changedwith a progress of rifting. Lavas from the back-arc knolls zone show more tholeiitic trend thanthose from the active rift zone (Fig. 5). The ba-salts from the back-arc knolls zone show signi¢-cantly higher K2O (Fig. 5) and lower SiO2 andNa2O compared to the active rift zone.In this study, we focused on dating of volca-

nism in the back-arc knolls zone. PreliminaryKÂr dating results gave general image of tempo-ral variation of volcanism in the back-arc region.However, it was not su⁄cient to elucidate theevolution of volcanism in the early stage of riftingespecially in terms of its space and mode andidentify the earliest volcanism associated with rift-ing. This is partly because some basaltic lavas

Fig. 3. Bathymetric map of the back-arc knolls zone produced by Multi Narrow Beam Echo Sounder System (HS-10) duringYK97-10 cruise. Mapped area is shown in Fig. 1B. Sampling location and obtained age data (shown in Ma) for volcanic rocksare designated on this map.

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from the back-arc knolls zone are vesiculated andcontain glassy portion, which is susceptible to Kremobilization and Ar loss. In this study we ap-plied stepwise-heating method of 40Ar/39Ar datingto obtain more reliable and precise ages by exam-ining age spectrum.

3. Samples studied

Samples studied here were lavas collected fromnorth^south-trending ridges and small knolls inthe back-arc knolls zone by dredging duringR/V Moana Wave (MW9507) and R/V Hakurei-maru cruises (Ishizuka et al., 1998; Hochstaedteret al., 2000). Descriptions of the dated samples arelisted in Table 1. Chemical compositions of thesamples are shown in Hochstaedter et al. (2000,2001) and Ishizuka et al. (1998). Sample selectionfor 40Ar/39Ar dating was based mainly on exami-nation of thin sections. Samples that are lessglassy (usually less than 5% of total groundmass)and have almost no visible groundmass alterationwere regarded as suitable for dating except in thecase of some dacites and rhyolites (very fresh, butwith glassy groundmass). Some of the basaltsshow swallow-tail texture of plagioclase and den-dritic growth of pyroxene in groundmass, suggest-ing rapid cooling of these lavas (e.g. MWD24-1).

This type of lavas often contain small amount ofglass in vesicules and groundmass. Glass-rich ves-icules were carefully removed from these samplesand longer acid treatment (see below) was ap-plied. Loss on ignition (LOI) was also takeninto consideration for sample selection, and sam-ples showing less than 2% LOI were determinedto be suitable.

4. Methods

Ages of volcanic rocks were determined by la-ser-heating 40Ar/39Ar dating system at the Geo-logical Survey of Japan (Uto et al., 1997). Slabsof thickness 1 mm were taken out from the fresh-est part of the samples with a water-cooled saw.This is partly because £at surfaces are required toprecisely monitor the thermal energy distributionon the samples during laser heating. The slabswere gently crushed into small pieces of about2^7 mg weight and then ultrasonically cleaned indistilled water. The sample pieces were carefullyexamined under microscope to make sure thatthey do not contain large phenocrysts and alter-ation products. The pieces were further treatedultrasonically in 3N HCl for 10^15 min to removealteration products. The samples were wrapped inaluminum foil packets each about 2 mm U2 mm

Fig. 4. Schematic cross section of the back-arc region in the middle part of the Izu-Bonin arc in WÊ direction. These cross sec-tions were produced based on the single-channel seismic re£ection pro¢les obtained by R/V Hakurei-maru cruise (upper section:GH84-2 cruise; lower section: GH79-2 cruise). Survey lines for these pro¢lings are shown in Fig. 1B.

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in size. The packets were piled up in a pure alu-minum (99.5% Al) irradiation capsule with £uxmonitor minerals. Sanidine separated from theFish Canyon Tu¡ (FC3), whose age is 27.5 Ma(Lanphere and Baadsgaard, 2001), was used forthe £ux monitor. Correction for interfering iso-topes was achieved by analyses of CaFeSi2O6and KFeSiO4 glasses irradiated together withsamples. Sample irradiation was done at two re-actors, the JMTR and the JRR3 reactors.

A continuous Ar ion laser was used for sampleheating. The groundmass samples were heated for3 min in each step keeping the laser power con-stant. Laser beam diameter was adjusted to 2 mmto ensure uniform heating of the sample. Ex-tracted gas was puri¢ed for 10 min with threeZrÂl getters (SAES AP-10) and one Zr^Fe^Vgetter (SAES GP-50). Two ZrÂl getters werekept at 400‡C and other getters were kept atroom temperature. Argon isotopes were measuredon a VG Isotech VG3600 noble gas mass spec-trometer. All the analyses were done using theDaly collector. The sensitivity of the collectorwas about 5U10310 ml STP/V. Mass discrimina-tion was monitored using diluted air. The blankof the system including the mass spectrometer andthe extraction line was 7.5U10314 ml STP for36Ar, 2.5U10313 ml STP for 37Ar, 2.5U10313 mlSTP for 38Ar, 1.0U10312 ml STP for 39Ar and2.5U10312 ml STP for 40Ar. The blank analysiswas done after every two- or three-step analysis.Plateau ages were calculated as weighted means

of ages of plateau-forming steps, where each agewas weighted by the inverse of its variance. Theage plateaus were determined following the de¢-nition by Fleck et al. (1977). Inverse isochronswere calculated using York’s least-squares ¢t(York, 1969). All errors for dating results are re-ported at one standard deviation. Error for agesincludes analytical uncertainties for Ar isotopeanalysis, correction for interfering isotopes and Jvalue estimation. An error of 0.5% was assignedto J values as a pooled estimate during the courseof this study.

5. Results

5.1. North^south-trending ridges

Four basalt samples from the north^south-trending ridges in the back-arc knolls zone weredated (Table 2, Fig. 6). From a ridge locatedsouth of the Genroku Seamount, two samples(MWD24-1 and MWD22-2: Fig. 1B) were ana-lyzed. Basalt MWD24-1 yielded a plateau age of2.63S 0.15 Ma comprising all the gas released(Fig. 6a). This plateau age is older than a KÂr

Fig. 5. FeO*/MgO^SiO2 and SiO2^K2O plots of the chemicalcomposition of lavas from the back-arc knolls zone. Compo-sitional range of lavas from the back-arc seamount chainsand active rift zone of the Izu-Bonin arc are also shown(data from Hochstaedter et al. (2000)). Line distinguishingthe tholeiitic (TH) and calc-alkaline (CA) ¢eld is from Miya-shiro (1974).

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Table 1Descriptions of the samples dated in this study

Sample No. Sample locations Description

Latitude (N) Longitude (E) Depth (m)

N^S-trending ridgesMWD24-1 30‡38.47P 139‡6.80P 1755^1780 Highly vesiculated (20^25%) and crystalline basalt with phenocrysts of cpx (6 0.6 mm, 10^15%) and

olivine (6 0.4 mm, 2^3%). Groundmass is composed of plagioclase, pyroxene and glass (6 2%).MWD22-2 30‡30.93P 139‡7.22P 1950^1865 Highly vesiculated (30%) basalt with phenocrysts of cpx (0.8 mm, 10%), plagioclase (6 3 mm, 7%) and

olivine (0.4 mm, 3^4%). Groundmass is composed of plagioclase, pyroxene and black glass.GHD616-1 31‡41.85P 139‡9.41P 1645 Vesiculated (15%) basalt with phenocrysts of plagioclase (6 8 mm, 5%), cpx (6 1 mm, 5%) and olivine

(6 0.5 mm, 1%). Pyroxene and olivine have oxidized rim. Groundmass is composed of plagioclase,pyroxene and black glass (3^4%). Vesicles are surrounded by glassy groundmass.

GHD368 31‡30.3P 139‡1.4P 1140^1220 Vesiculated (12%) basalt with phenocrysts of plagioclase (6 8 mm, 25%), cpx (6 3 mm, 3%) and olivine(6 3 mm, 2%). Olivine has iddingsite rim. Groundmass is composed of plagioclase, pyroxene, opaquemineral and brown fresh glass (2%). This basalt is a component of hyaloclastite.

KnollsMWD72-7 31‡21.4P 138‡58.04P 1590^1205 Highly vesiculated (20%) basalt with phenocrysts of plagioclase (6 0.4 mm, 20%), cpx (6 1 mm, 12%) and

olivine (6 0.5 mm, 1%). Groundmass is composed of plagioclase, pyroxene and small amount of brownglass (1^2%). One-mm-thick glassy rind is observed.

GHD621-11 31‡31.85P 138‡59.16P 830 Vesiculated (15%) basalt with phenocrysts of plagioclase (6 0.5 mm, 15%), cpx (6 0.5 mm, 5%) andolivine (6 0.2 mm, 1^2%). Groundmass is composed of plagioclase, pyroxene and fresh glass (3^4%). Verythin glassy rim is observed (6 2 mm).

GHD623-1 31‡34.08P 139‡0.74P 835 Fresh and moderately glassy (4%) dacite with phenocrysts of plagioclase (6 2 mm, 8%), hornblende (6 2mm, 1%), quartz (6 0.6 mm, 1%), biotite (6 0.2 mm, 6 1%) and opx (6 0.6 mm,6 1%). Groundmass iscomposed of plagioclase laths and transparent glass.

GHD623-14 31‡34.08P 139‡0.74P 835 Fresh and glassy (6%) andesite with phenocrysts of plagioclase (6 1 mm, 40%), hornblende (6 4 mm,25%). Groundmass is mainly composed of plagioclase lath and transparent glass.

MWD102-3 31‡2.31P 138‡58.59P 1400^780 Highly vesiculated (30%) basalt with phenocrysts of plagioclase (6 0.4 mm, 15%), cpx (6 1 mm, 4^5%)and olivine (6 0.4 mm, 3%). Groundmass is composed of feather-like plagioclase, pyroxene, olivine andfresh brown glass (2^3%).

MWD67-3 31‡38.15P 139‡8.01P 1255^1430 Moderately vesiculated (10%) basalt with phenocrysts of plagioclase (6 1mm, 20%), cpx (6 1 mm, 7^8%)and olivine (6 2 mm, 5^6%). Groundmass is composed of plagioclase, pyroxene and trace amount of glass(6 1%).

MWD66-1 31‡38.21P 139‡11.58P 905^895 Slightly vesiculated (5^6%) andesite with phenocrysts of cpx (6 0.6 mm, 12%) and plagioclase (6 1mm,1%). Groundmass is composed of plagioclase, minor amount of pyroxene and dark brown glass (6 5%).

MWD79-1 31‡12.39P 139‡11.02P 1210^960 Highly vesiculated (40^50%) rhyolite with phenocrysts of plagioclase (6 2 mm) and trace amount ofquartz and cpx. Groundmass is composed of transparent glass (30^40%), plagioclase and trace amount ofpyroxene.

MWD80-2 31‡21.82P 139‡5.87P 1410 Porphyritic dacite with phenocrysts of plagioclase (6 4 mm, 15%) and hornblende (6 2 mm, 6^7%).Groundmass is holocrystalline and composed of plagioclase.

MWD83-2 31‡29.48P 139‡20.05P 930^700 Vesiculated (10%) and nearly aphyric dacite. Acicular plagioclase (6 0.4 mm), hornblende (6 0.3 mm),lesser amount of cpx (6 0.3 mm) and transparent glass (6 5%) are observed.

MWD84-2 31‡31.41P 139‡27.69P 1180^800 Vesiculated (15%) and well-crystallized dacite with phenocrysts of plagioclase (6 2 mm, 5%) and traceamount of opx (6 1 mm) and cpx (6 0.6 mm). Groundmass is composed of plagioclase and opaquemineral.

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Table 240Ar/39Ar dating results on the volcanic rocks from the back-arc knolls zone

IrradiationNo.

AnalysisNo.

Sample No. Rock type Integrated age ( S 1c) Plateau age ( S 1c) KÂr age

Weightedaverage

Inv. isochronage

40Ar/36Arintercept

MSWD Fraction of39Ar

(Ma) (Ma) (Ma) (%) (Ma)

N^S-trending ridges9703-1 U98189 MWD24-1 cpxôl basalt 2.65S 0.24 2.62$ 0.14 2.58S 0.17 302S 9 0.73 100.0 1.91S 0.209702-2 U98090 MWD22-2 cpxôl basalt 2.42S 0.19 2.49$ 0.18 2.6S 0.5 290S 30 0.66 100.0 2.59S 0.059702-1 U98110 GHD616-1 cpxôl basalt 2.82S 0.22 2.77$ 0.21 3.0S 0.6 280S 40 0.31 100.0 3.0 S 0.69702-1 U98104 GHD368 cpxôl basalt 2.8S 0.3 2.7$ 0.3 2.5S 0.5 360S 240 1.19 100.0 2.68S 0.08Knolls9702-1 U98117 MWD72-7 cpxôl basalt 1.22S 0.07 1.29$ 0.07 1.40S 0.20 260S 80 0.92 100.0 1.49S 0.139601 U96209 GHD621-11 cpxôl basalt 1.31S 0.11 1.54$ 0.06 1.55S 0.16 293S 16 1.70 78.3 0.99S 0.119701-1 U98001 GHD621-11 cpxôl basalt 1.05S 0.09 1.38$ 0.07 1.29S 0.17 302S 14 1.20 80.5 0.99S 0.11

average for GHD621-11 1.47$ 0.059601 U9671 GHD623-1 hbôpx^cpx^bt

dacite2.50S 0.03 2.49$ 0.02 2.50S 0.03 295S 3 1.16 100.0 2.45S 0.04

9702-1 U98106 GHD623-14 hb^pl dacite 2.49S 0.08 2.51$ 0.07 2.52S 0.10 270S 80 0.95 100.0 2.39S 0.139702-1 U98115 MWD102-3 cpxôl basalt 0.72S 0.24 0.96$ 0.23 0.71S 0.97 310S 40 0.29 60.2 1.53S 0.149604-2 U97165 MWD67-3 cpxôl basalt 1.9S 0.4 1.6$ 0.3 0.05S 0.05 610S 520 0.54 100.09701-1 U98003 MWD66-1 augite andesite 1.95S 0.05 1.96$ 0.04 1.94S 0.09 299S 20 0.91 100.0 1.88S 0.059702-1 U98108 MWD79-1 rhyolite 1.99S 0.14 1.88$ 0.11 1.82S 0.13 316S 15 0.27 100.0 1.80S 0.059702-1 U98118 MWD80-2 hb dacite 2.24S 0.05 2.08$ 0.04 1.99S 0.10 304S 9 0.66 63.9 2.05S 0.079703-1 U98188 MWD83-2 dacite 1.58S 0.17 1.54$ 0.11 1.56S 0.16 290S 150 0.75 85.79702-1 U98109 MWD84-2 opx dacite 1.38S 0.09 1.33$ 0.07 1.11S 0.22 340S 40 0.72 100.0

Inv. isochron age: inverse isochron age.MSWD: mean square of weighted deviates ((SUMS/(n32))0:5) in York (1969).Integrated ages were calculated using sum of the total gas released.Weighted average ages and uncertainties are calculated using the following equations (Taylor, 1982):Tav =g(Ti/c2i )/g(1/c

2i ),

cav = (g(1/c2i ))30:5,

Tav : weighted average age, Ti : individual age,cav : uncertainty for the average age, ci : uncertainty for the individual age,VL =4.962U10310 yr31, Ve = 0.581U10310 yr31, 40K/K=0.01167% (Steiger and Ja«ger, 1977).

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71^8579

age of 1.91S 0.20 Ma (Table 2) slightly exceedingthe 2c error. A basalt lava of MWD22-2 gavean undisturbed age spectrum with a plateau ageof 2.49S 0.18 Ma (Fig. 6b), which is consistentwith a KÂr age. The plateau age of MWD24-1and MWD22-2 are concordant within analyticalerror.From north^south-trending ridges in the area

of the Enpo Chain extension, two basalts weredated (Fig. 1B). Basalt GHD616-1 yielded an un-disturbed age spectrum with a plateau age of2.77S 0.21 Ma (Fig. 6c), which is concordantwith a KÂr age of 3.0 S 0.6 Ma (Table 2) andhas much higher precision. Basalt GHD368 gavea plateau age of 2.7S 0.3 Ma (Fig. 6d) consistingof all the gas released. Four plateau ages from thenorth^south-trending ridges are all concordantwithin analytical uncertainty.

5.2. Knolls

Dating results are reported from west to east inthe back-arc knolls zone. A basalt from the south-ern slope of the Enpo Seamount (MWD72-7)yielded an undisturbed spectrum with a plateauage of 1.29S 0.07 Ma (Fig. 7a), which is consis-tent with a KÂr age of 1.49S 0.13 Ma withinanalytical error (Table 2). A basalt GHD621-11

from a knoll near the western margin of the back-arc knolls zone was analyzed for the two splitsirradiated in di¡erent reactors (the JMTR andJRR3 reactors). The two analyses yielded similarage spectra and both of them gave well-de¢nedplateaus (Fig. 7b,c). The plateau ages were1.54S 0.06 and 1.38S 0.07 Ma, respectively, whichare concordant within 2c error. The inverse iso-chron calculation further reinforces the validity ofthese plateau ages (Table 2). The inverse isochronage calculated for the plateau-forming steps foreach analysis is consistent with the plateau age,and the 40Ar/36Ar intercept is identical to the at-mospheric ratio within 1c error. The weightedaverage of the two plateau ages, 1.47S 0.05 Ma,is adopted for the best estimate age for this sam-ple. This basalt gave a KÂr age of 0.99S 0.11Ma (Table 2), which is younger than the inte-grated age and plateau age of the stepwise-heatinganalysis. Two lavas of dacite (GHD623-1) andandesite (GHD623-14) were dated from a knolleast of the GHD621 site. The dacite yielded aplateau age of 2.49S 0.02 Ma comprising 100%of the released gas (Fig. 7d). The andesite alsoyielded a completely undisturbed spectrum witha plateau age of 2.51S 0.07 Ma (Fig. 7e). A cli-nopyroxeneôlivine basalt (MWD102-3) from thenorthwestern slope of the Genroku Seamount

Fig. 6. 40Ar/39Ar age spectra with Ca/K plots for groundmass samples of lavas from the north^south-trending ridges in the back-arc knolls zone. Error for each step is given in 2c level, whereas plateau ages are given at the 1c level.

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Fig. 7. 40Ar/39Ar age spectra with Ca/K plots for groundmass samples of lavas from the knolls in the back-arc knolls zone. Errorfor each step is given in 2c level, whereas plateau ages are given at the 1c level.

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yielded a plateau age of 0.96S 0.23 Ma compris-ing three steps with 60.2% of the 39Ar released(Fig. 7f).A basalt from a knoll in the middle part of the

back-arc knolls zone (MWD67-3) yielded a pla-teau age of 1.6 S 0.3 Ma consisting of 100%of 39Ar released (Fig. 7g). An andesite lava(MWD66-1) from a knoll east to the MWD67station also yielded an undisturbed spectrumwith a plateau age of 1.96S 0.04 Ma (Fig. 7h),which is consistent with a KÂr age of1.88S 0.05 Ma. A rhyolite (MWD79-1) from aknoll northeast of the Genroku Seamount yield-ed a completely undisturbed spectrum with a pla-teau age of 1.88S 0.11 Ma (Fig. 7i). A dacite(MWD80-2) from a knoll located north-northwestof MWD79 gave a plateau age of 2.08S 0.04 Macomprising ¢ve steps with 63.9% of the 39Ar re-leased (Fig. 7j). A dacite (MWD83-2) from aknoll northeast to the dredge station of MWD80yielded a plateau age of 1.55S 0.11 Ma compris-ing four steps with 85.7% of the 39Ar released(Fig. 7k). A dacite (MWD84-2) from a knolleast-northeast of MWD83 station yielded a com-pletely undisturbed spectrum with a plateau ageof 1.33S 0.07 Ma (Fig. 7l).Two basalt samples (MWD24-1 and GHD621-

11) show discordance between the plateau age andKÂr age. Both of the samples are vesiculatedand contain small amount of glass. The possiblereason for the discordance is the removal of gasfraction trapped in the altered portion of a sampleby acid treatment or preheating of the sample incase of 40Ar/39Ar dating. In 40Ar/39Ar dating,samples were treated by HCl to remove alterationproducts before irradiation, while sample splitsfor KÂr analysis were not treated. Preheatingof samples after evacuation was done at lowerthan 150‡C in KÂr analysis, while preheatingat 250 ‡C was applied to stepwise-heating analy-sis. Preheating at higher temperature might havecaused more degassing of poorly retained radio-genic 40Ar from the alteration products beforeanalysis, which might have experienced radiogenic40Ar loss and show younger age than the unal-tered part. Thus, 40Ar/39Ar ages are consideredto be more reliable and accurate estimate of erup-tion age of these basalts.

6. Discussion

These new 40Ar/39Ar age data gave clearer andmore detailed information of temporal variationof volcanism than the preliminary KÂr results ofIshizuka et al. (1998). 40Ar/39Ar age data from thenorth^south-trending ridges and knolls in theback-arc knolls zone indicate that volcanism onthese ridges and knolls has been active during theperiod of around 2.8^1 Ma (Fig. 8). Previouslypublished KÂr age data (Ishizuka et al., 1998)indicate that volcanism on the back-arc seamountchains in the middle part (30‡30P^32‡30PN) of theIzu-Bonin arc has an age range from 12.5 to2.9 Ma, i.e. volcanism on the back-arc seamountchains had been active since the cessation of theback-arc spreading of the Shikoku Basin (Fig. 8).The dating results in this study clearly indicatethat the volcanism in the back-arc knolls zonepostdated the volcanism on the back-arc sea-mount chains and periods of active volcanism inthe two areas did not overlap (Fig. 8). Volcanismon the back-arc seamount chains seems to haveceased after the initiation of volcanism in theback-arc knolls zone.In the back-arc knolls zone, basalt lavas from

the north^south-trending ridges some 130 kmapart gave a narrow age range between 2.4 and2.8 Ma, and older than the lavas from the knolls.The detailed bathymetric map (Fig. 3) indicatesthat the north^south-trending ridge predates thesmall knolls. Thus, north^south-trending ridgesare presumed to have been formed by the earliestbasaltic volcanism related to rifting. The age of2.77 Ma for the oldest lava from the north^south-trending ridges approximates to the age of initia-tion of rifting-related volcanism. The back-arcknolls zone underwent extension at the initiationof rifting. North^south-trending normal faults arepervasively developed in the back-arc knolls zone,indicating extension in an east^west direction.Under this east^west extensional setting, the ba-salt lava is supposed to have erupted from ¢ssuresextending in a north^south direction and ventsaligned in this direction.After ca. 0.3 m.y. of initiation of rifting, the

volcanism in the back-arc knolls zone changedits mode of activity. The volcanism was more

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widely spread in the back-arc knolls zone than inthe earliest stage of rifting and became bimodal incomposition. The volcanism changed its style toforming small knolls rather than forming ridges.This type of volcanism continued until 1 Ma in

this area. After 1 Ma, volcanism and rifting seemto have ceased in the back-arc knolls zone andshifted to the east, i.e. the currently active riftzone.The sequence of temporal variation of volca-

nism in the Izu-Bonin arc revealed in this studyshows the evolution of the volcanism in the earlystage of rifting. Some modern analogs in the ini-tial stage of rifting give some supportive evidenceto the results in this study. Smith et al. (1990)reported the detailed observation of the bathy-metric features in the Sumisu Rift, which lies inthe active rift zone just east of the back-arc knollszone. In the Sumisu Rift, en echelon elongatedvolcanic ridges, basaltic cones and rhyolitic domesare major expressions of submarine volcanism re-lated to rifting. The en echelon ridge is formed asa result of coalescence of volcanic eruptive centeraligned north^south along rift-parallel faults andis dominated by pillow lava. This is consistentwith the bathymetric data from the northernpart of the back-arc knolls zone, which showsthat small cones aligned in north^south direction

form a ridge (Fig. 3). And the rock samples col-lected from the north^south-trending ridges in theback-arc knolls zone are mainly fragments of rap-idly quenched basalt lava with glassy rims. Thissuggests a similar origin for these ridges to thoseobserved in the Sumisu Rift, i.e. accumulation oferupted basalt lava and volcaniclastics from smallvolcanic centers. The size of the ridges in the Su-misu Rift is generally comparable (6^11 km long,2 km wide and 300^600 m in relief) to those in theback-arc knolls zone. In the Sumisu Rift, thesenorth^south elongated ridges are the sites of themost voluminous eruptions, except the volcanismin the cross-rift structural zone (Smith et al.,1990). The volcanic cones reported from the Su-misu Rift are also similar to those in the back-arcknolls zone in size and constituent materials.Their sizes are generally less than 4 km in diam-eter and 500 m in relief and are comparable tothose of the cones in the back-arc knolls zone(Fig. 2).In terms of the timing of the formation of the

elongated volcanic ridge, the observation in theOkinawa Trough provides some signi¢cant infor-mation (Sibuet et al., 1987). In the OkinawaTrough, which is thought to be in the incipientstage of back-arc rifting at a continental margin,elongated volcanic ridges up to 300 m high and

Fig. 8. Spatial variation of age of volcanism in the back-arc region of the Izu-Bonin arc. Error bar represents 1c error for 40Ar/39Ar age data. Age range for the volcanism in the back-arc seamount chains is from Ishizuka et al. (1998), and that for the ac-tive rift zone is from Hochstaedter et al. (1990a).

VOLGEO 2521 4-11-02


several tens of kilometers long were formed alongthe axis of depression where the amount of sub-sidence and extension is maximum. The ridges arecomposed of basalt lavas (Sibuet et al., 1987).This mode of volcanism in the Okinawa Troughsuggests that the earliest volcanism in the incipi-ent stage of back-arc rifting is structurally con-trolled and forms ridges extending along the ¢s-sures or faults along the axis of depression, i.e.normal to the direction of extension. Our resultsindicate that north^south elongated ridges wereformed by the basaltic volcanism at the earlieststage of back-arc rifting of the Izu-Bonin arc.These similarities suggest the same origin for theelongated ridges in the Okinawa Trough andback-arc knolls zone.Volcanic history of the back-arc knolls zone of

the Izu-Bonin arc is likely to be the typical of thecharacteristics of volcanism at the initial stage ofthe rifting. At the initial stage of the rifting, vol-canism is localized along the ¢ssures or faults andforms ridges elongating in the same direction. Asthe rifting progresses, volcanism spreads in acrossa wider area due to the further development ofnormal faults and ¢ssures and/or more upwellingof hotter mantle (including the production of alarger amount of magma). Further geochemicalwork on the back-arc knolls zone could give sig-ni¢cant information on sub-arc mantle processesduring the initial stage of rifting in the oceanicisland arc setting.

7. Conclusion

Laser-heating 40Ar/39Ar ages of volcanics fromthe back-arc rift zone in the middle part of theIzu-Bonin arc were obtained. The results madeclear the temporal variation of mode and spaceof the rifting-related volcanism. Back-arc riftingin this area initiated at 2.8 Ma, and arc volcanismin the back-arc seamount chains which had beenactive since the cessation of the spreading of theShikoku Basin ceased at this time. The volcanismin the earliest stage of rifting was strongly local-ized and is characterized by the basaltic volcanismfrom the north^south-trending ¢ssures and/orlines of vents. Then after ca. 0.3 m.y., as a prog-

ress of rifting, volcanism was widely spread in theback-arc knolls zone. In this stage of rifting, thevolcanism became compositionally bimodal andformed small knolls rather than ridges. This var-iation in mode and chemistry of volcanism alongwith the progress of rifting is presumed to berelated to the progressive development of exten-sional tectonic feature and corresponding changein source mantle region (input of hotter mantle,change of depth of melting, etc.) for the volca-nism.The observed stages of rifting in the studied

area can be partly recognized in the currently ac-tive rifts like the Sumisu Rift and OkinawaTrough. The volcano-tectonic evolution of theback-arc region of the Izu-Bonin arc revealed inthis study can be a typical process associated withrifting in the island arc.

Acknowledgements

We are grateful to the members of US^Japancooperative study on the back-arc region of theIzu-Bonin arc including J.B. Gill, T. Ishii, S. Mo-rita, B. Taylor, and A. Klaus. We also thank Mr.M. Narui for providing important information onneutron irradiation at the JMTR reactor. Manyhelpful suggestions on the geology of the Izu-Bo-nin arc from Drs. I. Sakamoto, A. Usui, F. Mu-rakami, T. Ishihara, and K. Fujioka are reallyappreciated. We appreciate R.W. Nesbitt andR.N. Taylor for improvement of the manuscript.We thank the o⁄cers, crews and onboard scien-tists of the R/V Moana Wave, Yokosuka and Ha-kurei-maru. This work was supported by Agencyof Industrial and Science Technology. The au-thors greatly appreciate the constructive com-ments of anonymous reviewers.

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Documents

Volcanism in the earliest stage of back-arc rifting in the Izu-Bonin arc revealed by laser-heating 40Ar/39Ar dating