Rex N. Taylor,2 Henriette Lapierre, Phillipe Vidal, Robert W ......Taylor, B., Fujioka, K., et al,...

Taylor, B., Fujioka, K., et al, 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 126

27. IGNEOUS GEOCHEMISTRY AND PETROGENESIS OF THE IZU-BONIN FOREARC BASIN1

Rex N. Taylor,2 Henriette Lapierre,3 Phillipe Vidal,4 Robert W. Nesbitt,2 and Ian W. Croudace2

ABSTRACT

Major element, trace element, and radiogenic isotope compositions of samples collected from Ocean Drilling Program Leg 126in the Izu-Bonin forearc basin are presented. Lavas from the center of the basin (Site 793) are high-MgO, low-Ti, two-pyroxenebasaltic andesites, and represent the products of synrift volcanism in the forearc region. These synrift lavas share many of thegeochemical and petrographic characteristics of boninites. In terms of their element abundances, ratios, and isotope systematicsthey are intermediate between low-Ti arc tholeiites from the active arc and boninites of the outer-arc high. These features suggesta systematic geochemical gradation between volcanics related to trench distance and a variably depleted source.

A basement high drilled on the western flank of the basin (Site 792) comprises a series of plagioclase-rich two-pyroxeneandesites with calc-alkaline affinities. These lavas are similar to calc-alkaline volcanics from Japan, but have lower contents ofTi, Zr, and low-field-strength elements (LFSE).

Lavas from Site 793 show inter-element variations between Zr, Ti, Sr, Ni, and Cr that are consistent with those predictedduring crystallization and melting processes. In comparison, concentrations of P, Y, LFSE, and the rare-earth elements (REE) areanomalous. These elements have been redistributed within the lava pile, concentrating particularly in sections of massive andpillowed flows. Relative movement of these two-element groupings can be related to the alteration of interstitial basaltic andesiteglass to a clay mineral assemblage by a post-eruptive process. Fluid-rock interaction has produced similar effects in the basementlavas of Site 792. In this sequence, andesites and dacites have undergone a volume change related to silica mobility. As a resultof this process, some lithologies have the major element characteristics of basaltic andesite and rhyolite, but can be related toandesitic or dacitic precursors by silica removal or addition.

INTRODUCTION

The Izu-Bonin arc-trench system is the product of about 50 Ma ofPacific lithosphere subduction. Magmatism during this time hasperiodically shifted from forearc to arc and backarc during arc-chainand rift-basin development, creating distinct volcanotectonic units.One of the aims of Leg 126 was to investigate the origin, timing, andproducts of the Izu-Bonin forearc rift-basin. The forearc basin is40 - 80 km across and can be traced as a basement feature for 2000 kmfrom south of Japan to east of Guam aside the Izu, Volcano, andMariana arcs. Drilling results from Leg 126 have shown that this basininitiated during the Oligocene (about 30-32 Ma) by the stretching offormerly contiguous outer-arc high and remnant arc terranes ofEocene and early Oligocene age (Leg 126 Shipboard Scientific Party,1989a; 1989b). The basin was rapidly filled by volcaniclasticmaterial derived from the uplifted blocks (Taylor, Fujioka, et al.,1990), which produced the 1-4-km-thick sedimentary cover.

To investigate the nature of the previously unsampled basementflooring the Izu-Bonin forearc basin, two deep holes were drilledthough the sedimentary fill (Figs. 1 and 2). The first, Hole 793B,penetrated 1404 m of sediment in the center of the basin beforeentering volcanic basement. In addition to basement, igneous materialwas recovered from a Neogene sill in Hole 793B and as clasts withinthe coarser volcaniclastic units. The second, Hole 792E, was locatedon a frontal-arc high in the west of the forearc basin (Figs. 1 and 2)and reached basement at 804 mbsf. This study details the petrologyand geochemistry of the igneous units recovered from the forearc, anddiscusses their petrogenesis and significance within the frameworkof intraoceanic arc evolution.

Taylor, B., Fujioka, K., et al, 1992. Proc. ODP, Sci. Results, 126: College Station,TX (Ocean Drilling Program).

2 Department of Geology, The University of Southampton, Southampton, SO9 5NH,United Kingdom.

3 URA-CNRS 1366,69, Université Joseph Fourier, Institute Dolomieu, 15 Rue MauriceGignoux, 38031 Grenoble Cedex, France.

4 Unite Associée no. 10, CNRS et Université Blaise Pascal, Clermont-Ferrand, France.

SITE 793

Rift Volcanism in the Forearc

Constraints from multichannel seismic surveys and Leg 126 drill-ing have shown that the Izu-Bonin forearc basin was initiated duringthe middle to late Oligocene (Taylor and Mitchell, this volume).Rifting of the Eocene basement, presently exposed on the BoninIslands and recovered at Site 786 (ODP Leg 125; Fryer, Pearce,Stokking, et al., 1990), produced a series of half-graben structures onthe eastern flank of the basin. Oligocene turbidites rapidly filled thesestructures and lapped onto footwall basement of the outer-arc high.Late Oligocene-Miocene strata show a decline in sedimentation rateswith deposition of more calcareous and fine-grained facies. Renewedarc activity between 6 Ma and the present is recorded in the Pliocene/Quaternary strata as mixtures of scoriaceous and pumiceous sandsand gravels. Growth of the currently active volcanic edifices is alsodisplayed as a downwarp of seismic reflectors toward the arc volcanoes.This is caused in part by the loading effect of the developing volcanicmass on the thin arc crust (Taylor et al., 1990).

Volcanic Stratigraphy

Beneath the forearc basin sediments, Hole 793B penetrated 279 mof volcanic basement, consisting of intercalated volcanic breccias(65%), pillowed flows (15%), and massive flows (20%), as illustratedin Figure 3. The volcanic breccias are predominantly monolithic,containing clasts of pillow and massive flow of similar compositionsto the adjacent lavas. The matrix to these clasts consists of relict glassshards, fine lava debris, and phenocrysts. Clast sorting and het-erolithic fragments within the uppermost breccia (Unit 1) indicatelocal reworking of the volcaniclastic material.

The basement sequence represents the product of a submarine-rift volcano. The high proportion of volcanic breccias can berelated to synvolcanic half-graben faulting, which would provide anumber of unstable scarps along which volcanic debris and lavaflows could accumulate. The Unit 1 breccia represents a declinein volcanic activity and a change from locally derived clastic

405

R. N. TAYLOR ET AL.

33°N

141' 142=

Figure 1. Bathymetric map of the Izu-Bonin arc-trench system between 30.5°and 33°N. The locations of sites drilled on Legs 125 (open circles) and 126(solid circles) are shown on the map as are the locations of site survey multichannel seismic lines. Contour interval = 500 m (numbers on contour linesindicate depth in km). From Taylor, Fujioka, et al. (1990).

deposition into the more distal, coarse-clastic sedimentation duringfurther basinal stretching.

Petrography

The transition from sedimentary sequence to basement is markedby a change from volcanogenic conglomerates to a heterolithicvolcanic breccia. The most apparent distinction between theseunits in the recovered material was the presence of highly plagio-clase-phyric andesites in the sediments and pyroxene-phyric lavasin the breccia. This distinction was amplified by the petrographicstudy of clast material in the conglomerates and breccia. Figure 3shows the change in phenocryst assemblage between cover andbasement. The clasts in sedimentary Unit V have a plagioclase-clinopyroxene-orthopyroxene-magnetite (plag-cpx-opx-mgte)crystallization assemblage, similar to the calc-alkaline andesitesrecovered in Hole 792E (see below). In contrast, the basementvolcanics show crystallization of the pyroxenes before Plagioclase(ol-opx + cpx-plag) and magnetite is either absent or limited tothe groundmass.

Basement lavas and breccia clasts are composed of four pheno-cryst assemblages (listed below in approximate abundance order)distributed randomly throughout the stratigraphy (see Fig. 3 andTable 1):

(i) cpx + opx + olivine (ol) + chrome spinel (porphyritic)(ii) cpx + opx (porphyritic)(iii) cpx + opx + plag (porphyritic)(iv) cpx + plag (aphyric)

Type (iii) is the dominant assemblage in the stratigraphy. Themineral chemistry from Hole 793B basement is discussed in a parallelcontribution (Lapierre et al., this volume). A typical texture from aType (i) lava is shown in Plate 1, and brief details of the phenocrystminerals follow.

Olivine

Only represented by saponite/celadonite/hematite/calcite pseudo-morphs after subhedral or anhedral grains, with inclusions of chrome

406

IGNEOUS GEOCHEMISTRY AND PETROGENESIS

30

20 -

10

α.LUQ

- 1 0

- 2 0

- 3 0

Shikoku- Basin

443I

-

SumisuRift

790/1i I

IzuBoninArc788 792

I

Fore-arcbasin

793

A

Fore—archigh

786 783I I

Bonintrench

100 200 300 400 500km

Figure 2. Schematic west-east section across the Izu-Bonin arc-trench system. Numbers refer to DSDP and ODP sites. Shaded

regions are MORB-type crust; stippled regions are arc-related crust.

spinel. Unit 13 (Orange Spot flow) contains about 8% olivine pseudo-morphs arranged as glomeroporphyritic clots.

Clinopyroxene

Present as fresh phenocrysts in all units of the basement. Euhe-dral-subhedral crystals are 1-10 mm in size, occasionally are twinned,and have minor compositional zoning. Compositions range fromEn39:Wθ44:Fs17 to En54:Wθ39:Fs7.

Orthopyroxene

Occurs as euhedral-subhedral phenocrysts 1-5 mm in size. Crystalmargins and sometimes whole crystals are replaced by smectite andceladonite. Limited compositional zoning is present, but rims tend tobe slightly more Fe-rich. Compositions range from En82 to En88.

Plagioclase

Subhedral, fresh phenocrysts 0.3-2 mm in size. Compositionsvary from An64 to An90 between phenocrysts, and from An60 to An86

in the laths and microlites of the groundmass.

Geochemistry

Major and trace element analyses were undertaken on 86 samplesfrom the basement volcanics and lava clasts from the lowest 350 mof Hole 793B (Table 2). A Philips PW1400 fully automatic X-rayfluorescence spectrometer fitted with a 3 kW Rh anode tube was usedfor the analysis of all samples at the Department of Geology, Univer-sity of Southampton. Major elements were obtained from fusionbeads prepared from 1 g calcined rock powder and 5 g of Spectroflux100B. Matrix effects were compensated for by using Philips' alphas(influence coefficients). Precision and accuracy are both high andnominally better than 1% (relative). Loss on ignition (LOI) wasmeasured for each sample as the weight lost on samples between100°C and 1000°C, and is excluded from the totals reported in Tables 2,5, and 7.

Trace elements were determined on 40 mm diameter powderpellets pressed to 10 tonnes. Twelve drops of an 8% w/v aqueoussolution of polyvinyl alcohol were used as a binder. Corrections for

matrix effects for trace elements were made for all wavelengthregions, using the Compton scatter technique (full procedural detailscan be found in Croudace and Gilligan, 1990). Modified corrections,necessary when crossing absorption edges, were also applied to Ba,V, and Cr. Concentration calibrations were performed using carefullychosen high-quality geological reference samples (basic-intermediatecomposition). Precision and accuracy are generally better than 1%(relative), when the element concentrations are well above theirdetection limits. Detection limits are approximately 1 ppm for Zr, Nb,Y, Ga, Rb, Sr, Ni, Cu, and Zn and 6 ppm for Cr, V, and Ba.

Phosphorus was measured both on fusion beads and powderpellets, although the latter data are preferred because the determina-tions are considerably more sensitive and precise. Using powderpellets, all samples were counted by recycling the measurements sixtimes, and counting at peak and background positions for 200 and 100 s,respectively. Matrix corrections were applied using mass absorptioncoefficients calculated from major element compositions. Precisionis better than 2% rel. at 100 ppm P.

Major and Trace Element Systematics

In Figure 4, major elements (recalculated anhydrous) are plottedagainst MgO for the basement volcanics. Negative correlations existwith A12O3 and TiO2, whereas CaO and Fe2O3 remain roughly con-stant with decreasing MgO. The plots in Figure 4 include the fieldsfor phenocryst compositions, a bulk extract (B) composition (a com-position weighted according to the average modal proportions ofphenocrystal minerals present in lavas with >7 wt% MgO), and abulk-extract control line.

The major element plots demonstrate that the majority of whole-rock compositions can be interrelated by the removal or addition ofthe bulk extract composition. This relationship is pursued further byleast-squares mixing calculations (Table 3). These calculations con-firm that extracting the observed modal proportions in a quantityequivalent to phenocryst abundances successfully links the whole-rock compositions. As such, it is possible that groundmass or liquidcompositions were similar throughout the majority of lavas, withphenocryst abundance causing whole-rock variations. This notion issupported by the lack of variation in clinopyroxene and Plagioclasechemistry from lavas with widely varying MgO contents (Lapierre etal., this volume).

407

R. N. TAYLOR ET AL.

phenocrysts ^ sparse (<2%) I • abundant (>2%)Depth oor No. I^CJ-T- / ^

M.B.S.F recovery graphic log u n i t cpx opx plag olrv mgte

1400

1450

1500

1600

1650

Hθtβrolithic conglomerate,blocks of porphyriticandesite up to 14 cm.

Sheared volcanic breccia

Hθtβrolithic breccia, dasts <1 cm

to >10 cm. dominantly composed

of porphyritic basaltic andesite.

Highly poΦhyritic basaltic andesitepillows and breccia.

Monolithic brecda. Sparse-phyric

dasts in a sand-size matrix of

glass fragments and zeolite cement

Porphyritic pillowed flow.

Porphyritic massive flow.

Monolithic brecda, sparse-phyric dasts.

Poiphyribc p ow d flow.

Heterolithic brecda, sparse-phyric and

porphyritic dasts.

Mononlithic breeds, porphyritic dasts.

PoΦhyritic massive flow

PoΦhyritic pillowed flows and brecda.

Monolithic brecda. Clasts of poΦhyritic

basaltic andesite pillows in a crystal

tuff matrix.

*Orang&-spot flow". Highly porphyritic

Massive flow.

PoΦhyritic massive flows.

Monolithic breccia. Sparse-phyric dasts.

Heterolithic breccia. Sparse-phyric and

Porphyritic dasts.

Pillowed flow Massive flow Volcanic breccia Sediment

Figure 3. Basement stratigraphy, Hole 793B. Phenocryst assemblage throughout the stratigraphy is shown in the columns on

right. Shaded = abundant (>2% modal), hatched = sparse (<2%), blank = absent. Abbreviations: cpx = clinopyroxene, opx =

orthopyroxene, plag = Plagioclase, olv = olivine, and mgte = magnetite/opaque.

408


Table 1. Modal mineralogy of basement rocks, Hole 793B.

Sample/analysis

044S-1903008305305]058033

>'071050039

Core, section,interval (cm)

97R-1.80110R-1, 192R-2,70112R-2, 57104R-1, 65102R-1, 128105R-192R-3, 5589R-2, 86109R-3, 50102R-1, 3694R-1, 100

Depth(mbsf)

1515.781633.831465.321661.381577.111562.511585.611468.501436.131633.311558.331483.21

Unit

5132

14109

1121

1283

Cpx

10.58.8

23.516.613.81.67.86.17.17.54.02.1

Opx

5.97.8

16.712.23.12.5

11.76.7

12.410.11.01.4

Plag

0.90.10.30.57.43.29.43.51.86.13.62.0

Oliv

1.28.4——

—0.7———

Vesic

4.30.72.72.93.76.55.42.43.70.22.44.8

Petrologicaltype

(i)0)(ü)(ii>

(in)

l i v i(iv)

Notes: All analyses performed by point counting (>800 counts per section). Reported values are percentages,with groundmass the residual percentage. Petrological types are defined in the text, unit numbers referto position in stratigraphy (see Fig. 3).

0.5

ra I—'O

20MgO wt%

Figure 4. Major element variation with MgO for Hole 793B basement lavas. Open circles are Group A lavas, and solid circles are Group B lavas (see text). Fieldsare shown for the range of phenocryst compositions present in Sample 033 (analyses from Lapierre et al., this volume). OP = orthopyroxene, CP = clinopyroxene,and PLAG = Plagioclase. Bulk extract (B) is a composition derived from average phenocryst analyses weighted according to their modal proportions with Sample 033.A mixing line between Group B lavas and Sample 050 (5.13 wt% MgO) represents between 2% and 55% addition of bulk extract to Sample 050.

Combining modal analyses of samples with geochemical dataindicates that the proportion of individual phenocryst types changeswith whole-rock chemistry. This is demonstrated in Figure 5, whichshows an increase in Plagioclase (as a percentage of phenocrystspresent in each sample) and a decrease in orthopyroxene with declin-ing whole-rock MgO content. As such, it is likely that whole-rockcompositions are a partial function of magma evolution. However,

phenocryst content appears to be the dominant factor in whole-rockgeochemical variations.

In Figure 6, trace elements are plotted against Zr as a fractionationindex. The samples are divided into two groups on these plots on thebasis of stratigraphic and geochemical differences. The first group(Group A) comprises the upper seven samples within Unit 1. Theselava clasts have higher Zr and lower Fe contents than all underlying

409

H- Table 2. Basement geochemistry, Hole 793B.

Core, sectionIntervalDepth (MBSF)Sample no.UnitLithology

Major elements (wt%)

SiO2TiO2AI2O3Fe2O3 *MnOMgOCaONa2OK2OP2O5(LOI)

Total

Trace elements (ppm)

PZrYGaRbSrBaVNbNiCrZnCuNdSm

100Mg/Mg+Fe(t)AI2O3/TiO2CaO/TiO2CaO/AI2O3Ti/ZrZr/Y

86R-1128-1311408.37

0201

Breccia

54.970.34

15.847.500.157.77

10.711.650.930.012.44

99.87

3142.2

8.514.212.9129

41760.555

2757718

69.546.631.50.68

485.0

86R-210-11

1409.50021

1Breccia

56.230.34

15.247.160.209.269.781.480.430.033.07

100.15

14243.411.511.55.2900

2550.352

28264

353

74.044.828.80.64

473.8

86R-226-30

1410.11S - 6

1Breccia

55.330.34

15.797.590.147.40

10.631.731.120.021.18

100.09

9342.310.0

1510.6125

01910.351

3166717

68.246.431.30.67

474.2

87R-329-34

1419.77024

1Breccia

57.820.33

14.877.290.168.689.041.660.820.034.47

100.70

14741.210.311.67.47615

2010.645

2365667

72.445.127.40.61

484.0

87R22-25

1420.60S - 7

1Breccia

55.030.34

15.217.740.168.55

10.611.810.660.031.69

100.14

15241.310.714.94.9123

42100.456

2966866

70.944.731.20.70

463.9

88R-165-68

1425.50S - 8

1Breccia

53.890.37

18.467.910.125.21

10.292.230.920.051.45

99.45

24044.216.718.212.4156

102090.440

1267426

4.641.48

59.249.927.80.56

472.6

88R-191-95

1426.61026

1Breccia

54.400.37

18.587.950.135.65

10.272.240.910.032.18

100.53

10844.312.518.110.8160

22130.244

1257924

61.050.227.80.55

503.5

89R-20 - 4

1434.44S - 9

1Breccia

54.980.28

13.629.090.149.239.231.781.090.021.92

99.46

6327.0

9.814.318.2147

131950.4103566

6816

69.148.633.00.68

582.8

89R-29-12

1434.60027

1Breccia

54.810.32

16.409.080.136.479.462.151.280.022.05

100.12

10333.710.818.122.1162

82090.359

1727222

61.151.329.60.58

573.1

89R-286-90

1436.13028

1Breccia

54.590.29

14.218.840.15

10.279.301.750.560.021.82

99.98

6827.5

8.815.69.9172

112030.0111347

7716

71.949.032.10.65

633.1

92R-120-24

1460.62029

1Breccia

54.770.26

12.438.640.16

12.ß29.001.570.460.012.51

99.62

6125.1

7.213.76.4157

42040.3127485

6823

75.847.834.60.72

623.5

92R-2 92R-270-74 113-117

1465.32030

2Pillow

53.460.25

12.078.740.17

12.5610.211.780.580.051.05

99.87

27524.918.314.610.9135

62210.0150748

7414

76.048.340.80.85

601.4

1466.34031

2Pillow

53.520.24

11.468.930.18

14.129.651.450.180.021.19

99.75

14024.2

9.112.33.2122

62210.4146669

6514

77.747.840.20.84

682.7

92R-325-28

1467.79S-10

2Pillow

54.110.30

13.878.440.158.86

10.242.300.880.361.35

99.51

166027.920.4

1515.3163

192210.4110399

6216

5.921.73

69.846.234.10.74

621.4

92R-331-35

1467.96032

2Pilbw

53.620.28

14.058.840.159.17

10.122.251.030.051.15

99.56

21229.521.615.118.4158

152340.1112438

6816

69.550.236.10.72

621.4

92R-355-59

1468.50033

2Pillow

54.190.31

15.978.700.158.829.442.030.420.021.90

100.05

7231.4

9.215.96.0163

72250.889

2047719

69.151.530.50.59

633.4

92R-cc11-12

1469.51034

2Pillow

54.540.32

16.458.640.137.269.102.310.890.032.33

99.67

11330.112.116.816.1222

72330.278

1867048

64.951.428.40.55

642.5

93R-163-67

1471.80035

2Pillow

53.030.28

13.208.960.19

11.9210.61

1.590.200.032.79

100.01

15826.911.514.44.4141

12160.0170561

6816

74.547.137.90.80

682.3

93R-185-87

1472.47S - 4

2Pillow

52.630.28

13.159.160.19

12.2010.28

1.570.260.031.64

99.75

12526.610.014.34.5141

22140.1168541

6518

74.647.036.70.78

662.7

r3

i—OPBmH>

Table 2 (continued).




Total



100Mg/Mg+Fβ(t)AI2O3/TO2CaO/TiO2CaO/AI2O3Ti/ZrZr/Y

93R-196-1001472.86

0363

Breccia

52.730.28

12.769.070.19

12.5010.04

1.540.230.021.68

99.36

12826.3

9.713.8

4.4141

22160.5

161491

6618

75.245.635.90.79

692.7

93R-222-25

1475.28037

3Breccia

53.510.37

17.908.480.157.06

10.372.290.370.032.55

100.53

14336.311.916.4

4.8190

02590.456

1747072

64.748.428.00.58

643.1

93R-227-29

1475.44S-11

3Breccia

53.410.37

17.718.420.156.94

10.292.190.470.021.93

99.97

13134.6

9.8

3.9184

132600.555

1448876

64.547.927.80.58

642.5

93R-3 94R-1 94R-CC60 100-105118-126

1476.47038

3Breccia

54.370.32

16.078.500.168.299.542.140.600.013.23

100.00

7031.410.3

152.8

1548

2140.278

2607119

68.250.229.80.59

633.0

1483.21039

3Breccia

53.710.37

18.178.650.146.429.962.190.470.031.47

100.11

11235.812.217.56.8189

52520.446747225

62.049.126.90.55

622.9

1488.29041

3Breccia

55.150.36

17.708.490.166.499.352.300.650.033.74

100.68

11534.611.116.3

6.6164

52310.142736745

62.749.226.00.53

603.1

96R-177-81

1504.07042

4Pillow

53.140.29

14.158.820.18

10.8910.42

1.830.330.032.76

100.08

13428.811.314.9

5.8149

02310.1

138569

8318

73.148.835.90.74

642.5

96R-181-83

1504.28S-12

4Pillow

53.480.29

14.468.980.16

10.7310.57

1.900.360.011.68

100.94

10129.111.115.1

7.2152

52300.6

140544

7919

3.631.20

72.549.936.40.73

602.6

96R-193-98

1505.16043

4Pillow

54.360.36

17.978.810.135.769.682.530.890.021.55

100.51

9735.211.618.813.4193

92220.045707619

59.049.926.90.54

613.0

97R-180-85

1515.78044

5Massive

52.640.28

13.979.130.20

11.779.971.820.220.031.27

100.03

15527.813.014.4

4.4149

12210.1127432

6722

73.949.935.60.71

622.1

97R-1 97R-198-102 124-1271517.31

0455

Massive

53.170.29

14.889.010.19

10.6110.01

1.970.260.031.65

100.42

10929.510.014.9

3.2151

02370.5

107390

6616

72.251.334.50.67

633.0

1517.50S-13

5Massive

53.190.28

13.838.980.20

11.589.921.760.210.021.55

99.97

11127.6

8.813.5

3.5140

32110.4

123488

6424

2.460.84

73.949.435.40.72

613.1

98R-463-67

1526.61046

6

99R-1 99R-154-55 140-144

1530.41S-14

7

1534.84048

7Breccia Cpx-pill Cpx—pill

55.030.32

16.918.600.156.978.192.350.770.024.07

99.31

4932.4

8.515.9

9.6176

232150.1393969

129

64.152.825.60.48

593.8

53.020.27

13.509.700.169.56

10.821.760.900.041.62

99.73

17926.611.515.617.2140

112560.6122596

7515

3.691.21

68.450.040.10.80

632.3

53.660.26

13.349.160.17

10.569.991.830.670.011.82

99.65

7726.1

9.314.512.8132

122290.5137491

7934

71.751.338.40.75

652.8

100R-216-19

1545.10049

8Pillow

53.960.33

17.199.470.135.889.522.430.910.011.83

99.83

7333.310.117.316.4187

02330.559628521

57.752.128.80.55

613.3

100R-228-30

1545.20S-15

8Pillow

53.500.35

18.139.060.135.93

10.202.460.720.020.81

100.50

7534.410.617.412.0197

42470.548878719

59.051.829.10.56

603.2

102R-1 102R-136-40 128-132

1558.33050

8Breccia

54.050.36

17.379.420.115.139.202.371.510.041.71

99.56

13937.513.618.327.1184

02440.451

1267518

54.548.325.60.53

592.8

1562.51051

9Breccia

55.260.34

16.378.710.157.058.802.400.730.042.42

99.85

15335.912.515.9

7.2152

1254

• 0.446

16265

164

64.048.125.90.54

582,9

£ Table 2 (continued).



SiO2TiO2AI2O3Fe2O3*MnOMgOCaONa2OK2OP2O5(LOI)

Total



100Mg/Mg+Fe(t)AI2O3/TD2CaO/TiO2CaO/AI2O3Ti/ZrZr/Y

103R-1123-1271569.78

0529

Breccia

53.950.37

16.839.630.147.278.902.310.550.021.24

99.97

8136.810.1

188.1170

02350.642637520

62.445.524.10.53

593.6

104R-1 104R-165-70 140-145

1577.11053

10Massive

52.800.30

13.689.360.169.76

11.661.780.280.021.30

99.80

10732.610.115.64.7161

42530.248

2448319

69.745.638.90.85

673.2

1578.49054

10Massive

52.880.31

14.289.630.178.99

11.222.030.290.031.42

99.83

16332.111.715.24.7174

32590.345

2408129

67.346.136.20.79

652.7

104R-249-54

1579.56S-16

10Massive

53.180.32

14.559.170.168.23

11.622.170.510.060.83

99.97

29233.515.416.39.6178

72590.549

2707618

5.611.78

66.445.536.30.80

602.2

104R-269-75

1579.92055

10Massive

53.370.31

14.458.900.147.41

11.572.270.910.131.69

99.46

75431.221.716.816.9201

142640.149

2787757

64.746.637.30.80

681.4

104R-395-1001583.11

05610

Massive

53.690.29

13.549.480.179.37

11.521.980.200.032.14

100.27

14530.1

9.814.32.5

23412

2440.544

2356653

68.546.739.70.85

653.1

105R-1 105R-1 105R-31-6127-131106-110

1585.61058

11Pillow

55.200.30

13.748.960.178.37

10.542.240.460.032.60

100.01

15033.010.615.25.0145

82390.434

18966

139

67.345.835.10.77

603.1

1588.38S-17

11Pillow

56.040.28

13.039.090.178.66

10.112.040.460.032.27

99.91

12531.210.412.75.8129

62310.538

2216472

2.770.97

67.746.536.10.78

573.0

1594.47063

11Pillow

54.410.29

13.469.350.178.61

10.751.770.640.032.31

99.48

14231.510.015.710.0157

102250.045

2807291

67.046.437.10.80

623.2

106R-228-33

1605.43062

11Pillow

54.010.29

14.579.510.178.28

10.632.150.240.031.32

99.88

20834.712.115.54.3180

102450.636

1597123

65.750.236.70.73

602.9

107R-110-14

1605.01064

11Pillow

57.260.29

15.267.680.134.649.612.741.390.510.48

99.51

235034.050.915.922.519738

2210.027

1395898

57.152.633.10.63

540.7

107R-117-20

1605.10S-18

11Pillow

53.510.33

16.789.350.147.34

10.202.260.500.021.42

100.43

7734.79.9

17.28.7183

72370.437

16286

169

63.350.830.90.61

583.5

107R-310-16

1610.26065

12Breccia

53.340.33

14.738.970.179.88

10.391.810.320.031.31

99.97

11833.311.214.66.2150

02350.61063207716

70.844.631.50.71

643.0

108R-155-60

1615.96068

12Breccia

53.280.27

14.648.490.16

11.269.421.890.260.021.99

99.69

9630.7

9.914.84.2153

42050.0134400

6555

74.554.234.90.64

583.1

108R-2 109R-20-7 127-132

1618.64069

12Breccia

52.530.30

14.708.590.16

10.6710.42

1.870.260.041.65

99.54

18128.111.615.85.0147

22330.0124351

7020

73.249.034.70.71

712.4

1631.40070

12Breccia

53.070.29

14.408.770.17

11.309.951.860.220.021.33

100.05

11428.110.016.45.0145

12180.2123345

6843

73.949.734.30.69

702.8

109R-350-55

1633.31071 (a)

12Breccia

56.130.27

14.048.010.149.838.412.050.580.031.96

99.49

10131.09.8

13.19.715328

1890.1123449

5968

73.052.031.10.60

503.2

109R-350-55

1633.31071 (b)

12Breccia

54.280.26

13.508.660.17

11.799.251.730.340.021.59

100.00

7429.710.1

134.8129

02000.0132490

64120

75.051.935.60.69

562.9

110R-11-3

1633.83S-19

13massive

54.560.25

13.029.140.15

10.159.832.290.690.040.77

100.12

20125.113.013.511.514720

2490.5124647

65102

3.461.06

71.052.139.30.75

621.9

70

•z%oTO

>





Total




413

110R-1 110R-2 110R-27-12115-125115-125

1633.96072

13Massive

53.990.25

12.338.800.15

11.0810.09

2.150.600.031.34

99.47

19524.112.313.710.1142

52190.0

139637

6428

73.549.340.40.82

652.0

1637.86073(a)

13Massive

55.440.26

12.938.940.149.149.632.420.770.041.19

99.71

20525.912.814.515.2151

142210.090

4535936

69.249.737.00.74

632.0

1637.86073(b)

13Massive

55.120.25

12.299.790.159.629.742.360.880.041.49

100.24

19424.412.513.717.8136

262150.3

114737

6445

68.449.239.00.79

622.0

110R-378-83

1638.97075

13Massive

54.410.26

12.669.180.159.649.992.320.750.041.87

99.40

19124.512.413.812.7143

182140.2

100563

57130

69.848.738.40.79

582.0

110R-4 110R-5 111R—157-62 128-132110-115

1640.48074

13Massive

53.720.24

12.248.940.16

10.4911.04

2.220.770.041.81

99.86

18523.211.313.913.2139

212480.3112606

5744

72.151.046.00.90

702.1

1643.20076

14Massive

56.710.31

15.928.590.114.248.863.111.480.091.12

99.42

43430.820.716.433.0191

132210.0409261

112

52.151.428.60.56

581.5

1645.89079

14Massive

54.820.26

13.179.120.139.789.012.420.960.051.68

99.72

24825.411.014.620.6152

202020.2126493

6514

70.250.734.70.68

652.3

111R-2 111R-232-37 111-116

1648.53077

14Massive

52.930.24

11.919.490.14

13.319.221.760.600.052.35

99.65

29223.015.413.113.3131

01880.6

146565

686

75.549.638.40.77

671.5

1650.61078

14Massive

54.300.24

11.378.590.16

13.619.071.910.330.031.60

99.61

16023.5

8.912.2

7.4130

192090.4

158670

6554

11147.437.80.80

662.6

112R-159-63

1654.72S-20

14Massive

55.510.30

15.088.000.127.069.282.711.190.690.90

99.94

358929.718.514.425.3187

222210.383

2395819

3.281.04

66.050.330.90.62

611.6

112R-163-68

1655.20080

14Massive

56.770.29

15.047.990.126.358.992.801.370.520.86

100.24

281530.521.314.931.7183

252160.071

2225322

63.651.931.00.60

591.4

112R-193-97

1656.33081

14Massive

55.530.28

13.968.610.127.239.272.521.510.661.48

99.69

342227.924.314.236.9172

191960.389

3665525

64.949.933.10.66

591.1

112R-2 112R-257-62 140-149

1661.38083

14Massive

53.750.25

12.588.840.16

12.519.032.040.350.111.46

99.62

63725.711.713.1

7.0143

162150.5143536

7012

75.750.336.10.72

632.2

1662.50084

15Massive

57.920.32

16.327.620.103.558.363.091.690.351.41

99.32

182933.449.716.333.5197

212190.036565121

50.651.026.10.51

570.7

113R-2 113R-2 113R-3 113R-313-17 133-138130-135137-141

1666.50085

15Breccia

52.810.30

16.369.230.158.849.372.150.310.021.91

99.54

9433.7

9.616.3

5.4184

82470.281

21974

142

67.854.531.20.57

603.5

1669.35086

15Breccia

52.970.31

16.868.570.138.079.612.470.500.011.68

99.50

8033.9

8.516.8

8.9182

82490.087

23875

361

67.554.431.00.57

634.0

1670.23087

16Massive

57.830.32

16.748.070.103.518.373.101.850.060.83

99.95

26935.521.919.135.4197

322370.228415815

48.952.326.20.50

561.6

1671.40S-21

16Massive

59.290.31

16.617.510.093.248.163.031.690.060.64

99.99

27934.421.618.531.5196

252270.223405138

7.362.32

48.753.626.30.49

551.6

113R-435-41

1672.30088

16Massive

58.840.30

16.837.510.093.288.223.141.720.061.04

99.99

25433.421.917.131.3195

172200.321335125

49.056.127.40.49

541.5

r\UJ

Zm

odGO(-\

m

sm

>θP

ET

RO

GE

NE

SIS

Table 3. Least squares mixing calculations, Hole 793B basement.

Model 1Sample 030 to sample 033(12.58 to 8.82 wt%MgO)

Parent Daughter Daughteranalysis analysis calculated

Residual

SiO2TiO2AI2O3Fe2O3MnOMgOCaONa2OK2OP2O5

53.530.25

12.098.750.17

12.5810.22

1.780.580.05

54.160.31

15.968.700.158.829.442.030.420.02

Sum

26.5% crystallization as:

Sample 030 contains41.6% phenocrysts as:

53.280.31

15.838.860.178.859.462.380.790.07

R squared

cp× : op×58 :42

cp× : opx : plag58 :41 :0.7

0.880.000.13

-0.16-0.02-0.03-0.03-0.35-0.37-0.05

1.08

Model 2Sample 030 to sample 050(12.58 to 5.16 wt% MgO)

Model 3Sample 033 to sample 050(8.82 to 5.16 wt% MgO)

Parentanalysis

53.530.25

12.098.750.17

12.5810.22

1.780.580.05

Daughteranalysis

54.290.36

17.459.460.115.159.242.381.520.04



Daughtercalculated

53.570.42

17.389.640.185.289.323.041.070.09

Sum R squared

cp×45 :

cpx37 :

Residual

0.72-0.06

0.07-0.18-0.07-0.13-0.08-0.66

0.44-0.05

1.22

: opx : plag40 :15

: opx : plag41 :22

Parentanalysis

54.160.31

15.968.700.158.829.442.030.420.02

Daughteranalysis

54.290.36

17.459.460.115.159.242.381.520.04



Daughtercalculated

54.830.42

17.569.450.165.169.272.570.570.03

Sum R squared

cpx17 :

cp×46 :

: opx49 :

: op×11 :

Residual

:plag34

: plag43

-0.54-0.06-0.11

0.02-0.05

0.00-0.03-0.18

0.950.01

1.25

Notes: Modal proportions calculated on a vesicle-free basis. Mineral compositions taken asaverages of phenocryst analyses from sample 033 (Lapierre et al., this volume).


8 10MgO wt%

« 60

üoS 50

40

Φ

S 3 0

20

o"ocoroαoa.

10

8 10MgO wt%

12 14 16

Figure 5. MgO contents of Hole 793B basement plotted vs. the proportion of Plagioclase and orthopyroxene expressed as a percentage of total phenocrysts. Modalanalyses are given in Table 1.

lavas of Group B (see also Fig. 4). Clinopyroxene analyses fromGroup A lavas reflect the whole-rock compositions in having appro-priately lower Fe/Mg ratios and total Fe contents (see Lapierre et al.,this volume). Both groups show positive correlations between Zr andTi, Ga, Sr, and A12O3, with inter-element ratios remaining roughlyconstant. Ni and Cr correlate negatively with Zr, reflecting the re-moval or addition of pyroxenes.

Aspects of Trace Element Enrichment

Relationships between Zr and certain other trace elements are not aseasily explained. On Figure 6, P and Y show no correlation with Zr andSm + Nda very weak one. Yttrium varies by a factor of 5 and P by afactor of 30 in samples ranging from 25 to 35 ppm Zr. Similar, thoughless pronounced discrepancies, are noted in the combined Nd and Smagainst Zr plot. Figure 7 demonstrates the anomalous behavior of certaintrace elements. On this plot, concentrations of a low-MgO (anomalous)sample (S-21: 126-793B-113R-3, 137-141 cm) are normalized to aprimitive high-MgO sample (S-13: 126-793B-98R-3, 124-127 cm).Also shown for comparison on Figure 7 are predicted trace elementconcentrations after removal of 30% and 80% of observed phenocrystsin S-13. Of the trace elements, only Sr, Ti, Zr, and Ga are close to predictedconcentrations after 30% crystallization. Levels of P, Y, Nd, Sm, and thelow-field-strength elements (LFSE) are all significantly higher, requiringin excess of 70% crystallization to generate the observed concentrations.REE patterns from two basement samples (Table 4 and Fig. 8) shownonparallel profiles, suggesting a process other than crystallizationrelates lavas within the sequence.

As elements such as P, Y, and the REE are normally considered tobe immobile during low-temperature hydrothermal alteration (e.g.,Saunders et al., 1980), the possibility of a primary magmatic originfor the trace element peculiarities must be examined. Because lowerMgO samples tend to be the most affected by P and Y enrichment, itis possible that an assimilation/crystallization or magma mixing pro-cess may be responsible. To test this hypothesis, a single massive flowcontaining a range of phenocryst abundances and MgO contents wasexamined. Subunit 14a (Fig. 3) is a suitable flow with recognizableupper and lower contacts and good recovery. Six whole-rock sampleswere analyzed from this flow with a view to examining its internalchemical variations.

Geochemical profiles throughout the Subunit 14a flow are shownin Figure 9. Phenocrysts tend to be concentrated in the center of theunit, reflecting a flowage differentiation during extrusion. Variations

within major element, Zr, Ga, Cr, Ni, Sr, and V profiles can beexplained by dilution of the erupted liquid by the observed phenocrystabundances. However, the profiles for P, Y, K2O, and Rb do not fit asimple dilution model. K2O and Rb are enriched by factors of 3-4between the low- and high-MgO samples, far greater than the enrich-ment factors of 1.3 -1.5 for Sr, Ti, Zr, and Ga. Phosphorus and yttriumare also over-enriched in the lower MgO samples, however, in con-trast to Rb and K2O, concentrations of these elements are greatest inthe two samples from the base of the flow. The enrichment in Pbetween the center and the base of the flow is by factors of 18-22.Similar P and Y anomalies were found throughout the section but areonly recognized within massive and pillowed flows and were absentin samples taken from breccia units (see P-Zr and Y-Zr plots, Fig. 6).

In light of the known mobility of elements such as K and Rb duringalteration, enrichment in the low-MgO sections of Subunit 14a canbe explained by hydrothermal redistribution of these elements withinthe flow. On a broad scale, samples with high K2O also tend to havegreater concentrations of P and Y. However, the over-enrichment ofP at the base of Subunit 14a suggests that another event or processmay have disturbed the original igneous concentrations. Clearly,assimilation of host lithologies, either before or after eruption, isunlikely to produce such extreme enrichment within a single flow.

The most likely explanation for anomalous P, Y, and the rare-earthelement (REE) concentrations relative to HFS elements is a low-tem-perature hydrothermal alteration process. Bienvenu et al. (1990) havedescribed the systematic removal of REE, Y, and P and retention ofZr and Ti during alteration of mid-ocean ridge basalt (MORB) glassesto smectite. In the case of these oceanic lavas, the trivalent elementsand P were removed from the system during fluid (seawater) interac-tion. In contrast, Zr and Ti remained in the glass/smectite systembecause of their high ionic potential and hydrolytic properties. Similarsmectitization took place in Hole 793B lavas during hydrothermalcirculation within the basement after eruption. However, the hetero-geneous nature of the Hole 793B basement (namely, the intercalationsof breccias and flows) has clearly affected fluid flow and elementredistribution. Breccia units, which lack the anomalous P, Y, and REEvalues (Fig. 6), may act as permeable pathways for the efficientremoval of fluids from the system without deposition of the solute.In contrast, massive and pillowed flows are less permeable and simplyredistribute the mobilized elements internally. This has producedregions within flows where element leaching has occurred, andother areas (notably the base of Subunit 14a) where precipitationis dominant. The behavior of Mg, Fe, and Al with the sequence

415

21

20

19

18

16

15

14

13

12

11

1010

30Zr (ppm)

I * '• ••• ft* *•«%••'

• a

0 °

0 O

20 30Zr (ppm)

200

150 -

100 -

50 -

10

•

•

-

#>.*%••

•J

•

•>

•

••

8 ' °8o° 8

800

700

600

500

400

300

200

100

010

20 30Zr (ppm)

40

30Zr (ppm)

40

50

•*

•a• ••• *•••

A-

-

•••• •

t

<

•J*••

°o°oo

o

50

O5 14

13

12

11

10

-

-

-

••

i

•

•

• ••

0

0 O

o

10 20 30Zr (ppm)

40 50

0.4

^•0.35

"CM

OH 0.3

0.25

0.2

•*•••• ••••a m• «••M > ••«M»•••*•

o

o o oo

20 30Zr(ppm)

40 50

10

9

8

? 7Q.

S 6w 5

§ 4

3

2

1

0

•

o

•• •

30Zr (ppm)

4 0

Figure 6. Trace elements (ppm) and Tiθ2 and AI2O3 (wt%) plotted vs. Zr (ppm) for Hole 793B basement. Open circles = Group A lavas, and filled circles = Group B lavas.


10

ICO

Q .

roCO

Ba Rb

Figure 7. Trace element concentration of Sample S-21 (3.24 wt% MgO)normalized to Sample S-13 (11.58 wt% MgO). Also shown are predictedprofiles for magmas generated by 30% and 80% crystallization (x'tal) ofSample S-13. Crystallization modeled using distribution coefficients fromHenderson (1982) and phase proportions cpx:opx:plag = 5:4:1.

appears to be commensurate with crystal fractionation/accumulationprocesses. This can be ascribed to the locking of these elements in thesaponite/celadonite/illite alteration products that replace the interstitialglass (for details of clay chemistry, see Lapierre et al., this volume).

Interestingly, Sr, which is normally considered to be mobile,shows a variation within the sequence commensurate with primaryigneous fractionation. This Sr behavior can be explained by itslocation within the whole-rock system. All lavas within Hole 793Bhave abundant Plagioclase laths and microlites in their groundmass.Strontium would behave incompatibly during ol-opx-cpx crystal-lization, concentrating in the liquid with Zr, Ti, P, and the REE.However, during final solidification, Sr would partition strongly intothe Plagioclase microphenocrysts, leaving the incompatible elementsin the interstitial glass. During alteration the glass underwent smecti-

Table 4. Rare earth element analyses, Hole 793B basement.

Sample S-12 S-20Core, section 96R-1 112R-1Interval (cm) 81-83 cm 59-63 cm Detection

Unit 4 14a limits

LaCeNdSmEuGdDyEr

YbLu

1.894.953.631.200.421.571.540.991.030.18

5.513.281.040.391.521.781.331.520.28

0.10.1

0.020.0150.015

0.20.020.020.02

0.015

All concentrations in parts per million.Analysis by isotope dilution mass spectrometryusing the technique described by Croudace andMarshall (1991), atthe Department ofGeology, University of Southampton, U.K.

_^_S-12 (10.89 wt% MgO, 11.3 ppm Y)_a_S-20 (7.03 wt% MgO, 18.5 ppm Y)

Figure 8. Chondrite-normalized, rare-earth element (REE) profiles for Hole793B basement lavas. Chondrite values from Kay and Gast (1973).

tization (and possibly illitization) causing partitioning of REE, P, andY into circulating fluids and leaving hydrolyzed Ti and Zr in or aroundclays, and the Sr locked in Plagioclase. Sr isotope results (see below)clearly indicate that a minor isotopic exchange between rock andseawater-based fluids has taken place. However, strontium concen-trations remain close to the original magmatic abundances.

Forearc Diabase Intrusive

A diabase sill intruding 14-Ma sediments was encountered at adepth of 586 m in Hole 793B. Petrographically, the intrusion containsphenocrysts of clinopyroxene, orthopyroxene, and Plagioclase andpseudomorphs of olivine. Titanomagnetite was found throughout theintrusion, but it is restricted to the groundmass. Further details of theinternal structure and petrography are given in Taylor, Fujioka, et al.(1990) and of the mineral chemistry in Lapierre et al. (this volume).

Major and trace element geochemistry of the intrusion (Table 5)indicates a tholeiitic affinity. Trace element ratios such as Ti/Zr (100),Zr/Y (2), and Ti/V (12) of the diabase are similar to those of the activeIzu arc volcanoes of Tori Shima and Sumisu Jima (Shipboard Scien-tific Party, 1990). A tholeiitic basalt clast derived from the active arcwas found in early to middle Miocene sediments (610 mbsf) ofHole 793B. This clast has a similar composition (Sample 012, Table 5)to the diabase, supporting the association of the sill with the active arc.

Laterally extensive seismic reflectors were found across the fore-arc region (B. Taylor, pers. comm., 1990), which may representintrusions such as the diabase. In light of these reflectors and thediabase chemistry, it is likely that the intrusion represents a continu-ation of arc magmatism into the forearc region, or the lateral injectionof magmas from the active arc into forearc sediments.

SITE 792

The Oligocene Arc

Multichannel seismic profiles across the Izu-Bonin arc have shownthe presence of a series of basement promontories, spaced about every60-100 km, and located 30-40 km east (trenchward) of the activearc chain (Taylor et al., 1990). These frontal-arc basement highs wererecognized by Honza and Tamaki (1985), who named them theShinkurose Ridge. Initial interpretations designated the ridge as theremnants of an earlier arc-volcano chain, now buried by sedimentsduring the formation of the forearc basin. Hole 792E drilled through

417

R. N. TAYLOR ET AL.

15 100 200 300 400 500 600 700

- MgO (wt %)

100 120 140 16'

g 1646 -079

190 200 210 220 230 240

MnO _,-TiO2 wt % -V (ppm)

- » - K 2 θ ( w t % ) -^Y (ppm) P2Os (wt%) _».Rb(p

Figure 9. Geochemical profiles through Subunit 14a, Hole 793B basement. Sample numbers used are shown on the left.

418


the marginal sediments of the basin and penetrated 82 m of basementvolcanics on a basement high at 32°24'N (Fig. 2). The sedimentsoverlying basement were constrained biostratigraphically and mag-netostratigraphically as late Oligocene (30-32 Ma), whereas base-ment volcanics were radiometrically dated as 33 Ma. Details of thematerials used, analytical procedures, and interpretation of the radio-metric ages are given in Taylor and Mitchell (this volume).

Sediments in the 10 m directly overlying basement are hydrother-mally altered volcaniclastic sandstones and conglomerates with clastsof andesite, claystone, and extremely fresh amphibole crystals. Thematrix is dominantly blue-grey smectite, replacing original vitricdetritus. These sediments suggest that post-eruptive hydrothermalcirculation above the basement was concentrated in a restrictedhorizon. Alteration was probably induced by low-temperature (<200° C)porewater circulation, in a similar fashion to the vitric-sediment/pore-water interaction envisaged for the overlying sediments by Egeberg(1990). Initial studies of the basement lavas indicate a similarlylow-temperature alteration with partial replacement of some originalphases by smectite, zeolite, and carbonate.

Volcanic .Stratigraphy and Eruptive Setting

Basement volcanics in Hole 792E are subdivided into five majorunits (Fig. 10). The dominant lithology is massive flows of two-pyroxene andesite (estimated at 5-10 m thickness Taylor, Fujioka, etal., 1990) with subordinate dacite, rhyolite, and basaltic andesite.These are intercalated with hyaloclastite breccia layers containingsimilar mineral species and proportions to the massive flows, but theyhave matrices of smectitized vitric shards. A single heterolithic volcano-genie breccia is found in the volcanic sequence (Unit 4). This containssubrounded clasts similar to the surrounding flows and a single clastof prehnite-chlorite-bearing meta-basalt. Detailed descriptions of thelithologic units are given in Taylor, Fujioka, et al. (1990).

The combination of hyaloclastitic layers and a heterolithic sedi-mentary horizon suggests that the volcanics were the products ofsubmarine eruptions. This is consistent with a number of scenarios,any or all of which are plausible: (1) that the volcanic edifice wasnever emergent above sea level, either because of arc-rifting duringconstruction, or cessation of arc volcanoes, (2) that Site 792 waslocated on the submerged flank of an arc volcano, and/or (3) that theoriginal volcano has been denuded to its submarine base by sub-sequent erosion.

Petrography

Basement volcanics within Hole 792E are petrographically similarto many calc-alkaline lavas in the active Japan Arc. Plagioclase is anabundant and ubiquitous phenocryst (20-30 modal%) throughout thesequence; it is found associated with two pyroxenes: titanomagnetiteand sometimes quartz (Plate 2). The crystallization sequence of plag+ oxide —» opx + cpx —> quartz is also typically calc-alkaline. Modalanalyses of the lavas are given in Table 6. Quartz is found in all lavas fromthe lowest 55 m of the stratigraphy (Fig. 10), regardless of their whole-rock SiO2 contents. A brief outline of the mineralogy is given below; amore detailed analysis is given in Lapierre et al. (this volume).

Magnetite

Subhedral grains 0.3-2 mm in size, existing as phenocrysts in thegroundmass and as inclusions within clino- and orthopyroxene.

Plagioclase

Euhedral to subhedral fresh phenocrysts 0.5-5 mm in size. Strongoscillatory zoning is common, with compositions ranging from An^to An84. Fluid and glass inclusions are abundant, but are restricted toa growth zone between core and rim.

Clinopyroxene

Euhedral to subhedral unaltered phenocrysts 0.3-3 mm in size.Zoning is prevalent in most crystals with compositions En49:Wo40:Fs11-En38:Wθ43:Fs19. Magnetite and glass are found as inclusions inthe larger phenocrysts.

Orthopyroxene

Subhedral phenocrysts 0.3-3 mm in size are found throughout thesequence; however, with the exception of the uppermost 15 m of thestratigraphy, are pseudomorphed by clays. The orthopyroxene in theupper flows of Unit 1 have fresh cores mantled by alteration zonescomprising a mixture of smectite and cristobalite.

Quartz

Occurs as subrounded to rounded euhedral grains < 0.1-0.7 mmin size.

Geochemistry

We analyzed a series of 26 basement volcanics from Hole 792Eto constrain the geochemical characteristics of the Oligocene Izu-Bonin arc. Major and trace element data are presented in Table 7. Themajor element data reveal a diverse compositional range within the82 m of basement drilled. Samples range from basaltic andesites,through andesites and dacites, to rhyolite (about 53-72 wt% SiO2 and1-7 wt% MgO). Evolutionary trends within the major element data(Fig. 11) are typical of calc-alkaline suites, with a decline in total ironand a steady increase in SiO2 with decreasing MgO contents.

As the basement lavas are to some extent altered, it is importantto assess how secondary processes have affected original chemicalcompositions. Petrographically, the freshest samples are in the upper20 m of Unit 1 and the most altered lavas are found in Units 3-5. Theconsistency of most immobile elements within the various flows ofUnit 1 also suggests that these andesites are close to their unalteredcompositions. The contrast between the upper and lower units of thesection is paralleled by chemostratigraphic variations, particularly inthe distribution of MgO, as shown in Figure 10. Units 3-5 areintercalations of basaltic andesite, andesite, and dacite/rhyolite whereasUnits 1 and 2 are entirely andesite. It is, therefore, a possibility thatthe extremes found in the lower units are simply a function of analteration process. Support for this comes from observations that thebasaltic andesites are fractured and friable, suggesting a fluid-relateddensity decrease, whereas the rhyolitic rocks are hard and havesilicified groundmasses, commensurate with bulk silica addition.

Lavas from Hole 792E show similarly anomalous trace elementfeatures to the forearc basin basement of Hole 793B. Notably,Hole 792E lavas show large variations in P and Y (Fig. 12) notcommensurate with the quantities of crystallization estimated frommajor element compositions. This is investigated further by com-paring average analyses of basaltic andesites, andesites, and dacites.Figure 13 shows the relative enrichment/depletion of trace ele-ments in average basaltic andesite and dacite relative to averageandesite. Contrary to normal fractionation systems, the basalticandesite has higher Zr and lower Ni and Cr compared with averageandesite. Furthermore, the basaltic andesite has a factor of 2-3less P, Y, Rb, K, and Ba. The average dacite could be derived fromthe andesite by fractional crystallization in light of the lower Ni andCr. However, the dacite profile in Figure 13 has similar levels of Ti,Zr, and Sr to the basaltic andesite, combined with a relative enrich-ment of P, Y, and the LFS elements.

The basaltic andesites have similar phenocryst mineral propor-tions to the andesites, with the exception of the andesites at the topthat lack quartz. As the basaltic andesites are more altered than theandesites, the possibility is raised that the basaltic andesites are simply

419

Table 5. Diabase sill and basalt pebble geochemistry, Hole 793B. Ti


Major elements


Total

Trace elements

PZrYGaRbSrBaVNbNiCrZnCuKMINQ

Sm

1R-11/35-39

586.79001

IDiabase

(wt%)

52.840.58

14.8510.610.168.56

10.191.940.260.060.48

100.05

(ppm)

28134.417.914.9

2.8116

372880.476

1676958

100Mg/Mg+Fe(t) 64.0AI203/Ti02CaO/TiO2CaO/AI2O3Ti/ZrZr/Y

25.617.60.691011.9

1R-11/73-77

587.23002

IDiabase

52.520.52

14.709.680.179.34

10.851.860.110.050.37

99.80

28732.817.215.4

1.8119

522520.099

2526684

68.028.320.90.74

951.9

1R-11/115-119

587.65003

IDiabase

52.650,57

14.4410.340.198.95

10.821.720.270.050.20

100.00

26135.218.414.7

3.61161032840.093

22572

109

65.625.319.00.75

971.9

1R-11/132-136

587.82004

IDiabase

52.070.53

14.4610.030.199.41

11.011.640.260.050.55

99.65

24932.617.414.9

4.1113

452660.0104260

70112

67.427.320.80.76

971.9

1R-22/15-19

588.15005

IDiabase

52.240.53

14.3710.170.199.68

10.941.630.200.050.46

100.00

25831.917.214.2

3.611246

2610.0113249

7084

67.727.120.60.761001.9

1R-22/25-31

588.27006

IDiabase

52.610.58

14.5110.440.199.10

10.801.700.180.050.33

100.16

28235.018.415.1

2.911764

2800.597

2337193

65.725.018.60.74

991.9

1R-22/61 -65

588.60007

IDiabase

53.100.58

14.689.800.168.99

10.491.880.320.060.30

100.06

29636.418.114.94.311666

2770.0100255

7399

66.925.318.10.71

962.0

1R-22/91 -95

588.92008

IDiabase

52.870.56

14.809.820.158.80

10.271.970.320.050.53

99.61

27335.317.314.8

4.2117

552650.189

2006797

66.426.418.30.69

952.0

1R-33/1-5589.36

009I

Diabase

52.990.59

14.8610.050.168.67

10.391.930.360.050.25

100.05

29335.517.914.8

5.8117

672800.392

2207295

65.525.217.60.701002.0

1R-33/27-31

589.58010

IDiabase

53.050.61

14.7110.050.168.72

10.282.030.340.060.36

100.01

30338.617.715.1

5.5116

502820.791

22472

105

65.624.116.90.70

952.2

1R-33/77-82

590.12011

IDiabase

52.580.56

14.639.650.179.26

10.761.960.280.060.68

99.91

27834.217.714.44.811445

2900.3106275

7288

67.926.119.20.74

981.9

3R-11/95-98

605.00S-1

IBasalt

52.810.66

17.0710.420.165.27

11.701.910.300.040.33

100.34

52.725.917.70.69

3R-11/98-100

605.28012

IBasalt

52.680.65

16.7610.240.185.33

11.612.130.340.060.00

99.98

17937.621.115.9

5.1140

323230.425

1027990

53.425.817.90.691041.8


core no. unit

graphic tog

VI

850

alteredsandstone

massiveflows ofporphyritictwo-pyroxeneandesitθ

quartz out

quartz in

hyaloclastitebreccia

massiveandesitβflows, withhyaloclastltθand alteredglassy lava.

volcanic breccia

massiveporphyriticandesiteflows withintercalatedhyaloclastitehorizons

MgO (wt%)

2 3 4 5

I I I I

sedimentarycover

massive 8|flows $ji

dark, friablelava horizon

$& volcanic§S breccia

hyaloclastite

Figure 10. Stratigraphic summary of basement rock sequence in Hole 792E, Cores 126-792E-70R

through -78R. Recovery within each core is proportionally expanded to fill the total core penetration.

MgO values are plotted vs. depth for all samples analyzed from coherent igneous units.

andesites that have undergone hydrothermal alteration. The net resultof the alteration is a decrease in silica, P, Y, and LFS elements with aresultant increase in the contents of the immobile elements Ti and Zr.Rhyolite and, to a limited extent, dacite compositions are likely to bethe result of net deposition of the solute elements along particularhorizons. As with the Hole 793B basement, Sr is apparently relativelystable during the leaching process and can again be explained by theretention of Sr in unaltered Plagioclase.

RADIOGENIC ISOTOPE GEOCHEMISTRY OFFOREARC VOLCANICS

Basement volcanics from Holes 793B and 792E together with asample from the Neogene intrusion in Hole 793B were analyzed forSr, Nd, and Pb isotopes. Lead isotopes were analyzed at RoyalHolloway and Bedford New College, London, using a VG354 five-collector mass-spectrometer with static data collection. Ratios werenormalized for mass fractionation to SRM981 by about 0.11%/amu.

Internal errors are estimated at better than 0.005%/amu and repro-ducibility at better than 0.05%/amu. Sr and Nd isotopes were meas-ured at Universite Blaise Pascal, Clermont-Ferrand, France. Pb, Nd,and Sr isotope data are presented in Table 8.

Sr-Nd Isotopes

The nine samples from Hole 793B basement have consistenti43Nd/i44Nd r a ü o s (Fig_ 1 4 ) ? w i t h ε N d ranging from 5.63 to 6.82 (t =30 Ma). 87Sr/86Sr ratios are slightly more dispersed (0.7038-0.7044)indicating that basalt-seawater interaction or hydrothermal fluidshave disturbed the original proportions of radiogenic Sr. However,143Nd/144Nd ratios do not correlate with enrichment of the LFSelements or the unusual P and Y variations. This suggests that despitethe apparent mobility of the REE during basement alteration, thefluids were of a similar Nd-isotopic composition to the lavas. As such,we propose that the fluids causing the redistribution of the REEobtained their REE solute from within the lava pile. If the fluids did

421

R. N. TAYLOR ET AL.

Table 6. Modal mineralogy of basement rocks, Hole 792E.

Sample/analysis

A -BCD S31E S-34

Core, section,interval (cm)

71R-CC, 1074R-1, 14674R-1, 14676R-1, 6178R-1, 134

Depth(mbsf)

812.6840.3840.3858.3877.9

Plag

36.523.130.735.532.8

Opx

5.00.15.96.56.5

Cp×

3.56.31.81.24.4

Qtz

—-

1.51.3

Opaques

1.78.92.41.21.3

Notes: All analyses performed by point counting (>400 counts per section).Reported values are percentages, with groundmass the residual percentage.Plag = Plagioclase, Opx = orthopyroxene, Cpx = clinopyroxene, Qtz = quartz.

A = Porphyritic andesite, Unit 1B = Basal 1.4cm-thick layer of hyaloclastite breccia, Unit 2C = Top of andesite flow - Unit 3, directly beneath sample b.D = Porphyritic quartz-bearing andesite, Unit 5.E = Porphyritic quartz-bearing andesite, Unit 5.

circulate through sediments or lower regions of the basement, eitherthey did not scavenge significant quantities of REE in these locations,or these lithologies had similar 143Nd/144Nd systematics.

In comparison to other lavas in Izu-Bonin arc-trench system (Fig. 14),the Hole 793B basement has lower εN d than MORB and the activearc, and it lies at the upper end of the spectrum of boninites from theIzu-Bonin outer-arc high.

Pb Isotopes

Lead isotopes were measured on six samples from Hole 793Bbasement, three samples from Hole 792E, and one sample from theNeogene diabase sill in Hole 793B. Figures 15 and 16 compare 207/204pt>206/204pb a n d 208/204pb.206/204pb ^ from h&g m ^ m a y s e s from

potential components and lavas from the western Pacific environs.Lavas from the forearc basement at Sites 792 and 793 lie within therange of isotopic compositions of Pacific MORB, on or slightly abovethe Northern Hemisphere reference line (Δ7/4 and Δ8/4 values rangefrom -1.74 to 3.3 and 7.3 to 37.4, respectively). The diabase sill liesbetween the Pacific MORB and western Pacific sediment fields onboth 207/204pb.206/204pb a n d 208/204pb_206/204pb p l o t s ( Δ ? / 4 a n j Δ 8 / 4 ^

7.2 and 52.4, respectively).Unlike the Bonin Islands (Dobson and Tilton, 1989), no clear

evidence for a 207/204Pb-rich sedimentary component was found in thebasement lavas of the forearc basin. However, the 2 0 7 / 2 0 4pb datumfrom the diabase sill (Fig. 15 and 16) suggests that sediments mayhave played a role in its petrogenesis. Contamination of the sillmagma could have taken place either in the mantle source by intro-duced slab sediments or by digestion of host sediments during intru-sion. The lack of available Pb data for Izu arc lavas of comparablecomposition as well as intruded sedimentary horizons precludes aninterpretation regarding the nature of contamination.

PETROGENETIC RELATIONS BETWEENARC-TRENCH MAGMAS

The occurrence of boninite series volcanics (BSV) in the forearcterrains of the Western Pacific has been well documented (e.g.,Dietrich et al., 1978; Hickey and Frey, 1982; Umino, 1986; Bloomerand Hawkins, 1987). However, the question remains as to whetherthe current location of boninite lavas bears any relation to theiroriginal position with respect to the subduction zone and arc. Drillingduring Leg 126 sampled igneous units within the forearc region

which, combined with age constraints, suggest that the boninites weregenerated within current tectonic framework.

The Hole 793B diabase sill is younger than about 12 Ma and wasrecovered from the center of the forearc basin. However, its similarcomposition to Torishima and other Izu arc volcanoes, together withthe possibility of lateral injection from the arc, do not allow thisintrusion to be constrained as forearc volcanism. Age relations be-tween basement at Sites 792 and 793 do provide an insight into thetemporal development of the arc-trench crust. The calc-alkaline vol-canic edifice drilled at Site 792, combined with the seismic evidence,demonstrates that during the Oligocene (about 30-33 Ma) a well-de-veloped arc-chain existed in a similar orientation to the modern arc.Basement lavas from Hole 793B, trenchward of Site 792, are inter-preted as being in the age range from 26 to 33 Ma, (Taylor andMitchell, this volume). These lavas were clearly produced in a forearcenvironment in response to basin formation between arc and outer-archigh. As such, Site 793B basement represents one of the few well-constrained examples of forearc volcanism.

Comparable tectonic structures and age relationships along thelength of the Izu and Mariana subduction systems suggest that littletectonic erosion has taken place between the outer-arc high and trenchin the duration of this subduction system. The absence of thrust-re-lated structures across the arc suggests that tectonic units within theforearc have not been juxtapositioned. Hence, it is likely that theouter-arc high lavas were also generated in a forearc location.

In light of the consistent relative positions between tectonic units,Taylor et al. (1992) examined spatial geochemical variations withinthe Izu-Bonin arc-trench region. These authors found that indexes ofmantle depletion (e.g., TiO2, Y, and Al2O3/TiO2) displayed a gradualtransition from the arc toward the trench. Figure 17 shows the declinein TiO2 contents of lavas from the backarc to the forearc, reflecting amore depleted mantle source toward the trench. On the active arc, thelowest-Ti lavas are primitive island arc tholeiites, similar to theHole 793B diabase intrusion (about 0.5 wt% TiO2 at 9 wt% MgO). Inthe forearc rift-basin, basement lavas are low-Ti tholeiites transitionalto boninites (0.3 wt% TiO2 at 9 wt% MgO). Boninitic volcanics closestto the trench have the lowest TiO2 concentrations (0.15 wt%). Thislateral variation in geochemistry can be related to a compositionallystratified mantle combined with variable melting location in responseto slab depth (Taylor et al., 1992).

Lavas from the Izu-Bonin forearc basin are transitionalbetween arc tholeiites and boninites in terms of their petrogra-phy and trace element depletion. However, an important char-

422


0.7

0.6 -

j 0.5

o

0.4 -

0.3 -

0.2

-

- <P

1

\ °o

1

•

\

1 1

*

3 4 5MgO wt %

3 4 5MgO(wt

80

75 -

70 -

1 65 -

60 -

55 -

50

45

0 °<P o

-

1 1

i

i i i i i

3 4 5MgO (wt%)

3 4 5MgO (wt %)

3 4 5MgO (wt%)

Major elements plotted vs. MgO for Hole 792E basement (all values in wt%).

423

Table 7. Basement geochemistry, Hole 792E.


70R-CC16-20805.00

0991

Andesite

71R-19-13

810.00S-24

1Andesite

71R-118-21810.38

1001

Andesite

71R-289-92815.66

1011

Andesfte

71R-334-38818.20S-25

1Andesite

72R-1100-104

824.59103

1Andesite

72R-1111-115

825.50S-26

1Andesite

73R-115-18829.50S-27

1Andesite

73R-118-21829.84

1041

Andesite

73R-172-76831.60

1051

Andesite

73R-23 - 7

835.12106

1Andesite

73R-253-57845.00S-28

3Andesite

74R-2104-108

847.10110

3Andesite

75R-118-22849.11

1123

Dacite

75R-149-51850.00

1133

Bas.and



57.470.53

17.807.260.313.339.982.410.190.151.77

57.480.54

18.926.900.153.289.672.310.230.111.01

57.120.58

18.427.310.253.179.992.550.280.110.96

57.440.55

17.698.190.323.568.822.370.550.070.93

57.550.56

18.208.650.144.179.062.210.150.050.81

56.770.54

18.327.420.133.359.792.430.180.471.26

57.480.55

19.047.840.083.689.502.180.130.131.05

57.390.56

17.939.260.104.688.991.980.140.041.10

56.550.55

17.648.990.134.498.702.400.160.060.98

57.790.56

18.158.050.133.768.842.470.200.060.53

56.830.53

18.538.070.183.829.402.450.170.091.30

59.910.54

17.507.170.053.418.212.570.200.041.44

59.390.52

17.657.950.093.908.062.660.170.061.17

64.200.53

17.705.240.091.168.342.550.280.090.72

52.570.51

17.1512.560.177.117.632.100.170.031.71

Total 99.43 99.59 99.78 99.56 100.74 99.40 100.61 101.07 99.67 100.01 100.07 99.60 100.45 100.18 100.00




63539.224.516.03.0172

182490.0122758

50.233.618.80.56

811.6

39.920.3

1.417631

2580.9

9128634

51.135.017.90.51

812.0

44441.125.715.83.317526

2670.1122974

48.831.817.20.54

851.6

28139.020.815.810.316624

2510.2122616

48.932.216.00.50

851.9

39.920.3

1.417631

2580.9

9128634

51.532.516.20.50

842.0

224440.446.816.33.118524

2530.2116666

49.833.918.10.53

800.9

42.228.1

1.2188

162601.3

9118228

50.834.617.30.50

781.5

41.421.8

1.3175

122441.1

9129327

52.732.016.10.50

811.9

25641.122.816.03.1170

192480.0139773

52.432.115.80.49

801.8

25741.822.515.83.117026

2540.3126372

50.732.415.80.49

801.9

37641.125.015.83.4177

102370.1112066

51.035.017.70.51

771.6

38.718.4

2.416628

2420.6

60

7939

51.132.415.20.47

842.1

22741.723.715.83.6167

151970.0101079

51.933.915.50.46

751.8

35541.827.216.04.316024

2430.0

61963

32.833.415.70.47

761.5

9245.3

7.214.22.81803911620.6

818

109

55.533.615.00.44

676.3





Total

Trace elements (pprn


100Mg/Mg+Fe(t)AI2O3/TO2CaO/TiO2CaO/AI2O3Ti/ZrZ •/Y

75R-150-55850.50S-29

3Bas.and

53.260.53

17.6612.370.166.477.851.800.120.022.23

100.24

43.85.9

1.615533

1860.5

50

13418

53.533.314.80.44

737.4

75R-269-73856.56

1143

Dacite

62.670.54

17.635.840.072.008.172.500.160.090.90

99.67

36842.326.016.23.616523

2350.1

81658

43.032.615.10.46

771.6

76R-143-46860.10S-30

5R.dacite

73.240.41

14.032.920.020.876.132.230.240.061.28

100.15

34.715.4

2.713442

1531.4

00

4126

39.634.215.00.44

712.3

76R-160-64861.00S-31

5Bas.and

55.280.56

19.3210.040.145.368.312.040.140.023.10

101.21

48.76.9

2.1166

22470.7

72

9817

54.034.514.80.43

697.1

76R-164-68860.52

1165

Bas.and

55.050.57

18.879.810.165.358.172.190.200.042.27

100.41

13653.8

7.315.73.4159

02160.3

93171

54.633.114.30.43

647.4

76R-185-87862.20S-32

5R.dacite

74.040.40

14.433.720.020.796.232.310.220.041.49

102.20

35.722.4

3.11382111251.0

42

7831

31.936.115.60.43

671.6

77R-13 - 7

867.57117

5Dacite

62.390.52

17.915.490.061.347.973.300.480.060.00

99.52

26843.121.116.66.017341

3380.4

62250

34.934.415.30.45

722.0

78R-171-74879.00

1185

Andesite

59.020.50

18.487.220.083.418.612.720.170.100.38

100.31

42245.219.216.03.3167

152050.0

92063

51.037.017.20.47

662.4

78R-180-85880.20S-33

5Andesite

58.410.51

18.747.940.073.968.882.300.140.041.62

100.99

43.710.3

1.2170

62600.7

58

8028

52.336.717.40.47

704.2

78R-1131-136

883.00S-34

5Dacite

64.870.49

17.775.080.031.758.352.450.370.170.69

101.33

42.330.6

3.916243

2331.2

44

5518

43.136.317.00.47

691.4

78R-1132-135

884.50119

5Dacite

63.060.51

18.095.080.061.458.402.680.400.180.00

99.91

66141.629.216.26.116336

2250.6

72540

38.635.516.50.46

731.4

o

R. N. TAYLOR ET AL.

0.4

S? 0.3

wt.

om

cf 0.2

0.1

0

-

$**•

o .?>«

•

•

°o

•

•oo»•

1 1

10

10 20 30Y(ppm)

40 50

60

55 -

50

E

N

40 -

35 -

30

*

*

*

* •

o

•

o

••o

°o °

•

10 20 30Y(ppm)

40 50

Figure 12. P2O5 and Zr plotted vs. Y for Hole 792E basement.

acteristic of boninitic volcanism is the nature of enrichments to theirsource. Figure 18 combines two parameters that distinguish boninitesfrom arc and oceanic basalt compositions. Western Pacific boniniteshave lower Ti/Zr ratios and εN d compared with MORB and lavas fromthe active Izu-Mariana arcs. These features have been ascribed toenrichment in light REE and Zr relative to middle/heavy REE, Ti, andY (Sun and Nesbitt, 1978; Hickey and Frey, 1982; Cameron, 1985;

T3

TO<Dσ>raα>TO

0.1

average dacite

average basaltic andesite

Ba Rb K Sr Zr Ti Y Ga V Ni Cr

Figure 13. Average trace element compositions for average dacite (Unit 5) andaverage basaltic andesite (Unit 5) normalized to average andesite (Unit 1).

Taylor and Nesbitt, 1988). Samples from the Hole 793B basement areintermediate between arc and boninite in terms of both parameters inFigure 18.

It is notable that the Site 793 lavas are geochemically very similarto Site 458 bronzite andesites from the Mariana forearc basin. Lavasfrom both these sites are intermediate in composition between respec-tive active arc and outer-arc high lavas (e.g., Fig. 18). This is taken toreflect their similar tectonic setting (forearc basin) and age (Hussong,Uyeda, et al., 1981; Taylor and Mitchell, this volume). Geochemicaltransects across the Izu-Bonin and Mariana arcs demonstrate a shal-lower source for the basin volcanics relative to the arc volcanics(Taylor et al., 1992), and a parallel mantle evolution along the lengthof the Izu-Mariana system.

CONCLUSIONS

Drilling during Leg 126 determined the composition and nature oflavas within the Izu-Bonin forearc basin. Basement at the center of thebasin (Site 793) consists of Oligocene synrift volcanics. These aretwo-pyroxene basaltic andesites, with HFSE contents lower than theactive Izu arc. On the western edge of the forearc basin (Site 792), lavaswere recovered from a basement promontory, postulated as a relict arcvolcano. The volcanics from this site are massive flows and breccias oftwo-pyroxene andesite and dacite with calc-alkaline affinities.

At Site 793, basement lavas show anomalous behavior of P, Y, andREE relative to HFS elements. These can be attributed to posteruptivefluid-rock interaction, which redistributed P, Y, and REE within thebasement system. Similar discrepancies are found in the Site 792 lavas,

Table 8. Nd, Sr, and Pb isotopic results from Hole 793B.

CoreIntervalSample numberLithology

1R-261-65

007diabase

86R-1128-131

020cpx-bas

88R-165-68

S-8aph bas

92R-270-74

030cpx-pül

92R-325-28S-10

cpx-pill

97R-1124-127

S-13cpomas

99R-154-55S-14

cpx-pill

104R-165-70

053cpomas

104R-249-54S-18

cpc mas

104R-269-75

055cpc mes

105R-1127-131

S-17cpx-pill

110R-11-3

S-19c-spot

111R-1110-115

079cpc mas

112R-159-63S-20

cpc mas

113R-3137-141

S-21aph bas

113R-435^11

088aph bas

SrNdSm

143/144Nd

87/86Sr

εNd

206/204pu207/204p.206/204 p k

116

18.3515.5638.33

129

18.2015.4937.93

1564.641.48

0.512960.70377

6.4

135

18.1815.4637.81

1635.921.73

0.512960.70410

6.4

1402.460.84

0.512960.70384

6.2

1403.691.21

0.512950.70397

6.1

161

18.2215.4837.91

1785.611.78

0.512990.70383

6.8

201

18.2115.4737.87

1292.770.97

0.513030.70394

5.6

1473.461.06

0.612960.70383

6.3

152

18.1615.4737.84

1873.281.04

0.512960.70439

6.2

1967.362.32

0.512970.70389

6.5

195

18.1715.4437.80

426


0.703 0.708

Sr

Figure 14. 8Nd plotted vs. 87Sr/86Sr for Hole 793B basement. Fields for Izuand Mariana arcs from Woodhead (1989) and Nohda and Wasserburg (1981);ODP Leg 125 fields for Hole 786B from Pearce et al. (1992); and Chichi Jimaand Haha Jima from Taylor, Nesbitt, and Vidal (unpubl. data).

with more extreme element redistribution. Groundmass changes andfluid movements have produced SiO2-rich and SiO2-poor meta-andesiteswith characteristic concentration and dilution of HFS elements.

Age relationships of basement lavas from the two forearc Sites(792 and 793) indicate that lavas from the forearc basin are youngerthan the Oligocene arc at Site 792. This demonstrates that the currentIzu-Bonin forearc crust was in a forearc location during Oligocenerifting and experienced synrift volcanism.

Trace element and isotopic evidence indicates that the Hole 793Bbasement volcanics are transitional between arc-tholeiite and boninitechemistry. This suggests that their intermediate location between theouter-arc high (boninitic) and the active arc (arc-tholeiite) can berelated to a gradual change in melting environment toward the trench.

ACKNOWLEDGMENTS

This work was supported by Natural Environment Research CouncilGrant No. GST/02/416 (RNT and RWN). RNT would like to thankall Leg 126 participants for a good cruise, and I. Chaplin for rockpreparation logistics. The reviewers are thanked for their helpful andconstructive comments.

REFERENCES

Bienvenu, P., Bougault, H., Joron, J. L., Treuil, M., and Dmitriev, L., 1990.MORB alteration: rare-earth element/non-rare-earth hygromagmaphileelement fractionation. Chem. Geol., 82:1-14.

Bloomer, S. H., and Hawkins, J. W., 1987. Petrology and geochemistry ofboninite series volcanic rocks from the Mariana trench. Contrib. Mineral.Petrol, 97:361-377.

Cameron, W. E., 1985. Petrology and origin of primitive lavas from theTroodos ophiolite, Cyprus. Contrib. Mineral. Petrol, 89:239-255.

Croudace, I. W., and Gilligan, J., 1990. Versatile and accurate trace elementdeterminations in iron-rich and other geological samples using X-rayfluorescence analysis. X-ray Spectrom., 19:117-123.

Croudace, I. W., and Marshall, S., 1991 Determination of rare-earth elementsand yttrium in nine geochemical reference samples using a novel groupseparation procedure involving mixed acid elution ion-exchange chroma-tography. Geostand. Newsl., 15:139-144.

Dietrich, V, Emmermann, R., Oberhànsli, R., and Puchelt, H., 1978. Geo-chemistry of basaltic and gabbroic rocks from the west Mariana basin andthe Marina trench. Earth Planet. Sci. Lett, 39:127-144.

15.8

15.7

15.4

15.3

• = 793B, o = 792E, + = diabase sill

18 18.5206 p f c )

19 19.5

Figure 15. 207Pb/204Pb-206Pb/204Pb for Leg 126 igneous rocks. Fields areshown for Pacific MORB (Tatsumoto, 1978; Vidal and Clauer, 1981); westernPacific sediment (WPS; Sun, 1980;Meijer, 1976; Woodhead and Fraser, 1985);Bonin islands (Taylor et al., unpubl. data); DSDP Site 458 (Hickey-Vargas,1989); and ODP Site 786 (Pearce et al., 1992).

Dobson, P. F., and Tilton, G. R., 1989. Th, U, and Pb systematics ofboninite series volcanic rocks from Chichi-Jima, Bonin Islands, Japan.In Crawford, A. J. (Ed.), Boninites and Related Rocks: London (UnwinHyman), 396-415.

Egeberg, P. K., and Leg 126 Shipboard Scientific Party, 1990. Unusualcomposition of pore-waters found in the Izu-Bonin fore-arc sedimentarybasin. Nature, 344:215-218.

Fryer, P., Pearce, J. A., Stokking, L. B.., et al., 1990. Proc. ODP, Init. Repts.,125: College Station, TX (Ocean Drilling Program).

Henderson, P., 1982. Inorganic Geochemistry: Oxford (Pergamon Press).Hickey, R. L., and Frey, F. A., 1982. Geochemical characteristics of boninite

series volcanics: implications for their source. Geochim. Cosmochim.Acta, 46:2099-2115.

Hickey-Vargas, R. L., 1989. Boninites and tholeiites from DSDP Site 458,Mariana forearc. In Crawford, A. J. (Ed.), Boninites and Related Rocks:London (Unwin Hyman), 339-356.

Honza, E., and Tamaki, K., 1985. The Bonin Arc. In Nairn, A.E.M., Stehli, F. G.,and Uyeda, S. (Eds.), The Ocean Basins and Margins (Vol. 7): New York(Plenum), 459-502.

Kay, R. W., and Gast, P. W., 1973. The rare-earth content and origin ofalkali-rich basalts. J. Geol, 81:653-682.

Leg 126 Shipboard Scientific Party, 1989a. Arc volcanism and rifting. Nature,342:18-20.

, 1989b. ODP Leg 126 drills the Izu-Bonin Arc. Geotimes, 34:36-38.Meijer, A., 1976. Pb and Sr isotopic data bearing on the origin of volcanic rocks

from the Mariana island-arc system. Geol Soc. Am. Bull, 87:358-369.Nohoda, S., and Wasserburg, G. J., 1981. Nd and Sr isotopic study of volcanic

rocks from Japan. Earth Planet. Sci. Lett, 52:264-276.Pearce, J. A., Thirlwall, M., Ingram, G., Mutton, B. J., Arculus, R. J., and Van

Der Lann, S. R., 1992. Isotopic evidence for the origin and evolution ofthe Izu-Ogasawara forearc at Sites 782 and 786. In Fryer, P., Pearce, J. A.,Stokking, L. B., et al., Proc. ODP, Sci. Results, 125: College Station, TX(Ocean Drilling Program).

Saunders, A. D., Tarney, J., Marsh, N. G., and Wood, D. A., 1980. Ophiolitesas ocean crust or marginal basin crust: a geochemical approach. In Panay-iotou, A. (Ed.), Ophiolites: Geol. Surv. Dept Cyprus, 193-204.

Shipboard Scientific Party, 1990. Sites 788/789. In Taylor, B., Fujioka, K., etal., Proc. ODP, Init. Repts., 126: College Station, TX (Ocean DrillingProgram), 97-126.

Sun, S. S., 1980. Lead isotopic study of young volcanic rocks from mid-oceanridges, ocean islands and island arcs. Philos. Trans. R. Soc. London, Ser. A,297:409-445.

Sun, S.-S., and Nesbitt, R. W, 1978. Geochemical regularities and geneticsignificance of ophiolitic basalts. Geology, 6:689-693.

427

R. N. TAYLOR ET AL.

40

39.5 -

37.5

37

• = 793B, o = 792E, + = diabase sill

18 18.5 19 19.5

Figure 16. 208Pb/204Pb-206Pb/204Pb for Leg 126 igneous rocks. Fields as forFigure 15.

Tatsumoto, M., 1978. Isotopic composition of lead in oceanic basalt and itsimplication to mantle evolution. Earth Planet. Sci. Lett., 38:63-87.

Taylor, B., Fujioka, K., et al., 1990. Proc. ODP, Init. Repts., 126: CollegeStation, TX (Ocean Drilling Program).

Taylor, B., Moore, G., Klaus, A., Systrom, M., Cooper, P., and MacKay, M.,1990. Multichannel seismic survey of the central Izu-Bonin Arc. In Taylor, B.,Fujioka, K., et al., Proc. ODP, Init. Repts., 126: College Station, TX(Ocean Drilling Program), 51-60.

Taylor, R. N., Murton, B. J., and Nesbitt, R. W., 1992. Chemical transectsacross intra-oceanic arcs: implications for the tectonic setting of ophiolites.Spec. Publ, Geol. Soc. London, 60:117-132.

Taylor, R. N., and Nesbitt, R. W., 1988. Light rare-earth enrichment ofsupra-subduction-zone mantle: evidence from the Troodos ophiolite, Cy-prus. Geology, 16:448-451.

Umino, S., 1986. Geological and petrological study of boninites and relatedrocks from Chichi Jima, Bonin Islands [Ph.D. thesis]. Univ. Tokyo.

Vidal, P., and Clauer, N., 1981. Pb and Sr isotopic systematics of some basaltsand sulphides from the East Pacific Rise at 21°N (Project RITA). EarthPlanet. Sci. Lett., 55:237-246.

Woodhead, J. D., 1989. Geochemistry of the Mariana arc (western Pacific):source composition and processes. Chem. Geol., 76:1-24.

Woodhead, J. D., and Fraser, D. G., 1985. Pb, Sr, and 10Be isotopic studies ofvolcanic rocks from the Northern Mariana Islands: implications for magmagenesis and crustal recycling in the western Pacific. Geochim. Cosmochim.Acta, 49:1925-1930.

Date of initial receipt: 26 December 1990Date of acceptance: 30 September 1991Ms 126B-146

400 300 200 100 0Distance from trench (km)

Figure 17. TiO2 content of lavas from the Izu-Bonin arc-trench system plottedvs. distance from the trench. Details of samples used and locations are givenin Taylor et al. (1992).

100 150Ti/Zr

Figure 18. Ti/Zr plotted vs. ENd for Hole 793B basement. Data fields for Izuand Mariana arcs from Nohda and Wasserburg (1981) and Woodhead (1989).Site 458, Site 1403, and Cape Vogel from Hickey and Frey (1982). Chichi Jimafrom Taylor, Nesbitt, and Vidal (unpubl. data).

428


Plate 1. Photomicrograph from Sample 126-793B-110R-1, 1 cm (S-19); width of field = 7 mm; cross-polarized light; basaltic andesite massive flow (Unit 13).Euhedral phenocrysts of orthopyroxene (upper right) and clinopyroxene (center left) set in a groundmass of altered glass and Plagioclase microlites. Vesicles (darkpatches) are coated with smectite.

429

R. N. TAYLOR ET AL.

Plate 2. Photomicrograph from Sample 126-792E-71R-3, 51 cm; width of field = 6 mm; plane polarized light; typical texture in a two-pyroxene andesitefrom Hole 792E basement. Large Plagioclase with fluid/melt inclusion zone occupies center of field. Other phenocrysts include clinopyroxene (center left) andorthopyroxene pseudomorphs (lower right).

430

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