deep sea drilling project initial reports volume 58

33. GEOCHEMISTRY OF BASALTS FROM THE SHIKOKU AND DAITO BASINS,DEEP SEA DRILLING PROJECT LEG 58

Nicholas G. Marsh, Andrew D. Saunders,1 and John Tarney, Department of Geological Sciences,University of Birmingham, U.K.

andHenry J. B. Dick, Department of Geology and Geophysics, Woods Hole Oceanographic Institution,

Woods Hole, Massachusetts

ABSTRACTLeg 58 successfully recovered basalt at Sites 442, 443, and 444, in

the Shikoku Basin, and at Site 446 in the Daito Basin. Only at Site442 did penetration reach unequivocal oceanic layer 2; at the othersites, only off-axis sills and flows were sampled. Petrographic obser-vations indicate that back-arc basalts from the Shikoku Basin, withthe exception of the kaersutite-bearing upper sill at Site 444, are min-eralogically similar to basalts being erupted at normal mid-oceanridges. However, the Shikoku Basin basalts are commonly very vesic-ular, indicating a high volatile content in the magmas. Site 446 in theDaito Basin penetrated a succession of 23 sills which include bothkaersutite-bearing and kaersutite-free basalt varieties.

A total of 187 samples from the four sites has been analyzed formajor and trace elements using X-ray-fluorescence techniques. Chem-ically, the basalts from Sites 442 and 443 and the lower sill of Site 444are subalkaline tholeiites and resemble N-type ocean-ridge basaltsfound along the East Pacific Rise and at 22° N on the Mid-AtlanticRidge (MAR), although they are not quite as depleted in certain hy-gromagmatophile (HYG) elements. They do not show any chemicalaffinities with island-arc tholeiites. The basalts from Site 446 andfrom the upper sill at Site 444 show alkaline and tholeiitic tendencies,and are enriched in the more-HYG elements; they chemically resem-ble enriched or E-type basalts and their differentiates found alongsections of the MAR (e.g., 45°N) and on ocean islands (e.g., Icelandand the Azores).

Most of the intra-site variation may be attributed to crystal set-tling within individual massive flows and sills, to high-level fractionalcrystallization in sub-ridge magma chambers, or, where there is evi-dence of a long period of magmatic quiescence between units, tobatch partial melting. However, the basalts from Sites 442 and 443and from the lower sill at Site 444 cannot easily be related to thosefrom Site 446 and the upper sill at Site 444, and it is possible that thedifferent basalt types were derived from chemically distinct mantlesources. From comparison of the Leg 58 data with those alreadyavailable for other intra-oceanic back-arc basins, it appears that themantle sources giving rise to back-arc-basin basalts are chemically asdiverse as those for mid-ocean ridges. In addition, the high vesicu-larity of the Shikoku Basin basalts supports previous observationsthat the mantle source of back-arc-basin basalts may be contami-nated by a hydrous component from the adjacent subduction zone.

INTRODUCTION Shikoku back-arc or marginal basin, lying between theIwo Jima and Kyushu-Palau Ridges, sampled basalts

During Leg 58, basaltic rocks were recovered at four are no older than 21, 17, and 15 m.y., respectively, fromof the five sites occupied in the Shikoku and Daito both sides of the north-south-trending axial zone. OnlyBasins. At Sites 442, 443, and 444, in the intra-oceanic at Site 442 were basalts penetrated which belong un-

equivocally to oceanic layer 2; at Sites 443 and 444, re-covered basalts probably represent off-axis intrusive or

1 Present address: Department of Geology, Bedford College, extrusive activity. At Site 446, in the Daito Basin, sillsUniversity of London. of post-early Eocene age were penetrated, but basement

805

N. G. MARSH, A. D. SAUNDERS, J. TARNEY, H. J. B. DICK

was not reached; in this region it is thought to beoceanic (Karig, 1975).

It has been established by detailed geophysical inves-tigation and by dredging operations that most back-arcor marginal basins are of extensional origin, and arefloored by basaltic crust broadly comparable in struc-ture and composition to that of the major ocean basins(Karig, 1971, 1974; Hart et al., 1972; Barker, 1972;Hawkins, 1974; Gill, 1976). However, there is still noconsensus as to whether all marginal-basin basalts areencompassed by the range of basalt compositions pres-ently being erupted at mid-ocean ridges, or whether theyhave geochemical characteristics which are, to a smalldegree, transitional towards orogenic basalts, as in-dicated by detailed chemical investigations of lavasfrom back-arc centers in the East Scotia Sea (Saundersand Tarney, 1979) and Bransfield Strait (Weaver et al.,1979). Similar studies of basalts from the MarianaTrough (Hart et al., 1972) and Lau Basin (Hawkins,1976, 1977), however, do not corroborate the evidencefrom the Scotia Sea and Bransfield Strait basins. It isimportant, therefore, to assess the compositions ofmagmas being erupted in several back-arc basins beforeany general conclusions about their genesis can bemade. The prominent role of back-arc basins in the for-mation of many ophiolites (Dewey, 1976; Saunders etal., 1979) emphasizes the importance of being able tochemically characterize and distinguish back-arc-basinbasalts from ocean-ridge basalts, and this can be achievedonly when sufficient data on modern back-arc-basin ba-salts are available.

Comprehensive studies of basalts from the North At-lantic Ocean have illustrated the chemical variability ofmagmas being erupted along the Mid-Atlantic Ridge(Schilling, 1973, 1975a,b; Hart et al., 1973; White et al.,1976; Bougault et al., 1979; Tarney et al., 1978, 1979, inpress; Wood et al., 1979). It is therefore important, be-fore any detailed comparisons between back-arc-basinand ocean-ridge basalts are made, that the compositionof ocean-ridge basalts be clearly delineated. Sun et al.(1979) and Tarney et al. (in press) have defined threemain basalt types presently being erupted in the NorthAtlantic: normal, or N-type, depleted ocean-ridge ba-salts; enriched, or E-type, basalts; and T-type basalts,which have a composition transitional between N- andE-type basalts. These terms will be defined in detaillater. The variations in chemistry appear to be in partdue to chemical heterogeneities in the mantle source, inaddition to variations caused by partial melting andfractional crystallization (see discussions by Bougault etal., 1979; O'Nions et al., 1976; Tarney et al., 1978,1979, in press; Wood et al., 1979). It is necessary toassess whether or not back-arc-basin basalts exhibit asimilar range in chemistry.

Leg 58 has recovered basaltic material which affordsan excellent opportunity to undertake detailed petrolog-ical and geochemical studies of basalts from a "typical"Western Pacific intra-oceanic back-arc basin. In thispaper we shall be concerned essentially with X-ray-fluorescence data on major and trace elements. Basaltsfrom Leg 49 in the North Atlantic were analyzed using

the same analytical equipment (Tarney et al., 1978), al-lowing regional comparisons without inter-laboratoryerror. Instrumental conditions were arranged to providethe highest precision possible, especially for the traceelements. Thus, reported geochemical differences, bothwithin and between sites, are significant. However, wehave confined ourselves to an essentially qualitativeassessment of the results.

REGIONAL AND TECTONIC SETTING

The Shikoku Basin

The Shikoku Basin (Figure 1) has the form of anelongate trough some 1200 km long, 500 km wide, and4.5 km deep, bounded by the I wo Jima Ridge to the eastand by the Kyushu-Palau Ridge to the west. The north-ern end of the basin is delineated by the Nankai Troughand by the Island of Shikoku. Southward, the basincontinues without any physical barrier into the PareceVela Basin.

The Shikoku Basin has been identified as an inter-arcbasin separating a composite frontal arc system — com-prising the I wo Jima Ridge, Bonin Trough, and BoninRidge — from the remnant Kyushu-Palau Ridge. Sub-duction of Pacific Ocean floor beneath the Bonin Ridgeis presently occurring along the Izu-Bonin Trench, andit is apparent that the Bonin Trough is an incipientback-arc basin (Karig, 1971). The Nankai Trough prob-ably represents the site of subduction of Shikoku Basinfloor beneath southern Japan (Karig, Ingle, et al., 1975;Kobayashi and Isezaki, 1976).

Figure 1. Map of the northern Philippine Sea, showingthe location of Leg 58 sites in the Shikoku andDaitoBasins.

806

GEOCHEMISTRY OF BASALTS

Geophysical studies have shown that the ShikokuBasin is floored by oceanic crust, although the absenceof detectable shallow earthquakes beneath the axialzone of the basin (Katsumata and Sykes, 1969) suggeststhat spreading is not now taking place. However, the ax-ial zone has the high heat flow and topography charac-teristic of many presently active oceanic spreading cen-ters (Kobayashi and Isezaki, 1976), although the mag-netic lineations observed within the basin are of loweramplitude than those found in most major ocean basins.Linear, weak magnetic anomalies running parallel to theKyushu-Palau Ridge are well established for the west-ern side of the basin, but because of rough topographythe interpretation of anomalies in the eastern side of thebasin remains equivocal (Watts and Weissel, 1975; Ko-bayashi and Isezaki, 1976; Shih, this volume). Availabledata suggest that spreading in the Shikoku Basin beganbetween 25 and 27 Ma and ceased about 17 Ma (Wattsand Weissel, 1975; Kobayashi and Isezaki, 1976). Thiseffectively separated the continental crustal slivers ofthe Kyushu-Palau and Iwo Jima Ridges (Murauchi etal., 1976). Alternative interpretations of the data aresuggested by Tomada et al. (1975), who consider thatthe spreading occurred later, between 22 and 10 Ma,and by Murauchi et al. (1976), who have proposed thatspreading occurred in two distinct episodes, 24 to 19 Maand 12 to 16 Ma. The interpretations agree, however, insuggesting that spreading is not now occurring in theShikoku Basin. It would appear that the locus ofspreading has migrated westward into what is now theBonin Trough.

Northeast Philippine Basin

The northeast corner of the Philippine Basin isroughly triangular; it is bordered by the Ryukyu Trenchto the northwest and by the Kyushu-Palau Ridge to theeast (Figure 1). The area comprises a complex group ofbasins and topographic highs, including the Oki-Daitoand Daito Ridges, which apparently are structurallysimilar to island arcs (Murauchi et al., 1968). The inter-vening basins (e.g., Daito Basin) are sufficiently deep tobe floored by old ocean-crust, possibly of early-Tertiaryor late-Cretaceous age (Karig, 1975). However, a thickpile of sediments now obscures the oceanic layer 2, al-though heat-flow measurements (Watanabe et al., 1970)show values similar to those of old ocean crust.

Rocks previously dredged from the Daito Ridge in-clude greenschists, hornblende schists, serpentinites, an-desite, and diorite. Andesite, basalt, and granodioritehave been recovered from the Oki-Daito Ridge. Fromthe geophysical evidence and dredge hauls it has beensuggested that the Daito and Oki-Daito Ridges formedas island arcs (Murauchi et al., 1968; Karig, 1975;Mizuno et al., 1975, in press). These arcs and the ad-joining pieces of ocean crust drifted some 2000 kmnorthward during the last 52 m.y. as a result of sea-floorspreading in the West Philippine Sea, before being trappedagainst the Kyushu-Palau Ridge.

ANALYTICAL TECHNIQUES

Sample Preparation

Minicores or quarter-core sections were washed indistilled water to remove surface contamination, anddried at 100°C prior to being ground to a fine powder(less than 60 µm) in the shipboard tungsten-carbideTEMA swing mill. Powder briquettes 46 mm in diame-ter were made by binding 15 grams of rock powder withabout 30 drops of a 7 per cent aqueous solution ofpolyvinyl alcohol, under a pressure of 15 tons againstthe polished, hardened faces of a steel die.

Representative samples from Sites 442, 443, and 444,and all the samples from Site 446, were made into fusionbeads for the purpose of major-element analysis. Thistechnique eliminates mineralogical effects and reducesmatrix absorption. The rock powders were dried over-night at 110°C, and ignition loss was determined byheating at 1000°C in an electric furnace for 1 hour.Beads were prepared by mixing sufficient rock powderand flux in a Pt/Au crucible to give an exact ratio of 1:5after fusion (0.9 gram rock, 4.5 gram flux, ignited mate-rial). Fusion was carried out at 1200°C, with frequentswirling, for 20 minutes in a vertical-tube electric fur-nace. The flux, 80 per cent Li metaborate, 20 per cent Litetraborate (Johnson Matthey Spectroflux 100B), is de-scribed by Bennett and Oliver (1976) and is designed todissolve the widest possible range of silicate and ceramicmaterials.

After fusion, the melt was pressed in aluminummolds, using equipment similar to that described byHarvey et al. (1973); the beads were transferred toheated insulating blocks for annealing at 200° C for 30minutes, and finally allowed to cool in the blocks. Thelithium borate beads are slightly hygroscopic and mustbe stored in sealed polypropylene bags in a desiccator toprevent atmospheric attack of the bead surface.

X-Ray-Fluorescence Analysis

The equipment used was a Philips PW1450 automaticspectrometer with a PW1466 60-position sample chang-er. Count data from the spectrometer were processedusing a 32K Digital PDP11-03 computer, and the Uni-versity of Birmingham ICL1906A computer. Trace ele-ments were determined on 46-mm powder briquettes,using molybdenum (Y, Sr, Rb, Th, Pb, Ga, Zn, Ba) andtungsten (Ni, Cr, Ce, La, Zr, Nb) anode X-ray tubes.Major- and minor-element analyses of the basalts fromSites 442, 443, and 444 were determined on 46-mm pow-der briquettes and representative fusion beads using achromium-anode X-ray tube. Major- and minor-ele-ment analyses of basalts from Site 446 were determinedon fusion beads, using a rhodium-anode X-ray tube. In-strumental repeatability was ensured by ratioing countsto a reference sample, re-analyzing every fourth sample(Rh and Cr tubes) or at the beginning of every loadingof 60 samples (Mo and W tubes).

807

N. G. MAR^H, A. D. SAUNDERS, J. TARNEY, H. J. B. DICK

Instrumental Conditions

Details of instrumental conditions for analysis oftrace elements are given in Tarney et al. (1978).

The Rh-anode X-ray tube, operated at 30 kV, 70 mA,provides equivalent or superior sensitivity for thoseelements previously analyzed using a Cr anode, with theexception of Ti, Ca, and K. However, these elements areextremely sensitive to Cr excitation; it is usually nec-essary to greatly reduce tube power to avoid saturatingthe detector, and therefore no problems are encounteredwith the reduced efficiency of the Rh anode. An addi-tional advantage is that the analysis of Mn is possiblewithout filtering out the Cr K radiation from the tubetarget.

The Kα wavelengths of all major elements were ana-lyzed using the flow-proportional detector (argon/meth-ane) with automatic pulse-height selection. The fine col-limator (0.15 mm) was used for all elements except Naand Al, for which the coarse collimator (0.55 mm) wasemployed. The crystals used were: LiF2Oo (Mn), LiF220

(Fe), PET (Ti, Ca, K, Si, Al), T1AP (Mg, Na), and G e m

(P). The germanium crystal was chosen for P to avoidthe overlap of the escape peak arising from the second-order CaKß on PKα. A symmetrical, 50 per cent widthwindow was used for the pulse-height discriminator forall elements, with the exception of Mg, for which a set-ting of 20 to 65 per cent is needed to reduce the overlapof the third-order CaKα. However, both these Ca in-terferences are considerably less than when using Cr ex-citation.

Calibration

Initial trace-element calibrations were prepared froma wide range of rock types spiked with an appropriatepure-element compound (Leake, Hendry, et al., 1969).These values were then related using either the WL^ orMoKα Compton scatter lines to provide correction fortotal mass absorption for all emission lines shorter thanthe iron absorption edge (Reynolds, 1967). Ce and Lacalibrations were prepared using basalts previously ana-lyzed by isotope dilution. The accuracy of the variouscalibration lines was monitored by analyzing severalinternational reference samples of suitable composi-tions.

For the major elements, primary calibrations and " α "(matrix-dependent constant) correction factors were cal-culated from binary and ternary mixtures of appropri-ate pure chemicals and referred to an average basaltcomposition. The validity of these calibrations waschecked against departmentally analyzed material andinternational reference standards. Blank values for mi-nor elements were checked using pure SiO2, A12O3, andBeO, and were found to be low but significant forNa2O, K2O, and P2O5. For these elements at very lowlevels the values produced from undiluted powder bri-quettes are preferred.

Precision and Accuracy

The precision of measurement and sample prepara-tion is illustrated by the standard deviation of six

separate beads of the internal ocean basalt referencesample BOB-1 (Table 1). The trace-element data inTable 1 were determined on powder briquettes. The in-strumental precision with the techniques used is high,even at relatively low trace-element concentrations.However, a potential problem of instrumental analysisis long-term drift when periods greater than 12 monthsare considered. This has been kept to a minimum by us-ing 30 samples from Leg 49 (Tarney et al., 1979) to-gether with other basaltic material, selected to give aswide a concentration range as possible for all elements.These samples are analyzed together with the unknowns,ensuring compatibility of data between different legs.For major-element analysis it is not desirable to re-analyze fusion beads over long periods of time, hence anumber of internal reference samples are fused and ana-lysed with each batch of unknowns.

Accuracy of analysis can only be judged against in-ternational reference samples which, for trace elements,do not cover the range of concentrations encountered inmost DSDP legs. Data for trace elements in basalt

TABLE 1Analyses of Standard Reference Samplesa

Component(% or ppm)

SiO2

TiO2

AI2O3F e 2 O 3

MnOMgOCaONa2OK 2 O

P2O5

Standard

BOB-1

51.02 ± 0.0781.28 ± 0.004

16.55 ± 0.0128.48 ± 0.0130.14 ± 0.0017.58 ± 0.030

11.39+ 0.0273.10+ 0.0740.37+ 0.0030.16 ± 0.001

AVG-1

60.191.01

17.446.980.101.564.874.402.990.49

JB-1

53.131.30

14.489.020.157.799.412.871.450.25

Total 100.07 100.03 99.85

BOB-1 BCR-1 JB-1

NiCrbRbSrYZ I L•

NbBa d

LaCeZnGa

106.4+ 2.1257± 10

4.8 ± 0.6202.1 ± 1.0

26.9 + 0.5107.6+ 3.0

5.3 ± 1.650.4+ 3.6

6.4 ± 1.916.6 ± 1.761.5 + 2.117.6 + 0.7

191848

32936

19112

6952655

12025

134359

42442

23150

35500

38668017

a F e 2 θ 3 is total iron as F e 2 θ 3 ; mean and 1 sigmastandard deviation of six fusion beads of BOB-1,and means of three fusion beads of internationalreference samples; all values are for ignited sam-ples; average concentration of BOB-1 determinedon 22 consecutive days (with 1 sigma variation);average concentration of two samples of interna-tional reference material.

t>Not corrected for V interference.cCorrected for Sr interference.^Corrected for Ce interference.

808

GEOCHEMISTRY Cg BASALTS

BCR-1 are given in Tarney et al. (1979) and for a widerrange of materials in Hendry (1975). Major elements forAGV-1 and JB-1 are given in Table 1.

SHIKOKU BASIN

Three sites, comprising a total of six holes, weredrilled in the Shikoku Basin during Leg 58. The igneousstratigraphy of the recovered sections from each ofthese sites is given in Figures 2, 17, and 27 in this paper,and although detailed petrographic descriptions aregiven in the relevant site chapters (this volume), briefresumes are included in the following sections.

Site 442

Site 442 (Figure 1) is approximately 50 km west of theaxial zone of the Shikoku Basin, at 28°59.00'N, 136°03.43 'E, on magnetic anomaly 6 (Kobayashi and Naka-ta, 1977; 19-20 m.y.: LaBrecque et al., 1977). Threeholes were drilled; Hole 442 was a test hole for the re-entry cone, and Holes 442A and 442B cored approxi-mately 158 meters of massive basalt flows and pillowbasalts beneath some 300 meters of sediment, at a waterdepth of about 4635 meters.

Site 442 is the only site of Leg 58 from which pillowbasalts typical of oceanic Layer 2 were definitelyrecovered. Two lithologic units have been recognized:Unit 1, comprising mainly massive flows or sills, andunit 2, the underlying pillow lava sequence (Figure 2).The basalts from both units are vesicular and sparselyolivine bearing. Unit 1 is separated from unit 2 by ap-proximately 2 meters of brown mudstone, for whichmicrofossils indicate an age between 18 and 21 m.y.This is in agreement with the magnetic-anomaly age

(19-20 m.y.) of the basement at this site. The oldestsediments overlying unit 1 are limestones containingmicrofossils giving an age between 15 and 17 m.y.

Unit 1 consists of approximately 59 meters of fine- tomedium-grained basalt, which has been divided into 16sub-units. Many of these sub-units have fine-grained oroccasionally aphanitic chill zones, and the absence ofglassy margins suggests that these basalts were emplacedas sills or as massive flows (Site 442 report, this vol-ume). However, the presence of glassy selvages and var-iolitic basalts in Core 6 of Hole 442B indicates that theunit may consist, at least in part, of pillow lavas.

The sub-units of unit 1 grade from fine-grained ba-salts to medium-grained dolerites with an inequigranu-lar, subophitic texture. Seriate textures are common. Inthe finer-grained samples, intersertal to intergranularaugite and plagioclase (An55_75) and interstitial magnet-ite are embedded in a cryptocrystalline groundmasswhich is often replaced by clays. Strongly zoned clino-pyroxene crystals, colored green in their centers tobrown at the edges, and "bow-tie" zoned clinopyrox-enes are present in the coarsest-grained sections of somesub-units. In these coarse-grained rocks, it is difficult toascertain whether such crystals are phenocrysts or mere-ly large groundmass crystals. However, they frequentlyform glomerocrystic intergrowths with zoned plagio-clase crystals (core compositions of An7O_85). Chromespinel is a common accessory phase throughout unit 1,generally appearing as inclusions in plagioclase pheno-crysts. Chrome-spinel inclusions also occur in clinopy-roxene phenocrysts. The basalts and dolerites of unit 1tend to be very vesicular, often with two distinct popula-tions of vesicles. Most sub-units have 10 to 30 per centsmall vesicles ( < l mm) and generally 1 to 5 per cent

Core

No.

Lithologic

Unit

Magnetic

InclinationZ f { p p m ) ( p p m ) ( p p m ) F e / M g Y / Z r T j / Z r

280 η

300-

320-

3. 3 4 0 -

Q.

Qc 3 6 0 -

Su

b-b

ott

oi

CO

4 0 0 -

4 2 0 -

4 4 0 -

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Mudstone and

Unit 1(sub-units A to P)

Massive aphyricvesicular basalts

Unit 2Aphyric vesicular

pillow basalts

+90°r . . . . . . .

— —

f -90°

'I

II"

r

80 100 120

I 1 1

.V

*

-

•*i r

20 40 601 1 1

V

.V•* •"•• *.

t

f

100 200 300 400I I I I

• •••*3»

••-V

•

*•i

I

*

1 2 3

I I 1

1

<V•

\

m

*

0.1 0.2 0.3 0.41 I 1 I

•*••*••*

•s

60 70 80 90

1 1 1 1

. 9

•

.

t

u

1

2

3

4

Figure 2. Down-hole variation in lithology, magnetic inclination, and selected geochemical parameters at Site 442.

809


large vesicles (2-5 mm). Pipe vesicles are also foundnear the chill margins of several sub-units.

The pillow basalts of unit 2 have glassy margins,generally less than 1 cm thick, and contain 1 to 2 percent small plagioclase laths at their inner margins. Theglassy zones grade abruptly into variolitic zones towardthe pillow interiors. The varioles consist of small anhe-dral plagioclase laths and clinopyroxene microlites in aglassy matrix. The cores of the pillows are usually finegrained, with an intersertal texture comprising 25 percent plagioclase, (An55_7O), 30 per cent augite, 2 to 3 percent granular magnetite, and a glassy or (more com-monly) a clay-rich matrix. Vesicles are again common,composing up to 40 per cent of the rock, and are usuallyless than 1 mm across. However, larger vesicles up to 10mm across do occur, and pipe vesicles are common,usually in a zone between 1 and 10 cm from the glassymargin.

Low-temperature alteration is a general feature of allthe basalts recovered at this site. The cryptocrystallineor glassy matrixes are commonly replaced by clay min-erals. The basalts are commonly cut by calcite- and clay-lined fractures or veins, and the vesicles are generallyfilled or lined by similar material. Zeolites are rare, andpyrite was found in small amounts in vesicles and frac-tures of sub-unit IK only, which also contained somechlorite and talc-like alteration products (Site 442report, this volume).

Major- and trace-element data for the basalts recov-ered at Site 442 are listed in Tables 2 and 3. In thefollowing discussion, emphasis will be placed on thebasalts recovered from Hole 442B, the range of com-positions of which generally encompass those fromHole 442A. The basalts have been grouped in differentgeochemical units according to variations in their mi-nor- and trace-element contents, as illustrated in Figure2. These variations broadly coincide with the lithologicand magnetic units, although lithologic unit 2 (Hole442B) has been subdivided into three geochemical units.The geochemical units and the lithologic units do notnecessarily coincide.

Site 442 basalts are hypersthene and olivine normative(subalkaline), with practically no normative nepheline orquartz (Figure 3); the minor transgressions into the Neand Q fields on Figure 3 may be due to the presence ofsecondary carbonate and clays, respectively. The basaltsalso have a high normative-plagioclase content, eventhough plagioclase phenocrysts are not a major phase,and this is shown in Figure 4, where the basalts plot asplagioclase tholeiites on the pyroxene-plagioclase-olivinefield of Shido et al. (1971). Geochemical unit 442-1 isgenerally more olivine normative than the underlyingpillow lavas (units 442-2 to 442-4), and this is alsoreflected in the lower Fe/Mg ratios, and higher Cr andNi abundances in Unit 442-1 (Figures 5 and 6).

There is considerable downhole variation in absoluteelement abundances at Site 442. The essential featuresof this variation are illustrated in Figure 2. There is aclear correlation between units based on magnetic in-clination and geochemistry, although geochemical unit

TABLE 2Analyses, Element Ratios, and CIPW Norms

for Basalts from Hole 442A&

CoreSectionInterval (cm)

SiO2 (%)TiO2

A12O3

Fe 2 O 3

MnOMgOCaONa2OK 2 Op 2 θ 5

Total

Ni (ppm)CrZnGaRb

SiYZr

Nb

BaLaCe

PbTh

Zr/NbTi/ZrY/ZrCe/ZrBa/ZrCeN/YN

Fetot/MgK/RbBa/Sr

QOrAbAnNeDi

HyOIMt11

Ap

442A-311

85

49.61.24

16.28.660.146.12

13.462.700.260.18

98.55

45

220

78

174

186

3384

337

4

10

5< l

28

89

0.390.120.440.741.64

5460.20

0.01.6

23.231.7

0.028.5

5.3

4.61.5

2.4

0.4

322

48.61.12

15.88.380.145.38

15.192.670.400.16

97.83

57234

6317

9

18328

864

28

211

3< l

21

780.330.130.330.961.81

3710.15

0.02.4

18.630.5

2.437.2

0.0

3.91.5

2.2

0.4

322

80

49.41.16

16.07.290.106.77

13.792.780.250.15

97.72

130

2406917

4182

26

86< l29

4

13

51

_

810.300.150.341.231.25

5150.16

0.0

1.524.131.1

0.030.7

1.0

7.1

1.32.30.4

a3i

124

47.51.09

15.47.960.146.16

16.712.400.240.16

97.77

5 3216

11616

4

18628

82

321

313

7< l

27

800.340.160.261.141.50

5060.11

0.01.5

12.131.2

4.7

42.80.03.11.42.1

0.4

332

57

49.91.23

15.99.660.136.53

11.472.770.380.15

98.13

49

2336318

7189

29

861

27

619

< l< l

86

860.340.220.311.611.72

4450.14

0.02.3

23.930.4

0.0

21.813.5

2.81.72.4

0.4

333

68

49.81.22

15.89.130.127.17

11.602.820.330.14

98.20

77245

6618

6182

26

89

330

313

3< l

30

820.290.150.341.231.48

451

0.16

0.02.0

24.330.2

0.022.410.7

5.31.62.4

0.3

334

111

49.91.20

15.210.09

0.136.55

11.742.840.520.14

98.32

35214

6519

26178

27

88

331

512

3< l

2982

0.310.140.351.091.79

167

0.17

0.03.1

24.427.8

0.0

25.08.4

5.9

1.8

2.3

0.3

341

78

49.81.15

16.18.620.117.04

12.372.720.230.14

98.30

6 4245

68

18

2

18627

87

322

511

2< l

29

790.310.130.251.001.42

9710.12

0.01.4

23.431.7

0.0

24.39.5

4.81.5

2.2

0.3

aNormative values based on Fe2θ3/FeO ratio of 0.15; Fe 2 θ3 is total iron as Fe2C>3.represents ratio of chondrite-normalized values.

442-1 exhibits a large amount of within-unit variation:Cr ranges from 193 to over 300 ppm and one lithologicsub-unit (442-IK) contains 412 ppm Cr and 201 ppm Ni.Zr varies between 67 and 106 ppm, and the Fe/Mg ratiovaries from 0.97 to 2.41. The singularly high Ni, Cr, andnormative 01 contents (Figures 3,5, and 6) and the lowFe/Mg ratio and Zr levels in lithologic sub-unit 442-IK(sample 442-8-1, 32 cm) may be due to the developmentof a within-flow cumulus assemblage. The rock at thisinterval has a doleritic appearance, and there is evidenceof a concentration of crystals where the sample was ta-ken.

Geochemical units 442-2 and 442-4 have significantlyhigher Zr contents and Fe/Mg ratios, and complemen-tary lower contents of Ni and Cr, than 442-1. Geo-chemical units 442-3 and 442-4 have very low Cr (35-64ppm) and Ni (32-63 ppm) levels, and a high Fe/Mg ratio(1.47-2.32), indicative of considerable fractionation ofolivine, pyroxene, and (or) chrome spinel prior to erup-tion and emplacement of the magma. Indeed, none ofthe geochemical units of Site 442 has the high MgO(>9.0%) and Ni (>200 ppm) indicative of the"primary" basaltic liquid of Kay et al. (1970), apart

810


from 442-8-1, 32 cm, which as we have seen could easilybe a cumulus assemblage derived by within-flow dif-ferentiation.

The chemical data for Site 442 may be convenientlyplotted against an index of fractionation to illustratevariations within and between the four distinct geo-chemical units. The Fe/Mg ratio is one such index thathas been used in the past, but the strong dependence ofthis ratio on the composition of the fractionating phases,rather than their quantity, makes it less desirable as afractionation index than a suitable incompatible traceelement. Incompatible or hygromagmatophile elementsare those with bulk-distribution coefficients (D) signif-icantly less than unity; ideally, D should approach zero.Hygromagmatophile (HYG) elements may be groupedaccording to their relative bulk-distribution coefficientsinto more-HYG (D 0.01) and less-HYG types (D ~0.1); in ocean-ridge basalts, the more-HYG elements in-clude Rb, K, Th, La, Ce, Nb, Ta (Zr, Hf); and the less-HYG elements include (Zr, Hf), Sm, Ti, P, Y, and theheavy rare-earth elements (REE) (Wood et al, 1979).Zirconium is thus considered to have a low D value inocean-ridge basalts, and probably back-arc basinbasalts also (Saunders and Tarney, 1979; Weaver et al.,1979), and in consideration also of its apparent im-mobility during the hydrous alteration of basalts (Cann,1970; Pearce and Cann, 1971), it therefore provides asuitable index of fractionation for the study of basalticsuites (cf. Weaver et al., 1972).

Figures 5 to 14 present a visual summary of the data,using zirconium as an index of fractionation. Figure 15shows down-hole variation in Zr, P2O5, and K2O.Although there is only a moderate range of Zr contentsin the Site 442 basalts, the salient points may be sum-marized as follows. TiO2, Ce, Nb, and Y (Figure 16) ex-hibit strong colinearity with Zr, and the ratios do notchange significantly from unit to unit. By inference, itmay be expected that the other hygromagmatophile ele-ments P, K, Rb, and Ba would also show a colinear rela-tionship with Zr, but in fact this is not the case (Figures10 to 14). P2O5 varies independently of Zr (and of theother incompatible elements) especially in geochemicalunits 442-2, 442-3, and 442-4 (Figure 10). This is clearlyillustrated in Figure 15, where P2O5 is shown to vary on-ly slightly, and colinearly with Zr, in unit 442-1, butshows erratic behavior in the other units. We suggestthat this variation is due to alteration of the pillowbasalts, possibly at very low temperatures (<25°C).K,Rb, Ba, and Sr show similar erratic behavior, which isprobably also due to alteration by sea water: the mobili-ty of these elements in oceanic basalts is well docu-mented (Hart, 1971; Hart et al., 1974).

Included on Table 4 are representative element ratiosof basalts being erupted at a present-day ocean ridge,and each biaxial plot in this paper contains ratio lines toaid comparison with Leg 58 basalts. Three distinct typesare presented for comparative purposes.

Normal, or N-type ocean ridge basalts, which are theso-called mid-ocean-ridge basalts (MORB) now beingerupted along most ocean ridges, have low initial 87Sr/

ratios (0.7023-0.7027; Hart, 1971), have low abun-

dances of many HYG elements (K, Rb, Ba, Th, U, lightREE, Nb and Ta; White and Schilling, 1978; Bougaultet al., 1979), and are depleted in the more-HYG ele-ments relative to chondrites (Schilling, 1971; Frey et al.,1974; Wood et al., 1979). Representative analyses ofN-type ocean-ridge basalts were taken from Rhodes etal. (1976).

Enriched, or E-type tholeiitic and alkalic basalts, arepresently being erupted along certain sections of themid-Atlantic Ridge (e.g., the Azores, parts of Iceland,and MAR 45° N). They are characterized by variable butgenerally high abundances of most HYG elements; ra-tios of the more-HYG elements are approximately chon-dritic (Wood et al., 1979), and initial 87Sr/86Sr ratios aremoderately high (0.7035; White et al., 1976).

Transitional or T-type ridge segments lie between thenormal and enriched areas, and include most of Iceland,the Reykjanes Ridge (60-64° N) and the FAMOUS area(MAR 35-39° N). These segments are erupting basaltswith trace-element abundances and isotope chemistryintermediated between N- and E-type basalts. In addi-tion to ratios for ocean-ridge basalts, representativeratios from basalts from the East Scotia Sea (Saundersand Tarney, 1979) and from the Bransfield Strait (Wea-ver et al., 1979) back-arc basins are presented in Table5. The data for the MAR 45° N (E-type) and MAR 63° N(T-type) basalts are from Tarney et al. (1979) and weredetermined using the same XRF spectrometer as the pre-sent Leg 58 samples, as were the Scotia Sea and Brans-field Strait lavas. Inter-laboratory error is thus avoided.

In Zr/Nb ratios (Figure 9), the basalts of Site 442closely resemble N-type ocean-ridge basalts recoveredduring DSDP Leg 34 from the Nazca plate (Rhodes etal., 1976). Ce/Zr and TiO2/Zr ratios are also similar toN-type ocean-ridge basalts, and also closely resemblethose from the East Scotia Sea (Figures 7 and 8). Thus,in relative abundances of Ti, Zr, Nb, and Ce, the Site442 basalts most closely resemble N-type ocean-ridgebasalts.

Full rare-earth-element data are not yet available forthe Site 442 basalts, althoughWood et al. (this volume)have presented some preliminary patterns for Sites 443,444, and 446. However, by using Y as a heavy REE, ap-proximately equivalent to Ho, a plot of Ce versus Y(Figure 16) shows that the Site 442 basalts have chon-drite-normalized Ce/Y ratios (represented as CeN/YN)of about 1 (see also Table 2).

Other chemical criteria frequently used to character-ize and discriminate between different basalt types arethe relative abundances of K, Rb, U, Th, and Ba. Un-fortunately, apart from Th and Ba, these elements areeasily mobilized during alteration by sea water (Bass etal., 1973; Hart et al., 1974; Mitchell and Aumento,1977) and from Figures 11, 12, and 13 it is apparent thatalteration precludes their use in the present study.However, Ba levels in the Site 442 basalts are low(generally less than 40 ppm), giving the low Ba/Zr ratios(Figure 13) typical of N-type ocean-ridge basalts. Woodet al. (this volume) have also shown that the Th/Hfratios of the Site 442 basalts are similar to those ofN-type basalts.

811


TABLE 3Analyses, Element Ratios, and CIPW Norms for Basalts from Hole 442B a


Siθ2 (%)TiC 2AI2O3Fe2θ3MnOMgOCaONa2θK 2 0P2O5Total

Ni (ppm)CrZnGaRbSrYZrNbBaLaCePbTh


Fetot/MgK/RbBa/Sr

QOrAbAnNcDiMy01Mt11Ap

33

40

50.21.38

15.99.230.176.08

12.152.720.360.18

98.44

44193

7620

81923298< l40

414

32

840.330.140.411.071.76

3700.21

0.02.1

23.430.7

0.024.113.9

0.21.62.70.4

33

140

50.01.23

16.59.070.136.63

12.492.760.290.15

99.19

57247

8318

5186

2989

124

216

7< l

89830.330.180.271.361.59

4760.13

0.01.7

23.532.0

0.024.1

8.45.11.62.40.4

34

13

50.01.27

15.99.980.146.39

11.692.910.330.15

98.83

52221

85187

1803096

137

512

11

9679

0.310.130.390.981.81

3940.21

0.02.0

24.929.80.0

22.910.44.61.82.40.4

41

57

49.51.18

16.58.870.156.00

12.822.880.310.17

98.40

60258

7018

7189

3089

422

31762

22790.340.190.251.391.71

3710.12

0.01.9

24.831.6

0.026.1

4.46.21.62.30.4

42

110

50.21.19

16.08.910.116.46

11.983.030.330.14

98.33

42222

6620

8187

2985

328

3123

< l

2884

0.340.140.331.021.60

3440.15

0.02.0

26.129.6

0.024.5

7.55.31.62.30.3

43

21

49.51.18

15.99.170.126.64

12.423.270.290.14

98.58

53216

67176

1842890

226

214

4< l

4579

0.310.160.291.231.60

4050.14

0.01.8

26.528.2

0.927.3

0.010.4

1.62.30.3

51

120

50.01.29

16.08.740.117.49

11.602.890.250.14

98.54

84222

6718

3188

2589<l33

513

< l2

870.280.150.371.281.35

6950.18

0.01.5

24.830.5

0.021.910.2

5.91.52.50.3

521

49.91.23

15.98.480.107.63

11.432.970.250.14

98.00

103237

7019

2183

2391

132

212

< l<l

9181

0.250.130.351.281.29

10210.17

0.01.5

25.630.00.0

21.99.36.81.52.40.3

52

60

50.21.26

15.98.660.107.93

11.112.800.220.13

98.37

157232

8419

2179

2791

435

211

1< l

2383

0.300.120.381.001.27

8970.20

0.01.3

24.130.7

0.019.815.2

3.81.52.40.3

52

106

49.81.15

15.88.390.117.32

12.302.790.340.13

98.16

80244

59155

1762684

330

211

1<l

2882

0.310.130.361.041.33

5660.17

0.02.1

24.030.2

0.025.3

7.06.71.52.20.3

53

94

49.61.18

15.68.590.126.71

12.842.780.310.14

97.86

74226100

155

1792790

332

3941

3079

0.300.100.360.821.48

5230.18

0.01.9

24.029.7

0.028.0

5.55.81.52.30.3

54

79

49.61.18

16.09.010.126.54

12.802.700.230.14

98.37

60233

7914

3185

2884

229

316

32

4284

0.330.190.351.401.60

6360.16

0.01.4

23.231.4

0.026.3

8.14.51.62.30.3

The colinearity exhibited by the hygromagmatophileelements (excluding those which have been subject tomobilization during alteration by sea water) in the Site442 basalts suggests that the magmas giving rise to thefour geochemical units were derived from similar paren-tal magmas, or by fractional crystallization of a com-mon parental magma. However, unit 442-1 could nothave been derived from the same magma that producedthe other units, because it is separated from them by 2meters of mudstone. This could have taken as long as 2m.y. to have been deposited (Site 442 report, this vol-ume). In addition, units 442-3 and 442-2 are separatedby a magnetic reversal, which suggests that these two

units may not be comagmatic, although they are prob-ably cogenetic.

Site 443

Site 443 (Figure 1) lies approximately 95 km east ofthe axial zone of the Shikoku Basin, at 29° 19.65'N,137°26.43 'E, at a water depth of 4372 meters. Approx-imately 116 meters of basaltic flows and 460 meters ofsediment were drilled in the single hole. The site is on apositive magnetic lineation tentatively identified asanomaly 6A (21-22 m.y.; LaBrecque et al., 1977; Koba-yashi and Nakata, 1977). The oldest microfossils recov-ered at this site indicate an age of 14 to 17 m.y., but it is

812


TABLE 3 - Continued

6153

49.71.19

15.68.980.126.89

12.942.780.250.15

98.63

6423775182

13028883293103

<l

29810.320.110.330.881.51

10420.22

0.01.5

23.829.70.0

28.05.56.41.62.30.4

6232

49.81.16

15.69.050.136.9712.882.730.260.15

98.73

71277106187

1522786237388

<l

43810.310.090.430.731.51

3110.24

0.01.6

23.429.80.027.76.66.01.62.20.4

62

147

49.81.19

15.88.490.118.19

10.972.720.200.13

97.64

822477120<l19328834284121

<l

21860.340.140.341.051.20_0.15

0.01.2

23.631.10.019.315.84.21.52.30.3

6328

50.11.24

16.38.030.137.14

11.902.990.290.15

98.20

4923276183

195268933241261

30840.290.130.361.131.30

8050.16

0.01.8

25.830.70.0

23.17.76.11.42.40.3

7146

50.31.20

15.79.920.167.94

10.192.820.200.16

98.61

5020975192

20231943444143

<l

31770.330.150.471.111.45

8220.22

0.01.2

24.230.20.016.419.63.21.82.30.4

71

110

50.21.35

15.810.050.167.5810.512.780.170.14

98.68

5921180182

16130934403105

<l

23870.320.110.430.821.54

6930.25

0.01.0

23.830.40.0

17.619.42.11.82.60.3

8132

47.60.9114.99.880.1811.839.382.090.110.12

96.96

2014125919<l13521676145131

<l

11820.310.190.211.520.97_0.10

0.00.618.231.80.0

12.419.612.61.81.80.3

8574

49.41.16

15.59.510.158.88

10.342.640.110.13

97.86

10025558162

15928841293134

<l

84830.330.150.351.141.24

4690.18

0.00.7

22.830.90.016.817.95.81.72.30.3

86

141

49.51.24

15.29.380.139.2810.132.700.150.16

97.84

1052586318<l169279113431252

91820.300.130.371.091.17—0.20

0.00.9

23.329.60.0

16.817.56.61.72.40.4

87

104

50.21.21

16.08.900.137.43

10.733.240.370.14

98.38

6622841175

15420944192136

<l

23770.210.140.201.601.39

6180.12

0.02.2

27.928.60.020.07.68.71.62.30.3

9134

49.31.14

16.18.310.147.7810.972.590.260.12

96.74

137300591811167247843551451

19880.310.180.451.431.24

1920.21

0.01.6

22.732.70.018.416.83.11.52.20.3

91

107

49.11.08

14.89.170.137.6412.502.660.520.13

97.80

11828261149

16425796352104

<l

13820.320.130.440.981.39

4780.21

0.03.1

23.027.60.0

28.52.9

10.01.62.10.3

9237

49.21.13

15.38.420.126.7313.982.720.330.14

98.07

10027085177

173288445221262

21810.330.140.621.051.45

3960.30

0.02.0

22.529.20.5

33.10.08.01.52.20.3

9354

50.21.47

15.011.080.147.8510.032.970.300.17

99.21

11825199203

1773110623132031

53830.290.190.291.581.64

8300.18

0.01.8

25.326.90.018.116.25.51.92.80.4

possible that some older sediments were not cored (Site443 report, this volume). Sites 442 and 443 lie on thesame spreading line, on opposite sides of the spreadingaxis of the basins.

The basalts recovered at Site 443 appear to be frommassive flows or sills (Figure 17). Essentially all thebasalts are olivine-bearing, and in general vesicles arescarce (although in lithologic unit 3 they reach 30%).On the basis of shipboard petrography, the basalt se-quence has been divided into six lithologic units. Unit 1consists of one small piece of glassy basalt immediatelyunderlying the sediments in Core 49. Unit 2 compriseseight sub-units, each of which is considered to be an in-

dividual, massive basalt flow. The various sub-unitshave similar mineralogies and textures, containing botholivine and plagioclase phenocrysts, having less than 5per cent vesicles, and ranging in texture from cryp-tocrystalline at chilled margins to intergranular andeven subophitic at the centers of the thickest flows. Themost massive sub-unit, 2E, shows accumulation of oli-vine in its middle and lower sections, indicative of grav-itational differentiation.

Unit 3 has been divided into nine sub-units, the sixuppermost comprising three pillow-basalt sub-units(3A, 3C, 3E) each with an underlying massive-basaltsub-unit (3B, 3D, 3F) of similar lithology. The pillow

813


TABLE 3 - Continued


101

110

111

146

112

70

113

24

121

52

131

34

132

92

133

19

151

40

161

24

161

47

161

111

SiO2 (%)TiO2

AI2O3Fe2θ3MnOMgOCaONa2OK2O

P2O5Total



Fetot/MgK/RbBa/Sr

QOrAbAnNeDiHy01Mt11Ap

49.91.62

16.010.94

0.175.26

12.182.890.340.21

99.52

50150

9420

7162

35105

325

613

51

3592

0.330.120.240.912.41

4040.15

0.02.0

24.629.8

0.024.410.2

2.51.93.10.5

48.81.60

16.411.890.194.52

12.432.660.410.29

99.18

58192108

239

18538

1143

306

153

<l

38840.330.130.260.973.05

3790.16

0.02.4

22.731.9

0.023.7

9.52.92.13.10.7

49.81.55

16.110.340.165.89

11.972.830.330.22

99.18

70195105

215

17537

1164

244

1871

2980

0.320.160.211.192.04

5530.14

0.02.0

24.130.5

0.022.911.1

3.11.83.00.5

49.81.59

16.310.310.185.56

12.172.860.330.23

99.31

71207110

215

17839

1155

394

144

< l

23830.340.120.340.882.15

5410.22

0.01.9

24.430.9

0.023.410.1

3.01.83.00.5

49.11.60

16.311.480.194.66

12.122.590.500.32

98.85

67194

9924

8181

40120

336

417

2< l

4080

0.330.140.301.042.86

5210.20

0.03.0

22.231.6

0.022.513.4

0.42.03.10.8

49.91.74

16.110.940.195.60

12.012.750.360.25

99.82

66201100

206

17143

1245

286

196

< l

25840.350.150.231.092.27

4950.16

0.02.1

23.330.6

0.022.713.0

1.51.93.30.6

49.21.66

16.111.450.164.66

12.452.730.390.33

99.16

48197103

216

18641

1144

406

1832

2887

0.360.160.351.082.85

5400.22

0.02.3

23.330.8

0.024.410.8

1.42.03.20.8

49.51.58

16.110.790.145.20

12.272.790.400.35

99.11

51195110

227

18239

1154

318

1821

29820.340.160.271.132.41

4730.17

0.02.4

23.830.6

0.023.610.6

2.31.93.00.8

49.71.23

16.39.880.165.52

12.652.450.390.40

98.73

32498321

7219

3188

333

715

3< l

2984

0.350.170.381.192.08

4640.15

0.42.3

21.032.8

0.023.214.3

0.01.72.40.9

50.01.31

16.710.670.165.58

11.702.830.480.19

99.55

3935882221

2083395

245

517

32

4883

0.350.180.471.272.22

1880.22

0.02.8

24.131.5

0.021.110.7

4.01.92.50.5

49.71.23

16.69.830.175.51

12.672.390.380.22

98.67

42527919

8210

3388

231

413

4< l

4484

0.380.150.350.972.07

3960.15

0.32.3

20.533.9

0.023.414.1

0.01.72.40.5

50.21.13

16.38.610.126.80

12.172.580.300.14

98.31

59508220

4200

2781

330

416

54

27840.330.200.371.461.47

6230.15

0.01.8

22.232.5

0.022.814.7

1.21.52.20.3

basalts are sparsely olivine- and plagioclase-phyric, andchrome spinel occurs as an accessory phase. Vesicles arecommon to abundant (15-30% ). The flows are severalmeters thick, and the thickest again shows subophitictextures and olivine accumulation in its middle andlower sections. The bottom three sub-units of unit 3 aresparsely phyric massive flows or sills, in lithologysimilar to sub-units 3A to 3F, although they do contain2 to 3 per cent resorbed plagioclase phenocrysts.

Unit 4 is a single 15-m-thick flow or sill consisting ofintersertal to subophitic basalt. Plagioclase and olivinephenocrysts are common, and pilotaxitic alignment ofgroundmass pyroxene and feldspar is observed. Unit 5,on the other hand, consists of five flows which are either

aphyric or sparsely plagioclase-phyric. Again, however,vesicles are scarce. The last unit drilled, unit 6, com-prises two olivine- and plagioclase-phyric basalt sub-units, both of which could be flows or sills. Only the topfew centimeters of subunit 6B were recovered.

The Site 443 basalts are generally fine to mediumgrained with intersertal to intergranular textures. Por-phyritic varieties contain 1 to 10 per cent plagioclasephenocrysts (An70 or greater), and 2 to 5 per cent olivinephenocrysts (probably Fo80_9o) Due to alteration, thepresence of groundmass olivine is uncertain. Plagioclase(Ang<µyo) and augite are the two major groundmassphases, with about 2 to 5 per cent interstitial magnetite.Finer-grained sections have a groundmass containing

814


TABLE 3 - Continued

162

15

171

14

171

64

171

142

181

46

191

45

192

36

192

51

201

50

49.41.26

15.89.390.136.26

11.312.610.420.15

96.71

3942

10920

8204

3093< l34

3184

<l

810.320.190.371.471.74

4400.17

0.02.6

22.831.2

0.021.116.4

0.51.72.50.4

49.61.24

16.510.15

0.155.54

12.662.370.370.21

98.79

43577820

6211

3394

224

416

52

4779

0.350.170.261.192.12

5060.11

0.22.2

20.333.8

0.023.414.5

0.01.82.40.5

aNormative values

50.11.21

17.19.490.155.38

12.952.700.480.19

99.78

5058

14920

8210

3187

339

515

4< l

2983

0.360.170.451.192.05

4950.19

0.02.8

22.933.2

0.024.7

7.63.61.72.30.4

based on

50.21.65

15.511.09

0.175.75

11.952.680.330.25

99.61

47599420

6185

37121

330

617

52

4082

0.310.140.251.132.24

4570.16

0.22.0

22.829.5

0.023.415.5

0.01.93.10.6

FeoO^/F<

50.21.63

16.010.210.165.53

12.312.820.290.26

99.42

48598822

3189

39123

131

516

1<l

12379

0.320.130.251.012.14

8000.16

0.01.7

24.030.4

0.024.213.0

0.41.83.10.6

:O ratio

50.71.65

15.810.330.156.05

11.413.240.320.18

99.81

5958

10622

5173

36123

542

317

1< l

2580

0.290.140.341.161.98

5250.24

0.01.9

27.527.7

0.022.9

9.44.41.83.10.4

of 0.15: F<

50.41.65

16.410.110.165.38

12.263.080.320.21

99.92

57598122

7189

38129

434

419

3< l

3277

0.290.150.261.232.18

3810.18

0.01.9

26.129.9

0.024.4

8.23.31.83.10.5

50.21.59

16.210.380.195.46

12.133.130.380.20

99.86

6360842414

18638

1233

344

1573

4177

0.310.120.280.972.20

2250.18

0.02.2

26.529.0

0.024.7

6.15.31.83.00.5

:oθ^ is total iron as

50.11.62

15.910.960.175.47

12.042.760.470.22

99.70

4164792010

17939

1241

296

19< l< l

12478

0.310.150.231.202.32

3870.16

0.02.8

23.429.8

0.023.712.1

1.81.93.10.5

23Ce^/YN represents ratio of chondrite-normalized values.

plagioclase and pyroxene microlites, with granular orskeletal magnetite set in a cryptocrystalline matrix,often partially altered to clays. Chrome spinel is an ac-cessory phase in units 3 and 4.

Veins and fractures lined by calcite and clays arecommon throughout the recovered section, as arecarbonate- and clay-filled amygdules. In unit 4 (Core60), native copper occurs in a carbonate vein togetherwith well-formed pyrite octahedra. Other veins in thisunit are lined by a talc-like mineral which has alsoreplaced the olivine phenocrysts. Olivine in other unitsis invariably partially or totally replaced by pseudo-

morphs of clay and chloritic material, with occasionalzeolites or calcite. In unit 2, the cores of some largeplagioclase phenocrysts are saussuritized.

The basalts recovered at Site 443 (Table 6) areolivine- and hypersthene-normative sub-alkaline tholei-ites; no nepheline- or quartz-normative varieties havebeen found (Figure 3). On the basis of their chemistry,the basalts have been divided into five geochemical units(Figure 17): units 443-1 and 443-2 correspond to litho-logic unit 2; geochemical unit 443-3 corresponds to mostof lithologic unit 3; geochemical unit 443-4 consists ofthe lower part of lithologic unit 3, and lithologic units 4

815


Site 443

Figure 3. Normative nepheline (Ne), olivine (Ol), diopside (Di), hypersthene (Hy),quartz (Q) tetrahedra for basalts from Sites 442 and 443. Normative values basedon Fe2O3/FeO ratio of 0.15.

Site 443

20

10

90 80

Figure 4. Normative plagioclase, pyroxene, olivine diagrams for basaltsfrom Sites 442 and 443. The Fo-An cotectic (C) is from Shido et al.(1971).

and 5; and geochemical unit 443-5 corresponds to litho-logic unit 6. Geochemical unit 443-4 cannot be disting-uished from 443-5 using the present XRF data, butbecause they are petrographically distinct and becausethe two units have different Th/Hf ratios (Wood et al.,this volume) we have retained the division for the pur-pose of the present discussion.

On the diagram of normative plagioclase-pyroxene-olivine, the basalts range in composition from olivinetholeiites (443-3, 443-4 and 443-5), through olivine-pla-gioclase tholeiites straddling the 1-atmosphere Fo-Ancotectic (443-2), to plagioclase tholeiites (443-1)(Figure4). This division broadly corresponds with the observedphenocryst assemblages, although many of the basaltsfrom Site 443 are only sparsely phyric or even aphyric.

Thus, the phenocryst assemblage of lithologic unit 2 isdominated by plagioclase, whereas in the lower litho-logic units, olivine phenocrysts are more abundant.

As with the Site 442 basalts, there is considerabledown-hole variation in chemistry at Site 443 (Figure 17).From the Fe/Mg ratio and Zr, Cr, and Ni abundances(Figures 17 and 18), it is evident that geochemical units443-4 and 443-5 contain the most-primitive basalts (withthe lowest Fe/Mg ratio and Zr, and highest Ni and Cr)and that units 443-1 and 443-3 contain the most-evolvedbasalts recovered at this site. Unit 443-2 occupies an in-termediate position. However, there is considerablewithin-unit variation. Thus, unit 443-3 basalts contain54 to 156 ppm Ni, one sample (Core 58-3, 29 cm) con-taining 405 ppm Ni, 428 ppm Cr, and 14.01 per cent

816


200 -

3 100 -

2.0

Zr (ppm)

Figure 5. Ni versus Zr, Site 442. The numerals indicategeochemical units in down-hole order (see Figure 2).

' ' ' ' 1 I

Site 442

• 442-1A 442-2x 442-3O 442-4

-

l,

l,

' . ' | i i i i

•

-

V; :

A

K. x αS>oX XX —

0 50 100 150Zr (ppm)

Figure 6. Cr versus Zr, Site 442.

MgO. Ni and Cr also show strong variations in Units443-1, 443-4, and 443-5. Whereas fractional crystalliza-tion (involving removal of pyroxene, olivine, and (or)chrome spinel from the melt) may be responsible forsome of the observed variations in Ni and Cr, it is evi-dent that some within-flow differentiation has also oc-curred, particularly in the thickest flows of lithologicunit 3. Thus, the high Ni and Cr levels observed in 58-3,29 cm are probably due to the presence of cumulus oli-vine and chrome spinel. However, the olivine accumula-

1.0 -

1 1 1 1—

• Site 442

• 442-1A 442-2x 442-3O 442-4

~S *

-I 1 1 1 1 1 1 r

X

A

•• *

•

i i i i i i i i

/

A

A

i

' ' '

d>

i i

<ß× -o

-

1 P

100

Zr (ppm)

Figure 7. TiO2 versus Zr, Site 442.

15 -

* Site 442

• 442-1A 442-2x 442-3O 442-4

-

-

•

X «XXX•

• x*

m» •«

xX

X

•• A

1••

i 1 i

àAà.

A

A

<A O

AOO

O

O J>

* ^ -

-

-

100Zr (ppm)

150

Figure 8. Ce versus Zr, Site 442.

10 I I , .

- Site 442

• 442-1- A 442-2

x 442-3O 442-4 *

_

-

-

-―

— i 1-

•

×

1

—i 1—

/

/

/

• x#_•_•_•

~1—1

••XX

A

•

i

A

AA4

A

CA

0 0

* Site 443 ^

|l/ _

/O /

x^_ • * '

-

50 100Zr (ppm)

150

Figure 9. Nb versus Zr, Sites 442 and 443. Individualdata points for Site 443 omitted for clarity.

tion which was observed petrographically in unit 2E hasnot been geochemically confirmed. In general, it is ap-parent that the basalts from Site 443 have undergone

817


1 1 1—i—

Site 442

ΦA

X

o

_

442-1442-2442-3442-4

^ Φ

X

X

ΦΦΦΦ

x m mm• ΦΦ Φ

Φ _,

1

X

X

Φ

ΦX ΦΦ>

1 '

A

Φ

A

A

A

A

A

,/-

y^ —

O A 9^-^^

oo

° ,,6/ivi L-^—

o —

-

-

100Zr (ppm)

Figure 10. P2O5 versus Zr, Site 442. The scatter of datapoints along the P2O5 axis in geochemical units 2, 3and 4 appears to be a function of alteration of thesebasalts. See also Figure 15.

0.5 -

- Site 442

Φ 442-1~ A 442-2

X 442-3O 442-4

-y×^^—

Φ

Φ

*

X

<

Φ

Φ

X

X

Φ

' • Φ

1

X

X

•Φ

Φ•

A AA

Φ

_

O ^-"^

A o

°o oo -

-

-

50 100Zr (ppm)

Figure 11. K2O versus Zr, Site 442. Scatter of datapoints is indicative of sea-water alteration.

20

10

n

Site 442

-

AX

O

442-1442-2442-3442-4

Φ

ΦΦXX

ΦΦXΦ

ΦX

ΦΦ

Φ«Φ

ΦΦ Φ

1 1

x

x ΦΦ

XΦ

. 1

A

Φ

A

A

A

A t

-

-

-

o

o

AO

O A

o

o

100Zr (ppm)

150

Figure 12. Rb versus Zr, Site 442. Scatter of data pointsalong Rb axis in geochemical units 3 and 4 is in-dicative of sea-water alteration.

Site 442

ΦA

X

o

-

442-1442-2442-3442-4

Φ

Φ

X

ΦΦ t

V. *•

Φ Φ

y

Φ

I

Φ

A

' ' ' ' ' \ ^

—

O

A

o ot °°2 ja£^°2^-A

50 100Zr (ppm)

Figure 13. Ba versus Zr, Site 442. As in figures 11 and12, scatter of data points may be a result of sea-wateralteration.

200

150

100

1

Site 442

ΦA

~ X

o

_

442-1442-2442-3442-4

Φ

//

/

1

X

ΦΦ

1 1 1

X

y

x× xX

Φ

Φ Φ

Φ>*

Φ *

Φ

Φ

I '

A

Imn

-1 T—7

/

A

1 '

k oA

i i i i

;

"V -o

•

-

-

-

I 1 1 1

150

Zr (ppm)

Figure 14. Sr versus Zr, Site 442.

considerable fractionation prior to their eruption: theirNi and Cr contents are too low, and Fe/Mg ratios toohigh, for the liquids to have been in equilibrium withupper-mantle olivine compositions (Fo88_90; Green,1970; O'Hara, 1973).

Using Zr as an index of fractionation, hygromagmat-ophile trace elements have been plotted in Figures 19 to25. TiO2, P2O5, Ce, La, Nb, and Y all show strong posi-tive correlations with Zr, similar to the Site 442 data. Ingeneral, the ratios of these elements between the dif-ferent geochemical units are constant, particularly forthe more-HYG elements (Ce, La, Nb, and Zr). The plotof Zr versus Y (Figure 20), however, suggests that geo-chemical units 443-2 and 443-5 have slightly higherY/Zr ratios than units 443-3 and 443-1; unit 443-4shows a range of Y/Zr ratios covering both extremes. Astrong correlation exists between P2O5 and Zr (Figure21): the incoherent scatter of P2O5 in lithologic unit 2 atSite 442 is not observed at Site 443.

818


Core P2O5 (wt. %)

0.1 0.2 0.3 0.4

K2O (wt. %)

0.1 0.2 0.3 0.4 0.5

• 442-1A 442-2x 442-3O 442-4 Λ•

•

60—i r

Zr (ppm)

80 100

\ .

J I I I I L

Figure 15. Down-hole variation at Site 442 in the abundancies ofP2O5, K2O, and Zr.P2O5 and Zr show a broad correlation in unit 442-1 but the distribution ofP2O5 isincoherent in units 2, 3 and 4. K2O also exhibits considerable scatter, probably in-dicative of sea-water alteration.

1 ' ' ' 1

- Site 442

• 442-1A 442-2

1 x 442-3O 442-4

^ — " " "

i • | i

. 1 1 , 1

• xx • A

Y (ppm)

Figure 16. Ce versus Y, Site 442. CeN/YN indicateschondrite normalized Ce/Y ratios (approximatelyequivalent to CeN/HoN ratio).

K2O, Rb, and Sr do not show clear correlations withZr (Figures 22 and 23) and, as with the Site 442 basalts,we suspect that this is essentially because of alterationby sea water. The least-altered samples show low K/Zr,Ba/Zr, and Rb/Zr ratios, similar to those of N-typeocean-ridge basalts (Table 4; Erlank and Kable, 1976).

CeN/YN ratios for the basalts of Site 443 are, withinthe error of XRF analysis at such low levels of Ce, con-stant throughout the five units, at about 1 (Figure 26).This is in agreement with preliminary, more-compre-hensive REE data which indicate that the CeN/YbN ratiois approximately 1.5 in unit 443-3, and 1.2 in unit 443-4(Wood et al., this volume; unpublished data of theauthors). The CeN/YN ratios for the Site 443 and 442basalts are practically indistinguishable, and are similarto the low values reported for N-type ocean-ridge ba-salts (e.g., Kay et al., 1970; Sun et al., 1979). Other

trace-element ratios, in addition to the K/Zr, Ba/Zr,and Rb/Zr ratios already mentioned, indicate closesimilarities between the Site 443, 442, and N-type ocean-ridge basalts. Zr/Nb ratios are high (25-40; Figure 9),Ce/Zr ratios are low (Figure 26), and Y/Zr ratios ap-proach chondritic values (Figure 20). However, neitherthe Site 442 nor Site 443 basalts show the strong deple-tion of all more-HYG elements that is observed inN-type basalts. Thus, CeN/YN is approximately 1, andnot significantly less than 1, as in N-type ocean-ridgebasalts (Sun et al., 1979).

With the exception of the slight variation in Y/Zrratio, the constant ratios of most hygromagmatophileelements among the units of Site 443 indicate that mostof the basalts could be derived from a common basalticparent by varying degrees of low-pressure fractionalcrystallization of olivine, plagioclase, and (or) chromespinel. The rapid decrease in Ni and Cr relative to Zr(Figure 18) supports this suggestion. However, to varythe Y/Zr ratio (and the Th/Hf ratio; Wood et al., thisvolume) requires either variable degrees of partial melt-ing, or open-system crystal fractionation (O'Hara, 1977),in order to retain Y in the refractory mineral assem-blage. We suggest, judging from Y and Zr distributionsin basalts recovered on DSDP Leg 49 in the NorthAtlantic Ocean (Tarney et al., 1978), that units 443-1and 443-3 represent magma batches derived by smallerdegrees of partial melting of the mantle source, and thatunits 443-2 and 443-5 represent slightly higher degreesof partial melting of the same mantle source. Unit 443-4occupies an intermediate position, and it is apparentthat most of the units subsequently underwent con-

819


TABLE 4Representative Analyses and Element Ratios of Basalts from the Daito Basin and from the Atlantic Oceana

Component

SiO2

TiO2

A12O3

F e 2 O 3

MnOMgOCaONa2OK2OP2O5L.O.I.

Total

NiCrRbSiYZrNbBaLaCe

Fe/MgZr/NbCe/ZrCe N /Y N

K/RbBa/SrP/ZrBa/Zr

446-1

45.83.67

14.710.880.364.80

11.143.162.040.372.53

99.40

291836

68927

20627

3132245

2.6380.224.09

4700.457.81.52

Daito Basin,Alkalic Group

446-4

44.84.9

13.311.850.196.25

12.442.811.5.50.990.48

99.53

456922

108826

18832

3622558

2.2060.315.48

5860.33

23.01.93

446-5

42.44.07

10.312.770.22

12.4311.25

1.490.740.732.88

99.33

2931137

16480

19150

24212

2242

1.1960.285.43

3830.44

21.21.41

446-10a

47.13.72

14.99.760.244.97

11.532.971.950.352.03

99.45

283030

75628

21125

3322448

2.2880.234.21

5390.447.21.57

446-6

49.63.61

13.013.01

0.176.339.882.690.270.341.07

100.02

72141

1367

31194

19121

1842

2.3810

0.223.33

22170.337.70.62

Daito Basin,Tholeiitic Group

446-8

47.84.16

13.114.490.215.82

10.162.500.200.430.94

99.86

3323

2447

37302

28143

3260

2.8911

0.203.98

8220.326.20.47

446-9

46.34.95

13.714.970.235.568.303.071.030.511.11

99.70

2039

45 341

37832

2253470

3.1212

0.194.19

9490.505.90.60

446-14

49.63.17

13.312.5 30.236.19

10.862.560.280.371.09

100.08

94175

11426

33229

241022650

2.3510

0.223.72

2090.247.10.45

1

50.011.37

16.1810.18—7.71

11.332.790.220.130.87

100.79

100200

1124

3390

31239

1.5330

0.100.67

18260.106.30.13

Atlantic Ocean Basalts

2

49.301.74

14.0112.130.198.09

11.652.300.180.220.83

100.64

106317

4103

35110

1370

619

1.7480.171.33

3740.688.70.64

3

47.635.22

12.5416.710.235.619.233.040.830.56n.d.

101.6

156616

44536

29031

1802857

3.4690.203.90

4310.408.40.62

4

46.452.94

16.6611.000.075.30

10.383.751.240.481.32

99.59

97022

693-

220-

3022951

2.41-0.23—

4680.449.51.37

aAnalyses of Daito Basin basalts from Tables 6 and 7. Atlantic Ocean basalts: 1, typical "depleted" N-type ocean-ridge basalt; major ele-ments from Engel et al. (1965); trace elements from Sun et al. (1979) and Bougault et al. (1979), with typical values for Ni and Cr; 2, tran-sitional or "T-type" ocean-ridge basalt, Reykjanes Ridge, Leg 49, Site 409, unit 3 (Wood et al., 1979); 3, "enriched" ferrobasalt, Eldgjá,Iceland, sample ISL79 (Wood et al., in press); 4, "enriched" alkali basalt, Fayal, Azores, sample FA47 (Joron et al., in prep.). F e 2 θ 3 istotal iron as F e 2 θ 3 . Ce^/Y^ represents ratio of chondrite-normalized values.

siderable fractional crystallization (particularly units443-3 and 443-1) prior to eruption.

Site 444

Site 444 (Figure 1) lies approximately 100 km southof Site 443, and along the same depositional, structural,and topographic trends. Located at 28°38.25'N, 137°41.03' E, in 4843 meters of water, Hole 444A drilled twobasaltic units, some 50 meters thick, and 285 meters ofsediments (Figure 27). The sediments above and be-tween the two basaltic units are approximately 15 m.y.old, and Waples (this volume), judging from changes inthe sedimentary organic chemistry profile, has sug-gested that the basalt of unit 1 was emplaced as a sillsome 13 m.y. ago. In addition, the sediment immediate-ly above unit 2 appears to have undergone thermal alter-ation, suggesting sub-sediment emplacement of unit 2also. Hence, it appears that both units were emplaced assills and are not true basement, which, from magnetic-anomaly data, has been tentatively dated as 21 to 22m.y. at this site.

Unit 1 is an aphyric dolerite sill with petrography dis-tinct from the other rocks recovered from the ShikokuBasin. The principal constituents are plagioclase (An60_82)and augite; olivine varies from less than 1 per cent at thetop of the sill to 15 per cent in the middle and lower sec-tions. The groundmass is essentially replaced by cryp-tocrystalline chloritic or clayey matrix. Opaque mineralsinclude magnetite and elongate, saw-toothed aggregatesof ilmenite. Apatite needles are found as occasional in-clusions in plagioclase, and chrome spinel occurs as in-clusions in some relict olivines. Small, green-brown,platey crystals have been tentatively identified as kaer-sutite amphibole (Dick et al., this volume). The inter-val 20-1, 53-81 cm is distinct from the remainder of theunit in containing less pyroxene (which has a strongpink-brown color indicative of a high TiO2 component),more magnetite and ilmenite, and aggregates of kaer-sutite up to 1 cm across. Although the very alterednature of the groundmass in this section suggests thatthe different mineralogy may be due to alteration, late-stage magmatic differentiation may also have producedthe observed mineralogy. In fact, the chemistry sup-

820


TABLE 5Representative Analyses and Element Ratios of Basalts from the Shikoku, East Scotia Sea,

and Bransfield Strait Back-Arc Basinsa

Component

Siθ2TiO2

A12O3

F e 2 O 3

MnOMgOCaONa2OK 2 OP2O5

Total

NiCrRbSrYZrNbBaLaCe

Fe/MgZr/NbCe/ZrCeN/YN

K/RbBa/SrP/ZrBa/Zr

442-1

50.21.26

15.98.660.107.93

11.112.800.220.13

98.37

157232

2179

2791

435

211

1.27230.121.0

8970.206.20.38

442-3

50.21.13

16.38.610.126.80

12.172.580.300.14

98.31

5950

4200

2781

330

416

1.4727

0.201.46

6230.157.50.37

Shikoku Basin

443-3

50.31.83

15.59.420.107.52

10.693.090.220.18

98.83

73225

3178

40126

750

717

1.4518

0.131.04

5950.286.20.40

443-4

49.71.22

16.39.130.137.95

11.622.710.100.11

98.99

113297

1154

2887

331

48

1.3329

0.090.70

8550.205.50.36

444-1

50.31.94

16.27.960.156.258.593.862.310.39

97.92

26127

16391

38209

31177

1842

1.4870.202.71

11980.458.10.85

444-2

50.21.61

16.110.150.176.87

11.362.820.160.23

99.73

53245

2160

40141

771

820

1.7120

0.141.23

6640.447.10.50

East Scotia Sea

D20.43B

50.71.29

16.68.40.167.67

11.123.150.340.19

99.63

63270

5193

29107

349

_16.1

1.2736

0.151.37

5640.257.80.46

D24.14

53.80.61

14.59.20.177.71

10.801.790.240.08

98.99

42295

4123

1440<l55

-6.45

1.38>40

0.161.13

4980.458.71.38

Bransfield Strait

B138.2

51.91.5

16.29.460.186.11

10.074.070.280.21

99.96

35141

3340

26144

288

822

1.8072

0.152.08

7750.266.40.61

P640.1b

52.90.64

17.77.430.136.14

10.303.540.470.06

99.27

40130

11332

1058< l70

28

1.40>58

0.140.97

3550.214.51.21

aAnalyses of shikoku Basin basalts taken from Tables 2 to 5. Analyses of East Scotia Sea basalts from Saunders andTarney (1979), those for Bransfield Strait basalts from Weaver et al. (1979). F e 2 θ 3 is total iron as F e 2 θ 3 •Cej\[/Y]S[ represents ratio of chondrite-normalized values.

CoreNo.

LithologicUnit

MagneticInclination

-90° 0° +90°

Zr (ppm)

80 120

Ba (ppm)

20 40 60

Cr (ppm)

100 400

Fe/Mg Y/Zr

1 2 3 0.1 0.2 0.3 0.4

Ti/Zrε ~

P60 70 80 90 O

460- 49

Claystone and Volcanicash

Unit 1

4 8 0 -Unit2

Plagioclase—olivinephyric basalts

~ 500-54

520

Unit 3Aphyric olivine-

bearing basalts andpillow basalts I

= 540- Unit4Plagioclase—olivine basalts

5 6 0 -

580-

UnitδAphyric and sparsely

phyric plagioclase and

pillow basalts

- ^ Unit 6 ^ - –Olivine-plagioclase basalts

X

ci

»

V\

•TI

_J k t• ~4rFigure 17. Down-hole variations in lithology, magnetic inclination, and selected geochemical parameters at Site 443.

821


TABLE 6Analyses, Element Ratios, and CIPW Norms for Basalts from Hole 443 a


SiO2 (%)TiO2AI2O3Fe2O3MnOMgOCaONa2OK 2 OP2O5Total



Fetot/MgK/RbBa/Sr


493

127

50.31.55

16.89.290.205.83

12.372.910.180.18

99.62

52219

7322

2162

35116

334

516

1<l

39800.300.140.291.121.85

7470.21

0.01.1

24.732.4

0.023.011.1

1.91.63.00.4

494

90

49.81.56

16.410.580.236.31

11.782.680.150.18

99.67

49216

8222

1155

34112

443

6146

< l

28840.300.130.381.011.94

12530.28

0.00.9

22.832.4

0.020.615.3

1.91.83.00.4

502

14

49.51.56

16.710.410.186.54

11.562.690.170.18

99.50

50198

7922

2151

35121

542

716

3< l

24770.290.130.351.121.85

7010.28

0.01.0

22.933.1

0.019.214.6

3.11.83.00.4

503

80

50.01.60

16.49.640.255.76

12.732.780.160.18

99.53

54224

7721

1161

37120

535

516

7< l

24800.310.130.291.061.94

13030.22

0.00.9

23.631.9

0.024.911.2

1.41.73.10.4

504

74

49.91.56

16.410.840.176.30

11.582.790.200.18

99.90

56231

9419

3152

35115

440

716

6<l

29810.300.140.351.122.00

5530.26

0.01.2

23.631.5

0.020.414.1

2.91.93.00.4

5110

49.71.58

14.710.160.137.77

11.422.650.440.18

98.73

107249

76186

16438

1167

587

144

< l

17820.330.120.500.901.52

6120.35

0.02.6

22.727.3

0.023.512.2

5.41.83.00.4

5212

50.11.58

16.59.130.355.77

12.792.860.160.18

99.40

60244488

192

16537

1143

384

136

< l

38830.320.110.330.861.84

6680.23

0.01.0

24.331.8

0.025.3

9.62.11.63.00.4

521

90

49.41.32

15.99.570.147.09

12.432.760.270.14

98.98

99249

9419

3168

3092

322

313

1<l

31860.330.140.241.061.57

7420.13

0.01.6

23.630.5

0.025.2

5.78.01.72.50.3

522

120

49.91.36

16.310.170.167.11

12.112.650.070.14

99.94

94263482

17<l

1643098

117

2124

<l

98830.310.120.170.981.66-0.10

0.00.4

22.432.4

0.021.913.04.31.82.60.3

524

16

49.81.35

16.410.000.167.04

12.042.700.080.15

99.75

86256

7617

< l163

33101

327

31351

34800.330.130.270.971.65-0.17

0.00.5

22.932.4

0.021.712.54.51.72.60.3

532

117

50.31.47

16.49.430.146.30

12.232.900.210.15

99.62

107263

9819

2171

35107

549

4154

< l

21820.330.140.461.051.74

8800.29

0.01.3

24.631.4

0.023.410.5

3.31.62.80.4

533

64

49.91.40

16.29.930.146.94

12.012.800.140.14

99.53

102263131

181

16534

1014

235

1352

25830.340.130.230.941.66

11540.14

0.00.8

23.831.3

0.022.610.7

5.11.72.70.3

ports the latter explanation, because this section hashigher contents of TiO2, P2O5, Zr, Ce, and Y than theremainder of lithologic unit 1.

Lithologic unit 2 consists of plagioclase-phyric basalt(10-15% plagioclase phenocrysts, composition aboutAn70), grading from aphanitic in the chill zone at its up-per margin to subophitic at its center. The uppermost 2meters are amygdaloidal, with 5 to 20 per cent clay- andzeolite-filled vesicles. Relict olivine phenocrysts are seenonly in the lower parts of the unit, and rarely exceed 10per cent of the rock. Augite and magnetite occur throughoutthe unit, and in the coarsest sections the augite ophiti-cally encloses plagioclase laths. Unit 2 appears to bequite distinct from unit 1, the former not containingsignificant ilmenite or apatite and no kaersutite ortitaniferous augite.

Major- and trace-element analyses of basalts recov-ered at Site 444 are given in Table 7. Geochemically, the

two lithologic units 1 and 2 are quite distinct; they aretherefore treated as two separate geochemical units (Fig-ure 27). The basalts of geochemical unit 1 are nepheline-normative and with a high normative olivine/diopsideratio (Figure 28), which is a function of their low CaOcontents. Geochemical unit 2, however, comprises sub-alkaline hypersthene- and olivine-normative tholeiiteswhich plot mainly in the plagioclase-tholeiite field on thenormative plagioclase-olivine-pyroxene diagram (Figure29). This is in agreement with the observed phenocrystassemblage (10-15% plagioclase phenocrysts, An70); thetwo samples which plot in the olivine-tholeiite field (Core23-2, 84 cm, and Core 27-5, 68 cm) both contain alteredgroundmass olivine. Core 27-5, 68 cm also contains rel-ict olivine phenocrysts, possibly in part of cumulusorigin.

Geochemical unit 444-1 comprises basalts which havelow absolute abundances of SiO2 and CaO compared

822


TABLE 6 - Continued

542

44

5434

544

106

545

31

54636

547

71

551

81

551

104

551

145

552

115

561

112

562

103

563

115

571

129

49.51.32

16.310.090.167.4911.862.630.080.14

99.53

1122549321

116130953

2541333

32830.320.140.261.061.56

6640.16

0.00.522.432.60.0

20.912.26.01.82.50.3

49.61.29

16.310.050.157.6511.742.740.080.15

99.76

1172536419<l16031943

266144

<l

31820.330.150.281.111.52_0.16

0.00.523.232.10.020.710.87.3

1.82.50.3

49.51.31

16.310.180.157.6111.762.700.110.14

99.75

110275107172

15830953

182

1261

32830.320.130.190.981.55

4400.11

0.00.622.932.00.0

20.910.97.21.82.50.3

49.61.33

16.510.080.157.43

11.782.780.100.14

99.84

10126271191

16130982

24410<l

1

49810.310.100.240.821.57

8130.15

0.00.623.632.20.0

20.810.07.41.82.50.3

49.61.32

16.39.890.157.1412.072.660.100.13

99.43

1032677219<l1653095424417<l<l

24830.320.180.251.391.61_0.15

0.00.6

22.632.50.0

22.012.04.91.72.50.3

49.71.33

16.310.030.157.2412.162.690.060.13

99.82

9927311218<l16131972

304126

<l

48820.320.120.310.951.61—0.19

0.00.422.832.30.022.311.15.71.72.50.3

49.71.6314.811.200.137.1511.382.630.390.17

99.24

8024481205

178361182

4571351

59830.310.110.380.891.82

6420.25

0.02.3

22.427.70.023.014.33.82.03.10.4

49.11.57

14.810.710.147.5211.872.480.360.17

98.72

11125279185

17033

1175374131

<l

23800.280.110.320.971.65

5930.22

0.02.1

21.328.60.0

24.511.85.51.93.00.4

49.41.66

14.810.980.147.3711.492.570.350.17

98.90

9824181194

16838120

7269193

<l

17830.320.160.221.231.73

7180.15

0.02.1

22.028.20.023.213.64.51.93.20.4

49.31.60

14.710.640.137.7911.602.540.320.17

98.78

12423479203

167361164314193

<l

29830.310.160.271.301.58

8800.19

0.01.9

21.828.00.0

23.813.05.3 .1.93.10.4

49.61.48

14.210.510.138.4411.222.530.400.17

98.67

15627876165

162341095

454134

<l

22810.310.120.410.941.44

6610.28

0.02.4

21.726.50.023.414.25.8

1.92.80.4

48.71.62

15.29.460.136.6513.882.550.240.18

98.57

8621975213

176381223

3161841

41800.310.150.251.161.65

6610.18

0.01.4

21.929.60.0

31.92.07.11.73.10.4

49.91.74

15.210.470.146.6111.412.850.470.19

99.05

54213821914

167391313

565162

<l

44800.300.120.431.011.84

2790.34

0.02.824.327.70.0

23.111.63.91.83.30.5

49.51.70

14.810.610.137.2811.672.720.330.18

98.92

10322883235

17139122

73651871

17840.320.150.301.131.69

5500.21

0.02.0

23.327.60.0

24.310.85.51.93.30.4

with unit 444-2 and the other Shikoku Basin basalts. Inaddition, the abundances of Na2O, K2O, P2θ5, Rb, Sr,Nb, Ba, La, and Ce are significantly higher than in unit444-2, and this has resulted in high Ba/Sr, Ba/Zr,Ce/Zr, and CeN/YN ratios in the 444-1 basalts.Although both K2O and Rb are high in these basalts(over 1.5% and 25 ppm, respectively), the strong correl-ation between these two elements, and between themand Zr (Figure 30), suggests that alteration is not entire-ly responsible for this enrichment. Indeed, the very highK/Rb ratio (generally greater than 1000) testifies againstlow-temperature alteration by sea water, which is knownto reduce the K/Rb ratio in oceanic basalts (e.g., Hart etal., 1974). The alkaline affinity of the basalts of geo-chemical unit 444-1 is also supported by the low Zr/Nb,Ti/Zr, and Y/Zr ratios (Table 7; Figures 31, 32, and

33), and of course by the presence of kaersutite andgest that significant fractionation of these phases hastitaniferous augite. In many respects, the basalts of geo-chemical unit 444A-1 closely resemble E-type ocean-ridge basalts from the North Atlantic (Tarney et al., inpress).

The high Ni and Cr contents of the unit 444-1 basaltsindicate that the overall high abundances of many of theHYG elements are not due to high-level fractional crys-tallization involving pyroxene, olivine, or chrome spin-el. However, within-unit variation (Figure 34) does sug-occurred locally, resulting in the low Ni and Cr and highHYG-element abundances observed in Core 20-1, 73cm. This interval is also mineralogically distinct fromthe remainder of lithologic unit 1, containing titan-augite, ilmenite, and abundant kaersutite, and it possi-

823


TABLE 6 - Continued


SiO2 (%)TiO2

A12O3

F e 2 O 3

MnOMgOCaONa2θK2O

P2O5

Total



Fetot/MgK/RbBa/Sr


5728

49.91.69

15.210.360.146.73

12.202.680.390.17

99.39

92237

982210

16640

1248

486

167

< l

15820.320.130.390.981.79

3210.29

0.02.3

22.828.3

0.025.710.7

3.81.83.20.4

572

109

49.41.71

14.811.340.186.46

12.282.510.470.17

99.33

90222

9418

6159

41123

549

611

21

25830.330.090.400.662.04

6560.31

0.02.8

21.427.8

0.026.610.9

3.82.03.30.4

581

58

49.71.68

15.310.280.146.91

11.072.860.410.18

98.56

79233

891917

16839

1245

495

161

< l

25810.310.130.401.011.73

2020.29

0.02.5

24.628.00.0

21.712.64.21.83.20.4

581

94

50.01.91

15.49.840.146.90

11.032.960.490.20

98.93

62227

802115

17544

1385

526

174

< l

2883

0.320.120.380.951.65

2700.30

0.02.9

25.327.7

0.021.411.6

4.21.73.70.5

582

99

50.31.83

15.59.420.107.52

10.693.090.220.18

98.83

73225

8422

3178

40126

750

717

52

1887

0.320.130.401.041.45

5950.28

0.01.3

26.528.2

0.019.714.0

3.91.73.50.4

583

29

46.11.36

11.113.850.17

14.017.481.730.190.16

96.13

405428102

124

972996

740

614

5<l

1485

0.300.150.421.191.15

3960.41

0.01.2

15.222.8

0.012.026.215.8

2.52.70.4

584

30

50.41.93

14.99.720.109.339.262.850.260.20

99.00

115246

8621

3154

40138

760

518

5< l

2084

0.290.130.431.111.21

7060.39

0.01.5

24.427.4

0.014.322.6

3.11.73.70.5

591

71

49.51.28

16.09.950.176.08

12.942.710.290.13

99.06

92286

6819

6173

2990

228

312

2< l

4585

0.320.130.311.021.90

3960.16

0.01.7

23.130.9

0.027.1

6.15.61.72.50.3

591

129

49.71.19

16.39.550.176.84

12.092.750.310.12

99.00

100285

6416

7165

2988

336

384

< l

2981

0.330.090.410.681.62

3740.22

0.01.9

23.531.5

0.023.1

8.46.51.72.30.3

592

15

49.71.25

16.29.060.137.84

11.582.680.100.12

98.66

104291

8718

1149

2791

323

411

51

30820.300.120.251.001.34

8630.15

0.00.6

23.032.4

0.020.413.5

5.11.62.40.3

592

91

50.01.24

16.48.940.137.34

12.072.720.060.12

99.02

108294

7819

< l156

2984

228

311

6< l

4288

0.350.130.330.931.41_

0.18

0.00.3

23.232.7

0.022.012.9

3.81.62.40.3

5934

49.71.22

16.39.130.137.95

11.622.710.100.11

98.99

113297

7917

1154

2887

331

484

< l

2984

0.320.090.360.701.33

8550.20

0.00.6

23.232.4

0.020.412.26.21.62.30.3

bly results from within-sill fractionation. The HYG-ele-ment ratios of Core 20-1, 73 cm, however, are indis-tinguishable from the rest of geochemical unit 444-1.

Geochemical unit 444-2 comprises basalts which arechemically similar to those recovered at Sites 442 and443. Thus, they contain neither normative nepheline nornormative quartz (Figure 28). They have low abun-dances of K2O, P2O5, Rb, Sr, and Ba, and have Ce/Zr,Ba/Zr, Ce/Y, and K/Rb ratios similar to those of theother Shikoku Basin basalts. Nevertheless, they haveslightly higher Nb (5-9 ppm), and consequently theyhave lower Zr/Nb ratios than the basalts from eitherSite 442 or Site 443, and they have significantly lowerTi/Zr ratios (Table 7; Figures 31 and 32). Cr and Ni areless than 100 ppm and 280 ppm, respectively, and themoderately high Fe/Mg ratios (greater than 1.5) testifyto the fractionated nature of these samples. The higherNi and Cr in Core 27-5, 68 cm, at the base of lithologic

unit 2 may be due to the presence of relict olivine pheno-crysts, possibly in part of cumulus origin. Within-unitfractionation of Ni is clearly indicated by Figure 34.

A plot of Ce against Y (Figure 35) shows that thebasalts of geochemical unit 444-1 are light-REE en-riched, CeN/YN ranging from 2.6 to 2.8, and CeN over30. Geochemical unit 444-2 is also enriched in lightREE, but to a much lesser degree (CeN/YN 1.12-1.46),and thus more closely resembles the basalts from Sites442 and 443 (Figure 35).

The available data suggest that it is unlikely that geo-chemical units 444-1 and 444-2 were derived from thesame parental melt by differing degrees of fractionalcrystallization. Ni and Cr are highest in 444-1 (apartfrom 20-1, 73 cm), which also contains basalts most en-riched in HYG elements. It is also unlikely that the mag-mas of the two units could have been derived from acompositionally identical source by varying the nature

824


593

101

594

138

59565

60121

60180

602

106

TABLE 6

6040

— Continued

606

104

61193

612

107

61313

61329

613

101

61469

49.71.27

16.09.280.137.66

11.702.770.230.11

98.80

14530175193

15029914313105

<l

23840.320.110.340.851.40

6280.21

0.01.4

23.730.90.022.09.87.01.62.40.3

49.11.26

15.98.580.128.71

11.792.680.070.12

98.26

12429161201

20026901243112

<l

90840.290.120.271.041.14

5730.12

0.00.423.131.60.021.98.99.11.52.40.3

49.61.25

16.38.300.138.54

11.522.690.280.11

98.75

14031180194

15127872292104

<l

43860.310.110.330.911.13

5850.19

0.01.7

23.032.00.020.39.88.31.52.40.3

48.81.18

15.89.110.159.1610.902.610.060.12

97.94

1222935718<l142268512321321

85830.310.150.271.231.15-0.16

0.00.422.531.90.018.212.89.11.62.30.3

49.31.23

15.89.390.169.11

10.672.690.090.11

98.52

1243035718<l144299133128

<l<l

30810.320.090.340.681.20_0.22

0.00.523.131.30.017.613.88.61.72.40.3

49.11.24

15.89.640.168.5711.072.520.090.12

98.38

10329970181

137289213131043

92810.300.110.340.881.30

7060.23

0.00.521.732.20.018.515.66.21.72.40.3

49.31.27

15.89.510.178.5611.352.560.070.12

98.64

9630566192

1472990333366

<l

30850.320.070.370.511.29

3110.22

0.00.422.031.90.020.113.57.01.72.40.2

48.61.14

15.99.150.169.0411.162.570.170.11

97.95

14729974143

17226821292123

<l

82830.320.150.351.131.17

4650.17

0.01.0

22.232.10.019.59.411.01.62.20.1

48.71.11

15.99.390.159.5611.062.470.070.11

98.58

18332461172

1402580<l1951164

_

830.310.140.241.081.14

2780.14

0.00.4

21.232.60.018.112.610.21.72.10.3

48.91.20

16.09.500.158.24

11.862.530.100.11

98.52

12729970193

199278322931051

42870.330.120.350.911.34

2790.15

0.00.6

21.732.40.021.610.38.21.72.30.3

49.41.21

16.19.390.157.0713.062.590.210.11

99.32

10830274195

1672889<l342124

<l

_820.310.130.381.051.54

3450.20

0.01.2

22.131.90.026.66.07.21.62.30.3

49.71.23

16.19.350.137.1712.342.600.210.10

98.98

11730468195

16727884284935

22840.310.100.320.821.51

3440.17

0.01.2

22.232.00.023.810.94.81.62.40.3

49.41.23

16.19.360.138.11

11.882.520.050.11

98.85

105308102181

1512986224213<l<l

43860.340.150.281.101.34

4480.16

0.00.321.632.70.021.313.55.51.62.40.3

49.71.22

16.29.490.147.7812.072.620.050.11

99.39

11731573171

15227853213164

<l

28860.320.190.251.461.41

4070.14

0.00.322.332.60.021.911.95.91.72.30.3

and degree of partial melting, because the low bulk-dis-tribution coefficients of K, Rb, Zr, La, Ce, and Nb (see,for example Wood et al., 1979) make it difficult tochange ratios of the more-HYG elements, even if verysmall, but still realistic, degrees of partial melting are in-voked. Instead, we suggest that geochemical units 444-1and 444-2 were derived from compositionally distinctmantle sources, the source of 444-1 being enriched inCe, La, Nb, K, Rb, Ba, and Sr relative to the source of444-2.

Both units 444-1 and 444-2 were emplaced as sillslong after basement formation; therefore, it is not sur-prising that the basalts of unit 444-1 are compositionallysimilar to E-type ocean-ridge basalts, which are charac-teristic of off-axis volcanic activity (see, for exampleTarney et al., 1979, in press; Wood et al., 1979). Evenunit 444-2, which is not as enriched as 444-1, is more en-riched than N-type ocean-ridge basalts, having higher

Ce/Y and lower Ti/Zr and Zr/Nb ratios; the 444-2 ba-salts more closely resemble T-type ocean-ridge basalts.The Th, Hf, and La data presented by Wood et al. (thisvolume) also support this comparison.

NORTHEAST PHILIPPINE BASINTwo sites are in the northeast corner of the Philippine

Basin. Site 445 is in a small basin on the southern side ofthe Daito Ridge (Figure 1), and the only igneous rocksrecovered at this site are a few clasts and pebbles insome of the conglomerates. These have been studied indetail by Mills (this volume).

Site 446 is in the Daito Basin, south of the DaitoRidge (Figure 1), at 24°42.04'N, 132°46.49'E, in 4952meters of water. Two holes were drilled, and basalts in-terbedded with sediments were encountered beneathabout 380 meters of sediment; the oldest sediment gavean age of early Eocene. From several lines of evidence,

825


TABLE 6 - Continued


SiO2 (%)TiO2

AI2O3F e 2 O 3

MnOMgOCaONa2OK 2 O

P2O5

Total



Fe tot/MgK/RbBa/Sr

QOrAbAnNeDi

Hy01Mt11

Ap

621

94

50.01.29

15.98.580.128.03

11.952.550.070.13

98.61

94290

6719

1158

2991< l37

311

3< l

_

85

0.320.120.410.931.24

6060.23

0.00.4

21.932.3

0.021.915.7

2.81.52.50.3

622

27

49.91.32

15.98.630.127.58

11.692.830.250.13

98.38

97293

6918

3161

3098

233

282

< l

4981

0.310.080.340.651.32

6950.20

0.01.5

24.330.5

0.022.410.6

5.51.52.50.3

623

56

49.91.25

15.99.280.157.66

12.322.560.250.11

99.41

127321

7519

4163

2890

228

31351

4583

0.310.140.311.141.40

5130.17

0.01.5

21.831.4

0.023.910.9

5.51.62.40.3

624

18

50.11.29

16.08.990.136.82

12.622.620.250.12

98.93

120310

9919

5165

3094

129

411

6< l

9482

0.320.120.310.901.53

4080.18

0.01.5

22.431.4

0.025.311.5

2.71.62.50.3

631

140

50.01.27

15.99.250.137.10

12.272.730.260.12

99.00

116306

7720

5165

2993

128

616

6< l

9382

0.310.170.301.361.51

4230.17

0.01.5

23.330.7

0.024.5

9.85.11.62.40.3

633

116

49.81.24

15.98.610.117.45

12.592.680.230.12

98.72

180307

6417

3166

2891

235

291

< l

45820.310.100.380.791.34

6420.21

0.01.4

23.031.1

0.025.5

8.16.01.52.40.3

6355

49.71.27

15.99.470.158.39

11.322.670.100.12

99.10

94308

8618

1146

3089

232

395

<l

4586

0.340.100.360.741.31

7970.22

0.00.6

22.831.4

0.019.813.7

6.41.72.40.3

636

12

49.51.29

15.89.450.168.52

11.372.650.090.13

99.02

97300

7719

1152

2891

233

210

7< l

4585

0.310.110.360.881.29

7640.22

0.00.5

22.631.4

0.020.113.0

7.11.72.50.3

6^8

54

50.01.30

16.09.440.158.11

11.532.810.090.14

99.56

96300

6321

2148

3090

326

414

22

3087

0.330.160.291.151.35

3690.18

0.00.5

23.931.0

0.020.811.5

7.01.62.50.3

641

78

48.71.15

15.89.590.159.47

10.672.380.110.12

98.17

139319

5520

2137

2782

230

412

61

4184

0.330.150.371.091.17

4650.22

0.00.7

20.532.7

0.016.6-16.4

8.11.72.20.3

642

97

48.41.11

15.69.470.15

10.0910.65

2.230.110.12

97.94

170323

6217

2137

2678

125

211

51

7885

0.330.140.321.041.09

4520.18

0.00.7

19.332.9

0.016.416.9

8.91.72.20.3

6430

48.71.14

15.79.450.159.82

10.712.310.130.12

98.20

144311

6317

3136

2684

133

511

5< l

8481

0.310.130.391.041.12

3570.24

0.00.8

19.932.6

0.016.816.5

8.41.72.20.3

6445

49.81.27

15.99.660.157.03

12.402.600.260.12

99.14

96310

7616

6173

3091

332

4114

< l

3084

0.330.120.350.901.59

3620.18

0.01.6

22.231.2

0.024.610.4

4.81.72.40.3

644

32

49.81.24

15.99.500.147.38

12.292.630.240.12

99.31

119304

7517

3193

2989

428

296

< l

2284

0.330.100.310.761.49

6720.15

0.01.4

22.431.2

0.024.010.0

5.81.72.40.3

aNormative values based on Fe2θ3/FeO ratio of 0.15. Fe 2 θ3 *s t o t a ^ ^on a s F e 2θ3 CeN/YN represents ratio of chondrite-normalized values.

but principally the abundance of thermally altered sedi-ments above and below most of the igneous units, all thebasalts at Site 446 are considered to have been emplacedas sills (Site 446 report, this volume) during post-earlyEocene time. Basement, which was not reached, isthought to have an early Eocene age at this site.

The basaltic rocks recovered from Hole 446A havebeen subdivided into 9 lithologic and 15 geochemicalunits (Figure 36), and 23 individual sills have been rec-ognized. These vary in thickness from 0.3 to 23 metersand can be delineated by aphanitic or occasionally evenglassy chill zones. Two main lithologic types of basaltand dolerite have been identified: a kaersutite, horn-blende-bearing variety (lithologic units 1, 4, 5), and ahornblende-free variety (lithologic units 2, 3, 6, 7, 8, 9).

Lithologic units 1, 4, and 5 contain 5 to 15 per cent ofprimary kaersutite, which occurs either as free-growinganhedral laths, as overgrowths on clinopyroxene, or as

fibrous clumps. The pyroxene in these kaersutite-bear-ing samples has a distinct purplish-pink coloration in-dicative of a high TiO2 content, and the rocks containabundant granular magnetite and skeletal ilmenite. Pla-gioclase and (in unit 5) olivine are the other main miner-al components; apatite crystals were observed inlithologic units 4 and 5.

Lithologic unit 1 comprises two equigranular, highlyvesicular (up to 30% vesicles), fine- to medium-grainedbasaltic sills. The sills have an intergranular texture, andappear to be fresh. Lithologic unit 4 also comprises twopetrographically similar sills, separated by a clay stoneinterbed. The thicker, upper sill has been divided intotwo sub-units by a break in recovery; the upper sub-unit(4A) consists of highly vesicular (25-40%), aphanitic tocoarse-grained basalt. Sub-unit 4B is a fine-grained,amygdaloidal basalt, and sub-unit 4C is a smaller-scaleversion of the upper sill. Unit 4 is cut by several clay-

826


200

150

?αα- 100

50

-

-

-

-

-

Site.

#

A

X

0

D

443-1443-2443-3443-4443-5

1 'C \1\\\\1

\

\

°\o\\

<faon o

O -Q.

ooo\ 1

1

I\

\

\\

Site 442 —»-\

t i

1

\\ x\

\\\

\ \

AA

A

1

-

-

-

-

-

\ x

\ x "

\x \

x ^x x X

X\ -

• ×l• / --

50 100Zr (ppm)

150

Figure 18. TV/ versws ZΛ Site 443. The dashed line out-lines the field of Site 442 basalts.

2.0

0.5

Site 443

• 443-1A 443-2

- x 443-3O 443-4G 443-5

/

<S>y

Ax

A

D

i i , I i ,

'/ x

x 4$y^

X X ^ y / ^

-

:

100Zr (ppm)

Figure 19. TiO2 versus Zr, Site 443. Note the strongcolinearity of the data points.

and carbonate-lined veins or fractures, and the vesiclesin the lower sections of both sills are filled by light-olive-green clay and calcite. Lithologic unit 5 is a single sill,containing up to 35 per cent dark, chloride material,which is occasionally recognizable as pseudomorphsafter olivine (0.1-1.0 mm across).

The kaersutite-free units essentially comprise plagio-clase, augite, titanomagnetite, and chloritic or clayeypseudomorphs after olivine. Lithologic unit 2 is a singleaphyric-basalt sill, the upper part of which is vesicular;the basalt is fine- to medium-grained, with intersertaltexture. The unit is cut by several veins lined with clay,quartz, and pyrite. Lithologic unit 3 comprises three

3 0 -

20

Site 443

• 443-1A 443-2x 443-3O 443-4G 443-5

-

A A«EAcA *

OOCCEO X

αx>αo o/ D OCD O

O

AA X .

A

xx

X X

• XX

•β

X

X

X

/

X

X

-

1 1 1

50 100Zr (ppm)

Figure 20. Y versus Zr, Site 443.

Site 443

• 443-1A 443-2x 443-3O 443-4D 443-5

—

I > i i i I i i

MX

X

A A AOA AAAOO AAO

D DDOO3C33D

ooooooO

1 . . . . 1 . .

i i | i i ' '

•

X —

X

XXX X X X ^ — '

\1i * \. --—'^}i•—

-

-

1 1

100Zr (ppm)

Figure 21. P2O5 versus Zr, Site 443. The colinearity ofthe data points should be compared with the scatterobserved in the data from Site 442 (Figures 10 and15).

Site 443

• 443-1A 443-2x 443-3O 443-4G 443-5

^ ^

1

X ~

X X

x " x x×

x x J^~-~o x X

°rf&6 o x

°° A 10 -

D A

i

100Zr (ppm)

Figure 22. K2O versus Zr, Site 443.

827


200

100

Site 443

• 443-1A 443-2x 443-3O 443-4D 443-5

' /

S i I I i

o o

o oo

o °

X

' l • ^ l 1 1

4/

A

/

/

x x × x

X X

** x* * × /*

-

I I

100

Zr (ppm)

Figure 23. Sr versus Zr, Site 443.

Site 443

• 443-1A 443-2x 443-3O 443-4D 443-5

100

Zr (ppm)

Figure 24. i?α versus Zr, Site 443.

20

1 0 r -

Site 443

• 443-1

A 443-2

x 443-3

O 443-4

H 443-5

•

• /

-

-

-

' A

O O

O Ax

CO OA • A

a Φ A J A

QO D CC3 O

O O 0 0 A

cno

CD O O

O

• 1 1 , 1

X X

X

X

• • • x

A •X XX

X

_

X

-

-

-

-

50 100

Zr (ppm)

150

20 30

Y (ppm)

Figure 25. Ce versus Zr, Site 443.

Figure 26. Ce VÉTSWS Y, Site 443. Field for Site 442shown for comparison.

sills: sub-unit 3A is a plagioclase (40%) and augite (1 °/o)microphyric basalt; 3B is a plagioclase (3-15%) andaugite (0.8%) phyric basalt; and 3C is a sparselyplagioclase-pyroxene phyric basalt. All sub-units con-tain a few clay pseudomorphs after olivine, and 3B and3C have vesicular upper portions.

Lithologic unit 6 comprises six fine-grained, sparselyphyric basalt sills. The basalt is essentially non-vesicu-lar, apart from the top of unit 6C, which contains 10 to15 per cent vesicles, and rare plagioclase phenocrysts.Lithologic unit 7, however, comprises two almost iden-tical plagioclase-pyroxene-phyric sills separated by aclaystone interbed. The upper sill, sub-unit 7A, has aglassy margin which contains a few red-brown pseudo-morphs, possibly after olivine. Lithologic units 8 and 9both comprise aphyric basalt sills, the former contain-ing rare plagioclase and augite microphenocrysts. Bothunits are cut by clay- and calcite-lined fractures.

Major- and trace-element data for the basalts recov-ered at Site 446 are listed in Tables 8 and 9. The ninelithologic units comprise at least 15 geochemical units(Figure 36), although lithologic units 6B and 6C werenot sampled. It is apparent that breaks between chemi-cal units usually coincide with breaks between lithologi-cal units or sub-units, but where a significant change inchemistry occurs without a change in petrography theunits are divided into geochemical sub-units.

Perhaps the most noticeable feature of the DaitoBasin samples, apart from their high contents of TiO2,total iron, K2O, P2O5, Sr, Ba, La, Ce, Zr, and Nb com-pared with the basalts from the Shikoku Basin, is theirwide range in chemical composition, both within andamong the various units. This is illustrated by the down-hole plot (Figure 36), and also by the normative basaltdiagram of Figure 37, where the range of chemis-try—from nepheline-normative basalts to quartz-nor-mative tholeiites—becomes apparent. Two interdigi-tated groups have been recognized: an alkalic-basaltsuite comprising geochemical units 446-1, 4, 5, 7a, and10a; and a tholeiitic-basalt suite comprising units 446-2,3,6, 7b, 8,9, 10b, 11, 12, 13, 14, and 15. This division isnot based solely on the presence or absence of norma-tive nepheline; indeed, most of the samples in unit 446-5(and all those in 446-7a) contain normative hypersthene.Rather, the alkalic basalts are characterized by lownormative-hypersthene contents, or the presence of nor-mative nepheline, and also by high abundances of cer-

828


Core Lithologic MagneticNo. Unit Inclination

-90° 0° +90° 100

Zr (ppm) Ba (ppm) Cr (ppm)

220 50 150 100 400

Fe/Mg Y/Zr Ti/Zr

1 2 3 0.1 0.4 60 70 80 90

E ~

15

1 1

240 33320

18 Mudstone and ClaystoneI " ~~~—I I I I I I I I I I 1 T -—i—i—i—i—r

Unit 1Equigranular dolerite ?

S 260-Q

oo 2 8 0 -

3 0 0 -

21

22

23"t2£|

25

26

27

Mudstoneand

claystone

Unit 2Plagioclase-pnyric

dolerite

Figure 27. Down-hole variations in lithology, magnetic inclination, and selected geochemical parameters, Site 444.

TABLE 7Analyses, Element Ratios, and CIPW Norms for Basalts from Hole 444Aa


SiO 2 (%)TiO 2

A1 2O 3

F e 2 O 3

MnOMgOCaONa2θK 2 OP2O5

Total


Zr/NbTi/ZrY/ZrCe/ZrBa/ZrCeN/YNF etot/ M gK/RbBa/Sr

QOrAbAnNeDi

HyOlMt11Ap

201

22

47.71.55

16.08.650.158.678.073.051.780.31

95.95

137222

672014

33630

15824

1411433

53

759

0.190.210.892.701.16

10550.42

0.011.025.725.8

0.611.30.0

19.51.63.10.8

201

73

50.31.94

16.27.960.156.258.593.862.310.39

97.92

26127

791816

39138

20931

1771842

44

756

0.180.200.852.711.48

11980.45

0.013.927.420.5

3.216.7

0.011.5

1.43.80.9

2020

47.21.41

16.48.490.167.708.433.621.900.34

95.68

180246

912017

29527

14624

1461530

43

658

0.180.211.002.731.28

9280.49

0.011.722.123.9

5.414.30.0

16.71.52.80.8

203

39

47.11.43

16.28.450.157.898.773.561.800.29

95.71

172234

621914

28427

14923

1351431

4<l

658

0.180.210.912.821.24

10670.48

0.011.121.324.0

5.515.8

0.016.4

1.52.80.7

204

37

46.71.43

16.58.360.157.698.103.922.060.26

95.18

182245

571816

28025

14524

1511126

52

659

0.170.181.042.551.26

10690.54

0.012.819.722.3

8.214.7

0.016.5

1.52.90.7

232

84

49.61.50

15.99.690.317.01

12.412.720.240.20

99.63

71253

7619

3163

35118

557

61741

2476

0.300.140.481.191.60

6640.35

0.01.4

23.130.6

0.024.4

8.46.11.72.90.5

241

87

49.71.57

15.910.03

0.166.89

11.272.710.170.21

98.63

59238

7020

2160

37130

865

822

3<l

1672

0.280.170.501.461.69

6930.41

0.01.0

23.231.3

0.019.617.01.61.83.00.5

242

73

50.01.59

16.010.320.177.25

11.092.720.220.22

99.56

59245

7324

3153

37134

970

821

51

1571

0.280.160.521.391.65

6060.46

0.01.3

23.130.9

0.018.717.0

2.71.83.00.5

251

118

50.21.61

16.110.15

0.176.87

11.362.820.160.23

99.73

53245

6320

216040

1417

718

2032

2068

0.280.140.501.231.71

6640.44

0.00.9

23.930.9

0.019.716.3

1.91.83.10.5

253

50

49.81.46

16.79.500.166.87

11.612.640.170.21

99.14

57235

6220

2159

33120

857

617

61

1573

0.280.140.471.271.60

6930.36

0.01.0

22.533.6

0.018.916.4

1.81.72.80.5

261

49

50.01.47

16.49.620.176.72

11.862.670.240.21

99.40

62234

8821

3159

36123

854

1952

1572

0.290.150.441.301.66

6590.34

0.01.4

22.732.4

0.020.814.7

2.21.72.80.5

2630

50.01.37

17.18.960.156.54

12.222.750.190.20

99.49

69273

8319

2167

32123

652

517

31

2067

0.260.140.421.301.59

7970.31

0.01.1

23.433.8

0.021.112.1

3.01.62.60.5

272

62

48.91.15

16.48.910.198.09

10.962.690.340.18

97.87

97262

6222

2266

29109

654

715

8<l

1863

0.270.140.501.271.28

14030.20

0.02.0

23.332.5

0.017.810.6

8.81.62.20.4

273

21

49.71.33

16.69.600.177.35

11.732.610.230.19

99.48

95277

7217

2159

33117

663

615

<l<l

1968

0.280.130.541.121.51

9590.40

0.01.4

22.232.9

0.019.813.5

4.71.72.50.5

275

68

49.11.38

15.610.02

0.177.46

12.672.490.220.20

99.29

114325

8519

2380

34128

762

616

4<l

1865

0.270.130.481.161.56

8920.16

0.01.3

21.230.9

0.025.4

8.56.91.82.60.5

aNormative values based on Fβ2θ3/FeO ratio of 0.15. F e 2 θ 3 is total iron a s F e 2 θ 3 represents ratio of chondrite-normalized values.

829


Figure 28. Normative nepheline (Ne), olivine (Ol), di-opside (Di), hypersthene (Hy) tetrahedron for Site444. Normative values based on Fe2O3/FeO ratio of0.15.

Px

10

2080

PI

70 60 50

Ol

30

Figure 29. Normative plagioclase-pyroxene-olivinediagram for basalts from geochemical unit 444-2(Site 444). The Fo-An cotectic (C) is from Shido etal. (1971).

tain hygromagmatophile elements (particularly K, Rb,Sr, Ba, Ti, Ce, and La), relative to the tholeiitic group.Note also that units 446-1, 4, and 5 contain kaersutite.

Intra-unit variation is well-illustrated by the range ofFe/Mg ratios (Figure 36) or, even better, by the Ni/Zrratio (Figure 38). With the exception of geochemicalunit 446-5, it is apparent from the high Fe/Mg ratios(generally in excess of 2.5), the low Ni abundances (less

2.5

2 . 0 -

1.5 -

1.0 -

0.5 -

1 ' . i | i . i

Site 444

•A

-

, ,

i 1

, ,

ii

1 ,

, .

. 1

i-

444-1444-2

Site 442 —>A

J, I i i •"~

i | > • .

,'2 \— I""* i i .

. | . . . . | . . . ,

•

•

••

|-<— Site 443

50 100 150Zr (ppm)

Figure 30. K2O versus Zr, Site 444. Field for Site 442from Figure 11; field for Site 443 from Figure 22.

30

I 20

Q

10

n

_

•

-

Site 444

• 444-1A 444-2

Site 442

. . . I

• . . . |

. . . . I

•

•

- Site 443

50

Zr (ppm)

Figure 31. Nb versus Zr, Site 444. Fields for Sites 442and 443 from Figure 9.

2.0

1.0

• Site 444

" 444-1" A 444-2

Site 4 4 2 — • ~ ^ ' '

• . . I . . .

1 . . . . 1

^ i/ /-*

/××/7 ± A >

i . . . . i

Site 443

•

-

-

100 150Zr (ppm)

Figure 32. TiO2 versus Zr, Site 444. Field for Site 442from Figure 7; field for Site 443 from Figure 19.

830


1 1 1 1 1

Site 444

• 444-1A 444-2

Site 442

. . . . I

I '

J

I i

i i i | i i i i | i i i i

/ Site 443

"AV A *

r

Zr (ppm)

Figure 33. Y versus Zr, Site 444. Field for Site 442 fromTables 2 and 3; field for Site 443 from Figure 20.

150

3 100

50

Site 444

• 444-1A 444-2

100Zr (ppm)

150 200

Figure 34. Ni versus Zr, Site 444.

50

?30hQ.

<3 20

10t-

. 1 . 1 1 . . .

Site 444

• 444-1A 444-2

•

. i . i

••

A i

i 1 •

•

àA

1

A

[

\ -

-

20 30Y (ppm)

50

Figure 35. Ce versus Y, Site 444.

than 100 ppm, decreasing to less than 50 ppm in units446-1, 3, 4, 7a, 7b, 8, 9, 10a, 10b, 12, and 15), and thelow Cr abundances (generally less than 150 ppm) thatthe basalts of Site 446 have undergone extensive frac-tionation and that they do not represent primary liq-uids. Unit 446-5, however, has significantly lowerFe/Mg ratios (0.90-1.68) and concomitant high MgO(7.3-17.7%), high Ni (90-627 ppm), and high Cr (305-1206ppm) contents. The consanguineous nature of the 446-5

suite is clearly illustrated on the Ni/Zr plot (Figure 38),and on several of the hygromagmatophile-elementplots. High contents of altered olivine crystals (up to30%) in parts of unit 446-5 strongly suggest that cumu-lus olivine may have caused the strong variation withinthis unit. However, implicit in the data is that cumuluschrome spinel, or pyroxene, also played an importantrole, because Cr increases in proportion to Ni (Table 9);olivine contains very little Cr. Units 446-2, 3, and 7balso show within-unit variation on the Ni/Zr diagram,although in units 446-2 and 446-3 Ni (and Cr) is practi-cally invariant (Figure 38). This suggests that fractionalcrystallization involving olivine or pyroxene has notplayed an important role in determining the abundancevariation of Zr (and other HYG elements) in units 446-2and 446-3.

Although the high contents of many of the HYG ele-ments are in part due to the fractionated nature of the446 basalts, it is also apparent that many HYG-elementratios are very different between the tholeiitic and al-kalic groups, and between the Site 446 and Shikoku Ba-sin basalts (apart from unit 444-1, which resembles thealkaline groups of 446). This is illustrated by plottingvarious HYG elements against Zr (Figures 39 to 45).

The alkalic basalt group has high K/Zr, Ba/Zr,Rb/Zr, and Sr/Zr ratios compared with the tholeiiticgroup. The Ba/Zr ratio, for example (Figure 39), is gen-erally between 0.98 and 2.08 in the former, and between0.38 and 1.08 in the latter (Ba/Zr is less than 0.4 in theleast-altered samples from the Shikoku Basin, althoughthe upper sill, unit 444-1, has a Ba/Zr ratio of 0.85-1.04;see Table 7).

Ti/Zr ratios (Figure 40) are high in all Site 446 ba-salts, especially so in the alkalic group; however, this isunlike unit 444-1, which has low Ti/Zr ratios (56-59).The alkalic group has the highest Ti/Zr ratios (up to163) of any basalts recovered during Leg 58, a reflectionof the high ilmenite and TiO2 contents of these samples.The alkalic group also has the highest CeN/YN ratios(4.09-5.92; Figure 41), although the tholeiitic groupalso exhibit relative light-REE enrichment (CeN/YNover 3). However, it is not only the light-REE contentswhich are high; it is apparent from the REE patterns forthe Site 446 basalts (Wood et al., this volume) that theheavy REE, at least in the alkalic group, are significant-ly lower than in the Shikoku Basin basalts, even thoughthe Site 446 rocks have undergone extensive fractiona-tion. Thus, although the Y/Zr ratios of the alkalic andtholeiitic suites are similar (0.12-0.17; Figure 42), theyare significantly lower than those of most of the Shiko-ku Basin basalts (0.3-0.4), apart from unit 444-1 (0.18).

All Site 446 basalts have low Zr/Nb ratios (less than12); in particular, units 446-4, 5, and 7a have Zr/Nb ra-tios less than 7 (Figure 43). Low Zr/Nb ratios are char-acteristic of "enriched" or "plume"-type basalts pres-ently being erupted along certain sections of the Mid-Atlantic Ridge (e.g., 45°N; Erlank and Kable, 1976;Tarney et al., 1979; Wood et al., 1979), on ocean islands(e.g., Iceland; Wood et al., in press), and in intra-continental alkali-basalt provinces (e.g., East AfricanRift; Weaver et al., 1972). The Ce/Zr ratios of the Site

831

00

to

380-

4 0 0 -

4 2 0 -

4 4 0 -

4 6 0 -

500-

520 H

5 4 0 -

5 6 0 -

580 -

600

620 -

Lithologic MagneticUnit Inclination

-90° 0° +90° 100

.1Zr (ppm) Ba (ppm) Cr (ppm) Fe/Mg Y/Zr Ti/Zr |

o400 100 300 500 200 600 1200 1 2 3 4 0.1 0.2 80 100 120 140 160 <S

Unit 1 Hb-basalt

Unit 2

Aphyric basalt

Unit 3Plagioclase—pyroxene

phyric basalt

Unit 4Hb-bearing vesicular

doleriteUnit 5 Ol-phyric basalt

Unit 6Aphyric basalts

Unit 7Pl-Cp× basalt

Unit8Aphyric to sparsly

plagioclase-phyric basalt

Unit 9Aphyric basalt

t

, g _10a,

-12-

13

14

15

Figure 36. Down-hole variation in lithology, magnetic inclination, and selected geochemical parameters, Site 446. Shaded areas repre-sent sediment.



SK)2 (%)TiO2

A12O3

Fe 2O 3

MnOMgOCaONa2OK 2 0

P2O5L.o.i.

Total



Fetot/MgK/RbBa/Sr

QOrAbAn

NeDiHy01Mt11Ap

Analyses,TABLE 8

Element Ratios, and CIPW Normsfor Basalts from Hole 446a

41CC20

46.43.58

14.79.850.334.55

11.393.092.390.382.59

99.25

2839952337

69328

21029

33024

5131

7102

0.130.241.574.472.51

5350.48

0.014.214.119.3

6.628.9

0.03.91.76.90.9

42CC24

45.83.67

14.710.880.364.80

11.143.162.040.372.53

99.40

2918992236

68927

20627

3132245

51

1070.130.221.524.092.63

4700.45

0.0

12.114.919.9

6.527.5

0.05.81.97.00.9

434

77

47.54.18

13.613.950.256.349.792.680.810.480.74

100.31

77124132

289

39642

29634

2063168

63

985

0.140.230.703.982.55

7510.52

0.04.8

22.622.5

0.018.915.1

2.82.47.91.1

441

19

49.74.02

12.813.39

0.205.709.392.860.670.431.43

100.62

73122110

279

35937

27533

1932862

71

8880.130.230.704.122.72

6180.54

2.83.9

24.020.0

0.019.416.3

0.02.37.61.0

442

53

50.54.17

12.913.78

0.194.719.592.610.850.460.11

99.87

69113130

2515

36840

28335

2313058

41

888

0.140.200.823.563.39

4700.63

5.45.0

22.120.9

0.019.814.1

0.02.47.91.1

444

56

49.33.92

12.614.46

0.265.549.362.620.530.450.59

99.71

69112119

2613

37339

28032

1793164

37

9840.140.230.644.033.03

3360.48

3.63.1

22.321.2

0.018.718.2

0.02.57.51.1

461

49

47.14.12

14.113.810.175.529.832.630.510.451.69

99.91

4053

12928

6424

38290

34151

3270

24

9850.130.240.524.522.90

7070.36

0.63.0

22.325.2

0.017.217.5

0.02.47.81.1

aNormative values based on Fe2θ3/FeO ratio of 0.15. Fe 2 θ3 is total iron as Fe2θ3.Cej\[/Yfv[ represents ratio of chondrite-normalized values.

446 basalts are also characteristic of E-type ocean-ridgebasalts (Figure 44) (-0.3; Tarney et al., 1979), beingsignificantly higher than in the Shikoku Basin basalts(less than 0.2), or in T-type and N-type ocean-ridgebasalts.

Units 446-4, 5, and 7a have high P/Zr ratios (higherthan 10; Figure 45), unlike the other units at Site 446,and unlike the basalts recovered from the ShikokuBasin. The wide variation in P/Zr ratios within unit446-5 basalts may be due to the presence of cumulusapatite, although the high phorphorus content in thealkalic units must be a function of the melt (and proba-bly the source) composition. Note, however, that theother alkalic units—446-1 and 446-10a—have P/Zr ra-tios similar to the tholeiitic suites, although there areslight differences in the P/Zr ratios of the various units.

The processes giving rise to the inter-unit chemicalvariations at Site 446 are difficult to quantify withoutdetailed major- and trace-element modeling. A compre-

hensive analytical survey of the constituent minerals isnecessary before this can be attempted. It is apparent,however, that the two main compositional types—al-kalic basalts and tholeiitic basalts—cannot easily berelated by fractional crystallization, because of thesignificant differences in the Zr/Nb, Th/Hf (Wood etal., this volume), and alkali-elements/Zr ratios of thetwo suites. In addition, it may be difficult to relate thetwo suites by varying degrees of partial melting of acompositionally homogeneous source, although furthermodeling is necessary to ascertain this.

SUMMARY AND DISCUSSION

One of the objectives of Leg 58 was to obtain samplesof basaltic basement from the Shikoku and Daito Ba-sins, in order to compare the petrological and chemicalcharacteristics of this basement with those of other mar-ginal basins and of the major ocean basins. Unfortun-ately, although substantial amounts of basalt were re-covered at all sites except 445, only drilling at Site 442and possibly Site 443 penetrated basalt belonging une-quivocally to oceanic layer 2; drilling at all the re-maining sites recovered post-basement basaltic materialwhich may represent, in part, off-axis activity. It isunlikely, however, that the sill and flow emplacement atSites 442, 443, and 444 post-dates true basement bysignificantly more than 2 m.y. (80 km from the ridgeaxis at a spreading rate of 4 cm/yr), and for the pur-poses of the present discussion it is considered that this"off-axis" activity tapped a mantle source similar tothat which gave rise to the underlying basement.

The crustal structure of the Shikoku Basin, and thecomposition of the basalts recovered during Leg 58,support the suggestion of Karig (1971) that the majorityof the western Pacific marginal basins are extensionaland formed by sea-floor spreading broadly analogous tothat occurring in the major ocean basins. The ShikokuBasin basalts exhibit considerable chemical variation,which may be due to a combination of several processes:low-temperature and hydrothermal alteration by seawater; within-flow and within-sill differentiation; high-level fractionation in sub-ridge magma chambers; vary-ing degrees of partial melting under varying P, PH2o>and Pθ2 conditions, hence different stable-phase assem-blages during partial melting; and heterogeneity in themantle source beneath the ridge axis. It is important toassess these effects before making any regional com-parisons with basalts from different tectonic environ-ments, and before evaluating the significance of basaltchemistry on the mechanisms of back-arc spreading.

It is apparent from Figures 10 to 14 and 22 to 24 thatthe compositions of the basalts recovered from Sites 442and 443 have been modified by sea water alteration,resulting in variably increased concentrations of K, Rb,P2O5, Sr, and to a lesser extent Ba. This limits the use ofthese elements in any petrogenetic discussions, althoughthe low abundances of K and Rb relative to Zr in someof the fresher samples do suggest that the magmas prob-ably had low K/Zr and Rb/Zr ratios originally. In addi-tion, Ba/Zr ratios are consistently low, which corrobo-rates previous evidence (Hart et al., 1974) that Ba is less

833


TABLE 9Analyses, Element Ratios, and CIPW Norms for Basalts from Hole 446Aa


SiO2 (%)TiO2

A12O3

Fe2O3

MnOMgOCaONa2OK2OP2O5L.o.i.

Total

NiCrZnGaRbSrYZrNbBaLaCePbTh


Fetot/MgK/RbBa/Sr

0OrAbAnNeDiHy01Mt11Ap

33

35

48.34.62

14.812.100.274.489.823.051.310.530.60

99.90

80133147

2919

42344

32838

35434744

<l

9840.130.231.084.133.13

5730.84

0.07.8

25.822.90.0

18.510.60.52.18.81.3

34

33

50.73.88

12.514.230.195.409.342.420.840.440.06

99.98

72115129

2420

35939

27132

2302863

43

8860.140.230.853.973.06

3480.64

5.45.0

20.520.70.0

18.917.30.02.57.41.0

41

39

50.43.89

12.514.180.185.119.392.490.830.440.24

99.59

67105127

2515

36540

27535

2223162

42

8850.150.230.813.813.22

4580.61

5.34.9

21.220.40.0

19.516.20.02.57.41.1

41

139

50.73.90

12.614.030.225.229.452.530.810.450.24

100.06

62111123

2515

36140

27934

2192967

73

8840.140.240.784.113.12

4470.61

5.24.8

21.420.5

0.019.516.30.02.47.41.1

52

26

49.53.92

12.514.460.225.769.682.480.560.450.38

99.98

65114128

249

37540

26936

2263167

87

7870.150.250.844.112.91

5150.60

3.83.3

21.021.40.0

19.718.10.02.57.41.1

62

34

50.93.91

12.614.220.185.099.252.550.910.450.04

100.10

69115122

2421

35741

27734

2272869

21

8850.150.250.824.133.24

3600.64

5.35.4

21.520.20.0

18.916.40.02.57.41.1

71

48

51.13.82

13.113.830.155.098.342.580.900.401.11

100.43

3446

1272515

37737

25832

2152762

43

8890.140.240.834.123.15

4980.57

6.25.3

21.721.50.0

14.318.20.02.47.20.9

73

97

50.54.07

13.113.390.164.889.342.570.790.420.72

99.93

4154

1402712

39836

27031

2062866

74

990

0.130.240.764.503.18

5490.52

5.74.7

21.821.80.0

18.214.90.02.37.71.0

81

66

50.63.92

13.013.380.165.009.372.800.610.410.71

99.95

4681

10827

9388

36258

301962757

23

9910.140.220.763.893.10

5590.51

5.03.6

23.721.10.0

18.915.10.02.37.51.0

91

79

49.84.02

12.713.530.244.449.452.700.920.451.60

99.81

3339

1192613

39040

28632

2132964

33

9840.140.220.743.933.54

5840.55

4.75.4

22.919.80.0

20.213.20.02.47.71.1

92

120

50.84.23

12.813.480.205.168.282.690.920.521.88

100.96

3840

1352813

40543

30535

2323274

61

9830.140.240.764.233.03

5890.57

6.05.4

22.520.0

0.014.417.10.02.38.01.2

93

97

47.54.16

12.915.190.215.559.712.490.550.470.69

99.39

3940

12924

6409

41281

301842968

54

9890.150.240.654.073.17

7560.45

1.53.2

21.222.60.0

19.018.70.02.78.01.1

105

107

44.84.90

13.311.850.196.25

12.442.811.550.990.48

99.53

4569982422

108826

18832

3622558

4<l

6156

0.140.311.935.482.20

5860.33

0.09.2

14.819.24.9

29.80.06.92.19.32.4

mobile than K, Rb, or Sr during low-temperature altera-tion by sea water. Halmyrolytic and hydrothermal activ-ity appear to have been less important in affecting thechemistry of the Site 444 (especially unit 444-1) and Site446 basalts, where K, Rb, Ba, and Sr behave in a coher-ent fashion. This may be a function of the generallymuch higher abundances of these elements at these sites.As a consequence of the alteration at Sites 442 and 443,we shall restrict our discussion primarily to those ele-ments which are considered immobile during hydrother-mal and halmyrolitic activity—particularly Ti, Zr, Nb,Y, Ni, and Cr (e.g., Pearce and Cann, 1971, 1973;Pearce, 1975), and the REE, although noting the possi-bility of Ce mobility illustrated by Ludden and Thomp-son(1979).

Mineralogically, the basalts from Site 442 and 443and unit 444-2 closely resemble abyssal basalts recov-ered from the major ocean basins. However, one impor-tant difference is the exceptionally high vesicularity of

all the Site 442 basalts, and some of the Site 443 and 444basalts. It is not unusual to find vesicular basalts onmid-ocean ridges, but vesicles are rare at the depthsfrom which the basalts at Sites 442 (4600 meters), 443(4400 meters), and 444 (4800 meters) were recovered,because of the inverse relationship between depth of ex-trusion and degree of vesicularity. It could be suggestedthat the basalts were emplaced in shallow water, beforeundergoing subsidence to their present depth, but this isconsidered unlikely. First, the basalts would have tohave been emplaced in very shallow water (less than 500meters) to produce the observed degree of vesicularity,but the sedimentary evidence indicates that the depth ofemplacement was in fact close to the carbonate-compen-sation depth (~ 4000 meters, near the present depth).Second, at Site 443 at least, not all the basalts are vesicu-lar.

It is more likely that the high vesicularity is a result ofa high volatile content in the magmas. Two suites of ba-

834


TABLE 9 - Continued

11227

48.5

2.9714.310.37

0.194.899.183.741.951.381.49

98.94

00

1122830

15532922946476348065

5780.130.352.086.78

2.46

5400.31

0.011.6

31.116.70.517.1

0.09.91.85.73.3

121

59

42.4

4.0710.312.77

0.2212.4311.251.490.740.732.88

99.33

2931137

801916

4801915024212224275

61630.130.281.41

5.431.19

3830.44

0.04.412.7

19.40.025.8

2.020.02.27.81.7

12272

41.43.60

9.413.690.27

15.439.091.130.670.694.11

99.43

4961206941916

41817

13825210234182

61560.120.301.525.92

1.03

3480.50

0.04.09.6

18.70.017.9

9.024.62.46.91.6

12436

45.6

3.7612.210.59

0.167.30

12.492.421.190.633.53

99.85

903058023217242216327304244431

6138

0.130.271.874.91

1.68469

0.42

0.07.0

17.7

19.01.6

31.7

0.08.21.87.21.5

12469

42.8

3.109.813.460.25

15.039.281.200.570.504.18

100.16

5631169861514230171292412720381

<l

5144

0.130.290.985.491.04

3410.55

0.03.4

10.119.60.018.7

11.821.7

2.35.91.2

131

109

41.4

2.269.113.680.35

17.698.120.870.25

0.365.61

99.71

6271070

90167

23316

13422172203842

6101

0.120.281.285.830.90

3020.74

0.01.57.4

20.30.0

14.315.027.22.44.30.9

14192

47.23.77

13.613.41

0.196.6210.302.650.550.351.13

99.83

77144122248

3792819823137224452

91140.140.22

0.693.862.35

5750.36

0.03.3

22.523.7

0.020.713.14.12.37.20.8

14364

49.63.61

13.013.010.17

6.339.882.690.270.341.07

100.02

72141115231

367311941912118423

<l

101120.160.220.62

3.332.38

22170.33

3.01.6

22.822.7

0.019.918.00.02.36.90.8

15170

49.63.58

13.013.100.176.329.87

2.600.450.341.22

100.30

72139105243

368301942013920392

<l

101110.150.200.72

3.192.40

1257

0.38

2.82.7

21.922.50.019.918.00.02.36.80.8

15375

49.33.60

13.213.07

0.166.359.992.500.420.341.08

100.03

72135113242

37332

20224133204152

8107

0.160.200.663.152.39

17240.36

2.82.5

21.2

23.60.019.5

18.30.02.36.80.8

16272

49.7

3.6013.212.300.195.8010.392.650.410.341.46

99.95

74131104235

374341981712820431

<l

12109

0.170.220.653.112.46

6870.34

3.42.4

22.422.80.0

21.914.80.02.16.80.8

18485

46.94.17

13.413.370.175.829.933.011.330.611.18

99.98

3848882414

67326191273342961<l2

71310.140.321.75

5.762.66

7910.50

0.07.9

25.519.20.021.71.99.92.37.91.4

18532

46.14.0813.712.840.185.9310.342.941.050.631.68

99.50

49521142412

720272003228431644

<l

6122

0.140.321.425.822.51

7280.39

0.06.2

25.021.20.0

21.71.5

10.02.27.81.5

19254

47.43.45

13.912.560.196.1810.083.010.690.421.63

99.53

761471202510

3853624322161235063

11850.150.210.663.41

2.36

5720.42

0.04.1

25.622.4

0.020.78.46.32.26.61.0

1947

49.5

3.9812.913.64

0.205.31

9.532.660.350.431.17

99.67

3113114268

4464130231228286562

10790.140.220.753.892.98

3590.51

4.82.1

22.622.20.018.616.40.02.47.61.0

salts from the East Scotia Sea back-arc basin exhibit ex-ceptionally high vesicularity (Saunders and Tarney,1979), not unlike the Site 442 basalts. It has been sug-gested (Saunders and Tarney, 1979) that this vesicularityis due to high volatile contents in the basaltic magmaand in the mantle source, possibly resulting from thepenetration of hydrous fluids from the adjacent subduc-tion zone. Subsequent studies of the East Scotia Sea ba-salt glasses have confirmed the hydrated nature of thevesicular basalts (Meunow et al., in press).

The suggestion that fluids arising from the subductedslab locally have contaminated the mantle source of theEast Scotia Sea basalts is supported by the fact that K,Rb, Sr, and Ba contents and 87Sr/86Sr ratios are cor-respondingly higher in the fresh vesicular basalts than inthe non-vesicular basalts, these elements presumably be-ing carried with fluids expelled from the slab. Unfor-tunately, because the Shikoku Basin basalts have under-gone excessive alteration by sea water, their K, Rb, Ba,

and Sr contents cannot be used as supporting evidence.Nonetheless, the high vesicularity of many of the Shiko-ku Basin basalts does indicate a high volatile content inthe source region beneath the basin. Basalts dredgedfrom the Mariana Trough, while not exhibiting the samedegree of vesicularity as the Shikoku Basin basalts, docontain significantly more H2O+ and CO2 than mid-ocean-ridge basalts (Garcia et al., 1979). Available datasuggest therefore that back-arc-basin basalts are derivedfrom a mantle source with a locally increased volatilecontent, and it may be presumed that the source forthese volatiles was an adjacent subduction zone (whichsince migrated eastward beneath what is now the BoninIslands).

It is apparent that none of the Leg 58 basalts can beconsidered as primary mantle melts: their Fe/Mg ratiosare too high, and their Cr and Ni contents too low, forthe melts to have been in equilibrium with predictedmantle compositions. Those basalts with high MgO, Ni,

835


TABLE 9 - Continued


SiO2 (%)TK)2A12O3

F e 2 θ 3

MnOMgOCaON 3 2 OK 2OP2O5L.o.i.

Total

Ni

CrZnGaRbSrYZrNbBaLaCePbTh


Fetot/MgK/RbBa/Sr


201

97

47.84.16

13.114.49

0.215.82

10.162.500.200.430.94

99.86

3323

12227

2447

37302

28143

3260<l

1

1183

0.120.200.473.982.89

8220.32

2.41.2

21.224.0

0.019.718.0

0.02.57.91.0

211

19

47.13.72

13.414.27

0.186.15

10.162.390.180.391.49

99.47

3628

11727

1439

35274

26105

2560

51

1181

0.130.220.384.212.69

14720.24

1.41.1

20.425.4

0.018.819.7

0.02.57.10.9

214

79

50.93.97

13.213.19

0.165.538.922.720.860.420.54

100.36

3527

1172410

43037

29027

1742765

51

1182

0.130.220.604.312.77

7100.40

4.65.0

22.921.2

0.016.717.1

0.02.37.51.0

221

29

46.34.95

13.714.97

0.235.568.303.071.030.511.11

99.70

203

13728

9453

41378

32225

3470

54

1279

0.110.190.604.193.12

9490.50

0.06.1

26.020.6

0.014.511.2

6.02.69.41.2

222

82

47.13.72

14.99.760.244.97

11.532.971.950.352.03

99.45

2830932230

75628

21125

3322448

5<l

8106

0.130.231.574.212.28

5390.44

0.011.618.021.6

3.927.6

0.04.81.77.10.8

224

26

46.14.91

13.115.52

0.215.73

11.082.440.270.480.86

100.67

2312

13426<l

44340

29930

1312969<l

3

1098

0.130.230.444.243.14_0.30

0.01.6

20.523.8

0.023.015.6

0.32.79.31.1

232

52

50.53.47

13.411.92

0.176.449.242.740.740.401.14

100.15

93176127

2712

41337

26828

1692554

82

1078

0.140.200.633.582.15

5130.41

3.14.4

23.122.0

0.017.418.2

0.02.16.60.9

235

73

50.53.61

13.611.65

0.146.099.432.830.860.411.17

100.26

93174

9527

9409

36273

27151

2358

4<l

1079

0.130.210.553.962.22

7930.37

2.85.1

23.921.8

0.018.216.3

0.02.06.81.0

241

19

46.44.46

13.713.80

0.455.16

11.102.340.420.540.66

98.95

5064

13523

3452

38341

42252

3670

21

878

0.110.210.744.523.10

11540.56

1.22.5

20.025.8

0.021.814.5

0.02.48.61.3

244

73

48.43.20

13.513.77

0.256.28

10.422.410.340.380.56

99.50

5383

10924

3415

35251

27129

2758

71

976

0.140.230.514.072.54

9360.31

1.42.0

20.525.1

0.020.219.6

0.02.46.10.9

252

51

51.03.42

13.113.28

0.155.389.022.610.770.400.50

99.60

5150

1162612

39337

25527

1792760

41

980

0.150.240.703.982.86

5300.46

5.34.5

22.121.8

0.017.117.7

0.02.36.50.9

261

33

48.13.36

14.412.04

0.255.88

10.802.770.730.391.19

99.81

88169111

289

42335

24327

1652454

62

983

0.140.220.683.792.37

6740.39

0.04.3

23.524.6

0.021.911.1

3.02.16.40.9

263

46

49.63.17

13.312.53

0.236.19

10.862.560.280.371.09

100.08

94175102

2111

42633

22924

1022650

2<l

1083

0.140.220.453.722.35

2090.24

2.51.6

21.723.8

0.022.816.4

0.02.26.00.9

271

112

45.94.45

13.215.01

0.266.259.492.510.800.471.40

99.72

3532

12124

8431

42312

33193

3068

84

985

0.130.220.623.982.78

8290.45

0.04.7

21.322.5

0.018.014.0

4.62.68.51.1

272

94

46.14.50

13.514.44

0.226.029.572.480.860.461.57

99.74

4236

12323

8442

41307

34206

3168

6<l

988

0.130.220.674.072.78

8890.47

0.05.1

21.023.3

0.017.615.2

2.82.58.61.1

aNormative values based on Fe2θ3/FeO ratio of 0.15. Fβ2θ3 is total iron as Fβ2θ3. Cejsf/YjM represents ratio of chondrite-normalized values.

and Cr invariably contain cumulus olivine, pyroxene, orchrome spinel, probably resulting from within-unit(within-flow or within-sill) crystal settling. This is espec-ially noticeable in geochemical units 442-1, 443-3, 444-2,and 446-5. Within-sill or within-flow differentiationmay play an important role in determining the observedwithin-unit chemical variation, but it is often difficult toascertain whether pre-eruptive differentiation, followedby contemporaneous emplacement of basalts of slightlydifferent compositions as single flow or sill units, is animportant factor. Within-sill or within-flow differentia-tion may be caused by gravitational settling of mineralphases with a high specific gravity, or crystal sortingduring magma flow through the sill (or flow) body. De-tailed studies of individual sills and flows (Marsh, inprep.) suggests that in situ crystal cumulation plays animportant role.

Compositional variation between individual litholog-ic and geochemical units is probably caused by pre-erup-tive differentiation, i.e., fractional crystallization and(or) partial melting. However, proving consanguinity ofmagmas at the same site is difficult, because of uncer-tainty surrounding the time between successive magmabatches. Thus, at Site 442 for example, geochemicalunits 442-2, 3, and 4 cannot be directly related to unit 1because of the 2-m.y. interval separating the twogroups. However, the four units at Site 442 appear io beessentially comagmatic, indicating derivation from acompositionally identical mantle source, althoughdistinct partial-melting episodes rather than magmabatches from a single fractionating magma chamber areinvolved. Similarly, the basalts from Site 443 appear tohave been derived from a single mantle source, although,unlike the units of Site 442, the less-HYG/more-HYG

836


446-1/J)

AlOa

• 446-1,2,4,5,9, 15

Δ 446-3O 446-6* 446-7X 446-8A 446-10T 446-11O 446-12• 446-13D 446-14

Hy

Figure 37. Normative nepheline (Ne), olivine (Ol), di-opside (Di), Hypersthene (Hy), quartz (Q) diagramfor Site 446. Normative values based on Fe2O3/FeOratio of 0.15.

100

z 50

Site 446

O 446-12

• 9

150 250Zr (ppm)

350 400

Figure 38. M versus Zr, Site 446. Separate geochemicalunits are labelled. Symbols as in Figure 37.

element ratios (e.g., Y/Zr, Th/Hf) show slight variationamong the various units (e.g., Figure 20; Wood et al.,this volume). It is considered unlikely that closed-systemfractional crystallization (Neumann et al., 1954) of abasaltic magma could produce significant changes inless-HYG/more-HYG ratios, unless the degree of frac-tionation becomes unreasonably high. Batch partialmelting may, however, change these ratios (e.g., data onLeg 49 basalts in Tarney et al., 1978), so it is reasonableto assume that the various units of Site 443 are relatedthrough different degrees of partial melting of the samesource. In addition, the compositional similarity of theSite 442 and 443 basalts suggests that the sources fromwhich the basalts were derived are very similar. This isnot unexpected in view of the fact that both sites are

250

E

α 200

100

150 200

Zr (ppm)

350

Figure 39. Ba versus Zr, Site 446. Field for Site 442from Figure 13; for Site 443 from Figure 24; and forSite 444 from Table 5.

5.0

4.0

3.0

1.0

Site 44610b A 446-9«

<0>12

50 150 2 0 0Zr (ppm)

Figure 40. TiO2 versus Zr, Site 446. Field for Site 442from Figure 7; for Site 443 from Figure 19; for Site444 from Figure 32.

located on the same mantle flow line on opposite sidesof the spreading axis.

Inter-unit variation at Sites 444 and 446 is less easilyexplained. Off-ridge basaltic material was penetrated atboth sites, and the basalts at both sites are composi-tionally heterogenous and distinct from the Site 442 and443 basalts. The within-unit variations at Site 444 maybe adequately explained by fractional crystallization insub-axis magma chambers, and perhaps also in individ-ual flow units. However, the inter-unit variation cannotbe explained by these processes, because of the widedisparity in less-HYG/more-HYG element ratios (e.g.,

837


40 50

Figure 41. Ce versus Y, Site 446. Field for Site 442 fromFigure 16; for Site 443 from Figure 25; for Site 444from Figure 35.

30

10200

Zr (ppm)

Figure 42. Y versus Zr, Site 446. Fields for Sites 442,443, and 444 from Figure 33.

Y/Zr, TiO2/Zr) and more-HYG element ratios (e.g.,Ce/Zr, Zr/Nb). More important, however, is that themore-HYG-element ratios cannot be changed during thepartial melting processes which may be expected duringthe formation of basaltic magmas (Tarney et al., 1979;Wood et al., 1979; Bougault et al., 1979). Detailedstudies of basalts from the North Atlantic Ocean haveshown that variations in more-HYG-element ratios(Bougault et al., 1979; Tarney et al., 1979; Wood et al.,1979) and variations in Sr-, Pb-, and Nd-isotope ratios

Figure 43. Nb versus Zr, Site 446. Fields for Sites 442,443, and 444 from Figures 9 and 31.

Figure 44. Ce versus Zr, Site 446. Fields for Sites 442,443, and 444 from Figures 8 and 26 and Table 5,respectively.

Site 446

-

1

1442/

• / '

/ / Vs

V/

/446-5/

\

^443

/ 444-1

• / f~~& 1 ^ -

• /

446-4

^ 7 a

446-1

~~g * a*"^P A 1 0 ε446-6

/ ^

10b\ 446-2

^ X X 446-15

200

Zr (ppm)

Figure 45. P2O5 versus Zr, Site 446. Fields for Sites 442,443, and 444 from Figures 10 and 21 and Table 5,respectively.

838


(e.g., O'Nions et al., 1977; Sun et al., 1979) are morelikely to have been caused by chemical heterogeneities inthe mantle source (e.g., Tarney et al., 1979). The natureand causes of these heterogeneities are still in dispute(Schilling, 1975a,b; Hart et al., 1973; White et al., 1976;Tarney et al., 1979, in press; Wood et al., 1979, inpress), but they appear to have arisen during successiveepisodes of crustal extraction and mantle metasomatism(e.g., Lloyd and Bailey, 1975; Frey et al., 1978; Pearceand Norry, 1979; Tarney et al., in press). In the ShikokuBasin, and possibly also the Daito Basin, there is the ad-ded possibility and complexity of a chemical componentbeing derived from an adjacent subduction zone.

Detailed modeling is required before partial meltingcan be ruled out as the dominant process governing theHYG-element distributions in the Site 444 and 446 ba-salts, and the variations attributed solely to mantleheterogeneity. The available data do suggest, however,that the various units at each of the two sites were de-rived from compositionally varied sources. The basaltsof unit 444-1 and Site 446 are chemically similar toevolved E-type basalts commonly found on oceanicislands (e.g., Iceland; Wood et al., in press), where suchbasalts occur in conjunction with T-type tholeiites: themantle source of the E-type basalts characteristicallyyields a mixture of enriched and depleted basalts in anyone area.

Representative analyses of the Shikoku Basin basalts,and of the basalts from the East Scotia Sea and Brans-field Strait back-arc basins, are given in Table 5; typicalanalyses of N-, T-, and E-type basalts from the AtlanticOcean are included in Table 4. Because of the effects ofpartial melting, fractional crystallization, and within-flow or within-sill differentiation, it is more valid toconsider HYG-element ratios, rather than absolute ele-ment abundances, when comparing basalt compositionsfrom different tectonic environments. In particular,ratios involving the more-HYG elements which inocean-ridge basalts are generally considered to includeLa, Ce, Nb, Ba, K, Rb, and Zr, are less responsive tofractionation processes, as mentioned before (see Woodet al., 1979).

The basalts from Sites 442 and 443, and unit 444-2,are chemically similar to basalts recovered by dredges 20and 23 (e.g., D20.43B, Table 5) in the East Scotia Sea(Saunders and Tarney, 1979). Although they tend incomposition toward N-type (depleted) ocean-ridge ba-salts, having high Zr/Nb ratios and low Ce/Y, Ce/Zr,and P/Zr ratios, they nonetheless have higher Ce/Y,Ba/Zr, Ba/Sr ratios, and probably higher K/Zr andRb/Zr ratios, than N-type ocean-ridge basalts. Equally,they have higher Zr/Nb and lower Ce/Y and Ce/Zrratios than transitional (T-type) basalts, for examplethose from 63°N on the Reykjanes Ridge (Tarney et al.,1979; Wood et al., 1979).

The transitional island-arc-tholeiite characteristicsexhibited by the basalts from dredge 24 in the EastScotia Sea (e.g., D24.14; Table 5) (high SiO2, high K/Zrand Ba/Zr ratios, and low Ni and Cr contents) are notobserved in any of the Leg 58 basalts. Island-arc tholei-ites and calc-alkaline basalts typically have low absolute

abundances of the high-field-strength (HFS) elements(Ti, P, Zr, Hf, Nb, Ta), and higher absolute abun-dances of the large-ion lithophile (LIL) elements (K, Rb,Ba, Sr, Th, U, Ce, and La) (Saunders et al., in press).Although some of the Site 444 basalts and all the Site446 basalts exhibit enrichment in the LIL elements, theyalso show significant enrichment in the HFS elements, acorrelation more characteristic of alkaline basalts thanorogenic basalts.

Lavas from Bransfield Strait, a small, young, intra-continental, back-arc basin at the north end of the An-tarctic Peninsula, exhibit chemistries which may be con-sidered transitional between ocean-ridge and calc-alka-line basalts (Weaver et al., 1979). Some of the lavashave significantly higher levels of K and Rb (BridgemanIsland) and Ba and Sr, and higher CeN/YN ratios thanthe Shikoku Basin basalts (with the exception of geo-chemical unit 444-1). Apart from their vesicularity,however, basalts recovered from the Shikoku Basin can-not be distinguished from basalts being erupted alongsome sections of ocean ridges in the major ocean basins.They do not show the extreme depletion in certain HYGelements (K, Rb, Ba, light REE, Nb) typical of N-typebasalts, and may be best considered as intermediate be-tween N- and T-type ocean-ridge basalts. Whereas theydo not show any chemical characteristics transitionaltoward orogenic basalts, their high vesicularity doessuggest that their mantle sources may have been en-riched in volatiles, possibly derived from the adjacentsubduction zone.

The HYG-element-enriched alkaline and tholeiiticbasalts from Site 446 and from unit 444-1 more closelyresemble enriched E-type basalts recovered from oceanislands and so-called "hot-spot" regions (e.g., 45°N) inthe major ocean basins (Table 4). However, there aresome significant differences, both between the 444-1and 446 basalts and between these and E-type basalts.For example, the basalts from Site 446 have muchhigher levels of TiO2 and total iron than basaltsrecovered from 45 °N on the Mid-Atlantic Ridge; theymore closely resemble HYG-element-enriched ferroba-salts commonly found on the oceanic islands (Table 4).In addition, the basalts of unit 444-1 have very highK/Rb ratios, a feature atypical of E-type basalts andtheir differentiates; nonetheless they have low Zr/Nbratios and high Ba/Zr, Ce/Y, and Ce/Zr ratios, similarto the Site 446 basalts.

It appears that the compositional range of basalts re-covered from the Shikoku and Daito Basins is as greatas for those being erupted at major oceanic spreadingcenters. Because the trace-element characteristics of ba-salts reflect, to a first approximation, those of theirmantle source regions, it may be assumed that an equaldiversity of mantle sources is available in the marginal-basin environment. This attains significance if "en-riched" mantle sources are attributed to the influence ofa deep mantle plume (e.g., Schilling, 1973), because,with the close association between marginal basins andsubduction zones, there is an inherent tectonic problemin circumventing the subducted slab. This problem maybe overcome if, for example, the "enriched" mantle

839


overlies the N-type basalt mantle source, or if the differ-ent mantle types are intimately associated by a veiningprocess, as suggested by Tarney et al. (in press). In addi-tion, the marginal-basin mantle source may contain acomponent from the subducted slab (Saunders and Tar-ney, 1979). On the basis of the available chemical data,this does not appear to be the case for the Shikoku Basinsamples. However, their high vesicularity may reflect ahigh volatile content in the mantle source, possibly dueto a hydrous component arising from the subductedslab.

ACKNOWLEDGMENTSWe would like to thank Dr. G. L. Hendry for providing

analytical facilities, and for assistance in drafting the sectionon analytical techniques. We also thank Nigel C. B. Donnellanfor drawing many of the text figures, and the DSDP techni-cians for their assistance in sample and thin-section prepara-tion. N. G. M. and A. D. S. would like to acknowledge theNatural Environment Research Council, U. K., for providingfinancial support during the tenure of this project.

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