29. PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALTIC ROCKS: LEG 811

R. K. Harrison and R. J. Merriman, British Geological Survey,with contributions by

Jane Evans, Dawn Hutchison, A. E. Davis, K. A. Holmes, P. Joseph,V. A. Judge, and C. W. Wheatley, British Geological Survey2

ABSTRACT

We detail the petrography and mineralogy of 145 basaltic rocks from the top, middle, and base of flow units identi-fied on shipboard along with associated pyroclastic samples. Our account includes representative electron microprobeanalyses of primary and secondary minerals; 28 whole-rock major-oxide analyses; 135 whole-rock analyses each for 21trace elements; 7 whole-rock rare-earth analyses; and 77 whole-rock X-ray-diffraction analyses.

These data show generally similar petrography, mineralogy, and chemistry for the basalts from all four sites; they aretypically subalkaline and consanguineous with limited evolution along the tholeiite trend. Limited fractionation is indi-cated by immobile trace elements; some xenocrystic incorporation from more basic material also occurred. Secondaryalteration products indicate early subaerial weathering followed by prolonged interaction with seawater, most likely be-low 150°C at Holes 552, 553A, and 554A. At Hole 555, greenschist alteration affected the deepest rocks (olivine-doler-ite) penetrated, at 250-300°C.

INTRODUCTION

Deep Sea Drilling Project (DSDP) Leg 81 drilled foursites transecting the rifted "passive" margin of the Rock-all microcontinent, with the object of studying the sedi-mentary and igneous evolution of this type of margin(Fig. 1). Of particular interest are the age and nature ofthe westward-dipping reflectors, which dominate shal-low seismic records of the Rockall type of margin (Rob-erts et al., this volume). Leg 81 located Sites 552 and553 at the midpoint of the transect, and both bottomedin basaltic rocks above or within the upper dipping re-flectors after passing through Paleocene and Tertiary suc-cessions. In Hole 552, 31.3 m of basalts were encoun-tered and in Hole 553A, 183.0 m of basalts, the lowest71.0 m corresponding to dipping reflectors.

Site 554 was located oceanward of these two sites onthe crest of the "outer high," separating the zone ofdipping reflectors from the oceanic crust to the west.The outer high lies at the edge of oceanic magnetic Anom-aly 24B (52 m.y. in age, corresponding to the initiationof rifting—spreading between Greenland and Rockall).Hole 554A bottomed in 72 m of probable pillow basaltsof normal and reverse polarity underlying Pleistoceneand Tertiary sediments. The basement of the "outer high"here is formed of overlapping normal-polarity basaltswhich may represent the first-formed oceanic crust. Site555 was located midway between Hatton and Edorasbanks, some 40 km east of the zone of dipping reflec-tors drilled at Site 553. The objective here was to com-pare the subsidence history of these banks with that

70°N

Greenland

Roberts, D. G., Schnitker, D., et al., Init. Repts. DSDP, 81: Washington (U.S. Govt.Printing Office).

2 Addresses: (Harrison, Merriman, Wheatley), British Geological Survey, ExhibitionRoad, London SW7 2DE, United Kingdom; (Evans, Hutchison, Davis, Holmes, Joseph,Judge) British Geological Survey, Gray's Inn Road, London, WC1X 8NG.

30cW

Figure 1. Map of the North Atlantic showing Leg 81 sites and otherlocalities referred to in the text.

found at the deeper sites. After penetrating Pleistoceneand Miocene sediments, a major hiatus was found be-tween the early Eocene and Miocene. The early Eocenesuccession contained tuffs and sediments, the former in-creasing downward and passing into pillow basalts inter-bedded with autoclastic breccias. These volcanics mayrepresent the dipping reflectors to the west. Late Paleo-cene marine mudstones underlie these rocks which restin turn on about 294 m of massive basalt of reversed po-larity (Roberts et al., this volume).

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R. K. HARRISON ET AL.

We record here the petrography and mineralogy ofsampled basalts and associated rocks from the four sites.However, since the basalts are mainly microcrystallineand their primary mineralogy relatively uniform, we havealso investigated their major and trace-element chemis-try, mainly to determine any inter- or intrahole varia-tions, to classify and characterize, and so to allow com-parison with other DSDP basalt sites. For this, we haverelied heavily on the very detailed shipboard logs (Rob-erts et al., this volume) to relate our (unavoidably re-stricted) samples to the flow units and sequences identi-fied therein for each of the sites.

PETROGRAPHYMETHODS

We selected samples from near the top, middle, and base of indi-vidual lava flow units where these were identified in the shipboard log.We examined 4 sections from the restricted sequence of Hole 552; 83thin-sectioned cores, at intervals ranging from 20 cm to 10 m, fromHole 553A; 8 sections of the small thickness penetrated in Hole 554Abasalts; and 51 sections from Hole 555 basalts and dolerites, at inter-vals from 50 cm to 15 m. We also examined 19 thin-polished sectionsof cores selected for microprobe analyses of individual minerals, basedon our initial petrographical examination. Because of alteration, espe-cially to smectite, many specimens required special treatment for sec-tioning. From our examination of the thin sections, we selected 31samples for X-ray diffraction analysis of secondary minerals. Twentymajor oxide analyses were made by beta-probe of the main and least-altered petrographical varieties, and we report analyses for 22 traceelements by direct reader spectrography of most of the sectioned sam-ples; rare-earth elements of 7 samples were analyzed by neutron acti-vation. We made 12 density determinations of the least-altered sam-ples from Hole 555 cores, but because of the prevalence of vesicles,further determinations were not justified.

Hole 552

A basalt sample from Subunit Va, (Sample 552-21-3,75-77 cm) is dark greenish gray (5GY4/1), vesicular,aphyric, and heavily smectitized. Spheroidal vesicles3 (av-eraging 1 mm diameter) are lined or partly filled with fi-broradial brown smectite and microframboidal pyrite.The texture is microphyric with common, randomly ori-ented labradorite laths (averaging 0.4 × 0.04 mm) (An^)set in a microcrystalline (0.06 × 0.006 mm) ground-mass. Clinopyroxene is colorless (0.25 mm across) andsubhedral but is rarely unaltered. The turbid smectiticbase is charged with opaque iron oxide. Microcrystallinefelts of skeletal iron oxide in a devitrified glass base sur-round vesicles. Two basalt samples of Subunit Vb (Sam-ples 552-22-1, 137-144 cm and 552-22-2, 35-38 cm) aredark gray (N3), contain abundant spheroidal to irregu-lar vesicles, averaging 1.5 mm diameter, partly filled withfibroradial smectite. The first basalt also shows sub-spherical regions 1-2 mm across, of devitrified micro-crystalline material charged with skeletal opaque ore,and resembling the mesostasis of tholeiitic dyke rocks.This material may also enclose smectite-filled vesicleswhich in turn are surrounded by tangentially arrangedplagioclase laths.

The possibility that these represent an immiscible Fe-rich liquid was investigated by electron microprobe scans

across the quench-textured areas and compared with sim-ilar scans of adjacent microcrystalline lava. These anal-yses, however, reveal no significant differences betweenthe contrasting textural portions, and it is concludedthat the quenched "spheres" represent volatile-rich bub-bles of liquid in a more viscous or partly solidified melt,which were quenched by sudden degassing. The textureis a random latticework of labradorite laths averaging0.6 × 0.06 mm with colorless anhedra of clinopyroxeneabout 0.4 mm across in a groundmass that is microcrys-talline or devitrified glass (intersertal texture) altered tosmectite with opaque dusty ore. Electron microprobe anal-yses show that plagioclase microphenocrysts have a com-position An63, the clinopyroxene is augite Mg39Fe2iCa4o,and the opaque is Ti magnetite. A second basalt samplefrom this subunit is coarse-grained, with labradorite laths(An57) averaging about 0.65 × 0.06 mm in partly sub-ophitic intergrowth with colorless to pale green clinopy-roxene (0.3 mm). Interstitial iron-titanium oxide and muchdevitrified, smectitized glassy mesostasis, some chargedwith opaque Fe-Ti oxide needles, are present. Abundantsmectite-lined vesicles interconnect and areas of quench-textured basalt again suggest sudden degassing of thelava.

Hole 553A

Textures and Lithologies

The term vesicle is used throughout and denotes cavities resulting from gas bubbles(vesicle: sensu strictö) and also those partly or wholly filled with minerals (amygdales).

Samples from the three subunits (Va,b,c) include: mas-sive, homogeneous aphyric basalts, with much variationin vesicle shapes, size distribution, orientation, and min-eral infilling; microphyric basalts; flow-brecciated ba-salts, vitroclastic tuffs, and possible agglomerates. Ba-salts (excluding vesicles) are mainly hues of gray, bluishgray, or more commonly greenish gray. Vitroclastic tuffsin Subunit Vb, in contrast, are dusky red (10R 2/2) toblackish red (near 5R/ 2/2); scoriaceous tops of certainflows (e.g., Sample 553A-52-3, 8-11 cm) may be of sim-ilar color, and autobrecciated material within flows (e.g.,Sample 553A-55-3, 71-74 cm) is pale purple as a resultof ferric oxide.

The texture of the basalts irrespective of position withinflow units is mainly trachytic and hyalopilitic with varia-ble content of plagioclase microphenocrysts. Samplesfrom near tops of flows may be aphyric or weakly tomoderately microphyric with less than 10% volume mi-crophenocrysts. In the former, plagioclase occurs as feltsof needles or laths in the range 0.1 × 0.01 mm to 0.02× 0.001 mm, with composition An57 to Ango. There arecoarser (0.4 × 0.06 mm) and finer aphyric (0.04 ×0.002 mm) patches (Sample 553A-46-4, 122-124 cm) andalso microphyric associated with aphyric material givinga pseudobreccia appearance (Sample 553A-45-4, 16-20cm; Plate 1, Fig. 3). In the microphyric or glomerophyr-ic examples, plagioclase laths range from 0.3 × 0.06 mm(An52), to 3.0 × 1.8 mm (An^) but are mainly about1.54 × 0.8 mm. The groundmass plagioclase laths aver-age about 0.04 × 0.003 mm across (An57 to An^). Fer-romagnesian and accessory minerals are mainly dispersedas minute anhedral, interstitial smectitized grains sel-dom exceeding 0.03 mm.

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PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

Samples from within flows differ to some extent fromthose from near the tops, being predominantly plagio-clase microphyric or glomerophyric (as in Subunit Vc,Samples 553A-51-1, 54-57 cm and 553A-59-4, 93-95 cm:see Plate 1, Figs. 5 and 6, respectively). Clinopyroxenerarely occurs in the microphenocrystic clusters, but moregenerally interstitially in the groundmass. Plagioclase mi-crophenocrysts range from 0.2 × 0.06 mm toward thetop of the sequence in Subunit Vc to 3.0 × 2.0 mm infeldspar-phyric samples near the base (Samples 553 A-56-3, 91-94 cm and 553A-59-4, 93-95 cm). The ground-mass feldspar laths do not co-vary in size with the mi-crophenocrysts. Thus in the feldspar-phyric samples thegroundmass needles average only 0.06 × 0.01 mm, andthese sizes are also common in the feldspar-microphyricvarieties. The midflow samples also differ from thosenear tops in generally containing less-altered clinopyrox-ene, which is nearly always interstitial to the ground-mass feldspar and averages about 0.02 mm.

Samples from near the base of flows range from aphyr-ic to feldspar-phyric (e.g., in Subunit Vb, Sample 553A-47-4, 81-84 cm, Plate 1, Fig. 4). In aphyric types, feltedplagioclase needles average 0.06 × 0.002 mm with in-tergranular euhedral clinopyroxene averaging 0.01 mm.Phyric and microphyric types have phenocrystic plagio-clase laths ranging from 3.0 × 2.0 to 0.5 × 0.3 mm, thegroundmass being very fine grained to the limits of opti-cal resolution. Clinopyroxene is interstitial, anhedral,and very fine grained (0.01-0.02 mm).

In general, therefore, samples near the tops and basesof flows tend to be aphyric (although phyric near somebases) and plagioclase-microphyric or plagioclase-phyricto glomerophyric elsewhere. Clinopyroxene and accesso-ries are mainly interstitial in the groundmass and onlyrarely form microphenocrysts. There does not appear tobe any direct relationship between size, shape, and abun-dance of vesicles and the textures of the basalts.

Mineral CompositionThe least-altered basalts indicate a simple assemblage

of primary minerals: essential plagioclase and clinopy-roxene as microphenocrysts (where present) and form-ing the groundmass, with accessory opaque ore, princi-pally magnetite, and variable proportions of devitrifiedglassy residuum. Mineralogically the basalts are "tholei-itic," although the use of this term in the context ofocean-floor basalts has been criticized (e.g., Cann, 1971).Rare (pseudomorphed) olivine was observed in only threesamples in Subunits Va and Vb (Samples 553A-40-1,79-82 cm; 553A-46-4, 122-124 cm; and 553A-48-6, 122-125 cm). A trace of possible orthopyroxene occurs inSample 553A-46-1, 121-125 cm. There is no discerniblevariation in mineral composition or proportions withsample depth or with position in a flow unit. We modal-ly analyzed a selection of the least-altered basalts bypoint counter (see Table 3), and these included the maintextural and mineralogical variants. In the phyric sam-ples, phenocrysts range from 1 to 5% (sparsely to mod-erately phyric) and are principally plagioclase and rarelyaugite. In the aphyric varieties, and in the groundmassof the phyric types, clinopyroxene generally exceeds pla-

gioclase: plagioclase (32-41% volume augite 41-51%).Titaniferous magnetite and other opaques are mainlyminor (4-16%). The remainder of the rock is mainlysecondary smectite, but it is not possible to attributethis specifically to replaced primary ferromagnesians orto altered glassy mesostasis. An averaged compositionof the freshest basalts is approximately plagioclase (prin-cipally labradorite), 36% volume; clinopyroxene, 48%;titaniferous magnetite, 8%; and smectite plus other sec-ondaries, 8%. Olivine is absent or sparse, as in Samples553A-40-1, 70-82 cm; 553A-46-4, 122-124 cm; and553A-48-6, 122-125 cm. A trace of orthopyroxene oc-curs in Sample 553A-46-1, 121-125 cm.

There is no discernible variation in modal composi-tion or in mineral composition with sample depth in thehole, or in the flow units. We summarize primary min-eral data in Table 1, with many optical determinationsof the plagioclase composition, together with precise elec-tron microprobe analyses of plagioclase microphenocry-sts, groundmass plagioclase, clinopyroxene, and opaqueoxides. The average microphenocryst plagioclase com-position on optical data is Ango (range An51-An86: lab-radorite to bytownite), groundmass plagioclase An56(range An3o-An7o: andesine to labradorite). Microprobeanalyses show that microphenocrysts in feldspar-phyricbasalts are more anorthite rich than co-existing ground-mass plagioclase, the former reaching An86 toward thebase of the succession. A consistent disparity betweengroundmass and microphenocryst composition (see Ta-ble 6) combined with a lack of zonation and marginalresorption of some plagioclase microphenocrysts sug-gests that the latter are fractionated xenocrysts.

Of six samples analyzed for pyroxene in Units Vb andVc, five differ little within the augite range of Mg (42-48): Fe(18-25): Ca(33-36) (mean Mg 46 : Fe 20 : Ca 34)and indicate little variation in magma composition. Sam-ple 553A-46-1, 121-125 cm contains both augite (Mg 42:Fe 25:Ca 33) and Mg-pigeonite (Mg 52:Fe 34:Ca 14) in-dicating a tholeiitic affinity. The principal opaque phaseis titanium-magnetite disseminated as irregular grains inthe groundmass.

Vesicles and Secondary Minerals

Flow tops are mainly highly amygdaloidal and varia-bly filled with smectite, celadonite, or less commonly,silica. Vesicles range up to 1 cm across but may extendto give a pseudobreccia texture; some may be fluxioned.Vesicles may also be sparse near flow tops; they showsimilar variation toward flow centers, ranging up to2 cm in length. Ellipsoidal-spheroidal examples rangeup to about 7 mm across. Toward flow bases, vesiclesare mainly dispersed and less than 3 mm diameter, butexceptionally (for example, in Sample 553A-56-1, 3-6 cm)irregular vesicles and vugs up to 1 cm across are com-mon, indicating gas phases trapped beneath the lavaflow. Thin sections show that smectite and celadoniteare the most common fillings of vesicles throughout thesampled succession. Where both smectite and celadon-ite occur in a vesicle, smectite generally forms the radi-ally microfibrous or cryptocrystalline brown rim and cela-donite a radially fibrous or fibrous aggregate pleochroic

745

R. K. HARRISON ET AL.

Table 1. Summary of electron microprobe, optical, and X-ray diffraction analyses of basalts and associated rocks,Holes 552 and 553A.

Sample(interval in cm)

Hole 552

22-1, 137-141

Hole 553Subunit Va

38-1, 62-6538-2, 58-5939-1, 2-440-1, 9-1240-1, 79-8240-4, 80-8342-2, 42-4643-1, 20-2343-2, 111-11444-2, 48-51

Subunit Vb

45-1, 44-47

45-1, 87-91

45-2, 114-11845-3, 79-8345-5, 17-1946-1, 121-125

46-3, 56-5946-4, 53-5646-4, 122-12446-6, 33-3547-3, 73-7647-4, 81-8447-4, 128-13048-1, 13-1648-2, 60-6548-3, 97-10148-6, 53-5848-6, 74-7648-6, 35-8748-6, 122-12550-1, 67-7150-2, 20-2250-3, 32-3550-3, 71-75

Subunit Vc

51-1, 54-5751-2, 77-8052-1, 78-8252-3, 8-1152-3, 140-14353-1, 77-8053-2, 85-8853-4, 31-3453-4, 76-7954-1, 64-6754-1, 89-9254-4, 81-9554-5, 83-8655-1, 36-3855-3, 71-7455-3, 113-11655-6, 44-4756-1, 5-656-2, 81-8456-3, 91-9457-2, 35-3758-1, 89-9159-2, 117-12059-1, 36-38

Primary mineralsa

Optical and electron microprobe analyses

Plagioclase (%An)

Ground-mass

41

(30)(54)

50-62(54)(57)(54)(5470

(57)(54)

(tuff)

68 (inglass)(50)(54)(54)64

(54)——

(51)—

(30-59)——

(54)56

(57)(60)

(Tuff)(60)(57)(60)(59)—

68(57)(57)——

(56)(56)_

(60)——

(57)(57)——65

(57)(58)(54)(54)67

—(58)

Microphe-nocrysts

63

—82—

(54)(54)(57)76

(57)(57)

76 (inglass)

(54)(54)(54)—

(54)—

(52)(51)(52)(60)—

(54)(54)(52)(57)(57)

(54)————

82————

(57)—

(57)(60)(57)(57)—

(60)—

(57)77—

(57)——86

(60)(60)—

PyroxeneMg:Fe:Ca

Aug. 39:21:40

Aug. 48:18:34

Aug. 42:25:33Mg-pig. 52:34:14

Aug. 46:20:34

Aug. 47:17:36

Aug. 47:18:35

Olivine(alteration)

sap.-ps.

sap.-ps.

sap.-ps.sap.-ps.

sap.-ps.

Opaques

Ti-mag.

Ti-mag.

Ti-mag.

Ti-mag.

Secondary minerals

X-ray diffraction analyses

(sap.)

Na-sap.>cel.Na-sap. >Ca-sap., eel.Na-sap.>cel.

Na-sap.>Ca-sap., eel., cal.Na-sap.>cel.(sap. > eel.)Na-sap. eel.

(sap., eel.)

Na-sap.>Ca-sap. eel., hem. (tr)

Na-sap.>cel.Na-sap.>cel.Ca-sap. > Na-sap., eel.Na-sap.

(sap.)(sap.)Na-sap.>cel.(sap. > eel.)(sap. > eel.)Na-sap.>cel.Na-sap.>cel.( s a p . » eel.)Ca-sap. >Na-sap., eel.(sap.)(sap.)(sap. > eel.)Na-sap.>cel.(sap.)Na-sap.>cel.Na-sap.>cel.(sap.)Na-sap.>cel.

Ca-sap., nont., cal.(sap.)(sap. > eel.)Na-sap.>cel.Na-sap.>Ca-sap., eel.

Na-sap.>Ca-sap., eel.

Na-sap.>Ca-sap., eel.

eel., Ca-sap.Na-sap.>Ca-sap., eel.

eel., Ca-sap.

Na-sap., Ca-sap., eel.

Na-sap.>Ca-sap., eel.

Note: For list of symbols see Table 2.* Optical determination of anorthite content are shown in parentheses.

Optical identification of secondary minerals are shown in parentheses.

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PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

in hues of green, olive green, or blue green. In somesamples (e.g., Sample 553A-38-2, 58-59 cm) the outerradially fibrous smectite appears to be continuous withthe inner celadonite, but precise distinction optically isnot possible. In some vesicles smectite alone occurs asradially fibrous or structureless masses and in othersceladonite occurs alone as aggregates of microcrystal-line fibers. Toward the lower part of the succession inSample 553A-56-1, 5-6 cm, smectite and celadonite maybe finely intercalated as vermiform radial aggregates.Chalcedony is sparse, but fills the cores of celadonite-rimmed vesicles in Sample 553A-50-2, 20-22 cm. Calciteoccurs sporadically in larger vesicles (as in Sample 553A-53-2, 85-88 cm) forming dendritic intergrowths with smec-tite. In some of the lowest samples celadonite appears topredominate over smectite.

Fragmental Rocks

The two samples of vitroclastic tuffs from Samples553A-45-1, 44-47 cm and 553A-45-1, 87-91 cm (SubunitVb) are closely packed, poorly sorted aggregates of mi-crolithic lava, pumice with spheroidal smectite-filledvesicles showing little compression, and abundant iso-topic orange-red argillized glass shards (Plate 1, Fig. 2).Microphyric lava particles contain possible olivine pseu-domorphs. The fine matrix is mainly comminuted, veryangular orange-red glass particles averaging 0.08 mm,and olive-green clay material. The color of the argillizedglass results from dispersed hematite. A lithic tuff fromSample 553A-43-1, 62-66 cm (Subunit Va) shows basal-tic fragments up to 1.5 cm across, set in a matrix ofsmectite and celadonite, which also fill vesicles. The ba-salts include microlithic aphyric patches with plagioclaseneedles (averaging 0.06 × 0.003 mm) and nests of coarserplagioclase (0.4 × 0.04 mm), altered ferromagnesians,and disseminated ferric oxide. Some fragments are opaqueas a result of intense iron oxide impregnation. The lavafragments appear to have fractured during flow, withlater cementation by the clay minerals. Other samples(Samples 553A-45-2, 114-118 cm and 553A-45-4, 16-20cm, Subunit Vb) are brecciated, with clasts of micro-phyric highly vesicular basalt set in a fine matrix ofsmectitic material veined by celadonite; they probablyrepresent flow breccias (? cf. Aa-lavas). Sample 553A-52-3,8-11 cm (flow top in Subunit Vc) is reddened, with aphyr-ic, smectitic, hematitized basalt fragments in a fine claymatrix. Another (Sample 553A-54-1, 89-92 cm) proba-bly owes its complex basalt textures to admixture of het-erogeneous lava during flow and different cooling re-gimes, with no distinct clasts.

Hole 554A

Textures and Lithologies

Samples of basalt from Unit V are light gray (N7) tolight-olive gray (5Y6/1) and microcrystalline. Amygdalestoward the top of lava flows are abundant (as in Sample554A-7-1, 63-66 cm), spheroidal to ellipsoidal, up to8 mm across, and filled with goethite and smectite, orsparse (as in Sample 554A-8-1, 99-102 cm) in massive,light-olive gray (5Y 6/1) microcrystalline basalt. The baseof a flow unit (Sample 554A-7-4, 25-28 cm) contains

scattered spheroidal (1-2 mm diameter) vesicles. Thesamples from near the flow tops are sparsely plagio-clase, or plagioclase-clinopyroxene-microphyric with mi-crophenocrysts from 0.3 × 0.02 mm to 0.6 × 0.05 mm(Sample 554A-8-1, 99-102 cm). The groundmass is afinely microcrystalline felt of plagioclase needles averag-ing about 0.1 × 0.03 mm, with intergranular clinopy-roxene (0.01 mm) and minor magnetite. A trace of anolivine pseudomorph occurs in Sample 554A-8-1, 99-102 cm. From the base of a flow unit (Sample 554A-7-4,25-28 cm) the basalt is aphyric with altered glassy meso-stasis. Felted plagioclase needles average 0.05 × 0.01 mmwith conspicuous intergranular clinopyroxene (0.01 mm).

Sample 554A-14-1, 36-39 cm is homogeneously mi-crocrystalline basalt, medium light gray (N6), with rarevesicles.

Mineral Composition

The two samples from near flow tops are microphyricwith labradorite and clinopyroxene microphenocrysts (0.3× 0.2 mm) in Sample 554A-7-1, 63-66 cm; sparse labra-dorite (An68) laths attain 0.6 × 0.05 mm in Sample554A-8-1, 99-102 cm. The groundmass is a finely micro-crystalline (to 0.1 × 0.03 mm) felt of labradorite (An67)needles with conspicuous intergranular subcalcic fer-roaugite (0.01 mm) (Table 2) and minor opaque iron-ti-tanium oxide grains. Modal analysis indicates about35% volume plagioclase, 55% clinopyroxene, and 10%opaque oxides and secondary minerals. Deep red smec-tite (after olivine) occurs in the second sample, which al-so contains some carbonate.

The sample from the middle of a flow (Sample 554A-7-3, 10-12 cm) is closely similar to the flow-top samplesboth in texture and modal mineralogy (Table 3). Elec-tron microprobe analyses show that groundmass and phe-nocrystic plagioclase are closely similar in composition(An67 and An68, respectively) and that pyroxene micro-phenocrysts are subcalcic ferroaugites (Mg33Fe46Ca2i).

Sample 554A-7-4, 25-28 cm (from the base of a flow)is aphyric with altered glassy patches with carbonate;the felted plagioclase needles average 0.05 × 0.01 mm,and there is conspicuous intergranular (0.01 mm) clino-pyroxene.

Yellowish-gray (5Y 7/2) basaltic conglomerate in Sam-ple 554A-9-1, 75-78 cm, is plagioclase-phyric with lathsup to 1.2 × 0.9 mm in a groundmass of plagioclase nee-dles (0.06 × 0.004 mm) and interstitial clinopyroxene;the mode is obscured by much altered glassy mesostasisbut the proportion of clinopyroxene to plagioclase ap-pears to be less than in the flow units.

A sample from near the bottom of the hole (Sample554A-14-1, 36-39 cm) contains rare vesicles, is patchilymicrophyric with nests of slender plagioclase laths andclinopyroxene crystals (up to 0.2 mm) set in a microcrys-talline ground of plagioclase needles (0.03 × 0.002 mm),minutely interstitial clinopyroxene (0.03 mm), and glassymesostasis (Plate 2, Fig. 1).

Hole 555

Basalt flows, hyaloclastites, and tuffs, occur in thelowest Unit (IV); interbeds of detrital sediment occur in

747

R. K. HARRISON ET AL.

Table 2. Summary of electron microprobe, optical, and X-ray diffraction analyses of basalts and associated rocks,Holes 554A and 555.

Sample(interval in cm)

Hole 554A

7-1, 63-667-3, 10-12

7-4, 25-288-1, 99-1029-1, 75-7814-1, 36-39

Hole 555Subunit IVa

68-2, 85-8868-3, 122-12669-2, 37-4069-3, 113-11670-1, 135-13975-2, 91-9476-2, 67-7076-2, 117-120

76-4, 8-10

76-5, 22-24

77-4, 74-7777-5, 144-14779-3, 114-11781-2, 61-64

Primary minerals'Optical and electron microprobe analyses

Plagioclase (%An)

Ground-mass

67

(55)(48)(56)48

(Tuffs)

Microphe-nocrysts

68

(80)

(56)(57)7674

(Agglomerate)

PyroxeneMg:Fe:Ca

S.c.f. aug.33:46:21

E. di. 49:10:41Aug. 46:15:39

Olivine(alteration)

Sap.-ps.Sap.-ps.

Sap.-ps.Sap.-ps.Sap.-ps.

Sap.-ps.

Opaques

Ti-mag.

Secondary minerals

X-ray diffraction analyses

Cel., eel./sap.Cel., cel./sap.

Cel., cel./sap., cal.Cel., cel./sap., cal., hem.Cel., cel./sap., cal., hem.Cel., cel./sap.

Na-sap. >cel., dial.Ca-sap., cel., serp., cal.Ca-sap., cel., cal.Na-sap., cel., cal.Na-sap., cel.Anal.>Na-sap.>cel., hem.Anal. > Na-sap. > cel. > hem.Na-sap.>Ca-sap., anal., cel.

hem., ch., dial.Na-sap., Ca-sap., cel./sap..

cel., anal., hem., ch., dial.anal.>Na-sap.>Ca-sap., cel., hem.,

dial.(anal.>Na-sap., Ca-sap.>cel., hem.)Anal.>Na-sap.>cel., hem.Anal. >Na-sap. >cel., hem.Na-sap.>Ca-sap., cel., anal.,

ch., hem.81-2, 133-13681-3, 110-11382-1, 36-38

82-1, 136-13982-2, 79-81

83-1, 42-45

Subunit IVb

90-1, 67-6990-1, 125-12890-2, 65-7091-1, 94-9693-2, 136-138

Subunit IVc

95-1, 40-4395-1, 97-100

96-2, 95-9897-3, 36-3997-6, 80-8396-5, 85-8896-5, 108-110

(56)52

(57)(Tuff)

45

(60)(57)

74

(57)66

(56)

74

52

45

74

(57)66

(56)

74

Aug. 41:22:37

Aug. 46:22:32

Aug. 44:28:28

Aug. 47:14:39

Aug. 46:14:40

Fo82

Fo76

Ti-mag.

Ti-mag.

Ti-mag.

Ilmenite

Na-sap.>Ca-sap, anal., hem., ch.Na-sap.>Ca-sap., cel., anal.,

hem., ch.

Na-sap.>cel., Ca-sap., anal.,hem., ch.

Ca-sap. >cel.

Ca-sap. > cel.Ca-sap.>cel., ch.

Na-sap., Ca-sap.>cel., ch.Na-sap., Ca-sap.>cel., serp. ch.

(amph.)(sap., cel. serp., ch. amph.)(sap., cel., ch.)(sap., cel., ch.)(sap., cel., ch.)Ca-sap.>(cel., ch.)

Note: aug. = augite, s.c.f. aug. = subcalcic ferroaugite, e.di. = endiopside, Mg-pig. = magnesium pigeonite, Ti-mag. = titaniferousmagnetite, sap.-ps = saponite pseudomorphs after olivine, amph. = fibrous amphibole, dial. = chalcedony, cal. = calcite, Na-sap. = sodium-saponite, Ca-sap. = calcium-saponite, sap. = saponite (optically), cel. = celadonite, cel./sap. = celadonite/sap-onite mixed layer mineral, nont. = nontronite, serp. = serpentine, ch. = chlorite, anal. = analcime, hem. = hematite.

* Optical determinations of anorthite content are shown in parentheses.Optical identifications of secondary minerals are shown in parentheses.

the two upper subunits (IVa, IVb) and a massive basaltdolerite forms the lowest (Unit IVc). We sectioned 33samples of the basalt flows and 7 of volcaniclastic mate-rial from Subunits IVa and IVb, plus 12 samples of themassive basalt Subunit IVc. The 10 lava flow units of

Subunits IVa and IVb identified on shipboard are sim-ple or composite, commonly with pillows, and mainlyless than 1 m thick. The pillows show thin (5-mm) rindsof altered dark glass which may be brecciated and vesic-ular interiors coarsening toward their centers.

748

PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

Table 3. Modal analyses of basalts from Holes 553A and 554A and of basalts and dolerites from Hole 555.

Sample(interval in cm)

Hole 553A

40-1, 79-8242-2, 42-4645-5, 17-1946-4, 53-5648-3, 97-101

48-6, 122-125

51-2, 77-8055-3, 113-11658-1, 89-91

Hole 554A

7-3, 10-12

Hole 555

68-3, 122-126

69-2, 21-2469-4, 85-88

76-1, 69-7490-1, 100-10498-5, 56-5996-3, 110-113

Brief lithology

Sparsely phyric basaltSparsely phyric basaltSparsely phyric basaltAphyric basaltModerately phyric

basaltModerately phyric

basaltAphyric basaltSparsely phyric basaltSparsely phyric basalt

Sparsely phyric basalt

Moderately phyricbasalt

Glomerophyric basaltModerately phyric

basaltDoleriteAphyric basaltOlivine-doleriteOlivine-dolerite

Phenocrysts (vol.%)

Plag.

121

—3

5

_1tr

1

8

77

-

—

Cpx.

_——1

——

2

12

—

Total

121

_4

5

1tr

1

10

89

0000

Plag.

3541363229

33

333237

29

37

3029

38434946

Groundmass (vol.%)

Cpx.

5141514951

50

504337

53

36

4546

40333740

Oliv.

__———

tr

———

-

tr

trtr

0tr65

Magnet.

748

108

5

7164

13

7

77

1433

Smectite

612498

7

108

22

4

10

109

2120

56

Total

999899

10096

95

10099

100

99

90

9291

100100100100

Note: tr = trace, dash indicates absence.

General Features of Basalts of Subunits IVa and IVb

Basalts above the pyroclastic sequence of Subunit IVshow hues of greenish gray to greenish black (5G 2/1),with sporadic vesicles filled with smectite, chalcedony,calcite, or celadonite, and ranging from spheroidal to el-lipsoidal, as at the base of a flow in Sample 555-70-1,135-138 cm. Cumulophyric plagioclase laths (up to 2.1× 0.6 mm) and clinopyroxene plates up to 6 mm acrossoccur in the uppermost flow unit (Plate 2, Fig. 2), set ina microcrystalline groundmass of plagioclase needles av-eraging about 0.1 × 0.02 mm, unaltered intergranularclinopyroxene, and minor opaque ore. A trace of pseu-domorphed olivine (0.8 × 0.6 mm) occurs in Sample555-69-3, 113-116 mm. Mesostasis is altered mainly tosmectite and celadonite. Aphyric or sparsely microphyricbasalts occur in the composite pillow lava units, and ex-cept for a fine plagioclase needle mesh (0.1 × 0.02 mm)are heavily altered to smectite; glassy rinds are devitri-fied to smectite. Unaltered clinopyroxene granules oc-cur, for example, in Samples 555-76-1, 10-13 cm and555-76-5, 22-24 cm (Plate 2, Fig. 3). Beneath the volca-niclastic sequences of Subunit IVa, samples of compos-ite pillow lavas away from glassy selvedges coarsen tosubophitic dolerite (e.g., in the middle of a flow in Sam-ples 555-81-3, 110-113 cm and 555-82-1, 136-139 cm),in which plagioclase laths average 0.6 x 0.07 mm andpartly intergrown clinopyroxene averages 0.3 mm. Thereis a little accessory magnetite, sporadic analcime, andmuch secondary clay.

Samples of the three units in Subunit IVb are mainlyaphyric except for a plagioclase-microphyric (1.5 ×

0.6 mm) basalt in the middle of a flow in Sample555-93-2, 136-138 cm. These basalts are hues of green-ish gray becoming medium dark gray (N4) toward thelower part of the subunit. Vesicles rarely exceed 3 mmacross and are mainly sparse even toward flow tops.Coarser (ophitic) textures with plagioclase laths average1.0 × 0.1 mm and intergrown clinopyroxene occur withinthe flows as in Sample 555-83-1, 42-45 cm (Plate 2,Fig. 5), but elsewhere the basalts are microcrystallineand equigranular with plagioclase needles averaging 0.04× 0.004 mm (as in Sample 555-90-1, 67-69 cm), and in-tergranular clinopyroxene (0.03 × 0.06 mm) in a smec-tite base.

Pyroclastic Rocks of Subunits IVa and IVb

We report on six samples of pyroclastic rocks includ-ing hyaloclastites or vitric tuffs from Sections 555-77-4,77-5, 78-2, 79-3, 80-2, an agglomerate in Sample 555A-81-2, 61-64 cm, and from Subunit IVb, a tuff in Sample555-82-2, 79-81 cm. The hyaloclastic rocks consist ofhighly angular argillized glass particles up to 4 mm acrossvarying from deep olive to grass green with yellow sel-vedges, clear, or containing scattered vesicles and crys-tallites of plagioclase. The glass is otherwise uniform-ly isotropic (with n = 1.566 to 1.578 ± 0.003) to weak-ly anisotropic and shows networks of shrinkage cracks(Plate 2, Fig. 4); glass particles are generally separatedby matrix rather than being self-supporting. Fragmentsof microphyric to aphyric lava occur between the glassparticles and contain slender labradorite laths averaging0.3 × 0.03 mm and anhedra of clinopyroxene. These

749

R. K. HARRISON ET AL.

laths may be pseudomorphed by microcrystalline claymaterial (as in Sample 555-78-2, 97-100 cm).

An agglomerate in Sample 555-81-2, 61-64 cm con-tains angular fragments of dark brown cumulomicro-phyric devitrified glassy lava with fluxioned margins andaligned microliths and vesicles. Clusters of labradoritelaths and pseudomorphed(?) olivine grains reach 3 mmacross. The matrix is a finely clastic aggregate of glassparticles, feldspar crystals, and argillized material.

Tuff in Sample 555-82-2, 79-81 cm includes a particleof pillow lava showing a glassy rind and microphyric in-terior with plagioclase laths attaining 0.6 × 0.03 mm.The matrix consists of feldspar crystals, clinopyroxene,particles of altered glass, lava, quartz, apatite, biotite,and opaque oxides in a smectite-rich base. The feldsparsinclude sodic plagioclase, microcline, and orthoclase andindicate an input from a wide variety of source rockother than basalt.

X-ray diffraction (XRD) analyses (Table 2) show thatthe glassy fragmental rocks are altered to Na-saponitewith celadonite, analcime, and hematite. The agglomer-ate and tuff are altered similarly although they also con-tain chlorite.

Mineralogy of the Basalts of Subunits IVa and IVb

Optical, electron microprobe, and XRD data are sum-marized in Table 2 and modal analyses of selected sam-ples appear in Table 3. Plagioclase phenocrysts in themicrophyric basalts of Subunit IVa range from An45 (an-desine) to An76 (bytownite), and the groundmass pla-gioclase from An45 to An57 (labradorite). Coexistingplagioclases in the lower part of Subunit IVa show nocompositional difference between groundmass and mi-crophenocrysts (Table 4). Marked differences, however,occur in the upper part of the subunit and in the ab-sence of zoning and suggest that the microphenocrystsare fractionated xenocrysts. In the same subunit an au-

gitic pyroxene differs little from the lowest to the highestbasalts and shows a range of Mg(41-46) : Fe(15-22) :Ca(32-39), except for Fe-poor endiopside (Mg 49 : Fe 10 :Ca 41) in Sample 555-69-2, 37-40 cm. The opaque ore istitanomagnetite. Modal analyses of phyric basalts give8-10% volume phenocrysts (plagioclase 7-8%, clinopy-roxene 1-2%). Groundmass consists of 29-37% plagio-clase, 36-46% clinopyroxene, a trace of olivine, 7% ti-tanomagnetite, and 9-10% secondary minerals.

Subunit IVc

Sample 555-95-1, 40-43 cm, near the chilled top ofthis single massive unit, is a greenish gray, aphyric mi-crocrystalline basalt formed of a random mesh of pla-gioclase needles (0.12 × 0.02 mm) (-55%) and in-tergranular clinopyroxene (-30%) with minor opaqueoxide, sparse orthopyroxene, and interstitial material re-placed by smectite. In contrast to all other basic rocksof this and the other two sites (Sites 553 and 554), theunderlying rocks are olivine-bearing ophitic to subophiticdolerites becoming coarse grained in the central position(Plate 2, Fig. 6). They are also conspicuous by change ofcolor to bluish hues (mainly light bluish gray 5 to 7/1)compared with the greenish gray of most of the basalts.Vesicles are rare and reach 2 mm across. These doleritesconsist of an equigranular intergrowth of labradorite(averaging 0.7 × 0.07 mm) and clinopyroxene (0.3 mm),with fresh and altered olivine crystals up to 0.6 mmacross, minor opaque oxides, and an altered smectiticmesostasis. Plagioclase compositions are in the rangelabradorite to bytownite (An56-An74), and the clinopy-roxene is augite in the range Mg 44-47 : Fe 14-28 : Ca28-40. Microprobe analyses (Table 2) also show the oli-vine to be forsteritic (Fo76_82), and the opaque ore is tita-nomagnetite. Modal analyses (Table 3) give plagioclase46-49% volume; augite, 37-40%; titanomagnetite, 3%;smectite and other secondaries, 5-6%.

Table 4. Representative plagioclase analyses (Holes 552, 554A, and 555) normalized to eight oxygens.

SiO2A12O3FeO*CaONa2OK2O

Total

SiAlFeCaNaKAnAbOr

Note: ]

552-22-1,137-141 (MP)

52.7528.87

0.8912.424.080.05

99.06

2.421.560.030.610.360.01

62.5637.130.31

552-22-1,137-141 (G)

59.2924.46

1.228.096.440.19

99.69

2.671.300.050.390.560.01

40.5058.36

1.14

554A-7-3,10-12(G)

51.5229.13

1.0613.453.740.15

99.05

2.381.580.040.660.330.01

65.9433.170.89

Sample (interval in cm)a

554A-7-3,10-12 (MP)

52.3729.68

0.8713.803.640.09

100.45

2.381.590.030.670.320.01

67.3732.130.50

555-69-3,113-116 (G)

56.5325.97

0.819.435.760.16

98.66

2.581.400.030.460.510.01

49.0951.990.92

Dash indicates absence. FeO* includes Fe 2 θ3 recalculated as FeO.a Grain type is indicated in parentheses after sample number. G = groundmass, MP

O = ophitic.

555-69-3,113-116 (P)

49.7130.970.57

15.082.890.00

99.22

2.291.680.020.740.260.00

74.2525.75

—

555-83-1,42-45 (MP)

57.5726.24

0.949.306.260.07

100.38

2.581.390.040.450.540.01

44.9254.67

0.40

555-96-5,108-110(0)

49.5430.750.69

15.012.870.01

98.87

2.291.680.030.740.260.00

74.3325.67

—

= microphenocrysts, P = phenocrysts, and

750

PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

Basaltic Rocks from Sites 552-555

The samples of lava flows from all four sites are ba-salts showing similar ranges of textures and broadly sim-ilar primary mineralogy within the limits of sampling(see Tables 4-9). That is, most phenocrysts in phyric ba-salts are plagioclase mainly in the labradorite to by-townite range (with some andesine at Site 555), but afew are clinopyroxenes in the augite range. The ground-mass of these and the aphyric basalts is an intergrowthof plagioclase, in the range andesine to labradorite, and

mainly augite, although Mg-pigeonite was also found inHole 553A, and subcalcic ferroaugite in Hole 554A. Theopaque ore is titaniferous magnetite. The primary min-eralogy is generally similar in the lithologically distinctbasal olivine-dolerite (IVc) of Hole 555, with essentiallabradorite, augite, and minor Ti-magnetite. However,here olivine is the characteristic accessory, with minor il-menite. But traces of olivine also occur sporadically inthe lava flows of Holes 553A, 554A, and 555. Modalanalyses give similar ranges of primary minerals in thebasalts of the four holes but the olivine-dolerites of

Table 5. Representative clinopyroxene analyses (Holes 552, 554A, and 555) normalized to six oxygens.

SiO 2A1 2O 3FeO*MnOMgOCaOTiO 2C r 2 O 3

Total

S iivAlIVA 1 V I

FeMnMgCaTiCr

Mg/Mg + F e 2 + %WoEnFs

552-22-1,137-141 (G)

49.023.69

12.580.04

13.2719.21

1.920.20

99.93

1.850.150.020.400.000.750.780.060.01

65.2240.4738.8620.68

554A-7-3,10-12(G)

49.161.11

26.910.61

10.679.670.780.00

98.91

1.960.040.010.900.020.630.410.020.00

41.4021.2332.6146.16

Sample (interval in cm) a

555-69-2;37-40 (O)

53.152.076.150.05

16.9919.920.340.69

99.36

1.960.040.050.190.000.930.790.010.02

83.1341.2148.87

9.92

555-69-3,113-116 (G)

51.182.099.300.20

15.7118.170.830.22

97.70

1.940.060.030.300.010.890.740.020.01

75.0438.4146.2215.37

555-81-3,110-113 (G)

50.502.23

13.210.33

14.1517.490.820.18

98.91

1.920.080.020.420.010.800.710.020.01

65.6036.8441.4321.72

555-95-1,97-100 (O)

51.871.22

16.990.26

15.0013.650.430.00

99.42

1.970.030.030.540.010.850.560.010.00

61.1228.5943.6527.76

555-96-5,108-110(0)

52.721.898.770.20

16.0919.870.600.07

100.21

1.950.050.030.270.010.890.790.020.00

76.5640.4745.5713.95

Note: FeO* includes Fe 2 θ3 recalculated as FeO.a Grain type is indicated in parentheses after sample number. G = groundmass, O = ophitic.

Table 6. Representative plagioclase analyses (Hole 553A) normalized to eight oxygens.

SiO 2A1 2O 3FeO*CaONa 2 OK 2 O

Total

SiAlFeCaNaKAnAbOr

553A-39-1,2-4 (G)

52.2627.71.2.1712.164.050.06

98.41

2.431.520.080.610.370.01

62.1137.47

0.41

553A-39-1,2-4 (MP)

47.0631.27

0.7016.45

1.990.03

97.50

2.221.740.030.830.180.00

82.0517.95

—

553A-43-1,20-23 (G)

51.5729.33

0.9514.003.340.00

99.19

2.371.590.040.690.300.00

69.8430.16

—

Sample (interval in cm) a

553A-43-1,20-23 (MP)

48.9930.07

0.6914.942.570.01

97.27

2.301.670.030.750.230.00

76.2723.73

—

553A-46-1,121-125 (G)

52.5629.51

1.1313.404.200.08

100.88

2.381.570.040.650.370.01

63.4836.04

0.49

553A-51-1,54-57 (G)

49.8127.71

1.3713.153.410.07

95.52

2.381.560.060.670.320.01

67.8131.79

0.40

553A-51-1,54-57 (MP)

47.4132.68

0.6216.94

1.700.00

99.35

2.191.780.020.840.150.00

84.6815.32

—

553A-57-2,35-37 (G)

49.0729.65

1.0212.283.400.05

92.77

2.411.560.040.650.320.01

66.4333.26

0.31

553A-57-2,35-37 (MP)

45.9531.92

0.5016.37

1.490.03

96.26

2.191.790.020.840.140.01

85.6614.140.20

Note: Dash indicates absence. FeO* includes F e 2 O 3 recalculated as FeO.a Grain type indicated in parentheses after sample number. G = groundmass, MP = microphenocrysts.

751

R. K. HARRISON ET AL.

Table 7. Representative clinopyroxene analyses (Hole 553A) normalized to six oxygens.

SiO 2A12O3FeO*MnOMgOCaOTiO 2C r 2 O 3

Total

S iivAl™A 1 V I

FeMnMgCaTiCr

Mg/Mg + F e 2 + %WoEnFs

553A-45-1,44-47 (MP)

50.712.35

10.130.34

15.5815.260.47—

94.84

1.970.030.080.330.010.900.630.01

—

73.2534.0148.3417.65

553A-46-1,121-125 (G)

51.911.60

15.530.38

14.6315.680.540.00

100.27

1.950.050.020.490.010.820.630.020.00

62.6732.5442.2825.18

Sample (interval in cm) a

553A-46-1,121-125 (G)

52.931.01

20.970.56

18.336.830.470.00

101.10

1.970.030.010.650.021.020.270.010.00

60.9214.0452.3733.59

553A-51-1,54-57 (G)

50.752.06

12.350.44

15.4916.010.440.07

96.61

1.940.060.030.400.010.880.660.010.00

69.0933.9245.6620.42

553A-55-3,113-116(G)

51.801.81

10.260.21

16.4117.340.300.08

98.21

1.950.050.030.320.010.920.700.010.00

74.0635.9947.4016.61

553A-57-2,35-37(G)

48.326.799.270.18

13.4913.690.47

—

92.21

1.910.090.230.310.010.790.580.01—

72.1834.4847.2918.23

Note: Dash indicates no data. FeO* includes Fe 2 θ3 recalculated as FeO.a Grain type indicated in parentheses after sample number. G = groundmass, MP = microphenocryst.

Table 8. Representative olivine analyses

(Hole 555) normalized to four oxy-

gens.

SiO 2FeO*MnOMgOCaOTiO 2C r 2 O 3

Total

SiFeMπMgCaTiC r

Mg/Mg + F e 2 + %

Sample(interval in cm)

555-95-1,97-100

39.8416.790.31

42.580.410.000.00

99.93

1.010.360.011.610.01

81.87

555-96-5,108-110

38.6022.26

0.2138.59

0.330.010.07

100.07

1.000.480.011.490.010.000.01

75.53

Note: Dash indicates no data. FeO* includesF e 2 O 3 recalculated as FeO.

Hole 555 contain higher plagioclase and lower clinopy-roxene than the basalts. No systematic downhole varia-tions in mineralogy were detected at any of the sites.

Electron microprobe analyses reveal consistent differ-ences between groundmass and phenocrystic plagioclasein Hole 553A basalts and also in the one sample exam-ined from Hole 552. These differences are also apparentin the upper part of Subunit IVa but are absent from theremainder of Hole 555 basalts and dolerites, and they

do not appear in the one analyzed sample from Hole554A. The strongly calcic nature of the phenocrysts inHole 553A lavas and, in all cases, a lack of zonationcoupled with, in some cases, marginal resorption, wouldseem to rule out an intratelluric origin, suggesting in-stead that the phenocrysts were fractionated from a moreprimitive magma. Thus these xenocrystic lavas of Hole553A appear to share some specific petrographical char-acteristics with the poorly sampled lavas of Hole 552and the uppermost lavas of Hole 555 but not with theother basalts examined.

CHEMISTRY

Major Oxides

We report 16 whole-rock analyses, made by beta-probe,of the least-altered basalt samples of Hole 553A. Allhave Fe2O3 less than 7.0% and (H2O

+ + CO2) contentsof less than 5% (except two samples which slightly ex-ceeded that amount), which is an acceptable limit foraltered rocks. Three similarly least-altered basalts wereanalyzed from Hole 554A and two from Hole 555. Sixolivine-dolerites were analyzed from the basal Unit IVcof Hole 555. Two vitroclastic tuffs (from Holes 553Aand 555) were also analyzed for comparative purposes;they contain high Fe2O3 and H2O. Results are tabulatedin Tables 10-13, and expressed on a moisture-free basis;H2O (– 105°C) contents are listed separately.

There is little variation in major oxides in the basaltsof Hole 553A. No significant variation is apparent withdepth of sample or between flow tops, middles, or bases.The ranges noted in Table 10 reflect variations in sec-ondary minerals (vugs, mesostasis, alteration); the in-trasample variation in, for example, vesicle proportions

752

PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

Table 9. Representative analyses of opaque oxides from Holes 552, 553A, and

555.

Siθ2TiO2A12O3Cr2O3Fe 2O3FeOMnOMgO

Total

SiTiAlCr

Fe2 +

MnMgNo. of Os

552-22-1,137-141

0.4121.91

2.460.12

22.5749.06

0.481.56

98.57

0.124.910.860.035.06

12.210.120.69

32

553A-39-1,2-4

0.6517.39

1.050.17

30.7644.67

0.231.28

96.20

0.204.040.380.047.14

11.530.060.59

32

Sample (interval in cm)

553A-55-3,113-116

0.3513.20

1.930.00

38.5739.09

2.321.03

96.49

0.113.060.700.008.96

10.090.610.47

32

555-69-3,113-116

0.6026.33

1.500.23

12.1252.63

1.290.83

95.53

0.196.110.550.062.81

13.580.340.38

32

555-95-1,97-100

1.4023.13

1.260.01

17.7251.30

1.120.85

96.79

0.435.300.450.004.07

13.080.290.39

32

555-96-5,108-110

0.5550.11

0.360.16—

45.510.422.33

99.44

0.031.900.020.01—1.920.020.186

-5 _, Λ I

Note: Dash indicates no data. Fe and Fe estimated on the basis of 24 cations per unitcell.

Table 10. Whole-rock chemical analyses8 of basalts and a tuff from Hole 553A (wt.%). b

Core-Section[interval in cm)

38-2, 56-5940-1, 79-8245-1, 44_4745-5, 17-19

46-1, 121-12546-3, 56-5946-4, 53-5647-3, 73-7648-2, 60-6551-1, 54-5751-2, 77-8053-1, 77-8054-4, 81-8555-5, 92-9555-6, 44-4756-1, 3-659-2, 115-118

Lithology

BasaltBasaltTuffBasalt,

cpx-richBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasaltBasalt

Mean''Ranged

SiO2

52.054.752.150.3

50.350.850.249.749.649.150.249.850.050.050.448.749.5

50.36.0

A12O3

14.014.011.413.8

13.513.713.513.213.413.713.813.913.213.613.713.815.6

13.82.4

Fe2O3

6.486.65

16.35.50

3.473.624.086.265.106.344.35.804.784.194.043.665.06

4.963.18

FeO

6.826.211.327.52

9.549.348.618.189.036.497.896.229.648.658.878.335.12

7.904.52

MgO

7.615.245.137.28

7.357.327.386.906.867.927.858.266.567.187.027.188.12

7.253.02

CaO

9.968.574.11

11.3

11.511.511.510.110.411.011.210.910.211.111.111.110.7

10.73.03

Na2O

2.462.882.112.16

2.012.102.032.342.202.082.062.172.332.082.162.162.16

2.210.87

K2O

0.160.101.140.06

0.050.060.060.080.060.060.060.060.060.050.060.060.04

0.070.12

H2O +

1.611.314.951.44

0.870.810.90

(

.61

.38

.75

.45

.62

.19).89.08.85

>.26

.37

.45

CO2

0.13cc

0.00

0.06cc

0.020.000.050.060.300.140.200.160.960.00

0.160.96

TiO2

(

.32

.46

.45

.19

.13

.16

.13

.52

.50).99.01.07.35.15.17.18.08

1.280.47

MnO

0.320.380.130.42

0.470.490.370.380.530.380.380.400.540.500.500.650.33

0.440.33

P 2 O 5

0.100.140.040.13

0.170.110.120.120.120.080.070.080.120.080.090.080.07

0.110.10

Total

102.97101.64100.18101.10

100.42101.0199.88

100.41100.1899.94

100.33100.58100.1199.67

100.3599.71

100.04

100.503.30

H 2 O~

2.501.558.552.02

0.790.851.102.181.652.231.663.241.941.781.322.482.95

1.892.16

* Analysts: Dawn Hutchison, A. E. Davis and K. A. Holmes.Results expressed on a moisture-free basis.

c Not determined (insufficient sample).Excluding the tuff sample.

and infillings might otherwise suggest more variationsin bulk chemistry. The greatest ranges in terms of themeans of the six major oxides are shown by Fe2O3, FeO,MgO, and CaO. The means of the basalts compare fa-vorably with tholeiitic basalt (Nockolds' means of 137analyses, 1954) and ocean-floor tholeiitic basalt (Nock-olds et al., 1978) (Table 11).

The K2O of the Hole 553A basalts is considerablylower than that of geographically close olivine-basaltsof DSDP Leg 12, Sites 116 and 117 (Laughton et al.,1972), but compares closely with a basalt from the Mid-Atlantic Ridge, from DSDP Leg 49 (Luyendyk, Cann,

et al., 1979). Many other basalts from the site, however,contain much higher Fe2O3 than do the present samples.

Analyses of three basalts from Hole 554A (Table 12)are closely similar except for higher Fe2O3 in Sample554A-7-1, 63-66 cm. Their means are close to the Hole553A basalts except for significantly higher K2O andlower MgO in the former.

Two Hole 555 basalts (Table 13) approach the meansof the other two sites. MgO contents of Hole 555 ba-salts, however, are closer to those of Hole 554A; alsotheir Na2O and TiO2 contents are a little higher than atthe other sites. Five olivine-dolerites from Hole 555 con-

753

R. K. HARRISON ET AL.

Table 11. Comparison of major oxides of basalts from Holes 409 and 553A and ocean floor and tholeiitic basalts.

SiO2 A12O3 Fe2O3 FeO MgO CaO Na2O K2O H 2 O+ TiO2 MnO P 2 O 5

Hole 553A basaltsTholeiitic basaltsOcean-floor tholeiitic

basaltsHole 409 basalts

50.350.8349.99

49.91

13.814.0715.65

14.09

4.962.881.74

2.46

7.909.068.06

8.14

7.256.347.98

8.15

10.710.4211.36

11.89

2.212.232.70

2.21

0.070.820.19

0.07

1.370.910.60

0.69

1.282.031.40

1.17

0.440.180.19

0.17

0.110.230.23

0.14

Table 12. Whole-rock chemical analysesa of basalts from Hole 554A (wt.%) .

Core-Section(interval in cm) Lithology SiO2 Al 2 θ3 Fe2θ3 FeO MgO CaO Na2O K2O H 2 O

+ CO2 TiO2 MnO P 2 θ5 Total H2O"

7-1,7-3,7-4,

63-6610-1225-28

BasaltBasaltBasalt

MeanRange

50.851.551.251.2

0.7

13.614.013.813.80.4

6.343.853.854.682.49

6.877.798.247.631.37

6.586.706.676.650.12

11.411.711.511.50.3

2.142.282.242.220.14

0.340.190.240.260.15

0.740.770.720.740.05

0.060.020.040.040.04

1.081.061.061.070.02

0.300.310.340.320.04

0.080.070.100.080.03

100.33100.24100.00100.19

0.33

0.740.770.720.740.05

Analysts: Dawn Hutchison, A. E. Davis and K. A. Holmes.Results expressed on a moisture-free basis.

Table 13. Chemical analysesa of basalts, a tuff and olivine-dolerites from Hole 555A (wt.°7o).

Core-Section(interval in cm)

69-2, 113-11677.4, 74-7790-1, 125-12895-1, 40-4396-2, 95-9897-3, 36-3997-6, 80-8396-5, 108-110

Lithology

BasaltTuffBasaltOlivine-doleriteOlivine-doleriteOlivine-doleriteOlivine-doleriteOlivine-dolerite

Meanc

Rangec

SiO2

49.947.149.549.749.650.349.649.6

49.70.8

A12O3

14.012.413.213.913.813.714.214.1

13.81.0

Fe2O3

3.829.665.812.823.624.163.433.26

3.852.55

FeO

8.613.227.208.798.228.237.708.16

8.131.59

MgO

5.969.496.997.507.256.317.747.76

7.071.80

CaO

11.42.8

10.912.512.412.212.612.5

12.11.7

Na2O

2.604.822.422.102.112.502.132.18

2.290.50

K2O

0.110.120.070.090.090.050.050.06

0.090.07

H2O +

1.408.862.131.191.371.151.622.12

1.570.98

co2

0.260.340.000.100.000.060.050.00

0.050.26

TiO2

1.68.26.34.08.15.42.01.07

1.250.57

MnO

0.420.260.390.340.400.340.350.39

0.360.07

P2O5

0.140.080.080.080.080.080.070.06

0.080.08

Total

100.30100.41100.0399.49

100.09100.50100.70101.26

100.351.77

H 2 O~

1.339.202.260.560.661.171.180.94

1.161.60

* Analysts: Dawn Hutchison, A. E. Davis and K. A. Holmes.Results expressed on a moisture-free basis.

c Excluding the tuff sample.

tain higher CaO on average than all other basalts butotherwise are closely similar chemically.

The vitroclastic tuff sample from Hole 553A has mark-edly higher Fe2O3, H 2 O

+ , and K2O, and lower FeO, CaO,MgO, P 2 O 5 , and MnO compared with basalts. However,SiO2, A12O3, Na2O, and TiO2 are similar. The tuff fromHole 555 is similarly enriched and depleted relative tothe basalts, except that Na2O is here greater in the tuffand P 2 O 5 similar to that of the basalts.

Plotted on a SiO2-alkali diagram (Fig. 2), all of theanalyzed basalts and dolerites fall in the subalkaline ba-salt field of Irvine and Baragar (1971) ("tholeiitic" se-ries of Kuno, 1960). One high-SiO2 sample (Sample553A-40-1, 79-82 cm) from Hole 553A falls in the an-desite range of the proposed classification of the IUGS(basalt: andesite limit at 52% SiO2). There is little sys-tematic variation of alkalies with SiO2, with between 2and 3% (Na2O + K2O). In this range also lie the SiO2-poorer, mainly subaerial, Blosseville Coast basalts ofEast Greenland, from the rifted counterpart of the Green-land-Rockall continent (Brooks et al.* 1976) and the op-posing Faeroes basalts (Bollingberg et al., 1975). A tufffrom Hole 555 lies in the alkaline basalt field whereasthat from Hole 553A lies at the basalt/andesite bound-ary.

Plots of the major oxides on the solidification index(SI) variation diagram (Fig. 3) reveal some positive andnegative linear correlations which suggest that at somesites the flows are related by fractionation from a com-mon parent magma. This is clearly displayed by basaltsfrom Hole 553A which show negative linear correlationfor FeO, MnO, Na 2O, TiO2, and P 2 O 5 , and correspond-ingly positive linear correlation for MgO. Poor correla-tion between SI and CaO or A12O3 may reflect the wide-spread occurrence of xenocrystic plagioclase in the Hole553A basalts. The K2O-deficient nature of these basaltsis emphasized by the barely discernible negative correla-tion with SI. Similar trends were obtained by combiningplots of Hole 555 basalts and dolerites, suggesting thatthe two rock types are, respectively, more and less frac-tionated portions of the same parental magma. The smallgroup of basalts from Hole 554A possess higher SiO2values, higher K2O, and generally show anomalous cor-relations with SI compared to the other basalt groups.Figures 4 and 5 illustrate the negative linear correlationbetween MgO and both alkalis and TiO2 for all the ba-salts analyzed.

In the FMA diagram (Fig. 6), the Leg 81 basalticrocks cluster closely in the FeO 45-60% range (Na2O +K2O) 10-20%, MgO 30-46%; they show some align-

754

PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

Z 2

Hole 553A basaltsHole 553A tuffsHole 554A basaltsHole 555 basalts

Hole 555 tuffs

Hole 555 doleritesBlosseville Coast basaltsE. Greenland (Brooks et al.,1976)Faeroes basalts (Bollingbergetal. , 1975)IUGS proposed classification,1981; normative nepheline

R. K. HARRISON ET AL.

Basalts from Hole 553A35 40 45 50

Basalts from Hole 554 •Dolerites from Hole 555 T35 40 45

0.60.50.40.3

3.0

2.0

9.08.07.06.05.0

0.150.100.05

12.011.010.09.0

8.0

0.15

0.10

0.05

14.0

13.0

12.0

54.0

52.0

50.0

48.0

1.51.41.31.21.11.00.90.8

8.0

7.0

6.0

5.0

.Na-,0

-K2O

_CaO

_ P ,

MnO

_AI2O3

SiO-

MgO

FeO

< . t—• r32 . . *

MnO

Na,O.

FeO.

K,O_

CaO.

SiO •

T i 0 o -

MgO

35 40 45 50 35 40Solidification index = (MgO/MgO + FeO + IMa2O + K2O) x 100

45

0.4

0.3

2.5

2.0

8.07.06.0

0.300.250.200.150.100.05

12.011.010.09.08.0

0.15

0.10

0.05

14.0

13.0

12.0

54.0

52.0

50.0

48.01.6

1.4

1.2

1.0

0.8

8.0

7.0

6.0

5.0

Figure 3. Variation diagram for major oxides versus solidification index (SI = MgO /MgO + FeO +Na2O + K2O) for basalts from Holes 553A, 554A, 555 and for dolerites from Hole 555.

the argillized volcaniclastics. Although published datafor Li in seafloor basalts are very sparse, most Rb val-ues in Hole 553A basalts are of similar range to these,for example, from the Franco-American Mid Ocean Un-dersea Study (FAMOUS) area (Roex et al., 1981), ornormal mid-ocean ridge basalts (MORB) (22-25°N) (Bry-an et al., 1981), or from International Phase of OceanDrilling (IPOD) Leg 49 (Wood et al., 1979).

Group IB: Cu, AgThere is no systematic variation of these with depth,

position in flows, lithology, or mineralogy. The Cu val-ues are considerably higher than those from the FAMOUS

area (Roex et al., 1981) and from "normal MORB"(22-25°N) (Bryan et al., 1981). No data for Ag are avail-able for comparison, although the levels are extremelylow.

Group IIA: Be, SrThe Be contents are all extremely low, and no signifi-

cance is attached to their apparent variation. Sr valuesare very constant throughout the basalts, but a littlelower in some of the volcaniclastic rocks. Because pla-gioclase (the main host of Sr) remains little altered ineven the most heavily smectitized basalt, the contents ofSr do not vary with alteration. There is no apparent var-

756

PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

2.5

Hole 553A basalts

Hole 554A basalts

Hole 555 basalts

Hole 555 dolerites

5.5 6.0 7.0MgO (wt.%)

8.0

Figure 4. Alkali:MgO diagram for basalts from Holes 553A, 554A,and 555 and for dolerites from Hole 555.

Hole 553A basalts

Hole 554A basalts

_ Hole 555 basalts

- Hole 555 dolerites

1.6

1.5

1.4

i 1.3

1.2

1.1

6.0 7.0MgO (wt.%)

'8.0 9.0

Figure 5. TiO2:MgO diagram for basalts from Holes 553A, 554A, and555 and for dolerites from Hole 555.

iation with position in flow. The basalt Sr values areslightly lower than for normal MORB (22-25 °N) andfor basalts from the FAMOUS area but fall in the rangeof Mid-Atlantic Ridge (MAR) basalts from the CentralNorth Atlantic (White and Schilling, 1978).

Group IIB: ZnThis shows no systematic variation throughout the

samples. The range is similar to that for normal MORBand seafloor basalts elsewhere (e.g., IPOD Leg 49).

Group III A: B, GaBoron, being an oxyphile, is commonly enriched in

volcanic emanations. The highest values here occur inthe volcaniclastic rocks (53.3-60.0 ppm) in the upper se-quence. But other relatively high values occur sporadi-cally in the basalts, some (but not all) relating to flowtops. Gallium, on weathering, tends to concentrate inhydrolyzates. However, the values are relatively constantirrespective of position in flows, although perhaps slight-ly lower in the volcaniclastics.

Group IIIB: Y, La

Values of Y vary relatively little throughout the se-quence. The highest value (100.2 ppm) relates to an ag-glomeratic basalt; other peaks relate to vitroclastic tuffsor basalts, irrespective of lithology or flow position. Themean Y content (31.32, s.d. 11.65) lies in the range forIPOD Leg 49 basalts. La values are sporadic (mean 3.41,s.d. 4.6) without systematic variation with Y. In view ofthe low La results no inferences can be drawn; the meanlies in the range for normal MORB (22-25 °N); the ele-ment is discussed below in the context of the rare earthsuite. In the discriminant plot of Y (× 3) versus Ti (÷ 100)versus Zr (Fig. 10), the Hole 553A basalts plot mainly inthe ocean-floor basalt and calc-alkali basalt fields, withscatter to the within-plate basalt field.

Group IVA: Sn, PbThe Sn values vary little at very low levels throughout

the sequence (mean 4.68, s.d. 1.78). The Pb values arealso very low (mean 2.73, s.d. 4.48); the highest Sn con-tents appear to relate to some higher volcaniclastic rocksas well as to some tops of flows, but there is no system-atic relationship. There is a sparsity of data on Sn andPb in seafloor basalts for comparison.

Group IVB: ZrValues vary relatively little throughout the sequence

(mean 112, s.d. 29.7), and there is no relationship to li-thology, position in flow, or alteration. The values fol-low those for normal MORB (22-25°N), and for IPODLeg 49 basalts, but are mainly twice those for FAMOUSbasalts. There is a positive correlation between Zr and Y(Fig. 11), but none between Zr and Nb, which are al-leged to covary in cogenetic suites (Erlank and Kable,1976).

Group VA; BiThe values reported are all very low and there is no

relationship between the sporadic variation and litholo-gy or other factors.

Group VB: V, NbV and Nb show positive correlation (Fig. 12) as ex-

pected geochemically. There is no correlation between Vand Ti, although V is commonly enriched in Ti-Fe ox-ides during initial crystallization of a magma. Group Vis probably mainly held in the secondary hydrolyzates,although there is no apparent correlation between de-gree of basalt smectitization and V content. The mean(316, s.d. 65.6) falls in the range recorded for IPOD Leg49 basalts and normal MORB (22-25 °N). The positivelinear correlation with Nb suggests that it too is mainlyheld in the secondary clay minerals; in this respect thesalts of Nb (and Ta) are readily hydrolyzed during weath-ering (Rankama and Sahama, 1950).

Group VIB: Cr, MoThe Mo values are very low and variable (mean 1.4,

s.d. 1.9). Cr (mean for basalts 83.95, s.d. 56.23) is rela-tively enriched in the lowest samples (attaining 257 ppm)

757

R. K. HARRISON ET AL.

FeO* (includes Fe2O3 recalculated as FeO)

• \ Field of Reykjanes Ridge (MAR)•• J basalts, Leg 49 (Wood et al., 1979)

\ Fe-enrichment trend ofx "tholeiite" type

• Hole 553A basalts

o Hole 553A tuff

• Hole 554A basalts

Λ Hole 555 basalts

T Hole 555 dolerites

Hole 555 tuff

Na2O + K2O 50 MgO

Figure 6. FMA diagram for basalts from Holes 553A, 554A, and 555 and for dolerites from Hole 555.

100 90

i

8°i

• A

T

70

•• •

i ...

Plagioclase (%An)

60

T Ai

50

•A Ai

40

i

30 20Microphenocrysts

Groundmass

Hed

Pyroxene

Hole 552 basalts +

Hole 553A basalts

Hole 554A basalts •

Hole 555 basalts A

Hole 555 dolerites T

Tie lines to /co-existing phases /

Fs

90 80 70 60 50 40Olivine (%Fo)

30 20 10

Figure 7. Composition of primary silicates in basalts from Holes 552, 553A, 554A, and 555 and in doler-ites from Hole 555.

758

PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

1000

700

500

300

100

50

10

IAT

i I l l 1 I I I I5

•

o•4

10

Hole 553A basalts

Hole 553A tuffs

Hole 554A basalts

Blosseville Coast Basalts

20Y (ppm)

T

•

Δ

30 50

Hole 555 dolerites

Hole 555 basalts

Hole 555 tuffs

E. Greenland (Brooks et al., 1976)

MORB Mid-ocean ridge basaltsWPB Within plate basaltIAT Island arc tholeiite

Figure 8. Discriminant diagram of Cr versus Y for basalts from Holes

553A, 554A, and 555; dolerites from Hole 555; and for East Green-

land basalts.

Table 14. CIPW norms of basalts from Hole 553A, Leg 81.

100

50

20

Ea io

5

3

2

-

-

_

_

/

1

-

r~ ~I

i

11

f

Θ \ Hole 553A basalts

of• | Hole 554A basalts

A Hole 555 basalts

T Hole 555 dolerite

× ' ! \

•~-ty

Composite values for 9 chondrites (Haskin et al., 1968)

0.3301

0.85

I10.181

0.60 I 0.069i I I

0.047\ 0.070l |

. -^©~Θ

g

0.200| 0.034

ILa Ce Nd Sm Eu Tb Ho Yb Lu

Figure 9. Chondrite-normalized REE patterns for basalts from Holes

553A, 554A, and 555 and for a dolerite from Hole 555.

of slightly or partly saponitized feldspar-phyric basalts.The lower values occur in the upper volcaniclastic rocks(down to 20.3 ppm). Cr generally correlates positivelywith Ni; both elements tend to concentrate in hydroly-zates during weathering, but there is no apparent rela-tionship in the present samples between Cr (and Ni) anddegree of argillization of the basalts. Apart from the ba-salt units noted, the Cr contents are considerably lowerthan those recorded for IPOD Leg 49 basalts, or fromnormal MORB (22-25 °N), but approach those of pla-gioclase-pyroxene basalts from the FAMOUS area.

Group VIII: Co, Ni

Although geochemically related, these elements be-have differently in petrogenesis and alteration processes(Rankama and Sahama, 1950). However, the mean val-ues are the same (117) in the basalts, although differ inthe vitroclastic rocks (Co 85 ppm, s.d. 26.32; Ni 105 ppm,

Normativemineral

Q

°Oab Fanjdi] p

hyj VmtilapccAgpaicity coefficientDifferentiation index

38-2,56-59

6.970.93

20.5126.33

- 16.7216.229.772.470.230.290.30

54.75

40-1,79-82

13.410.59

24.2624.9013.2410.869.612.760.320.000.35

63.16

45-5,17-19

5.060.36

18.3227.8822.1815.608.002.270.300.000.26

51.62

46-1,121-125

3.270.30

17.0627.8022.9320.86

5.062.160.400.140.25

48.43

46-3,56-59

3.270.35

17.7127.7323.3019.915.242.200.250.000.26

49.07

46-4,53-56

3.970.36

17.3327.8323.6018.45

5.982.170.280.000.25

49.49

Core-Section (interval in cm)

47-3,73-76

5.810.48

20.0225.5919.5316.109.192.920.280.050.30

51.90

48-2,60-65

4.750.36

18.8226.8420.0218.537.492.880.280.000.27

50.76

51-1,54-57

4.800.36

17.9028.3921.0115.929.361.920.190.120.25

51.45

51-2,77-80

3.860.36

17.6128.5521.6819.366.311.940.160.140.25

50.38

53-1,77-80

4.920.36

18.5328.3119.0317.388.502.050.190.690.26

52.12

54-4,81-85

4.730.36

19.9125.6619.5019.617.012.590.280.320.30

50.66

55-5,92-95

4.370.30

17.8021.9121.1519.386.152.210.190.460.26

50.43

55-6,44-47

4.240.36

18.3927.7221.3419.215.902.240.210.370.26

50.70

56-1,3-6

3.550.36

18.6528.3917.2121.66

5.422.290.192.230.26

50.96

59-2,115-118

4.490.24

18.6733.5016.0917.217.502.100.170.000.23

56.90

759

R. K. HARRISON ET AL.

Table 15. CIPW norms of basalts and dolerites from Holes 554A and 555A, Leg 81.

Normativemineral

Q

ab Fanjdi")hyjmtilapccAgpaicity coefficientDifferentiation index

554A-7-1,63-66

6.722.02

18.1626.6123.4911.359.232.060.190.140.29

53.51

554A-7-3,10-12

4.681.13

19.3727.5624.5614.825.612.020.160.050.28

52.74

554A-7-4,25-28

4.471.43

19.0727.0924.0115.925.622.030.230.090.29

52.06

555A-69-3,113-116

3.330.66

22.2226.5022.9714.545.603.230.330.600.31

52.70

Sample (interval in cm)

555A-9O-1,125-128

4.720.42

20.8925.4923.4813.578.612.600.190.000.31

51.52

555A-95-1,97-100

0.820.54

17.9528.5527.4418.284.142.070.190.000.26

47.86

555A-96-2,95-98

2.010.54

18.0628.2827.2516.115.322.210.190.000.26

48.89

555A-97-3,36-39

3.140.30

21.2726.1927.5512.416.072.720.190.140.30

50.90

555A-97-6,80-83

1.200.30

18.2029.3626.7816.905.031.940.160.110.25

49.05

555A-96-5,108-110

0.530.36

18.5828.7627.1317.654.772.050.140.000.26

48.24

Ti / 100

Field A Low-potassium tholeiites(also in field B)Ocean-floor basaltsCalc-alkali basalts(also in field B)

Field D: Within-plate basalts(after Pearce and Cann, 1973)

Field B:

Field C:

Hole 553A basalts and tuffs

Hole 554A basalts

Hole 555 basalts, dolerites,and tuffs

Blosseville Coast basalts,E. Greenland (Brooks et al., 1976)

Zr Y x 3

Figure 10. Zr:Ti/100:Y × 3 diagram for basalts and tuffs from Hole 553A, basalts from Hole 554A, andof basalts, dolerites, and tuffs from Hole 555.

s.d. 39.55). Similar enrichment and depletion with depthare shown between Co and Ni, although the lowest ba-salts have low Co contents. The Co values are approxi-mately twice those of other seafloor basalts—of IPODLeg 49, of the FAMOUS area, and of normal MORB(22-25°N). The Ni results fall in the general ranges ofthose seafloor basalts.

Interflow Variation: Hole 554A

Six samples only were available for analysis and sincethese are lithologically closely similar basalts, little sig-

nificance can be deduced from the trace-element data(Table 17). The mean values for Li and Rb are morethan twice those of Hole 553A basalts (Tables 16, 17).In this respect, K2O contents are also much higher thanin the latter. Boron is considerably higher in the Hole554A basalts (approaching the vitroclastic rocks of theother sites), suggesting much interaction with seawateror other supply of B. Other trace elements (except Pb)are similar to those of Hole 553A basalts; thus the com-parisons with published data on seafloor basalts notedabove relate also to Site 554 basalts.

760

PETROLOGY, MINERALOGY, AND CHEMISTRY OF BASALT

200 -

150

100

•• A

• ••

I I I I

10 50Y (ppm)

100

Figure 11. Zr:Y diagram for basalts from Hole 553A.

40 -

30

20

10

. :t

•v •

150 200 300V (ppm)

400 500

Figure 12. Nb:V diagram for basalts from Hole 553A.

Interflow Variations: Hole 555 and Comparisons

Li, RbThere is a general correspondence between high-Li

and volcaniclastic rocks and with some highly altered(saponitized) basalts; this enrichment is attributed to in-corporation in the main alteration products—saponiteand celadonite. Low Li values relate especially to the ol-ivine-dolerites of Unit Vc, which, significantly, are littlealtered. The Rb data are more erratic, although the high-

est value (26.6 ppm) is for a saponitized lapilli tuff,while the olivine-dolerites all show low values (less than2.9 ppm Rb). The mean Li content of Hole 553A ba-salts (5.32) is, however, a little lower than that (6.0) forthe fresh olivine-dolerites of Hole 555. Both alkali met-als are relatively enriched in the volcaniclastic rocks ofthe two holes. Basalts of Hole 554A are more enrichedin Li and Rb than the basalts of the other two holes,although the former are, on average, no more highlysaponitized.

Cu, AgThere is little significant variation in Cu, although

the lowest values occur in some of the volcaniclasticrocks. The mean Cu content for the basalts (232.6) is alittle higher than that for Hole 553A basalts and dis-tinctly higher than for Site 554 basalts. The Cu contentsof Hole 555 basalts and olivine-dolerites are similar; sa-ponitization appears to have little general effect on Cucontents. The volcaniclastics of Holes 553A and 555have closely similar mean Cu contents, which are stillappreciably higher than for Hole 554A basalts. Ag ismainly below the limit of detection in Hole 555.

Be, SrThe Be contents are all low and there is no significant

variation with lithology or other factor. The mean Srcontent (103.3 ppm) for the basalts is a little higher thanfor Holes 553A and 554A basalts. The Sr values for thevolcaniclastics are lower than for the basalts but are sim-ilar to those of Hole 553A; those for the olivine-doler-ites are closely similar to the volcaniclastics and altera-tion has not appreciably affected the Sr contents.

ZnZn contents range widely in the samples; there is no

discernible variation with lithology, petrography, or de-gree of alteration. The means are close to those of theother two sites.

B, GaThe highest B values occur in the volcaniclastic rocks

with one exception. Some very saponitized basalts arealso enriched in B. The mean B content is close to thatof the volcaniclastic rocks of Hole 553A. The mean Bcontent of the Hole 555 basalts is also close to that ofthe basalts of Hole 553A, but both are considerablylower than that of Hole 554A basalts. The olivine-dol-erites of Hole 555 have higher B contents than the ba-salts. Ga shows little variation irrespective of lithologyat Site 555 (in common with the analyzed rocks fromHoles 553A and 554A).

Sn, PbThe Sn contents are very low and within the limits of

error, apparently erratic without any relation to petrog-raphy, degree of alteration, or position within flows.The values of Sn are of similar order to those of theother sites. The Pb values, however, show a marked en-richment in the olivine-dolerite (Unit Vc), whereas the

761

Table 16. Trace elements of basalts and pyroclastic rocks from Hole

Core-Section(interval in cm)

38-1, 62-6538-2, 56-69

39-1, 2-440-1, 9-1240-1, 79-82

40-4, 80-83

42-2, 42-4643-1, 20-2343-1, 62-6643-2, 111-11444-2, 48-5145-1, 44-47

45-1, 87-9145-2, 114-11845-3, 79-8345-4, 16-2045-5, 17-19

46-1, 121-125

46-3, 56-59

46-4, 53-56

46-4, 122-12446-6, 33-3647-3, 73-76

47-4, 181-18447-4, 128-13048-1, 13-1648-2, 60-65

48-3, 97-10148-6, 53-58

Brief lithology

Sap. basalt, aphyricPartly sap. basalt, aphyric

Partly sap. basalt, f-mphyricPartly sap. basalt, aphyricSlightly sap. basalt (?olivine-)

f-cpx-mphyricPartly sap. basalt, f-cpx-

mphyricPartly sap. basalt, f-mphyricSap. basalt, f-mphyricBasaltic tuff (?)Basalt, f-mphyricPartly sap. basalt f-mphyricVitroclastic tuff

Vitroclastic tuffSap. ?tuffSap. ?tuffBasaltic breccia/tuffBasalt, (cpx-rich), f-mphyric

Basalt, (cpx-rich), f-mphyric

Basalt, (cpx-rich), f-mphyric

Basalt, mcryst., aphyric

Sap. basalt, (?olivine-),Partly sap. basalt f-mphyricBasalt f-mphyric

Sap. basalt, aphyricSap. basalt, f-mphyricPartly sap. basalt f-mphyricBasalt f-mphyric

Slightly sap. basalt, f-mphyricSlightly sap. basalt, f-mphyric

Flowposition

(B)(M)

(T)(T)(M)

(B)

(?)(T)

(?)(?)

(?M)

(?)

(?)(?)(?)

(B)

(T)(?M)(?M)

(B)(T)(M)

(?M)

(?M)(?M)

IA

Li

6.76.26.45.33.38.58.27.2

5.24.16.56.47.2

16.916.618.16.95.03.54.95.25.05.85.35.45.86.26.24.05.85.25.84.33.86.06.16.55.0

Rb

0.03.23.16.4

13.40.70.74.7

0.015.814.50.50.3

17.616.612.216.33.60.00.10.20.00.00.00.00.80.71.70.50.20.32.0

10.30.00.00.10.10.0

553A (ppm).

IB

Cu

260.899.0

109.6154.5138.4179.4173.7132.4

193.8125.8282.1181.567.296.695.4

289.2221.8184.0164.8124.8112.4284.6315.2140.8147.9164.1163.5307.5

78.385.686.7

232.797.494.0

242.8211.2151.3178.7

Ag

0.01.50.40.00.00.20.20.0

0.00.00.10.00.00.00.20.00.00.00.00.00.60.00.00.00.00.20.60.00.00.60.10.10.00.00.00.00.20.6

Be

0.01.30.70.20.10.80.70.2

0.00.50.60.10.41.00.80.50.00.10.40.50.90.40.40.40.40.70.80.30.20.90.70.40.70.30.50.50.50.2

IIA

Sr

108.889.390.691.393.385.083.681.1

87.286.473.172.481.372.873.478.273.388.468.078.682.479.876.177.877.979.781.893.580.186.483.990.371.284.983.380.479.482.0

IIB

Zn

139.259.060.2

107.550.2

102.589.1

109.1

84.361.4

113.055.385.061.665.773.359.092.860.584.862.291.5

103.976.968.778.382.358.175.489.965.890.054.284.6

105.881.372.072.1

IIIA

B

0.00.07.80.03.12.10.01.8

0.817.00.08.32.2

60.059.953.3

1.10.5

22.310.16.09.7

14.516.914.82.52.12.92.30.0

13.80.05.20.0

11.711.36.91.3

Ga

17.516.116.213.914.316.216.914.4

15.013.210.79.6

14.810.511.09.99.9

13.210.514.714.414.114.414.214.113.714.712.914.015.115.215.59.3

14.016.714.913.814.7

IIIB

Y

23.525.231.121.433.640.042.640.5

31.630.812.229.946.828.133.259.418.425.821.424.223.828.530.231.936.523.525.343.349.637.340.530.79.5

34.948.333.046.538.5

La

0.014.411.70.00.06.53.00.0

0.00.00.00.00.05.78.32.90.00.00.00.06.70.00.40.82.27.0

11.80.00.0

12.26.87'. 90.50.04.72.7

10.20.0

IVA

Sn

4.30.02.44.45.14.64.66.7

3.26.95.07.95.3

11.611.07.07.15.49.65.65.36.27.05.86.86.24.07.45.33.66.44.25.95.36.26.24.64.9

Pb

0.00.00.00.00.04.20.10.0

0.00.00.00.00.00.00.00.00.00.00.04.82.04.3

19.14.5

19.68.10.00.00.03.26.60.40.00.0

10.55.30.00.0

IVB

Zr

13090

110100140170180180

1101401201301701101701309090708090

1001001101309080

11016013013013090

110180130170130

VA

Bi

0.08.04.00.00.14.52.90.0

0.85.41.53.72.82.84.51.60.00.51.92.35.91.33.81.84.84.33.71.11.97.94.87.13.10.22.92.86.70.9

VB

V

379.0483.6426.7299.7266.6370.0350.5239.5

240.4282.7309.0253.5359.1247.0260.0164.6194.5295.6163.1318.8379.1316.4275.2340.0273.3255.7369.2342.7295.4423.1367.9385.8298.7305.4292.4346.2390.5299.9

Nb

13.640.025.814.617.430.831.620.0

16.916.717.016.026.09.0

14.89.3

10.016.613.326.830.130.225.940.232.826.236.717.817.436.831.021.813.515.421.428.437.718.7

VIB

CT

100.595.493.586.078.326.025.421.0

117.839.333.737.453.123.523.020.335.759.037.549.851.449.049.349.644.946.452.249.246.846.448.548.660.993.984.786.9

112.0102.1

Mo

0.00.03.70.00.01.01.30.0

1.20.00.01.80.96.87.31.40.00.02.93.80.94.12.87.93.81.41.91.40.21.45.90.00.00.04.44.33.20.0

VIII

Co

114.0140.0152.2107.070.7

117.2104.381.9

77.085.686.9

126.791.362.651.972.4

101.283.1

130.8106.9526.6127.9111.0116.7103.194.894.698.292.8

105.2129.489.8

106.4106.7108.6109.8100.286.6

Ni

112.893.0

100.9116.979.073.169.781.2

113.176.677.4

114.197.071.862.186.9

131.2120.9158.1121.0119.8128.7118.1127.9112.0104.9107.4111.098.486.7

111.397.2

120.1143.4121.3134.4124.2112.0

R. K

. HA

RR

ISON

ET

1

r

48-6, 74-7648-6, 85-8748-6, 122-12550-1, 67-7150-2, 20-2750-3, 32-3550-3, 71-7551-1, 54-57

51-2, 77-80

52-3, 8-1152-3, 140-14353-1, 77-80

53-2, 85-8853-4, 31-3453-4, 76-7954-1, 64-6754-1, 89-9254-5, 81-85

54-5, 83-8655-1, 36-3855-3, 71-7455-3, 113-11655-5, 92-95

55-6, 44-47

56-1, 3-6

56-2, 81-84

56-3, 91-94

57-2, 35-3758-1, 89-9158-2, 117-12059-1, 36-38

59-2, 115-118

59-4, 93-95

Partly sap. basalt, f-mphyπcBasaltic tuffSlightly sap. basalt, f-mphyricSap. basalt, aphyricPartly sap. basalt, aphyricSlightly sap. basalt, aphyricSap. basalt, aphyricBasalt, f-phyricBasalt, f-phyric

Basalt, f-phyric

Basaltic sap. ?flow-brecciaSap. basalt, mcryst.Slightly sap. basalt, f-mphyric

Sap. basalt, aphyricSap. basalt, f-mphyricPartly sap. basalt, f-mphyricSap. basalt, f-mphyricSap. basalt, f-mphyricSlightly sap. basalt, aphyric

Sap. basalt, f-mphyricSap. basalt (?agglomeratic)Sap. basalt (?agglomeratic)Partly sap. basalt, f-phyricPartly sap. basalt, f-mphyric

Slightly sap. basalt, f-phyric

Slightly sap. basalt, f-phyric

Slightly sap. basalt, sparselymphyric

Slightly sap. basalt, sparselymphyric

Partly sap. basalt, f, cpx-phyricPartly sap. basalt, f, cpx-phyricPartly sap. basalt, f, cpx-phyricPartly sap. basalt, f., cpx-

phyricPartly sap. basalt, f-mphyric

Partly sap. basalt, f-phyric

(?M)(?M)(?M)(T)(M)(B)(T)(M)

(M)

(T)(T)

(B)

(T)(M)(M)(M)

(M)(T)

(B)(T)(M)

(?)

(B)

(M)

(B)

(M)(M)(T)

(T)

(?M)

5.44.94.83.04.55.03.66.13.03.45.05.22.21.55.65.04.04.46.66.94.24.75.37.53.33.44.85.75.15.65.76.76.25.9

5.3

5.74.14.14.5

6.96.99.3

0.45.30.00.10.00.0

24.50.01.21.30.00.05.87.30.00.00.03.00.37.40.00.00.00.27.74.70.00.00.00.30.20.40.20.0

0.0

0.02.00.00.0

0.20.10.0

534.7130.0318.721.8

115.5101.5497.0270.4324.6340.4199.9213.5138.0145.5236.8224.5468.2440.6200.2243.2270.1158.8172.7140.4198.9162.783.2

345.0355.6127.4113.3240.4250.6141.4

676.5

247.158.0

198.6419.5

167.8157.395.4

0.00.00.00.50.00.00.00.00.00.30.00.00.00.00.00.00.00.00.00.00.01.00.90.20.40.40.00.81.10.00.10.90.00.1

0.0

0.00.00.00.0

0.20.10.0

0.40.00.10.10.20.10.20.20.30.60.40.30.40.30.60.30.10.00.40.30.20.50.70.40.70.90.10.90.80.40.80.60.40.6

0.1

0.10.10.10.4

0.20.40.3

92.294.481.480.175.278.277.274.177.279.776.078.082.674.088.083.590.586.886.288.885.477.879.786.291.770.790.475.984.485.1

5.889.484.383.7

83.6

85.382.584.382.5

89.688.169.9

98.5160.280.065.678.153.459.679.3

105.391.594.680.362.469.074.679.495.886.873.777.3

132.383.280.983.747.975.995.885.449.793.768.588.082.961.8

87.5

94.958.986.972.2

71.469.352.0

0.00.02.50.00.20.0

14.23.37.5

11.00.05.18.2

11.12.82.10.00.52.00.00.03.41.10.00.30.03.04.00.05.64.20.07.50.0

0.0

0.02.20.00.0

6.310.56.5

15.415.014.112.19.9

11.48.4

13.213.313.614.013.59.58.8

15.713.412.913.415.514.716.314.115.816.612.510.315.214.014.016.314.616.815.313.5

13.1

13.913.013.113.1

14.714.511.2

26.844.936.334.724.025.26.4

24.923.128.727.226.319.741.130.833.125.026.440.628.626.528.735.529.8

100.213.728.527.629.531.032.127.437.022.8

27.1

28.526.746.419.1

27.732.221.0

0.00.00.00.00.00.00.00.00.08.00.00.70.00.08.20.90.00.04.36.33.10.0

11.18.8

17.29.40.0

10.06.02.6

14.311.20.06.7

0.0

0.00.00.00.0

1.53.45.9

4.53.05.76.55.31.87.35.45.54.74.46.07.26.80.13.03.14.33.56.23.55.53.25.24.96.57.32.70.05.32.82.15.52.6

2.0

3.42.63.53.8

3.42.54.6

0.00.00.00.50.00.00.00.06.84.56.09.90.00.00.00.00.00.00.05.10.39.54.90.00.00.04.46.80.09.03.6

12.613.90.0

0.0

0.00.00.00.0

6.32.70.0

100110120807080808080

1009090

1001401201107090

17016011090

1101009070

1209090

140120110130100

100

90909070

13013070

1.70.02.36.91.10.90.01.41.44.52.04.00.00.20.50.70.00.12.88.00.15.98.17.35.44.91.48.74.71.94.16.53.20.5

0.0

0.00.00.00.9

3.22.21.9

377.0250.6309.4252.3278.8249.3212.5263.7264.2252.4340.7364.7189.0178.7380.6272.8313.0211.3381.2268.4322.4347.7451.7342.1322.0252.5266.6392.0421.4276.5404.0454.3252.4404.4

276.0

356.9261.2270.2305.7

293.9226.3284.5

21.33.4

23.118.613.810.19.5

16.917.717.120.522.5

9.911.823.612.514.65.9

23.810.913.517.233.519.023.319.315.733.437.913.525.029.117.524.2

6.3

16.215.517.117.6

16.511.324.3

110.083.0

100.2104.5110.1110.163.491.684.295.693.797.954.143.390.181.679.823.747.522.939.325.726.228.199.667.462.775.782.075.583.681.870.090.5

79.6

152.7257.7241.4245.1

235.2246.6252.3

0.00.00.00.00.00.01.40.02.44.60.01.11.86.21.42.20.00.01.50.00.01.91.50.00.00.00.03.61.80.01.30.02.40.0

0.0

0.00.50.00.0

4.36.71.4

89.491.090.385.5

112.989.696.997.3

100.992.3

111.4113.2124.0145.9124.2111.4102.7108.5102.588.5

109.9109.691.482.589.374.2

106.093.892.6

105.6100.1104.692.2

111.4

98.5

97.292.296.198.0

78.170.797.0

126.6145.0115.8125.4152.9147.6128.2139.0136.3122.1125.2131.8154.6172.0123.0120.0137.894.693.977.098.388.075.085.787.392.