18. geochemistry and petrography of cretaceous sills and lava

Larson, R. L., Lancelot, Y., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 129

18. GEOCHEMISTRY AND PETROGRAPHY OF CRETACEOUS SILLS AND LAVA FLOWS,SITES 800 AND 8021

P. A. Floyd,2 J. A. Winchester,2 and Paterno R. Castillo3

ABSTRACT

On the basis of their respective eruptive environments and chemical characteristics, alkalic dolerite sills from the northernPigafetta Basin (Site 800) and tholeiitic pillow lavas from the Mariana Basin (Site 802) sampled during Ocean Drilling ProgramLeg 129 are considered to represent examples of the widespread mid-Cretaceous volcanic event in the western Pacific. Bothgroups of basic rocks feature mild, low-grade, anoxic smectite-celadonite-carbonate-pyrite alteration; late-stage oxidation is verylimited in extent, with the exception of the uppermost sill unit at Site 800. The aphyric and nonvesicular Site 800 alkalic doleritesills are all well-evolved mineralogically and chemically, being mainly of hawaiite composition, and are similar to ocean islandbasalts. They are characterized by high contents of incompatible elements (for example, 300-400 ppm Zr), well-fractionated rareearth element patterns ([La/Yb]N 18-21) and HIMU isotopic characters. They probably represent deep-sea, lateral, intrusiveoff-shoots from nearby seamounts of similar age. The olivine-plagioclase ± clinopyroxene phyric tholeiitic pillow lavas and thinflows of Site 802 are nonvesicular and quench-textured throughout. Relative to normal-type mid-ocean ridge basalt, they areenriched in large-ion-lithophile elements, exhibit flat (unfractionated) rare earth element patterns and have distinctive (lower)Zr/Nb, Zr/Ta, La/Ta, and Hf/Th ratios. Overall they are compositionally and isotopically similar to the mid-Cretaceous tholeiitesof the Nauru basin and the Ontong-Java and Manihiki plateaus. The Site 802 tholeiites differ from the thickened crustal segmentsof the oceanic plateaus, however, in apparently representing only a thin veneer over the local basement in an off-axis environment.

INTRODUCTION

One of the major problems encountered by previous attempts tosample Jurassic ocean crust in the western Pacific Ocean were theproducts of extensive mid-Cretaceous volcanism that filled manyocean basins with seamount-derived volcaniclastic debris and proveddifficult to penetrate completely (Lancelot, Larson, et al., 1990). Of thethree sites drilled during Ocean Drilling Program (ODP) Leg 129(Fig. 1), only Site 801 penetrated Jurassic ocean crust; although bothSites 800 and 802 drilled acoustic "basement," these proved to beCretaceous in age. The clear extrusive nature of the Cretaceous-agedpillow basalts at Site 802 was intriguing, especially as the magneticanomaly patterns and water depth indicated that the ocean crust in thispart of the western Pacific Ocean should be of Jurassic age (Lancelot,Larson, et al., 1990). Was this really Cretaceous ocean crust or anotherexpression of mid-Cretaceous volcanism which spread a thin veneerof extrusives over Jurassic crust lying immediately below? Currently,we prefer the latter tectonic situation. There is little doubt that themid-Cretaceous volcanic event was a major feature of the central-west-ern Pacific (Larson, 1991) and is represented by different volcanicforms, such as reef-capped seamounts and ocean islands (with theirattendent slumped volcaniclastic aprons), enormous oceanic plateaus,such as the Ontong-Java Plateau, and deep-sea sill-flow complexes, asin the Nauru Basin (for example, Winterer, 1973; Schlanger et al.,1981; Larson and Schlanger, 1981; Haggerty et al., 1982; Rea andVallier, 1983; Schlanger and Moberly, 1986; Floyd, 1989). Leg 129also demonstrated the existence of other deep-sea expressions of themid-Cretaceous event in the form of sills intruding Early Cretaceoussediments (Site 800) and pre-Aptian lava flows (Site 802). This paperdescribes the petrography and geochemistry of these Cretaceous vol-canics and briefly discusses their origin and tectonic setting. Isotopicand radiometric age data are discussed in companion papers by Castilloet al. (this volume) and Pringle (this volume), respectively.

' Larson, R.L., Lancelot, Y, et al., 1992. Proc. ODP, Sci. Results, 129: College Station,TX (Ocean Drilling Program).

2 Department of Geology, University of Keele, Staffordshire, ST5 5BG, UnitedKingdom

Scripps Institution of Oceanography, La Jolla, CA, 92093-0220, U.S.A.

LITHOSTRATIGRAPHY AND PETROGRAPHY

The recovery of igneous rocks from Sites 800 and 802 was limited,with only 7 m of dolerites (from 58 m penetration) from the formerand 16.6 m of basalts (from 5 1 m penetration) from the latter.Although a number of volcanic cooling units were identified at eachsite (Lancelot, Larson, et al., 1990), in both cases the rocks sampledshow little major lithological and chemical downhole variation, un-like the longer Jurassic crustal sequence at Site 801 (Floyd andCastillo, this volume). However, each location is distinctive in thatSite 800 acoustic "basement" is composed of alkalic dolerite sills,whereas Site 802 exhibits tholeiitic basalt lavas. Various petrographicfeatures displayed by the dolerites and basalts from these sites areshown in Figure 2.

Site 800 Alkalic Dolerite Sills

Mode of Emplacement and Age

Although actual contacts were not recovered, it is inferred that thethree cooling units recognized at this site probably represent intrusivesills rather than lava flows (Lancelot, Larson, et al., 1990). However,interpretation of the emplacement mode of thick, massive, relativelycoarse-grained cooling units only seen in incompletely cored sectionscan often be equivocal, such that intrusives can be confused with thickflows and visa versa. Nevertheless, in areas of rapid sedimentation,where magmas may only occasionally reach the seafloor as lavaflows, sill-like intrusives are relatively common. For example, in theGuaymas Basin (Gulf of California) numerous intrusive bodies wereemplaced into, and clearly hydrothermally altered, poorly consoli-dated claystones (Einsale et al., 1980). However, massive bodies ofsimilar thickness and textural variety sampled at the mouth of the Gulfof California were all interpreted as flows rather than sills on variouscriteria, including absence of intrusive relationships (Griffin et al.,1983). Because of the lack of observable contacts in Hole 800A, theintrusive nature of the three dolerite units was inferred from thefollowing features: (1) basal and interbedded cherty and radiolaritesediments were baked, bleached, and brecciated relative to litholog-ically similar, but unaltered, sediments higher in the sedimentarysequence; (2) the dolerites were medium textured throughout, with

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P. A. FLOYD, J. A. WINCHESTER, P. R. CASTILLO

55°N

130°E 150°E 170°E 170°W

Figure 1. Map of the western Pacific Ocean showing the position of Leg 129 Sites800, 801, and 802 (Lancelot, Larson, et al., 1990). Unshaded area representsnormal Pacific ocean crust with magnetic lineation contours, whereas shadedareas represent thickened crustal sections and younger crust to the west of thePacific subduction zones. Feature abbreviations are as follows: Caroline Islands(Cl), Ontong-Java Plateau (OJP), Marshall Islands (MI), Nauru Basin (NB),Mid-Pacific Mountains (MPM), Shatsky Rise (SR), Hawaiian Ridge (HR) andEmperor Seamounts (ES).

finer-grained chilled margins, holocrystalline and nonvesicular quenchtextures and glassy margins characteristic of rapid cooling by seawaterwere conspicuously absent towards the margins; (3) the variablepresence of late-stage secondary biotite and actinolite in all sills isinterpreted as the result of contact metamorphism caused by sillintrusion. This feature is particularly well developed in the top of Unit 3where both minerals replace earlier smectite-dominated alteration; and(4) magnetic results indicate that the least altered dolerites have a(primary) reversed polarity, whereas those portions affected by biotite-actinolite alteration exhibit a superimposed normal polarity whichprobably developed after initial emplacement.

The sills were emplaced into sediments of lowermost Cretaceousage (Berriasian) and on the basis of magnetic polarity correlationsmay have been intruded during pre- or early Aptian times (Lancelot,Larson, et al., 1990), whereas Ar-Ar dating (Pringle, this volume)indicate an intrusion age of 126 Ma. These data confirm the intrusivenature of the dolerites relative to the older enclosing sediments andtheir association with the mid-Cretaceous volcanic event.

Primary and Secondary Petrography

All three cooling units are represented by aphyric, nonvesicular,holocrystalline gray-green dolerites with medium-grained intersertalto interlocking textures. Background secondary alteration is domi-nated by smectite-carbonate-celadonite assemblages throughout. Di-rectly below the basal sediments, however, Unit 1 is represented bya fine-grained highly altered basalt with a granular texture (chilledmargin facies) and a reddish oxidized top that grades downwards intothe more normal coarser-grained greenish dolerites.

Pale pink, poorly pleochroic titanaugite is the main mafic phase andforms stubby to elongate prims variable replaced by greenish smectites

and carbonate, leaving large skeletal, optically-continuous relicts. Noolivine or its pseudomorphs appear to be present, although a few rareragged subhedral crystals of a pale brown primary amphibole arepresent in Unit 2. Albite-twinned plagioclase laths commonly exhibitnormal zoning (An55_60 to An3O_35), although cores are often partiallyreplaced by brownish smectite and opaque granular material (analciteor clays). Titanomagnetite is common throughout, forming smallopaque granules in the finer grained portions of the dolerites andlarger, variably corroded, subhedral grains and needles in the coarserinteriors. At the top of Unit 1 this phase is completely oxidized toeither red hematite or a limonitic stain that also impregnates adjacentsmectites. Acicular apatite prisms are a common accessory through-out all units and the only primary phase to remain unaltered. Apartfrom patchy carbonate replacement, small alteration domains in thematrix are represented by either (1) granular dark brown smectite withpale green fibrous smectite, or (2) pale green fibrous smectite oftenwith strong blue-green celadonite plates. In both cases, secondarybiotite and actinolite may be seen concentrated in these domains andgrow across them unaltered. This suggests that biotite and actinolitegrowth was later than the main smectite-dominated alteration andprobably represents a higher grade of metamorphism superimposedon the dolerites by the thermal effects of adjacent intrusions. Biotiteis more abundant than actinolite and may also nucleate on titanomag-netite grains. Rarely, biotite may be present as zoned euhedral plateswith slightly darker rims relative to the optically uniform cores. Insome highly altered domains, biotite plates may have ragged orsplayed ends parting along the cleavage and are then partially replacedby blue-green celadonite or green smectite. This may reflect a sub-sequent alteration during cooling after thermal metamorphism.

The apparent lack of olivine and the generally intermediate nature ofthe plagioclase (andesine-labradorite) indicate that the dolerites are rela-tively evolved and (as substantiated by their major element chemistry)have an overall hawaiite composition rather than that of an alkali basalt.

Site 802 Tholeiitic Lava Flows

Mode of Emplacement and Age

The basalts of Hole 802A are clearly extrusive submarine flowsbeing characterized by numerous thin, nonvesicular cooling units(about 1 m or less) with quench textures throughout and glassyspherulitic margins. The preservation of typical rapid cooling texturalzones and, in a number of cases, curved glassy selvages (Fig. 44 inLancelot, Larson, et al., 1990) indicates that the sequence here ispredominantly composed of a series of pillow lavas. Hole 802A,however, terminated in a coarser-grained, hypocrystalline unit whichis considered to be a thin (<5 m) flow and thicker than most of themeasurable units above. The top of the lava sequence is not exten-sively reddened and indicates that the time interval, before beingcovered by sediment, was relatively short so that alteration by oxy-genated seawater was restricted. Glassy fragments spilled off theuppermost pillow lavas during cooling by seawater are incorporatedinto the basal sediments (volcanogenic claystones) of late Aptian age,and provide a minimum age for the lavas. Ar-Ar dating of the basaltsgives an age of about 115 Ma (Pringle, this volume) that correspondsto the uppermost Aptian (time scale by Harland et al., 1982).

Primary and Secondary Petrography

One of the features of the lava flow sequence at Hole 802A is thatall cooling units are petrographically very similar, largely beingcomposed of sparsely olivine-plagioclase or olivine-plagioclase-cli-nopyroxene microphyric tholeiitic basalts. However, up to about 20%of collected core is broadly aphyric, although even here scatteredolivine and plagioclase phenocrysts may be encountered. Microphe-nocrysts are not uniformly distributed between or within coolingunits, with some showing a concentration in the marginal highlyquenched zones relative to the phenocryst-poor interiors. Larger

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CRETACEOUS SILLS AND LAVA FLOWS, SITES 800 AND 802

0.1 mm ß 0.1 mm 0.1 mm

0.1 mm 0.1 mm

0.5 mm H 0.1 mm 0.1 mm

0.1 mm 0.1 mm

Figure 2. Photomicrographs illustrating petrographic and textural features of Site 800 alkalic dolerite sills (numbers 1-6) and Site 802 tholeiitic flows (numbers7-12). (ppl = plane polarized light; cp = cross polars). A. Small brownish glass inclusions in titanaugite prism, Sample 129-800A-60R-1, A3-AA cm (ppl). B.Carlsbad twinned and zoned plagioclase lath with core replaced by dark granular smectite and analcite, Sample 129-800A-57R-2, 89-90 cm (cp). C. Acicularapatite crystals, Sample 129-800A-60R-1,71-74 cm (ppl). D. Large plate of poorly zoned actinolite with later fibrous fringe in brown smectite alteration domain,Sample 129-800A-58R-1, 46-47 cm (ppl). E. Primatic, fibrous actinolite growing unaltered across a brown smectite domain, Sample 129-800A-58R-1, 46-47cm (ppl). F. Group of biotite plates in smectite alteration domain, with frayed terminations partially replaced by blue-green celadonite, Sample 129-800A-60R-1,71-74 cm (ppl). G. Dark brown, partly coalesced spherulites nucleated on plagioclase microlites set in fresh glass (sideromelane), Sample 129-802A-58R-2,145-149 cm (ppl). H. Smectite pseudomorphed olivine microphenocrysts in spherulitic zone of lava, Sample 129-802A-58R-2, 145-149 cm (ppl). I. Double-fanvariolite growth on plagioclase microlite surrounded by opaque magnetite granules, Sample 129-802A-58R-4, 55-57 cm (ppl). J. Coarse sheaf and plumosevariolites of clinopyroxene typical of the interior of flows, Sample 129-802A-57R-2, 126-128 cm (ppl). K. Group of clinopyroxene and zoned plagioclasephenocrysts, sample 129-802A-57R-2, 126-128 cm (cp). L. Glomerophyric cluster of anhedral clinopyroxene grains and plagioclase laths in quenched matrix,Sample 129-802A-57R-2, 126-128 cm (cp).

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dimensioned plagioclase phenocrysts (about 1 mm in length) mayalso be observed and are termed "megaphenocrysts" to distinguishthem from the common smaller type.

Textural variation within units is largely the result of differentialquenching by seawater, together with the random distribution ofglomerophyric clumps and individual phenocrysts. Rapidly chilledselvages are glassy with variably coalesced spherulites that may oftennucleate on plagioclase microlites, whereas the matrix of flow inte-riors grade from sheaf and plumous variolites to coarser intersertaland granular textures that may still exhibit quench morphologies(Fig. 2). Microphenocrysts of olivine are always replaced by brownfibrous smectite and occur as individual crystals scattered throughoutboth glassy and quenched crystal matrices. Plagioclase is often freshand may be present as either broken and magmatically corroded zonedmegacrysts (with cores of around An80), or, more usually, as mi-crophenocrysts (commonly An65_70) in glomerophyric clumps withclinopyroxene. Clinopyroxene (augite) is rarely seen as individual"free" microphenocysts being invariably concentrated in glomero-crystic clumps. There are textural phenocryst associations that oftencharacterize the more crystallized variolitic zone just below the glassyselvage of most units. On the one hand, there may be scattered,subhedral plagioclase megaphenocrysts and relatively abundant oli-vine microphenocrysts, whereas on the other are finer-grainedglomerophyric clumps of quenched plagioclase laths and granularclinopyroxene. Both are set in a poorly birefringent matrix of brownplumous variolites and scattered magnetite granules. The two sizes ofphenocryst imply different cooling histories, with the plagioclasemegaphenocrysts and olivine developing in the magma chamber andthe glomerophyric clumps at a higher level possibly just prior to actualextrusion. Any remaining melt on contact with seawater will thenrapidly quench to produce a variolitic and/or glassy matrix. Thetextural diversity and association of glomerophyric clumps and vari-olites is common to all flows and is only replaced in the coarserinteriors of larger cooling units with intersertal textures composed ofgranular clinopyroxene and serrated plagioclase laths, such as thethick olivine-plagioclase microphyric flow at the base of Hole 802A.

In terms of secondary alteration, the Cretaceous flows are remark-ably fresh, being characterized by mainly smectite-carbonate-pyriteassemblages, both within the bulk rock and veins. This assemblage istypical of low-grade anoxic conditions. Color changes in variousshades of gray within the core represent alteration haloes usuallyassociated with areas of veining and more extensive matrix alterationby smectites. The preservation of fresh yellow sideromelane in somepillow selvages (for example, Sample 129-802A-58R-2,99-102 cm)is rare, as most glass is replaced by brownish smectite. Olivinemicrophenocrysts are always pseudomorphed by brown smectites orin some glassy selvages by green smectite. Carbonate replacement islater than smectite alteration and occurs in a random, patchy fashionthrough the matrix. Plagioclase is sometimes replaced by carbonate,whereas clinopyroxene always remains fresh.

GEOCHEMISTRY

Major oxide and trace element data for Hole 800A dolerite sills andHole 802A basalt lavas are shown in Tables 1 and 2, respectively.Analytical procedures and analysts are referenced in Floyd and Castillo(this volume). Geochemical data for both holes are plotted on the samediagrams to illustrate their main features and highlight the significantdifferences between the alkalic dolerites and tholeiitic basalts.

Apart from the main chemical differences between the alkalicdolerites and the tholeiitic basalts that are typical of their respectivemagma types, both groups feature very little chemical variationthroughout the sampled sequences. This uniformity is also mirroredin the similarity of petrographic composition within each hole andmay, in part, reflect the relatively short segment sampled in each case.However, small, relatively insignificant differences with depth (me-ters below sea floor, or mbsf) have been noted. The uppermost part

of Hole 800A (cooling Unit 1) is marginally more primitive (withslightly higher Cr and lower Zr abundances) than the rest of the sectionbelow (cooling Units 2 and 3), although there is no systematic variationwith depth (Fig. 3). Hole 802A, on the other hand, shows a chemicalbreak in Zr content at 536 mbsf (Fig. 3) that corresponds to theboundary between cooling Units 14 and 15. Both the lower and upperchemical groups show a poorly defined trend towards higher Zr valueswith height in the "basement" (the highest value of 101 ppm Zr beingat the top of cooling Unit 1). The significance of the Zr hiatus is notclear as other stable trace elements (incompatible and compatible) donot show this feature as strongly as Zr (Fig. 3) and the lavas above andbelow show broadly the same petrographic variation throughout.

The development of secondary minerals is a common feature of bothHoles, and in particular, influences the distribution of most large-ion-lithophile (LIL) elements (for example, Humphris and Thompson, 1978).Hole 800A alkalic dolerites are more hydrated (higher H2O+) andoxidized (higher Fe2O3/FeO ratio) than the mildly altered Hole 802Atholeiitic basalts (Fig. 4). Part of the high water content of the formergroup probably reflects the presence of extensive contact metamorphicbiotite. Both groups exhibit considerable non-magmatic variation in theirLIL-element contents and with ratios indicative of low-grade submarinealteration rather than the effect of magmatic processes; for example K/Rbratios range from 400-1300 (Hole 800A dolerites) and 160-950 (Hole802A basalts).

The Cretaceous alkalic dolerites (Hole 800A) and tholeiitic basalts(Hole 802A) exhibit the following primary chemical features (Figs.5 and 6):

1. As might be expected all the alkalic dolerites are markedlyenriched in incompatible elements (except Y) and exhibit differentincompatible element ratios to the tholeiitic basalts. For example,alkalic dolerites have Zr contents >300 ppm and Zr/Y ratios of 13-16,whereas the tholeiitic basalts have about 60-70 ppm Zr and very lowZr/Y ratios between 2.5-2.8.

2. Both dolerites and basalts are characterized by very narrowranges of incompatible elements such that both suites show littlechemical variation (Fig. 5) and feature relatively uniform incompat-ible element ratios. Compatible elements, such as Cr and Ni, show aslightly greater diversity (Fig. 5) and together with a moderate rangeof FeO*/MgO ratios for dolerites (0.8-2.0) and basalts (1.0-1.6),indicate that only very minor mafic fractionation (olivine and clino-pyroxene) has taken place throughout both groups. Compare, forexample, the range in elemental compositions exhibited by tholeiiticlavas from the Nauru Basin and Ontong-Java plateau with that of Hole802A tholeiites (Fig. 5).

3. Although chondrite-normalized rare earth element (REE) pat-terns (Fig. 6) are dissimilar for the alkalic dolerites and tholeiiticbasalts, the range of variation and degree of REE fractionation is againvery limited in each group. The dolerites show strong light REEenriched patterns, with (La/Yb)N from 17.5-20.0, whereas most ofthe basalts are characterized by relatively flat REE patterns ([La/Yb]N

0.9-1.2) with about 10-12 times the chondritic abundances. Some ofthe slightly more fractionated samples (with lowest Cr and Ni) exhibitminor light REE enrichment ([La/Yb]N 1.5-2.0) that is a function ofclinopyroxene fractionation. However, the general similarity andparallelism of the REE patterns in both dolerites and basalts reflectsthe precipitation of phases that do not fractionate the REE to anydegree and conforms with the observed presence of olivine andplagioclase as phenocrystic phases.

In general terms, the incompatible element-enriched features ofthe alkalic dolerites are characteristic of ocean island basalts (OIB).Although the low abundances of these elements and their distinctiveratios in the tholeiitic basalts are akin to some types of mid-oceanridge basalt (MORB) (Bougault et al., 1980; Saunders, 1984), theirflat REE patterns distinguishes them from typical N-type MORB.These distinctive chemical features find their counterpart in clearly

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different isotopic ratios, which implies the two groups were derivedfrom different mantle sources (Castillo et al., this volume). This issubstantiated by markedly different ratios of highly incompatibleelements (La, Nb, Ta, Th) that reflect the nature of the source ratherthan variable mantle melting (Bougault et al., 1979). For example, La/Thand La/Ta ratios are consistently higher in the basalts (about 15 and 18,respectively) than the dolerites (about 10 and 12.5, respectively).

ERUPTIVE ENVIRONMENT AND ORIGIN

In this section we consider the general geochemical features of thedolerites and basalts in an attempt to determine their tectonic settingrelative to other basic submarine volcanics, especially those consid-ered part of the widespread mid-Cretaceous volcanic event. In termsof age and postulated eruptive setting, both Hole 800A dolerites andHole 802A basalts are strong contenders for additional expressions ofmid-Cretaceous volcanism (Lancelot, Larson, et al., 1990).

Alkalic Dolerite Sills

Thick massive doleritic units, generally interpreted as sills, have beendrilled in a number of deep sea locations in both the Atlantic and Pacificoceans (for example, Leg 43, Tucholke, Vogt, et al., 1979; Leg 58, deVriesKlein, Kobayashi, et al, 1980; Leg 61, Larson, Schlanger, et al., 1981;Leg 64, Curray, Moore, et al., 1982), although with a few exceptions mosthave tholeiitic compositions. Sills forming part of the oceanic crust ofsmall basins, such as the Gulf of California (Saunders et al., 1982) andthe Daito and Shikoku basins in the western Pacific (Marsh et al., 1980)are mainly tholeiites, although in a few cases relatively abundant alkalicbasalt units may be present, such as in Hole 446A (Daito Basin). In thedeeper water, open ocean environment, such as the Nauru Basin, theupper portion of the basaltic sequence constitutes a series of sills, althoughagain they are tholeiitic in composition (Floyd, 1989). hi some casessubmarine sills appear to be closely associated with seamounts or oceanislands. For example, alkalic sills with high incompatible element con-tents (200-400 Zr ppm) were recovered from Leg 43 (Houghton, 1979),some of which were developed on the flanks of the New EnglandSeamounts. In the Central Pacific Basin various doleritic rocks (someconsidered to be sills) with an alkalic parentage and containing primarybrown amphibole and biotite, were probably derived from the nearbyLine Islands or adjacent seamounts (Bass et al., 1973). It is apparentthat the alkalic composition of some sills and their relatively closegeographic position to nearby seamounts and ocean islands suggeststhey may be structurally related and form minor intrusions on theirflanks. A Seamount model based on the dissected La Palma SeamountSeries (Canary Islands) suggests that small sill-like intrusions are afeature of early development and may well extend out beyond theSeamount base (Staudigel and Schmincke, 1984).

As there is little chemical data on submarine Cretaceous-agedalkalic sills, we have compared the Hole 800A sills with oceanicislands, seamounts, and submarine sills of different ages but of alkalicorigin. As seen in Figure 7, alkalic basalts from submarine sills showgood covariance for individual suites, with Ti/Zr ratios ranging be-tween 120 and 59. Note that the upper sequence Jurassic alkalicdolerites from Site 801 also plot within this range. Probably becauseof the opportunity to sample a wider range of material, oceanic islandalkalic basalts, on the other hand, show a far wider range of TiO2 andZr abundances, with a TiO2 peak that roughly separates the strictlybasaltic compositions from the more evolved basic differentiates. TheHole 800A dolerites also show a minor peaked pattern (due totitanomagnetite fractionation), but differ from the "main alkalic ar-ray" with very low Ti/Zr ratios (about 36). In comparison the appar-ently evolved compositions of Hole 800A dolerites are also supportedby the petrography (see above) which indicates that they are mainlyof hawaiite composition. The dolerites also differ from other OIB inexhibiting highly fractionated REE patterns (Fig. 6) with (La/Yb)N

ratios of between 18-21. As shown in Figure 8, these sills are more

fractionated than most OIB alkalic rocks and are only matched bysome of the Society Seamount lavas (Devey et al., 1990). Otherincompatible element ratios are also distinctive, such as high Zr/Y(about 14) and La/Th (about 10) relative to other OIB, as well as tothe Site 801 alkalic dolerites.

On the basis of the above data, the Site 800 alkalic dolerites havea chemical identity of their own, but are more akin to evolved alkalicOIB from islands and seamounts than alkalic basalts that appear toform part of the oceanic basement (for example Site 801). The particu-lar chemistry of the alkalic dolerite (especially the high (La/Yb)N andZr/Y ratios) may suggest that their parental composition was generatedby smaller degrees of partial melting relative to some of the other OIB,or that the source was generally more enriched in some incompatibleelements. Site 800 is surrounded by a number of seamounts includingHimu and Golden Dragon, both of which have isotopically distinctivecompositions with a high proportion of the HIMU mantle end memberand are almost isotopically identical to the Site 800 alkalic dolerites(Castillo et al, this volume). A HIMU component is also suggested bythe specific chemical composition of the dolerites, such as the lowBa/Nb ratio (c. 5-8). As demonstrated by Sun and McDonough (1989)low Ba/Nb ratios correlate with low 87Sr/86Sr ratios that are generallytypical of OIB with a HIMU component relative to other OIB withenriched mantle (EMI and EMU) components.

Both chemical and isotopic parameters suggest the dolerite sillsare tapping the same mantle source as some of the nearby seamounts.In view of the seamount-sill relationships suggested elsewhere, Site800 sills may also be volcanologically and genetically related, repre-senting deep intrusive offshoots from the Seamount flanks. Site 800is surrounded by seamounts between 50-120 km away (Fig. 2,Lancelot, Larson, et al., 1990) and well within the distance of possiblevolcanic activity associated with active ocean islands. For example,the widespread alkalic volcanism associated with the Hawaiian SouthArch (Lipman et al., 1989) occurs c. 200 km to the south of HawaiiIsland, and in a similar manner the Site 800 sills may represent leakageof flank melts from plumes feeding the adjacent seamounts.

Tholeiitic Basalt Lavas

The extrusive pillow lavas of Site 802 with an age of about 115 Maare probably a deep-sea expression of the mid-Cretaceous volcanicevent, especially as they are about 50 m.y. younger than that predictedfor the basement (mid-Jurassic) in this part of the Mariana Basin(Lancelot, Larson, et al., 1990). Apart from numerous seamountsgenerated in mainly Aptian times, large-scale volcanic provinces inthe western Pacific generated during the mid-Cretaceous includeNauru Basin and various oceanic plateaus, such as Ontong-Java,Manihiki, Hess Rise etc. (Winterer, 1973; Schlanger et al., 1981).These submarine provinces are generally considered to representepisodes of crustal thickening soon after or as a continuation of axialactivity (Saunders, 1986; Mahoney, 1987) generated above the in-flated heads of large mantle plumes during the early Cretaceous(Richards et al., 1989). The tholeiitic basalts of the oceanic plateaushave a broad MORB-like geochemistry, although they exhibit anumber of distinctive features that set them apart (Batiza et al., 1980;Floyd, 1989). In particular they are enriched in LIL elements, arecharacterized by generally flat (or less depleted) REE patterns andhave different incompatible element ratios, in particular lower Zr/Nb,Zr/Ta, La/Ta, and Hf/Th, relative to normal-type mid-ocean ridgebasalt (N-MORB).

Chemical tectonic environment discrimination diagrams, such asHf-Ta-Th (Wood, 1980), Nb-Y-Zr (Meschede, 1986), and Zr/Y-Zr(Pearce and Norry, 1979), all group the Site 802 basalts as N-MORB.However, in this case, these diagrams are unreliable as the basalts arecharacterized by (1) flat REE patterns (Fig. 6) and (2) normalizedmulti-element distributions exhibiting LIL element and Nb-Ta enrich-ment (Fig. 9) relative to N-MORB. Highly incompatible elementratios are also different (Fig. 10) to Jurassic N-MORB (Site 801) with

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Table 1. Major oxide and trace element data for Hole 800A alkalic dolerite sills.

Core, section:Interval (cm):Depth (mbsf):Cooling unit:

57R-117-24

498.17Al

Major oxides (wt%)SiO2

TiO2

A1,O3

Fe2O3

FeOMnOMgOCaONa2OK20P ALOI

Total

H2O+

co2

49.562.17

15.869.010.370.155.043.962.665.640.554.94

99.92

3.381.34

57R-140-45498.40

Al

45.721.95

14.776.841.060.124.01

10.273.633.690.558.04

100.64

2.215.10

Trace elements by XRF (ppm)BaCeCrCuGaLaNbNdNiPbRbSSrVYZnZr

47505

2036321485256

1223

93:2334010522

137348

25174

1403022314837834

8212846618825SO

330

Trace elements by INAA (ppm)CsHfScTaThU

———

—

Rare earth elements (ppm)LaCePrNdSmEuGdTbDyHoErYbLu

—

——

————

———

3.537.51

16.33.183.571.93

37.1079.80

—36.007.962.48—0.94

—1.440.197

57R-286-91499.36

Al

49.092.01

15.975.632.820.077.924.832.943.220.565.30

100.38

3.611.38

339109127652354506082

120

105449318523

144340

0.147.12

17.83.213.641.58

36.5078.30—

37.007.752.46—0.87

—1.290.177

58R-112-17

507.32Al

49.342.19

15.914.514.920.156.726.003.922.550.543.49

100.26

2.381.18

33781

134542231512742

131

2735661872777

356

0.157.70

20.33.264.201.25

43.0687.329.90

40.257.962.576.84—5.330.862.251.670.26

58R-147-51507.67

A2

49.882.21

15.695.554.010.156.7674.533.860.620.623.21

99.36

3.040.22

38810480242252545843

122

26352518327

101360

0.158.10

18.63.334.301.23

46.0793.8710.8143.77

8.42.727.44—5.550.912.361.590.24

58R-186-91508.06

A3

50.602.13

15.894.585.120.196.445.924.120.540.542.63

100.42

2.130.60

31780

155362231533541

219

3326081862672

390

—

—

—

—

—

—

58R-232-37508.05

A3

48.942.11

15.835.723.780.206.466.344.070.550.553.50

99.46

2.500.64

3689581832441534546

224

42157819225

117362

—

—

—

—

———

———

58R-267-73509.20

A3

50.402.13

16.004.894.860.156.734.893.870.530.532.77

99.99

2.410.38

35991

120742444514944

225

3155481872582

344

0.108.10

19.13.194.101.23

43.6089.0010.0640.34

7.802.586.87—5.270.862.251.550.24

58R-252-60509.35

A3

50.522.23

16.154.714.600.186.314.744.050.560.562.93

99.74

2.490.34

4028082422243554539

123

30554418525

104379

0.068.29

18.03.534.061.37

41.4086.60

—38.008.342.64—0.945———1.500.204

58R-2106-110509.59

A3

50.322.03

15.966.473.380.166.733.923.792.930.463.54

99.70

3.140.18

421111814324615865433

2726847319024

114390

0.108.19

17.13.714.221.48

39.0080.40

—37.007.582.38—0.882—

—1.480.201

58R-2142-150509.95

A3

51.212.04

16.224.874.300.196.424.754.072.920.632.62

100.23

2.220.16

4138193652329564043

329

2745261542590

400

——————

—————————————

58R-335-41510.38

A3

51.222.14

16.564.144.840.216.125.204.412.240.482.15

99.71

1.970.12

31688

326642442564842

128

2715972112596

394

0.108.30

19.33.634.601.39

45.9992.5610.5341.51

7.852.576.85—5.370.882.361.670.26

59R-14-9

516.64A3

49.732.17

15.924.505.300.166.315.444.221.940.553.65

99.89

2.700.43

3919038382139583839

121

35057418925

115386

——————

—————————————

Note: Dash (—) indicates element not determined.

values intermediate to more enriched types, such as transitionalMORB from the FAMOUS area of the Mid-Atlantic Ridge. Note thatthe only Pacific MORB with Zr/Ta ratios similar to the Site 802 basaltsis from a failed rift (the Mendoza Rise), off the East Pacific Rise,drilled during Leg 92 (Leinen, Rea, et al., 1986). Although the basaltsfrom Site 597 of this Leg have many depleted MORB-type charac-teristics (Pearce et al., 1986) they are very different to most Pacificand Atlantic N-MORB in terms of ratios of highly incompatibleelements as seen in Figure 10. However, in many cases incompatible

element ratios overlap or are similar in the Site 802 lavas to the NauruBasin and Ontong-Java tholeiites. Compatible element contents arealso generally similar at the same level of magmatic evolution,although the range of Cr and Ni contents (up to high values) is notseen at Site 802. Isotopic ratios (Sr, Nd, and Pb) are also comparableto the Nauru Basin basalts (Castillo et al., this volume).

Overall, the Site 802 pillow lavas are chemically similar to otherlarge-volume mid-Cretaceous volcanic provinces in the Pacific Oceanand we suggest that they represent a further example of this widespread

350


Table 1 (continued).

60R-115-20

526.05A3

49.902.12

16.064.334.960.14

6.365.944.042.290.483.17

99.78

2.220.95

30892

114

32

22

41

524X

43

2

25

194

585181

24

76

357

0.117.80

17.43.364.401.25

43.9089.16

9.9539.71

7.522.506.57—

5.220.852.291.630.26

60R-144-46526.30

A3

50.672.14

16.115.274.870.16

6.485.004.002.500.552.51

100.26

1.880.20

380

104

107

77

23

44

54

54

44

327

334

559

178

27

95

370

0.098.03

19.43.453.991.43

39.0081.80

—

40.007.962.53—

0.950———1.520.215

60R-169-74526.59

A3

50.802.09

16.095.663.850.146.754.644.022.490.463.23

100.22

2.500.16

419

71

64

49

24

3155

34

44

1

24

263

518

184

24

111

379

0.077.87

17.43.463.981.36

36.0076.20

—36.00

7.512.43—

0.883———1.420.197

61R-110-16

535.40A3

50.682.12

15.994.894.860.126.535.264.042.290.472.59

99.83

2.200.29

331

92

333

22

21

47

52

48

43

125

159

572

200

25

85

356

0.108.50

20.63.404.401.22

42.3785.60

9.7838.00

7.422.516.69—

5.260.862.311.630.26

61R-117-24

535.47A3

50.212.29

15.994.994.890.126.675.054.062.330.603.04

100.24

2.830.20

371

96

48

24

21

36

55

41

391

23

168

542

194

26

111

363

0.117.83

15.63.504.061.27

40.4085.40

—41.00

8.362.60—

0.964—

—

—

1.570.219

5 0 0 -

volcanic event. Isotopically, Site 801 lavas fall outside typical PacificMORB compositions (Castillo et al., this volume) and are similar tolarge oceanic plateaus (Castillo et al., 1991; Mahoney and Spencer,1991) with OIB-like compositions. They differ, however, in vol-canological development and environment to the oceanic plateaus(that appreciably thicken the oceanic crust near spreading axes), inthat the Site 802 basalts probably represent a thin veneer over the localJurassic crust and were developed in an off-axis environment aconsiderable time after the local basement was formed. An alternative(but less likely) possibility is that Site 802 tholeiites actually represent

α

Q

5 2 5 -

Zr ppm60 70

i i 1

-

Th ppm0.1 0.3 100

l i i

>101 ppm

•if

u

••

Units 1-14

Units 15-17

• *HOLE 802A

Cr

#

*•*

;•'

, : .

ppm150

1 1200

Zr ppm300 400

1 I

Unit 1

v

+ -N-

HOLE 800A

550 -

Figure 3. Distribution of Zr, Th, Cr (Hole 802A) and Zr (Hole 800A) withdepth below sea floor (mbsf). The Zr hiatus in Hole 802A occurs betweencooling Units 14 and 15, although this is not marked by other trace elements,such as Th and Cr.

enriched ocean crust in the East Mariana Basin, developed andfocussed by the local rifting of pre-existing Jurassic crust (for exam-ple, Castillo et al., 1991). Whatever the case, the nature of the mantlewas represented by an enriched MORB-source composition, ratherthan a typically depleted composition.

ACKNOWLEDGMENTS

Thanks are due to co-chiefs Roger Larson and Yves Lancelot forimpressing on us the extent of the mid-Cretaceous volcanic event inthe Pacific and the difficulties it posed for reaching "true basement."All shipboard support by the technical staff is gratefully acknowl-edged, together with shore-based analysts at Keele, Bedford NewCollege, Risley Research Reactor (in the U.K.), Washington (U.S.A.),and Vandoeuvre (France). We also acknowledge the helpful com-ments of Drs. R. Batiza and M. Storey.

REFERENCES

Basaltic Volcanism Study Project, 1981. Basaltic Volcanism on the TerrestrialPlanets: New York (Pergamon Press).

Bass, M. N., Moberly, R. M., Rhodes, J. M., Shih, C, and Church, S. E., 1973.Volcanic rocks cored in the Central Pacific, Leg 17, Deep Sea DrillingProject. In Winterer, E. L., Ewing, J. I., et al., Init. Repts. DSDP, 17:Washington (U.S. Govt. Printing Office), 429-503.

Batiza, R., Larson, R. L., Schlanger, S. O., Shcheka, S. A., and Tokuyama, H.,1980. Trace element abundances in basalts of the Nauru Basin. Nature,286:476^78.

Batiza, R., and Vanko, D. A., 1985. Petrologic evolution of large failed rifts inthe East Pacific: petrology of volcanic and plutonic rocks from the Mathe-matician Ridge area and the Guadalupe Trough. J. Petrol, 26:564-602.

Blanchard, D. P., Rhodes, J. M., Dungan, M. A., Rodgers, K. V., Donaldson,C. H., Brannon, J. C, Jocobs, J. W., and Gibson, E. K., 1976. The chemistryand petrology of basalts from Leg 37 of the Deep Sea Drilling Project. J.Geophys. Res., 81:4231-4246.

Bougault, H., Cambon, P., Corre, M., Joron, J. L., and Treuil, M., 1979.Evidence for variability of magmatic processes and upper mantle hetero-geneity in the axial region of the Mid-Atlantic Ridge near 22°N and 36°N.Tectonophysics, 55:11-34.

Bougault, H., Joron, J. L., and Treuil, M., 1980. The primordial chondriticnature and large-scale heterogeneitites in the mantle: evidence from highand low partition coefficient elements in ocean basalts. Philos. Trans. R.Soc. London, 297:203-213.

Castillo, P. R., Carlson, R. W, and Batiza, R., 1991. Origin of Nauru Basinigneous complex: Sr, Nd and Pb isotope and REE constraints. EarthPlanet. Sci. Lett, 103:200-213.

351


Chaffey, D. J., Cliff, R. A., and Wilson, B. M., 1989. Characterization of the St.Helena magma source. In Saunders, A. D., and Norry, M. J. (Eds.), Magma-tism in the Ocean Basins. Geol. Soc. Spec. Publ. London, 42:257-276.

Curray, J. R., Moore, D. G., et al., 1982. Init. Repts. DSDP, 64: Washington(U.S. Govt. Printing Office).

Devey, C. W., Albarede, F., Cheminee, J. L., Michard, A., Muhe, R., and Stoffers,P., 1990. Active submarine volcanism on the Society hotspot swell (WestPacific): a geochemical study. J. Geophys. Res., 95:5049-5066.

Einsale, G., Gieskes, J. M., Curray, J., Moore, D. M., Aguago, E., Aubry, M. P.,Fornari, D., Guerrero, J., Kastner, M., Kelts, K., Lyle, M., Matoba, Y,Molina-Cruz, A., Niemitz, J., Rueda, J., Saunders, A., Schrader, H., Simoneit,B., and Vacquier, V., 1980. Intrusion of basaltic sills into highly poroussediments and resulting hydrothermal activity. Nature, 283:441—445.

Floyd, P. A., 1986. Petrology and geochemistry of oceanic intraplate sheet-flow basalts, Nauru Basin, Deep Sea Drilling Project, Leg 89. In Moberly,R., Schlanger, S. O., et al., Init. Repts. DSDP, 89: Washington (U.S. Govt.Printing Office), 471-497.

, 1989. Geochemical features of intraplate oceanic plateau basalts.In Saunders, A. D., and Norry, M. J. (Eds.), Magmatism in the OceanBasins. Geol. Soc. Spec. Publ. London, 42:215-230.

Griffin, B. J., Neuser, R. D., and Schmincke, H.-U., 1983. Lithology, petrog-raphy and mineralogy of basalts from DSDP Sites 482, 483, 484 and 485at the north of the Gulf of California. In Lewis, B.T.R., Robinson, P., et al.,Init. Repts. DSDP, 65: Washington (U.S. Govt. Printing Office), 527-548.

Haggerty, J. A., Schlanger, S. O., and Premoli-Silva, I., 1982. Late Cretaceousand Eocene volcanism in the southern Line Islands and implications forhotspot theory. Geology, 10:433^Φ37.

Harland, W. B., Cox, A. V, Llewellyn, P. G., Pickton, C.A.G., Smith, D. G.,and Walters, R., 1982. A Geologic Time Scale: Cambridge (CambridgeUniv. Press).

Houghton, R. L., 1979. Petrology and geochemistry of basaltic rocks recov-ered on Leg 43 on the Deep Sea Drilling Project. In Tucholke, B. E., Vogt,P. R., et al., Init. Repts. DSDP, 43: Washington (U.S. Govt. Printing Office),721-738.

Humphris, S. E., and Thompson, G., 1978. Hydrothermal alteration of oceanicbasalts by seawater. Geochim. Cosmochim. Acta, 42:107-126.

Joron, J. L., and Treuil, M., 1989. Hygromagmaphile element distributions inocean basalts as fingerprints of partial melting and mantle heterogeneities:a specific approach and proposal of an identification and modellingmethod. In Saunders, A. D., and Norry, M. J. (Eds.), Magmatism in theOcean Basins. Geol. Soc. Spec. Publ. London, 42:277-299.

Klein, G. déV., Kobayashi, K., et al., 1980. Init. Repts. DSDP, 58: Washington(U.S. Govt. Printing Office).

Lancelot, Y, Larson, R. L., et al., 1990. Proc. ODP, Sci. Results, 129: CollegeStation, TX (Ocean Drilling Program).

Larson, R. L., 1991. Latest pulse of Earth: evidence for a mid-Cretaceoussuperplume. Geology, 19:547-550.

Larson, R. L., and Schlanger, S. O., 1981. Geological evolution of the NauruBasin and regional implications. In Larson, R. L., Schlanger, S. O., et al.,Init. Repts. DSDP, 61: Washington (U.S. Govt. Printing Office), 841-862.

Larson, R. L., Schlanger, S. O., et al., 1981. Init. Repts. DSDP, 61: Washington(U.S. Govt. Printing Office).

Leinen, M., Rea, D. K., et.al., 1986. Init. Repts. DSDP, 92: Washington (U.S.Govt. Printing Office).

Lipman, P. W, Clague, D. A., Moore, J. G., and Holcomb, R. T., 1989. SouthArch volcanic field—newly identified young lava flows on the seafloorsouth of the Hawaiian Ridge. Geology, 17:611-614.

Mahoney, J. J., 1987. An isotopic survey of Pacific oceanic plateaus: implica-tions for their nature and origin. In Keating, B. H., Fryer, P., Batiza, R.,and Boehlert, G. W (Eds.), Seamounts, Islands and Atolls. Am. Geophys.Union, Geophys. Monogr., 43:207-220.

Mahoney, J. J., and Spencer, K. J., 1991. Isotopic evidence for the origin ofthe Manihiki and Ontong-Java oceanic plateaus. Earth Planet. Sci. Lett.,104:196-210.

Marsh, N. G., Saunders, A. D., Tarney, J., and Dick, H.J.B., 1980. Geochem-istry of basalts from the Shikoku and Daito Basins, Deep Sea DrillingProject Leg 58. In Klein, G. deV., Kobayashi, K., et al., Init. Repts. DSDP,58: Washington (U.S. Govt. Printing Office), 805-842.

Mattey, D. P., 1982. The minor and trace element geochemistry of volcanicrocks from Truk, Ponape and Kusaie, Eastern Caroline Islands: the evolu-tion of a young hot spot trace across old Pacific Ocean crust. Contrib.Mineral. Petrol., 80:1-13.

Meschede, M., 1986. A method of discriminating between different types ofmid-ocean ridge basalts and continental tholeiities with the Nb-Zr-Ydiagram. Chem. Geol, 56:207-218.

Moberly, R., Schlanger, S. O., et al., 1986. Init. Repts. DSDP, 89: Washington(U.S. Govt. Printing Ofice).

Pearce, J. A., and Norry, M. J., 1979. Petrogenetic implications of Ti, Zr, Yand Nb variations in volcanic rocks. Contrib. Mineral. Petrol, 69:33-47.

Pearce, J. A., Rogers, N., Tindle, A. J., and Watson, J. S., 1986. Geochemistryand petrogenesis of basalts from Deep Sea Drilling Project Leg 92, EasternPacific. In Leinen, M., Rea, D. K., et al., Init. Repts. DSDP, 92: Washington(U.S. Govt. Printing Office), 435^57.

Rea, D. K., and Vallier, T. L., 1983. Two Cretaceous volcanic episodes in thewestern Pacific Ocean. Bull. Geol. Soc. Am., 94:1430-1437.

Richards, M. A., Duncan, R. A., and Courtillot, V. E., 1989. Flood basalts andhot-spot tracks: plume heads and tails. Science, 246:103-107.

Saunders, A. D., 1984. The rare earth element characteristics of igneous rocksfrom the ocean basins. In Henderson, P. (Ed.), Rare Earth Element Geo-chemistry: Amsterdam (Elsevier), 205-236.

, 1986. Geochemistry of basalts from the Nauru Basin Deep SeaDrilling Project Legs 61 and 89: implications for the origin of oceanic floodbasalts. In Moberly, R., Schlanger, S. O., et al., Init. Repts. DSDP, 89:Washington (U.S. Govt. Printing Office), 499-517.

Saunders, A. D., Fornari, D. J., Joron, J. L., Tarney, J., and Treuil, M., 1982.Geochemistry of basic igneous rocks, Gulf of California, Deep Sea Drill-ing Project Leg 64. In Curray, J. R., Moore, D. G., et al., Init. Repts. DSDP,64: Washington (U.S. Govt. Printing Office), 595-642.

Schlanger, S. O., Jenkyns, H., and Premoli-Silva, I., 1981. Volcanism andvertical tectonics in the Pacific basin related to global Cretaceous trans-gressions. Earth Planet. Sci. Lett., 52:435^149.

Schlanger, S. O., and Moberly, R., 1986. Sedimentary and volcanic history: EastMariana Basin and Nauru Basin. In Moberly, R., Schlanger, S. O. et al., Init.Repts. DSDP, 89: Washington (U.S. Govt. Printing Office), 653-678.

Staudigel, H., and Schmincke, H.-U., 1984. The Pliocene Seamount series ofLa Palma/Canary Islands. J. Geophys. Res., 89:11195-11215.

Sun, S. S., and McDonough, W. E, 1989. Chemical and isotopic systematicsof ocean basalts: implications for mantle composition and processes. InSaunders, A. D., and Norry, M. J. (Eds.), Magmatism in the Ocean Basins.Geol. Soc. Spec. Publ. London, 42:313-345.

Tucholke, B. E., Vogt, P. R., et al., 1979. Init. Repts. DSDP, 43: Washington(U.S. Govt. Printing Office).

White, W. M., Tapia, M.D.M., and Schilling, J.-G., 1979. The petrology andgeochemistry of the Azores Islands. Contrib. Mineral. Petrol, 69:201-213.

Winterer, E. L., 1973. Regional problems. In Winterer, E. L., Ewing, J. I., et al.,Init. Repts. DSDP, 17: Washington (U.S. Govt. Printing Office), 911-922.

Winterer, E. L., Ewing, J. I., et al., Init. Repts. DSDP, 17: Washington (U.S.Govt. Printing Office).

Wood, D. A., 1980. The application of a Th-Hf-Ta diagram to problems oftectonomagmatic classification and to establishing the nature of crustalcontamination of basaltic lavas of the British Tertiary Volcanic Province.Earth Planet. Sci. Lett., 50:11-30.

Date of initial receipt: 23 May 1991Date of acceptance: 27 January 1992Ms 129B-128

352


wt.%

4

3

2

1

-X

X_ X

XX

X XX

××

— X

×

•×Hole•Hole

i

800A802A

1 2 3 4Fe2θ3/FeO

Figure 4. Alteration parameters for Holes 800A and 802A basic rocks. Apartfrom sections of the 800A dolerite sills, oxidation is low.

353


Table 2. Major oxide and trace element data for Hole 802A tholeiitic basalt lavas.

Core, sectionInterval (cm)Depth (mbsf)Cooling unit

57R-2109-115509.19

Al


TiO2

A12O3

Fe2O,FeOMnOMgOCaONa2OK2OP2O5

LOI

Total

H2O+CO2

48.661.42

16.535.914.75ü.Ui7.428.532.780.840.102.70

99.79

2.050.58

Trace elements by XRF (ppm)BaCeCrCuGaLaNbNdNiPhRbSSrVYZnZr

5617

16716220377

8848

234161423

28119101

Trace element INAA (ppm)CsHfScTaThU

0.092.30

48.80.460.620.17

Rare earth elements (ppm)LaCePrNdSmELI

(idTbDyHoErYbLu

9.0719.882.65

12.733.191.224.16—4.610.872.522.360.39

57R-31-6

509.61Al

49.501.17

14.804.106.430.207.87

11.702.650.100.041.17

99.73

1.110.12

1012

15615715244

10354

40098

357259264

—

———

———

—

57R-337-42509.97

Al

49.961.18

14.503.567.260.217.79

11.692.570.090.031.02

99.86

0.630.22

2610

138155

17345

10062

275104354249067

0.201.76

49.40.190.220.40

3 .178.60

15.002.320.88—0.57———2.370.356

58R-19-13

516.09A2

50.561.13

14.453.757.330.197.68

11.722.100.080.031.00

100.091

0.570.18

98

125153

1623

1696

61

189102359

259166

0.101.90

46.50.1860.280.076

3.939.931.467.942.250.893.34—4.080.822.472.490.43

58R-296-100516.96

A2

49.551.09

14.213.078.580.218.05

11.762.000.070.030.98

99.61

0.540.10

1625

150153

1754

2398

62

25299

332249565

——

—

———————

———

—

58R-218-23517.68

A2

50.761.11

14.203.318.290.187.50

11.692.010.100.030.97

100.13

0.440.10

3212

1311592144

139561

30297

342259164

0.101.60

47.80.1650.260.09

3.8310.02

1.467.762.230.873.28

4.080.832.442.380.41

58R-291-96518.41

A2

49.181.19

14.273.468.020.207.69

11.732.100.240.031.00

99.11

0.640.16

1126

1511481624

2596

66

27998

346269465

———

—

—————————————

58R-299-102518.49

A3

49.401.22

14.392.548.830.217.66

11.781.950.180.020.86

99.03

0.540.11

2010

155157

1544

109942

28495

358259765

——————

—————————————

58R-2117-122518.67

A4

49.361.15

14.383.758.170.217.68

11.871.950.140.031.08

99.77

0.900.13

147

159150

16248

10154

32294

355259465

0.151.76

49.00.170.250.50

3.088.50—6.202.220.87—0.56———2.410.36

58R-323-28519.23

A5

50.461.22

14.783.696.360.167.34

11.962.330.080.021.50

99.90

0.650.62

187

13415616246

10041

217106349259365

0.0451.80

48.60.160.260.097

4.0810.31

1.518.212.380.913.50—4.170.832.482.510.43

58R-390-95519.90

A5

49.341.10

14.254.407.560.207.71

11.732.040.250.021.09

99.69

0.650.15

128

141144

192 •

41099

68

27299

351249263

0.191.64

47.70.1650.180.50

3.008.00—6.002.250.814—0.54———2.280.357

58R-3134-140520.34

A5

50.661.14

14.353.437.280.178.03

11.931.990.030.100.90

100.10

0.510.06

283

13616116255

9921

24299

363249463

——————

—————————————

Note: Dash (—) indicates element not determined.

354



58R-424-29520.74

A5

50.641.17

14.103.018.350.177.56

11.652.060.080.020.95

99.76

0.460.10

1310

136154

18234

9371

31596

340259063

—————

———————-—————

58R-465-70521.15

A5

49.321.12

14.553.878.230.217.57

11.962.040.130.031.05

100.06

0.940.08

22I0

180156

17345

IK)41

33045

367259765

0.301.83

49.00.170.230.30

3.188.30—9.00———0.57———2.410.358

59R-17-12

525.47A7

49.461.21

14.723.607.630.197.98

11.972.080.080.030.98

99.93

0.640.13

78

139163

15139

10341

293100373259766

0.101.50

47.60.170.240.112

3.9610.281.478.302.350.913.34—

4.320.852.592.570.43

59R-170-75526.10

A8

49.591.22

14.232.418.570.207.94

12.131.960.060.101.11

99.56

0.430.26

194

131164

15358

9611

244<-ü

IMi254364

————

—

———————————

—

59R-178-82526.18

A8

49.761.14

14.092.738.630.207.92

12.021.910.080.031.04

99.53

0.530.18

3411

14415514437

9341

24998

34425

65

—————

—————

——

——

59R-1130-136526.70

A9

49.761.10

14.263.558.520.217.66

11.851.920.140.021.03

100.02

0.920.07

224

1661521844

12105

41

24593

345259664

0.081.65

48.10.1770.220.30

3.168.10

5.002.350.863

0.567

2.370.359

59R-1138-142526.78

A9

49.271.10

14.433.997.880.217.64

11.582.070.180.021.09

99.45

0.730.24

145

148152

1733

10100

54

25510035025

10065

———

—

—

—

59R-28-13

526.98A9

49.951.24

14.292.978.290.197.68

11.852.000.200.091.05

99.80

0.580.13

236

138157

16358

10044

24996

349259464

——

—

—

59R-215-19527.05

A9

49.581.11

14.313.308.830.217.71

12.021.920.140.020.90

100.06

0.540.13

176

1551511734

12100

64

26396

338259565

—

———

——

—

—

59R-2100-105527.90

A10

49.521.21

14.422.608.340.208.07

12.122.010.050.020.99

99.56

0.500.27

97

1331571714

1598

61

24699

340259064

0.101.60

47.60.1680.260.074

4.8610.82

1.517.942.200.883.15—4.050.802.432.470.42

59R-2123-127528.13

All

49.381.10

14.373.658.470.217.67

11.851.950.130.020.91

99.72

0.710.07

165

165151

18154

10552

25996

349259764

0.161.69

48.90.190.200.40

3.148.70—8.002.310.88

0.56—

2.430.364

59R-317-22

528.49A12

50.481.16

14.593.187.830.187.51

11.842.130.110.020.87

99.91

0.510.24

178

134151

17247

9862

407100346

269264

————

—

————————

—

—

59R-340-44528.72

A13

49.981.23

14.272.039.170.217.67

11.852.000.100.020.84

99.37

0.500.18

328

1501501824

10100

42

26695

348259563

—————

————————————

59R-3127-132529.59

A13

50.341.20

14.242.658.350.217.72

11.781.970.080.021.08

99.64

0.860.10

2910

145156

1623

20102

62

26495

346259363

0.201.76

48.50.170.200.30

3.138.30—

17.002.250.86—0.60———2.360.372

355



Core, sectionInterval (cm)Depth (mbsf)Cooling unit

60R-111-16

534.81A14


TiO2

A12O3

Fe2O3

FeOMnOMgOCaONa2OK2OP 2 O 5LOI

Total

H2O+CO2

50.011.11

14.442.858.790.237.89

12.151.990.040.020.89

100.40

0.450.18

Trace elements by XRF (ppm)BaCeCrCuGaLaNbNdNi

PbRbSSrVYZnZr

20

5146

15116234

10281

22396

3412490

63

Trace element INAA (ppm)CsHfScTaThLI

0.171.68

48.20.180.270.80

Rare earth elements (ppm)LaCePi

NdSmEuGdThDyHoErYbLu

3.108.00—

7.002.290.87

0.57—

—

2.320.369

60R-178-84535.48

A14

51.001.20

16.284.154.770.127.16

10.972.520.080.041.27

99.55

0.850.26

214

159159

18247

100111

239118416

2310170

0.201.95

54.30.190.260.60

3.529.20—

19.002.410.90

0.53——

1.960.281

61R-132-37544.52

A15

49.311.19

14.643.717.700.187.56

11.722.080.250.031.10

99.48

0.750.13

204

160150

1524

1595

56

264100361

2610166

0.161.70

48.70.180.280.097

3.779.531.347.762.300.923.33—

4.170.822.472.440.41

61R-1139-144545.59

A16

50.621.09

14.133.288.210.187.73

11.541.990.090.020.95

99.83

0.510.12

252

123153

172

31595

32

27598

332249564

—

—

—

—

—

61R-214-21545.84

A16

50.311.22

14.053.197.870.177.84

11.282.350.100.031.14

99.53

0.910.09

292

140156

17135

9822

29493

342259064

0.301.74

48.40.1710.250.40

3.288.30

7.002.360.90

0.57

2.480.375

62R-10-7

550.60A17

50.761.17

14.403.417.520.187.78

11.292.240.090.031.07

99.92

0.750.11

241

134159

16341

9652

19496

356259164

0.051.60

50.10.180.240.076

4.2110.17

1.547.942.260.893.37—

4.080.832.482.460.44

62R-1112-118551.72

A17

50.771.06

14.113.398.290.197.79

11.721.960.080.020.84

100.22

0.460.09

379

143151

1724

1099

61

31095

323258562

0.301.63

48.00.170.190.70

3.028.10—

9.002.210.85

0.55——

2.300.361

62R-230-35552.36

A17

50.481.12

13.883.088.460.187.85

11.682.040.080.091.08

100.02

0.520.04

254

127162

15243

9422

33994

328238862

—————

—

—————

——————

—

62R-245-50552.51

A17

49.351.28

14.434.537.100.247.87

11.742.100.170.031.41

100.14

0.600.78

305

130161

17246

9634

30095

340239063

—————

—

————————————

—

62R-253-59552.59

A17

50.791.09

14.253.108.490.207.76

11.791.930.070.020.83

100.31

0.460.12

334

134157

1824

1695

51

304

98330

248863

0.101.60

48.60.1860.270.069

3.659.401.507.762.260.893.24—4.110.822.492.490.42

62R-312-18553.60

A17

50.411.08

14.273.787.620.187.61

11.631.920.070.021.18

99.76

0.820.10

293

147159

16235

9931

34894332

239462

0.201.73

48.20.180.230.50

3.137.90—8.002.340.83—0.54———2.340.367

62R-327-33553.75

A17

50.651.08

14.173.318.350.187.64

11.821.990.060.020.94

100.22

0.450.20

294

133157

15249

9451

31395

329259263

—————

—

————————————

—

356


400

Cr ppm

300

200

100

150

Ni ppm

100

50

200 400Zr ppm

200 400Zr ppm

2.5

Tiθ2 wt.%

2.0

1.5

1.0

—

- f)11

I /

X

i

X

x×* \

× ×××

×

1

35

Y ppm

30

25

20

-

- 1'i ii '

1 /— / M /

hrV- ui

XXX

X X

X X XX X×××

X XX

X

X

1

200 400Zr ppm

200 400Zr ppm

60

Nb ppm

30 -

XX

X X × *V X

X

1

100

Ce ppm

50

Hole 802A

Hole 800A

Nauru Basin

Ontong-Java

200 400Zr ppm

200 400Zr ppm

Figure 5. Distribution of Cr, Ni, TiO2, Y, Nb, and Ce relative to Zr in Hole 800A dolerites and Hole 802A tholeiites. Apart from the clear differencesin incompatible element contents (excepting Y) between the magma types, note the restricted chemical range illustrated, especially relative to tholeiitesfrom the Nauru Basin (data compiled from Larson, Schlanger, et al., 1981; Moberly, Schlanger, et al., 1986) and Ontong-Java Plateau (Leg 130; M.Storey, pers. comm.).

357


-.§100CO

Φ

C

o

jl>

Q.

ECO

10

Hole 800A

i i i i i i i i

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

Figure 6. Envelopes incorporating the range of chondrite-normalized REE

patterns for Hole 800 A alkalic dolerites and Hole 802 A tholeiitic flows. Within

each group fractionation of the REE is very limited; the tholeiites typically

exhibit virtually flat patterns.

TiO2

wt.%

5 h

4 -

Society Seamounts

St. Helena

100 200 300 400 500

Zr ppmFigure 7. Distribution of TiO2 and Zr in alkalic basalts from deep-sea subma-

rine sills (envelopes) and oceanic islands (trends) compared with Hole 800A

dolerite sills and Leg 129 Site 801 alkalic flows (Floyd and Castillo, this

volume). Data sources: Holes 167 and 170 (Bass et al., 1973); Holes 382 and

385 (Houghton, 1979); Holes 446A (Saunders et al, 1982); Mathematician

failed rift (Batiza and Vanko, 1985); St. Helena (Chaffey et al., 1989); Caroline

Islands (Mattey, 1982) and Society Seamounts (Devey et al., 1990).

40

La/Yb)N

30

20

10

Society Seamounts

\

St. Helena

Mathematicianfailed rift

TiO2 wt.%

Figure 8. Degree of light REE fractionation (chondrite-normalized La/Yb

ratio) and TiO2 distributions for various oceanic alkalic basalts relative to Hole

800A alkalic dolerites. Note the marked difference between the Leg 129 Site

801 alkalic basalts and the Hole 800A samples. Data sources: Hawaii (Basaltic

Volcanism Study Project, 1981); Mathematician failed rift (Batiza and Vanko,

1985); Azores (White et al., 1979); St. Helena (Chaffey et al., 1989); Society

Seamounts (Devey et al., 1990).

CQ

o

100

10

I 1CO

• 129-802A-57R-2, 109-115 cm

+ 129-802A-62R-2, 53-59 cm

0 -gl I I I I I I I I I I I I I I I I I I I I I I ICs RbBa Th U K Nb Ta La CePb SrNd P SmZr Hf EuTi Dy Y Yb Lu

Figure 9. N-MORB normalized multi-element patterns for typical Hole 8002A

tholeiite (Sample 129-802A-62R-2, 53-59) and most fractionated variant

(Sample 129-802A-57R-2, 109-115). Note the enrichment in LIL elements,

together with Nb and Ta. N-MORB normalization factors from Sun and

McDonough (1989).

358


10La ppm

8

6

4

2

>

Ay' \ Nauru /^T /

/ tt) . Basin / /

i i i i

x?Ta/

i

0.2 0.4 0.6 0.8 1.0

Th ppm

0.6>pm0.5

20 60 100 140Zr ppm

Figure 10. La versus Th and Ta versus Zr plots showing the distribution of Hole 802A tholeiites relative to Jurassic N-MORB (Leg 129, Site 801; Floyd andCastillo, this volume), enriched MORB from the FAMOUS area of the Mid-Atlantic Ridge (Blanchard et al., 1976; Joron and Treuil, 1989), and the Nauru Basin(Saunders, 1984; Floyd, 1986). Mendoza failed rift data from Pearce et al. (1986).

359

18. geochemistry and petrography of cretaceous sills and lava

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