Pisciotto, K. A., Ingle, J. C, Jr., von Breymann, M. T., Barron, J., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 127/128, Pt. 1

8. SEDIMENTOLOGY, PHENOCRYST CHEMISTRY, AND AGE—MIOCENE "BLUE TUFF": SITES794 AND 796, JAPAN SEA1

David A. Barnes,2 Peter Thy,3 and Paul Renne4

ABSTRACT

Distinctive, massive to stratified, pale blue volcaniclastics, initially referred to as the "blue tuff," were encountered at all four sitesdrilled during ODP Leg 127 in the Japan Sea. Detailed vertical sequence analysis, Plagioclase chemistry, Plagioclase 87Sr/86Sr isotopiccomposition, and 40Ar/39Ar age dating indicate that thick sequences of the blue tuff are not genetically related.

Blue tuffs at Hole 794B were apparently deposited by density flows at ambient temperature. Deposition was penecontemporaneouswith a large submarine phreatomagmatic eruption at 14.9 Ma in bathyal or deeper water depths. The blue tuffs at this location comprisemostly reworked hydroclastic glass shards and lesser amounts of Plagioclase crystals. Pyrogenic Plagioclase has an average An mole%of 18± 3. Comparison of blue tuff Plagioclase compositions with the composition of Plagioclase from acoustic basement at Site 794suggests that these rocks are not genetically related. As such, the extrapolation of sediment accumulation rate data in conjunction withthis more precise age for the blue tuff corroborates previous minimum age estimates of 16.2 Ma for acoustic basement at Site 794.

Blue tuffs at Hole 796B were probably deposited at ambient temperatures by downslope slumping and density flow of reworkedpyrogenic debris. This debris includes abundant bubble wall glass shards and Plagioclase crystals, with variable admixture of volcaniclithic and intrabasinal fragments. Pyrogenic fragments were produced by subaerial or shallow submarine, magmatic eruptions datedat 7.6 Ma. Blue tuffs contain a heterogeneous mixture of unrelated fragments including a mixed population of Plagioclase crystals.The average An mole% of the predominant, probable comagmatic, Plagioclase population is 30± 4.

The two sequences of blue tuff studied are distinct in age, mineral composition, and the eruptive origin of pyroclastic fragments.Preliminary 87Sr/86Sr isotopic compositions of Plagioclase, however, indicates that blue tuffs at both locations are the product of typical,subduction-related island arc magmatism. Based on the results of this study, there is no justification for stratigraphic correlation ofwidespread, Miocene, blue to blue-gray bentonitic tuff and tuffaceous sandstones nor the interpretation that these strata are indicativeof regional, explosive submarine volcanism genetically related to rifting and formation of the Japan Sea. Rather, these reworkedpyroclastic strata of intermediate composition were deposited over a protracted 6-8 m.y. period in association with widespread,subduction-related submarine to subaerial volcanism in the Japan Sea backarc basin.

INTRODUCTION

Miocene strata informally called the "blue tuff were recoveredfrom all sites during Ocean Drilling Program Leg 127 in the JapanSea (Figs. 1 and 2). At Sites 794 and 796 (over 300 km apart) thesestrata comprise lithologically distinctive, massive to stratified, paleblue to medium pale gray volcaniclastics that were initially describedas primary submarine, vitric- and crystal-rich pyroclastic deposits(Tamaki, Pisciotto, Allan, et al., 1990). At all Leg 127 sites blue tuffsare the dominant lithic type. They range from less than 1 m to over11 m in thickness. The characteristic blue color and thick bedding aswell as consistent occurrence within a regionally recognizable cal-careous and siliceous claystone lithofacies (Unit D of Tamaki, Pis-ciotto, Allan, et al., 1990; Fig. 2) initially suggested lithostratigraphiccorrelation of these blue tuffs.

Preliminary analysis (Tamaki, Pisciotto, Allan, et al., 1990) sug-gested that the pyroclastic material comprising the blue tuffs wasadmixed with intrabasinal debris during a regionally significant pe-riod of mafic to intermediate explosive submarine volcanism. Epi-clastic debris was not identified in the blue tuffs, suggesting that Leg127 drilling sites were isolated from subaerially exposed land masses.Strata associated with blue tuffs ranges in age from early Miocene (atSite 797) to middle Miocene (at Site 794) and possibly late Miocene(in Site 796) (Tamaki, Pisciotto, Allan, et al., 1990).

Pisciotto, K. A., Ingle, J. C, Jr., von Breymann, M. T., Barron, J., et al, 1992. Proc.ODP, Sci. Results, 127/128, Pt. 1: College Station, TX (Ocean Drilling Program).

2 Department of Geology, Western Michigan University, Kalamazoo, MI 49008,U.S.A.

3 Department of Geology, University of Botswana, Private Bag 0022, Bagarone,Botswana.

4 Institute of Human Origins Geochemistry Center, 2453 Ridge Road, Berkeley, CA94709, U.S.A.

The stratigraphic proximity of blue tuffs to subjacent mafic extru-sive and intrusive rocks that constitute acoustic basement in theYamato and Japan basins could indicate magmatic cogenesis. If thisbasement igneous complex formed during a period of intense, earlyto middle Miocene magmatic activity that was genetically related torifting and subsequent subsidence in the Japan Sea region (Tamaki,Pisciotto, Allan, et al., 1990), then blue tuffs might also be related torifting. This purported genetic relationship between rift-related vol-canism and widespread blue tuff deposition would support diachro-nous rifting during the tectonic evolution of the Japan Sea region.

We first address the validity of correlation of blue tuff sequences basedon the comparison of sedimentologic and petrologic characteristics. Wealso studied the megascopic and microscopic structures and textures, andthe major element chemistry, isotope chemistry, and radiometric ages ofpyrogenic fragments in order to interpret the emplacement mechanisms,depositional setting, and the petrologic affinity of the blue tuffs. Ourresults define the general magma type and the paleogeography duringemplacement of the blue tuffs and their relationship to the voluminousvolcanic rocks purportedly formed during the rifting and formation of theYamato and Japan basins (Tamaki, Pisciotto, Allan, et al., 1990).

METHODS AND MATERIALS

Sedimentology

Thick-bedded, coarse-grained intervals of blue tuff are best rep-resented in cores drilled at Sites 794 and 796 located in the northernYamato and Japan basins, respectively (Fig. 1). Traditional, sedimen-tologic vertical sequence analysis of volcaniclastic rocks is useful forinterpreting the conditions during magmatism and deposition (Cassand Wright, 1987). Detailed description of intervals dominated byblue tuffs were compiled from cores taken in Sites 794 and 796. Thesedescriptions include sedimentary structures, grain-size variation, andlithology. Eruptive mechanisms and the environmental conditions

115

D. A. BARNES, P. THY, P. RENNE

45°N

40c

130°E 135° 140c

Figure 1. Location map showing ODP Leg 127 (large dots) and DSDP Leg 31 (small dots) drilling sites in theJapan Sea.

during deposition of the tuffaceous units were interpreted based onthese observations in conjunction with the analysis of petrographicmicrotextures observed in thin section.

Pyrogenic Crystal Chemistry

Although lavas and intrusive rocks in submarine volcanic successionsare commonly hydrothermally altered, the pyrogenic crystal componentof associated tuffaceous strata may escape the effects of such alteration.The major element chemistry, isotope chemistry, and absolute age ofunaltered, pyrogenic minerals in tuffs provides information for the inter-pretation of magmatism that is not otherwise available. The majorelement chemistry of Plagioclase in blue tuffs and related strata in Sites794 and 796 was analyzed initially to evaluate the homogeneity offragmental particles in individual samples. Isotopic analysis of plagio-clase crystal fractions from individual samples is relevant only if thesesamples contain a homogeneous crystal fraction and are, therefore,considered cogenetic, juvenile crystal ejecta. Wavelength-dispersive,electron probe microanalysis (EPMA) was used to determine the majorelement composition on polished thin sections or grain mounts of crystalseparates from poorly consolidated samples. Six samples were analyzedfrom Site 794, Hole B, and 13 were analyzed from Site 796, Hole B. Datawere generated by microprobes in two laboratories (accelerating volt-ages: 12 kV, 15 kV; focused beam approximately 10 µm; beam currents:7 nAand 25 nA; respectively). Although some inconsistency was recog-nized in the data generated from the two labs, clustering amongst relatedsamples was clearly recognizable in the combined data sets.

40Ar/39Ar Dating

Glassy fragments in tuffs are ubiquitously replaced by alterationproducts so that neither the chemistry of glass nor whole-rock chemi-cal analyses are pertinent to the interpretation of magma genesis. Theage of volcanism and tectonic activity represented by blue tuffs in theJapan Sea region was investigated using the 40Ar/39Ar dating tech-nique on Plagioclase, hornblende, and biotite grains separated fromappropriate samples. The friable, clay-rich, tuffaceous rocks wereeasily disaggregated by gentle crushing and ultrasonic agitation.

Hand-picked grains from the 200- to 500-µm size fraction wererinsed in 10% HC1, followed by 7% HF, for 5 min each in an ultrasonicbath. Grains were selected to be free of visible alteration, inclusions,or adhering material. Ten to 30 grains from each sample were loadedinto cylindrical pits in an Al disk along with grains of Fish Canyonsanidine (Cebula et al., 1986), which was used as a neutron fluxmonitor. Samples were submitted to the Omega West reactor at LosAlamos National Laboratory for a 30 min irradiation, but they wereirradiated for approximately 29 hr due to an error at the reactor facility.

All samples were analyzed by total fusion on the IHOGC auto-mated 40Ar/39Ar laser microprobe using procedures similar to thosedetailed by Deino and Potts (1990) and Renne et al. (1991). Gas waspurified for 200 s with two Zr-Fe-V getters run at 3.0 amp. Systemblanks were analyzed between every three samples, and interpolatedvalues used to correct the sample data.

Ten aliquots of purified atmospheric Ar from an automated pipettesystem were run over the course of the sample analysis, yielding a

116

SEDIMENTOLOGY, CHEMISTRY, AND AGE OF MIOCENE BLUE TUFF

mean 40Ar/36Ar of 288.42 ± 3.01 (sigma) corresponding to massdiscrimination of 1.0061+ 0.0026 (sigma) per mass unit with refer-ence to atmospheric 40Ar/36Ar = 295.5 (Nier, 1950). Five grains ofFish Canyon sanidine from the 590- to 840-µm fraction were ana-lyzed individually to yield a J-value of 2.8634 × I0"2 ± 3.34 × I0"5

based on an age of 27.84 Ma.

87Sr/86Sr Isotopic Composition

A split of crystal residues was also provided for isotopic analysis of87Sr/86Sr ratios in Plagioclase to interpret source magma affinities. ThePlagioclase crystal separates were purified by magnetic separation, hand-picked for visual homogeneity, and ultrasonically agitated to removeadhered material. The samples were then washed in warm (-50° C) 2.5 MHC1 to further clean the surfaces before dissolution. Chemical processingconsisted of dissolution in HF and HC1O4 followed by cation-exchangeseparation of Sr using Dowex 50 resin. Isotope analysis were made witha Finnigan MAT 261 mass spectrometer operated in the static mode. The87Sr/86Sr ratios were normalized within a run to 86Sr/88Sr = 0.119400,and further adjusted to a value of 0.710200 for the NBS-987 interlabo-ratory reference standard.

SEDIMENTOLOGY OF VOLCANICLASTICS

Graphic representation of two sections of blue tuffs described fromSites 794 and 796 are presented in Figures 3A and B (respectively). Eachsequence contains distinct, blue to blue-gray beds of coarse-grainedvolcaniclastics defined by sharp, scoured bases, well-defined internal,planar lamination, and complex, internal size grading. These beds areinterstratified with laminated to burrowed, diatomaceous and calcareous,hemipelagic claystone. Intraformational claystone rip-up clasts to severalcentimeters are common in the thicker, coarser-grained beds.

Blue Tuffs in Site 794, Hole B

Volcaniclastic strata a few decimeters to several meters thick inCores 127-794B-21R through 127-794B-23R occur in eight discreteunits defined by sharp, commonly scoured basal contacts (Fig. 3A).The base of units 3 and 5 contain pronounced inverse to normal(symmetrical) size grading. The massive, unstratified basal portionsof these units are gradational upward to well developed planar strati-fication and then to convolute lamination and bioturbation at the top.Abundant, intraformational rip-up clasts of siliceous claystone occurat the base of the massive and structureless units. Elongate rip-upclasts are aligned parallel to bedding in the basal portion of unit 3.These clasts are randomly oriented higher in this unit and are, again,horizontally aligned at the top of the massive portion of the unit.Horizontal clast alignment is common throughout unit 5. The upper-most portions of these thick units are bioturbated and gradational tooverlying claystone and silty claystone. The maximum grain sizewithin these thickly bedded units is 10-20 mm and size sorting isgenerally poor.

Planar bedding is pronounced in the upper portion of the thickunits and is defined by interstratified layers that are alternatelydominated (on a decimeter scale) by low-density tube pumice andthen more dense, vesicular, vitric hydroclasts and/or Plagioclasecrystals (see the following petrographic descriptions). Adjacent layersthat contain clasts of predominantly different density are not clearlygradational, however. The basal, massive portion of unit 3 contains asubstantial crystal component, up to 10%. These crystals are alsoirregularly distributed, in lesser proportions, in the upper, stronglyplanar laminated layers of this unit. The upper part of unit 5 containsonly a trace of Plagioclase crystals. Overall, dense, slightly vesicularhydroclasts and Plagioclase crystals are more abundant at the base ofthese thick units whereas less dense, tube pumice is more abundantat the top. These relationships are interpreted as crude density gradingin the massive beds.

Thinner bedded units 1 and 2 (Fig. 3A) at the base of the sequenceare disrupted by drilling. They contain mostly coarse-grained, slightlyvesicular hydroclasts and Plagioclase crystals and were apparently trun-cated as a result of the emplacement of subsequent layers. Finer grainedand thinner bedded units higher in the section (units 4, 6, 7, and 8;Fig. 3A) contain better preserved structures and are characterized byfine- to very-fine-grained beds that overly sharp, scoured contacts withunderlying beds. Normal size grading is vague to distinct. The upper partsof these beds grades into overlying claystone. Backfilled burrows arecommon in the upper, finer grained parts of these beds and are bothhorizontally and obliquely oriented. Deformational structures are com-mon including flames, and convolute laminations.

Blue Tuffs in Site 796, Hole B

Coarse-grained Miocene volcaniclastics occur within lithofaciesD (Tamaki, Pisciotto, Allan, et al., 1990) at Site 796 in the easternJapan Basin (Fig. 1). The succession of pale blue to blue-grayvolcaniclastics consists of seven distinct units identified by sharp,scoured basal contacts, complex size and density grading, and planarbedding (Fig. 3B). The grain size ranges from rare granule- (3^4 cm)size fragments (mainly intraformational siliceous mudstone rip-upclasts) to very-fine-grained ash. These units are interbedded withbioturbated, hemipelagic, siliceous claystone.

The thickest unit (unit 2; Fig. 3B) is approximately 2.6 m thick and ischaracterized by sharp basal scour, a thin zone of inverse size grading,and abundant intraformational rip-up clasts. The upper portion ofthis unit contains gradational contacts between individual, normal-size-graded, centimeter- to decimeter-thick layers with pronounced planarbedding megascopically defined by color banding (PL 1, Fig. 1). Colorbanding is due to gradational variation in grain composition from dark-colored, lithic- and crystal-rich layers below to lighter colored, vitric-richlayers above. Thinner, normal-size-graded layers in the section are alsocolor banded as a result of crystal and lithic fragment concentrationrelative to the vitric components (PI. 1, Fig. 2).

Particle size sorting in the sequence is moderate to poor withininversely and normally graded units, although the density segregation ofthe particles is moderately well developed. The basal portions of all unitsare dominated by denser particles including lithic fragments, Plagioclase,and minor amphibole crystals. The upper, complexly size-graded portionof unit 2 contains density-segregated layers with gradational contactswithin the overall fining-upward interval. These structures indicate amal-gamation of density flows deposited in rapid succession.

Petrography of Blue Tuffs: Site 794, Hole B

The composition of clasts in blue tuffs at Site 794 was determined byexamination of 20 thin sections and is generally uniform through thesection. Abundant long-tube pumice (PI. 2, Fig. 1) and shards show noevidence of pre- or syndepositional physical/chemical alteration. Thesefragment types, along with abundant euhedral or broken euhedral plagio-clase crystals, are a good indication of a pyrogenic origin and attest tocontemporaneous, active volcanism (Fiske, 1969). Long-tube pumice is,in general, most common in proximal, subaqueous pyroclastic depositsbecause of its greater permeability and susceptibility to water saturationand rapid sinking (Fiske, 1969). Long-tube pumice is more common atthe top of thick units of blue tuffs and was apparently less dense than thecrystal component. Long-tube pumice is present in smaller proportionsthan other vitroclasts throughout the sequence in Cores 127-794B-21Rthrough 127-794B-23R.

Vesicular glass shards with spherical vesicles are the major vitro-clastic constituent of tuffaceous rocks (PI. 2, Fig. 2). These vitricfragments are characterized by equant shapes and sharp, arcuateboundaries that commonly transect vesicles. Vesicle size, as indicatedby the radius of curvature of partial vesicles, is commonly less thanthe size of the vitric fragments (PI. 2, Fig 3). Original void space istypically less than 30%-50%. Vitroclastic particles of this shape have

117

D. A. BARNES, P. THY, P. RENNE

SITE 797(Yamato Basin)

WD = 2862 m

SITE 794(Yamato Basin)

WD= 2811 m

SITE 795(Japan Basin)WD = 3300 m

SITE 796(Okushiri Ridge)

WD = 2571 m

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SEDIMENTOLOGY, CHEMISTRY, AND AGE OF MIOCENE BLUE TUFF

been ascribed to hydroclastic eruption by Heiken (1972). Subsequentwork by Wohletz (1983), however, has shown that the distinction ofblocky shards as the unique product of phreatomagmatic eruptionsmay be too simplistic (Cass and Wright, 1987).

Vesicular hydroclasts in subaqueous pyroclastic successions havebeen interpreted to result from eruptions above the pressure compen-sation level (PCL, the depth at which magma vapor pressure exceedswater pressure; Fisher, 1984). This depth is a function of the depth ofthe water column and the volatile composition of the magma. Thisdepth may exceed 1000 m for volatile-rich alkalic magmas (Fisher,1984). Preliminary results of the petrologic affinities of sills recov-ered in the basement complex at Site 794 (Tamaki, Pisciotto, Alan, etal., 1990) suggest a moderate to very low magmatic water content forthese magmas. Such lavas, if they are related to the blue tuffs, mightbe expected to have a moderate to shallow (<IOOO m) PCL. Recentobservations from the Izu-Bonin Island arc (Gill et al., 1990) suggestthat the PCL of backarc basin basalts can be deeper than 2000 m andresult in explosive hydroclastic eruptions at this depth.

The proportions of vitric fragments and Plagioclase crystals in bluetuffs varies irregularly through the sequence as a result of complex densitysegregation (see the preceding). All originally vitric-rich fragments arenow altered to a uniform greenish brown, translucent, and slightly bire-fringent alteration product resembling palagonite. This material is com-posed predominantly of dioctahedral smectite (on the basis of preliminaryX-ray diffractograms of clay separates). Euhedral and broken euhedralPlagioclase crystals are also common pyrogenic fragments but compriseno more than 10% of any sample (PI. 2, Fig. 4). Rip-up clasts ofintraformational, siliceous claystone and disseminated claystone are theonly other significant components of the tuffaceous strata.

Petrography of Blue Tuffs: Site 796, Hole B

Eleven thin sections were examined to determine the compositionand textures of volcaniclastics in Core 127-796B-18R. The composi-tion of these strata is more heterogeneous, compared to blue tuffs atSite 794, and includes abundant (20%-40%) nonvolcanic material insome samples. Most nonvolcanic material consists of intraforma-tional siliceous claystone clasts and a disseminated, microfossilifer-ous matrix. Rare green grains, probably glauconitic, are also present.

The dominant component in most tuffaceous strata is an interpartide"paste" now altered to moderate birefringence clay. X-ray diffractionanalysis of clay-size separates indicates that this clay is dioctahedralsmectite. However, the clast composition of any sample varies as afunction of position within flow units (see the preceding description ofdensity segregation). Indistinct portions of the clay-rich matrix retainaltered intersertal textures with Plagioclase microphenocrysts clearlyindicating a volcanic, lithic-fragment origin. Other areas in the matrixcontain indistinct clasts with possible long-tube pumice texture, althoughmost of the matrix is probably altered fine-grained ash.

Discrete bubble-wall glass shards are also a major component ofblue tuff at Site 796 and comprise between 10% and 30% of mostsamples. Shards are present throughout the section, but are concen-trated in the upper portions of all flow units as a result of densitysegregation. Shards are ubiquitously altered to lath-shaped zeolite,identified as clinoptilolite-heulandite group minerals (on the basis ofpreliminary X-ray-diffraction analyses). The shard morphology iswell preserved with consistent bubble wall shape having commoncuspate junctions and a large vesicle size compared with fragmentsize (as determined by the radius of curvature of partial vesicles; PI. 2,Fig. 5). Bubble wall shards are typically interpreted as the product ofexplosive vesiculation during magmatic eruption without substantialinfluence of water interaction with the melt (Heiken, 1972, 1974).

The basal portions of most units in Core 127-796B-18R arepyrogenic crystal rich (10%-30%) including abundant Plagioclase,minor amphibole, and trace biotite and quartz. Most crystals are euhedral

or euhedral and broken (PL 2, Fig. 6). Individual thin units in thesequence typically contain concentrations of crystals and intermedi-ate volcanic lithic fragments at their base, although unit 2 containscomplex size and density grading in the upper, planar-bedded portion(PI. 1, Fig. 1). Both size and density grading throughout the blue tuffand especially in unit 2 indicate multiple, density flows and complexamalgamation. The abundance of lower density vitric particles, com-pared with more dense crystal and lithic particles, has apparentlyresulted in the unusual (compared with typical epiclastic density flowdeposits) size and density grading in the unit.

Depositional Mechanisms: Site 794, Hole B

Blue tuffs at Site 794 are interbedded with fossiliferous hemipelagicmudstone deposited in middle bathyal depths (Tamaki, Pisciotto, Allan,et al., 1990). Abundant long-tube pumice and moderately vesicular,blocky shards suggest a hydroclastic origin for these tuffaceous strata inassociation with a subaqueous phreatomagmatic eruption at Site 794.The overall symmetrically (inverse to normal) graded, thick units 3 and5 are interpreted as single submarine density-flow events followed, inabrupt succession by smaller flow pulses defined by density grading inthe planar-bedded upper portion of these units. The lower, massiveportions of units 3 and 5 is unusually thick (>60% of the entire beds)compared to typical, coarse-grained epiclastic density flow deposits andare overlain by planar-bedded and indistinctly density-graded layers.These features suggest a subaqueous pyroclastic flow origin wherein theoverall upward-fining grain size of the thick flow units resulted from thewaning force of a contemporaneous, subaqueous eruption (Fiske andMatsuda, 1964; Fisher, 1984; Fisher and Schmincke, 1984). The separa-tion of individual flow units by marine claystone layers reflects periodsof quiescence and suspension sedimentation between eruptive events.

Recent findings regarding the PCL (Fisher, 1984) for magmaticwater-rich, backarc basin basalts (Gill et al., 1990) indicate that vesicularhydroclasts do not clearly constrain the maximum depth of explosive,submarine phreatomagmatic eruptions. Backarc spreading center ba-salts and associated basalt breccia clasts in the Izu-Bonin arc areobserved to contain vesicles caused by explosive eruption at emplace-ment depths greater than 1800 m (Gill et al., 1990). As such, there areno unequivocal indications of paleobathymetry that can be interpretedfrom vesicular submarine, pyroclastic flow deposits of the blue tuffat Site 794.

Depositional Mechanisms: Site 796, Hole B

Abundant bubble wall shards in blue tuffs at Site 796 suggest that thispyroclastic debris may have originated as a result of explosive vesicula-tion and pyroclastic transport during a large magmatic eruption. Bubblewall shard texture is in strong contrast to the common hydroclasts ofapparent phreatomagmatic eruption origin in blue tuffs at Site 794. Thepyroclastic debris in Core 127-796B-18R was probably generated bysubaerial or shallow subaqueous eruptions, deposited in marine waters,and subsequently remobilized by density flow. Density flows wereprobably generated by slumping on over-steepened slopes adjacent to themagmatic centers although a considerable distance of pyroclastic trans-port may have preceded marine deposition and subsequent downsloperemobilization. There is no clear indication of penecontemporaneousdensity transport of blue tuffs at Site 796 following the magmaticeruptions that produced the tuffaceous debris. These units may have beenemplaced by density-flow mechanisms that were essentially unrelated tothe eruption that formed the majority of the tuffaceous detritus. Thedepositional mechanism responsible for complexly graded units com-posed of reworked, dominantly pyroclastic material and emplaced byepiclastic slumping and density flow is a poorly documented but typicalaspect of marine deposits associated with marginal basin volcanism(Fisher, 1984).

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D. A. BARNES, P. THY, P. RENNE

SYMBOLS

blue tuff

mudstoneburrows

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grading

V >/ V V

v v

GRAIN SIZE AND

SEDIMENTARY STRUCTURES

UNIT 5

UNIT 3

UNIT 2

UNIT 1

Figure 3. Graphic representation of the blue tuff. A. Cores 127-794B-21R through 127-794B-23R. B. Core 127-796B-18R.

120

SEDIMENTOLOGY, CHEMISTRY, AND AGE OF MIOCENE BLUE TUFF

B

SYMBOLS

blue tuff

mudstoneburrows

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Figure 3 (continued).

PETROGENESIS

Pyrogenic Crystal Chemistry

The composition of pyrogenic Plagioclase crystals in blue tuffs atboth Sites 794 and 796 was investigated using the EPMA. In sampleswith relatively homogeneous Plagioclase feldspar compositions thesefeldspars are interpreted as cogenetic, juvenile crystal ejecta. Thecomposition of these cogenetic Plagioclase may then be interpretedin terms of the general magma type. Cogenesis of Plagioclase grainsin individual samples is a prerequisite for the use of other analytical

SEDIMENTARY STRUCTURES

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techniques, including 40Ar/39Ar dating and 87Sr/86Sr analysis, to inter-pret the origin of blue tuffs.

EPMA analysis of 6-19 individual crystals in 19 tuffaceous samplesis reported in Table 1. Most of the analyses are from Plagioclase in bluetuffs although Plagioclase in several additional samples of coarse-grainedtuffaceous sandstone from Site 796 were also analyzed.

The variation in An mole% of Plagioclase crystals from Site 794is indicated in Figure 4A. The mean An mole% for Plagioclase fromblue tuffs at this site is 18 ± 3. Because low-temperature alteration ofPlagioclase feldspar normally results in nearly pure albitic phases(Boles, 1982) these Plagioclase are probably not significantly altered

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D. A. BARNES, P. THY, P. RENNE

Anorthite mole%100

50 100 150Sample Number

200

B

Aπorthitθ mole%80

0 1 2 3 4 5

Orthoclase mole%Figure 4. A. Anorthite mole% for all samples vs. sample number (Table 1). Sample numbers in Table 1 are arrangedfrom top to base in each site. Crosses and stars are samples from blue tuffs at Sites 794 and 796, respectively. Boxesare non-blue tuff samples at Site 796. The circled fields indicate the inteΦreted cogenetic Plagioclase from bluetuffs at both sites. B. Orthoclase mole% vs. anorthite mole% for blue tuff samples. The circled fields indicate

genetic Plagioclase from blue tuffs at Sites 794 and 796. Data from Table 1. Symbols as in A.

SEDIMENTOLOGY, CHEMISTRY, AND AGE OF MIOCENE BLUE TUFF

by diagenesis. Figure 4B also suggests relative homogeneity of thePlagioclase crystal component in blue tuffs at Site 794.

Considerable scatter exits for all other samples analyzed from Site796, including blue tuffs in Core 127-796B-18R (Fig. 4A). The plagio-clase in blue tuffs at Site 796 have mean An mole% of 36 ± 12. Thisscatter in Plagioclase composition is also represented in Figure 4B. Adistinct crystal population can be defined, however, for Plagioclase withAn mole% < 40, low Ca/K values (< 15), and moderately high K values(> 2.5%). The average An mole% of these Plagioclase is 30 ± 4. Theseplagioclases are the dominant, probably comagmatic, juvenile crystalejecta component in blue tuffs in Site 796. Other Plagioclase crystals areinterpreted as detrital contamination or mixing of other unrelated pyro-clastic material.

The compositions of Plagioclase crystals in blue tuffs at Site 794suggest little or no contamination of cogenetic, juvenile particlesconsistent with the sedimentologic interpretation that the blue tuff isa reworked but essentially uncontaminated pyroclastic deposit. Thecomposition of Plagioclase in Site 794 blue tuffs is sodium-rich(An]5_24) and is unlike the composition of feldspars in Plagioclasephyric basalt recovered from acoustic basement in this site (An45_90;Thy, this volume). The comparison of blue tuff and basement igneouscomplex Plagioclase compositions at Site 794 suggests that these unitsare unrelated.

Considerable mixing of disparate Plagioclase in blue tuffs is inter-preted at Site 796. The dominant Plagioclase population in Site 796 bluetuffs is more calcic in composition (An25_3g) with respect to the Site 794blue tuff Plagioclase but may constitute cogenetic, juvenile crystal ejecta.Site 796 blue tuffs contain a substantial component of reworked frag-ments, however, and are significantly adulterated by accidental or unre-lated detrital components.

Results of 40Ar/39Ar Dating: Site 794, Hole B

Fifteen individual grains from two tuff samples in Site 794 (Table2) yield dates that range between 12.65 ± 0.82 and 16.82 ± 0.79 Ma.The individual grain dates appear normally distributed rather thanmultimodal, suggesting that xenocrystic contamination is not present.37Ar/39Ar values for the Plagioclase grains imply Ca/K atomic ratiosof 3.2-9.0, with all but one less than 6.7. This is in close agreementwith the EPMA data (Table 1). Mean dates and standard errors of themean for the two samples are 14.87 ± 0.16 and 14.85 ±0.18 Ma; theages of the two units are therefore analytically indistinguishable basedon the present data.

These radiometric ages for blue tuffs in Site 794 are geologicallyreasonable compared to the age indicated by biostratigraphic data pre-sented in Tamaki, Pisciotto, Allan, et al. (1990). Biochronology andsediment-accumulation rate extrapolation indicates a middle Miocene(13-14 Ma) age for blue tuffs. Basement ages (Tamaki, Pisciotto, Allan,et al., 1990) at Site 794 range from 14.8 to 16.2 Ma, again, on the basisof biochronology and sediment-accumulation rate extrapolation. Thedeposition of blue tuffs at this site is interpreted here (see the preceding)as essentially contemporaneous with its magmatic origin at approxi-mately 14.9 Ma and suggests two possible interpretations. The firstalternative is that the basement igneous complex at this site may becontemporaneous with eruption and emplacement of blue tuffs at14.9 Ma. The second alternative is that basement rocks are unrelated toblue tuffs and the actual age of the basement at this site may be evaluatedby sediment-accumulation rate extrapolation from the 14.9 Ma age of theblue tuff. This extrapolated age corroborates initial estimates of theminimum age for basement of 16.2 Ma (Tamaki, Pisciotto, Allan, et al.,1990). The dissimilarity in Plagioclase composition in blue tuffs com-pared to basement sills and flows (see the preceding) tentatively favorsthe second interpretation presented here.

Results of Dating: Site 796, Hole B

Four grains of Plagioclase from one sample at Site 796 were analyzedafter preheating at 0.3 W. Three grains (Table 2) yielded nearly concordantdates with a mean and standard error of 7.61± 0.56 Ma. Ca/K for the threegrains ranged from 6.4 to 6.6 consistent with EPMA data for interpretedcomagmatic Plagioclase (Table 1). The fourth Plagioclase grain yieldeda considerably younger date of 4.91+ 0.57 Ma and had a slightly lowerCa/K of 5.9. The two biotite grains were not preheated with the laser;these gave statistically indistinguishable dates of 5.15 ±0.28 and5.07± 0.08 Ma, despite a significant difference in the extent of atmos-pheric contamination (see Table 2). These dates are also similar to thatobtained from the anomalous Plagioclase grain. Hornblende from Site796 was analyzed by total fusion of 12 grains, resulting in a date of9.79 ±0.50 Ma.

The significance of the limited data from Site 796 is uncertain.However, comparison with biostratigraphic data and the extrapolationof sediment-accumulation curves (Tamaki, Pisciotto, Allan, et al.,1990) indicates that the three older Plagioclase grains date the maxi-mum age of deposition of the tuff at 7.61± 0.56 Ma and the youngerdates may reflect grain alteration. The apparent contemporaneity ofages in these younger grains would be fortuitous in this interpretation.Alternatively, the two biotite grains and the anomalously youngPlagioclase grain are cogenetic and the tuff is 5.05± 0.31 Ma basedon the mean (and standard error) of the three grain dates. This isunlikely, however, in light of reliable diatom dates from nearly 100m uphole of 6.4 Ma (Tamaki, Pisciotto, Allan, et al., 1990). In eithercase, the hornblende is likely to have been derived from reworkingof one or more older sources. Further clarification of the age of thisunit would require the analysis of many more grains in order to definethe distribution of apparent ages, which would help distinguish be-tween contamination and alteration to explain the data.

87Sr/86Sr Isotopic Composition

Observed 87Sr/ 86Sr measurements from Plagioclase crystal separateswere undertaken in order to infer the tectonic affinity of source magmasfor blue tuffs from Sites 794 and 796. Values are presented in Table 3 andshow a range from 0.704199 to 0.709547 for samples from both Sites794 and 796. We believe that the analysis from Sample 127-794B-23R-CC, 10-13 cm, of 0.709547 may not be representative of the truestrontium ratio in pyrogenic Plagioclase but is a function of contaminationby clay-rich matrix or the alteration of feldspar crystals. More analyseswould be needed to verify this preliminary interpretation, however. Weinterpret all other samples to represent the true range of observed stron-tium isotope ratios for pyrogenic feldspars in the blue tuff from bothsites (0.704512-0.704199).

The general homogeneity of strontium ratios from the samples sug-gests similar magmatic derivation in a plate tectonic context. The rangeof values for these samples is very close to the average value of 0.70437for island arcs derived from the upper mantle (Faure, 1986).

SUMMARY

Depositional Setting

The deposition of Miocene blue tuff successions encountered atSites 794 and 796 (Tamaki, Pisciotto, Allan, et al., 1990) was inter-preted based on associated strata, sedimentary facies characteristics,and glass shard morphology. Blue tuffs at Site 794 are composed ofjuvenile pyroclasts and minor intraformational material apparentlydeposited by cool or cold density flows that formed contemporane-ously with a large submarine phreatomagmatic eruption. The mini-mum water depth at the site of eruption is poorly constrained but was

D. A. BARNES, P. THY, P. RENNE

Table 1. Electron probe microanalysis of Plagioclase from blue tuffs and related strata, Leg 127.

Elemental data are presented as atomic fractions normalized to eight oxygens.

Analysisno.

Hole 794B

1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.24.30.31.32.33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.51.52.53.54.55.56.

Hole 796B

57.58.59.60.61.62.63.64.65.66.67.

Hole, section,interval (cm)

21R-5, 11921R-5,11921R-5, 11921R-5, 11921R-5, 11921R-5, 11921R-5,11921R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 1621R-6, 13421R-6, 13421R-6, 13421R-6, 13421R-6, 13421R-6, 13421R-6, 13421R-6, 1321R-7, 5021R-7, 5021R-7, 5021R-7, 5021R-7, 5021R-7, 5021R-7, 5021R-7, 5021R-7.5021R-7, 5021R-7, 5021R-7, 5021R-CC, 121R-CC, 121R-CC, 121R-CC, 121R-CC, 121R-CC, 121R-CC, 121R-CC, 121R-CC, 122R-1,6922R-1.6922R-1.6922R-1,6922R-1.6922R-1.6922R-1,6922R-1,6922R-1, 69

llR-1,27llR-1,27llR-1,27llR-1,27llR-1,2711R-1, 27llR-1,2711R-1,61HR-1,61HR-1,61HR-1,61

Si

2.772.762.752.762.772.752.762.832.852.792.832.822.852.802.842.832.752.822.852.872.852.842.852.872.822.852.852.842.842.842.852.842.812.822.842.842.852.822.812.802.792.802.802.812.762.782.822.852.832.812.812.842.852.832.842.85

2.372.092.092.152.372.542.172.552.582.182.64

Al

1.261.27.26.27.251.281.26.171.151.20.16.17.15.20.16.16.24.17.15.13.14.16.15.12.18.14.14.15.16.15.14.15.18.17.17.15.15.18.20.21.23.21.21.20.25.23.20.15.17.19.19.15.14.16.15.14

.62

.90

.91

.84

.62

.46

.81

.45

.41

.81

.35

Fe

0.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.01

0.020.020.020.020.030.010.020.010.010.020.01

Ca

0.210.210.230.210.220.220.210.170.160.210.160.160.150.190.160.160.250.170.140.130.160.160.160.140.180.160.140.150.160.160.150.160.180.170.180.160.140.190.170.170.180.180.160.160.200.180.150.160.170.200.180.180.150.160.170.15

0.630.890.900.840.630.450.820.440.420.810.33

Na

0.660.690.680.690.670.670.680.750.770.740.770.780.780.740.780.780.710.780.780.800.770.780.770.790.770.770.790.790.780.780.790.780.770.780.730.780.780.750.740.760.710.720.760.770.730.750.750.780.770.750.760.770.790.790.770.79

0.360.100.090.160.360.530.180.530.560.180.64

K

0.040.040.040.040.040.040.040.040.050.040.040.050.050.060.050.050.030.040.050.060.050.040.050.050.040.050.050.050.050.050.050.050.040.040.040.050.050.040.050.050.050.050.050.050.050.050.050.050.040.040.040.040.050.050.050.05

0.010.010.000.000.010.010.000.020.020.010.04

Total

4.954.974.974.974.964.974.964.984.984.994.984.994.994.994.994.995.005.004.984.994.984.984.984.984.994.984.994.994.994.994.994.995.005.004.964.994.984.994.995.004.974.975.005.004.995.004.984.984.994.994.994.994.995.004.994.99

5.005.015.015.015.005.005.015.005.005.005.02

An(%)

2322242223242318162117161619161625171513161616151816151516161516181719161519181719191716211916161720181815161715

6389908464458245428132

Ab(%)

7273727472727378797579797975797972798081798079807879808079798079777977798077777876767878757679797976777880797880

361091636541854561863

Or(%)

44445545545556553556545545555555444554655555556544445555

11001102214

124

SEDIMENTOLOGY, CHEMISTRY, AND AGE OF MIOCENE BLUE TUFF

Table 1 (continued).

Analysisno.

68.69.70.71.72.73.74.75.76.77.78.79.80.81.82.S3.84.85.86.87.88.89.90.91.92.93.94.95.96.97.98.99.100.101.102.103.104.105.106.107.108.109.110.111.112.113.114.115.116.117.118.119.120.121.122.123.124.125.126.127.128.129.130.131.132.133.134.135.136.137.138.139.140.141.

Hole, section,interval (cm)

11R-1,6112R-3, 112R-3, 112R-3, 112R-3, 112R-3, 116R-1, 1016R-1, 1016R-1, 1016R-1, 1016R-1, 1016R-1, 1016R-1, 1016R-1, 1016R-1, 1017R-1, 3017R-l,3017R-l,3017R-17R-17R-17R-17R-17R-17R-17R-17R-17R-17R-18R-18R-18R-18R-18R-18R-18R-18R-

,30.30.30.58.58.58,58,58,58,58,58,21,21.21,21.21,21.21.21

18R-3,13818R-3,13818R-3, 13818R-3, 13818R-3, 13818R-3, 13818R-3, 13818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 3818R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4, 9018R-4,11618R-4,11618R-4,11618R-4,11618R-4,11618R-4, 116

Si

2.162.572.372.432.562.652.552.592.572.552.502.542.452.492.432.612.492.522.452.582.552.702.382.422.582.352.532.642.702.302.562.292.712.712.552.552.432.622.642.752.672.712.702.662.462.762.772.732.692.792.692.292.732.492.752.532.732.712.752.542.682.662.702.722.742.772.772.642.672.712.682.742.672.70

Al

.82

.43

.62

.56

.45

.36

.46

.42

.44

.46

.50

.46

.56

.52

.57

.39

.50

.48

.55

.43

.45

.31

.61

.57

.43

.65

.48

.37

.31

.70

.45

.70

.30

.30

.46

.46

.57

.39

.37

.27

.34

.30

.31

.34

.53

.24

.22

.27

.30

.21

.30

.69

.26

.50

.24

.45

.26

.29

.24

.45

.31

.33

.29

.27

.26

.23

.23

.36

.34

.31

.33

.27

.33

.32

Fe

0.030.010.020.020.020.010.010.010.010.010.020.010.010.020.020.020.020.020.020.010.020.010.020.020.010.030.010.010.010.020.010.030.010.010.010.010.020.010.010.010.010.010.010.010.020.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.010.01

Ca

0.840.420.610.570.430.340.440.400.410.430.470.440.530.490.550.370.510.460.550.410.440.280.620.620.400.640.450.340.270.660.410.700.280.280.440.430.550.360.340.230.330.290.290.320.540.250.240.280.320.220.310.710.280.500.270.470.280.300.260.450.320.340.310.300.260.240.250.360.330.290.310.260.330.31

Na

0.150.550.370.400.510.590.480.530.530.500.480.500.420.460.400.590.460.480.410.520.500.670.340.330.520.320.490.580.640.300.520.290.630.650.480.490.410.560.580.690.580.630.620.610.430.690.700.660.630.710.640.280.650.470.670.520.660.640.680.520.630.610.630.650.680.680.660.600.580.590.600.650.610.57

K

0.000.020.010.020.030.010.030.040.030.030.030.030.020.020.020.030.010.020.020.030.020.040.020.000.030.010.020.030.050.010.020.010.040.030.020.020.010.030.030.040.040.040.040.040.010.050.050.040.040.050.040.010.040.020.050.020.040.040.050.020.030.030.030.040.040.040.040.020.040.040.040.050.040.04

Total

5.005.005.015.004.994.974.974.994.984.985.004.984.994.994.995.004.994.994.994.984.985.004.994.964.984.994.984.984.985.004.985.014.974.984.974.974.994.984.984.984.974.984.974.994.994.994.984.984.984.984.995.004.984.994.985.004.994.984.995.004.994.984.984.984.994.984.974.994.964.954.974.984.984.95

An(%)

8543615744364641424548455550563852485643462864654266473628684370292946465737362435303033552525293223317129512747283026453335323027252636353233273434

Ab(%)

1555374053635155555249514347426046504255526834355533516167315429666851524259617261666563437071676472652867476851676669536462656669706961626463686262

Or(%)

02123134333322231223242031245131433313344444155445414252445233444453444544

125

D. A. BARNES, P. THY, P. RENNE

Table 1 (continued).

Analysisno.

142.143.144.145.146.147.148.149.150.151.152.153.154.155.156.157.I5S.L59.160.161.162.163.164.165.166.167.I6S.169.170.171.172.173.174.

Hole, section,interval (cm)

18R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11618R-4, 11619R-1, 10519R-1, 10519R-1, 10519R-1, 10519R-1, 10519R-1, 10522R-1, 11822R-1, 11822R-1,11822R-1, 11822R-1, 11822R-1, 11822R-1, 11822R-1, 11822R-1, 11822R-1, 11822R-1, 11822R-1,11822R-1, 11822R-1,118

Si

2.712.722.712.752.682.712.752.232.682.692.712.692.692.172.132.242.222.212.192.242.242.472.232.482.432.302.322.512.322.182.192.232.27

Al

.29

.29

.31

.26

.33

.30

.26

.78

.33

.33

.31

.32

.33

.82

.85

.74

.77

.79

.80

.75

.75

.53

.74

.51

.57

.69

.68

.48

.66

.80

.79

.76

.71

Fe

0.010.010.010.010.010.010.010.010.010.010.010.010.010.030.030.030.030.030.020.030.030.020.020.040.020.020.020.020.020.020.030.030.03

Ca

0.300.280.290.250.300.280.240.720.310.300.290.300.290.820.860.760.770.760.790.740.740.520.760.490.520.680.660.490.670.790.790.750.71

Na

0.630.620.600.670.600.630.670.260.600.600.620.620.600.170.140.220.210.200.190.230.230.460.230.460.440.310.320.490.310.220.210.220.27

K

0.040.040.040.050.040.050.050.010.040.040.040.040.040.010.000.010.010.010.010.010.010.020.010.030.020.010.010.020.010.010.010.010.01

Total

4.984.974.964.994.974.984.985.014.974.974.974.984.965.015.015.005.004.995.005.005.005.015.005.005.015.015.005.005.005.025.025.005.00

An(%)

312931253229257333323031318286767878807676527650536867496878787772

Ab(%)

656664696466702663646565641714232121192424462347453132503121212327

Or(%)

455545514444410111111213211211111

Note: The sample numbers are sequential from the top to the bottom of each hole. See text for description of sampleacquisition parameters. Elemental data are presented as atomic fractions normalized to eight oxygens. Anorthite(An), albite (Ab), and orthoclase (Or), mole% are also presented.

probably at bathyal depths or deeper. Maximum water depth for thiseruption is not constrained.

The blue tuff at Site 796 consists of more heterogeneous tuf-faceous detritus including abundant bubble wall glass shards formedby probable shallow-marine or subaerial magmatic eruption. Overallfacies characteristics suggest that blue tuffs at this location weredeposited by a series of density flows generated on an unstabledepositional slope. There is no clear evidence to indicate that themagmatic eruption that produced the majority of pyrogenic detritusin blue tuffs was contemporaneous with downslope reworking bydensity flows. The admixture of accidental fragments (material otherthan comagmatic pyroclasts), including glauconitic pellets, suggestsmore protracted reworking prior to redeposition of blue tuffs atSite 796 compared to Site 794.

Pyrogenic Crystal Chemistry

Microprobe analysis indicates that the average An mole% fromPlagioclase feldspar in blue tuffs from Site 794 is 18± 3. Thishomogeneous composition suggests that the Plagioclase crystal popu-lation is essentially cogenetic. The low An mole % of these feldsparsis in strong contrast to the composition of Plagioclase in intrusiverocks recovered from acoustic basement at this site (An mole% =45-90). It is unlikely that the blue tuff at Site 794 is genetically relatedto subjacent intrusives.

The average An mole% of Plagioclase from blue tuff samples atSite 796 is 36± 12. These plagioclases are clearly a mixture of at leasttwo populations. The dominant population with average An% of30 ± 4 is interpreted as the probable comagmatic Plagioclase popula-tion, distinct in composition from Plagioclase at Site 794. The oligo-

clase to andesine composition of these Plagioclase is consistent withderivation from andesitic magmas.

40 A r / 39 A r Dating

The results of 40Ar/39Ar dating of Plagioclase, biotite and horn-blende crystals from blue tuffs at Sites 794 and 796 are consistentwith the biochronology reported by Tamaki, Pisciotto, Allan, et al.(1990). The ages for multiple crystal analysis in two samples fromSite 794 are concordant and average 14.86 Ma. This age is consistentwith ages based on sediment-accumulation rate extrapolation (Tamaki,Pisciotto, Allan, et al., 1990) and corroborates initial estimates of theminimum age of basement at this site at 16.2 Ma (Tamaki, Pisciotto,Allan, et al., 1990).

The results of multiple crystal analyses, including Plagioclase, biotite,and hornblende, from one sample from Site 796 are more complex. Threeconcordant Plagioclase crystal ages constrain the maximum age of bluetuffs at 7.61 Ma, generally consistent with interpretations based onbiostratigraphy and extrapolations of sediment accumulation rates(Tamaki, Pisciotto, Allan, et al., 1990). Other mineral ages are incompat-ible with biostratigraphic data and are considered either older, inheriteddetrital ages or younger ages dating alteration.

The observed age of blue tuffs recovered during Leg 127 drillingin the Japan Sea are comparable to 40Ar/39Ar ages of trachyandesiteto basalt samples dredged from the Yamato Seamount chain andreported by Kaneoka et al, (1990). The composition of Plagioclasephenocrysts in these generally alkaline rocks from seamounts isprobably comparable to Plagioclase from blue tuffs (Anlo_5O). Mag-matism related to the emplacement of the Yamato Seamount chainmay be related to blue tuff magmatism.

126

SEDIMENTOLOGY, CHEMISTRY, AND AGE OF MIOCENE BLUE TUFF

Table 2. Results of 40Ar/39Ar age dating.3

Sampleno.

794B-21R-5, 112-115 cm4402.014402.024402.034402.044402.054402.064402.074402.08

Mean± SEM

794B-21R-1, 69-72 cm4404.014404.024404.034404.044404.054404.064404.07

Mean± SEM

796B-18R-4, 90-93b4403.01c4403.02b4403.03b4403.04c4403.05c4403.064403.07

Mean ± SEM (b samples)Mean ± SEM (c samples)

Mineral

PlagioclasePlagioclasePlagioclasePlagioclasePlagioclasePlagioclasePlagioclasePlagioclase

PlagioclasePlagioclasePlagioclasePlagioclasePlagioclasePlagioclasePlagioclase

PlagioclasePlagioclasePlagioclasePlagioclase

BiotiteBiotite

Hornblende

40Ar(nA)

0.55520.26570.33200.23610.45120.18730.22880.3316

0.31170.36220.37250.22500.79500.42480.3925

1.03450.74120.25812.94621.79580.55400.6504

37Ar/39Ar

4.47181.79791.85673.30812.93052.33451.58291.9234

2.28542.21762.07472.00861.68202.69242.9322

3.28132.90573.29183.18260.04220.01469.3093

38Ar/39Ar

0.01140.01050.01090.01130.01180.01130.01100.0113

0.01070.01130.01140.01170.01150.01140.0114

0.01230.01220.01220.01250.01570.01570.0427

40Ar/39Ar

0.39620.35560.36000.42360.72500.33370.33330.3331

0.41320.33090.46880.34960.33560.35070.3615

1.04110.56790.47211.59660.68630.14060.2835

40Ar(%)

78.990.076.664.235.094.780.886.8

79.089.052.481.282.186.779.8

16.216.829.1

8.514.669.966.6

Age(Ma)

16.1316.4614.2114.0313.0716.2913.8814.8914.87

16.8215.1712.6514.6214.1915.6614.8714.85

8.714.947.117.005.155.079.797.615.05

± σ

0.510.820.681.051.031.010.850.630.16

0.790.530.820.880.270.520.550.18

0.530.570.720.610.280.080.500.560.31

d Ar isotopic data are derived from total laser fusion. Plagioclase grains from Site 794B were degassed at 0.3 W laser power outputbefore fusion to help remove atmospheric Ar contamination. These data are corrected for mass discrimination, radioactive decaynucleogenic interferences, procedural blanks, and atmospheric contamination." Ar, Ar, and Ar as used to refer to the proportionof these isotopes derived by neutron activation of Ca, Cl, and K, respectively. Reported errors reflect uncertainties accrued fromall corrections plus regression of isotopic data. Errors in the apparent ages do not reflect uncertainty in the age of the standard.SEM = standard error of the mean.

b Mean ± SEM.c Mean ± SEM.

Table 3. Results of 87Sr/86Sr isotopic compositionanalysis. See text for description of acquisitionparameters.

Sample

794B-21R-6, 16-19 cm794B-21R-6, 69-72 cm794B-23R-CC, 10-13 cm796B-18R-4, 90-93 cm796B-18R-4, 131-134 cmStandard NBS-987

An(%)

15.7-25.214.9-20.3

NA25.2^5.5

NANA

87Sr/86Sr

0.70451210.704390 ±0.709547 ±0.704464 ±0.704199 ±0.710200 ±

132313202013

Note: Errors are 2 sigma uncertainties in observed run. Noaccuracy is better than ± 2 from experience with replicateruns. NA = not analyzed.

8 7Sr/8 6Sr Isotopic Composition

Limited data on the 87Sr/ 86Sr isotopic composition of Plagioclasecrystal separates from blue tuff samples indicates consistentratios ranging from 0.7045 to 0.7042. One anomalous value of0.7095 is discounted as attributable to probable contamination byclay alteration products. These data suggest an origin for blue tuffsat both Sites 794 and 796 from mantle-generated magmas in anisland arc setting. Strontium isotope ratios are inconsistent with thederivation of magmas from more primitive mantle sources analo-gous to oceanic spreading centers.

The isotope geochemistry of basement rocks in the Yamato basin(Cousens and Allen, this volume) indicates virtually indistinguish-able strontium isotopic signature for Japan arc rocks compared toJapan Sea basement. The range of isotopic composition of thesesuites of igneous rocks also encompasses the isotopic compositionof blue tuffs. As such, little can be inferred regarding the petrologicaffinity of blue tuffs relative to regional trends in strontium isotopegeochemistry of arc and backarc rocks in the Japan Sea region. Thestrontium isotope geochemistry of Yamato Seamount chain volcanicrocks (0.70357-0.70388; Kaneoka et al., 1990) suggests 87Sr/86Srisotopic composition significantly lower than that observed forPlagioclase grains in blue tuffs. The inferred relationship betweenYamato Seamount chain volcanic rocks and blue tuffs, suggested bysimilar age, composition, and geographic location is not supportedby the disparity in strontium isotope chemistry, however.

Correlation of Blue Tuffs in the Yamato and Japan Basins

On the basis of sedimentologic facies characteristics, Plagioclasecomposition, and age, distinctive, coarse-grained, Miocene blue tuffsrecovered at Sites 794 and 796 are not closely related. Sedimentologiccharacteristics suggest generally similar depositional conditions forthe two successions. Both sequences were deposited by gravity flowsin probable bathyal or deeper water depths. But the initial magmatismresponsible for the formation of tuffaceous debris at Site 794 wasprobably submarine phreatomagmatic compared with subaerial orshallow submarine magmatic at Site 796.

127

D. A. BARNES, P. THY, P. RENNE

The age of blue tuffs at Site 794 is 7 m.y. older than at Site 796.Although these units were clearly not deposited as a result of contempo-raneous magmatism, further differences in Plagioclase crystal chemistryand sedimentology indicate that there is no clear genetic relationshipbetween these units. The initial identification and correlation of coarse-grained, blue to blue-gray volcaniclastics (Tamaki, Pisciotto, Allan, et al.,1990) was made on the basis of gross physical appearance and thesimilarity of associated strata. The gross similarity in physical appearanceof the blue tuff is mainly a function of the similarity of alteration productsof the dominantly glassy vitroclast-rich strata to blue-colored, clay altera-tion products. Also, the generally massive to stratified and graded struc-tures in these successions are a function of crudely similar, density flowdepositional mechanisms.

Lithostratigraphic correlation of blue tuffs and the implication thatsuch strata are indicative of magmatism associated with backarcspreading in the Japan Sea region are considered invalid by the authors.Rather, Miocene blue to blue-gray bentonitic tuff and tuffaceoussandstones of intermediate composition are a product of a protracted,6- to 8-m.y., period of subduction-related submarine to subaerialvolcanism in the Japan Sea backarc basin.

REFERENCES

Boles, J. R., 1982. Active albitization of Plagioclase, Gulf Coast Tertiary. Am.J.Sci., 282:165-180.

Cass, R.A.F., and Wright, J. V., 1987. Volcanic Successions—Modern andAncient: London (Allen and Unwin).

Cebula, G. T., Kunk, M., Mehnert, H. H., Naeser, C. W., Obradovich, J. D.,and Sutter, J. F., 1986. The Fish Canyon Tuff: a potential standard for the40Ar-39Ar and fission-track methods. Terra Cognita, 6:139-140.

Deino, A. L., and Potts, R., 1990. Single-Crystal 40Ar/39Ar dating of theOlargesaile Formation, Rift. J. Geophys. Res., 95:8453-8470.

Faure, G., 1986. Principles of Isotope Geochemistry (2nd ed.): New York (Wiley).Fisher, R. V., 1984. Submarine volcaniclastic rocks. In Kokelaar, B. P., and

Howells, M. F. (Eds.), Marginal Basin Geology: Volcanic and Associated

Sedimentary and Tectonic Processes in Modern and Ancient MarginalBasins. Geol. Soc. Spec. Publ. London, 16:5-27.

Fisher, R. V, and Schmincke, H. U., 1984. PyroclasticRocks: Berlin (Springer Verlag).Fiske, R. S., 1969. Recognition and significance of pumice in marine pyro-

clastic rocks. Geol Soc. Am. Bull, 80:1-8.Fiske, R. S., and Matsuda, T, 1964. Submarine equivalents of ash flows in the

Tokiwa Formation, Japan. Am. J. Sci., 262:76-106.Gill, J., Torssander, P., Lapiere, H., Taylor, R., Kaiho, K., Koyama, M., Kusukabe,

M., Aitchison, J., Cisowski, S., Dadey, K., Fujioka, K., Klaus, A., Lovell, M.,Marsaglia, K., Pezard, P., Taylor, B., and Tazake, K., 1990. Explosive deepwater basalt in the Sumisu Backarc Rift. Sci. Rep., 248:1214-1217.

Heiken, G. H., 1972. Morphology and petrology of volcanic ashes. Geol Soc.Am. Bull., 83:1961-1988.

, 1974. An Atlas of Volcanic Ash. Smithsonian Contrib. Earth Sci., 12.Kaneoka, I., Notsu, K., Takigami, Y, Fujioka, K., and Sakai, H., 1990.

Constraints on the evolution of the Japan Sea based on 40Ar/39Ar ages andSr isotopic ratios for volcanic rocks of the Yamato Seamount chain in theJapan Sea. Earth Planet. Sci. Lett., 97:211-225.

Nier, A. O., 1950. A redetermination of the relative abundances of the isotopes ofcarbon, nitrogen, oxygen, argon, and potassium. Phys. Rev., 77:789-793.

Renne, P. R., Fulford, M. M., and Busby-Spera, C. J., 1991. High resolution40Ar/39Ar chronostratigraphy of the Late Cretaceous El Gallo Formation,Baja California del Norte, Mexico. Geophys. Res. Lett, 18:459-462.

Tamaki, K., Pisciotto, K., Allan, J., et al, 1990. Proc. ODP, Init. Repts., 127:College Station, TX (Ocean Drilling Program).

Wohletz, K. H., 1983. Mechanisms of hydrovolcanic pyroclast formation:grain-size, scanning electron microscopy and experimental studies. /.Volcanol. Geotherm. Res., 17:31-63.

Date of initial receipt: 22 May 1991Date of acceptance: 20 January 1992Ms 127/128B-185

128

SEDIMENTOLOGY, CHEMISTRY, AND AGE OF MIOCENE BLUE TUFF

120

125

130

135

140 -

Or-

10

15

20 -

25 ~

Plate 1. 1. Well defined planar stratification in the overall normally graded, medium-grained blue tuff in Section 127-796B-18R-3. Variation in color corresponds tochanges in dominant clast type as a result of density segregation. See text for discussion. 2. Stratification in fine-grained blue tuff defined by normal size-grading anddensity segregation in Section 127-796B-18R-3. Darker colored layers are more crystal- and lithic-rich compared with lighter colored, vitric fragment-rich layers

129

D. A. BARNES, P. THY, P. RENNE

Pi;. J

l‰MiP&iB à ̂ m i l

I^ • • - • • < i l 1" ' 1

; «^ ^ • " . - * -

••• • • • • • .

. • •

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J

V 'fi

I

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Plate 2. 1. Long-tube pumice from Sample 127-794B-21R-3, 56-58 cm. Scale bar is 0.25 mm. 2. Vesicular hydroclast from Sample 127-794B-23R-1, 23-25cm. Note that the margins of the grain transect small vesicles (smaller than the grain). Scale bar is 0.25 mm. 3. Typical vesicular hyaloclasts from Sample127-794B-23R-CC, 10-12 cm. Scale bar is 0.25 mm. 4. The dominant clast types in the blue tuff at Site 794 are long-tube pumice (upper left), vesicular hydroclast(center), and a euhedral Plagioclase crystal (lower right) in Sample 127-794B-21R-6, 16-18 cm. Scale bar is 0.35 mm. 5. Delicate bubble-wall shards and otherfragmented material in Sample 127-796B-18R-1, 21-23 cm. Scale bar is 0.25 mm. 6. Several bubble-wall shards (center) with vesicle size (based on radius ofcurvature) significantly larger than the shards themselves in Sample 127-796B-18R-1, 21-23 cm. Other common clast types include euhedral Plagioclase crystals(upper left) and siliceous claystone rip-up clast (lower right). Scale bar is 0.25 mm.

130