andre pouclet, kantaro fujioka, jacques charvet, and jean
TRANSCRIPT
18. PETROGRAPHY AND GEOCHEMISTRY OF VOLCANIC ASH LAYERS FROM LEG 87A,NANKAI TROUGH (SOUTH JAPAN)1
Andre Pouclet, Kantaro Fujioka, Jacques Charvet, and Jean-Paul Cadet2
ABSTRACT
In the Leg 87A holes, 45 ash layers were sampled in Recent to upper Pliocene strata. The main volcanogenic depositscame from single eruptions or subcontemporaneous eruptions of cognate volcanoes. Some of them are mixed ashes pro-duced from multiple eruptions and accumulated in reworked sediments. The petrographic and geochemical patterns in-dicate rhyolitic and dacitic compositions; andesitic glasses are scarce. We infer a magmatic affinity with calc-alkalinesources and a possible origin from the volcanic arc of southwestern Japan. A few samples may originate from the alka-line volcanism of southwestern Japan or the area south of Korea and the Sea of Japan.
INTRODUCTION
Leg 87A was devoted to the study of subduction-re-lated characteristics and processes operating in the NankaiTrough, off southwestern Japan. There, the trench fill isaccreted and the large-scale geometry of the accretion-ary prism is well defined by the seismic profiles. Twodrill sites were occupied (Fig. 1). Site 582 is located inthe undeformed sediments filling the trench, about 2 kmsouth of the deformation front (beginning of the pro-tothrust zone); it offers a lithologic reference. Site 583 islocated on the lowest structural terrace of the landwardslope, at the toe of the basal thrust (Karig et al., 1983).
In the different holes drilled at both sites, several vol-canic ash layers were crossed and sampled. This chapterreviews the results of the petrographic and geochemicalstudy of those layers (Fig. 2).
ASH LAYERS AT LEG 87A SITES
Site 582Two main lithologic units were recognized at Site 582
(Fig. 3). Unit 1 is approximately 560 m of trench-fill tur-bidites of the Recent to early Quaternary times. Unit 2,drilled from 566 to 749.4 m sub-bottom, consists of hemi-pelagic mud of the Shikoku Basin sequence of the earlyPleistocene to late Pliocene times. Most likely, the ap-proximate age at the base of the trench fill is about 0.86Ma (site chapter, Site 582, this volume).
Volcanogenic sediment is common throughout the en-tire sequence and increases in the upper part of Unit 1and in Unit 2, as reflected by the number of ash layers.
Unit 1 contains i3 ash layers, 1-25 cm thick. The twoupper layers are the thickest (15-25 cm, Fig. 2). They in-clude up to 25% of silt-size to sand-size glass shards("bubble-wall" type and pumice type), generally color-less to pale brown. A shard content of 5 to 15%, includ-
Kagami, H., Karig, D. E., Coulbourn, W. T., et al., Init. Repts. DSDP, 87: Washing-ton (US. Govt. Printing Office).
2 Addresses: (Pouclet) Département des Sciences de la Terre, Université d'Orléans, 45046Orleans, Cedex, France; (Fujioka) Ocean Research Institute, University of Tokyo, Minami-dai, Nakano-ku, Tokyo 164, Japan; (Charvet and Cadet) Département des Sciences de laTerre, Université d'Orleans, 45046 Orleans Cedex, France.
30°N
130° 140°E
Figure 1. Geographic and structural setting of Leg 87A (Sites 582 and583). Quaternary volcanoes and tectonic lines from Geological At-las of Japan (Geological Survey of Japan, 1982). Open triangles =tholeiitic basalt; solid triangles = andesites; solid squares = da-cite or rhyolite; open squares = alkaline rocks; stars = large ex-plosive calderas.
ing micropumice and grains (lithic fragments) of vol-canic glass and clastic-free minerals, is common in manyother terrigenous layers. A concentration in the upperstrata (0-20 m sub-bottom) may be late Pleistocene toHolocene (less than 0.21 Ma, according to biostratigraphicdata). A second concentration, located above the mid-dle of Unit 1 (220-230 m sub-bottom), is middle to earlyPleistocene, below the 0.44-Ma biostratigraphic marker.
Unit 2 contains 16 ash layers, 1-10 cm thick. Theycorrespond to an important phase of early Quaternaryvolcanic activity.
Holes 583B, 583C, and 583DDrilled just at the toe of the basal thrust, which was
intercepted near 155 m sub-bottom (site chapter, Site 583,this volume), Hole 583B, 583C, and 583D contain sparselayers of dispersed ashes in hemipelagic muds and silts.Two upper Quaternary ash layers in Hole 583D (Fig. 2)occur above the thrust fault.
695
A. POUCLET, K. FUJIOKA, J. CHARVET, J.-P. CADET
1.21 -1.23-2.25-3.26-
28.85-
58.81 -119.78-184.49-220.17 -220.22-224.40-226.47 -234.27 -251.60-261.88-442.10-557.93-566.86 -569 73 -588.15-589.13 -595.72-
608.68 -615.78 -624.35 -626.08-637.35-645.59 -
654.87 -655.31 -711.73-712.28-712.48-
1.55-15.42-29.29 -3 1 . 1 1 -
3.20-3.27-6.68-
27.70 -27.78-29.40 -34.29 -37.71 -37.88-
128.10-258.65 -
340.00 -34625 -356.23 -
Age
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3
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ù
£3
σ
c
£CO
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9>o
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£
a
Lithology
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N.D.
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N.D.N.D.
rmi
N.D.N.D.
N.D.
N.D.N.D.
N.D.Glass shards
N.D.
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N.D.
C::v:.vts?sl—N.D.
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Core-Section
level in cm)
Hole 582
1-1, 121-1231-1, 123-1251-2,751-3, 26
3,CC
Hole 582B
2-1.61-628-3, 2815-1, 89-9018-6, 17-1818-6, 22-2319-2, 70-7219-4, 77-7820-2, 87-8822-1, 5033-4,88-10042-3, 6054-1, 143-14455-1,8655-3, 7357-2, 13557-3, 83-8458-1, 82-8358-4, 27-2859-3, 118-12060-2, 18-2061-1,6561-2,8862-3, 10563-2, 11 9
64-2, 131-13270-1, 11370-2, 18-1970-2, 38-39
Hole 583
1,CC (5-6)3-1,424-5, 29-304-6,61-62
Hole 583A
1-3, 20-211-3, 27-282-2, 1186-3, 70-716-3, 78-797-1, 408-1,29-308-3,71-738-3, 88-89
Hole 583D
9-3, 140-14122-7, 35
Hole 583G
4-3, 1405-1,956-1, 123
m
Sam
ple
1
2—
—
3—
4
5
67
89—
——
10-
_11
12131415__
—
-
16—
17
18
19—
20
21
22
23_
24
25—
2627
28
29
-
-
-
Gla
ss (
8390
359540
7575_
959555
70_
2515—
2540
_
-
602098—
702555
90—
—
100
/u
858560—
4295
9025
—
85
75
ysta
lsFr
ee c
r
+ + +
+ + +++ + +
++ + +
+ + +
+ ++ +
+ ++ +
+ ++
Mineral
(mafics)
Hy,Ho,Au,PyHy,Au
Bi.Py
Hy,Au,Ho
Hy,Au,Ho,Bi
Hy.AuHy
HyHy,Au
Hy,AuHy,AuGr,Ho,Hy,Au
Hy,Au,Ho
Hy.AuHy.Au.Ho
Hy,AuHy,Au
Gr.Ho.HyHo.Hy
Hy,AuHy,Au,HoHy.Au
Hy.Au
Volcanic glass
bw.pmbw.pm
bw
bw
bwbw.pmpmpm
pm
pm
pm.bwpm.bw
pm.bwpm.bw
pmpm.bw
bw.pm
pmpm
pm
pm.bw
pmpm
pmpm
pm
pm.bw
cl.p.brcl.p.br
cl
cl
clpbr.clcl.pbrcl.br
pbr
clcl
clcl
clcl
cl.pbr
cl.pbr
clcl
cl
cl
clpbr
cl.pbr
cl.pbr
Max.grainsize(mm)
0.6
0.3
0.4
0.1
0.60.2
0.30.2
0.2
0.2
0.4
0.30.15
0.30.2
0.3
0.80.5
0.50.7
0.4
0.3
0.5
0.50.5
0.6
Refractive index
Range
1.508-1.5121.505-1.512
1.499-1.501
1.499-1.5001.499-1.5001.511-1.5121.511-1.5171.507-1.515
1.520-1.522
1.503-1.5051.504-1.5061.499-1.5011.500-1.5031.501-1.503
1.503-1.5051.499-1.501
1.508 1.512
1.509-1.513
1.499-1.5011.499 1.500
1.500-1.5021.500-1.503
1.510-1.5121.513-1.5171.506-1.508
1.514 1.518
Meanrange
1.508-1.512
1.512-1.5151.507-1.509
1.501-1.502
1.510-1.512
1.515-1.517
Mode
1.500
1.521
1.504
1.500
1.500
1.5011.502
1.511
1.507
SiO 2
(wt.%)<Σ = 94)
70.470.0-70.7
72.8-73.8
59.073.773.970.7 70.168.2 72.2(64.5) 71.0-71.3
64.3
71.4-73.873.1-74.2
73.8-73.9
69.7-72.271.8-72.5
(66.9) 70.1-70.3
69.5-70.1(61.9) (68.6)
73.3-73.5(62.5) 72.4-73.5
73.3-73.9
65.371.2-71.8
(58.0) (66.3)
Notes
15 cm thick, like K-Ah25 cm thick, like K-AhSilty to sandy quartz, 50%Sandy volcanic grainsSandy, quartz mica
3 cm thick, like At
Basalt, terrigenous grains4 cm6 cm1 cmHeteromagmaticGlass poor
Silty, alkaline-rich glass
3 cm10 cm, low K
1 cm, low K
Heteromagmatic5 cm, high K
Heteromagmatic, like K-Ah
3 cm, like K-AhHeteromagmatic
41 cm, large ash layerwith cross-bedding
28 cm, large ash layer;Ho-rich, low-Na glass
8 cm1 cm, alkaline-rich glass
4 cm, alkaline-rich glass
1 cm, white ash layer0.7 cm, light gray15 cm
Figure 2. Petrographic characteristics of tephra layers, Leg 87A.
Holes 583, 583E, 583F, and 583G
These holes were drilled about 500 m farther land-ward than Holes 583B, 583C, and 583D. Three ash lay-ers (3-5 cm thick) and many volcanic glass grains (10-15%) are present in hemipelagic sandy mud of the up-per part of this composite hole (upper Pleistocene). Layer19, in Core 583-1 (Fig. 3), is located above the 0.21-Ma
marker; its approximate age is 0.10 to 0.15 Ma, if we as-sume a sediment accumulation rate of 220 m/Ma (de-duced from the Hole 583A sequence and the lack of up-per Quaternary sediments). Layers 20 and 21, in Core583-4, should be dated at about 0.25 Ma, if a sedimentaccumulation rate of 330 m/Ma is assumed. Three ashlayers in the lower part (Hole 583G) may be lowerPleistocene.
696
Hole 583A Holes 583,583E,583F Holes 583B,583C,583D Holes 582,582A,582B
Hole
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Slope deposits andtrench turbiditesMud to silty sand
Hemipelagic mud
400 J
Figure 3. Simplified stratigraphic columns. Arrows and sample numbers identify ash layers; dashed arrows locate dispersed volcanic sediment. For corresponding DSDP sample number designa-tions, see Figure 2.
A. POUCLET, K. FUJIOKA, J. CHARVET, J.-P. CADET
Table 1. Selected microprobe analyses (in wt.%) of glass shards, Leg 87A.
Hole
Core-SectionInterval in cmOur sample number
SiO2
TiO2A12O3
EFeOMnOMgOCaONa2OK2OCr2O3
Total
582
1-1121-123
1
70.970.54
12.442.280.060.441.623.792.640.01
94.79
1-1123-125
2
70.980.56
12.642.500.100.431.613.722.800.05
95.39
2-161-62
3
75.650.11
11.991.170.130.130.983.653.050.00
96.86
15-189-90
4
56.031.46
14.4610.550.183.807.462.880.650.00
97.47
15-189-90
4
56.940.95
19.626.630.311.107.934.220.800.03
98.53
18-617-18
5
72.740.25
11.181.040.000.180.993.402.990.04
92.81
18-622-23
6
73.240.26
11.260.920.020.200.993.262.950.02
93.12
19-270-72
7
69.600.40
12.412.030.210.431.653.832.690.08
93.33
19-477-78
8
68.200.50
13.132.290.170.512.124.272.440.00
93.63
582B
19-477-78
8
71.660.17
12.121.320.160.141.403.522.790.00
93.28
20-287-88
9
70.520.40
12.191.760.090.301.283.672.700.00
92.91
20-287-88
9
62.680.72
14.532.300.040.882.073.984.070.05
91.32
54-1143-144
10
65.440.65
14.643.270.130.621.963.925.010.03
95.67
57-383-84
11
72.830.32
11.901.240.030.331.423.872.660.03
94.63
58-182-83
12
73.170.36
10.851.080.120.271.203.312.310.00
92.67
58-427-28
13
73.210.20
11.690.940.100.211.243.672.950.00
94.21
Hole 583A
Located 400 m farther landward from Holes 583, 583E,583F, and 583G, this hole cut into the younger sediments,which were not present in other holes. In this sequence,the time marker of 0.21 Ma is near the end of the hole,at 47 m. Above it, seven ash layers (1-41 cm thick) arelate Quaternary to Recent. A very large deposit (theyoungest of this site) is in Core 1 (Plate 1). This last lay-er has an estimated age of 14,000 yr., if we assume asediment accumulation rate of 220 m/Ma.
Glass Analysis
GEOCHEMISTRY
MethodTwenty-nine samples were geochemically investigated
(ten major elements) using a Jeol (JCXA 733) electron-probe microanalyzer (EPMA) of the Ocean Research In-stitute (Tokyo University), with 15-kV accelerating volt-age, 20-µm average defocused beam diameter, 0.01 -µAsample current, and 10-s count time. Ninety-seven anal-yses were performed on twenty-seven samples. (The anal-yses of Samples 15 and 16 were discarded because oftechnical failure.)
SignificanceTable 1 gives the data from 34 analyses, including the
most different compositions. The total amount of oxidecomponents generally ranges from 92-96% (extremes:91.32 and 98.53%). The deficit is due to hydration ofglass in seawater (Sheidegger et al., 1978). Ninkovitch(1979) demonstrates that hydration in glass shards reach-es a saturation point of 4-5% in 250,000 to 400,000 yr.Our recent studies on samples from DSDP Legs 67 (Ca-det et al., 1982) and 84 (Pouclet et al., 1985) show thathydration occurs more rapidly and leads to a greater con-tent in acidic glass (5-8%) than in basic glass (3-5%).However, the initial water content is unknown, and mag-matic acidic glasses may be richer in H2O (5% for rhyo-litic glass in the Osaka group in Japan; Yoshikawa, 1978).Fujioka and others (1980) found a total ignition loss ofnear 6% in an upper Pliocene acidic glass (DSDP Leg 57).
SiO2 content ranges from 56 to 75.6%. Most analyseshave dacitic to rhyolitic composition (69 < SiO2 < 74).
We analyzed many shards (5-15) from each ash layer.The composition is often homogenous (our Samples 3,6, 9, 11, 12, 13, 18, 20, 24, 28). (See Fig. 2 for DSDPdesignations of samples.) In the nomenclature of Huang(1980), they belong to type I, because they originatedfrom single eruptions or subcontemporaneous eruptionsof cognate volcanoes. Some layers are "heteromagmatic":Samples 8, 17, and 19 are dacitic and ryolitic; Sample 21is andesitic and dacitic. Corresponding to type II, theseare mixed ash from frequent, closely spaced eruptionsoriginating from multiple or differentiated chambers, evenwithin a single volcano. Alternatively, as in the case ofSamples 22-23, the ash accumulated in reworked sedi-ments (Plate 1).
Magmatic CompositionsBecause of their alkali and silica contents (Fig. 4),
volcanic glasses show a weak calc-alkalic to tholeiitic af-finity. For the petrographic typology, because the mag-matic composition is not water-free, the sum of the ox-ides is recalculated to 94%, the average for acidic-glassanalyses (Figs. 5, 6).
We registered 77 rhyolitic analyses, most of them hav-ing a medium K2O composition. The highest silica con-tents (SiO2 > 75) occur in Samples 3 (582B-2-1, 61-62cm), 22 (583A-1-3, 20-21 cm), and 23 (583A-1-3, 27-28cm). One ash layer, Sample 18 (582B-70-2, 38-39 cm),has a higher potassium content. This sediment, from theupper Pliocene strata of the Nankai Trough site, is theoldest sampled. Otherwise, two lower Quaternary ashlayers (Samples 12 1582-58-1, 82-83 cm] and 14 [582B-59-3, 118-120 cm]) have low K2O contents. Sample 24(583A-6-3, 70-71 cm), a recent large ash layer, is classi-fied as Na2O-poor; however, it contains abundant freecrystals of hornblende.
Twelve samples have a medium K2O dacitic composi-tion.
Two analyses (linked dots, Figs. 4, 6) of Sample 4(582B-15-1, 89-90 cm) indicate basaltic andesite of tho-leiitic affinity; they consist of terrigenous grains in a darkgray sandy mud. One of them, with high A12O3 and Na2Ocontents and low FeO and MgO contents, may be con-taminated with microliths of labradorite (Fig. 4).
Six analyses of Samples 9, 10, 27 and 29 from Holes582B, 583A, and 583D show andesitic and dacitic speci-mens with very high K2O contents (crosses, Figs. 5, 6).
698
PETROGRAPHY AND GEOCHEMISTRY, VOLCANIC ASH LAYERS, LEG 87A
Table 1. (Continued).
59-3118-120
14
72.810.22
11.071.020.070.221.313.422.400.08
92.62
582B
70-218-19
17
69.200.13
14.290.800.100.251.754.102.740.00
93.36
70-218-19
17
72.090.25
12.881.000.130.271.233.272.800.00
93.92
70-238-39
18
71.930.09
11.941.070.070.020.583.454.590.08
93.82
583D
9-3140-141
29
56.460.95
14.785.820.222.134.443.822.830.08
91.53
9-3140-141
29
66.120.56
14.232.080.100.451.274.164.620.20
93.79
l.CC5-619
67.390.38
15.022.080.130.483.193.862.180.00
94.71
l.CC5-619
71.160.51
12.611.800.050.431.874.222.660.05
95.36
583
4-529-30
20
69.590.39
12.901.970.090.441.603.912.660.06
93.61
4-661-62
21
63.481.01
16.213.990.091.244.973.631.800.05
96.47
4-661-62
21
69.150.59
13.232.560.100.522.383.902.400.00
94.83
1-320-21
22
75.420.11
11.801.020.060.161.133.653.280.12
96.75
1-327-28
23
64.590.85
14.775.420.111.414.293.841.870.04
97.19
1-327-28
23
74.110.12
11.891.100.110.140.973.573.160.00
95.17
583A
6-370-71
24
73.100.22
11.251.040.000.271.392.872.910.00
93.05
8-371-73
27
66.420.72
14.742.260.060.691.804.294.620.00
95.60
8-371-73
27
68.140.77
15.122.630.110.871.924.214.310.00
98.08
8-388-89
28
72.170.29
12.021.770.100.281.373.832.940.00
94.77
8 -
6 -
2-
S
At
IB * •*
' i 2 + 24
Aπ-B An Dc Rh
50 55 60
SiO,
65 70 75
Figure 4. SiO2/(Na2O + K2O) diagram, raw data. Dots represent tho-leiitic to calc-alkaline glass compositions; crosses represent alkalinecompositions. Area of feldspar compositions: L = labradorite, A= andesine, O = oligoclase, At = anorthosite, S = sanidine. IBis the alkaline/subalkaline lava boundary (Irvine and Baragar, 1971)and K, the calc-alkaline/tholeiitic lava boundary (Kuno, 1968). Blocates basalt; An-B, basaltic andesite; An, andesite; Dc, dacite;Rh, rhyolite. Sample 12, low K2O; Sample 18, K2O rich; Sample24, Na2O poor.
75
Figure 5. SiO2/K2O diagram, normalized sum to 94. Symbol conven-tions as in Figure 4; T locates trachyte. Area limits are shown formedium-potassium and high-potassium lavas. Samples 12, 18, and24 as in Figure 4. Sample 14 is K2O poor.
+
OCM
CO
1 0 -
-
8 -
-
6 -
-
4 -
2 -
o-
•
A A
A . *
Δ
Δ
*
^ . * '^ " * . e
A *A A
A
TholeiiticΔ
45 50 55 60
SiO
65 70 75
Figure 6. SiO2/(Na2O + K2O) diagram, normalized sum to 94. Datasymbols and alkaline, calc-alkaline, and tholeiitic boundaries as inFigure 4. On-land data for comparison (table 23 of Isshiki, 1977,average chemical composition of aphyric volcanic rocks of Japan);open triangles = the Izu-Hakone tholeiitic series, solid triangles= the Izu-Hakone and southwestern Japan arc calc-alkaline se-ries, and stars = the southwestern Japan alkaline series.
They may be calc-alkalic glasses containing andesine andoligoclase microliths or may belong to an alkaline mag-matism (mugearite and trachyte); they are not too richin TiO2.
Six samples (6, 7, 13, 22, 27, 28) were selected andanalyzed for trace elements by the instrumental neutronactivation (INA) method, together with 18 samples fromLegs 57 and 87B (Fujioka et al., this volume). Sample27 (583B-8-3, 71-73 cm) is very rich in Th, U, and hy-gromagmaphile-elements (Rb, Zr, Cs, La, Hf, Ta) andmay be related to trachytic magma. The other samplesare rich in these elements and have ratios indicating acalc-alkaline character.
Comparison with On-Land Volcanism
Glasses of rhyolitic to dacitic composition probablyhave their source in Kyushu, a region of large explosiveacidic volcanism (Aso, Aira, Kikai), and in the Izu pe-ninsula and islands (Fig. 1). Because the ash and pumicefall mainly spread eastward, the southwestern Japan vol-canic arc is the most likely origin for aerial tephra. De-trital volcanic fragments dispersed in terrigenous layers
699
A. POUCLET, K. FUJIOKA, J. CHARVET, J.-P. CADET
(large pumice samples and basaltic or andesitic grains)may originate from the volcanic terrane of Izu, consid-ering the southward axial transport of turbidite sedimentsof the trough (for example, Sample 4; see discussion inthe site chapter, Site 582, this volume).
With regard to their major (and some minor) elementchemistry, Leg 87A acidic glass shards may belong tothe calc-alkaline series of southwestern Japan (Fig. 6).We are attempting correlations of tephra layers with on-land volcanism, using mineralogic features, refractive in-dex, and chemical analysis, following Machida's meth-ods (1981). Some recent layers have the same character-istics as the widespread tephras found around the JapanSea: Kikai-Akahoya, 6300 yr., and Aira-Tn, 21,000-22,000 yr. (K-Ah and AT tephra-fall deposits; Machidaand Arai, 1981). Samples 1 and 2 (582-1-1) and 19 (583-1,CC) may correlate with K-Ah, and Sample 3 (582B-2-1)with AT. Unfortunately, if we accept the biostratigraph-ic dates of these layers, these samples seem to be olderthan K-Ah and AT ash. The Recent and Holocene ashlayer of Leg 87A, Core 583A-1 (our Samples 22-23), isheterogeneous (Plate 1) and includes some glass shardsof AT-ash composition.
For trace elements determined from IN A analyses,Samples 6, 7, 13, 22, and 28 belong to the calc-alkalinesouthwestern Japan arc, based on their Th and K con-tents (Sato and Sato, 1981). Sample 27, with its high hy-gromagmaphile-element content, should originate fromalkaline volcanism of the southwestern Japan arc or southof Korea and the Sea of Japan.
Because of the very similar composition of volcanicproducts from this region, future accurate correlationsof volcanic products and ash layers require further de-velopment of analytical techniques like INA (cf., Rose etal., 1982) and more precise dating of ash layers.
CONCLUSIONThe ash layers deposited off southwestern Japan (Nan-
kai Trough area) denote explosive activity from the latePliocene to early Quaternary, middle Quaternary, and latePleistocene to Recent.
Glasses are mainly acidic with a weak calc-alkalineaffinity and a medium K2O content. Characterization ofa few layers is possible. As a whole, they belong to thesouthwestern Japan arc, but some alkaline products mayalso originate from south of the Sea of Japan.
The sparse core coverage and the lack of recovery ofupper Quaternary strata do not allow accurate correla-tion of these sites with recent widespread tephra horizons.
ACKNOWLEDGMENTS
We would like to acknowledge Dr. W. Coulbourn and Ms. J. Bla-keslee for their very useful comments and their corrections of themanuscript.
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Date of Initial Receipt: 25 July 1984Date of Acceptance: 8 February 1985
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35 r
PETROGRAPHY AND GEOCHEMISTRY, VOLCANIC ASH LAYERS, LEG 87A
583A-1-3, 35-60 cm
Plate 1. Lower part of the 41 cm ash layer in Core 583A-1 (Samples 22-23). Note graded bedding and sedimentary rework.
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