6. volcanic rocks and tertiary carbonates dredged · pdf filevolcanic rocks and tertiary...

Rangin, C., Silver, E., von Breymann, M. T., et al., 1990Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 124

6. VOLCANIC ROCKS AND TERTIARY CARBONATES DREDGED FROM THE CAGAYAN RIDGEAND THE SOUTHWEST SULU SEA, PHILIPPINES1

H. R. Kudrass,2 P. Müller,2 H. Kreuzer,2 and W. Weiss2

INTRODUCTION

The Cagayan Ridge divides the Sulu Sea into two basins. The1500-2000 m deep northwest basin is underlain by a several ki-lometer thick layer of partly deformed sediments. In the 4000-5000 m deep southeast basin the assumed oceanic basement iscovered by sediments only 1 km thick (Murauchi et al., 1973;Mascle and Biscarrat, 1978; Rangin, 1989; Hinz et al., this vol-ume). The northeastern part of the Cagayan Ridge faces thesteep slope of the accretionary wedge of the subduction zone ex-tending along the eastern rim of the southeast basin (Figs. 1 and2). The northern section of the ridge is capped by numerous reefislands, where 400-m-thick reef deposits were drilled at the Tub-bataha Island. The central and southern part of the ridge is notas pronounced because some broad seamounts lie along theeastern side of the ridge.

Several hypotheses have been published explaining the originof the Cagayan Ridge and the two basins of the Sulu Sea. Ac-cording to Holloway (1981) the southeast basin of the Sulu Seaoriginated in back-arc spreading coincident in part with theopening of the South China Sea. In this model, the CagayanRidge formed in the Oligocene to mid-Miocene as a volcanic arcsoutheast of the accretionary wedge of Palawan.

Alternatively, the northward subduction of the Celebes Seaunder the Sulu Archipelago is thought to have induced back-arcspreading forming the Sulu Sea (Hamilton, 1979; Mitchell etal., 1986; Rangin, 1989). The Cagayan Ridge originated in theOligocene/early Miocene as a related volcanic arc along thesouthern margin of a fragment of the proto-China mainland.Lee and McCabe (1986) and others suggest entrapped Eoceneoceanic basement as the origin of the southeast basin.

So far only one sample from the Cagayan Ridge was avail-able, consisting of andesitic basalt with a K/Ar age of 14.7 ±0.6 Ma (Kudrass et al., 1986). In this paper we describe dredgesamples from the Cagayan Ridge and the southeast Sulu Basinobtained during two pre-site cruises with the Sonne (1987, 1988)(Table 1).

Dredge hauls were performed and sampling sites were deter-mined as described by Kudrass (1986). Prospective sites werechosen along seismic records (Hinz et al., this volume) wherebasement outcrops were indicated, or on nearby steep slopes.Recovery ranges between some chips of rocks to 500 kg. In onecase, basement could be sampled by piston coring when twopebbles at the base of a slump deposit were recovered.

VOLCANIC ROCKSVolcanic rocks were recovered from 12 sites on the Cagayan

Ridge and from two seamounts in the southeast basin of theSulu Sea. The rocks were analyzed by petrographic, geochemi-cal, and K-Ar methods.

1 Rangin, C , Silver, E., von Breymann, M. T., et al., 1990. Proc. ODP, Init.Repts., 124: College Station, TX (Ocean Drilling Program).

2 Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 3000 Han-nover 51, Federal Republic of Germany.

According to their chemical composition the volcanic rockscan be classified as basalt, basaltic andesite, andesite, and da-cite (Table 2, Fig. 3). Basalt was recovered from two seamountsin the southeast basin (Fig. 1): SO58-65 near ODP Site 769 andSO58-90 near Site 768. At the southern end of the CagayanRidge, near Maeander Reef, samples of basaltic andesite weredredged (SO23-15) or cored (SO49-96) as phosphatized pebblesfrom a slump deposit. Volcanic rocks from the northern end ofthe Cagayan Ridge (Fig. 2) are richer in silica and range frombasaltic andesite to dacite (Fig. 3). Within this small sub-area,the composition varies independently of water depth or the po-sition of the sample along the ridge.

In general, the basalts of the seamounts have a tholeiiticcomposition, whereas the other volcanic rocks from the Ca-gayan Ridge are predominantly calc-alkaline (Fig. 4).

Except for some volcanic sandstone and tuff, the volcanicrocks consist of mostly vesicular, phorphyritic rock commonlybearing phenocrysts of plagioclase, clinopyroxene, and someorthopyroxene (Table 3). Altered olivine occurs in the basalt ofthe seamounts and in the basaltic andesite from the southernend of the Cagayan Ridge. Phenocrysts of plagioclase up to 2cm in length are present in each rock type, quite often includingsome glass aligned parallel to the zonal texture (Fig. 5). The pla-gioclase of the basaltic andesite of SO58-55-3 is especially richin glassy inclusions that contain small bubbles of fluid and gas(Fig. 5).

The groundmass usually contains fine to microcrystallinelaths of plagioclase and sometimes clinopyroxene. In some sam-ples, light brown to light-colored glass is present. As a maincomponent, glass occurs in the dacitic pumice (SO58-43-3) andis partly devitrified in smectitic sphaerolites (SO58-47-1).

The massive dacite (SO49-59) contains numerous large am-phiboles of ferrian tschermakitic hornblende (after Leake, 1978and Rock and Leake, 1984; Fig. 6; for chemical composition seeTable 2), which are surrounded by a thin layer of opaque grains,except at the contact of the amphiboles with the labradoritephenocrysts (for chemical composition see Table 2). The unu-sual occurrence of this amphibole, which otherwise occurs inmesozonal metamorphic rocks, may be explained either by a re-sorption of an amphibolite by a dacitic lava or by a volcano-genie mobilization of amphibole-bearing metamorphic rocks.The divergent K-Ar data of the amphibole (Table 4) also proveits xenolithic character.

GEOCHEMISTRYThe geochemical composition of the various volcanic rocks

was used to characterize their volcano-tectonic origin.As the samples have been obtained from outcrops, which

were exposed for long periods to seawater, diagenetic alterationmay have changed their composition. However, the petrographicanalysis and the relative low concentration of LOI (mostly 2-4%, Table 2) indicate that a severe alteration is limited to somesamples only, which were excluded from the discussion. Anotherlimitation is the X-ray fluorescence detection limit of about 5 to15 ppm for rare-earth elements and some other trace elements.

93

H. KUDRASS, P. MULLER, H. KREUZER, W. WEISS

1OC

Suiu SeaBathymetryDepth in fathoms

• ODP Sites• Dredge Sites

117°E 118° 119° 120

Figure 1. Location map of the Sulu Sea drill sites of ODP Leg 124.

The distribution of trace elements in the basalt samples fromthe two seamounts (SO58-65, -90, Fig. 7) is typical for transi-tional mid-ocean ridge basalt (E-MORB) after Pearce (1982).The great deviation of P in the samples of SO58-65 is presum-ably caused by a diagenetic phosphatization during the long-term exposure to seawater. The high variability of P205 from 0.7to 2.6 wt %, which correlates with an increase of CaO from 9.2to 12.3 wt %, indicates this alteration. Two pebbles from theMaeander Reef (SO49-96) were also phosphatized (P2O5 = 6.06wt °7o). An 8-cm-thick manganese crust covering the outcrops ofbasalt of sample SO58-90 may have prevented this alteration(P205-concentration 0.31 wt %). The SO58-90 deficit of Cr withrespect to E-MORB is presumably caused by early magmaticdifferentiation.

The intermediate and acid volcanic rocks from the CagayanRidge itself are difficult to interpret, as their original composi-tion, indicative of their volcano-tectonic environment, is maskedby fractional crystallisation and contamination. The basaltic an-desite dredged from the Maeander Reef (SO23-15) fits to calc-alkaline volcanic-arc basalts (Fig. 7); the rest of the higher dif-

ferentiated rocks from the northern end of the Cagayan Ridgecannot be classified. Typically these volcanic rocks are extremelylow in Nb and Cr, usually below detection limits, poor in TiO2(0.7-0.4 wt °7o) and Ni (usually below 40 ppm).

K-Ar DATAK-Ar data were obtained from andesite (SO49-55) and dacite

(SO49-59) samples dredged from the northern end of the Ca-gayan Ridge (Table 4, Fig. 2). In SO49-55 three varieties of an-desite and in SO49-59 two varieties of dacite were investigated.Plagioclase (pi), amphibole (am), and rough-ground whole-rock(wr) were analyzed in different grain-size fractions, as well asone magnetic fraction (mf) of whole rock.

Usually, the relative standard deviations of the determina-tions of radiogenic argon and potassium are less than 1%. Butin the present case, the large scatter of the K-Ar dates and thedifferences between the dates for different-size fractions of thesame material suggest the presence of inhomogeneous material.Indeed, three duplicate analyses yielded differences in radio-genic argon of two to four times the possible analytical uncer-

94

VOLCANIC ROCKS AND TERTIARY CARBONATES

10°20'N

10 00

Dredge SO 58Dredge SO 49Depth contour (m)

Table 1. Coordinates and water depths of dredge sam-ples.

Number

SO23-15 KD

SO49-55 KD

SO49-56 KD

SO49-57 KD

SO49-58 KD

SO49-59 KD

SO49-96 KL

SO58-33 KD

SO58-43 KD

SO58-44 KD

SO58-46 KD

SO58-47 KD

SO58-65 KD

SO58-66 KD

SO58-90 KD

Coordinates (North/East)

8°10.390'119°21.800'10°00.608'

121°35.709'9°59.860'

121°39.065'9°55.522'

121°36.187'9°56.076'

121°35.052'10°10.791'

121°41.339'8°11.10'

119°28.05'10°18.26'121°44.059°59.11'

121°35.97'9°59.90'

121°35.80'10°05.59'

121°40.92'9°48.10'

121°34.02'8°59.74'

121°22.52'9°OO.3O'

121°23.45'8°16.07'

121°22.26'

to

to

to

to

to

10°121

9°121

10

01.900'°35.890'

56.130'°34.700'

°18.62'121°43.879

121

10121

9121

°59.33'° 36.06'

°O5.37'°40.97'°49.20'03342-

Water depth(corrected)

3312 m

2683 to 2014 m

3780 m

2677 m

2167 to 2065 m

2400 to 2100 m

3634 m

1650 to 1620 m

2700 to 2580 m

2220 m

3580 to 3700 m

3220 to 2890 m

3500 m

3400 m

3600 m

9°50'

121°30'E 121°40' 121°50'

Figure 2. Detail map of the area outlined in Figure 1, Cagayan Ridge.

tainty. We therefore calculate the error of the K-Ar date with anuncertainty of the radiogenic argon arbitrarily enhanced by afactor of four. The only exceptions are the plagioclase dates forSamples SO49-55-3 and SO49-55-9, which have already largeanalytical uncertainties because of the relatively low concentra-tions of radiogenic argon.

The amiphibole (ferrian tschermakitic hornblende) in SO49-55 exhibits the widest range in K-Ar dates (36-158 Ma). Accord-

10

5 -

Trachyte

Rhyolite

90

Basalt Basalticandesite

35 411 1

45 52 57

SiO2

63

Figure 3. Chemical composition plot of volcanic rocks recovered from two seamounts in the southeast Sulu Basin.

77

95

H. KUDRASS, P. MLJLLER, H. KREUZER, W. WEISS

Table 2. Chemical composition of volcanites.

SO23-15

wr

SO49-55-1wr

SO49-55-2lap

SO49-55-3a

wr

SO49-55-3b

wr

SO49-55-3c

wr

SO49-55-3d

wr

SO49-57-2a

wr

SO49-57-2b

wr

SO49-59-la

wr

SO49-59-laam

SO49-59-la

pi

SO49-59-la

gm

SO49-59-lb

wr

SO49-59-lb

am

SO49-59-lb

Pi

SO49-59-lb

gm

SO49-59-lb

wr

SO49-96wr

SiO (wt.<Fo)TiO2

A12O3

Fe2O3

MnOMgOCaONa2OK2OP2O5SO 3

LOITracesTotalTraces (ppm)AsBaBiCeCoCT

CuLaMoNbNiPbRbScSnSrTaThUV

wYZnZr

52.130.85

16.629.620.175.09

10.421.671.040.210.301.380.18

99.68

1174

14712458337

15

308

4646

1388

861

2971

198345

53.700.80

15.4910.600.163.608.252,530.890.080.123.130.13

99.49

179

1111

9722

51

10162637

1149

111

35522268646

56.980.53

14.538.720.145.479.201.940.510.070.001.560.13

99.79

155

111

18040

151

749

1240

1138

111

2471

268856

59.160.57

14.227.530.144.378.092.241.290.160.101.500.12

99.49

21108

111

8763

151

26192733

1173

611

2266

205570

57.550.70

15.787.980.144.337.642.370.730.090.002.440.10

99.85

161

111

3025

143

191

1329

1160

111

2161

248366

54.950.44

15.878.050.135.819.201.800.560.030.002.550.10

99.50

140

11

215949

113

261

153727

143111

2221

196651

59.390.54

13.987.490.145.118.452.191.130.070.001.180.13

99.80

782

11

24131

71111

30152335

1155

111

2711

215874

48.010.48

12.699.940.048.742.942.451.540.090.00

12.800.10

99.83

1111

3115241

111

541

22383089

166

1441

125820

47.340.46

12.359.500.048.182.642.341.720.090.24

14.750.10

99.75

1444

111

14248

111

46163332

189

171

14711167026

60.480.38

17.345.540.192.156.063.351.410.200.072.440.12

99.73

24234

9111

131598

153610

1366

171

951

166395

44.371.22

11.6415.290.60

12.1110.671.870.390.160.001.190.11

99.61

11

122121281922

13

1417

571

87111

2321

6214253

52.030.02

28.010.490.040.10

12.323.970.550.320.001.890.12

99.85

193

1111

111141

19113

29820

1111169

23

53.280.10

24.401.650.101.21

10.723.770.890.300.003.040.12

99.56

10137

1011

115

1111

1121

51

642111

281

112542

61.520.40

17.235.590.142.204.804.030.900.190.002.580.10

99.67

1136

1111

1411757

1510

1307

691

1041

154993

44.011.22

11.5215.340.58

12.4910.882.010.390.130.001.010.11

99.68

171

172023

12121

45

12108

551

78115

2231

6814358

49.940.04

27.610.710.060.989.383.321.170.600.005.580.14

99.53

1978

1111

12011

371

4317

1114599

1211119

7422

52.350.06

26.421.100.051.177.783.561.240.100.005.750.09

99.67

1103

1111711411

1941

561111119

2133

61.310.40

17.544.850.172.364.633.681.350.220.073.040.11

99.73

1201

1111

1326

541

271591

3061

101

921

154788

38.790.77

17.585.590.122.23

18.562.490.866.060.326.100.23

99.69

9135

1130

1157768

11

33231430

1711

11242

28424

1039540

Notes: X-ray fluorescence analyses by J. Lodziak and D. Requard. Subsamples of one rock type are indicated by a, b, c...; LOI = loss on ignition; wr = whole rock; lap lapilli; gm = groundmass; am =amphibole; pi = plagioclase. Values of 1 indicate concentrations below detection limits.

ing to the petrographic analysis, this mineral is atypical forvolcanites and may be derived as xenocrysts from a metamor-phic rock. Apparently the transfer of the amphibole did not re-set the K-Ar clock of the mineral, or the excess Ar from themetamorphic rock was not released. In any case, the old datesof the amphibole suggest the inheritance of argon from an oldermetamorphic rock.

The dates of the phenocrysts of plagioclase in SO49-59 havea smaller range (22 to 26 Ma), with the better-preserved plagio-clase of SO49-59-7 yielding the younger dates. The dates forwhole-rock fractions lie between the dates of the amphibole andthe plagioclase in Sample SO49-59-1, but are comparable oreven younger than those of the plagioclase in Sample SO49-59-7.

In the dilemma of proved excess argon and possible losses ofargon from the plagioclase and even more from the whole-rockfractions, the youngest dates of SO49-59 (i.e., 22-26 Ma) pro-vide only doubtful limits for the time of volcanic activity at thenorthernmost position of the Cagayan Ridge.

The K-Ar dates of SO49-55 are generally younger than thoseof SO49-59, but they also show a wide range (11 to 20 Ma) anda significant scatter for different fractions of the same sample.We have no certain criteria for the reliability of the different re-sults. If the single date of the magnetic fraction is not consid-ered, we have a cluster of two whole-rock samples, one plagio-clase date around 15 Ma, and a single one at about 11 Ma thatmight indicate the time of volcanic activity at the northern endof the Cagayan Ridge. The K-Ar date of 14.7 ± 0.6 Ma for abasaltic andesite (Kudrass et al., 1986: SO23-15) suggests a sim-ilar time of volcanic activity at the southern end of the CagayanRidge. The deposition of the early to early middle Miocene

shallow-water carbonates (see below) could be associated withthis volcanic activity.

SHALLOW-WATER CARBONATESAt the northern end of the Cagayan Ridge, shallow-water

carbonates were dredged at four localities with water depthranging from 3800 to 1600 m (Fig. 2). The carbonates that con-sist of algal packstone, biomicrite or foraminifer sandstone con-tain shallow-water faunal assemblages of larger foraminifers(Lepidocyclina, Cycloclypeus, Table 5). The larger foraminifersthat are frequently reworked indicate, at least for the Lepidocy-clina species, an early Miocene to early middle Miocene age.The planktonic foraminifers from the younger sediments are oflate Miocene (SO58-43) or of probable early Pliocene age(SO49-57).

The occurrence of early Miocene to early middle Mioceneshallow-water carbonates, even when they are reworked intoyounger sediments, indicates that the Cagayan Ridge existed asa shallow area of presumably volcanic islands at that time.Early to middle Miocene carbonates were also dredged near thereef islands of the central Cagayan Ridge (Kudrass, 1987).

SUMMARYThe tholeiitic basalt samples dredged from two seamounts

near ODP Sites 768 and 769 have a trace-element distributioncomparable to transitional mid-ocean ridge basalts (afterPearce, 1982). The volcanic rocks obtained from the CagayanRidge on which Site 769 was drilled are richer in silica and rangefrom basaltic andesite to dacite. Most of the rocks belong to thecalc-alkaline series, possibly indicating that the Cagayan Ridge

96


Table 2 (continued).

SO58-43-la

lap

52.530.67

16.029.590.115.736.623.120.390.100.004.380.12

99.37

148

11

421

58111

101

1345

1167

111

3651

236932

SO58-43-lb

lap

56.530.73

14.217.370.162.454.003.151.340.200.009.500.09

99.72

194

1111

3514411

2124

1194

111

1011

368977

SO58-43-3wr

63.670.53

13.073.780.151.193.344.461.570.120.057.760.07

99.75

196

1111

2715411

2614

1153

111

271

306486

SO58-43-6wr

22.710.273.466.241.289.67

27.450.980.320.260.36

26.400.27

99.68

13217

161

79458

111

58779

131

195111

1171

104119

SO58-44-la

wr

60.370.47

14.306.830.144.427.912.461.290.060.001.210.12

99.58

1104

11

3412555

166

251

3029

1170

116

1891

255075

SO58-44-lb

wr

59.230.46

14.257.290.145.228.612.431.020.070.000.990.13

99.84

1101

11

2415759

161

26111938

1153

111

2101

205757

SO58-44-lc

wr

59.390.46

14.306.960.154.648.152.651.210.070.001.750.12

99.84

1111

11

319870

131

28172629

1179

111

2001

244866

SO58-46-la

wr

65.310.65

15.324.930.111.264.973.920.800.170.002.250.08

99.77

1484

1111

1411117

1419

1193

111

701

327086

SO58-46-lb

wr

66.410.62

15.484.150.070.864.624.541.150.140.001.590.08

99.71

1591

1111

251111

132519

1185

111

801

335389

SO58-46-5wr

50.540.72

13.497.950.204.178.183.143.730.380.456.630.29

99.87

1295

11

54491

77111

1391

3929

1724

111

2371

147553

SO58-47-1lap

47.760.78

15.839.030.124.945.302.230.940.080.00

12.630.09

99.73

1111

251

6011511

1529

1180

111

1871

148046

SO58-47-3wr

43.831.98

16.7214.400.217.756.034.360.070.150.003.910.13

99.53

11

111

744710

114

3714

521

114111

3971

396385

SO58-65-la

wr

43.951.18

18.379.300.092.78

11.292.851.001.340.007.300.18

99.65

166

111

3009629

51760

11537

1272

111

2481

3412080

SO58-65-lb

wr

45.851.12

17.859.120.083.489.202.991.470.700.007.680.18

99.71

184

]

;

;

33!7(

:•

[

It))1I

n69

12439

1248

117

2071

3710875

SO58-65-lc

wr

43.961.15

18.539.350.122.74

11.013.050.881.340.007.210.20

99.54

2270

111

38389

11

1265

11535

1280

111

2601

2611677

SO58-65-ld

wr

46.231.08

17.069.990.623.019.702.600.991.070.006.640.20

99.18

1169

12339

21311023

41394142235

1269

191

2251

5711374

SO58-65-2wr

43.171.08

16.749.370.512.77

12.342.720.852.580.037.340.19

99.70

22147

11

23226103

11

1385

11433

1302

111

2251

3311670

SO58-90-la

wr

46.131.22

19.0610.010.225.069.792.840.540.290.004.510.16

99.82

182

11

256361

11

1354

17

301

338117

3001

19103

95

SO58-90-lb

wr

47.161.26

20.239.190.144.30

10.012.880.460.310.003.200.15

99.28

170

111

1552

11

12199

113223

35410

16

3111

2410074

SO58-90-2wr

4S.141.41

19.7610.400.143.879.963.160.640.340.004.680.15

99.66

174

1111

5914

1326119

321

349111

3251

24108

86

SO58-90-3wr

44.811.38

19.4410.380.184.16

10.692.980.530.320.004.790.16

99.82

154

111

21135

14

1436

18

351

359161

3171

2211590

FeO*

Na,0

<>Seamount basalts• Cagayan Ridge volcanites

MgO

Figure 4. Classification of volcanic rocks from the two southeast SuluBasin seamounts and the Cagayan Ridge. Separating line between tho-leiitic (upper part) and calk-alkaline basalts is after Irvine and Baragar(1971).

originated as a volcanic arc. The K-Ar dates of basaltic rocksand sites from the northern end of the Cagayan Ridge are in-consistent but suggest that the volcanic activity occurred in aperiod between 10 to 20 Ma. At approximately the same time,shallow-water carbonates of early to early middle Miocene agewere deposited on the slopes of presumably volcanic seamountsor islands in the central and northern Cagayan Ridge. Dacitesfrom the northernmost Cagayan Ridge (SO48-59) bear xeno-

crysts of possible metamorphic origin, which obviously containexcess argon.

REFERENCES

Bideau, D., and Hekinian, R., 1984. Segregation vesicles of ocean floorbasalts, 1. Petrological study of the segregation products. J. Geo-phys. Res., 89:7903-7914.

Hamilton, H., 1979. Tectonics of the Indonesian region. (U.S.) Geol.Surv. Prof. Pap., No. 1078.

Holloway, N. H., 1981. The North Palawan Block, Philippines: its rela-tion to the Asian mainland and its role in the evolution of the SouthChina Sea. Bull. Geol. Soc. Malays., 14:19-58.

Irvine, T. N. and Baragar, H.R.A, 1971. A guide to the chemical classi-fication of the common volcanic rocks. Can. J. Earth Sci., 8:523-547.

Kudrass, H. R., 1987. Improvement of deep-sea dredging operationsthrough the use of a depth meter. J. Sediment. Petrol., 54:39-43.

Kudrass, H. R., Hiedicke, M., Cepek, P., Kreuzer, H., and Müller, P.,1986. Mesozoic and Cenozoic rocks dredged from the South ChinaSea (Reed Bank area) and Sulu Sea and their significance for plate-tectonic reconstructions. Mar. Pet. Geol., 3:19-30.

Leake, B. E., 1978. Nomenclature of amphiboles. Can. Mineral., 16:501-520.

Lee, C. S., and McCabe, R., 1986. The Banda Celebes Sulu Basin: atrapped piece of Cretaceous—Eocene oceanic crust? Nature, 322:51-54.

Le Bas, M. J., Le Maitre, R. H., Streckeisen, A., and Zanettin, B.,1986. A chemical classification of volcanic rocks based on the totalalkali-silica diagram. J. Petrol., 27:745-750.

Mascle, A., and Biscarrat, P. A., 1978. The Sulu Sea: a marginal basinin Southeast Asia. In Watkins, J. S., et al. (Eds.), Geophysical In-vestigations of the Continental Margins. AAPG Mem., 29:373-381.

Mitchell, A.H.G., Hernandez, R, and Dela Cruz, A. P., 1986. Ceno-zoic evolution of the Philippines Archipelago. J. Southeast AsiaEarth Sci., 1:3-22.

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Murauchi, S., Ludwig, H. J., Den, N., Hotta, H., Asanuma, T., Yoshi,T., Kuboten, A., and Agiwara, K., 1973. Structure of the Sulu Seaand Celebes Sea. J. Geophys. Res., 78:3437-3447.

Pearce, J. A., 1982. Trace-element characteristic lavas from destructiveplate boundaries. In Thorpe, R. S. (Ed.), Orogenic Andesites andRelated Rocks: New York (Wiley-Interscience), 525-548.

Rangin, C , 1989. The Sulu Sea, a back-arc basin setting within a Neo-gene collision zone. Tectonophysics, 161:119-141.

Rock, N.M.S., and Leake, B. E., 1984. The International MineralogicalAssociation amphibole nomenclature scheme: computerization andits consequences. Mineral. Mag., 48:211-227.

Seidel, E., Kreuzer, H., and Harre, H., 1982. A late Oligocene/earlyMiocene high pressure belt in the external Hellenides. Geol. Jahrb.,SerE: 165-206.

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Ms 124A-106

Table 3. Description of the mineralogical composition of volcanites.

n

Figure 5. Photograph of phenocrysts of plagioclase with glass inclu-sions in basalts and basaltic andesite from southeast Sulu Basin sea-mounts and the Cagayan Ridge. Sample SO49-55-3.

SampleNumber Description

•Φ •

- - » *>

* « .

SO58-33-4SO58-43-1

SO58-43-3

SO58-43-6

SO58-44

SO23-15 Porphyritic basaltic andesite, some vesicles.Pc: pi, cpx, some altered ol.Gm: microcrystalline, pi, pyroxene in light-brown glass.

SO49-55-1 Vesicular porphyritic andesite, fine and large vesicles.Pc: pi with inclusions of glass and small cpx.Gm: brown glass with laths of pi and cpx, fluidal texture.

SO49-55-2 Lapilli tuff, lapilli consisting of fine-vesicular porphyritic andesite.Pc: pi with inclusions of glass; some cpx; cpx with inclusions of glass and op.Gm: fine crystalline with pi, cpx, op.

SO49-55-3 Vesicular porphyritic andesite.Pc: zonal pi with inclusions of glass, fluid, and gas; cpx; some opx (hyper-

sthene) with inclusions of glass.Gm: glass with some pi and cpx, devitrifiaction sphaerolites.Glass in pc darker than in gm.

SO49-55-9 Fine-vesicular porphyritic andesite.Pc: zonal pi with inclusions of glass, cpx, and op; opx; cpx; agglomerates of

pi, cpx, and opx.Gm: fine crystalline with needles of pi, op; smectite in vesicles.

SO49-57-2 Tuff with lapilli of basaltic andesite, lapilli with fluidal texture.SO49-59-1 Massive porphyritic dacite.

Pc: green am (ferrian-tschermakitic hornblende) and pi (labradorite), both pccontain glassy inclusions, rim of op surrounds am, however not when amcontacts pi, large pi have often smecitic or carbonate cores, other pc areapatite, op.

Gm: microcrystalline with ?quartz, pi, op, and am.SO49-59-7 Massive porphyritic dacite similar to SO49-59-1, but somewhat better preserved

(no carbonate in pi pc). Gm with chlorite aggregates partly around phlogo-pite, am pc are not always surrounded by op, some cm-sized microdiorite-like restites.

SO49-96 Two pebbles of highly vesicular porphyritic andesite.Pc: pi, some cpx, altered (?) ol.Gm: microcrystalline laths of pi, grains of cpx, op, brown glass.The rim of the pebbles is phosphatized.Reddish or grey tuffite.Basaltic or andesite grey ash with lapilli consisting of highly fine-vesicular

andesite or basaltic andesite.Pc: some pi and cpx.Gm: small laths of pi exhibit fluidal texture.White to light gray ash with dacitic pumice, pumice with faint fluidal texture.Pc: pi, cpx, op.Gm: light colored glass.Greenish carbonaceous, smectitic, sandy tuffite (pi, cpx, green am, brown am,

quartz).Rare grains of calcareous algae.Fine-vesicular andesite.Pc: pi (up to 2 cm in diameter), opx, cpx, op, large pi are corroded and

contain many inclusions of glass; medium-sized pi contain some glassinclusions.

Gm: brown glass, pi needles, op grains.SO58-46-1 Fine-vesicular porphyritic dacite.

Pc: pi, opx, some cpx, op; pc often in aggregates.Pm: brown glass, very fine crystalline with small needles of pi, some cpx,

fluidal texture.SO58-46-5 Zeolithic, smectitic volcanic sandstone consisting of pi, light-green cpx, green

and brown am, biotite, and rare planktonic foraminifers, calcareous algae.SO58-47-1 Ash with various lapilli, light-brown lapilli consist of andesite to dacitic

devitrified glass, vesicles filled with smectite; dark-brown lapilli consist ofbasaltic andesite with small laths of pi in cryptocrystalline gm.

SO58-47-3 Greenish slightly porphyritic metamorphite (prasinite-like) with chemicalcomposition of basaltic andesite, intersertal texture.

Pc: laths of pi, altered cpx.Gm: fine crystalline, pi, chlorite, epidote, leucoxene, op, am, vesicles filled

with epidote, chlorite, one pebble only, probably an artifact.SO58-65 Slightly porphyritic basalt with large and small round vesicles (segregation

vesicles corresponding to those described by Bideau and Hekinian (1984)).Pc: altered ol, some pi.Gm: cryptocrystalline, intersertal texture, many laths of pi, some cpx, op.Vesicles partly lined by zeolites.Outcrop covered by a 8-cm-thick manganese crust.

SO58-90-1 Slightly porphyritic basalt with large vesicles and microporosity.Pc: pi, cpx, altered ol, some opx.Gm: fine crystalline intersertal texture, smectitic glass with laths of pi, cpx,

op, some skeletal opx.Large vesicles lined by zeolite, small vesicles partly filled by smectite.

SO58-90-2 As SO58-90-1, but in gm am (kaersutite) in radial aggregates.

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Table 4. K-Ar data. Argon determined by standard total fusion mass-spectrometric isotope-dilution analysis and potassium by flame photo-metry.e.g., (Seidel et al., 1982).

Sample no.Rock type

SO49-55-1Porphyritic

andesite

SO49-55-3Basaltic

andesite


andesite


dacite


dacite

SO23-15Porphyritic

basalticandesite

Materialsize 0*m)

wr (200-100)wr (100-63)

mf (200-100)pi (100-63)

pi (100-63)

wr (600-400)wr (400-250)am (250-160)am (160-80)pi (250-160)pi (160-80)

wr (630-315)wr (315-200)am (200-125)am (125-63)pi(200-125)pi (125-63)

wr(Kudrass et al., 1986)

K-Ar date(Ma)

14.9 ± 0.516.6 ± 0.9

19.8 ± 0.714.7 ± 1.1

10.7 ± 1.5

36.5 ± 0.750.5 ± 1.6158 ± 5

36.5 ± 1.325.2 ± 1.325.8 ± 1.1

21.8 ± 0.722.0 ± 0.766.1 ± 2.360.5 ± 2.122.3 ± 0.923.9 ± 1.0

14.7 ± 0.6

K(wt%)

0.4760.442

1.1810.115

0.147

0.9300.9130.3470.3350.9790.978

1.501.460.3310.3230.4640.446

0.863

^ r a d(nl/g)

0.280.29

0.9140.066

0.061

1.331.822.230.480.980.98

1.281.260.870.770.400.42

0.49

Aratm(STP)

1.281.09

0.240.38

0.32

0.740.770.230.302.932.88

0.470.410.340.340.550.52

0.18

Figure 6. Photograph of dacitic Sample SO49 showing large amphibolesof ferrian tschermakitic hornblende. Sample SO49-59-1.

Notes: IUGS recommended constants used (Steiger and Jàger, 1977). Argon innannoliters per gram at standard pressure and temperature. Error estimates ata level of 95% confidence in the analytical value (see text). Abbreviations: wr= sieve fraction of crushed whole-rock, mf = magnetic fraction of wr, am =amphibole, pi = plagioclase.

99


3020 -

10 -

Φ

I 0.10ΦΦoo 0.04

1 -

o SO23-15

0.10

Figure 7. Trace elements in basalts. Upper part: The two seamounts of the southeast Sulu Basin, normalized to E-MORB basalt after Pearce(1982). Lower part: Andesitic basalt from the southwestern Cagayan Ridge, normalized to volcanic arc calc-alkaline basalt after Pearce (1982).

Table 5. Microfacies, foraminifer fauna, and age of shallow-water carbonates from thenorthern edge of the Cagayan Ridge.

Sampleno. Microfacies and foraminifer fauna Age and remarks

SO 49-56 Algal packstone with larger foraminifers reworked,such as Lepidocyclina (Nephrolepidina) aff. ferreriProvale, Cycloclypeus sp., Operculina sp., Amphis-tegina sp.

SO 49-57 Algal-foraminifer wacke-/packstone with reworkedLepidocyclina specimens, and with Amphisteginasp., Heterostegina sp., Gypsina sp., Calcarina sp.,Globorotalia tumida (Brady), Sphaeroidinellaspecimens

SO 49-58 Biomicrite with algal fragments and larger forams,such as L. aff. verbeeki (Newton and Holland),Cycloclypeus sp. and Amphistegina sp. and withplanktonic foraminifers, such as Globigerinoidessp.sp. and Orbulina universa D'orbigny

SO 58-43 Slightly cemented planktonic foraminifer sand with rarebenthic and larger foraminifers; with Globoquad-rina altispira (Cushman and Jarviz), Globoquad-rina dehiscens (Campman, Parr and Collins),Globorotalia aff. merotumida Blow and Banner,Globorotalia aff. plesiotumida Blow and Banner,Orbulina suturalis Brönnimann and others, andwith reworked specimens of Cycloclypeus sp.,Lepidocyclina sp., and Planorbulina sp.

early Miocene to earlymiddle Miocene

Probably early Pliocene withreworked early to middleMiocene

early middle Miocene

Probably late Miocene withreworked early to middleMiocene

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