SITE 445, PHILIPPINE SEA · Location of Site 445 and other DSDP sites in the Philippine Sea. through the unit: in sub-unit Va, they measure several millimeters (average diameter),

14. CONGLOMERATE AND SANDSTONE PETROGRAPHY, DEEP SEA DRILLING PROJECTSITE 445, PHILIPPINE SEA

Hidekazu Tokuyama, Ocean Research Institute, University of Tokyoand

Makoto Yuasa and Atsuyuki Mizuno, Marine Geology Department, Geological Survey of Japan

ABSTRACT

Conglomerates and sandstones in lithologic unit V at DSDP Site445 comprise lithic clasts, detrital minerals, bioclasts, and authigenicminerals. The lithic clasts are dominantly plagioclase-phyric basaltand microdolerite, followed by plagioclase-clinopyroxene-phyric ba-salt, aphyric basalt, chert, and limestone. A small amount of horn-blende schist occurs. Detrital minerals are dominantly plagioclase,augite, titaniferous augite, olivine, green to pale-brown hornblende,and dark-brown hornblende, with subordinate chromian spinel, epi-dote, ilmenite, and magnetite, and minor amounts of diopside, en-statite, actinolite, and aegirine-augite. Bioclasts are Nummulites bo-ninensis, Asterocyclina sp. cf. A. penuria, and some other larger for-aminifers.

Correlation of cored and dredged samples indicates that the DaitoRidge is mainly composed of igneous, metamorphic, ultramafic, andsedimentary rocks. The igneous rocks are mafic (probably tholeiitic)and alkalic. The metamorphic rocks are hornblende schist, tremoliteschist, and diopside-chlorite schist. The ultramafic rocks are alpine-type peridotites. Mineralogical data suggest that there were twometamorphic events in the Daito Ridge. The older one was interme-diate- to high-pressure metamorphism. The younger one was contactmetamorphism caused by a Paleocene volcanic event, possibly re-lated to the beginning of spreading of the west Philippine Basin. Theultramafic rocks suffered from the same contact metamorphism.

During the Eocene, exposed volcanic and metamorphic rocks onthe uplifted Daito Ridge may have supplied pebble clasts to the sur-rounding coast and shallow sea bottom. The steep slope offshoremay have caused frequent slumping and transportation of the pebbleclasts and shallow-water benthic organisms into deeper water, form-ing the conglomerates and sandstones treated here.

INTRODUCTION

The purpose of this study is to examine the petro-graphy of middle-Eocene conglomerate and sandstonecored in DSDP Hole 445, and to discuss the provenanceof these rocks in relation to early-Tertiary geologicevents on the Daito Ridge.

Hole 445 is in an east-west-trending, sediment-filleddepression within the Daito Ridge, at a depth of about3400 meters below sea level (Figures 1 and 2). The de-pression opens both to the northwest and to the south-east, to the Kita-Daito Basin and the Minami-DaitoBasin, respectively. The depression is bounded by top-ographic highs to the north and south with relativeheights of as much as 2000 meters.

Sediments from Hole 445 are dominantly redepositedbiogenic components from the upper middle Eocene tothe Quaternary (0-645 m; lithologic units I to IV), anddominantly redeposited terrigenous components in themiddle Eocene (645-892 m; lithologic unit V). The mid-

dle Eocene of unit V is characterized by thin-bedded tolaminated nannofossil mudstone and sandy mudstonewith many graded interbeds of conglomerate and sand-stone. The lower boundary of this lithosome was notreached. However seismic data indicate that the contactbetween unit V and the basement lies only a few tens ofmeters below the bottom of the hole.

Unit V is subdivided into three sub-units (Va, Vb,and Vc, in descending order), on the basis of the pro-portions of biogenic and terrigenous components andgrain size of terrigenous components (Figure 3). Sub-unit Va is dominantly greenish-gray to dark-greenish-gray nannofossil mudstone to sandy mudstone and nan-nofossil limestone, with interbeds of conglomerate andsandstone containing larger foraminifers. Sub-unit Vbconsists of dark-greenish-gray, sandy mudstone; mud-stone; and conglomerate. Sub-unit Vc is almost exclu-sively dark-greenish-gray conglomerate and sandstone;the conglomerate is of debris-flow origin. Lithic clastsof the conglomerate tend to increase in size downward

629

H. TOKUYAMA, M. YUASA, A. MIZUNO

60°N

15C

PHILIPPINE SEA ‰> V \ ò

^ δ j PHILIPP

2 J_^0 U^3000

JoV ,—-3000 i

135°W 150° 165° 180°W

Figure 1. Location of Site 445 and other DSDP sites in the Philippine Sea.

through the unit: in sub-unit Va, they measure severalmillimeters (average diameter), with a maximum ofabout 1.5 cm; in sub-units Vb and Vc, the maximum di-ameter increases to 3 cm and 15 cm, respectively. Bio-clasts are contained in sub-units Va and Vb. Micro-spheric tests of Nummulites boninensis as large as 3 cmin diameter are visible in considerable numbers in con-glomerates of sub-unit Va. The lithology and the pri-mary sedimentary structures of unit V indicate that itformed by typical canyon or upper-fan resedimentationof shallow-water deposits from the Daito Ridge (Site445 report, this volume).

A multichannel reflection record (Nasu et al., 1977,1978; Mizuno et al., 1979) shows that the sedimentarysequence in the depression, as thick as 0.9 to 1.0 seconds

in two-way reflection time, can be divided into threeacoustic layers (Figure 4). The upper layer, acousticallytransparent, is about 0.2 seconds thick and can be corre-lated with unit I. The middle layer is well-stratified, withhigh-frequency reflections and a thickness of about 0.54seconds; we believe that this layer corresponds mostly tothe extensive calcareous turbidite deposits of units IIand III. The reflectors appear to abut onto the southernwall of the depression. The lower layer, more than 0.2seconds thick, is characterized by low-frequency reflec-tions and is correlated with unit V. Some of reflectors ofthe lower layer appear to be intermittently traceable to-ward the main peak of the Daito Ridge along its north-ern slope. In addition, velocity data (Nasu et al., 1978)suggest the distribution of materials equivalent to unit V

630

CONGLOMERATE AND SANDSTONE PETROGRAPHY

Figure 2. Bathymetry around Site 445, compiled by H.Tokuyama. Contours in meters below sea level.

along the slope. This is consistent with the inferred re-sedimentation of unit V.

Although the geology of the Daito Ridge is not en-tirely clear, previous work on dredge samples (Mizunoet al., 1977) has shown that the topographic highs arelikely underlain by pre-Eocene basement rocks (includ-ing metamorphic rocks); middle-Eocene Nummulites-bearing limestone, presumably overlying the basementrocks; and various igneous rocks of unknown age. Cit-ing both geologic and seismic-reflection data, Mizuno etal. (1979) discussed the geologic history of the DaitoRidge during the early to middle Tertiary. They inferredthat metamorphic rocks probably underlie the middle-Eocene shallow-water strata, and that there was volcan-ic activity on the Daito Ridge during the Eocene, or lat-er, which raised the ridge.

Petrographic study of the cored middle-Eocene con-glomerate and sandstone serves to elucidate their prove-nance geologic events on the Daito Ridge. For thisstudy, 18 samples were selected throughout unit V (Fig-ure 3). They are medium sandstone, coarse sandstone,very coarse sandstone, and granule conglomerate, eithergreenish gray or dark greenish gray (Table 1). Some ofthe samples contain bioclasts {Nummulites boninensisand other larger foraminifers). All samples were ob-served under a microscope, and some were analyzed. Inorder to correlate with the cored samples, parts of sam-ples dredged from the Daito Ridge (Mizuno et al., 1975;Yuasa and Watanabe, 1977) were re-examined with amicroscope and were chemically re-analyzed (Table 2).

Q.QJ

QEoo-9is

6 5 0 -

7 0 0 -

-

7 5 0 -

-

-

8 0 0 -

850-

-

Age

ene

oLU

iddk

Sub-unit

Va

Vb

Vc

Samples (interval in cm)

-. AAR RO 9 Of! 019

ΛΛG. 71 1 7R 77

445-72-2, 6-8

^^445-74-1,87-89—' 445-74-1,96-97= = ^ ^ - 445-74-3, 59-71

^^445-74-3,92-94

___——445-77-5, 37-38445-77-5, 45-46

445-81-1, 143-145

_ _ _ ^ 445-83-5, 27-28— 445-83-4, 48-49

• ΔΔR OR 1 o n 99

44R fifi-1 7R 77

/1 Λ r, 00 9 99 9/1

AΔR 0 1 1 RO RI

—445-93-2, 16-18— 445-93-2, 105-107

Figure 3. Stratigraphy of lithologic unit V, Hole 445,with sample positions.

PETROGRAPHY OF CONGLOMERATE

General Descriptions

Petrography is summarized in Table 3. Some photo-micrographs are shown in Figure 5. The conglomerateand sandstone of unit V contain lithic clasts, bioclasts,and detrital and authigenic minerals. They have an ac-cessory amount of matrix, comprising nannofossil lime-stone, nannofossil mudstone, and zeolite which may bea product of alteration of volcaniclastic materials. Inmany cases, there is grain to grain contact, with no ma-trix.

The lithic clasts are largely igneous rocks, partlysandstone and chert, and rarely metamorphic rocks.

631


sw NE- 2

- 3

SP4100 SP3900 SP3800

0 2 km

Figure 4. SW-NE multichannel seismic-reflection profile across the depression on the Daito Ridge (profile track isplotted on Figure 5). (The upper layer, middle layer, and lower layer can be correlated with lithologic units I, II andIII, and V, respectively. The reflector between the middle and lower layers may represent a chert layer in the middlepart of unit IV. Site 445 is about 1 nautical mile northwest of shot point 3960. Its approximate projection onto theseismic line is indicated by an arrow.

They are generally subangular to angular, with varyingsize. Generally, detrital minerals are dominant in sub-unit Va, whereas lithic clasts increase downward. Thesize of lithic clasts is less than 2 mm in sub-unit Vb, and2 to 5 mm in sub-unit Vc. Clasts of limestone with Num-mulites boninensis is rarely found. Authigenic compon-ents are chert, recrystallized calcite, glauconite, chlorite,and zeolite.

Many bioclasts are present in the samples from sub-unit Va and Vb, but very few in those from sub-unit Vc.Most abundant are megalospheric tests of Nummulitesboninensis, with sizes of about 0.5 cm or less; followedby microspheric tests of the same species, as large as 1cm; and microspheric tests of Asterocyclina sp. cf. A.penuria. Fragmental bryozoans are also found. Sometests, particularly larger ones, are considerably broken,suggesting their incorporation into the conglomerateand sandstone by resedimentation.

Clasts Of Igneous And Metamorphic RocksClasts of igneous and metamorphic rocks are grouped

into five types on the basis of mineral assemblage andtexture. All the clasts are moderately to highly altered.They are plagioclase-phyric basalt, microdolerite, pla-gioclase-clinopyroxene-phyric basalt, aphyric basalt andhornblende schist.

Plagioclase phyric basalt with hyalopilitic to interser-tal texture is the dominant type of lithic clast through-out unit V, with lengths of 0.5 to 8 mm. Phenocrysts ofplagioclase are lath-shaped and up to 1 mm in length;compositional zoning is not strong. The groundmass

consists of lath-shaped plagioclase, clinopyroxene, andopaque minerals. The opaque minerals are mainly mag-netite, with small amounts of ilmenite and hematite.The clinopyroxene is augite, often altered to chlorite.Glass appears as mesostasis, which is largely altered toglauconite and clay minerals.

Microdolerite, with subophitic texture, is a commonlithic clast, with lengths of 0.5 to 4 mm. Phenocrysts ofplagioclase are lath-shaped and up to 1 mm in length;they show slight compositional zoning. Microlites areplagioclase, clinopyroxene, and opaque minerals. Theclinopyroxene (augite) is often altered to chlorite. Opaqueminerals are mainly titanomagnetite, with small amountsof hematite.

Plagioclase-clinopyroxene-phyric basalt is a rare lith-ic clast throughout unit V, with lengths of 5 to 6 mm. Itsmineral assemblage is similar to that of the plagioclase-phyric basalt, but it has clinopyroxene (augite) pheno-crysts. The clinopyroxene is euhedral to subhedral andhas a pale-brownish tint.

Aphyric basalt clasts, rare in general, but common at445-74-3, 92-94 cm, have lengths of 0.5 to 5 mm. Micro-lites and groundmass consist of plagioclase, opaqueminerals, and clinopyroxene. The plagioclase is acicularto lath-shaped. The opaque minerals are acicular ordust-like. The plagioclase and opaque minerals oftenform spherulitic to dendritic texture. The opaque miner-als are mainly titanomagnetite, with small amounts ofhematite. The clinopyroxene is augite, often altered tochlorite. Glass appears as a mesostasis, but is largely al-tered to clay minerals.

632


TABLE 1Sample Descriptions

Sample(interval in cm)

Strati-graphicPosition Visual Description

445-69-2, 90-92

71-1,76-77

72-2, 6-8

74-1,87-89

74-1,96-97

74-3,59-61

74-3, 92-94

77-5, 37-38

77-5, 45-46

81-1,143-145

83-4, 27-28

83-4, 48-49

85-1,20-22

86-1,75-77

88-2, 32-34

91-1,50-51

93-2,16-18

93-2, 105-107

Va, upper

Va,upper

Va, upper

Va, middle

Va, middle

Va, middle

Va, middle

Va, lower

Va, lower

Va, lowest

Vb, upper

Vb, upper

Vb, middle

Vb, middle

Vb, lower

Vc, upper

Vc, lower

Vc, lower

Greenish-gray granule conglom-erate with Nummulites

Greenish-gray, coarse-grainedsandstone with Nummulites

Greenish-gray, coarse-grainedsandstone with granule clastsand calcareous tests

Dark-greenish-gray, very coarse-grained sandstone with granuleclasts and Nummulites

Grayish, very coarse-grainedsandstone with Nummulites

Greenish-gray, conglomeratic,very coarse-grained sandstonewith granule clasts andNummulites

Greenish-gray granule conglom-erate with Nummulites andpebble clasts of light-grayNummulites limestone

Greenish-gray, medium-grainedsandstone with granule clastsand calcareous tests

Greenish-gray granule conglom-erate with Nummulites

Greenish-gray, medium-grainedsandstone with calcareous tests

Dark-greenish-gray, medium-grained sandstone with granuleclasts and fragmental calcare-ous tests

Dark-greenish-gray, very coarse-grained sandstone with frag-mental calcareous tests

Greenish-gray, medium-grainedsandstone with calcareous tests

Dark-greenish-gray, medium-grained sandstone

Dark-greenish-gray, coarse-grained sandstone

Dark-greenish-gray, medium- tocoarse-grained sandstone

Dark-greenish-gray conglomer-ate with granule and pebbleclasts

Dark-greenish-gray conglomer-ate with granule and pebbleclasts

Hornblende schist occurs as rare lithic clasts withlengths up to 2 mm. It is observed only at 445-83-4,27-28 cm. This schist (schistosity is not well developed)is composed of green hornblende, albite, quartz, andunidentified dust-like minerals. Green hornblende, elon-gate parallel to schistosity, has lengths up to 1 mm.

Detrital MineralsSeveral kinds of detrital minerals were identified: green

to pale-brown hornblende, dark-brown hornblende,plagioclase, augite, titaniferous augite, olivine, chromi-an spinel, opaque minerals, epidote, diopside, aegir-ine-augite, actinolite, and enstatite.

Green to pale-brown hornblende and dark-brownhornblende, with lengths of less than 1 mm, are com-mon in sub-unit Va, but rare in sub-units Vb and Vc.

Plagioclase, with lengths of less than 1.5 mm, is thedominant detrital mineral throughout unit V. Somelaths are strongly zoned. It is often altered to chlorite orclay minerals in sub-units Vb and Vc.

Augite and titaniferous augite are common minerals,particularly dominant at 445-83-4, 27-28 cm. Cleavageis developed in both types. Titaniferous augite is palebrown, with weak pleochroism.

Olivine pseudomorphs (replaced by serpentine) arealso common, with lengths of less than 0.6 mm.

Chromian spinel and opaque minerals are rare through-out unit V, except at 445-83-4, 27-28 cm, where they areabundant. The chromian spinel is picotite, with a dark-reddish-brown tint and lengths of about 0.5 mm. Theopaque minerals, with lengths of up to 0.6 mm, aremainly ilmenite and magnetite, with small amounts ofhematite.

Epidote, with lengths up to 0.4 mm, is also rare; it isobserved at 445-68-1, 93-94 cm and 445-83-4, 27-28 cm.It is pale greenish and appears as aggregates.

Diopside, enstatite, actinolite, and aegirine-augite wereobserved, although they occur in very minor amounts inthe studied samples.

DREDGE HAULS FROM THE DAITO RIDGEDuring the last several years, many rocks have been

dredged from 13 sites on the crest, the uppermost slopes,and the northern peak of the Daito Ridge (Mizuno etal., 1977) (Figure 6). The rocks include igneous rocks,metamorphic rocks, sedimentary rocks, and Nummu-//tes-bearing rocks.

Various kinds of igneous rocks have been reported.Dolerites were collected from the eastern edge of themain body and northern peak of the Daito Ridge (loca-tion GDP 15-7). Two-pyroxene andesite, hornblendeandesite, hornblende andesite, augite andesite, horn-blende-biotite granodiorite, and small amounts of pyro-clastic rocks have been also found (locations GDP 15-1,2, 3, 7, and others) (Mizuno et al., 1975; Ishibashi,1975; Ishikawa and Aoki, 1978).

Metamorphic rocks which were dredged from theeastern part of the Daito Ridge (location GH 183) (Mi-zuno et al., 1975) were identified mainly as hornblendeschist (Yuasa and Watanabe, 1977). The hornblendeschist, whose schistosity is represented by fine alterna-tions of felsic and mafic layers, is mainly composed ofhornblende, actinolite, plagioclase, and epidote, withminor amounts of quartz, calcite, ilmenite, and sphene.Yuasa and Watanabe reported that this schist is charac-terized by low-alkali hornblende, and suggested that itmay have formed under low pressure. Tremolite schistwas reported as clasts in dredged conglomerate from thesame site; it is composed of tremolite, diopside, chlo-rite, hornblende, and opaque minerals (Yuasa and Wa-tanabe, 1977). In addition, the conglomerate is accom-panied by clasts of diopside-chlorite schist and ultra-mafic rocks. The ultramafic rock contains serpentine,

633


TABLE 2Chemical Analyses (%) and Calculated Atomic Proportions of Minerals from DSDP Site 445 Cores

and Dredged Ultramafic Rocks, Daito Ridge

Com-ponent

SiO 2

A1 2 O 3

TiC 2FeOMnOMgOCaONa2OK 2OCr 2 O 3

Total

SiAlTiFe+2Fe+3MnMgCaNaKCr

1

ChromianSpinel

_

37.32-

15.970.13

14.490.13

—-

31.88

99.92

10.112_

2.9770.0930.0244.9670.033

—_

5.795

2

ChromianSpinela

_

38.12—

16.360.02

14.840.12

——

30.55

100.01

_

10.267—

2.9120.2150.0045.0540.030

—_

5.518

3

ChromianSpinela

_

48.59—

13.78_

17.600.04

_—

18.96

98.97

_

12.488—

2.2700.2440.0005.7210.009

__

3.268

4

Olivineb

42.070.080.075.770.37

51.030.030.000.000.08

99.50

1.01450.00240.00120.1163

—0.00751.83450.00070.00000.00000.0016

5

Olivineb

42.230.090.105.850.32

50.880.090.000.000.14

99.70

1.01730.00270.00180.1178

—0.00651.82720.00230.00000.00000.0027

6

Sρinelc

(core)

0.2564.36

0.0012.580.24

21.140.030.05_

1.45

100.10

0.05115.3320.0001.6890.4370.0416.3700.0060.018_

0.231

7

Spinelc

(margin)

0.2764.67

0.0012.41

0.3022.20

0.000.02—

0.30

100.17

0.05515.342

0.0001.4790.6100.0516.6620.0000.010_

0.048

8

Hornblende^

49.2910.050.945.140.33

19.4312.47

1.04—

0.06

98.73

6.6921.6070.0960.581—

0.0383.9001.8131.273

0.007

9

Hornblende^

53.136.980.705.130.18

19.6112.550.68—

0.00

98.96

7.141.1051.1050.577—

0.0213.9281.8070.177—

0.000

^Core 445-83-4, 27-28 cm; 32 oxygens.Dredged ultramafic rocks; 4 oxygens.

'Dredged ultramafic rocks; 32 oxygens.Dredged ultramafic rocks; 22.5 oxygens.

hornblende, chlorite, spinel, and magnetite, with minoramounts of olivine. The chemical compositions of oliv-ine, spinel, and hornblende are listed in Table 2. Thespinel is characterized by greenish spinel (Cr2O3 0.3wt. %) rimming a core of reddish-brown spinel (Cr2O3

1.5 wt. °7o). The olivine appears as tiny, irregular rem-nants within aggregates of serpentine (antigorite) orchlorite. Its composition is Fo93 to Fo9 4. The hornblendeis characterized by low alkali content and occurs spo-radically as aggregates or associated with antigorite.The low-alkali hornblende and the antigorite may havebeen formed by thermal metamorphism (Arai, 1975).This suggests that the ultramafic rocks may have beenaffected by thermal metamorphism.

Fragments of Nummulites boninensis-beanng lime-stone, many individual specimens of the species, andAsterocyclina have been reported from several sites atthe Daito Ridge (locations GDP 15-4, 7, and others).These in particular indicate upraising of the crestal partsof the Daito Ridge into a shallow-sea environment (Shi-ki et al., 1977, Mizuno et al., 1979). Oligocene nanno-fossil chalk also has been teported.

CORRELATION OF CORES ANDDREDGED SAMPLES

The clasts of igneous rocks from Hole 445 are mainlyplagioclase-phyric basalt, plagioclase-clinopyroxene-phy-ric basalt, and microdolerite. Dolerite is also found in

the dredged samples. However, dredge hauls from theDaito Ridge contain two-pyroxene andesite and horn-blende granodiorite, which were not found in Hole 445.The heavy mineral analysis of sandstone from lithologicunit V (Sato, this volume) shows that hyperthene, a con-stituent mineral of two-pyroxene andesite, is rare through-out the unit. Therefore, it is assumed that mafic volcan-ic activity characterized by plagioclase-phyric basalt,plagioclase-clinopyroxene-phyric basalt, and microdol-erite (perhaps a tholeiitic series) was the main volcanicevent on the Daito Ridge during or before the deposi-tion of unit V.

This volcanism, coupled with a period of alkali-seriesvolcanism, is suggested by detrital aegirine-augite andtitaniferous augite in unit V. Ozima et al. (this volume)report that the age of the plagioclase-clinopyroxene-phyric basalt in a clast from Site 445 (Core 93, Section2; unit Vc) is about 59 m.y. (Âr/Âr method). There-fore, the age of this event is taken to be Paleocene. Onthe other hand, drilling at Site 446, in the Daito Basin,recovered alkali sills, and the age of these sills is about60 m.y. (Âr/Âr method; Ozima et al., this volume).Therefore, volcanic activity occurred in the Paleocenenot only on the Daito Ridge but also in the Daito Basin.

Because east-southeast-trending magnetic lineationssouth of the central basin fault were identified as anom-alies 18 through 21 (Louden, 1976), spreading of thePhilippine Basin is thought to have started before 55

634


TABLE 3Petrography of Igneous and Metamorphic Clasts and Detrital Minerals in Conglomerate, Site 445


Igneous and Metamorphic Clasts

Rock Type Minerals Remarks Mineral

Detrital Minerals

Remarks

Unit Va445-69-2, 90-92

71-1, 76-77

72-2, 6-8

74-1, 87-89,96-97

74-3,59-61

Pl-phyric basalt

Microdolerite

Microdolerite

Pl-phyric basalt

Pl-phyric basalt

Pl-phyric basalt

Microdolerite

Aphyric basalt

Pl-cpx-phyricbasalt

Phenocryst:plagioclase

Groundmass:plagioclaseclinopyroxeneopaque minerals


Microlite:plagioclaseclinopyroxeneopaque minerals


Microlite;plagioclaseclinopyroxeneopaque minerals









Microüte togroundmass:

plagioclaseopaque mineralsglass

Phenocryst:plagioclaseclinopyroxene


Dominant lithic type;hyalopilitic to inter-sertal texture;size < 1 mm

Rare lithic type;subophitic texture;size 1-3 mm

Common lithic type;subophitic texture;size 0.6-4 mm

Common lithic type;hyalopilitic texture;size 0.4-1.5 mm

Common lithic type;hyalopilitic to inter-sertal texture;size 0.2-0.5 mm

Dominant lithic type;hyalopilitic to inter-sertal texture;size ~ 1 mm

Common lithic type;subophitic texture;size ~ 2 mm

Rare lithic type; plagi-oclase and opaqueminerals showing den-dritic texture; glasshighly altered to clayminerals; size < 0.5 mm

Dominant lithic type;hyalopilitic to inter-sertal texture;size 5-6 mm

Plagioclase

Green hornblende

Olivinepseudomorph

Plagioclase

Pale-greenhornblende

Brown hornblende

Plagioclase

Green hornblende

Green hornblende

Plagioclase

Titaniferousaugite

Augite

OlivinepseudomorphBrown hornblende

Green hornblende

Common mineral; often highlyzoned; size < 0.8 mm

Common mineral; size < 0.5 mm

Rare mineral; replaced by ser-pentine; size < 0.3 mm

Common mineral;size 0.2-0.4 mm

Rare mineral; size < 0.3 mm


Common mineral; often slightlyzoned; size 0.2-0.3 mm


Common mineral; size < 1 mm

Common mineral; often highlyzoned; size < 1 mm

Common mineral; pale brownish;size < 0.5 mm




Common mineral; slightly tohighly altered to chlorite;size < 1 mm

Ma. The volcanic activity in the Daito Ridge and Basinprovince might have been related to the beginning ofspreading of the west Philippine Basin.

Clasts of metamorphic rocks are rare throughout unitV; however, some metamorphic minerals are recognized(hornblende, actinolite, epidote, and diopside; magne-

635


TABLE 3 - Continued


Unit Va

445-74-3, 92-94

77-5, 37-38

77-5, 45-46

81-1, 143-145

Rock Type

Pl-phyric basalt

Microdolerite

Aphyric basalt

Pl-phyric basalt

Microdolerite

Pl-phyric basalt

Microdolerite

Pl-phyric basalt


Minerals


Groundmass:plagioclaseclinopyroxeneopaque mineralsglass (±)



Microlite togroundmass:

plagioclaseclinopyroxeneopaque mineralsglass


Microphenocryst:plagioclase




plagioclaseclinopyroxeneopaque minerals






Remarks

Dominent lithic type;size 1-3 mm

Common lithic type;subophitic texture;size 3-4 mm

Common lithic type;clinopyroxene andglass are highly alteredto clay; size 1-5 mm

Rare lithic type; inter-sertal texture; clinopy-roxene is highly al-tered; size ~ 0.2 mm(Lithic clast are rarein this sample.)

Common lithic type;subophitic texture;size 1-1.5 mm

Common lithic type;intersertal texture;size 2-3 mm

Common lithic type;subophitic texture;size 1-2 mm

Common lithic type;intersertal texture;size 1.5-2.0 mm

Mineral

Plagioclase

Plagioclase

Pale-greenhornblende

Opaque minerals

Plagioclase

Detrital Minerals

Remarks

Rare mineral; partly altered toclay minerals; size < 0.3 mm(Detrital minerals are rare in thissample.)

Rare mineral; size 0.3-0.4 mm

Rare mineral; size ~ 0.2 mm

Rare mineral; size < 0.2 mm(Detrital minerals are rare in thissample.)


tite and ilmenite are partly of metamorphic origin).Hornblende schist, tremolite schist, and diopside-chlo-rite schist were found among dredged rocks. There seemsto be good correlation between the metamorphic miner-als from cored and dredged samples. In the hornblendeschist from the Daito Ridge, hornblende and epidote oc-cur together with albite-andesine (Yuasa and Watan-

abe, 1977). Hornblende coexists with albite in interme-diate- and high-pressure regional metamorphism (Miya-shiro, 1973). The hornblende in this schist is character-ized in its low alkali content and it coexists with smallamounts of K-feldspar; such hornblende occurs in con-tact metamorphic aureoles. On the other hand, it is as-sumed that the schistosity of this rock has resulted from

636


TABLE 3 - Continued


UnitVb

445-83-4, 27-28

83-4,48-49

85-1, 20-22

86-1, 75-77


Rock Type

Pl-phyric basalt

Hb schist

Pl-phyric basalt

Microdolerite

Aphyric basalt

Pl-phyric basalt

Microdolerite

Micro dolerit,e

Pl-phyric basalt

Minerals



HornblendeAlbiteQuartz






clinopyroxeneopaque mineralsglass


Groundmass:plagioclaseclinopyroxeneopaque mineralsglass



Microphenocryst:plagioclaseclinopyroxene

Microlite;plagioclaseclinopyroxeneopaque minerals



Remarks

Rare lithic type;size ~ 2 mm(Lithic clasts are rarein this sample.)

Rare lithic type;highly to moderatelyaltered; size 2 mm

Dominant lithic type;intersertal texture;opaque minerals acicu-lar; size ~ 2 mm

Common lithic type;subophitic texture;size < 1.5 mm

Rare lithic type;plagioclase showsquench morphology;size ~ 2 mm

Rare lithic type;hyalopilitic texture;clinopyroxene andglass are highlyaltered; size ~ 2 mm

Rare lithic type;subophitic texture;clinopyroxene altered;size ~ 0.5 mm

Common lithic type;subophitic texture;size < 0.5 mm

Common lithic type;hyalopilitic to inter-sertal texture; highlyaltered; size ~ 0.5 mm

Mineral

Opaque mineralsChromian spinelAugiteOlivinepseudomorph

Titaniferous augiteGreen hornblendeEpidote

Plagioclase

Plagioclase

Plagioclase

Opaque minerals

Detrital Minerals

Remarks

Dominant mineral; size < 0.6 mmCommon mineral; size ~ 0.5 mmCommon mineral; size < 0.2 mmCommon mineral; altered to ser-pentine or clay minerals;size < 0.6 mmRare mineral; size < 0.2 mmRare mineral; size < 0.2 mmRare mineral; size < 0.4 mm(Detrital minerals are abundantin this sample.)

Dominant mineral; often slightlyzoned; size < 1 mm


Dominant mineral; often slightlyzoned; size 0.2-0.4 mm


intermediate- to high-pressure regional metamorphism,rather than contact metamorphism. Therefore, the meta-morphic rock in the Daito Ridge may have undergonetwo different types of metamorphism: intermediate- to

high-pressure regional metamorphism in the pre-Paleo-cene, and contact metamorphism related to Paleocenevolcanic activity. The age of this schist is 49 ±3.7 m.y.(Mizuno et al., unpublished data; determined by the

637


TABLE 3 - Continued



Rock Type Minerals Remarks Mineral

Detrital Minerals

Remarks

UnitVb445-88-2, 32-34 Microdolerite

Pl-phyric basalt

Unit Vc

445-91-1,50-51 Apyric basalt

Pl-phyric basalt

Microdolerite

Microphenocry st:plagioclase







Groundmass:plagioclaseclinopyroxeneglass

Microphenocryst:plagioclaseclinopyroxene

Microlite:plagioclaseclinopyroxene

Common lithic type;subophitic texture;size 1-1.5 mm

Common lithic type;intersertal texture;clinopyroxene highlyaltered; size < 1.2 mm

Common lithic type;plagioclase and opaqueminerals show den-dritic texture; glasshighly altered to clayminerals; size < 2 mm

Common lithic type;hyalopilitic to inter-sertal texture; glasshighly altered;size 0.5-1.0 mm

Common lithic type;subophitic texture;clinopyroxene moder-ately altered;size 0.7 mm

Plagioclase

Opaque minerals

Dominant mineral;size 0.1-0.4 mm


93-2,16-18 Pl-phyric basalt

Aphyric basalt

93-2,105-107 Pl-phyric basalt

Microdolerite

Aphyric basalt


Groundmass tomicrolite:






Phenocryst:plagioclaseclinopyroxeneopaque mineralsglass (±)


plagioclaseclinopyroxeneopaque mineralsglass

Dominant lithic type;intersertal to hyalopi-litic texture; skeletalopaque grains;size 2-8 mm

Rare lithic type;plagioclase showsquench morphology;glass mostly altered toglauconite;size < 1 mm

Dominant lithic type;intersertal to hyalopi-litic texture;size 2-3 mm

Common lithic type;subophitic texture;size ~ 2 mm

Rare lithic type;size ~ 1 mm

Plagioclase Rare mineral; size ~ 0.3 mm(Detrital minerals are rare in thissample.)

638


1.00 mm 0.05 mm

0.025 mm 0.05 mm

Figure 5. Photomicrographs of (A) plagioclase phyric basalt (445-69-2, 90-92 cm); (B) microdolerite (445-93-2, 105-107 cm); (C) aphyric basalt (445-74-1, 96-97 cm); and (D) picotite and other detrital minerals (445-83-4, 27-28 cm).

K/Ar method, using hornblende separated from the sam-ple). This age may indicate the second metamorphicevent.

The 10-m.y. difference between the ages of the basaltand the schist can be attributed to alteration of thehornblende. The age of the schist (49 ±3.7 m.y.) shouldbe taken to be the minimum age of the metamorphism.

Some ultramafic detrital minerals are recognized(chromian spinel, serpentine pseudomorphs after oliv-ine, magnetite, and enstatite), although clasts of ultra-mafic rocks were not found in the cores from Site 445.The chemical composition of the chromian spinels isshown in Table 2. The analyses were recast in structuralformulas on the basis of 32 oxygens. Total Fe was parti-tioned between A and B sites as Fe2+ and Fe3+ in theproportions required for stoichiometry (Finger, 1972),yielding an estimate of the amount of ferric iron in thespinel. From the covariation of Fe3 + /(Fe3+ + Cr + Al)

and Mg/(Mg + Fe2+) in spinel (Figure 7), it can be seenthat these chromian spinels are characterized by lowFe3+ content, and they plot in the field of alpine-typeperidotites.

Considering that chromian spinel in the cores musthave been derived from the Daito Ridge, the dredged ul-tramafic rocks are assumed to be thermally metamor-phosed alpine-type peridotite, and the basement of theDaito Ridge is assumed to be partly composed of thisperidotite. This thermal metamorphism also is assumedto have been caused by the Paleocene volcanic activity.Therefore, the age of the ultramafic rocks may be pre-Paleocene.

The bioclasts from unit V (Nummulites boninensis,Asterocyclina sp. cf. A. penuria, etc.) are quite consis-tent with the dredge-sample data. This implies that unitV (at least sub-units Va and Vb) and the Nummulites-bearing strata on the ridge are nearly the same age, in-

639


Figure 6. Dredge sampling sites in the vicinity of Site445. Line A-B is the track of the multichannel seis-mic-reflection record of Figure 4.

<+ 0.2o

-

u.- 0.1

Iherzolites

( \

i i i

alpine peridotites

0.9 0.8 0.7 0.6

Mg/(Mg + Fe2+)

0.5 0.4

Figure 7. Compositions of chromian spinel from Site445 (lithologic unit Vc; Core 83, Section 4). The com-positional field of spinels from Iherzolites is fromO'Hara et al. (1971) and Littlejohn and Greenwood(1974); the field for spinels from alpine-type perido-tites is from Loney et al. (1971).

dicating transport of the sedimentary materials fromshallow water to the bottom of the depression.

The available data (Table 4), demonstrate that theclastic and bioclastic materials of the conglomerate andsandstone in unit V must have been supplied from theupraised Daito Ridge. In the upraised ridge or island,crystalline schist, such as hornblende schist of interme-diate- to high-pressure type, and alpine-type peridotites,whose age is assumed to be pre-Tertiary, have sufferedcontact metamorphism caused by tholeiitic and alkalicvolcanism of Paleocene age.

Detritus from metamorphic and volcanic rocks fromthe elevated area was distributed, in the surroundingshallow sea, together with Nummulites and other organ-isms. Steep offshore slopes may have caused frequentslumping and transport of the pebble clasts and organicremains from shallow to deeper waters.

ACKNOWLEDGMENTS

We are grateful to Assoc. Prof. H. Kagami, Dr. K. Fuji-oka, and Dr. T. Furuta for critical reading of the manuscript.We are indebted to Prof. N. Nasu for critical suggestions, andalso to Miss M. Nash, who helped with English.

REFERENCES

Arai, S., 1975. Petrology of alpine-type ultramafic complexesin the Sangun zone, western Japan, Ph. D. thesis, Geol. In-stitute, University of Tokyo.

Finger, L. W., 1972. The uncertainty in the calculated ferriciron content of a microprobe analysis. Ann. Rept. Dir.Geophys. Lab. Yearbook, 71, 600-603.

Ishibashi, K., 1975. Igneous rocks collected from the Daito,Oki-Daito Ridges and from the Okinawa trough. In Kaga-mi, H. (Ed.), Preliminary Report of the Hakuho-MaruCruise KH-72-2 (The Southwest Japan Arc and RyukyuArc Areas), pp. 110-111.

Ishikawa, M , and Aoki, H., 1978. On the volcanic rocksdredged in the Daito Ridge and Oki-Daito Ridge regions,northwestern Philippine Sea with special reference to petro-chemical characters on the GDP-15 and GDP-21 Cruises.Earth ScL, 32, 244-253.

Littlejohn, A. L., and Greenwood, H. J., 1974. Lherzolitenodules in basalts from British Columbia, Canada. Can. J.Earth Sci., 11, 1288-1308.

Loney, R. A., Himmelberg, G. R., and Coleman, R. G., 1971.Structure and petrology of the alpine-type peridotite atBurro Mountain, California, U.S.A.. J. Petrol., 12, 245-309.

Louden, K. E., 1976. Magnetic anomalies in the west Philip-pine Basin. In .4m. Geophys. Union Monogr., 19, pp. 253-267.

Miyashiro, A., 1973. Metamorphism and Metamorphic Belts:London (George Allen and Unwin).

Mizuno, A., Okuda, Y., Nagumo, S., Kagami, H., and Nasu,N., 1979. Subsidence of the Daito Ridge and associated ba-sins, north Philippine Sea. Am. Assoc. Petrol. Geol. Mem.,29, 239-243.

Mizuno, A., Shiki, T., and Aoki, H., 1977. Dredged rock andpiston and gravity core data from the Daito Ridges and Ky-ushu-Palau Ridge in the north Philippine Sea. In Geologi-cal Studies of the Ryukyu Islands (vol. 2), pp. 107-119.

Mizuno, A., Okuda, Y., Tamaki, K., Kinoshita, Y., Nohara,M., Yuasa, M., Nakajima, N., Murakami, F., Terashima,S., and Ishibashi, K., 1975. Marine geology and geologichistory of the Daito Ridges area, northwestern PhilippineSea. Marine ScL, 7, 484-491, 543-548.

Mizuno, A., Yuasa, M., and Tokuyama, H., in press. K-Ardating of hornblende schist from the Daito Ridge, northPhilippine Sea. Bull. Geol. Surv. Japan.

Nasu, N., Tomoda, Y., Kobayashi, K., Kagami, H., Uyeda,S., Nagumo, S., Kushiro, I., Ozima, M., Nakazawa, K.,Takayanagi, Y., Okada, H., Murauchi, S., Chujo, J., andIshii, Y., 1977. Multichannel seismic reflection data acrossthe Shikoku Basin and the Daito Ridges, 1976. IPOD-Ja-pan Basic Data Series, 1.

, 1978. Multichannel seismic reflection data acrossthe Shikoku Basin and the Daito Ridges, 1976. IPOD-Ja-pan Basic Data Series, 2.

O'Hara, M. J., Rechardson, S. W., and Wilson, G., 1971. Gar-net peridotite stability and occurrence in crust and mantle.Contr. Mineral. Petrol., 32, 48-68.

640


TABLE 4Summary of Core Samples from Site 445 and Dredge Hauls from the Daito Ridge

Rock Type

Igneous

Ultramafic

Metamorphic

Sedimentary

Biogenic

Basalt

Alkalibasalt

Andesite

Grano-diorite

Core Samples, Site 445

Clasts

Plagioclase-phyric basalt

MicrodoleriteApyric basaltPlagioclase-

clinopyroxene-phyric basalt

None

None

None

Hornblende schist

ChertNummulite s-beanng

limestoneSandstone

Nummulitesboninensis

Asterocyclina sp. cf. Apenuria

Operulinoides sp.

(lithologic unit V)

Detrital Minerals

PlagioclaseAugite

Aegirine-augiteTitaniferous augite

?

?

PicotiteEnstatiteMagnetiteSerpentine(pseudomorphsafter olivine)

Green hornblendeEpidoteDiopsideActinolite

(magnetite,ilmenite)

9

Dredge Hauls from theDaito Ridge

Dolerite

Two-pyroxene andesiteHornblende andesiteAugite andesite

Hornblende-biotite granodiorite

Peridotite (metamorphosed)

Hornblende schistDiopside-chlorite schistTremolite schist

NummuHtes-beanng limestoneSandstoneClaystoneCalcareous rock

Nummulites boninensisAsterocyclina sp.Discocyclina sp.

Ozima, M., Kaneoka, I., and Ujiie, H., 1977. Ar^-Ar39 age ofrocks, and the development mode of the Philippine Sea.Nature, 267, 816-818.

Shiki, T., Konda, I., Misawa, Y., and Nishimura, A., 1977.Geology and. geohistory of the northwestern PhilippineSea, with special reference to the results of the recent Ja-

panese research cruises. Mem. Fac. Sci. Kyoto Univ., Ser.Geol. Mineral., 43 (1), 67-78.

Yuasa, M., and Watanabe, T., 1977. Pre-Cenozoic metamor-phic rocks from the Daito Ridge in the northern PhilippineSea. J. Japan Ass. Mineral. Petrol. Econ. Geol., 72 (6),241-251.

641

Documents

SITE 445, PHILIPPINE SEA · Location of Site 445 and other DSDP sites in the Philippine Sea. through the unit: in sub-unit Va, they measure several millimeters (average diameter),