27. METALLOGENESIS IN THE PARECE VELA MARGINAL BASIN COMPLEX OF THEPHILIPPINE SEA, DEEP SEA DRILLING PROJECT LEG 59

Martha R. Scott, Department of Oceanography, and George W. Bolger, Department of Geology,Texas A&M University, College Station, Texas

This chapter addresses the question of whether back-arc spreading centers leave the same fingerprint ofmetal-enriched sediments that mid-ocean ridge spread-ing centers are so well known to leave. DSDP Legs 59and 60 made an east-west traverse across the SouthPhilippine Sea. Samples were collected in the PareceVela Basin (Sites 449 and 450), the West Mariana Ridge(Site 451), and the Mariana Trough (Sites 452-460). Nosamples from the West Philippine Basin were analyzed,because original basal sediments had been eroded andreplaced with coarse volcaniclastic breccia derived fromthe igneous basement itself (see Site 447 report, thisvolume). We analyzed basal-sediment samples fromHoles 449 and 450 in the Parece Vela Basin (Leg 59) andfrom Holes 453, 459B, and 460A (Leg 60) for alumi-num, calcium, magnesium, total iron, manganese, cop-per, colbalt, and nickel. Data and discussion of theresults of Leg 59 are included in this report; those forLeg 60 are forthcoming in that volume.

The samples consisted of 1 to 5 cm plugs that werehomogenized in an agate mortar and pestle. Aliquots offrom 0.3 to 0.5 g were weighed to the nearest 10 µg inTeflon beakers and dissolved in concentrated reagentgrade HC1, HNO3, and HF. The samples were allowedto dry, redissolved in concentrated hydrochloric acid,and diluted to 100 ml final volume (1% HC1 matrix).Additional samples of USGS standards of basalt (JB-1)and marine mud (MAG-1) were digested concurrentlyfor accuracy determinations.

The analyses were performed utilizing both flame(air-acetylene for Mg, Fe, Mn, and Cu; nitrous oxide-acetylene for Al and Ca) and flameless (HGA-2100 forCo and Ni) atomic-absorption spectrophotometry. Con-centrations were determined by comparison with ap-propriate dilutions of Fisher atomic-absorption stan-dards, with a precision of better than ±5% for allelements. Comparison of experimental results withpublished values for JB-1 (Flanagan, 1973) and MAG-1(Manheim et al., 1976) showed that recoveries werewithin ±6% for Al, Mn, Ca, Mg, Fe, and Cu and± 10% for Co and Ni. The analytic results are listed inTable 1.

Parece Vela Basin Sites 449 and 450 are respectivelylocated west and east of the trough, which is identifiedas the now inactive spreading center that formed thebasin (Kroenke and Scott, 1978). The sediment at Site449 consists of 111 meters of upper-Oligocene to Pleis-tocene normal oceanic clays and nannofossil oozes. Itshould be noted that the last core [Core 13] above base-ment had a cored length of 6.5 meters, yet 9.0 meters of

core were recovered. Probably the upper part of thecore consists of material displaced from higher in thesection; in all likelihood, the basal part of the coreabove igneous basement is in place.) The basalt base-ment at Hole 449 is reported by Kroenke and Scott(1978) to be "most likely of late Oligocene age." Karig(1975) has also suggested that the western side of thebasin appears to be older than the Miocene. The sedi-ment contact with basement was not preserved wellenough in the coring process to study; the underlyingbasalt was reported to show moderate low-temperaturealteration (Site 449 report, this volume).

Site 450 was drilled in the eastern Parece Vela Basinon the western flank of the West Mariana Ridge. Thecore penetrated a thick sediment wedge, 90 meters ofPleistocene to middle-Miocene pelagic clay, whichgrades downward to basaltic tuffs that continue to adepth of 330 meters (these tuffs were derived fromvolcanism on the West Mariana Ridge [Kroenke andScott, 1978]). The sediment terminates in basalt, but it isnot representative of basal sediment, because a bakedmetamorphic contact suggests an intrusive basalt. Truebasement is probably several hundred meters lower. Theage of sediments above the intrusion are several millionyears younger than basement age based on eithermagnetic-age estimates (Langseth and Mrozowski, thisvolume) or geologic evidence (see Site 450 report, thisvolume).

The metal to aluminum ratios for samples from Hole449 are plotted against depth in Figure 1. The verticaldashed lines represent average Pacific pelagic-clay(APC) ratios (Table 2). The sediments are generallyenriched in Fe, Cu, Co, and Ni with respect to averagepelagic clays. The distribution of ratios shown in Figure1 indicates that the greatest enrichment is found in thebasal sediments—in the lower 2.5 meters of the core.This fits the pattern of metal enrichment delineated byBoström's data (1970, 1975) and suggests a strong paral-lelism between mid-ocean ridge spreading and back-arcspreading. That is, sediment forming at spreadingcenters of either type is enriched in metals by sea waterthat has circulated through the oceanic crust; as thespreading moves this newly formed basal sediment awayfrom the center, the metal-enriched sediment is buriedunder normal, nonmetalliferous sediment. Kroenke andScott (1978) have suggested that the IPOD Trough inthe enter of the Parece Vela Basin may have been asmall segment of the spreading center here.

Boström and Peterson (1969) have determined that avalue of less than 0.4 for the A1/(A1 + Fe + Mn) ratio is

649

M. R. SCOTT, G. W. BOLGER

Table 1. Composition of basal sediments collected during DSDP Leg 59 at Holes 449 and 450.

Sample Depth below Sea Floor Fe Mn Ca Mg Al Cu Co Ni(intervals in cm) (m) (%) (%) (<7o) (%) (%) (ppm) (ppm) (ppm)

449-13-2,449-13-2,449-13-3,449-13-3,449-13-3,449-13-3,449-13-4,449-13-4,449-13-5,449-13-5,449-13-5,449-13-5,449-13-5,449-13-5,449-13-5,449-13-5,449-13-6,449-13-6,449-13-6,449-13-6,449-13-6,449-13-6,449-13-6,449-13-6,

450-35-1,450-35-1,450-35-1,450-35-1,450-35-2,450-35-2,450-35-3,450-35-3,450-35-3,450-35-3,450-35-3,450-35-3,450-35-3,450-35-3,450-35-3,450-35-4,450-36-1,450-36-1,450-36-1,450-36-1,450-36-2,450-36-2,450-36-2,450-36-2,450-36-2,450-36-2,450-36-2,

28-30128-13019-2065-67114-116124-12519-2181-8316-1820-2262-6475-7790-9294-96135-137142-1445-721-2338-4048-5056-5895-97119-121132-134

4-626-2858-6085-8947-50128-1320-420-2533-3742-4551-5459-6364-6780-8389-920-52-548-51112-113144-14737-4186-8995-99106-109115-118118-121121-124

Core 13a (104.5-111.0)

Core 35 (321.0-330.5)

Core 36 (330.5-340)

5.183.294.844.618.824.574.142.624.573.844.813.505.362.563.432.583.623.243.383.462.802.923.173.58

4.925.004.265.264.895.065.095.074.394.294.875.724.232.622.943.247.656.535.736.206.906.846.827.378.816.785.82

1.190.6201.020.9270.3090.7110.6780.4200.9290.8441.320.7871.740.5180.6950.7920.8080.7220.7260.7560.8840.7750.7600.816

0.1110.1120.1150.1150.1150.1120.1120.1220.1230.1190.2570.7360.5870.4050.3220.1740.9170.4960.3980.3490.4440.3730.4450.5191.230.8480.751

20.623.520.021.04.5319.320.425.520.523.621.726.323.629.420.321.425.726.625.822.625.823.825.423.1

3.883.603.553.892.733.113.283.032.592.732.803.562.721.891.982.051.181.277.024.821.321.211.231.331.241.131.04

1.040.9881.071.071.631.151.040.881.080.940.920.740.840.671.490.930.910.820.811.250.891.001.041.25

2.021.941.851.872.022.052.051.921.781.621.552.371.641.381.331.372.442.292.052.112.342.292.282.132.372.422.42

2.232.112.312.316.002.732.661.702.292.022.121.361.701.072.531.631.781.511.602.371.431.501.521.84

7.567.697.228.638.528.648.718.107.627.617.497.917.536.917.147.138.739.837.848.289.669.379.339.138.859.069.28

204125166169206163149102175155202120232103207139152140146187132143147182

57.865.381.050.643.058.258.864.273.651.376.8147.1102.6110.1111.457.7257.2212.8175.8203.6245.7237.7223.0277.2562.5472.9414.6

48.639.351.052.292.554.251.535.749.944.746.046.052.044.748.833.139.751.344.437.748.548.733.743.3

25.325.823.125.829.029.429.327.325.224.925.534.026.223.822.224.946.742.133.939.047.247.046.848.570.356.962.5

365347366335395366381320347357340528393377533342379330353389440363426421

34.332.535.938.339.240.031.428.046.030.950.050.542.341.330.330.6175134110113148150153188497455451

a Although the cored interval is 6.5 meters, 9.0 meters of sediment were recovered; see Figure 1 caption for anexplanation.

indicative of metal enrichment in sediments. All of thesamples analyzed from Site 449 fall below the 0.4 valueof this ratio.

The metal to aluminum ratios for samples fromCores 35 to 36 of Hole 450 are shown in Figure 2. Thesesediments, described earlier as not being true basaldeposits, show no evidence of metal enrichment whencompared to average pelagic clay. In all cases the ratioA1/(A1 + Fe + Mn) is greater than 0.4. In fact, the ver-tical average pelagic-clay ratio lines in Figure 2 suggestthese deposits to be unusually low in metals except foriron. The sedimentation rate for this site is listed as 50

meters per m.y. (Site 450 report, this volume), or 5 to 10times greater than for Site 449. The mass of basaltic tuffdelivered to Site 450 from the West Mariana Ridge ap-parently accumulated so rapidly that the chemistry ofthis material dominates the sediment chemistry in thispart of the basin and obscures any sign of hydrothermalmetal enrichment in the data. It even conceals metalenrichment by normal hydrogenous processes (Bonattiet al., 1972) that account for the relatively high metalcontent of Pacific sediment.

The metal to aluminum ratios for Site 450 are notwithout texture, however (Fig. 2). Just above the in-

650

METALLOGENESIS

HOLE 449, CORE 13 Core Section HOLE 450, CORES 35-36

103.5 -

104.0

3 e.Q

Q% 106.5

o

4 J=

C/)

108

^APC

m

•

1

LAPC

h

h

I*

r\\[i

^APC

•

t

LAPC1

^APC

i*

1 2 3 0 1

Fe/AI Mn/AI

3 4 5 6 7 8 9101112131 2 3 4 0 10 20 30

Cu/AI x 103

Figure 1. Metal to aluminum ratios for Hole 449. (Vertical dashedlines represent ratios for average Pacific pelagic clay [APC]—seeTable 2. Although only a 6.5-m interval was drilled for Core 13,the entire 9.0 m of core barrel were filled with soft sediment. Ap-parently the upper part of Core 13 actually consists of oozes in-troduced by flow of material from higher in the sequence. A voidexists between the last 50 cm of Section 4 and the first 50 cm ofSection 5 and may separate the admixture of flow materialrepresenting the interval immediately above the basalts. Thus thehigher metal to aluminum ratios observed at the base of Core 13probably represent the cored interval.)

Table 2. Average pelagic-clay metal to aluminum ratios.

Fe/AI Mn/AI Cu/AI × 103 Co/Al × 103 Ni/Al × 103

Average Pacific 0.61 0.11 4.8 1.3 3.6Pelagic Clay (APC)a

Average Atlantic 0.56 0.04 1.3 0.4 0.9Pelagic Clayb

a Turekian and Imbrie, 1966; Landergren, 1964, from Bischoff and Rosenbaum, 1977;Dymond et al., 1973.

b Turekian and Imbrie, 1966; Cronan, 1972; Boström and Valdes, 1969; Horowitz andCronon, 1976; Van der Weidjen et al.. 1970; El Wakeel and Riley, 1961.

trusive basalt, all of the metals increase abruptly in whathas been referred to as the metamorphic aureole (Kro-enke and Scott, 1978). There is also a strong Mn spike inthe middle of Core 35 that is associated with Fe, Cu,and Co increases. However, neither of these featurescan be interpreted unequivocally as the result of hydro-thermal activity at a spreading center.

We conclude that basal sediments at Site 449 in thewestern Parece Vela Basin have been enriched in metals,probably by a hydrothermal process related to back-arcspreading. Metal-enriched basal sediments have beenfound by Bonatti et al. (1979) in the West PhilippineBasin, which is west of the Parece Vela Basin andseparated from it by the Palau-Kyushu Ridge. Bonatti etal. (1979) have also concluded that the metal enrichmentis the result of hydrothermal processes related tospreading.

The lack of mirror-image metal enrichments in basalsediments from Site 450 may be attributed to two fac-

322.5

εg 325.52

1

36 3 3 8 • 5

2

3400 .5 0 .05 .1 0 5 0 .5 1.0 0

Fe/AI Mn/AI Cu/AI × 10J Co/Al x 10° Ni/AI x 10°

Figure 2. Metal to aluminum ratios for Hole 450. (Vertical dashedlines represent ratios for average Pacific Pelagic Clay [Table 2].)

tors: (1) these sediments probably lie well above the truebasement and are in contact with an intrusive basalt,and (2) the accumulation rate of tuffaceous sedimentfrom the West Mariana Ridge may have been so great asto obliterate any evidence of hydrothermal metal enrich-ment.

ACKNOWLEDGMENT

We thank Drs. Andrew Hajash and Robert Scott for reviewing thispaper.

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Bonatti, E., Kolla, V., Moore, W. S., et al., 1979. Metallogenesis inmarginal basins: Fe-rich basal deposits from the Philippine Sea.Mar. Geoi, 32:21-37.

Bonatti, E., Kraemer, T., and Rydell, H., 1972. Classification andgenesis of submarine iron-manganese deposits. In Horn, D. R.,(Ed.), Ferromanagese Deposits of the Ocean Floor: Washington(IDOE), pp. 149-165.

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, 1975. The origin and fate of ferromanganoan ridge sedi-ments. Stockholm Contrib. Geol., 27:149-241.

Boström, K., and Peterson, M. N. A., 1969. Origin of aluminum-poor ferromanganoan sediments in areas of high heat flow on theEast Pacific Rise. Mar. Geol., 1:421-441.

Boström, K., and Valdes, S., 1969. Arsenic in the ocean floor. Lithos,2:351-360.

Cronan, D. S., 1972. The Mid-Atlantic Ridge near 45°N: XVII. Al,As, Hg, and Mn in ferrugenous sediments from the median valley.Can. J. Earth Sci., 9:319-323.

Dymond, J., Corliss, J. B., Heath, G. R., et al., 1973. Origin ofmetalliferous sediments from the Pacific Ocean. Bull. Geol. Soc.Am., 84:3355-3372.

El Wakeel, S. K., and Riley, J. P., 1961. Chemical and mineralogicalstudies of deep-sea sediments. Geochim. Cosmochim. Ada, 25:110-146.

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Flanagan, F. J., 1973. 1972 values for international geochemicalreference samples. Geochim. Cosmochim. Ada, 37:1189-1200.

Horowitz, A., and Cronan, D. S., 1976. The geochemistry of basalsediments from the North Atlantic Ocean. Mar. GeoL, 20:205-228.

Kang, D. E., 1975. Basin genesis in the Philippine Sea. In Karig,D. E., Ingle, J. C , Jr., et al., Init. Repts. DSDP, 31: Washington(U.S. Govt. Printing Office), 857-879.

Kroenke, L., and Scott, R., 1978. Old questions answered—and newones asked. Geotimes, 23:20-23.

Manheim, F. T., Hathaway, J. L., Flanagan, F. J., et al., 1976.Marine mud, MAG-1, from the Gulf of Maine. In Flanagan, F. J.(Ed.), Descriptions and Analyses of Eight New USGS Rock Stan-dards: Geological Survey Professional Paper 840: WashingtonU.S. Govt. Printing Office).

Turekian, K. K., and Imbrie, J., 1966. The distribution of traceelements in deep-sea sediments of the Atlantic Ocean. EarthPlanet. Sci. Lett., 1:161-168.

Van der Weijden, C. H., Schuiling, R. D., and Das, H. A., 1970.Some geochemical characteristics of sediments from the North At-lantic Ocean. Mar. GeoL, 9:81-99.

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