26. METAMORPHISM OF ULTRAMAFIC CLASTS FROM …K. L. SABODA,P. FRYER, H. MAEKAWA 45* N I I I J I 125*...
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Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 125 26. METAMORPHISM OF ULTRAMAFIC CLASTS FROM CONICAL SEAMOUNT: SITES 778, 779, AND 780 1 Kristine L. Saboda, 2 Patricia Fryer, 2 and Hirokazu Maekawa 3 INTRODUCTION The Mariana arc-trench system, the easternmost of a series of backarc basins and intervening remnant arcs that form the eastern edge of the Philippine Sea Plate (Fig. 1), is a well-known example of an intraoceanic convergence zone. Its evolution has been studied by numerous investi- gators over nearly two decades (e.g., Kang, 1971; Uyeda and Kanamori, 1979; LaTraille and Hussong, 1980; Fryer and Hussong, 1981; Mros- owski et al., 1982; Hussong and Uyeda, 1981; Bloomer and Hawkins, 1983; Karig and Ranken, 1983; McCabe and Uyeda, 1983; Hsui and Youngquist, 1985; Fryer and Fryer, 1987; Johnson and Fryer, 1988; Johnson and Fryer, 1989; Johnson et al., 1991). The Mariana forearc has undergone extensive vertical uplift and subsidence in response to sea- mount collision, to tensional and rotational fracturing associated with adjustments to plate subduction, and to changes in the configuration of the arc (Hussong and Uyeda, 1981; Fryer et al., 1985). Serpentine seamounts, up to 2500 m high and 30 km in diameter, occur in a broad zone along the outer-arc high (Fryer et al., 1985; Fryer and Fryer, 1987). These seamounts may be horsts of serpentinized ultramafic rocks or may have been formed by the extrusion of serpentine muds. Conical Seamount, one of these serpentine seamounts, is located within this broad zone of forearc seamounts, about 80 km from the trench axis, at about 19°30'N (Fig. 2). The seamount is approximately 20 km in diameter and rises 1500 m above the surrounding seafloor. Alvin submersible, R/V Sonne bottom photography, seismic reflection, and SeaMARC II studies indicate that the surface of this seamount is composed of unconsolidated serpentine muds that contain clasts of serpentinized ultramafic and metamorphosed mafic rocks, and authigenic carbonate and silicate minerals (Saboda et al., 1987; Haggerty, 1987; Fryer et al., 1990; Saboda, 1991). During Leg 125, three sites were drilled (two flank sites and one summit site) on Conical Seamount to investigate the origin and evolution of the seamount. Site 778 (19°29.93'N, 146°39.94'E) is located in the midflank region of the southern quadrant of Conical Seamount at a depth of 3913.7 meters below sea level (mbsl) (Fig. 2). This site is located in the center of a major region of serpentine flows (Fryer et al., 1985, 1990). Site 779 (19°30.75'N, 146°41.75'E), about 3.5 km northeast of Site 778, is located approximately in themidflank region of the southeast quadrant of Conical Seamount, at a depth of 3947.2 mbsl (Fig. 2). This area is mantled by a pelagic sediment cover, overlying exposures of unconsolidated serpentine muds that contain serpentinized clasts of mafic and ultramafic rocks (Fryer et al., 1985, 1990). Site 780 (19°32.5'N, 146°39.2'E) is located on the western side of Conical Seamount near the summit, at a depth of 3083.4 mbsl (Fig. 2). This area is only partly sediment covered and lies near active venting fields where chimney structures are forming (Fryer et al., 1990). Fryer, P., Pearce, J. A., Stokking, L. B., et al, 1992. Proc. ODP Sci. Results, 125: College Station, TX (Ocean Drilling Program). 2 Planetary Geosciences Division, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, U.S.A. 3 Department of Earth Sciences, Faculty of Science, Kobe University, Nada, Kobe, 657, Japan. The ultramafic rock types present in the seamount are primarily harzburgite with subordinant dunite that show evidence of melt extraction in an island arc environment (Fryer, Pearce, Stokking, et al., 1990; Parkinson et al., this volume; Ishii et al., this volume). The purpose of this paper is to describe the mineralogic relationships observed in the ultramafic clasts and to present an interpretation of the metamorphic history of the rocks. Petrography of Ultramafic Clasts In hand specimen, the serpentinized, tectonized harzburgites range in color from a deep blackish-green to a light gray. Many exhibit small-scale color variations caused by veining of serpentine and carbonate. Although most clasts are massive, some show small-scale foliation caused by alignment of bastitic orthopyroxene. The primary minerals have been altered pervasively to serpentine. The texture of the serpentine phases and their interrelationships throughout clasts recovered in all of the drillholes on this seamount fall within the range observed in the dredge and Alvin samples (Saboda, 1991). Thus, the suite of clasts recovered attests to a general uniformity in very low- to medium-grade metamorphism. Original olivine, orthopyroxene, clinopyroxene, and apparently fresh chrome-spinel are still preserved (Table 1). Serpentinized olivine (about 75-95 modal % of the rock) forms a massive mesh-textured fabric and has no obvious crystal outlines. Bastitic orthopyroxene (5-25 modal %) is typically kink- banded and sometimes elongate, probably as the result of metamor- phically induced stress. Clinopyroxene (1-3 modal %) occurs as exsolution lamellae along the (100) plane of orthopyroxene and usually displays undulose extinction. Relict clinopyroxene (less than 1 mm in size) is also present in the margins of some orthopyroxene crystals (Girardeau and Lagabrielle, this volume). Chromium-spinel (less than 1 mm in size) occurs as disseminated and isolated anhedral grains. Chromium-spinels also sometimes occur as dumbbell-shaped crystals and occasionally form stringers (Girardeau and Lagabrielle, this volume). The degree of serpentinization varies from 40% to 100%. In the strongly serpentinized rocks, all of the original chromium-spinel (translucent in shades of deep reddish-brown/green when fresh) has been replaced by an opaque oxide mineral, probably magnetite. In some samples, this transformation process is incomplete. These par- tially altered spinels have original chromium-rich translucent cores, rims of magnetite, and radially arranged halos of microcrystalline chlorite (probably penninite). This type of alteration also has pene- trated cracks in the original spinel grains. There are numerous veins of serpentine (up to about 40 mmwide) cutting the harzburgites, many of which record a multistage history of filling. The degree of serpentinization of the harzburgite is usually greater near the veins. The veins contain chrysotile, and antigorite or lizardite is present in most. Some veins contain chrysotile that has grown both parallel to the original fracture walls and as discontinuous, crosscutting subsets. Subordinant brucite also is present. Some spinel trains were dislocated during the formation of serpentine veins. Orthopyroxene grains, adjacent to the veins in some samples, have been altered to chlorite. Typically, serpentinization affects olivine the most, orthopyroxene less, and clinopyroxene the least; however, in 431
Transcript of 26. METAMORPHISM OF ULTRAMAFIC CLASTS FROM …K. L. SABODA,P. FRYER, H. MAEKAWA 45* N I I I J I 125*...
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Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 125
26. METAMORPHISM OF ULTRAMAFIC CLASTS FROM CONICAL SEAMOUNT: SITES 778, 779,AND 7801
Kristine L. Saboda,2 Patricia Fryer,2 and Hirokazu Maekawa3
INTRODUCTION
The Mariana arc-trench system, the easternmost of a series of backarcbasins and intervening remnant arcs that form the eastern edge of thePhilippine Sea Plate (Fig. 1), is a well-known example of an intraoceanicconvergence zone. Its evolution has been studied by numerous investi-gators over nearly two decades (e.g., Kang, 1971; Uyeda and Kanamori,1979; LaTraille and Hussong, 1980; Fryer and Hussong, 1981; Mros-owski et al., 1982; Hussong and Uyeda, 1981; Bloomer and Hawkins,1983; Karig and Ranken, 1983; McCabe and Uyeda, 1983; Hsui andYoungquist, 1985; Fryer and Fryer, 1987; Johnson and Fryer, 1988;Johnson and Fryer, 1989; Johnson et al., 1991). The Mariana forearc hasundergone extensive vertical uplift and subsidence in response to sea-mount collision, to tensional and rotational fracturing associated withadjustments to plate subduction, and to changes in the configuration ofthe arc (Hussong and Uyeda, 1981; Fryer et al., 1985). Serpentineseamounts, up to 2500 m high and 30 km in diameter, occur in a broadzone along the outer-arc high (Fryer et al., 1985; Fryer and Fryer, 1987).These seamounts may be horsts of serpentinized ultramafic rocks or mayhave been formed by the extrusion of serpentine muds. Conical Seamount,one of these serpentine seamounts, is located within this broad zone offorearc seamounts, about 80 km from the trench axis, at about 19°30'N(Fig. 2). The seamount is approximately 20 km in diameter and rises 1500m above the surrounding seafloor. Alvin submersible, R/V Sonne bottomphotography, seismic reflection, and SeaMARC II studies indicate thatthe surface of this seamount is composed of unconsolidated serpentinemuds that contain clasts of serpentinized ultramafic and metamorphosedmafic rocks, and authigenic carbonate and silicate minerals (Saboda etal., 1987; Haggerty, 1987; Fryer et al., 1990; Saboda, 1991).
During Leg 125, three sites were drilled (two flank sites and onesummit site) on Conical Seamount to investigate the origin andevolution of the seamount.
Site 778 (19°29.93'N, 146°39.94'E) is located in the midflankregion of the southern quadrant of Conical Seamount at a depth of3913.7 meters below sea level (mbsl) (Fig. 2). This site is located inthe center of a major region of serpentine flows (Fryer et al., 1985,1990). Site 779 (19°30.75'N, 146°41.75'E), about 3.5 km northeast ofSite 778, is located approximately in the midflank region of the southeastquadrant of Conical Seamount, at a depth of 3947.2 mbsl (Fig. 2). Thisarea is mantled by a pelagic sediment cover, overlying exposures ofunconsolidated serpentine muds that contain serpentinized clasts of maficand ultramafic rocks (Fryer et al., 1985, 1990). Site 780 (19°32.5'N,146°39.2'E) is located on the western side of Conical Seamount near thesummit, at a depth of 3083.4 mbsl (Fig. 2). This area is only partlysediment covered and lies near active venting fields where chimneystructures are forming (Fryer et al., 1990).
Fryer, P., Pearce, J. A., Stokking, L. B., et al, 1992. Proc. ODP Sci. Results, 125:College Station, TX (Ocean Drilling Program).
2 Planetary Geosciences Division, Department of Geology and Geophysics, Schoolof Ocean and Earth Science and Technology, University of Hawaii, 2525 Correa Road,Honolulu, HI 96822, U.S.A.
3 Department of Earth Sciences, Faculty of Science, Kobe University, Nada, Kobe,657, Japan.
The ultramafic rock types present in the seamount are primarilyharzburgite with subordinant dunite that show evidence of meltextraction in an island arc environment (Fryer, Pearce, Stokking, etal., 1990; Parkinson et al., this volume; Ishii et al., this volume). Thepurpose of this paper is to describe the mineralogic relationshipsobserved in the ultramafic clasts and to present an interpretation ofthe metamorphic history of the rocks.
Petrography of Ultramafic Clasts
In hand specimen, the serpentinized, tectonized harzburgitesrange in color from a deep blackish-green to a light gray. Many exhibitsmall-scale color variations caused by veining of serpentine andcarbonate. Although most clasts are massive, some show small-scalefoliation caused by alignment of bastitic orthopyroxene. The primaryminerals have been altered pervasively to serpentine. The texture ofthe serpentine phases and their interrelationships throughout clastsrecovered in all of the drillholes on this seamount fall within the rangeobserved in the dredge and Alvin samples (Saboda, 1991). Thus, thesuite of clasts recovered attests to a general uniformity in very low-to medium-grade metamorphism. Original olivine, orthopyroxene,clinopyroxene, and apparently fresh chrome-spinel are still preserved(Table 1). Serpentinized olivine (about 75-95 modal % of the rock)forms a massive mesh-textured fabric and has no obvious crystaloutlines. Bastitic orthopyroxene (5-25 modal %) is typically kink-banded and sometimes elongate, probably as the result of metamor-phically induced stress. Clinopyroxene (1-3 modal %) occurs asexsolution lamellae along the (100) plane of orthopyroxene andusually displays undulose extinction. Relict clinopyroxene (less than1 mm in size) is also present in the margins of some orthopyroxenecrystals (Girardeau and Lagabrielle, this volume). Chromium-spinel(less than 1 mm in size) occurs as disseminated and isolated anhedralgrains. Chromium-spinels also sometimes occur as dumbbell-shapedcrystals and occasionally form stringers (Girardeau and Lagabrielle,this volume).
The degree of serpentinization varies from 40% to 100%. In thestrongly serpentinized rocks, all of the original chromium-spinel(translucent in shades of deep reddish-brown/green when fresh) hasbeen replaced by an opaque oxide mineral, probably magnetite. Insome samples, this transformation process is incomplete. These par-tially altered spinels have original chromium-rich translucent cores,rims of magnetite, and radially arranged halos of microcrystallinechlorite (probably penninite). This type of alteration also has pene-trated cracks in the original spinel grains.
There are numerous veins of serpentine (up to about 40 mm wide)cutting the harzburgites, many of which record a multistage historyof filling. The degree of serpentinization of the harzburgite is usuallygreater near the veins. The veins contain chrysotile, and antigorite orlizardite is present in most. Some veins contain chrysotile that hasgrown both parallel to the original fracture walls and as discontinuous,crosscutting subsets. Subordinant brucite also is present. Some spineltrains were dislocated during the formation of serpentine veins.Orthopyroxene grains, adjacent to the veins in some samples, havebeen altered to chlorite. Typically, serpentinization affects olivine themost, orthopyroxene less, and clinopyroxene the least; however, in
431
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K. L. SABODA,P. FRYER, H. MAEKAWA
45* N I I I
J I125* E 130' 135* 140* 145* 150*
Figure 1. Location map showing the regional setting of the Mariana arc system. Solid square depicts Leg 125 drilling area.
432
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METAMORPHISM OF ULTRAMAFIC CLASTS
I46°4O'
I9β3θ' N - - I9°3O' N
I0 KM
I9β30 N - r - I9°3O' N
I46β4θ' E
Figure 2. Locations of Leg 125 drill sites on Conical Seamount in the Mariana forearc.
several samples serpentinization may have affected orthopyroxenemore than olivine.
In hand specimens, the serpentinized and tectonized dunites aregenerally massive, vary from deep greenish-black to dun-brown, andsometimes grade to fresh harzburgite over distances of a few milli-meters. The primary mineralogy consists of 90%-99% olivine, 1%-9% orthopyroxene, and up to 1% chromium-spinel (Table 1). Mostdunites have been extensively serpentinized and exhibit mesh texture
and splays of antigorite. The dunites also are cut by numerousserpentine-rich veins. Some samples contain crudely aligned, elon-gate (less than 15 mm), and irregularly shaped olivine that tends toform a sheared fabric. Microgranulation of olivine, kink-banding inbastite, elongation of spinel, and deformation of clinopyroxene exso-lution lamellae in orthopyroxene all provide evidence of penetrativedeformation. An unusually large amount of fresh olivine persists in afew samples (Table 1), despite transformation of almost all of the
433
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K. L. SABODA,P. FRYER, H. MAEKAWA
Table 1. Modal mineralogy of ultramaflc rocks from Conical Seamount, Leg 125 samples.
Samplesite-core
125-778A-2R-1125-778A-2R-1125-778A-2R-1125-778A-7R-1125-778A-7R-1125-778A-7R-1125-778A-7R-2125-778A-7R-2125-778A-7R-CC125-778 A-7R-CC125-778A-8R-125-778A-8R-125-778A-9R-125-778A-12R-2125-778A-12R-2
125-779A-3R-CC125-779 A-4R-1125-779 A-4R-125-779 A-5R-125-779 A-5R-2125-779 A-5R-2125-779 A-5R-2125-779 A-5R-2125-779 A-5R-2125-779 A-6R-1125-779 A-8R-1125-779A-8R-1125-779A-8R-I125-779A-8R-I125-779A-9R-2125-779A-10R-125-779 A-10R-125-779A-I0R-125-779A-10R-125-779A-1IR-125-779A-1IR-125-779A-11R-125-779A-12R-125-779A-13R-
2
125-779A-13R-2125-779A-13R-3I25-779A-14R-1125-779A-14R-2125-779A-14R-2I25-779A-14R-2125-779 A-15R-1125-779 A- 15R-2125-779 A-15R-2125-779A-15R-2125-779A-15R-2125-779A-I6R-125-779A-I6R-125-779A-I6R-125-779A-I6R-125-779 A-I6R-125-779A-I6R-2125-779A-16R-2125-779 A-16R-2125-779 A-16R-2125-779 A-17R-1125-779A-I7R-2125-779A-17R-2125-779A-I7R-3125-779A-17R-3125-779A-I7R-4125-779A-19R-2125-779A-22R-1125-779A-22R-1125-779A-22R-2125-779A-22R-3125-779 A-24R-1125-779 A-25R-1125-779 A-26R-2125-779 A-26R-2125-779 A-26R-3125-779 A-28R-2I25-779A-28R-3
125-780C-6R-125-780C-8R-1125-780C-10R-125-780C-I0R-I25-780C-I0R-125-780C-I6R-125-780C-18R-
125-78OD-7X-1125-780D-7X-5
interval
57-5974-7578-8112-1613-1625-2855-6074-750-20-3
39-4047-4857-5843-4573-75
13-1527-3042-44
108-10914-1534-3740-4365-69
116-12018-2027-2957-6082-8390-9352-5426-2939-4354-5728-316-9
14-1834-3638-42
2-550-549-11
74-7721-2477-79
139-14115-1718-2024-2737-40
110-11116-1919-2337-4050-52
127-12944-4761-6474-77
117-120144-14517-2021-2477-8080-8344-4797-9958-6063-6518-2055-5736-3885-8750-5271-75
101-103109-11326-28
61-6298-10113-1636-3736-3753-5960-61
25-2694-96
p#
256
78
23512
3334
10a
25a5b8a8b4b
356123561
11
5a
38164
33517336819699111733
8b8b4
13II11
276
10b2b2c3b
2a
7a102667
2a
rockname
SHSHSHSHSHSHSHSHSHSHSHSH
ASHSHSH
SDSHSHSD
ASHSTHSHSDSHSH
ASHSH
STDSTDSTHSTD?STDSTDSHSHSHSDSHSHSHSHSDSDSHSHSHSHSDSHSDSHDHH
SDSHSHH
SHSDHHHH
SH
DASHSH
SDSDSD
SDH
ASHSH
ASDSH
SHSD
STHSDSDSHSH
SSH
olivineextant
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METAMORPHISM OF ULTRAMAFIC CLASTS
Table 1 (continued).
Samplesite-core
125-778 A-2R-1125-778 A-2R-1125-778A-2R-1125-778 A-7R-1125-778 A-7R-1125-778 A-7R-1125-778A-7R-2125-778A-7R-2125-778A-7R-CC125-778A-7R-CC125-778A-8R-1125-778A-8R-1125-778 A-9R-1125-778 A-12R-2125-778 A-12R-2
125-779A-3R-CC125-779 A^R-1125-779 A-4R-1125-779A-5R-1125-779A-5R-2125-779 A-5R-2125-779 A-5R-2125-779 A-5R-2125-779 A-5R-2I25-779A-6R-1I25-779A-8R-I125-779 A-8R-I125-779 A-8R-1125-779A-8R-1125-779A-9R-2125-779A-10R-125-779 A-10R-125-779A-10R-I25-779A-IOR-125-779 A-11R-I25-779A-11R-I25-779A-11R-125-779 A-12R-125-779 A-I3R-
2
125-779 A-13R-2125-779A-13R-3125-779A-14R-1125-779A-14R-2125-779A-14R-2125-779A-14R-2125-779 A-15R-1I25-779A-15R-2125-779A-15R-2125-779A-15R-2125-779 A-15R-2125-779 A l 6R-125-779A-I6R-125-779A-16R-125-779A-16R-I25-779A-16R-125-779 A-16R-2125-779 A-16R-2125-779 A-16R-2125-779 A-16R-2125-779 A-17R-1125-779 A-I7R-2125-779 A-I7R-2125-779 A-17R-3125-779 A-I7R-3125-779 A-17R-4I25-779A-19R-2I25-779A-22R-125-779 A-22R-1125-779A-22R-2125-779A-22R-3125-779 A-24R-I125-779 A-25R-1125-779 A-26R-2125-779 A-26R-2125-779A-26R-3125-779A-28R-2125-779A-28R-3
125-780C-6R-1125-780C-8R-1125-780C-10R-1125-78OC-1OR-1125-780C-10R-1125-780C-16R-1
125-780D-18R-1125-780D-7X-1125-780C-7X-5
mt
1322
2022321
1-201-22
11
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K. L. SABODA,P. FRYER, H. MAEKAWA
Table 2. Major-element (in weight % oxides) and trace-element (in ppm) data for ultramafic
rocks from Hole 778A.
Core:Interval (cm):
SiO2TiO2
A12O3Fe 2 O 3MnOMgOCaO
Na2OK2OP 2 O 5LOINiO
Cr 2 O 3Total
NhZrYSrRbZnCuNiCrVTi
CeBa
2R-153-56
38.170.000.746.730.10
36.590.070.090.010.00
15.060.290.36
98.20
tr1090
523
22812496
460trtr
2R-189-92
35.690.000.006.770.10
35.883.520.090.000.01
15.550.390.07
98.08
tr2
nd595nd283
3039501220trnd
3R-CC1-7
40.930.001.097.210.10
36.850.740.000.010.00
11.490.280.37
99.08
tr0151
3611
21732523
400
ndtr
7R-273-78
34.140.000.858.660.14
39.320.020.090.010.00
14.760.390.41
98.79
tr1
nd2
nd392
30342786
380trnd
7R-CC7-13
37.460.000.767.860.13
39.150.030.000.050.00
13.660.360.33
99.79
tr1
nd31
422
28182261
410
ndnd
8R-136-44
34.610.000.688.000.16
39.210.080.000.000.00
16.060.330.37
99.50
tr1
nd81
644
26382510
480
nd10
12R-243-45
41.520.000.718.950.08
39.610.060.000.000.006.010.360.42
97.70
tr019
nd428
28132866
480trnd
12R-273-75
39.290.001.017.640.06
37.040.070.000.000.00
11.990.370.53
98.00
tr1
nd120
402
29053603
620
ndnd
Oxide and elemental abundances are from whole-rock analyses by XRF; LOI = loss on ignition between 150°C and1030°C; tr = below detection limits (< 5 ppm for Nb;
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METAMORPHISM OF ULTRAMAFIC CLASTS
Table 3. Major-element (in weight % oxides) and trace-element (in ppm) data for ultramafic rocks from Hole 779A.
Core:Interval
(cm):
SiO2TiO2
A12O3Fe2O3MnOMgOCaONa2OK2O
P 2 O 5
NiOCr2O3LOITotalMg#
%SERP
NbZrYSrRbZnCuNiCrVCeBa
3R-CC1 3 - 1 5
36.740.000.397.380.10
40.310.140.000.020.000.280.42
14.1899.2791.54
80
tr1
nd21
311
22382903
25trtr
8R-190-93
39.470.000.287.630.11
41.210.150.000.000.000.280.399.55
98.3991.45
78
tr1020362
22082650
22tr13
10R-140-43
38.140.000.638.080.12
41.190.600.000.000.000.300.419.28
98.0690.99
55
tr122
nd357
2396279642trtr
14R-174-77
38.300.000.387.330.10
41.710.440.000.010.000.310.359.00
97.2791.85
70
tr0141
34nd
24462396
22trtr
14R-221-24
35.250.000.077.140.09
41.350.100.000.000.000.300.21
14.6498.6491.98
85
tr0nd00221
2398114412trtr
15R-224-27
35.540.000.075.660.08
42.140.210.000.020.000.450.19
16.7198.4293.65
85
tr1
nd41
33nd
35271319
11trnd
Dunites
16R-119-23
40.800.000.608.050.10
43.430.540.000.010.000.320.334.77
98.3191.45
17
tr1000375
2515223632tr13
19R-297-99
39.210.000.198.150.12
44.360.250.000.000.000.320.226.22
98.5091.51
35
tr11
ndnd334
2520150913trnd
22R-218-20
39.470.000.407.360.10
40.950.150.000.000.000.270.309.26
97.7491.68
85
tr111
nd312
21632024
29tr12
22R-355-57
38.090.000.107.760.11
42.960.130.000.000.000.300.189.17
98.3291.65
60
tr1041
323
23821216
8trtr
24R-136-38
34.500.000.467.310.11
38.940.370.004.000.000.290.25
16.27101.9791.3593
tr10
21nd462
2667168823trtr
25R-185-87
34.440.000.127.020.09
43.400.200.000.000.000.340.32
12.7798.0592.45
85
tr1
nd30333
24862167
8trnd
averageDunite
37.500.000.317.410.10
41.830.270.000.340.000.310.30
10.9998.5891.80
69
tr1040342
24962004
21trtr
Oxide and elemental abundances are from whole-rock analyses by XRF; tr = below detection limit (
-
K. L. SABODA,P. FRYER, H. MAEKAWA
Table 4. Major element (in weight % oxides) and trace element (in ppm) data for
ultramafic rocks from Hole 780C.
Core:Interval (cm):
SiO3TiOi
AI 2 O 3Fe^O,MnOMgOCaONa,O‰OP,O5NiO
C r ^LÒITotalMg#
% SERP
NbZrYSrRbZnCuNiCrVCeBa
6R-161-62
40.700.000.477.910.11
42.940.600.000.000.000.29
30.245.28
98.5591.49
65
tr0nd30333
2319162025ndtr
10R-11 3 - 1 6
38.420.000.607.450.11
39.040.790.000.020.000.280.31
12.3199.3191.21
65
tr1091
352
21882103
24ndtr
Harzburgites
16R-153-59
37.760.000.547.440.11
39.970.690.000.010.030.290.31
12.0698.6091.41
70
tr10111
458
22642126
30trtr
18R-154-57
38.560.000.637.800.11
39.130.860.000.000.000.290.28
11.1398.7890.86
75
tr11
ndnd3310
2318190725ndnd
18R-158-61
38.850.000.667.810.11
38.870.890.000.000.000.280.34
11.1298.9290.79
75
tr11003312
22072303
33trtr
avg.harz.
38.860.000.587.680.11
39.990.770.000.010.010.290.30
10.3898.8391.16
70
tr1050367
22592012
27tr13
Dunite
8R-198-101
38.430.000.657.840.11
39.780.500.000.010.000.290.34
11.9099.8490.95
55
tr1060321
22802295
41nd
Oxide and elemental abundances are from whole-rock analyses by XRF; LOI = loss on ignitionbetween 150 deg. C and 1030 deg. C; tr = below detection limits (< 5 ppm for Nb;
-
METAMORPHISM OF ULTRAMAFIC CLASTS
Table 6. XRD results of group 2 samples (45-89% secondary minerals).
Samplesite-core
rockinterval p# ol opx cpx sp C L br ml (ale magn cal arag chl clay amph hem il
778A-2R-1778A-7R-CC
779A-3R-CC779A-5R-2779A-8R-I779A-9R-2779A-10R-I779A-12R-I779A-13R-2779A-14R-2779A-14R-1779A-14R-2779A-15R-2779A-22R-1779A-22R-2779A-22R-2779A-25R-1779A-26R-3
780C-6R-1780C-8R-1780C-10R-1780C-18R-1780C-18R-1
89-92 77-13
13-1534-3790-9352-5440-4338-4250-5421-2474-77139-141
24-27
63-65
18-20
53-57
85-87
101-103
61-62
98-101
13-16
54-57
58-61
23
8b4b56135 a
3 b
7aI022a2a
S
sSD80SH 80
STD53STH50STD56SH80SH60SD76SD57SH49SD89SH52SD85SD61SD79SH51
SH63SD61SH63SH69SH69 0
0 0 0
0 O
S = serpentinite; SD = serpentinized dunite; SH = serpentinized harzburgite; STD = serpentinized, tectonized dunite; STH = serpentinized, tectonized harzburgite (numbers after rock namecorrespond to % of secondary minerals); ol = olivine; opx = orthopyroxene; cpx = clinopyroxene; sp = spinel; C = chrysotile; L = lizardite; A = antigorite; br = brucite; mt = magnetite;magn = magnesite; cal = calcite; arag = aragonite; chl = chlorite; clay = clay minerals; amph = amphibole; hem = hematite; il = ilmenite; coal = coalingite; (O = definitely present, 0= probably present, o = possibly present).
Assuming a central conduit for the seamount, Phipps and Ballotti (thisvolume) suggest that the clasts are either protruded and become partof the flows that blanket the seamount or they may settle back intothe conduit during periods of quiescence between eruptions. Theultramafic clasts in the seΦentine muds of Conical Seamount repre-sent a depleted supra-subduction zone mantle affected by very low-to medium-grade metamoΦhism.
The temperature and pressure conditions of the metamoΦhismrepresented by the ultramafic rocks from Conical Seamount arecomplex. On the basis of mineral assemblages and textural relation-ships it is possible to determine general conditions of metamoΦhismand history of metamoΦhic events. A detailed examination of struc-tures and textural relationships (Girardeau and Lagabrielle, this vol-ume) indicates a general retrograde event coincident, Girardeau andLagabrielle suggest, with the onset of subduction. The thermal historyof the Mariana convergent margin as it pertains to this event isdiscussed by Fryer (this volume). Our observations for this report dealprimarily with the mineral assemblages in very low- to moderate-grade metamoΦhism of the rocks studied.
Serpentinization of ultramafic rocks composed mainly of forsterite,Mg-rich pyroxenes and amphiboles depends on the availability of water.Serpentinization occurs in both retrograde or prograde events. It isgenerally agreed, on the basis of field work, petrographic evidence, andlaboratory experiments, that antigorite is stable in the high temperature-pressure range of serpentinization (~200°C-600°C) and chrysotile andlizardite are stable in the lower temperature-pressure range of serpentini-zation (0°C-200°C) in prograde events (Coleman, 1971; Moody, 1976;Bonatti et al., 1984). Formation of antigorite requires less water than doesthat of lizardite or chrysotile because the structure of antigorite accom-modates less Mg(OH)2 than does that of either lizardite or chrysotile(Whittaker and Wicks, 1970; Moody, 1976). Therefore, the generallyhigher temperature characteristic of antigorite foπnation is attributed byMoody (1976) to the greater degree of dehydration of the host rock andto a lower activity of water as temperatures rise. Solely on the basis oflaboratory experiments, the different temperature-pressure stability con-ditions for chrysotile relative to lizardite cannot be distinguished (Bonattiet al., 1984). Textural relationships between these phases in thin section
can elucidate the relative sequence of their formation but still cannotbe used to determine temperature-pressure history.
Wenner and Taylor (1971) were able to estimate formation tempera-tures of seΦentine minerals in prograde events in oceanic ultramaficrocks on the basis of 18O/16O fractionation between coexisting seΦentineand magnetite. Approximate equilibrium temperatures are 125°C forlizardite, 180°C for chrysotile, and 220°C to 460°C for antigorite (Wennerand Taylor, 1971). The most common seΦentine textures in the mediumto high temperature range of seΦentinization (220°C-600°C) are antigo-rite ± magnetite in interpenetrating configurations that form eitherthrough the recrystallization of lizardite ± chrysotile pseudomoΦhictextures (formed at lower temperatures), or less frequently through theseΦentinization of previously unserpentinized primary minerals (Wicks,1979). According to Wicks (1979), inteΦenetrating antigorite tex-tures represent intense metamoΦhism and form at up to 510°C at2 kbar water pressure. Thus, the reported range of antigorite forma-tion at 2 kbar spans about 220°C (Wenner and Taylor, 1971) to 510°C(Wicks, 1979). The reported range of chrysotile formation spansabout 180°C (Wenner and Taylor, 1971) to 440°C at 2 kbar (Wicks,1979). The reported range of lizardite formation spans about 120°C(Wenner and Taylor, 1971) to 375°C (Wicks, 1979).
According to Wicks (1979), at temperatures below approximately375°C at 2 kbar water pressure, lizardite ± chrysotile replaces olivine,pyroxene, amphibole, chlorite, and talc. A common mineral assemblageof serpentinization below 375°C is lizardite ± chrysotile ± magnetite,where lizardite ± chrysotile form mesh-textured pseudomoΦhs afterolivine and form bastite after pyroxenes and the magnetite is a commonbyproduct of the seΦentinization of the olivine (Wicks, 1979). Lizardite± chrysotile has been reported as replacing preexisting antigorite in alower temperature-pressure regime than is observed for antigorite forma-tion (Coleman, 1971; Mumpton and Thompson, 1975). However, antigo-rite can persist into a retrograde metamoΦhic regime (Mumpton andThompson, 1975). Thus, its presence cannot be used exclusively as anindicator of higher temperatures and pressures of metamoΦhism. Thetexture of the common very low grade (120°C-180°C) seΦentinitecontaining lizardite ± chrysotile is pseudomoΦhism of the preexistingprimary minerals (Winkler, 1976; Wicks, 1979). Wicks (1979) also notes
439
-
K. L. SABODA,P. FRYER, H. MAEKAWA
Table 7. XRD results of group 3 samples (90% secondary minerals).
Samplesite-core p#
rockname opx cpx br magn cal arag chl clay amph hem
778A-3R-CC778A-12R-2778A-12R-2
779A-5R-2779A-8R-1779A-11R-1779A-28R-3
1-743-4573-75
40-4357-6014-1826-28
35b32a
SSH98SH99
SH96SH98SH97SH98
00
0
000O
000
0
000
0
0
0
0
O0o
ooo0
0
0o0
o0
0
0
0
0
p # = piece number, where assigned (not all samples were assigned a piece number aboard the ship); S = serpentinite; SH = serpentinized harzburgite (numbers after rock name indicatethe % secondary minerals); ol = olivine; opx = orthopyroxene; cpx = clinopyroxene; sp = spinel; C = chrysotile; L = lizardite; A = antigorite; br = brucite; mt = magnetite; magn =magnesite; cal = calcite; arag = aragonite; chl = chlorite; clay = clay minerals; amph = amphibole; hem = hematite; il = ilmenite; coal = coalingite; sm-k = smectite-kaolinite; kao =kaolinite; gr = greenalite; cron = cronstedite; ch-Ib = chomasite-Ib; sjog = sjogrenite (O = definitely present, 0 = probably present, o = possibly present).
Table 8. Variation in mineralogy of ultramafic rocks with depth in Hole 778A, ODP Leg 125.
Serpentine Grp Layered Grp Sjog GrpSample rocksite-core Interval p# name alteration C L A G cron talc br chl ch-Ib clay sm-k kao coal sjog
o o o
778A-1R778A-2R778A-3R-CC778A-4R778A-5R778A-6R778A-7R-CC778A-8R778A-9R778A-10R778A-11R778A-12R-2778A-12R-2
89-921-7
7-13
43—4-573-75
7 SS
s
SH98SH99
44
4
44
0O
O
o0
0O
0
o0
0
0
o0
p # = piece number, where assigned (not all pieces were assigned a number aboard the ship); S = serpentinite; SH = serpentinized harzburgite (numbers after rock nameindicate % secondary minerals); C = chrysotile; L = lizardite; A = antigorite; G = greenalite; cron = cronstedite; br = brucite; chl = chlorite; ch-Ib = chamosite-Ib; clay= clay minerals; sm-k = smectite-kaolinite; kao = kaolinite; coal = coalingite; sjog = sjogrenite; cal = calcite; arag = aragonite; magn = magnesite; ol = olivine; opx =orthopyroxene; cpx = clinopyroxene; sp = spinel; amph = amphibole; mt = magnetite; hem = hematite; il = ilmenite; (O = definitely present, 0 = probably present, o= possibly present) (under alteration, 4 = heavily altered).
that serpentine phases can persist metastably within broad tempera-ture/pressure ranges. Thus, although these criteria (Wicks, 1979) can helpdetermine the grade of metamorphism of the rocks in this study, withoutfurther detailed analysis the metamorphic grade of the rocks can only beconstrained to very low to medium grade, i.e., 120°C-600°C.
Brucite is produced by serpentinization with the temperature rangeof approximately 250°C to 450°C (Wicks, 1979). The mineral assem-blage then becomes lizardite ± chrysotile ± brucite ± antigorite ±magnetite. Brucite is difficult to determine petrographically becauseit often assumes a habit as fine grains intimately associated with thelizardite. It is easier to identify in veins. The importance of brucite isthat it may define a lower temperature of serpentinization than thepresence of antigorite alone or antigorite and chrysotile (Moody,1976; Wicks, 1979).
There are several different alteration assemblages observed in therocks examined in this study. Twenty-seven samples (of the 36samples studied by X-ray diffraction) contain the assemblage chrys-otile + lizardite + magnetite + brucite + antigorite. Six samples containchrysotile + lizardite + magnetite + brucite. Two samples contain theassemblage chrysotile + magnetite + antigorite. One sample containschrysotile + lizardite + magnetite + antigorite. One sample containschrysotile + magnetite + antigorite + brucite. One sample containslizardite + magnetite + brucite. There are only three samples thatcontain antigorite in the absence of brucite. These samples are allextremely serpentinized (>98%).
Although mineral assemblages alone can be used to determine therange of temperature-pressure conditions in rocks that have under-
gone prograde events, it is extremely difficult to use them to determinethe range for retrograde events.
Talc is present to some degree in most of the samples analyzed byX-ray diffraction. The occurrence of talc in the same thin sections thathave brucite is an apparent contradiction because the stability fieldsof these two minerals do not overlap. By knowing which of these twomineral phases developed first it would be possible to discuss thehistory of the rock. However, it is extremely difficult optically toidentify the brucite and its interrelationship with talc in thin section.We can say that the rocks are in disequilibrium on a relatively fine(centimeter-size) scale.
The vast majority of thin sections studied demonstrate a pseudo-morphic mesh and bastitic texture of lizardite and chrysotile afterolivine and orthopyroxene, suggesting recrystallization between125°C and about 500°C. Rocks containing antigorite indicate for-mation at a higher temperature between 200°C and 600°C. Modelingof the thermal history of the Mariana forearc (Fryer and Fryer, 1987;Fryer et al., 1989) suggests that a regional reduction in temperatureof the outer forearc occurs within the first several hundred thousandyears after initiation of subduction. If this modeling is correct, themetamorphism of these rocks would have to be constrained to a veryearly stage in the evolution of the Mariana subduction system.
Brucite can only be observed petrographically as large cross-cut-ting veins in both types of the above-mentioned rocks. In almost allrocks analyzed by X-ray diffraction from Sites 779 and 780 (Tables9 and 10), brucite is definitely present. However, in the samples fromSite 778, brucite is less prevalent, and these generally contain more
440
-
METAMORPHISM OF ULTRAMAFIC CLASTS
Table 7 (continued).
Samplesite-core
778A-3R-CC778A-12R-2778A-12R-2
779A-5R-2779A-8R-1779A-11R-1779A-28R-3
il
0
0
0
coal
00
sm-k
35b32a
kao
SSH98SSH99
SH96SH98SH97SH98
gr
O0
0
000O
cron
00
0
00
0
0
ch-Ib
O00
0000
sjog
0
O0
0
0
0
fields (Maekawa et al., this volume). Widespread metamorphism ofthe forearc probably occurred within several hundred thousand yearsafter subduction began (Fryer, this volume). This early event ispreserved in the relatively higher temperature/pressure serpentineassemblages noted in the rocks. Subsequent metamorphism occurredin very low to medium-grade conditions of temperature and pressure.
ACKNOWLEDGMENTS
We are grateful to John Mahoney for his insightful review of thischapter and for suggestions for improvement of the manuscript. Wealso thank Barbara Jones for her editorial review. The work performedfor this study was supported in part by National Science Foundation grant
Table 8 (continued).
Samplesite-core cal
Carbonate Grp
arag magn ol opx
Others
cpx sp amph
Opaques
mt hem il
778A-1R778A-2R778A-3R-CC778A-4R778A-5R778A-6R778A-7R-CC778A-8R778A-9R778A-10R778A-11R778A-12R-2778A-12R-2
0 o o 0
O o
o o o 0 O o O o oo o o 0 0 O o
antigorite. Brucite forms within a temperature range of 250°C to450°C. Thus, the last metamorphic event must have occurred withinthe temperature range of brucite formation.
Detailed textural analyses by spectral reflectance, electron micros-copy, or microbeam X-ray diffraction are needed in order to furtherdefine the intricate mineralogical interrelationships that may indicatethe metamorphic history.
SUMMARY
Several important factors based on these studies and related workhave changed concepts of processes active at non-accretionary con-vergent margins. Evidence suggests that mobilization of the forearcmantle wedge by serpentinization and diapiric intrusion is an impor-tant phenomenon in the region between the outer-arc high and thetrench. Emplacement of large serpentine seamounts is a commonoccurrence in the outer half of the Mariana forearc (Fryer et al., 1985;Fryer and Fryer, 1987). One such seamount, Conical Seamount, iscomposed of unconsolidated serpentine mud flows that have en-trained serpentinized ultramafic and metamorphosed mafic rocks.The extent of metamorphism in the samples from this seamount isvariable, but all of the ultramafic samples recovered are serpentinizedto some degree. Most of the rocks studied from this seamount areharzburgites with very small proportions of clinopyroxene and lowcontents of alumina. This indicates a depleted mantle rock thatrepresents the residue from a high degree of partial melting or morethan one melting event (Ishii et al., this volume).
The nature and magnitude of fluid flow within the forearc atnon-accretionary margins are still poorly understood. In the Marianaregion, the hydration of the crust and upper mantle of the forearcwedge is probably facilitated by the escape of fluids from the sub-ducting Pacific Plate (Fryer and Fryer, 1987). Large regions beneaththe forearc lie within the chlorite, greenschist and blueschist stability
OCE8411717 and by a grant from the U.S. Science Advisory Com-mittee of the Joint Oceanographic Institutions, Inc. This is Universityof Hawaii SOEST Contribution no. 2604, PG contribution no. 662.
REFERENCES
Bloomer, S. H., and Hawkins, J. W., 1983. Gabbroic and ultramafic rocks fromthe Mariana Trench: an island arc ophiolite. In Hayes, D. E. (Ed.), The tectonicand geologic evolution of Southeast Asian seas and islands. Part 2: Wash-ington (Amer. Geophys. Union), Geophys. Monogr. Ser., 27:294—317.
Fryer, P., and Hussong, D. M., 1981. Seafloor spreading in the Mariana Trough:results of Leg 60 drill site selection surveys. In Hussong, D. M., and Uyeda, S.(Eds.), Init. Repts. DSDP, 60: Washington (U.S. Govt. Printing Office), 45-55.
Fryer, P., Ambos, E. L., and Hussong, D. M., 1985. Origin and emplacementof Mariana forearc seamounts. Geology, 13:774-777.
Fryer, P., and Smoot, N. C , 1985. Morphology of ocean plate seamounts inthe Mariana and Izu-Bonin subduction zone. Mar. Geol, 64:77-94.
Fryer, P., Haggerty, J., Tilbrook, B., Sedwick, P., Johnson, L. E., Saboda, K.L., Newsom, Y, Kang, D. E., Uyeda, S., and Ishii, T, 1987. Results ofMariana forearc serpentinite diapirism. Eos, 68:1537. (Abstract)
Fryer, P., Saboda, K. L., Johnson, L. E., Mackay, M. E., Moore, G. F, and Stoffers,P., 1990. Conical Seamount: SeaMarc II, Alvin submersible, and seismicreflection studies. In Fryer, P., Pearce, J., Stokking, L., et al., Proc. ODP Init.Repts., 125: College Station, TX, (Ocean Drilling Program), 69-80.
Fryer, P., and Fryer, G., 1987. Origins of nonvolcanic seamounts in a forearcenvironment. In Keating, B. H., Fryer, P., Batiza, R., and Boehlert, G. W.(Eds.), Seamounts, islands, and atolls: Washington (Amer. Geophys.Union) Geophys. Monogr. Sen, 43:61-72.
Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990. Proc. ODP, Init. Repts.,125: College Station, TX (Ocean Drilling Program).
Haggerty, J. A., 1987. Petrology and geochemistry of Neogene sedimentaryrocks from the Mariana forearc seamounts. In Keating, B. H., Fryer, P.,Batiza, R., and Boehlert, G. W. (Eds.), Seamounts, islands, and atolls:Washington (Amer. Geophys. Union) Geophys. Monogr. Ser., 43:175-186.
Hsui, A. T, and Youngquist, S., 1985. Adynamic model of the curvature of theMariana Trench. Nature, 318:454-457.
441
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K. L. SABODA.P. FRYER, H. MAEKAWA
Table 9. Variation in mineralogy of ultramafic rocks with depth in Hole 119A, ODP Leg 125.
Samplesite-core Interval p #
rockname alt.
Serpentine Grp Layered Grp Sjog Grp Carbonate Grp
C L A G cron talc br chl ch-Ib clay sm-k kao coal sjog cal arag magn
779A-1R779A-2R779A-3R-CC779A-4R779A-5R-2779A-5R-2779A-6R779A-7R779A-8R-1779A-8R-1779A-9R-2779A-10R-1779A-11R-1779A-12R-1779A-13R-2779A-I4R-1779A-14R-2779A-14R-2779A-15R-2779A-16R-I779A-16R-2779A-I7R-2779A-17R-3779A-19R-2779A-20R779A-21R779A-22R-1779A-22R-2779A-22R-2779A-23R779A-24R779A-25R-1779A-26R-2779A-26R-3779A-27R779A-28R-3
13-15
34-3740-43
57-6090-9352-5440-4314-1838-4250-5474-7721-24
139-14124-2719-2374-7714-1777-8097-99
63-6518-2053-57
85-87
50-52
101-103
26-28
SD80
3 SH80 33 SH96 4
5 b8b4b5361
5 a
3
3
9
3
8b13b
II
SH98STD53STH50STD56SH97SH80SH60SD57SD76SH49SD89D20H37H36H36D38
SH52SD85
7 SD61
SD792b H363b SH51
2a SH98 0
o oo o0 O
O 0
0 0O 0
O O
00
0o
0o0000()O000
0
0
()
0
0
0
O0
Ooo00
o0oo0o00
o00
ooo
0oo
()0
000
o
0
0o0
0
O0
ooo
0
00
o
o o 0
p # = piece number, where assigned (not all rocks were assigned a number aboard the ship); SD = serpentinized dunite: SH = serpentinized harzburgite; STD = serpentinized. tectonizeddunite; STH = serpentinized, tectonized harzburgite; D = dunite; H = harzburgite; (numbers after rock name correspond to the c/c of secondary minerals); alt = degree of alteration (I =freshest, 2 = slightly altered, 3 = moderately altered, 4 = heavily altered); C = chrysotile; L = lizardite; A = antigorite; G = greenalite; cron = cronstedite; br = brucite; chl = chlorite; ch-Ib= chamosite-Ib; clay = clay minerals; sm-k = smectite-kaolinite; kao = kaolinite; coal = coalingite; sjog = sjogrenite; cal = calcite; arag = aragonite: magn = magnesite: ol = olivine; opx= orthopyroxene; cpx = clinopyroxene; sp = spinel; amph = amphibole; mt = magnetite; hem = hematite; il = ilmenite (O = definitely present. 0 = probably present, o = possibly present).
Table 10. Variation in mineralogy of ultramafic rocks with depth in Hole 780C, ODP Leg 125.
Samplesite-core
780C-1R780C-2R780C-3R780C-4R780C-5R780C-6R-1780C-7R780C-8R-1780C-9R780C-10R-1780C-11R780C-12R780C-13R780C-14R780C-15R780C-16R-1780C-17R780C-18R-1780C-18R-1
Interval
61-62
98-101
13-16
53-59
54-5758-61
p#
7a
I0
2
7
2a2u
rockname
SH63
SD61
SH63
SH65
SH69SH69
alt.
2
2
i
33
C
0
0
o
o
oo
L
o
o0
o
0
o
Serpentine Grp Layered Grp Sjog Grp Carbonate Grp
A G cron talc br chl ch-Ib clay sm-k kao coal sjog cal arag magn
o O α
o o o
0 O o
p # = piece number, where assigned (not all rocks
442
-
METAMORPHISM OF ULTRAMAFIC CLASTS
Table 9 (continued).
Samplesite-core
779A-1R779A-2R779A-3R-CC779A-4R779A-5R-2779A-5R-2779A-6R779A-7R779A-8R-1779A-8R-1779A-9R-2779A-10R-I779A-I1R-I779A-12R-I779A-I3R-2779A-I4R-1779A-14R-2779A-14R-2779A-15R-2779A-16R-1779A-16R-2779A-17R-2779A-17R-3779A-19R-2779A-20R779A-2IR779A-22R-1779A-22R-2779A-22R-2779A-23R779A-24R779A-25R-1779A-26R-2779A-26R-3779A-27R779A-28R-3
ol
0
O0
000000000
o00o000
o0
o
0o0
0
opx
o
o0
0
0
0
O000
00
0000
00
0
00
0
Others
cpx
0
00
0i)
000000
0o00000
00o
000
0
sp
0
o0
00
00
00o0
O0
0
0
0
0
O0
0
0
0
0
0
amph
o
0
o0
0
0
(1
mt
0
0o
0
00
00
0000
00
0
0
0
o()
00I)
0
0
0
0
Opaques
hem
o
0
0
0
0
0
0
il
0
0
()
()
0
0
Table 10 (continued).
Samplesite-core
Others
opx cpx sp amph
Opaques
mt hem il
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Hussong, D. M., and Uyeda, S., 1981. Tectonic processes and history of theMariana arc: a synthesis of the results of Deep Sea Drilling Program ProjectLeg 60. In Hussong, D. M., and Uyeda, S. (Eds.), Init. Repts. DSDP, 60:Washington (U.S. Govt. Printing Office), 909-929.
Johnson, L. E., and Fryer, P., 1988. Oceanic plate material on the Marianaforearc. Eos, 69:1471. (Abstract)
Johnson, L. E., and Fryer, P., 1990. Petrography, geochemistry and petrogene-sis of igneous rocks from the outer Mariana forearc. Earth Planet. Sci.Lett., 100:304-316.
Johnson, L. E., Fryer, P., Taylor, B., Silk, M., and Jones, D. L., 1991. Newevidence for crustal accretion in the outer Mariana forearc: Cretaceousradiolarian cherts and MORB-like lavas. Geology.
Kang, D. E., 1971. Structural history of the Mariana island arc system. Geol.Soc. Am. Bull, 83:323-344.
Karig, D. E., and Ranken, B., 1983. Marine geology of the forearc region,southern Mariana island arc. In Hayes, D. E. (Ed.), The tectonic andgeologic evolution of Southeast Asian seas and islands. Part 2: Wash-ington (Amer. Geophys. Union), Geophys. Monogr. Sen, 27:266-280.
LaTraille, S. L., and Hussong, D. M., 1980. Crustal structure across theMariana island arc. In Hayes, D. E. (Ed.), The tectonic and geologicevolution of Southeast Asian seas and islands: Washington (Amer. Geo-phys. Union), Geophys. Monogr. Ser., 23:209-222.
McCabe, R., and Uyeda, S., 1983. Hypothetical model for the bending of theMariana arc. In Hayes, D. E. (Ed.), The tectonic and geologic evolution ofSoutheast Asian seas and islands. Part 2: Washington (Amer. Geophys.Union), Geophys. Monogr. Sen, 27:291-293.
Michael, P. J., and Bonatti, E., 1985. Petrology of ultramafic rocks from Sites556,558, and 560, in the North Atlantic. In Bougault, H., and Conde, S. C,et al., Init. Repts. DSDP, 82: Washington (U.S. Govt. Printing Office),523-528.
Mrosowski, C. L., Hayes, D. E., and Taylor, B., 1982. Multichannel seismicreflection surveys of Leg 60 Sites, Deep Sea Drilling Project. In Hussong,D. M., and Uyeda, S., Init. Repts. DSDP, 60: Washington (U.S. Govt.Printing Office), 57-71.
Saboda, K. L., 1991. Petrology of ultramafic rocks from Conical Seamountbased on Alvin Submersible and Ocean Drilling Program studies [M.S.Thesis]. University of Hawaii, Honolulu.
Saboda, K. L., Fryer, P., and Fryer, G., 1987. Preliminary studies of metamor-phic rocks collected during Alvin studies of Mariana forearc seamounts.Eos, 68:1534. (Abstract).
Uyeda, S., and Kanamori, H., 1979. Back-arc opening and the mode ofsubduction. J. Geophys. Res., 84:1049-1061.
Date of initial receipt: 12 September 1991Date of acceptance: 8 October 1991Ms 125B-127
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