5 J.F. Dewey and J.M. Bird, Origin and emplacement of the ophiolite suite: .... 45 P. Jake~ and A.R.J. White, Major and trace element abun- dances in volcanic ...
Earth and Planetary Science Letters, 39 (1978) 127-144 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
127
GEOCHEMISTRY OF BASALTIC AND GABBROIC ROCKS FROM THE WEST MARIANA BASIN AND THE MARIANA TRENCH V. DIETRICH 1, R. EMMERMANN 2, R. OBERHANSLI 3 and H. PUCHELT 2 1 lnstitut ffir Kristallographie und Petrographie, ETH Zfirich Zentrum, CH-8092 Z~irich (Switzerland) 2 lnstitut ffir Petrographie und Geochemie der Universitdt Karlsruhe, D- 75 Karlsruhe 1 (Federal Republic o f Germany) 3 Labor far MikrorOntgenspektroskopie der phil. nat. Fakultdt der Universita't Bern, CH.3012 Bern (Switzerland)
Revised version received December 8, 1977
This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V "Dmitry Mendeleev" 1976 cruise in the western Pacific. The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project "Ophiolites" and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust. The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an "ophiolite-schuppenzone", probably due to subduction of Pacific oceanic crust. Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts. These rocks (together with the dolerites) are interpreted as partsof the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).
1. Introduction Very little is at present known about the structure and evolution of marginal basins, although various authors (e.g. [1 ]) have pointed out the importance of marginal basins in the development of the complex island arcs of the western Pacific. It is also possible that many large ocean basins originally began as marginal basins which grew away from their initiating subduction zones. One purpose of the 17th cruise of the "Dmitry Mendeleev" * was to search for steep escarpments in * The results are based on the "Initial Report of the Geological Study of Oceanic Crust of the Philippine Sea Floor" by the InternationalWorking Group on the IGCP Project "Ophiolites" [ 16 ]. According to the decision of the International Scientific Shipboard Committee (A. Peive, chief scientist and N. Bogdanov, co-chief scientist) the Swiss shipboard scientists
the "axial zone" of the West Mariana basin (Fig. 1) and to dredge material from the deepest parts. The other purpose was to test the proposal that subduction underneath island arcs produces thrust sheets in front of island arcs (see Dickinson [2]) composed of flysch and sheared-off slices of oceanic crust. The best localities in the western Pacific to dredge material from these "flysch-wedges" [3] are the slopes of the Mariana and Yap trenches. In this paper we present detailed geochemical descriptions of basaltic and gabbroic rocks dredged from these areas. These rocks include both common were authorized to carry out geochemical work on rocks dredged in the first part of the cruise between Tokyo and Guam and publish their results. A full geological compilation of all data obtained from the R/V "Dmitry Mendeleev" 1976 cruise is in preparation.
128
135°E
140°E
145°E
20°N
.......
P,
~ 8 / " " 8..~/ :?
MWeSr#ana
~t
:".
Basin
.'~'}
[~/'i 15°N
Plii /
'!i>,. .
10°N
WNW Kyushu Palau Ridge
(19°N)
~
Yap Zone
West M a r i a n a
Basin
~ 1398
West M a r i a n a East Parece ~ Vela Basin
ESE
- Ridge
Mariana Trough ~ | Guam
(12°N) 1404 1403 1402
km L..
~,:~:.-,~
,,
~o ~ : ~ ~ " ; ~ ' - ~ 20
3o 40
/
,,'v I
mantle
i
a a c t
~
i
.....,;.>:......~......~'....'...'..v . v -. . . . . . . . . . . . . - - - - - - ~ ':~'~":~"~'""~"""'
"""'
~oho
',, :'
,/
' '~i
:' v e
f2f~"
,"j;-',/- ~ na r e a
I
a c t
i
~" v e
a r
e
[F '
Fig. 1. Tectonic framework and schematic cross-section (P-P') of the southeastern part of the Philippine Sea. The map shows simplified bathymetric features (e.g., 3 = 3000-fathom contour) indicating the general tectonic setting of the sea floor. The tracks and stations 1398, 1402, 1403 and 1404 (described in detail in the text) are from the R/V "Dmitry Mendeleev" cruise 17, JuneAugust 1976, in the Philippine Sea.
oceanic lithologies and also some very unusual rock types which may be unique to this environment, The results may help continental geologists to interpret some o f the more enigmatic features of
ophiolite complexes, which have variously been thought o f as slices of ocean crust (e.g. [4~5]), as uplifted basal parts of-island arcs [ 7 - 9 ] and as obducted parts o f marginal ocean floor [ I 0,11 ].
129 2. Tectonic setting of the Philippine Sea floor and
adjacent areas The tectonic framework, general bathymetry and a schematic cross-section of the southeastern part of Philippine Sea floor are shown in Fig. 1. Over a distance of nearly 2000 km three submarine ridges, from west to east the Palau-Kyushu ridge, the West Mariana-Iwo Jima ridge and the Bonin-Mariana ridge, divide the area into the large West Philippine basin, the West Mariana-Parece VelaShikoku basin, and the Mariana trough. The submarine ridges have been interpreted by Karig [12] to be eroded and subsided remnant arcs. The evolution of the Philippine Sea floor probably started in the upper Cretaceous [13] and was certainly underway by the Eocene. Karig [1] provided a model of basin generation which involved the formation of oceanic crust as a result of upwelling mantle originating from the descending Pacific lithosphere. Later geophysical surveys and deep-sea drilling in the Philippine Sea by Karig [14] have tested and improved this model. Uyeda and Miyashiro [15] have proposed a plate tectonic model for the western Pacific by combining data on relative Pacific plate motions with geological evidence obtained from island arcs and the Japanese islands. The extension of the different basins probably took place in three tectonic cycles which were associated with volcanic activity and formation of island arcs: The WestPhilippine basin spread during the first cycle between 45 and 37 m.y., the WestMarianaParece Vela-Shikoku basin during the second cycle between 25 and 18 m.y. and the Mariana trough during the present cycle starting 2 - 3 m.y. ago. The WestMariana basin is divided into two different basins by an "axial zone of high relief" [14]. The morphology of the western part of the basin is characterized by small ridges and by valleys between 4500 and 5500 m depth, whereas in the eastern part of the basin, this irregular pattern is smoothed out by more than 1000 m of detrital sediments derived from the West Mariana ridge. One of the most active island arc systems in the western Pacific is the Bonin-Mariana ridge. According to sedimentary records from the volcanic islands of Saipan, Guam and Palau, the volcanic activity started at least before the late Eocene. At that time the
Bonin-Mariana and Kyushu-Palau ridges were probably combined (see the short review in Karig [ 14]). Whereas the volcanic and tectonic activity continued to the present in the Bonin-Mariana frontal arc, it terminated during the upper Tertiary in the KyushuPalau ridge. The latter ridge subsided during the spreading of the West Mariana-Parece Vela basin. The age of the intermediate West Mariana volcanic ridge is not clear. It was probably active during the Pliocene. The volcanic arcs are asymmetric with steep slopes pointing eastward, and deep-sea trenches occur in front of the arcs [16]. These data indicate that the oceanic crust has always been subducted towards the west (Fig. 1, cross-section). Seismic, magnetic, heat flow, and petrologic data indicate that oceanic crust exists in the marginal basins. The seismic compressional wave velocities are comparable with those in normal oceanic crust [17, 18] and suggest that the crust of the marginal basins is up to 6 - 7 km thick. Parallel magnetic anomalies, which would indicate the generation of oceanic crust from mid-oceanic ridge systems, have been recognized in the Shikoku basin [19,20] and in the western part of the Philippine basin [21]. High heat flow values are characteristic of the Mariana trough and of the central parts of the West Mariana basin [22], indicating an active tectonic environment and the existence of newly formed oceanic crust. Only the West Philippine basin shows normal values similar to those of the older parts of the Atlantic oceanic crust. Direct evidence for the existence of oceanic crust in the West Mariana basin comes from DSDP drilling, Leg 6, site 54 (Fig. 1). Here the uppermost part of the crust consists of high-alumina olivine-tholeiites with flat REE patterns and low dispersed trace element contents [23]. Although similar basalts have been dredged from the Mariana trough [24], to date structural and petrological evidence of a mid-oceanic ridge system has not been found within the marginal basins.
3. Dredge localities and petrography This investigation concentrates on stations in a part of the R/V "Dmitry Mendeleev" cruise 17 in the West
basalt basalt pillow basalt pillow basalt ?pillow basalt ?pillow basalt basalt
pillow basalt pilotaxitic (core) pillow basalt hyaline (rim) ?pillow basalt ~ ?pillow basalt / sparsely phyric ?pillow basalt
1398-D2-1A 1398-D2-3 1398-D2-6 1398-D2-10 1398-D2-11 1398-D2-12 1398-D2-17
1402-D2-XZ
?contact rock
1403-D1-24
1403-D1-40 1403-D1-44 1403-D1-45
1403-D1-39
1403-D1-34
1403-D1-26
"boninite"type rocks
troctolite mylonite gabbroic mylonite
1403-D1-50
1402-D2-Z
basalt with shoshonitic affinities
1402-D2-11 ] 1402-D2-13 /
1402-D2-7 1402-D2-17 1402-D2-19
1402-D2-XR
hyaloclastic/ vitrophyric amygdaloidal amygdaloidal porphyritic
porphyritic, dendritic porphyritic
brecciated mylonitic
mylonitic
~ amygdaloidal / to vitrophyric ~
aphyric porphyritic aphyric porphyritic aphyric sparsely phyric porphyritic
ophitic granular
metagabbro metagabbro
1398-D2-2
1398-D2-9
Texture
Rock type
Stationdredgesample
(5-6)
plag(Anso)
enst enst enst
enst
enst
enst, cpx
plag plag
(3-4) (1-2) (1-2)
(3-4)
(1-2)
(1-2) (1)
plag (1-2) plag(An8o) (3-4)
(4-5)
plag(An9o)
Macrophenocrysts (size, mm)
enst, clinoenst, cpx, chromite, pigeonite, altered ol enst, pigeonite, chromite cpx, (ol) enst, clinoenst, pigeonite, cpx enst, cpx, pigeonite enst, clinoenst, cpx, pigeonite, O188_92, chromite
plag, cpx, Fe-Ti oxides, ap
ol, cpx
?ol, cpx
sanidine with plag(Anss_60) rims, groundmass alkali-feldsp, ilm
plag, cpx, mgt plag, cpx, mgt plag(AnTo_8o), cpx, mgt
plag(AnTo_8o), Ti-cpx, ol, mgt
plag(AnTo-8o), Ti-cpx, ol, mgt
plag(AnTo_80), cpx, mgt plag(An6o), mgt plag(An8o), ol, cpx, mgt plag(An6o), cpx, mgt plag, cpx, ?ol, mgt plag(AnTo-8o), cpx, mgt plag, cpx, ol, mgt
plag(AnTo-8o), cpx br-hbl, mgt-ilm
Primary minerals microphenocrysts (groundmass)
wair, mont mont/chl mont
heul, wair
mont, amph, biot
mont, amph, biot, qtz
dps, chl, amph, act, preh, serp
serp, chl, amph, ?sulfate
apatite, ?mont smect, hem
cc, smect smect mont/chl
mont/chl
?talc(ol) smect mont/chl, ?zeol smect
smect mont/chl, ?zeol
dps, act, chl ?alb, ?epd
Secondary minerals
1-5% 5-10%
few amygdaloids few amygdaloids 10-15%
15-20%
15-20% 10-20%
few vesicles
few vesicles
(1-2) (1-2) (1-3)
(1-3)
(1-3)
(0.1)
(0.1-0.5)
(0.2-1) (1-5)
(0.1-0.3)
(2 -3)
Vesicles + amygdules (0, mm)
Mineralogy of the basaltic and gabbroic rocks from the West Mariana basin and the Mariana trench (determination by optical methods, X-ray and electron microprobe)
TABLE I
L~
dolerite (diabase) dolerite (diabase) meta basalt (?pillow) meta basalt (?pillow)
dolerite (diabase) meta basalt
variolitic + glomeroporphyritic
arborescent
ophitic-pegmatoid ophitic
porphyritic
ophitic
plag, cpx
plag
(0.5-1)
(1-2)
plag(AnTo-Ts), cpx
plag(An5o_7o), cpx, mgt mgt, Cr-spinel plag, cpx
p l a g ( A n s 0 _ 7 0 ) , pigeonite, cpx,
plag, cpx, m g t
plag, qtz, Ti-cpx, mgt
m o n t / c h l , serp(ol)
smect
chl, ?act, ?alb
chl, act, ?alb, serp(ol)
wair, ?anal, m o n t , talc, cc
chl, ?act few vesicles
(1-2)
zeol = zeolites.
Abbreviations: act = actinolite, alb = albite, a m p h = amphiboles, anal = analcime, ap = apatite, biot = biotite, br-hbl = br-hornblende, cc = calcite, chl = chlorite, cpx = clinopyroxenes, dps = diopside, enst = enstatite, epd = epidote, h e m = hematite, heul = heulandite, ilm = ilmenite, m g t = magnetite (partly Ti = magnetite), m o n t / ehl = mixed layers montmorillonite/chlorite, ol = olivine, plag = plagioclase, preh = prehnite, qtz = quartz, serp = serpentine minerals, smect = smectite, wair = wairakite,
1404-D1-F
1404-D1-E
1404-D1-D
1404-D1-A-C
1403-D1-48
1403-Di-37
132 Mariana basin and in the Mariana trench (Fig. 1). Table 1 lists the mineral assemblages and textural features of basaltic and gabbroic rocks dredged from four stations, and these data are discussed below. 3.1. West Mariana basin
In the West Mariana basin seismic investigations [16] reveal a southern extension of the "axial zone of high relief" into the Yap trench. This zone consists of a V-shaped central valley with 20 ° steep flanks and reaches depths betwen 6000 and 7000 m. Whereas serpentinized spinel-lherzolites and gabbros were brought up at station 1409 [16], gabbros and basalts were dredged at station 1398. Station 1398 (lower part of the western flank of the central valley). Position: 17°8.8'N, 139 ° 10.2'E (5600 m depth) to 17°7.1 'N, 140°11.1 'E (5300 m). The dredge contained 100 kg of broken pillow basalts, porphyritic basalts and low-grade metamorphosed gabbros with cataclastic and mylonitic textures. The basalts are partly altered except for some glassy pillow basalts. In most cases the glassy matrix is altered and consists of mixtures of montmorillonite, chlorite and zeolite.
(1) Fragments of probable pillow lavas, including pieces with typical hyaloclastic rims and fragments with aphanitic to phyric textures. These rocks are fairly fresh, but some contain alteration products of smectite, chlorite, chlorophaeite and calcite. (2) Vesicular vitrophyric to amygdaloidal basaltic rocks (chemically showing affinities with shoshonires). Alkali feldspar occurs as groundmass grains and as cores of intergranular grains (sanidine) with labradorite rims. Secondary apatite occurs in veins and pores. Amygdules (0.1-0.5 mm in diameter), partly filled with smectite, comprise up to 20% of the rock volume. Vesicles are as large as 3 - 4 mm in diameter and give the rock a scoria-like appearance. Stations 1403 and 1404 (island arc slope of the Mariana trench). Position 1403-D1:12°7.5'N, 144°32.1 'E (8400 m depth) to 12°7.8'N, 144°30.6'E (8100 m); position 1404-D1: 12°16.4'N, 144°21.2'E (5500 m depth) to 12°16.2'N, 144°20.3'E (5400 m). Dredge 1403 is from the lower part of the slope and contained 80 kg of flysch-like sediments and various rocks from a typical "ophiolite suite": serpentinized and partly mylonitized peridotites, troctolite (1403-50, Table 1), gabbro, brecciated and mylonitized gabbro (e.g. 1403-26, a rodingitized troctolitic gabbro) and basaltic rocks.
3.2. Mariana trench
During cruise 17 of R/V "Dmitry Mendeleev" two sections of the 2000 km long Mariana trench were investigated [16]: the more central part southeast of Guam (stations 1402-1404) and the western end at the intersection with the Yap trench. Station 1402 (Pacific slope of the Mariana trench). Position: 13°33.0'N, 146°59.4'E (5600 m depth) to 13°33.0'N, 146°59.7'E (5400 m), west of a series of seamounts probably aligned in a "Horst-Graben" system parallel to the Mariana trench (Fig. 1). The dredge (50 kg) contained mostly altered basaltic and mugearitic type rocks associated with volcanosedimentary rocks (breccias, hyaloclastites, phosphoritic hyaloclastites and phosphorites) and limestones. The ages of these rocks are indicated by Globorotalia (Eocene)- and Globotrunca (Upper Cretaceous)bearing limestone within the volcanosedimentary breccias. Two different basaltic rock groups were selected for detailed petrographic and chemical investigation:
Fig. 2. "Boninite"-type rock from the inner slope of the Mariana trench (sample 1403-39, Tables 1 and 4). These volcanic rocks contain macrophenocrysts of enstatite (with inclusions of trapped liquid and chromite) in a matrix of glass and quenched microphenocrysts of enstatite, pigeonite and augite. Optically fresh glass contains up to 3 wt.% K20 and up to 7% H20. A petrologic study of these unusual rocks is in preparation.
133 TABLE 2 Chemical analyses from the basaltic and gabbroic rocks dredged in the West Mariana basin (station 1398) Metagabbro
Basalt
Pillow basalt
?Pillow-basalt
Basalt
1398-2
1398-9
1398-1A 1398-3
1398-6
1398-10
1398-11
1398-12
1398-17
45.90 0.58 19.66 2.32 5.10 0.13 9.51 11.98 1.94 0.05 0.03 3.17 0.00 0.07
47.93 0.47 17.64 3.17 4.50 0.13 9.88 11.18 2.33 0.04 0.04 3.18 0.00 0.05
50.02 1.67 16.13 6.57 4.03 0.18 6.01 11.62 2.99 0.16 0.15 1.13 0.18 0.03
48.00 1.48 21.79 4.82 1.10 0.04 8.14 7.23 2.99 0.34 0.15 4.17 0.20 0.04
48.83 1.88 15.77 6.45 5.40 0.18 5.84 11.43 3.10 0.16 0.24 1.33 0.00 0.03
48.18 1.62 17.40 6.46 4.10 0.17 5.33 12.06 2.89 0.16 0.18 1.68 0.00 0.03
50.42 1.69 16.01 5.65 4.50 0.16 6.23 11.47 2.87 0.19 0.15 0.86 0.00 0.03
49.98 1.72 15.81 6.50 4.50 0.16 6.09 11.45 3.06 0.20 0.16 1.15 0.30 0.03
50.23 1.65 16.43 5.51 4.42 0.16 6.10 11.69 3.07 0.12 0.15 0.56 0.30 0.03
Total
100.44
100.54
100.87
100.49
100.64
100.26
100.23
101.11
100.42
F(ppm) Li Rb Sr V Zn Cu Co Ni Cr Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Zr Hf Nb
50 8 14 53 146 65 14 41 208 536 26 17 0.8 2.5 0.4 3.8 1.3 * 0.6 2.2 0.4 * 3.5 0.6 1.7 0.2 1.5 0.2 20 0.6
