20. mafic and ultramafic rocks of leg 84: … · 20. mafic and ultramafic rocks of leg 84:...

20. MAFIC AND ULTRAMAFIC ROCKS OF LEG 84: PETROLOGY AND MINERALOGY1

Jacques Bourgois, Département de Géotectonique, Université Pierre et Marie CurieAlain Desmet, Faculté des Sciences, Laboratoire de Pétrologie, Université de Nancy I

Jean Tournon, Laboratoire de Pétrographie, Université Pierre et Marie Curieand

Jean Aubouin, Département de Géotectonique, Université Pierre et Marie Curie2

ABSTRACT

Ultramafic rocks were recovered from the lower parts of holes at four sites (567, 566, 569, 570) drilled on Leg 84.These sites are on the landward slope of the Middle America Trench off Guatemala. Harzburgites, cumulative textureperidotites, gabbros, dolerites, and amphibolite facies rocks were recognized by pétrologie and geochemical analysis (X-ray fluorescence, microprobe). Some of these rocks belong to an ophiolitic sequence. They form the basement of thelandward slope of the Guatemala Trench upon which lower Eocene sediments were deposited. As a whole, the ophioliticrock has much in common with various ophiolitic massives known on land all over the world (e.g., Antalya, Oman),particularly the ophiolitic sequence of the Santa Elena Peninsula of Costa Rica. The age, the abundance of harzburg-ites, and the structural position of the mafic and ultramafic rocks of Leg 84 suggest that they could represent an off-shore equivalent of the Santa Elena ultramafic rocks. In any case, the first ophiolites to have been recovered from theslope of an active margin are those of the drilled mafic and ultramafic rocks of Leg 84.

INTRODUCTION

Mafic and ultramafic rocks were drilled at Sites 567,566, 569, and 570 of Leg 84 on the landward slope ofthe Middle America Trench off Guatemala. The simpli-fied stratigraphic columns of Figure 1 show the generallithostratigraphy of the drill holes and the main resultsof Legs 67 and 84 (Aubouin, von Huene, et al., 1979;von Huene, Aubouin, et al., 1980; Coulbourn et al.,1982).

Site 567 is located 100 m from the position of Hole494A of Leg 67 (Maury et al., 1982). Below the Pleisto-cene and Pliocene slope sediments, blocks of serpentin-ite were recovered in Miocene sediments. Beneath theMiocene section, drilling recovered two cores of Late Cre-taceous limestone, followed by acoustic basement of maficand ultramafic rocks (harzburgites, gabbros, dolerites,basalts; Fig. 2). It is not possible to establish a strate-graphy of the mafic and ultramafic basement from thesuccession of samples. Harzburgites and mafic rockssometimes alternate in the same core; no apparent se-quences were recovered. Poor recovery and drilling brec-cias further obscure the section. Tectonic breccias couldalso be recognized because of the Cataclastic texture ofsome samples in thin section.

Underneath the Pliocene-Pleistocene and the upperMiocene slope deposits in Holes 566A and 566C (Fig. 2)only serpentinized harzburgites were recovered.

von Huene, R., Aubouin, J., et al., Init. Repts. DSDP, 84: Washington (U.S. Govt.Printing Office).

2Addresses: (Bourgois and Aubouin>Département de Géotectonique, Université Pierre etMarie Curie, T. 26, 1st Floor, 4 Place Jussieu, 75230 Paris Cédex 05, France; (Desmet) La-boratoire de Pétrologie, Université de Nancy I, B.P. 239, 54506 Vandoeuvre les Nancy Cédex,France, (Tournon) Laboratoire de Pétrographie, Université Pierre et Marie Curie, T. 26, 3rdFloor, 4 Place Jussieu, 75230 Paris Cédex 05, France.

The rocks recovered from the base of Hole 569A (Fig.2) are amphibolites with evidence of retrograde meta-morphism in the greenschist facies. These amphibolitesappear under approximately 350 m of sediments wherea normal succession of Pleistocene through Eocene mi-cro fossils was identified.

Site 570 reached serpentinized peridotites with cumu-lative texture beneath sediments that are lower Eoceneto Pleistocene.

METHODS

A selection of samples suitable for geochemical studies was madefrom the pétrographie study of 64 thin sections. Nineteen chemicalanalyses on rocks were carried out at the Laboratoire de Pétrologie ofNancy by X-ray fluorescence and neutron activation analyses; the cor-rected values were established according to the Irvine and Baragar (1971)method (Fe3+ and Fe2+ determination). This method, especially es-tablished for mafic volcanic rocks, may be used for all mafic rocks.The content of Fe2O3% is calculated as follows: Fe2O3% = TiO2% +1.50, and FeO is adjusted after correction for loss on ignition.

A study of the primary minerals (olivine, orthopyroxene, clinopy-roxene, spinel, amphibole, Plagioclase) of 19 samples was made withthe Camebax microprobe (natural standard, 14 kv, 10 s) of the Muse-um National d'Histoire Naturelle de Paris.

ULTRAMAFIC ROCKS

The ultramafic rocks recovered (Fig. 2) from Hole567A are (1) reworked serpentinite blocks in the lowerMiocene sediments between 260 and 330 m; (2) a thickblock of serpentinized peridotites (Core 567A-16) abovethe Upper Cretaceous limestones; (3) igneous basementrocks under the Upper Cretaceous limestones between380 and 500 m.

From holes 566, 566A, and 566C, only ultramaficrocks were recovered beneath the thin canyon sedimentdeposits where 30, 5, and 25 m of this basement weredrilled, respectively.

633

σ\

Guatemala slope key

Siliceous mud—mudstone

Hemipelagic mud—mudstone

Breccia andconglomerate layers

Sand—sandstone

Unconformity

Limestone

Basic and ultrabasicslope basement

567Water depth

5529 m

501.0 m499 5 0 0 '

Water depths.Water depth.6127 m 6123 m

500"

.upper"--~Pleist.—=*—, ~-

upper Plio.upperMio.

Basement

446.5 m

/ / / upperV / Oligocene

Cocos Plate key

Trench-fill turbidites with—'•'•'— 'A siliceous biogenic admixture

Hemipelagic mud withsiliceous biogenic admixture

-IHHHHj] Brown abyssal clay

Nannofossil chalk

Manganiferous chalk

BasaltCocos Plate basement

Middle AmericaTrench axis

0 50 km

-I 6 km

91°00W 90°00

Figure 1. Cross section of the Guatemala slope, Leg 67 and Leg 84 summary results. (Inset map shows bathymetry in meters.)

0 -

50

100-

150 -

200-

E 250 -

300-

350

400-

450 •

500 J

1 4

16Mio.

28

14-

16 '

20

28-

501.0 m

1-13 (SP)50-52 (SH)50-56 (SH)

566CP l e i s t 3675.0 m

toPI IO

MAFIC AND ULTRAMAFIC ROCKS

569 & 569A2799.9 m

Serp.

8 3 - 8 6 (SH)104-106 (SH)

, ( 2 3 - 2 5 (SH)' ( 1 1 0 - 1 1 2 (SH)

4 1 - 4 3 (S)4 2 - 4 5 (S)

(S)

(S)

104 -106 (G)108-110 (G)

1, 118 -120 (B)143-147 (G)145-149 (G)

1 ( 4 0 - 4 3 (G)(CC (G)

f ,0-10 (G),1,132-42 (bB)j * 92-95 (bB)2,{ 90-96 (B)

133-138-b (B + G). 59-64 (D)

'(108-112 (D)-1,{i02-106 (D)

(47-51 (SH)'(117-121 (SH)

7-10 (D)15-19 (B?)36-40 (SH)78-85 (SH)1 15-119 (SH)

(569A1

\-10-

r—11 —

6-9 (A)21-23 (A)

13-19 (A)33-36 (A)57-60 (A)97-100 (A)128-131 (A)137-139 (A)

12, { 23-26 (A)x * 37-40 (A)

28-31 (SP)143-146 (SP)36-39 (SP)

25-28 (SP)104-107 (SP)145-149 (SP)7-10 (SP)

Figure 2. Location of studied samples of Leg 84 (after Aubouin, von Huene, et al., 1982). A = amphibolite, B = basalt, bB = basaltic breccia, D =dolerite, G = gabbro, S = serpentinite, SP = serpentinized peridotite, SH = serpentinized harzburgite. (See Fig. 1 legend for explanation ofsymbols.)

At Site 570, only ultramafic rocks were recovered inthe basement drilled down to 401 m from the base ofsediments at 376 m.

All these ultramafic rocks are widely serpentinized.Their primary mineralogy (spinel, olivine, orthopyrox-ene, and clinopyroxene) appears as relic textures. Becausethe serpentinization of the pyroxene is different fromthat of the olivine, it is possible to identify the originaltextures even in wholly serpentinized samples.

Two groups of ultramafic rocks thus become distinct:(1) peridotites (harzburgites) with a panxenomorphic tex-

ture (Holes 567A, 566, and 566C); (2) peridotites with acumulative texture (Hole 570).

The Harzburgites

The ultramafic rocks with a panxenomorphic textureare generally intensely serpentinized. In most cases spinelalone remains of the original minerals. Remains of oliv-ine and pyroxene occur in a sample from Hole 567A andin many others from Holes 566 and 566C. Minute isletsof olivine are spared as relicts in a mesh-textured serpen-tine. A fibrous-textured serpentine is the main alteration

635

J. BOURGOIS, A. DESMET, J. TOURNON, J. AUBOUIN

product of the orthopyroxene. In some samples thereare large relic sections of orthopyroxene. In a few sam-ples, there are small sections of clinopyroxene.

The harzburgites from Holes 566, 566A, and 566Chave a very similar chemical composition (Table 1, Anal-ysis numbers 2 to 6).

One harzburgite from 567A is also of similar chemi-cal composition and slightly more aluminous but lesscalcic (Table 1, Analysis number 1). Indeed, this sam-ple, although rich in orthopyroxene, is without clinopy-roxene.

All the samples have a remarkably constant mineral-ogical composition. The olivines are Fo 89-Fo 92 (Table2) and the orthopyroxenes are Mg-rich bronzite En 87-En 91 (Table 3). The clinopyroxenes from Hole 566 and566C samples are chromiferous diopsides (Table 3). Fromone sample to another, the clinopyroxenes show obviousvariations in Al and Ca content, but Cr is present in sig-nificant quantities. However, the spinels have variablechemical compositions, and those of Hole 567A are richor very rich in Al (Table 4). All the analyzed spinels havea positive correlation between Cr/Al and Mg/Fe ratios(Fig. 3).

Table 1. Major element composition (%) and CIPW norms.

The rock and mineralogical chemical compositions ofLeg 84 harzburgites are similar to those of many ophio-litic complexes, for example from Troodos and fromOman (Boudier and Coleman, 1981). The clinopyroxenechemical compositions of Leg 84 harzburgites are Wo479,En48 4, Fs3.7, and those of Oman (Pallister and Hopson,1981) are Wo45, En51, and Fs4.

The spinels (average composition of 6 analyses) ofLeg 84 harzburgites also have a chemical compositionsimilar to those of Oman (Pallister and Hopson, 1981),but different from those of the peridotites of Santa ElenaPeninsula in Costa Rica (Tournon et al., in press). Thespinels are more aluminous and magnesian (Table 3)and have less Cr and Fe.

Cumulative Texture Peridotites

The texture of the ultramafic rocks sampled at Site570 differs from that of rocks sampled at the other Leg84 sites (Holes 567A, 566, 566A, 566C). The olivine ap-pears in automorphic crystal aggregates cemented by xeno-morphic crystals of pyroxene.

The olivine and pyroxene are completely serpentinized,but the cumulative texture is still obvious. Relic crystals

1

Chemical analysis

SiO2A12O3FeMnOMgOCaONa2OK2OTiO2

Loss onignition

Total

Ni (ppm)Cr (ppm)

37.811.596.660.14

37.290.120.060.010.01

15.32

99.01

19734386

Dry recalculated data

SiO2

A12O3Fe2O3FeOMnOMgOCaONa2OK2OTiO2

CIPW norms

OrAbAnNeDiFsEnFaFoMa11

45.451.911.815.630.17

44.800.140.070.010.01

0.600.590.70

2.3128.355.27

58.562.620.02

2

37.040.607.430.16

36.680.720.090.020.02

16.37

99.13

19822283

45.050.731.856.540.19

44.610.880.110.020.02

0.120.931.44

2.302.06

21.306.67

62.472.680.04

3

37.450.626.790.14

34.581.870.250.020.02

17.36

99.10

19212455

46.090.761.875.900.17

42.562.300.310.020.02

0.122.620.62

8.461.78

19.855.75

58.042.710.04

4

36.170.517.400.13

37.710.660.100.010.02

16.23

99.04

19302540

43.960.741.856.500.16

45.840.800.120.010.02

0.061.021.45

1.981.44

15.507.06

68.772.680.04

5

36.760.587.610.15

36.500.790.080.010.01

16.31

98.80

19902030

44.870.711.846.770.18

44.550.960.100.010.01

0.060.841.46

2.602.05

20.416.99

62.902.680.02

6

36.650.727.380.15

36.570.960.060.000.02

16.47

98.98

23103612

44.910.881.856.500.18

44.611.170.070.000.02

0.592.09

2.921.88

19.616.72

63.472.680.04

7

34.130.627.140.15

37.830.340.210.000.01

18.19

98.62

21302515

42.700.781.896.410.19

47.330.430.260.000.01

2.200.96

0.920.727.837.57

77.072.740.02

8

44.3415.636.280.17

11.7311.562.190.190.25

7.06

99.40

166519

48.2517.001.904.480.18

12.7612.572.380.210.27

1.2417.0435.04

1.6721.68

3.7016.372.760.51

9

47.4917.805.470.156.72

11.243.270.430.22

6.92

99.71

67155

51.3719.251.863.690.167.27

12.153.540.470.24

2.7827.8935.20

1.1020.02

2.347.512.700.46

10

42.5614.8111.320.19

11.156.962.530.251.59

7.97

99.33

392417

46.9716.343.418.270.21

12.307.682.790.281.75

1.6623.5831.20

5.542.176.775.44

15.389.943.33

11

47.6116.469.240.166.238.174.560.521.98

4.83

99.76

100230

50.4217.433.695.550.176.608.654.830.552.10

3.2536.2324.22

2.4914.88

2.017.573.353.99

12

42.8415.4510.230.20

10.676.922.780.711.67

8.11

99.58

334312

47.1617.003.497.080.22

11.757.623.060.781.84

4.6125.8630.31

5.98

5.1218.245.063.50

13

41.5814.5011.810.20

12.006.792.400.211.51

8.75

99.76

434535

46.1016.073.348.870.22

13.307.532.660.231.67

1.3622.4831.19

4.951.604.776.926.304.843.17

14

45.1314.7811.140.197.019.823.020.472.41

5.29

99.26

153421

48.3615.844.197.050.207.51

10.523.240.502.58

2.9627.3827.16

20.000.722.422.07

11.486.084.90

15

45.4617.495.390.13

11.2113.201.390.200.22

4.97

99.66

157255

48.1818.541.823.540.14

11.8813.99

1.470.210.23

1.2412.4243.32

20.640.814.942.085.362.640.44

16

48.0515.419.740.186.029.445.920.400.66

4.00

99.82

62170

50.5316.182.277.250.196.339.926.220.420.69

2.4926.5514.9714.1027.96

3.973.043.291.31

17

47.3416.875.720.135.969.637.170.080.81

5.98

99.69

28037

50.6818.062.473.320.146.38

10.317.680.090.87

0.5325.9714.5221.1029.00

0.602.323.581.65

18

46.5814.438.920.175.329.817.440.090.60

5.98

99.34

6937

50.2215.562.266.700.185.74

10.588.020.100.65

0.5922.156.17

24.7337.83

1.73

3.281.24

19

42.9115.985.970.115.34

19.032.750.020.41

6.61

99.13

71137

46.5617.342.074.030.125.80

20.642.980.020.44

0.120.54

33.8413.3641.05

3.000.84

Note: Analysis numbers represent samples as follows: harzburgites—1 = 567A-29-2, 36-40 cm ( + Co normative = 1.53), 2 = 566-9-1, 50-52 cm; 3 = 566A-1-1, 10-12 cm; 4 = 566A-9-1,50-52 cm; 5 = 566C-6-1, 110-112 cm; cumulated periodites—7 = 570-41-3, 104-107 cm; gabbros—8 = 567A-20-1, 104-106 cm; 9 = 567A-20-1, 143-149 cm; dolerites—10 = 567A-26-1, 59-61 cm; 11 = 567A-27-1, 108-112 cm; 12 = 567A-29-2, 7-10 cm; 13 = 567A-26-1, 40-43 cm, basalts—14 = 567A-25-2, 90-96 cm; 15 = 567A-20-1, 118-120 cm; amphibo-lites—16 = 569A-10-1, 33-36 cm; 17 = 569A-10-1, 118-120 cm; 18 = 569A-10-1, 57-60 cm; 19 = 569A-10-1, 57-60 cm ( + Wo normative = 7.26).

636


Table 2. Olivines from ultramafic rocks. Table 4. Analyses of ultramafic rock spinels (%).

SiO 2

FeOMnOMgOCaONiO

Total

FoSiFeMnMgCaNi

Total

1

41.177.730.07

50.520.020.31

99.82

91.751.0030.1580.0011.8340.0010.006

3.003

2

41.118.460.11

49.570.000.43

99.68

90.771.0070.1730.0021.1800.0000.008

3.000

3

40.059.090.22

49.570.010.35

90.29

90.150.9900.1880.0051.8270.0000.007

3.017

4

40.429.000.73

49.570.000.15

99.87

89.930.9930.1850.0151.8140.0000.003

3.010

Note: Formula basis: 4 oxygens. Analysisnumbers represent the samples as follows:1 = 567A-19-2, 36-40 cm; 2 = 566C-6-1,110-112 cm; 3 = 566-9-1, 50-56 cm; 4 =566C-7-1, 83-86 cm.

Table 3. Analyses of ultramafic rock pyroxenes.

SiO 2

A12O3

FeOMnOMgOCaONa 2 OC r 2 O 3

TiO 2

Total

WoEnFsSiAlA 1 V I

FeMnMgCaNaCrTi

Total

54.984.494.760.09

33.210.930.021.070.00

99.55

1.8390.74

7.431.8990.1010.0820.1370.0031.7090.0340.0010.0290.000

3.996

55.962.436.140.13

33.570.970.030.530.00

99.76

1.8588.84

9.311.9390.0610.0380.1780.0041.7330.0360.0020.0150.000

4.005

56.902.135.750.07

34.000.730.030.710.00

100.32

1.8989.97

8.631.9540.0460.0400.1650.0021.7400.0270.0020.0190.000

3.995

55.502.136.180.13

33.021.610.000.510.01

99.09

3.0887.53

9.401.9410.0590.0290.1810.0041.7210.0600.0000.0140.000

4.008

52.932.592.150.11

17.1023.56

0.030.970.00

99.44

47.9748.43

3.591.9340.0660.0460.0660.0031.9310.9220.0020.0280.000

3.998

52.642.601.990.06

17.0824.11

0.000.800.06

99.34

48.7348.03

3.241.9270.0730.0390.0610.0021.9320.9460.0000.0230.002

4.004

52.432.292.450.00

17.1924.06

0.010.550.00

98.98

48.2447.93

3.841.9300.0700.0290.0750.0000.9430.9490.0010.0160.000

4.013

Note: Total iron as FeO. Formula basis: 6 oxygens. Analysis numbers representthe following samples: orthopyroxenes—1 = 567A-29-2, 36-40 cm; 2 =566-9-1, 50-56 cm; 3 = 566C-6-1, 110-112 cm; 4 = 566C-7-1, 83-86 cm;clinopyroxenes—5 = 566C-6-1, 110-112 cm; 6 = 566-9-1, 50-56 cm; 7 =566C-7-1, 83-86 cm.

A12O3

C r 2 O 3

F e 2 O 3

FeOMnOMgO

Total

AlCrF e + + +F e + +

MnMg

Total

1

41.1529.88

0.3713.310.10

16.70

101.51

10.7275.2230.0622.4610.0195.503

23.994

2

53.3314.20

1.2510.540.13

19.89

99.34

13.3132.3770.1991.8660.0236.277

24.055

3

24.7544.08

1.6416.610.24

12.43

99.75

7.1548.5450.3033.4060.0504.542

23.999

4

31.4137.72

1.1515.890.07

13.75

99.99

8.7517.0470.2053.1400.0144.842

23.999

5

28.0340.22

2.6316.180.22

13.15

100.43

7.9187.6190.4743.2420.0454.696

23.994

6

29.8939.08

1.9015.270.03

14.02

100.19

8.3507.3220.3393.0260.0064.951

23.995

Note: Formula basis: 32 oxygens. Analysis numbers represent thesesamples: 1 = 567A-29-2, 36-40 cm; 2 = 567A-16-1, 22-23 cm; 3 =566C-6-1, 110-112 cm; 4 = 566-9-1, 50-56 cm; 5 = 566C-7-1, 83-86 cm; 6 = 570-4-1, 78-31 cm.

60

50

40

o 30

20

10

y

• Harzburgite (567, 566, 566C)

+ Peridotite (570)

Δ Harzburgite (Santa Elena)

80 70 60 50 40

Mg × 100/Mg + Fe + +

Figure 3. Spinel chemical compositions of ultramafic rocks of Leg 84(diagram after Coleman, 1977).

of a very chromiferous spinel alone have survived (Table4). These peridotites have a harzburgitic chemical com-position, nevertheless it differs from the harzburgites ofpanxenomorphic texture on the basis of its less siliceousand higher magnesian content (Table 1, Analysis num-ber 7). The low Ca content suggests that the pyroxene isan orthopyroxene.

The ultramafic cumulates of ophiolitic complexes(Antalya, Troodos, Oman) are composed of rhythmicand recurrent layered dunites, lherzolites, and wehrlites

tending toward troctolites and gabbros. Harzburgiticcumulates are extremely rare. One must therefore con-sider the cumulative harzburgites of Leg 84 as a specialcase, but this is not a new case. The ophiolitic complexof the Santa Elena Peninsula in Costa Rica is also com-posed of harzburgitic cumulates with olivine, orthopy-roxene, and clinopyroxene exsolutions.

Thus the more uniform composition of the Leg 84 ul-tramafic sequences could represent a peculiarity of theophiolitic complexes of Central America. There is insuf-

637


ficient evidence to make firm conclusions because of thepoor recovery of Leg 84 igneous rock and also the factthat serpentinization can impart a change in the originalchemical composition.

THE GABBROS

Gabbros were found only in Hole 567A. These arerocks of coarse granular texture, mainly composed ofPlagioclase and green amphibole with occasional clino-pyroxenes. The parageneses are: (1) Plagioclase + clino-pyroxene + pseudomorph orthopyroxene; (2) Plagioclase+ clinopyroxene + amphibole. Opaque minerals are ab-sent in all the samples studied, as in the gabbros of mostof the differentiated ophiolitic complexes (Antalya, Troo-dos), as well as in those of the Santa Elena Peninsula.

One sample exhibits a centimeter-thick layer of pla-gioclases intercalated between two layers packed with am-phiboles; this regular undeformed layering originatedfrom magmatic phenomena.

The Plagioclase is commonly locally weathered, andmost samples exhibit calcic veinlets as well as zoisite ag-gregates.

Chemical Compositions

Two gabbros were analyzed; both are aluminous andcalcic but have slight Fe, Ti, and K contents. A low CIPWnorm nepheline content could be explained by the sec-ondary calcite (Table 1, Analysis numbers 8 and 9).

The nonzoned pyroxenes are calcic and magnesian(Table 5, Analysis numbers 1 and 2). Two amphibolesare magnesian hornblendes (Table 6, Analysis numbers1 and 2). On the other hand, a weathered gabbro con-tains a faintly colored amphibole of tremolitic composi-tion (Tables 6 and 4) and zoning in the very basic (An86-An76) plagioclases is slight.

The chemical compositions of both the clinopyroxene(W044, En45 5, Fs10) and plagioclases (AN86-76) of Leg 84layered gabbros are close to those of Oman ophiolites(W043, En47, Fs10, An88_64; Pallister and Hopson, 1981)and of those from Antalya (Juteau and Whitechurch,1980).

THE DOLERITES

Hole 567A gave dolerites from several levels with smallcrystals and a subophitic texture. The olivine is serpen-tinized; it is mostly of large automorphic crystals orsmaller ones enclosed in pyroxene. However, there is noolivine in one of the samples. The clinopyroxene is col-orless to pale brown; it has large poikilitic crystals en-closing olivine and also acicular crystals of slightly weath-ered automorphic Plagioclase. The opaque minerals arealso automorphic. The order of crystallization in thedolerites is therefore olivine, Plagioclase, ores andclinopyroxene.


The dolerites have homogeneous chemical composi-tions (Table 1, Analysis Nos. 10-13). They have highMg, Fe, and Ti contents and low concentrations of K.The calculation of the CIPW norms indicates the pres-ence of orthopyroxene and olivine.

The clinopyroxenes are calcic (Table 5, Analysis num-bers 3-7; Fig. 4). They have the same chemical composi-tion in the four samples analyzed. The clinopyroxenesare zoned because of variation in the Fe/Mg and Al ra-tios. The plagioclases are also calcic and zoned, with acomposition of An 80 to An 40. The opaque minerals areilmenite and titanomagnetite.

These dolerites have tholeitic affinities because of theirlow K content high Fe content of clinopyroxene (Fig. 4),and olivine + normative orthopyroxene, but the high Ticontent gives them an alkaline character.

One of the dolerites (Table 1, Analysis number 11)differs from the others by containing less Fe and Mg (noolivine) and more Ti and Na (2.49 CIPW norm nephe-line). This dolerite therefore has no specific tholeitic af-finity. However, the plagioclases of this sample are high-ly weathered, which suggests a possible secondary sodicenrichment.

THE BASALTS

Most of the basalts sampled from hole 567A werestudied at the University of Brest (Bellon et al., this vol-ume), and only two basalts are included in this study.One sample is a breccia intruded by calcite and hematiteveins. The rock is strongly weathered, but its microlitictexture is still observable. The second is a basalt withdoleritic texture (Table 1, Analysis number 14), made upof very elongated automorphic zoned plagioclases (An^,47) and automorphic ilmenite enclosed within xenomor-phic and poikilitic crystals of zoned pyroxenes (Table 5,Analysis numbers 8 and 9). The matrix is a devitrifiedpaste in which acicular crystals of Plagioclase appear.

The chemical compositions, the Fe and Ti contents,the texture, and the zonation of the minerals are similarto those of the dolerites. Because of the few samplesstudied and their brecciated and weathered state, it isnot possible to go into more detail.

THE AMPHIBOLITES

Hole 569A encountered metamorphic basement un-derlying lower Eocene sediments at a depth of 350 m be-neath the seafloor. Of the 13.5 m that were drilled in thebasement, only 2 m of metamorphic rocks were recov-ered in Cores 569A-10 and 569A-11. Ten samples ofCores 569A-10 and 569A-11 were studied, all of whichare amphibolites.

These rocks give evidence of a low-temperature retro-grade metamorphism. They are finely crystallized andhave a granoblastic texture; a number of samples are fo-liated. One of these granoblastic rocks (Table 1, Analy-sis number 16) has a well-preserved amphibolite faciesparagenesis: green hornblende is a magnesiohornblende,twinned basic Plagioclase in An82_58 and ilmenite. Zeo-lites partly replace the plagioclases. In other amphibo-lites, hornblende is preserved, the natrolite completelyreplaces the placioglases, the amphiboles are of the tre-molite-actinolite group, prehnite and colorless phyllitesoccur as clots. Prehnite, sometimes associated withquartz, appears also in veinlets. One sample (Table 1,Analysis number 19) is full of prehnite, and only a fewhornblende crystals are preserved. Thus these rocks have

638


Table 5. Clinopyroxenes from gabbros, dolerites, and basalts (%).

SiO2

A12O3

FeOMnOMgOCaONa2OK2OCr 2 O 3

TiO2

Total

WoEnFsSi.vA CA1V I

FeMnMgCaNaKCrTi

Total

1

51.512.776.400.11

15.4822.190.240.000.170.41

99.28

45.4844.1210.401.9160.0840.0380.1990.0030.8580.8850.0170.0000.0050.011

4.016

2

52.422.666.360.23

15.6121.690.310.010.040.53

99.85

44.6744.7310.601.9330.0670.0480.1960.0070.8580.8670.0220.0000.0010.015

4.004

3

49.784.096.780.15

14.8721.070.330.000.271.69

99.02

44.6743.8711.46

1.8610.1390.0410.2120.0050.8280.8440.0240.0000.0080.047

4.009

4

48.584.539.170.07

13.8420.650.510.000.112.46

99.92

43.8240.8615.31

1.8220.1780.0220.2880.0020.7740.8300.0370.0000.0030.069

4.025

5

48.883.45

10.760.30

12.4720.700.480.000.162.06

99.25

44.3437.1618.50

1.8600.1400.0150.3430.0120.7070.8440.0350.0000.0050.059

4.018

6

46.795.669.310.20

12.8520.720.650.000.113.08

99.41

45.0338.8316.141.7730.2270.0270.2950.0060.7260.8410.0480.0000.0030.088

4.034

7

48.723.28

12.590.25

11.8519.760.420.000.002.06

98.93

42.7135.6321.66

1.8700.1300.0180.4010.0080.6780.8130.0310.0000.0000.060

4.009

8

49.382.86

10.210.00

14.5219.920.370.000.001.59

98.84

41.4242.0016.581.8740.1260.0020.3240.0000.8210.8100.0270.0000.0000.045

4.029

9

51.401.63

10.860.31

15.1818.330.400.020.000.84

99.02

38.0543.8418.111.9410.0590.0130.3430.0100.8540.7410.0290.0010.0000.024

4.015

Note: Total iron as FeO. Formula basis: 6 oxygens. Analysis numbers represent the following samples: clino-pyroxenes of gabbros—1 = 567A-20-1, 104-106 cm; 2 = 567A-20-1, 101-111 cm; clinopyroxenes of dol-erites—3 = 567A-26-1, 108-112 cm; 4 = 567A-21-1, 40-46 cm; 5 and 6 = 567A-26-1, 59-64 cm; 7 =567-29-2, 7-10 cm; clinopyroxenes of basalt—8 and 9 = 567A-25-2, 90-96 cm.

Table 6. Analysis of amphiboles from gabbros and amphibolites (%).

SiO2

A12O3

FeOMnOMgOCaONa2OK2OCr 2 O 3

TiO2

Total

S i . . .A 1 ^A1 V I

FeMnMgCaNaKCrTi

Total

1

48.997.66

10.590.23

16.1212.41

1.330.080.000.24

97.65

7.0590.9410.3671.2760.0283.4621.9160.3710.0150.0000.026

15.467

2

48.946.17

13.140.25

15.3411.570.920.150.111.12

97.71

7.1190.8810.1761.5990.0303.3261.8030.2590.0270.0130.234

15.356

3

51.933.97

10.960.19

16.4411.970.390.000.040.26

96.15

7.5430.4570.2231.3310.0233.5591.8620.1090.0000.0050.028

15.140

4

47.506.99

16.000.31

12.4710.920.810.300.051.03

96.38

7.0990.9010.3291.9990.0392.7791.7490.2330.0580.0060.116

15.308

5

48.975.54

16.690.27

13.7210.690.870.140.070.74

97.70

7.2150.7850.1782.0570.0343.0141.7070.2480.0260.0080.082

15.354

6

47.616.87

16.850.28

13.2610.400.950.200.020.96

97.40

7.0590.9410.2602.0890.0352.9311.6530.2740.0370.0020.107

15.388

7

48.657.46

12.710.37

14.9011.310.850.230.000.97

97.44

7.0730.9270.3511.5460.0453.2301.7620.2390.0420.0000.106

15.321

Note: Total iron as FeO. Formula basis: 23 oxygens. Analysis numbers repre-sent the following samples: amphiboles from gabbros—1 = 567A-20-1,101-111 cm; 2 = 567A-21.CC; 3 = 576A-20-1, 143-147 cm; amphibolesfrom amphibolites—4 = 569-10-1, 33-36 cm; 5 = 569-10-1, 137-139 cm;6 = 569-10-1, 57-60 cm; 7 = 569-10-1, 128-130 cm.

a low-temperature (probably hydrothermal) paragenesisproducing zeolite, actinolite, prehnite, and phyllite. Prod-ucts of the primary high-temperature paragenesis (magne-siohornblende, calcic Plagioclase, ilmenite) of amphibo-lite facies are partially preserved.


The SiO2, A12O3, FeO, MgO, and TiO2 contents (Ta-ble 1, Analysis numbers 16-18) of these amphibolitesare those of basalts. Iron and TiO2 contents in amphib-olites are consistent with those of the gabbros and ba-salts, but not with those of the dolerites. MgO contentis lower. We believe that the amphibolites may representmetamorphosed rocks of primary intermediate basalticcomposition between the dolerites, which are alkaline,and the basalt (Table 1, Analysis number 15). The K2Ocontents are very low and suggest tholeiitic affinities.The Na2O contents are very high (normative nepheline)and suggest a possible secondary sodic enrichment be-cause of the zeolites that appear in place of plagioclases.Lastly a prehnitized sample (Table 1, Analysis number19) has an abnormal CaO content.

Thus the amphibolites of Hole 569A may correspondto tholeiitic basalts metamorphosed to the amphibolitefacies. They give evidence of low-temperature retrogrademetamorphism with sodic or calcic enrichment.

Clinopyroxene Chemical Composition: A Comparison

The clinopyroxene chemical compositions of gabbros,dolerites, and basalts (Table 5) give detailed informationabout the magmatic affinities of these rocks. The chem-

639


• Harzburgiteα GabbroΔ DoleriteA Basalt"fc Antalya*Oman

3070

Figure 4. Clinopyroxenes of ultramafic rocks of Leg 84 in Wo-En-Fs diagram. Antalya data from Juteauand Whitechurch (1980); average composition of (1) ultramafic cumulates, (2) clinopyroxenes, and (3)gabbros. Oman data from Pallister and Hopson (1981); average composition of (1, 2) ultramafic cu-mulates, (3) mafic cumulates, and (4) gabbros and dolerites.

ical character of the pyroxenes is shown in the Wo-En-Fs diagram (Fig. 4). The gabbros, dolerites, and basaltsof Leg 84 have distinct clinopyroxenes. The clinopyrox-enes of the gabbros are diopsides. The clinopyroxenesof the basalts have low Ca- and high Fe- content augites.The clinopyroxenes of the dolerites are high-Ca augites.This provides evidence for the tholeiitic affinities of thegabbros and basalts and the alkaline character of somedolerites.

According to Leterrier et al. (1982), the Ni and Crcontent of clinopyroxenes in rocks can be used to char-acterize their magmatic affinities and geodynamic con-text: (1) they may be alkaline or nonalkaline rocks (Tiversus Ca + Na; Fig. 5A); (2) nonalkaline rocks are di-vided into orogenic and nonorogenic rocks (Ti + Crversus Ca; Fig. 5B); (3) orogenic rocks may be of calk-alkaline or tholeiitic affinity (Ti versus Al; Fig. 5C).

CONCLUSION

Igneous basement from Leg 84 drill sites was foundto consist of mafic and ultramafic rocks: basalts, doler-ites, gabbros, and peridotites, as well as rocks of am-phibolite facies metamorphic grade (569). The serpen-tinized harzburgites have mineralogical and chemicalcompositions similar to those of the tectonic harzburg-ites more often described in the ophiolitic complexes.However, the petrofabrics of the Leg 84 harzburgites re-veals slight deformation. The cumulative texture of peri-dotites from Site 570 could be associated with the basalpart of a layered igneous mass. Thus these ultramaficcumulates may be linked to the same ophiolitic complexas the harzburgites. The gabbros may have a similar ori-gin, as evidenced by their magmatic layering, their weakFe and Ti contents, and their lack of opaque minerals.

The dolerites and the basalts differ in chemical com-position from the gabbros. The Fe and Ti contents andthe mineralogical evolution of the clinopyroxenes andplagioclases are particularly different. Thus it is not pos-

sible to conclude much about the magmatic affiliationof the mafic and ultramafic rocks of Leg 84.

However, there are similarities between the chemicalcompositions of the dolerites and basalts of Oman andLeg 84 (Fig. 4). The dolerites and basalts of Leg 84 couldthus be a sheeted complex associated with the ophiolitesin a more developed geodynamic context as in Oman.

Metamorphic rocks consisting of retrograde metamor-phosed amphibolites were recovered from the basementof Hole 569A. The rocks are of basaltic chemical com-position and have been subjected to high-temperaturemetamorphism (amphibolite facies). Amphibolites aresometimes found associated with ophiolitic complexes as atectonic sole under the ophiolitic rocks (Parrot, White-church, 1978) or inside the ophiolitic complex (Malpaset al., 1973; Bourgois, Calle, et al., 1982; Girardeau andMevel, 1982). Thus the basement rock of the landwardslope of the Middle America Trench off Guatemalacould be a disrupted ophiolitic complex.

Such ophiolites crop out on land in Central America.They are linked to the Polochic-Motagua zone (Bertrandand Vuagnat, 1975, 1976, and 1977) of Guatemala andcrop out in the Santa Elena Peninsula of Costa Rica.These two outcrops of mafic and ultramafic rocks (Har-rison, 1953; Dengo, 1962 and 1973; Butterlin, 1977; Weyl,1980; Kuijpers, 1980; Azema and Tournon, 1980, and1982; Bourgois, Azema, et al., 1982; Aubouin, Azema,et al., 1982) have the following similarities with the drilledbasement of the landward slope of the Middle AmericaTrench off Guatemala: the major component is harz-burgite; the plutonic material includes ultramafic cumu-lates and layered gabbros; there are dolerites and am-phibolites; and the lower Eocene-Upper Cretaceous sed-iments overly the ophiolites. So ages and petrographicand chemical compositions do not discriminate betweenthese ophiolites. Only their structural positions give dis-criminating evidence (Azema et al., this volume; Aubouinet al., this volume). The Santa Elena and Leg 84 ophiolites

640


1.0-

0.5-

A BasaltΔ Doleriteü Gabbro

0.5 1.0Ca + Na

0.8-

0.6^

0.4-1

0.2-

NO

0.4

0.2-

0.5 0.6 0.7 0.8Ca

0.9

CA

TH

0.1Al

0.2

Figure 5. Clinopyroxene chemical compositions of basalts, gabbros, and dolerites of Leg 84 in Leterrier etal. (1982) diagrams. A. Alkaline field (A); non-alkaline field (NA); B. orogenic field (O); nonorogenicfield (NO); C. calc-alkaline field (CA); tholeiitic field (TH).

are south of a cratonic shield that crops out in Guate-mala and Honduras. The Polochic-Motagua ophiolitesare north of this shield.

On the other hand, the major structural feature ofSanta Elena ophiolites of Costa Rica is their overthrust-ing southward (Azema and Tournon, 1980, 1982; Bour-gois, Azema, et al., 1982) onto the radiolarites of Mata-palo unit (Azema et al., this volume). The N90°E toN120°E trend of the Santa Elena ophiolites leads us topropose that they are involved, together with the Leg 84rocks South of Honduras shield, in the same pre-LateCretaceous orogen. We therefore suggest after Aubouin,Bourgois, et al. (1984) that Leg 84 ophiolites of Guate-mala are the offshore equivalent of the Santa Elena ophio-lites of Costa Rica.

ACKNOWLEDGMENT

The authors have greatly profited from the extensive discussions ofRoland von Huene who made helpful comments on a draft of themanuscript. We also thank Miriam Baltuck, Sherman Bloomer, andThomas W. Donnelly for their thoughtful and thorough reviews.Helpful technical guidance was provided by Jean Barandon and OscarSancini during microprobe analysis.

REFERENCES

Aubouin, J., Azema, J., Carfantan, J. Ch., Demant, A., Rangin, C ,Tardy M., and Tournon, J., 1982. The Middle America Trench in

the geological framework of Central America. In Aubouin, J., vonHuene, R., et al., Init. Repts. DSDP, 67: Washington (U.S. Govt.Printing Office), 747-755.

Aubouin, J., Bourgois, J., Azema, J., 1984. A new type of active mar-gin: the convergent-extensional margin, as exemplified by the Mid-dle America Trench off Guatemala. Earth Planet. Sci. Lett., 67:211-218.

Aubouin, J., von Huene, R., Azema, J., Coulbourn, W. T., Cowan,D. S., Curiale, J. A., Dengo, C. A., Faas, R. W, Harrison, W.,Hesse, R., Ladd, J. W., Muzylev, N., Shiki, T, Thompson, P. R.,and Westberg, J., 1979. Premiers résultats des forages profondsdans le Pacifique au niveau de la fosse du Guatemala (fosse d'A-mérique Centrale) (Leg 67 du "Deep Sea Drilling Project" mai-juin 1979). C. R. Acad. Sci. Paris, 289:1215-1220.

Aubouin, J., von Huene, R., Baltuck, M., Arnott, R., Bourgois, J.,Filewicz M., Kvenvolden, K., Leinert, B., McDonald, T., McDou-gall, K., Ogawa, Y., Taylor, E., and Winsborough, B., 1982. Leg84 of the Deep Sea Drilling Project, subduction without accretion:Middle America Trench off Guatemala. Nature, 297:458-460.

Azema J., and Tournon, J., 1980. La péninsule de Santa Elena, CostaRica: un massif ultrabasique charrié en marge pacifique de l'A-mérique Centrale. C. R. Acad. Sci. Paris, 290:9-12.

, 1982. The Guatemalan margin, the Nicoya complex, andthe origin of the Carribean Plate. In Aubouin, J., von Huene, R.,et al., Init. Repts. DSDP, 67: Washington (U.S. Govt. Printing Of-fice), 739-745.

Bertrand, J., and Vuagnat, M., 1975. Sur la presence de basaltes encoussins dans la zone ophiolitique méridionale de la Cordillére cen-trale du Guatemala. Bull. Suisse Mineral. Petrogr., 55:136-142.

, 1976. Etude pétrographique de diverses ultrabasites ophio-litiques du Guatemala et de leurs inclusions. Bull. Suisse Mineral.Petrogr., 56:527-540.

641


1977. Données chimiques diverses sur les ophiolites du Gua-temala. Bull. Suisse Mineral. Petrogr., 57:466-483.

Boudier, R, Coleman, R. G., 1981. Cross section through the peri-dotite in the Samail ophiolite, South Eastern Oman, J. Geophys.Res., 86(B4):2573-2592.

Bourgois, J., Azema, J., Tournon, J., Bellon, H., Calle, B., Parra, E.,Toussaint, J. R, Glaçon, G., Feinberg, H., De Wever, P., and Ori-glia, I., 1982. Ages et structures des complexes basiques et ultraba-siques de la facade pacifique entre 3°N et 12°N (Colombie, Pana-ma et Costa Rica). Bull. Soc. Geol. Fr., 3:545-554.

Bourgois, J., Calle, B., Tournon, J., and Toussaint, J. E, 1982. TheAndean ophiolitic megastructures on the Buga-Buenaventura trans-verse (Western cordillera - Valle Colombia). Tectonophysics, 82:207-229.

Butterlin, J., 1977. Geologie Structurale de la Region des Caraibes,Mexique, Amérique Centrale, Antilles et Cordilléres Caraibes: Paris(Masson).

Coleman, R. G., 1977. Ophiolites. Ancient Oceanic Lithospherel Ber-lin (Springer-Verlag).

Coulbourn, W. T., Hesse, R., Azema, J., and Shiki, T , 1982. A sum-mary of the sedimentology of Deep Sea Drilling Project Leg 67sites: the Middle America Trench and slope off Guatemala. An ac-tive margin transect. In Aubouin, J., von Huene, R., et al., Init.Repts. DSDP, 67: Washington (U.S. Govt. Printing Office), 759-774.

Dengo, G., 1962. Estudio geologico de la region de Guanacaste, CostaRica. Inst. Geogr. Costa Rica.

, 1973. Estructura geologica, historia tectonica y morfologiade America central. Inst. Centroamericano de Investigacion y Tec-nologia Industrial (ICAITI) (2nd ed.): Mexico, Buenos Aires (Cen-tro Regional de Ayuda Tecnica).

Girardeau, J., and Mevel, C , 1982. Shear zones in ophiolitic cumulategabbros as indicators of the evolution of the oceanic crust, Bay ofIslands, Newfoundland. Earth Planet. Sci. Lett., 61:151-165.

Harrison, J. V., 1953. The geology of the Santa Elena Peninsula inCosta Rica, Central America. Proc. 7th Pac. Sci. Congr., pp. 102-114.

Irvine, T. N., and Baragar, W. R. A., 1971. A guide to the chemicalclassification of the common volcanic rocks. Can. J. Earth Sci., 8:523-548.

Juteau, T , and Whitechurch, H., 1980. The magmatic cumulates ofAntalya (Turkey): evidence of multiple intrusions in an ophiolitic

magma chamber. Proc. Int. Ophiolite symp., Cyprus 1979, CyprusGeol. Surv. Dpt., pp. 377-391.

Kuijpers, E., 1980. The geologic history of the Nicoya ophiolite com-plex, Costa Rica, and its geotectonic significance. Tectonophysics,68:233-255.

Leterrier, J., Maury, R., Thonon, P., Girard, D., and Marchal, M.,1982. Clinopyroxene composition as a method of identification ofthe magmatic affinities of paleovolcanic series. Earth Planet. Sci.Lett., 59:139-154.

Malpas, J., Stevens, R. K., and Strong, D. E, 1973. Amphibolite as-sociated with Newfoundland ophiolite: its classification and tec-tonic significance. Geology, 1:45-47.

Maury, R. C , Bougault, H., Joron, J. L., Girard, D., Treuil, M.,Azema, J., and Aubouin, J., 1982. Volcanic rocks from Leg 67sites: mineralogy and geochemistry. In Aubouin, J., von Huene,R., et al., Init. Repts. DSDP, 67: Washington (U.S. Govt. PrintingOffice), 557-576.

Pallister, J. S., and Hopson, C. A., 1981. Samail ophiolitic plutonicsuite: field relations, phase variation, cryptic variation and layer-ing and a model of spreading ridge magma chamber. J. Geophys.Res., 86:2593-2644.

Parrot, J. E, and Whitechurch, H., 1978. Subduction antérieure aucharriage N-S de la croüte téthysienne: facteur de métamorphismede series sédimentaires et volcaniques liées aux assemblages ophio-litiques syro-turcs en schistes verts et amphibolites. Rev. Geogr.Phys. Geol. Dyn., 20:153-170.

Tournon, J., Azema, J., and Desmet A., in press. The Santa Elenaophiolitic nappe (Costa Rica). Its significance for the geologicalhistory of Central America. Can. J. Earth Sci.

von Huene, R., Aubouin, J., Azema J., Blackinton, G., Carter, J. A.,Coulbourn, W. T., Cowan, D. S., Curiale, J. A., Dengo, C. A.,Fass, R. W., Harrison, W., Hesse, R., Hussong, D. M., Ladd, J.W, Muzylov, N., Shiki, T , Thompson, P. R., and Westberg, J.,1980. Leg 67: The Deep Sea Drilling Project Mid-America Trenchtransect off Guatemala. Geol. Soc. Am. Bull, 91:412-432.

Weyl, R., 1980. Geology of Central America: Berlin-Stuttgart (Gebr.Borntraeger).

Date of Initial Receipt: 15 July 1983Date of Acceptance: 6 February 1984

642

20. mafic and ultramafic rocks of leg 84: … · 20. mafic and ultramafic rocks of leg 84:...

Documents