28. MINERAL CHEMISTRY OF FOREARC VOLCANIC ROCKS … › publications › 126_SR › VOLUME › CHAPTERS › sr126_28.pdfHole 793B arc-tholeiitic igneous rocks. The 30-32 Ma porphyritic,

Taylor, B., Fujioka, K., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 126

28. MINERAL CHEMISTRY OF FOREARC VOLCANIC ROCKS FROM THE IZU-BONIN ARC,HOLES 792E AND 793B1

Henriette Lapierre,2,3 Rex N. Taylor,4 Olivier Rouer,3 and Hervé Chaisemartin3

ABSTRACT

The Izu-Bonin forearc basement volcanic rocks recovered from Holes 792E and 793B show the same phenocrysts assemblage(i.e., Plagioclase, two pyroxenes, and Fe-Ti oxides ±olivine), but they differ in the crystallization sequence and their phenocrystchemistry. All the igneous rocks have suffered low-grade hydrothermal alteration caused by interaction with seawater. As a result,only clinopyroxenes, plagioclases, and oxides have preserved their primary igneous compositions.

The Neogene olivine-clinopyroxene diabasic intrusion (Unit II) recovered from Hole 793B differs from the basement basalticandesites because it lacks Cr-spinels and contains abundant titanomagnetites (Usp38 5 ^ 6 4) and uncommon FeO-rich (FeO = 29%)spinels. It displays petrological and geochemical similarities to the Izu Arc volcanoes and, thus, can be considered as related toIzu-Bonin Arc magmatic activity.

The titanomagnetites (Usp2g.5-33) in the calc-alkaline andesitic fragments of the Oligocene volcaniclastic breccia in Hole793B (Unit VI) represent an eariy crystallization phase. The Plagioclase phenocrysts enclosed in these rocks show oscillatoryzoning and are less Ca-rich (An78 6_67 8) than the Plagioclase phenocrysts of the diabase sill and the basement basaltic andesites.Their clinopyroxenes are Fe-rich augites (Fs < 19.4; FeO = 12%) and thus, differ significantly from the clinopyroxenes of theHole 793B arc-tholeiitic igneous rocks.

The 30-32 Ma porphyritic, two-pyroxene andesites recovered from Hole 792E are very similar to the andesitic clasts of theNeogene breccia recovered in Hole 793B (Unit VI). Both rocks have the same crystallization sequence, and similar chemistry ofthe Fe-Ti oxides, clinopyroxenes, and plagioclases: that is, Ti-rich (Usp25 5_30.4) magnetites, Fe-rich augites, and intenselyoscillatory zoned plagioclases with bytownitic cores (Ang6_63) and labradorite rims (An73_68). They display a calc-alkalinedifferentiation trend (Taylor et al., this volume).

So, the basement highly porphyritic andesites recovered at Hole 792E, and the Hole 793B andesitic clasts of Unit VI showthe same petrological and geochemical characteristics, which are that of calc-alkaline suites. These Oligocene volcanic rocksrepresent likely the remnants of the Izu-Bonin normal arc magmatic activity, before the forearc rifting and extension.

The crystallization sequence in the basaltic andesites recovered from Hole 793B is olivine-orthopyroxene-clinopyroxene-pla-gioclase-Fe-Ti oxides, indicating a tholeiitic differentiation trend for these volcanic rocks. Type i is an olivine-and Cr-spinelbearing basaltic andesite whereas Type ii is a porphyritic pyroxene-rich basaltic andesite. The porphyritic plagioclase-rich basalticandesite (Type iii) is similar, in most respects, to Type ii lavas but contains Plagioclase phenocrysts. The last, and least commonlava is an aphyric to sparsely phyric andesite (Type iv). Cr-spinels, included either in the olivine pseudomorphs of Type i lavasor in the groundmass of Type ii lavas, are Cr-rich and Mg-rich. In contrast, Cr-spinels included in clinopyroxenes andorthopyroxenes (Types i and ii lavas) show lower Cr* and Mg* ratios and higher aluminium contents. Orthopyroxenes from allrock types are Mg-rich enstatites. Clinopyroxenes display endiopsidic to augitic compositions and are TiO2 and A12O3 depleted.All the crystals exhibit strong zoning patterns, usually normal, although, reverse zoning patterns are not uncommon. Theplagioclases show compositions within the range of An90_64. The Fe-Ti oxides of the groundmass are TiO2-poor (Usp16_17).

The Hole 793B basaltic andesites show, like the Site 458 bronzites from the Mariana forearc, intermediate features betweenarc tholeiites and boninites: (1) Cr-spinel in olivine, (2) presence of Mg-rich bronzite, Ca-Mg-rich clinopyroxenes, andCa-plagioclase phenocrysts, and (3) transitional trace element depletion and εN d ratios between arc tholeiites and boninites. Thus,the forearc magmatism of the Izu-Bonin and Mariana arcs, linked to rifting and extension, is represented by a depleted tholeiiticsuite that displays boninitic affinities.

INTRODUCTION

One of the major goals of Ocean Drilling Program Leg 126 wasto examine the nature and composition of the Izu-Bonin forearcbasement (Leg 126 Shipboard Scientific Party, 1989a, 1989b; Taylor,Fujioka, et al., 1990). Drilling reached basement at Sites 792 and 793,located, respectively, on a basement high upslope from a fork in AohaShima Canyon and in the center of the basin. Sedimentary rocksoverlying the basement have biostratigraphic ages of about 31 Ma,indicating that the forearc basin was not formed earlier than themid-Oligocene. The volcanic rocks recovered from the Izu-Boninforearc region are mostly two-pyroxene andesites. The understanding

1 Taylor, B., Fujioka, K., et al, 1992. Proc. ODP, Sci. Results, 126: College Station,TX (Ocean Drilling Program).

2 URA-CNRS 69, Université Joseph Fourier, Institut Dolomieu, 15 Rue MauriceGignoux, 38031 Grenoble Cedex, France.

3 URA-CNRS 1366, Université cTOrléans, Laboratoire de Géologie Structurale, B. P.6759, F. 45067, Orleans Cedex 2, France.

4 Department of Geology, University of Southampton, Hampshire, SO9 5NH UnitedKingdom.

of the nature and origin of these andesites may add constraints to thetectono-magmatic evolution of intraoceanic island arcs and to the roleof the upper mantle in arc-magma genesis.

This paper presents mineralogical (microprobe) and petrographicdata from the recovered forearc basin lavas, compares them with otherforearc volcanic rocks and island-arc tholeiites, and discusses theirgeodynamic implications. Because the characteristics of volcanicrocks differ between the two forearc sites, they are discussed sepa-rately. General descriptions of lithologies are summarized from theinitial report (Taylor, Fujioka, et al., 1990). Lithologic unit numbersused in the text are those of the initial report.

METHODS

Minerals from 50 samples of the Izu-Bonin forearc basementrecovered in Holes 793B and 792E were analyzed for major andminor elements. All elements were analyzed on a CAMEBAX mi-crobeam electron microprobe at the Bureau de Recherches Géo-logiques et Minières (BRGM) and Université d'Orleans using thematrix corrections of Hénoc andTong (1978). Analytical conditions

431

H. LAPIERRE, R. N. TAYLOR, O. ROUER, H. CHAISEMARTIN

are 15 kV and 10 nA. Counting times range from 6-10 s (major elements)to 30 s (Cr in clinopyroxenes). Under these conditions, concentrations

MINERAL CHEMISTRY OF FOREARC VOLCANIC ROCKS

A Labradortte Bytownite \ Anorthite

80

An(%)

Figure 1. Plagioclase composition in the orthoclase (Or)-albite (Ab)-anorthite(An) diagram (after Deer et al., 1964, 1980), Hole 793B. A. Diabase sill. B.Andesitic clasts of the volcaniclastic breccia (Unit VI). C. Chromite-olivinebasaltic andesite (Type i). D. Porphyritic pyroxene-rich basaltic andesite (Typeii). E. Porphyritic plagioclase-rich basaltic andesite (Type iii). F. Aphyric tosparsely basaltic andesite (Type iv). Filled squares = microlites, filled circles= phenocryst cores, and open circles = phenocryst rims.

they are more Ca-rich, compared with the middle and upper partswhere compositions are restricted to labradorite.

The plagioclases of the andesitic fragments in the volcaniclasticbreccia (Unit VI) are more Na-rich (An79_69; Fig. IB) than those inthe diabase. In these crystals, oscillatory zoning is ubiquitous.

The Plagioclase compositions (An90_64; Figs. 1C-1E) of the base-ment basaltic andesites range from anorthite to labradorite. No chemi-cal differences were observed between the four lava types definedabove. The only chemical differences recognized are between phe-nocrysts and microlites in the Types ii and iii lavas, where themicrolites are enriched in Na (Figs. 1C-1E).

Clinopyroxene Compositions

Clinopyroxenes occur as either phenocrysts, microphenocrysts, orquenched microlites. Compositions, determined by microprobe, arelisted in Table 2. Their chemistry reflects the geochemical differencesand magmatic affinities of the rocks in which they are included(Kushiro, 1960; Le Bas, 1962; Coombs, 1963; Nisbet and Pearce,1977; Leterrier et al., 1982; Mollard et al., 1983).

In the Ca-Mg-(FeT+ Mn) diagram (Poldervaart and Hess, 1951),the diabase sill (Unit II) clinopyroxenes display endiopsidic andaugitic compositions (Fig. 2A), and are Ca-rich (Wo31^3) and Na-poor (Na2O < 0.3%). The most Fe-rich augites (Fs13_]7) have thehighest TiO2 (0.20% < TiO2 < 0.40%) and lowest Cr (0.15% < Cr2O3< 0.40%) contents. Some phenocrysts show chemical differencesbetween core and rim, with either (1) a normal evolution marked byan iron enrichment and a magnesium depletion from core to rim withan endiopside core and an augite rim, or (2) a reversed evolution withan iron depletion from core to rim (Fig. 2A). These chemical differ-ences within single phenocrysts are not associated with significantchanges in the Ca contents.

In the basement, there are no chemical differences between cli-nopyroxene phenocrysts or microphenocrysts, regardless of whetherthey are included in massive or pillowed lavas or in breccia fragments.The clinopyroxenes of the andesitic clasts in the volcaniclastic breccia(Unit VI) are mainly SiO2-rich (SiO2 = 52%), Fe-rich (Fs < 19.4; FeO= 12%; Table 2 and Fig. 2B), Ti-poor (TiO2 = 0.35%), Al2O3-poor(1.82%), and Cr-poor (Cr2O3 = 0.10%) augites (Fig. 2B). They showa reversed zoning from core to rim, marked by an Mg enrichment anda Ca and Fe depletion. Their chemical compositions, as well as thepresence of titanomagnetite inclusions, are features of calc-alkalinevolcanic rocks.

The clinopyroxene composition of the basement andesites is veryhomogeneous and does not exhibit significant differences between thelava types defined on the ship, with the exception of the clinopyroxenesof the uppermost breccia (Unit 1), which show higher total Fe contents(FeO = 7%-8%; Table 2, Sample no. 20). All the clinopyroxenes clusterin the endiopside and augite fields; most of them plot along the joindividing the two fields (Figs. 2C-2F). They are SiO2-rich (53% < SiO2< 54%) and MgO-rich (MgO < 18%), and very depleted in TiO2 (

H. LAPIERRE, R. N.TAYLOR, O. ROUER, H. CHAISEMARTIN

Table 1. Plagioclase representative compositions, Hole 793B.

Core, sectionInterval (cm)

SiO2AI2O3FeOCaONa2OK2O

Total

%An

State

Rock type

1R-1 (4)132-136

45.8635.00

0.5718.030.860.04

100.36

91.8

Freshphenocrystcore

Diabase

1R-1 (4)132-136

50.9231.34

1.0914.36

2.720.04

100.47

74.3

Freshphenocrystπm

Diabase

1R-1 (4)132-136

49.8731.50

1.0215.15

2.460.06

100.06

77.0

Freshmicrolite

Diabase

1R-2 (7)61-65

49.5631.52

0.9714.91

2.620.01

99.59

75.8

Freshmicrolite

Diabase

1R-2 (7)61-65

54.5827.94

0.7711.614.260.13

99.29

59.6

Freshmicrolite

Diabase

85R-2 (18)49-33

49.8231.86

1.0315.23

2.290.01

100.24

78.6

Freshphenocrystcore

Unit VI

85R-2(18)49-33

52.7930.01

0.6212.92

3.350.06

99.75

67.8

Freshphenocrystπm

Unit VI

99R-1 (48)140-144

47.0933.59

0.6417.32

1.07

99.71

89.9

Freshphenocrystπm

Type I

99R-1 (48)140-144

47.8333.32

0.6317.23

1.120.03

100.16

89.3

Freshphenocrystcore

Type I

99R-1 (48)140-144

52.9829.01

1.0313.16

3.170.12

99.47

69.1

Freshphenocrystπm

Type I

93R-3 (36)96-100

52.3728.81

1.3213.01

3.840.17

99.52

64.5

Freshmicrolite

Type I

Notes: The structural formula was calculated on the basis of eight oxygenes. The numbers in parentheses correspond to the sample numbers analyzed for major and trace element chemistry(Taylor et al, this volume).

En/ Fs

40/

A

/L

50// Diopside

/_ /Endiopside /

// Saüte

Augite

>

35

40>

B

/

5O/ Diopside

Endiopside /

/

/ Salite

Augite35

10 15Fs (%)

20 25 0 10 15Fs (%)

20

Figure 2. Clinopyroxene compositions in the wollastonite (Wo)-enstatite (En)-ferrosilite (Fs) diagram (after Poldervaart andHess, 1951), Hole 793B. A. Diabase sill. B. Andesitic clasts of the volcaniclastic breccia (Unit VI). C. Chromite-olivine basalticandesite (Type i). D. Porphyritic pyroxene-rich basaltic andesite (Type ii). E. Porphyritic plagioclase-rich basaltic andesite(Type iii). F. Aphyric to sparsely basaltic andesite (Type iv). Filled circles = microphenocrysts and microlites, filled squares =phenocryst cores, and open squares = phenocryst rims.

enrichment of their rims may be very important (Figs. 2C-2E). Thesechemical variations within single crystals are probably linked to thecooling rate. In particular, the rate of increase of the Ti and Al contentsvaries directly with the cooling rate (Gibbs, 1973; Walker et al., 1976;Groove and Bence, 1977).

Clinopyroxene Composition as a Function of the Fractionation Process

The clinopyroxene-bearing basement host lavas range from oli-vine-Cr spinel (Type i) to aphyric-sparsely basaltic (Type iv) an-desites. The andesites of Type i (chromite-olivine) and Type ii(clinopyroxene-rich) represent the most mafic rocks, whereas theplagioclase-rich andesites (Type iii) and the sparsely aphyric basalticandesites (Type iv) are the most evolved rocks of the forearc basin.

In Figure 4, Cr, Al, Ti, Si, Mg, Ca, and Na elements of the clinopy-roxene phenocrysts are plotted against XFe (i.e., Fe/[Fe*+Mg]) ratio,considered as a differentiation index. Negative correlations exist with Cr,Mg, and Si, whereas positive correlations exist with Ti and Al, fromolivine-Cr spinel (Type i) and clinopyroxene-rich (Type ii) basalticandesite to plagioclase-rich basaltic andesite (Type iii). In contrast, Caand Na remain roughly constant. In all the diagrams, the plots of theclinopyroxene phenocrysts of the aphyric to sparsely phyric andesite

(Type iv) and the diabase sill (Unit II) are randomly distributed alongthe differentiation trend. Finally, whatever diagram used, the plots ofthe clinopyroxene phenocrysts of the andesitic clasts of Unit VI forma distinct group, characterized by the highest XFe ratios.

Such chemical behavior in the clinopyroxenes of Hole 793Bandesites show the following:

1. The clinopyroxenes of the andesitic clasts of Unit VI exhibitvery different compositions and represent the most evolved mineralsamong all of the basement andesites;

2. The clinopyroxenes of the basement andesites display a regularevolution from Type i to Type iii basaltic andesites, compatible withcrystal fractionation;

3. No significant chemical differences are present between theclinopyroxenes of the chromite-olivine (Type i) and the clinopy-roxene-rich (Type ii) andesite lavas.

These results are in agreement with the whole-rock major chem-istry presented by Taylor et al. (this volume). The clinopyroxenechemistry and the whole-rock compositions of their host rocks are afunction of both phenocryst abundance and magma evolution. Thedifferences in the petrology and chemistry of Types i and iii basaltic

434


Table 1 (continued).

92R-3 (33)55-59

48.7732.080.44

15.512.43

99.23

77.9

Freshmicrolite

Type II

92R-3(33)55-59

47.0533.600.87

17.121.610.05

100.30

85.2

Freshphenocrystcore

Type II

92R-3(33)55-59

46.6533.320.47

17.341.51

99.29

86.4

Freshphenocrystrim

Type II

92R-3 (33)55-59

47.7733.00

1.0116.14

1.830.18

99.93

82.1

Freshphenocrystcore

Type II

96R-1 (42)77-81

47.5933.150.59

16.871.740.04

99.98

84.1

Freshphenocrystrim

Type II

86R-1 (20)128-131

55.3826.85

1.5212.193.210.24

99.39

66.7

Freshmicrolite

Type III

86R-1 (20)128-131

50.0030.80.8

15.222.020.11

98.95

80.1

Freshphenocrystcore

Type HI

88R-1 (26)91-95

48.0632.940.87

16.411.9

100.18

82.7

Freshphenocrystcore

Type ffl

86R-1 (26)91-95

47.6232.860.72

16.201.810.08

99.29

82.8

Freshphenocrystintermediate

Type III

86R-1 (26)91-95

46.9833.480.54

16.771.520.03

99.32

85.8

Freshphenocrystrim

Type III

109R-3(71)50-55

47.5833.350.36

17.131.410.01

99.84

87.0

Freshphenocryst core

Type III

5Q

/ Diopside / Salite

Endiopside Augite

10Fs (%)

E50/ Diopside / Salite

Endiopside Augite

25 0

35

10Fs (%)

25

Figure 2 (continued).

andesites are likely related to crystal fractionation, whereas crystalaccumulation is responsible for the genesis of the clinopyroxene-rich(Type ii) lavas.

Orthopyroxenes

Orthopyroxenes are found only in the diabase sill and in thebasement andesites. They exhibit very homogeneous, Mg-rich,bronzite (MgO 31%) compositions (Fs125_18; Wo ^; Table 4 andFig. 5) very similar to orthopyroxenes from the Mariana Arc tholeiites(Bougault et al., 1982). However, small chemical variations arenoticed within phenocrysts (Fig. 5).

Chromium Spinels

Chromium spinels (Table 5 and Fig. 6) occur as inclusions in theolivine pseudomorphs of the basement basaltic andesites (Type i). Theyare also found within the pyroxene phenocrysts of the clinopyroxene-rich

basaltic andesites (Type ii). Contents of Cr* [0.758 < (Cr/Cr+Al) <0.887] and Mg* [0.407 < (Mg/Mg+Fe2+) < 0.531] are high (Table 5).The high Cr* ratio (>0.6) is a feature of island-arc tholeiites and isprobably linked to a high oxygen fugacity (Kushiro, 1969).

The Cr-spinels included in the olivine pseudomorphs, or present inthe groundmass of the Type ii andesites, are the most Cr-rich (69.9% <Cr2O3 < 49%) and Mg-rich (0.508 < Mg* < 0.531) of all the analyzedspinels (Table 5) and cluster in the field of the Cr-spinels of the Troodosboninitic basalts (Fig. 7; Cameron et al., 1979). However, the Cr-spinelsincluded in the clinopyroxenes of the same rock type (Types i and ii) showlower Cr* and Mg* ratios and higher aluminium contents (10.9% < A12O3< 8.9%; Table 5). The most Cr-depleted spinels (Cr2O3 = 50.9; Cr* <0.758) are included in orthopyroxene phenocrysts of the Type ii lavas.

These differences in the chemistry of the Cr-spinels suggest thatthe chromites included in the olivine phenocrysts or in the ground-mass of Type i andesite represent an equilibrium phase of the forearctholeiites, whereas the Cr-spinels that are richer in alumina andincluded in the pyroxenes reacted with the liquid (Fig. 7).

435




SiO2AI2O3FeOCaONa2OK2O

Total

%An

State

Rock type

109R-3(71)50-55

49.831.58

0.5715.652.000.04

99.64

81.0


Typeiπ

100R-2(49)50-55

47.6233.57

0.4917.24

1.210.03

100.16

88.6

Freshphenocrystrim

Typeiπ

109R-3(71)50-55

52.7129.33

0.8613.55

2.890.12

99.43

71.6

Freshmicrolite

Type III

100R-2(49)16-19

50.980.470.78

14.572.890.08

99.77

73.2

Freshmicrolite

Type IV

100R-2(49)16-19

47.3233.18

0.5517.18

1.450.07

100.25

86.1

Freshmicrophenocrystcore

Type IV

100R-2(49)16-19

46.8533.53

0.6117.45

1.29

99.73

88.2

Freshmicrophenocrystrim

Type IV

113R-4(88)35-41

53.2928.86

0.9912.92

2.910.37

99.34

69.4

Freshmicrolite

Type IV

%Wo

EnZ

Figure 3. Comparison of the Izu-Bonin forearc andesitic basement clinopy-roxenes with lavas from various settings. 1 = Izu-Bonin andesite-clinopy-roxene field; 2 = Site 458 Marianas boninite-clinopyroxene field; 3 = BoninIslands clinopyroxene field; 4 - Site 458 Marianas boninite-clinopyroxenefield; 5 = Site 458 Marianas arc tholeiite-clinopyroxene field.

Iron Spinels and Magnetites

The diabase groundmass includes FeO-rich spinels (FeO = 29%;He96; Table 6 and Fig. 6) and titanomagnetites (13% < TiO2 < 15.5%;Usp38 5 ^ 4), close to the ulvospinel pole (Fig. 8 and Table 6; Buddingtonand Linsley, 1964). Magnetites in the groundmass of the basementbasaltic andesites are less TiO2-rich (TiO2 3.56%; Usp16_17; Table 6)than those of the diabase sill.

Titanomagnetites included in Fe-rich augites of the andesiticfragments of the volcaniclastic breccia (Unit VI) differ from those ofthe basement andesites by having higher contents of TiO2 (4.3%< TiO2 < 6.6%; Usp2 8 5_33) and MgO (


0.03

0.02-

U

0.01-

0

0.15

0.10-

0.05.

0

0.15

0.10

0.05-^

0.06-

0.04-

0.02-

°Δoo

> oo

A A

° .+A AA

• A

O io +

« OB 4 1 M4O0OC& AiO

OOAf OO fihA O O A Δ

o oo á

*****•A oó β^öjA A A


Table 2. Clinopyroxene representative compositions, Hole 793B.


1R-1 (4)132-136

1R-1 (4)132-136

lR-2(7)61-65

lR-2(7)61-65

1R-2 (7)61-65

85R-2 (18)49-53

85R-2(18)49-53

86R-1 (20)128-131

99R-1 (48)140-144

SiO2AI2O3TiO2FeOMgOCaOMnOCr2O3Na2O

Total

%En%Fs%Wo

XFeA11VA1V1Ti+CrCa+Na

State

Rock type

54.112.410.166.33

18.2818.720.33

0.20

100.77

51.110.538.0

0.1630.0450.0580.0110.739

Freshphenocrystcore

Diabase

53.682.490.145.92

17.8818.660.120.490.16

99.54

51.69.8

38.6

0.1570.0410.0660.0180.741

Freshphenocrystrim

Diabase

53.971.800.20

10.0718.3115.010.20.080.10

99.78

52.516.630.9

0.2360.0210.0570.0080.597

Freshmicrolite

Diabase

54.530.990.074.61

19.5018.970.060.460.08

99.27

54.57.3

38.2

0.1170.0150.0270.0150.746

Freshphenocrystcore

Diabase

53.092.850.237.06

17.0618.880.210.260.15

99.80

49.111.839.1

0.1880.0550.0680.0140.752

Freshphenocrystrim

Diabase

52.541.400.26

11.1014.4819.690.500.030.26

100.26

41.218.540.3

0.3010.0370.0250.0080.807

Freshphenocrystcore

Unit VI

51.922.230.129.60

16.1618.840.070.180.20

99.32

46.015.438.6

0.2500.0620.0360.0080.768

Freshphenocryst

rimUnit VI

52.871.720.118.29

15.9619.570.140.230.16

99.05

45.913.640.5

0.2260.0320.0430.0100.793

Freshphenocrystcore

Typel

53.662.040.105.39

17.3620.22

0.090.350.15

99.36

49.68.8

41.6

0.1480.0340.0540.0130.805

Fresh corephenocrystcore

Typel

Notes: The structural formulas was calculated on the basis of six oxygenes. The numbers in parentheses correspond to the sample numbers analyzed for major and trace elementchemistry (Taylor et al., this volume).

%He

Wo (%)

Bronzite

10 15Fs (%)

Wo (%)

B

Bronzite

10 15

Fs (%)

Figure 5. Orthopyroxene composition in the wollastonite (Wo)-enstatite (En)-fer-rosilite (Fs) diagram (after Poldervaart and Hess, 1951), Hole 793B. A. Circles =chromite-olivine basaltic andesite (Type i) with filled circles = phenocryst coreand open circles = phenocryst rim; filled squares = porphyritic pyroxene-richbasaltic andesite (Type ii); and open squares = porphyritic plagioclase-rich basalticandesite (Type iii). B. Aphyric to sparsely basaltic andesite (Type iv) with filledsquares = phenocryst core and open squares = phenocryst rim.

%Ma >/oCh

Figure 6. Oxide classifications in the magnetite (Ma)-chromite (Ch)-hercynite(He) diagram (after Stevens, 1944), Hole 793B. Filled triangles = diabase; opentriangles = andesitic clasts of the volcaniclastic breccia (Unit VI); open circles =chromite-olivine basaltic andesite (Type i); filled squares = porphyritic pyroxene-rich basaltic andesite (Type ii); filled circles = porphyritic plagioclase-rich basalticandesite (Type iii) and aphyric to sparsely basaltic andesite (Type iv).

Zeolites

Zeolites are only found as infills of fractures and vesicles (andesiteType iii). They do not occur as a product of glass alteration. They formlarge crystals up to 3 mm long. Their very homogeneous compositioncorresponds to heulandite with a low Si/Al ratio (



99R-1 (48)140-144

110R-3 (75)78-83

110R-3 (75)78-83

110R-3 (75)78-83

93R-1 (36)96-100

92R-2 (30)70-74

92R-2 (30)70-74

92R-2 (30)70-74

92R-2(31)113-117

92R-3 (33)55-59

54.361.860.095.55

16.9320.120.170.210.04

99.33

48.99.3

41.8

0.1550.0110.0690.0080.792

Freshphenocrystrim

Type I

53.592.020.105.22

17.6719.920.250.470.20

99.44

50.48.8

40.8

0.1420.0380.0490.0170.795

Freshphenocrystcore

Type I

53.591.80

5.2018.4219.490.050.720.10

99.45

52.18.3

39.6

0.1370.0410.0370.0230.770


Type I

51.953.980.389.76

15.5217.890.09

0.38

99.95

45.816.337.9

0.2610.0810.0920.0110.735

Freshphenocrystrim

Type I

53.572.100.115.23

18.3220.660.090.200.13

100.41

50.78.3

41.0

0.1380.0550.0350.0090.813

Freshphenocrystrim

Type I

54.121.730.075.03

18.4220.220.070.180.11

99.95

51.48.0

40.6

0.1330.0330.0410.0070.795

Freshphenocrystcore

Type II

54.061.520.054.92

17.9520.360.260.690.19

100.00

50.68.2

41.2

0.1330.0320.0330.0210.807


Type II

53.682.280.095.97

17.3120.340.350.360.35

100.63

48.810.041.2

0.1620.0490.0490.0120.810

Freshphenocrystrim

Type II

54.051.380.054.83

17.9719.880.140.710.31

99.08

49.39.3

41.4

0.1550.0440.0610.0250.802

Freshmicrophenocryst

Type II

53.311.880.226.47

17.6020.12

—0.15

99.75

49.310.240.5

0.1920.0320.0520.0060.789

Freshphenocrystcore

Type II

uoo

100 Mg*

Figure 7. Chromite composition in the 100C/Cr+Al vs. 100Mg/Mg+Fe2+

diagram (after Dick and Bullen, 1984), Hole 793B. The polygon represents theboninite chromium-spinel field (after Cameron et al., 1979).

canic rocks (Boles, 1972). Heulandite precipitates at very low tem-peratures (50°-100°C) and is replaced by laumontite (Ca-rich zeolite)when the temperature exceeds 100°C (Winkler, 1974).

HOLE 792

Petrography

The igneous rocks recovered from the basement at this site consistof five units: three levels of andesitic flows interbedded with hyalo-clastite beds, and two major hyaloclastitic-volcanic horizons. Thedominant volcanic lithology recovered from this hole is a highlyporphyritic, two-pyroxene andesite.

Unit 1 (Samples 126-792E-70R-1, Piece 1, through -74R-1, Piece3) is a porphyritic andesitic flow that contains about 30% euhedralfresh Plagioclase, 0.5-4 mm in diameter, with




SiO2A12O3TiO2FeOMgOCaOMnOCr2O3Na2O

Total

%En%Fs%Wo

XFeA11VA1V1Ti+CrCa+Na

State

Rock type

92R-3 (33)55-59

53.402.300.116.03

17.9020.160.140.360.23

100.63

49.99.7

40.9

0.1590.0600.0390.0130.801

Freshphenocrystrim

Type II

93R-1 (36)96-100

53.401.800.105.25

18.3820.770.050.460.12

100.33

50.78.2

41.1

0.1380.0570.0200.0160.818

Freshphenocrystcore

Type II

96R-1 (42)77-81

54.011.200.044.66

19.2319.840.160.560.15

99.85

53.17.50

39.40

0.1200.0360.0150.0170.784

Freshphenocrystcore

Type II

96R-1 (42)77-81

53.321.960.156.03

17.6120.340.280.190.13

100.01

49.29.9

40.9

0.1610.0490.0360.0090.806

Freshphenocrystrim

Type II

86R-1 (20)128-131

53.391.390.148.03

16.3519.780.140.220.16

99.60

46.513.040.5

0.2160.0260.0350.0100.795

Freshphenocrystcore

Type III

86R-1 (20)128-131

53.861.590.197.78

16.1219.660.280.210.24

99.93

46.413.040.6

0.2130.0190.050.0110.792


Type III

86R-1 (20)128-131

53.971.420.178.26

16.6918.940.240.250.12

100.06

47.613.638.8

0.2170.0180.0430.0120.754

Freshphenocrystrim

Type III

88R-1 (26)9-12

53.531.910.084.22

18.5720.770.160.900.23

100.37

51.66.8

41.6

0.1130.0590.0230.0280.823

Freshphenocrystcore

Type III

88R-1 (26)9-12

53.702.080.207.03

17.0519.440.190.210.24

100.01

48.611.639.8

0.1880.0390.0510.0120.780

Freshphenocrystrim

Type III

RutileTiO2

Ulvospinel Fe2TiO4

FeOWustite

Ilmenite FeTiO3

Fe3O4Magnetite

Fe2O3Hematite

Figure 8. Fe-Ti oxide composition in the TiO2-FeO-Fe2O3 diagram, Hole 793B. Open squares= diabase; filled squares = andesitic clasts of the volcaniclastic breccia (Unit VI); and opencircles = aphyric to sparsely basaltic andesite (Type iv).

In all the Hole 792B andesites, reaction rims around the quartzxenocrysts are systematically absent.

Igneous Mineralogy

Plagioclases

Plagioclases (Table 9 and Fig. 10) are the dominant phenocrysts,forming up to 25-40 modal%). They always include orthopyroxenes,clinopyroxenes, and Fe-Ti oxides. They are either clustered into glomero-

porphyritic aggregates that may or may not be associated with pyroxenes,or they mantle large orthopyroxenes pseudomorphs. Oscillatory zoning iscommon with bytownitic cores (An86_83) and labradorite rims (An•^g).Locally, the rims may be as Ca-rich as the cores (An81). Textural relationsdemonstrate the progressive Na-enrichment during the crystallizationprocess. Indeed, plagioclases included in the clinopyroxene phenocrystsshow anorthitic compositions (An90_g7 8) whereas, when they are jacketingthe orthopyroxenes, they are Na- enriched (An76-66). Microphenocrystsand microlites present in the groundmass display a wide compositionalrange from An82 to An65.

440



104R-1 (53)65-70

54.221.440.128.65

15.7919.040.440.100.20

100.00

45.714.839.5

0.2350.0040.0580.0060.775


Type III

104R-2 (55)64-75

54.530.730.033.91

19.2420.31

0.130.510.11

99.50

53.36.3

40.4

0.1020.0160.0150.0160.800

Freshphenocryst

coreType m

104R-2(55)64-75

52.421.930.178.71

16.1419.430.270.290.27

99.63

45.914.339.8

0.2320.0530.0320.0140.792

Freshphenocryst

rimTypeiπ

109R-5(71)50-55

53.222.660.125.88

16.6020.02

0.200.470.13

99.30

48.39.9

41.8

0.1660.0440.0710.0170.797

Freshmicrolite

Type III

109R-5 (71)50-55

54.921.530.084.22

17.9819.540.200.920.11

99.50

52.17.2

40.7

0.1160.0070.0580.0280.768


Type IE

100R-2 (49)16-19

54.091.460.045.08

18.1520.37

0.250.480.06

99.98

50.78.4

40.9

0.1360.0310.0320.0150.799


Type IV

114R-2(88)35-41

53.402.310.095.89

16.6320.05

0.240.470.09

99.17

48.210.041.0

0.1660.0350.0650.0160.796


Type IV

Al VI

Figure 9. Clay mineral composition in the AlVI-Mg-Fe+Mn diagram (after Thorette, 1987), Hole793B. Filled squares = illite, open squares = saponite, and open circles = celadonite.

Clinopyroxenes

The clinopyroxenes (Table 10 and Fig. 11) are always fresh andform up to 15% of the mode. They occur as large phenocrysts eitherwith the plagioclases in glomeroporphyritic aggregates or included inthe orthopyroxenes. However, orthopyroxenes are also often jacketedby clinopyroxene microphenocrysts. Despite the visible zoningaround the clinopyroxene phenocryst rims, the variations of compo-sition from core to rim are not important and consist of a slight Fe andAl enrichment.

The chemical composition of the clinopyroxenes varies with respectto their textural relationship with the orthopyroxene phenocrysts. Whenthey are included in the orthopyroxenes or associated with the plagio-

clases, their composition is that of a Fe-rich (FeO 17%) augite. Incontrast, they show a Mg enrichment (17.15% < MgO < 15.60%;En464_49) and cluster at the limit of the augite-endiopside field (Fig. 11;Poldervaart and Hess, 1951) when they rim the orthopyroxenes.

Such a Mg-enrichment of the clinopyroxenes when they mantleorthopyroxene phenocrysts has been described in calc-alkaline vol-canic rocks (Sakuyama, 1979; Fichaut, 1986) and is interpreted to berelated to magma mixing.

Titanomagnetites

Titanomagnetites (Table 11) are widespread in the Hole 792Ebasement andesites as inclusions in clinopyroxene, orthopyroxene,

441


Table 3. Average clinopyroxene compositions of basement, diabase, and breccia (Unit VI) andesites, Marianas

boninites, and arc and forearc tholeiites, Hole 793B.

Rock type

SiO2AI2O3TiO2FeOMgOCaOMnOCr2O3Na2O

%En%Fs%Wo

I

54.001.740.095.27

17.7520.01

0.130.460.17

50.58.60

40.90

II

53.771.750.095.39

18.0220.17

0.140.550.16

50.608.70

40.70

HI

53.351.780.137.12

17.2819.250.190.270.17

48.9011.6039.40

IV

54.001.860.075.57

17.4020.20

0.150.420.12

49.519.16

41.33

Diabase

53.582.240.196.65

17.8318.620.180.210.14

51.0010.1438.86

Unit VI

52.161.830.26

11.1414.9418.960.290.090.19

42.7218.3338.95

Boninite

51.014.530.29

16.9317.430.280.030.29

49.0014.0037.00

Arc tholeite

48.802.410.329.32

12.7817.680.200.020.46

38.0024.0038.00

Forarc lavas(Leg 59,

Hole 458)

52.702.300.209.70

16.2518.300.170.220.16

46.515.937.6

Notes: The structural formulas was calculated on the basis of six oxygenes. The numbers in parentheses correspond to the samplenumbers analyzed for major and trace element chemistry (Taylor et al., this volume). Boninite and arc tholeiite-clinopyroxenecompositions after Natland (1981); forearc-clinopyroxene compositions after Meijer et al. (1980).

and plagioclases phenocrysts, or as microphenocrysts in the ground-mass. Their composition is remarkably constant (Table 11). Their highTiO2 content (8.17% < TiO2 < 10.3%, Usp25.5_30.4; Table 11) ischaracteristic of titaniferous magnetite (Buddington and Linsley,1964; Deer et al., 1964, 1980).

The occurrence and composition of titanomagnetites in the Hole792E basement andesites resemble the same characteristics of oxidesin the andesitic clasts of the volcaniclastic breccia of Hole 793E (UnitVI; Table 6). In both rocks, the titanomagnetites represent an earlycrystallized phase.

DISCUSSION

The Izu-Bonin forearc volcanic rocks recovered from Holes 792E and793B show very different mineralogy and magmatic affinities, but similarsecondary mineralogy related to low-grade hydrothermal activity.

The highly porphyritic andesite recovered from Hole 792E isformed of intensely oscillatory zoned plagioclases, Fe-rich augites,orthopyroxene pseudomorphs, titanomagnetites, and local olivinepseudomorphs and quartz. The titanomagnetite is systematically in-cluded in the phenocrysts. The presence of a Mg-enrichment in theclinopyroxenes that mantle the orthopyroxene phenocrysts suggestmagma mixing. The calc-alkaline feature of these porphyritic an-desites is confirmed by their major and trace element chemistry(Taylor, Fujioka, et al., 1990; Taylor et al., this volume). Indeed, theanalyzed samples of Hole 792E show an evolutionary trend, typicalof calc-alkaline suites with a decline in total iron and titanium,correlated with an increase in SiO2, with decreasing MgO contents(Taylor, Fujioka, et al., 1990; Taylor et al., this volume). The andesiticclasts of the Hole 793B volcaniclastic breccia (Unit VI) have the samemineralogy as the Hole 792E andesite, and in both rocks, the chem-istry of the plagioclases, clinopyroxenes, and titanomagnetites aresimilar. The early precipitation of titanomagnetites, the high Fe con-tent in the augites, and the intense oscillatory zoning in the Plagioclasephenocrysts suggest that these andesites are calc-alkaline. Thus, thebasement highly porphyritic andesites, recovered at Hole 792E, andthe Hole 793B andesitic clasts of Unit VI show the same petrologicaland geochemical characteristics, which are that of calc-alkaline suites.These volcanic rocks likely represent the products of Izu-Bonin Arcmagmatic activity during Oligocene times.

The Hole 793B middle Neogene diabase sill and basement andesi-tic lavas are completely different. The diabase sill mineralogy is thatof an arc tholeiite with the following crystallization order: olivine(now pseudomorphs), Mg-rich orthopyroxene, Mg-Ca-rich clinopy-roxenes (endiopside-augite), Ca-rich Plagioclase, and finally, ti-tanomagnetite, the last mineral to crystallize. Its similar petrologicaland geochemical composition to Toroshima and other Izu Arc volca-noes (Taylor, Fujioka, et al., 1990; Taylor et al., this volume), togetherwith its stratigraphic position and age, allow this diabase intrusion tobe considered as related to Izu-Bonin Arc magmatic activity.

The very homogeneous composition of the Plagioclase, clinopy-roxene, and orthopyroxene phenocrysts in all the Hole 793B base-ment basaltic andesites suggests that the petrological differencesbetween the four lava types are linked to processes of crystal frac-tionation and accumulation. Indeed, the chemistry of the bronzitephenocrysts is constant in all the lava types. The behavior of Cr, Mg,Al, and Ti, relative to XFe (differentiation index) in the clinopyroxenephenocrysts suggests that the petrological differences between an-desite Types i and iii are related to crystal fractionation. The petrologi-cal differences between lava Types i and ii, and Types iii and iv, whichconsist mainly of a difference in the modal proportions of the py-roxenes and plagioclases respectively, are likely a result of clinopy-roxene and/or Plagioclase accumulation. Taylor et al. (this volume)have shown that (1) the whole-rock compositions of the Hole 793Bbasaltic andesites depend on both the phenocryst abundance and themagma evolution, and (2) these rocks have uniquely consistenti43Nd/i44Nd r a t i o S 5 w i m ^ ε N d r a nging from 5.63 to 6.82 (T = 30 Ma).

These results suggest that the Hole 793B basaltic andesites are cogenetic.Our data on the clinopyroxene chemistry support these conclusions.

The Hole 793B basaltic andesites show intermediate featuresbetween arc tholeiites and boninites. Their crystallization sequence isthat of an arc tholeiite but their petrology and mineral chemistry showsimilarities with boninites: Cr-spinel in olivine, and the presence ofMg-rich bronzite that is commonly rimmed by Ca-Mg-rich endiop-side augite. They differ by containing Ca-plagioclase as phenocrystsin all rock types but Type i lavas. Moreover, they exhibit transitionaltrace element depletion and εN d ratios between arc tholeiites andboninites (Taylor et al., this volume).

The Hole 793B basement lavas from the Izu-Bonin Arc and Site 458bronzite andesites from the Mariana forearc are similar in most respects.They have the same forearc tectonic setting (Hussong and Uyeda, 1982;Taylor, Fujioka, et al., 1990). They are dated as mid to late Oligocene(Tagikami and Osima, 1982; Taylor and Mitchell, this volume). Theirpetrology and mineralogy are similar (Meijer et al., 1982; Natland, 1982).Finally, they show identical geochemical (Bougault et al., 1982; Hickeyand Frey, 1982; Taylor et al., this volume) and isotopic characteristics(Hickey-Vargas, 1989; Taylor et al., this volume).

Thus, the syn-rift magmatism related to extension of the forearcof the Izu-Bonin and Mariana arcs is represented by the Hole 793Bbasement andesites and the Site 458 andesites that belong to anisland-arc-depleted tholeiitic suite, displaying boninitic affinities. TheHole 792E calc-alkaline andesites, apparently somewhat older at30-32 Ma (Taylor and Mitchell, this volume), may represent theremnants of a pre-extensional phase of normal arc magmatism.

The low-grade hydrothermal alteration was the result of the interac-tion of seawater with the igneous rocks. This is clearly shown by the highvalues of %,. (-11.26 to -3.41; T = 30 Ma; Taylor et al, this volume).Evidence for the hydrothermal activity comes from saponite-celadonitepseudomorphs of olivine, the illite or saponite + celadonite replacementof glass, and the presence of heulandite in vesicles, veins, and cracks.

442


An%

Figure 10. Andesite-plagioclase composition in the orthoclase (Or)-albite (Ab)-anorthite (An)diagram (after Deer et al., 1964, 1980), Hole 792E. Filled squares = microphenocrysts andmicrolites, filled circles = phenocryst core, and open circles = phenocryst rim.

This alteration took place under very low pressures and temperatures(


Table 4. Basement andesite-orthopyroxene representative compositions, Hole 793B.


SiO2AI2O3T1O2FeOMgOCaOMnOCr2O3Na2O

Total

%En%Fs%Wo

XFeA1IVA1VI

State

Rock type

110R-3(75)78-83

55.571.180.08

10.1130.92

1.830.130.21

—

100.03

81.4015.103.50

0.1550.0410.008

Freshphenocrystcore

Type I

110R-3 (75)78-83

55.071.510.00

10.6730.85

2.020.420.340.06

100.94

80.1016.203.70

0.1630.063

—

Freshphenocryst

rimTypel

93R-1 (36)63-67

56.341.140.08

10.0230.88

1.760.120.290.06

100.69

81.6015.003.40

0.1540.0300.017

Freshphenocrystcore

Type I

93R-1 (36)63-67

56.291.090.119.77

31.531.670.22

0.02

100.70

82.214.63.2

0.1480.0350.010

Freshphenocryst

rimType I

92R-2 (30)70-74

57.031.060.089.14

31.151.810.250.350.05

100.92

82.614.03.40

0.1410.0190.024

Freshphenocrystcore

Type II

92R-2 (30)70-74

57.280.720.008.93

31.411.950.27

0.08

100.64

82.713.63.7

0.1380.0080.022

Freshphenocryst

rimType II

92R-2(31)113-117

56.131.180.03

10.0930.22

1.910.220.260.01

100.05

80.815.53.7

0.1580.0240.025

Freshphenocrystcore

Type II

92R-2(31)113-117

56.521.320.03

10.5629.77

1.630.120.34

—

100.29

80.616.23.2

0.1660.0150.040

Freshphenocryst

rimType II

92R-3 (33)55-59

57.450.430.028.00

32.282.000.150.240.04

100.61

84.311.93.6

0.1220.0100.006

Freshphenocrystcore

Type II

92R-3 (33)55-59

56.471.380.11

10.5729.83

1.840.330.140.03

100.70

80.016.43.5

0.1660.0220.035

Freshphenocryst

rimType II

112R-2 (83)57-62

56.651.070.079.62

31.211.760.170.05

—

100.60

82.214.53.3

0.1470.0230.021

Freshphenocrystcore

Type II

112R-2(83)57-62

56.830.88

—9.72

30.721.710.210.07

—

100.14

81.914.93.2

0.1510.0090.027

Freshphenocryst

rimType II

%Wo

EnZ- AFs

Diopside Salite

Endiopside Augite

10Fs (%)

15 20 25

Figure 11. Andesite-clinopyroxene composition in the wollastonite (Wo)-en-statite (En)-ferrosilite (Fs) diagram (after Poldervaart and Hess, 1951), Hole792E. Filled squares = clinopyroxene-microphenocryst rims around orthopy-roxene, filled circles = phenocryst cores, and open circles = phenocryst rims.

, 1978. Rock-forming Minerals (Vol. 2A): Single-chain Silicates (2ded.): London (Longman Group Ltd.).

1980. An Introduction to the Rock-forming Minerals: London(Longman Group Ltd.).

Dick, H.J.B., and Bullen, T., 1984. Chromian spinel as apetrogenetic indicatorin abyssal and alpine-type peridotites and spatially associated lavas. Con-trib. Mineral. Petrol, 86:54-76.

Duplay, J., and Buatier, M, 1990. The problem of differentiation of glauconiteand celadonite. Chem. Geol., 84:264-266.

Fichaut, M., 1986. Magmatologie de la Montagne Pelée (Martinique) [Thesede doctorat]. Univ. de Brest, France.

Gasparik, T, 1984. Two pyroxene barometry with new experimental data inthe system CaO-MgO-Al2O3-SiO2. Contrib. Mineral. Petrol, 87:87-94.

Gibbs, G. P., 1973. The zoned pyroxenes of the Shiant Isles sill, Scotland. J.Petrol, 4:203-230.

Gill, J. B., 1981. Minerals and Rocks (Vol. 16): Orogenic Andesites and PlateTectonics: Berlin (Springer-Verlag).

Groove, T. L., and Bence, A. E., 1977. Experimental study of clinopyroxene—liquid-interaction in quartz normative basalt 15597. Proc. 8th Lunar Sci.Conf, 18.

Hénoc, J., and Tong, M., 1978. Automatization de la microsonde. J. Microsc.Spectrosc. Electron., 3:247-254.

Herzberg, C. T, 1978. Pyroxene geothermobarometry and geobarometry:experimental and thermodynamic evaluation of some solidus phase rela-tions involving pyroxenes in the system CaO-MgO-Al2O3-Siθ2 Geochim.Cosmochim. Acta, 42:945-957.

Hickey, R. L., and Frey, F. A., 1982. Rare-earth element geochemistry ofMariana forearc volcanics: Deep Sea Drilling Project Site 458 and Hole

Table 5. Basement andesite Cr-spinel compositions, Hole 793B.

Core, sectionInterval in cm

TiO2AI2O3Cr2O3FeO*MnOMgO

Total

Fe2O3FeO

%Magnetite% Hercynite% Chromium spinel

OccurrenceRock type

93R-1 (36)96-100

0.125.99

69.9324.49

0.278.94

109.74

2.3822.34

2.811.086.2

OlivineTypel

93R-1 (36)96-100

0.156.78

67.6425.66

0.278.73

109.23

3.2222.67

3.912.583.6

OlivineTypel

99R-1 (48)140-144

0.288.87

50.2127.65

0.189.84

97.03

11.0317.72

14.217.967.9

OlivineTypel

100R-2(49)16-19

0.235.42

57.3823.95

0.238.96

96.17

6.5318.08

8.711.380.0

GroundmassType IV

109R-8(71)50-55

0.259.01

52.1726.26

0.2726.26

95.96

6.9220.03

9.118.672.3

GroundmassType in

112R-2 (83)57-62

0.1610.8750.8833.48

0.368.64

104.39

12.7821.98

15.420.564.1

OrthopyroxeneType II

444


Table 6. Igneous rock Fe-Ti oxide compositions, Hole 793B.


T1O2AI2O1Cr2θ3FeO*MnOMgO

Total

Fe2θ3FeO

% Magnetite%Hercynite%Ulvöspinel

OccurrenceRock type

1R-1 (4)132-136

15.052.510.06

77.430.240.69

95.98

37.0744.07

90.39.6

44.81

GroundmassDiabase

lR-2(7)61-65

15.432.170.00

75.740.310.46

93.98

35.0944.17

91.28.8

41.13

GroundmassDiabase

85R-2(18)49-53

6.612.700.09

77.760.212.25

89.62

50.4933.39

90.59.1

28.85

ClinopyroxeneUnit VI

85R-2U8)49-53

7.691.690.09

65.390.350.33

75.54

37.9832.21

93.36.5

33.01

ClinopyroxeneUnit VI

H2R-2(83)57-62

3.562.350.04

86.930.070.79

93.74

59.9233.38

94.15.8

17.58

GroundmassType II

Notes: The structural formulas was calculated on the basis of 32 oxygenes. The numbers in parenthesescorrespond to the sample numbers analyzed for major and trace element chemistry (Taylor et al.,this volume).

Table 7. Clay-mineral representative compositions, Hole 793B.


SiO2AI2O3TiO2FeOMgOMnOCaONa2OK2OCrzOj

Total

XFeA1IVA1VI

TypeOccurrence

Rock type

1R-1 (4)132-136

51.703.980.009.11

19.940.320.330.370.71

—

86.46

20.4000.3780.314

SaponiteOlivine

Diabase

lR-2(7)61-65

54.001.930.006.82

18.940.140.320.490.920.08

83.64

16.8000.0000.341

SaponiteOlivine

Diabase

92R-2 (30)70-74

51.905.020.008.66

21.010.161.570.070.04

—

88.43

16.9000.5100.416

SaponiteGroundmass

Type II

93R-1 (36)96-100

48.805.120.047.11

19.650.260.610.200.49

—

82.28

18.4000.4600.458

SaponiteOlivine

Type I

110R-4 (75)78-83

42.673.850.02

14.7917.830.211.360.170.880.05

81.83

31.8000.7480.000

SaponiteOlivine

Type I

93R-1 (36)96-100

56.015.950.13

12.789.850.130.140.068.330.05

93.43

42.0000.0001.003

CeladoniteOlivine

Typel

96R-1 (42)77-81

55.594.460.11

15.407.380.020.090.038.720.11

91.91

53.9000.0000.776

CeladoniteCracks in OPX

Type II

1O9R-8(71)50-55

56.575.290.01

16.267.200.181.250.154.950.15

92.01

55.900.0000.904

CeladoniteVesicles

Type III

100R-2(49)16-19

55.725.280.30

13.406.100.020.180.039.650.10

90.78

55.200.0000.925

CeladoniteVesicles

Type IV

100R-2(49)16-19

49.8013.360.155.674.440.083.682.131.35

—

80.34

41.700.2852.170

IlliteGlass inclusions

inCPXType IV

96R-1 (42)77-81

47.4014.880.148.608.340.285.681.410.160.02

86.91

36.600.9901.604

IlliteGroundmass

Type II

109R-8(71)50-55

56.0012.950.215.247.200.272.500.420.68

—

85.47

29.000.0042.176

IlliteGroundmass

Type III

Notes: The structural formulas was calculated on the basis of 32 oxygenes. The numbers in parentheses correspond to the sample numbers analyzed for major and trace element chemistry(Taylor et al., this volume). OPX = orthopyroxene and CPX = clinopyroxene.

459B. In Hussong, D. M., Uyeda, S., et al., Init. Repts. DSDP, 60:Washington (U.S. Govt. Printing Office), 735-742.

Hickey-Vargas, R. L., 1989. Boninites and tholeiites from DSDP Site 458,Mariana forearc. In Crawford, A. J. (Ed.), Boninites and Related Rocks:London (Unwin Hyman), 339-356.

Hussong, D. M., and Uyeda, S., 1982. Mariana arc and forearc backgroundand objectives. In Hussong, D. M., and Uyeda, S., Init. Reports DSDP, 60:Washington (U.S. Govt. Printing Office), 251-254.

Kuroda, N., Shiraki, K., and Urano, H., 1978. Boninite as a possible calc-al-kalic primary magma. Bull. Volcanol., 41:563-575.

Kushiro, I., 1960. Si-Al relations in clinopyroxenes from igneous rocks. Am.J. Sci., 258:548-554.

, 1969. The system forsterite-diopside-silica with and without waterat high pressures. Am. J. Sci., Schairer Vol., 267A:269-274.

Leg 126 Shipboard Scientific Party, 1989a. Arc volcanism and rifting. Nature,342:18-20.

, 1989b. ODP Leg 126 drills the Izu-Bonin Arc. Geotimes, 34:36-38.Le Bas, M. J., 1962. The role of aluminum in igneous clinopyroxenes with

relation to their parentage. Am. J. Sci., 260:267-288.Leterrier, J., Maury, R. C , Thonon, P., Girard, D., and Marchal, M., 1982.

Clinopyroxene composition as a method of identification of the magmaticaffinities of paleo-volcanic series. Earth Planet. Sci. Lett, 59:139-154.

McBirney, A. R., 1984. Igneous Petrology: San Francisco (Freeman, Cooperand Co.).

Meijer, A., Anthony, E., and Reagan, M., 1982. Petrology of volcanic rocksfrom the forearc sites. In Hussong, D. M., Uyeda, S., et al., Init. Repts.DSDP, 60: Washington (U.S. Govt. Printing Office), 709-729.

Mollard, J. P., Maury, R. C, Leterrier, J., and Bourgeois, J., 1983. Teneurs enchrome et titane des clinopyroxenes calciques des basaltes: application àF identification des affmités magmatiques de roches paléovolcaniques. C.R. Acad. Sci. Sen 2, 296:903-908.

Natland, J. H., 1982. Crystal morphologies and pyroxene compositions in bon-inites and tholeiitic basalts from Deep Sea Drilling Project Holes 458 and 459B in the Marana fore-arc region. In Hussong, D. M., Uyeda, S., et al., Init.Repts. DSDP, 60: Washington (U.S. Govt. Printing Office), 681-707.

Nisbet, E. G., and Pearce, J. A., 1977. Clinopyroxene composition in mafic lavasfrom different tectonic settings. Contrib. Mineral. Petrol, 63:149-160.

Parron, C, and Amouric, M., 1990. Crystallochemical heterogeneity of glau-conites and the related problem of glauconite-celadonite distinction. Geo-chemistry of the Earth's Surface and of Mineral Formation, 2nd Inter.Symp., Rome.

Poldervaart, A., and Hess, H. H., 1951. Pyroxenes in the crystallization ofbasaltic magma. J. Geol, 19:472-489.

Sakuyama, M., 1984. Magma mixing and magma plumbing systems in islandarcs. Bull. Volcanol., 47:685-703.

Stevens, R. E., 1944. Composition of some chromites of the Western Hemi-sphere. Am. Min., 29:1-34.

Tagikami, Y., and Ozima, M., 1982. 40Ar-39Ar dating of rocks drilled at Sites458 and 459 in the Mariana forearc region during Leg 60. In Hussong, D.M., Uyeda, S., et al., Init. Repts. DSDP, 60: Washington (U.S. Govt.Printing Office), 743-736.

Tatsumi, Y., and Ishizoka, K., 1982. Magnesian andesite and basalt fromShodo-Shima island, southwestern Japan, and their bearing on the genesisof calc-alkaline andesites. Lithos, 15:161-172.

Taylor, B., Fujioka, K., et al., 1990. Proc. ODP, Init. Repts., 126: CollegeStation, TX (Ocean Drilling Program).

Thorette, J., 1987. Contribution à 1'étude de 1'hydrothermalismeocéanique: example du district mineralise de York-Harbour (ophiolitede Blow-Me-Down, Bay-Of-Island, Terre-Neuve) [These]. Ecole desMines de Paris, France.

Walker, D., Kirkpatrick, R. J., Longhi, J., and Hays, J. F., 1976. Crystallizationhistory of lunar picritic basalt sample 12002: phase equilibria and cooling-rate studies. Geol. Soc. Am. Abstr. Programs, 18:195.

Winkler, H.GP., 1974. Petrogenesis of Metamorphic Rocks: New York (Sprin-ger-Verlag).

Wood, CP. , 1980. Boninite at a continental margin. Nature, 288:692-694.

Date of initial receipt: 10 January 1991Date of acceptance: 12 September 1991Ms 126B-147

445


Table 8. Zeolite (heulandite) representative compositions, Hole 793B.


104R-2 (55)69-75

104R-2 (55)69-75

104R-2 (55)69-75

104R-2(55)69-75

Siθ2AI2O3FeOMgOCaONa2O

K2O

Total

SiteZ

Si/AlRock typeOccurrence

65.5214.92

3.932.070.26

86.70

36.50

3.73

Type IIIVesicles

64.9514.960.05

4.031.900.26

86.15

36.53

3.68

TypefflVesicles

66.1614.65

3.982.680.28

88.30

36.39

3.69

Type IIICracks

65.9914.65

3.753.160.27

87.82

36.27

3.82

Type πiCracks

Notes: The structural formulas was calculated on the basis of 72 oxygenes. The numbers in parentheses

correspond to the sample numbers analyzed for major and trace element chemistry (Taylor et al.,

this volume).

Table 9. Andesite Plagioclase compositions, Hole 792B.

Core, sectionInterval (cm) or piece no.

71R-1 (100)18-20

71R-1 (100)18-20

71R-1 (100)18-20

76R-1 (108)Piece 4

76R-1 (108)Piece 4

76R-1 (108)Piece 4

76R-1 (108)Piece 4

76R-1 (108)Piece 4

SiO2AI2O3FeOCaONa2O

K 2 O

Total

%An

State

Unit

47.2232.870.87

16.131.880.01

98.96

82.6

Freshphenocrystcore

Unit 1

47.0733.56

0.6116.15

1.82

99.19

83.1


Unitl

50.1530.68

0.5213.872.970.04

98.23

71.9

Freshphenocrystrim

Unitl

47.9232.400.41

15.732.07

98.53

80.8

Freshphenocrystcore

Unit 2

49.6931.41

0.5114.222.86

98.76

73.3


Unit 2

48.9031.740.62

14.932.570.01

98.76

76.2


Unit 2

47.8732.53

0.6315.562.07

98.46

80.6


Unit 2

51.6430.29

0.5313.283.510.05

99.10

68.7

Freshphenocrystrim

Unit 2

Notes: The structural formulas was calculated on the basis of eight oxygenes. The numbers in parentheses correspond to the sample numbers analyzed for major and traceelement chemistry (Taylor et al., this volume). OPX = orthopyroxene and CPX = clinopyroxene.



SiO2AI2O3FeOCaONa2OK 2 O

Total

%An

State

Unit

75R-2(114)69-73

51.9630.04

0.5813.523.790.05

99.92

66.2

Freshmicrophenocrystaround Opx

Unit 3

75R-1 (112)18-22

47.1233.56

0.7017.69

1.36

100.43

87.8

Freshmicrophenocrystincluded in Cpx

Unitl

71R-1 (100)18-20

48.5832.22

0.9616.172.020.03

99.97

81.5

Freshmicrolite

Unitl

73R-1 (104)18-21

50.3132.23

0.5914.693.170.01

99.98

71.9

Freshmicrolite

Unitl

446


Table 10. Andesite-clinopyroxene compositions, Hole 792E.


SiO2AI2O3TiO2FeOMgOCaOMnOCl2θ3Na2O

Total

%En

%Wo

A1IVA1VIXFe

State

UnitRock type

71R-1 (100)18-21

52.441.770.32

10.6613.9820.12

0.370.150.25

100.07

40.417.941.8

0.0400.0380.300

Fresh

phenocryst

core

Unit 1Andesite

71R-1 (100)

18-21

52.662.000.339.66

14.3420.70

0.390.070.36

100.50

41.116.242.7

0.0470.0410.274

Fresh

phenocryst

rim

Unit 1Andesite

73R-1 (104)18-21

51.092.270.47

10.7013.5220.63

0.41

0.30

99.38

39.118.042.9

0.0690.0320.307

Fresh

phenocryst

core

Unit 1Andesite

73R-1 (104)18-21

52.721.480.39

20.720.44

0.15

100.50

40.717.042.3

0.0390.0260.286

Fresh

phenocryst

rim

Unit 1Andesite

76R-1 (108)Piece 4

52.351.63

11.0514.3319.440.61

0.27

100.26

41.118.840.1

0.0440.0280.302

Fresh

phenocryst

core

Unit 2Andesite

76R-1 (108)Piece 4

51.234.34

6.8917.1519.410.080.260.12

99.80

49.011.239.8

0.1170.0710.184

Fresh

microphenocryst

around Opx

Unit 2Andesite

75R-1 (112)18-22

52.701.710.31

10.6213.8020.57

0.36

0.27

100.34

39.717.742.8

0.0350.O400.302

Fresh

phenocryst

rim

Unit VIDacite

75R-1 (112)18-22

52.521.620.35

10.6213.6120.72

0.27

0.21

99.92

39.317.743.0

0.0340.0380.305

Fresh

phenocryst

rim

Unit 1Dacite

75R-1 (112)18-22

52.891.830.28

10.5713.6620.58

0.43

0.21

100.43

39.517.842.7

0.0320.0480.303

Fresh

microphenocrysl

included in

PlagioclaseUnit 1Dacite

75R-2(114)Piece 4

52.021.880.34

20.410.390 020.24

100.14

39.718.242.0

0.0520.0310.307

Fresh

phenocryst

rim

Unit 3Dacite

75R-2(114)Piece 4

51.991.960.30

10.1813.99

0.430.030.21

99.31

40.617.342.2

0.0450.0420.290

Fresh

phenocryst

core

Unit 3Dacite

Notes: The structural formulas was calculated on the basis of six oxygenes. The numbers in parentheses correspond to the sample numbers analyzed for major and trace element chemistry

(Taylor et al., this volume). OPX = orthopyroxene and Plag = Plagioclase.

Table 11. Basement anesite Fe-Ti oxides representative compositions, Hole 792E.


TiO2AI2O3Cr 2O 3FeO*MnOMgO

Total

Fe 2O 3FeO

% Magnetite% Hercynite% Ulvöspinel

OccurrenceRock type

71R-1 (100)18-21

8.172.83

78.110.191.57

90.86

47.7336.10

91.58.5

25.5

Inclusion in CpxUnit 1

71R-1 (100)18-21

9.152.420.09

77.660.561.00

91.76

46.9635.41

92.57.4

28.0


71R-1 (100)18-21

10.292.450.01

78.220.301.28

92.55

46.9635.41

92.17.9

30.4

Inclusion in OpxUnitl

73R-1 (104)18-21

8.712.71

75.490.411.96

89.27

45.8034.28

91.58.5

27.5

Inclusion in CpxUnitl

73R-1 (104)18-21

9.522.320.07

77.890.381.90

92.07

46.5835.98

92.67.2

29.0

Groundmass (core)Unitl

73R-1 (104)18-21

9.352.450.09

80.910.391.58

94.77

48.5637.22

92.57.3

27.8

Groundmass (rim)Unit 1

75R-2 (114)69-73

9.082.31

78.600.471.75

92.20

47.5635.80

92.97.1

27.6


75R-2 (114)69-73

9.972.450.14

79.250.311.65

93.76

46.5137.40

92.17.6

30.0

Inclusion in OpxUnit 3

75R-2 (114)69-73

9.732.210.05

80.880.511.46

94.84

48.1037.60

93.26.7

28.8

GroundmassUnit 3

Notes: The structural formulas was calculated on the basis of 32 oxygenes. The numbers in parentheses correspond to the sample numbers analyzed for major and trace element chemistry (Taylor et al., this volume).OPX = orthopyroxene and CPX = clinopyroxene.

447

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28. MINERAL CHEMISTRY OF FOREARC VOLCANIC ROCKS … › publications › 126_SR › VOLUME › CHAPTERS › sr126_28.pdfHole 793B arc-tholeiitic igneous rocks. The 30-32 Ma porphyritic,