significance of the ca-na pyroxene-lawsonite-chlorite …2007)415-30.pdf · parageneses and...

International Geology Review, Vol. 49, 2007, p. 416–430.Copyright © 2007 by V. H. Winston & Son, Inc. All rights reserved.

Significance of the Ca-Na Pyroxene-Lawsonite-ChloriteAssemblage in Blueschist-Facies Metabasalts: An Example

from the Renge Metamorphic Rocks, Southwest Japan T. TSUJIMORI1 AND J. G. LIOU

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115

Abstract

Paleozoic lawsonite-bearing low-grade metabasalts with rare flattened pillow structures occur inthe Chugoku Mountains, Southwest Japan. The meta-mafic rocks are divided into meta-pillow basaltcore (PBC), meta-pillow basalt rim (PBR), and metabasaltic breccia (BB). The PBC have MORB-like major and trace element concentrations, contain 5 wt% water, and a Na-amphibole–free mineralassemblage: Ca-Na pyroxene (maximum 29% jadeite component) + lawsonite + chlorite + quartz. Incontrast, the PBR and BB contain a lawsonite-blueschist–facies mineral assemblage: Na-amphibole ±lawsonite + pumpellyite ± chlorite + albite + quartz. Parageneses and compositions of minerals yieldequilibrium P-T conditions of T< 280°C and P = 0.6–0.7 GPa for all three rock types. Phase rela-tions in the 2Al–Ca–2Fe3+–(Fe+Mg) tetrahedron suggest that the Na–amphibole-free mineralassemblage in PBC occurs only under relatively oxidized conditions. This mineral assemblage hasbeen described from low-grade metabasalts in several high-P–low-T terranes, and might be a low-Tequivalent of the lawsonite eclogite mineral assemblage. We propose that interaction of fluids withsubducting old oceanic crust first produces the Ca-Na pyroxene + lawsonite + chlorite assemblageby hydration reactions at shallow depths in a cold subduction zone. With subduction to greaterdepths, such intensely hydrated metabasalts with the Na-amphibole-free mineral assemblage trans-forms to lawsonite eclogite below 300°C.

Introduction

LAWSONITE, CaAl2Si2O7(OH)2 ⋅ H2O, is an indexmineral for high-P–low-T (HP-LT) blueschist andeclogite-facies mafic rocks. Nitsch (1968) and Liou(1971) independently determined the minimumpressure stability of lawsonite to be P = ~0.3 GPa atT = ~300°C by a decompression reaction of laumon-tite to lawsonite + quartz + H2O. After these earlyexperiments, subsequent experimental studiesrevealed the upper pressure stability of lawsonite upto 8–10 GPa at temperatures of 750–900°C (Pawley,1994; Schmidt and Poli, 1994; Schmidt, 1995;Okamoto and Maruyama, 1999). Consequently, law-sonite has been considered as the most importanthydrous mineral in the recycling of volatiles andselective trace elements in a subduction zone andthe cooling history of the Earth (Schmidt and Poli,1998; Maruyama and Liou, 2005; Ohtani, 2005;Tsujimori et al., 2006b). Recently, petrologic char-

acteristics of natural lawsonite-eclogites containingthe mineral assemblage garnet + omphacite + lawso-nite (Tsujimori et al., 2006b) have been documentedfrom both regional HP-LT terranes (e.g., Zack et al.,2004; Tsujimori et al., 2005, 2006a) and ultrahigh-pressure (UHP) xenoliths (e.g., Usui et al., 2003,2006). In both cases, however, the early processprior to lawsonite eclogitization of subductingoceanic crust is not clear. Specifically, what is theprecursor mineral assemblage for lawsonite-eclog-ites? In this paper, we describe petrologic and min-eralogic characteristics of low-T metabasalt andassociated blueschist-facies mafic rocks in theChugoku Mountains, SW Japan, and suggest thatthe mineral assemblage Ca-Na pyroxene + lawsonite+ chlorite (Okay, 1982; Hirajima, 1983; Sakakibara,1991) of the low-T blueschist-facies metabasalt isthe most likely candidate, in which chlorite decom-poses to produce garnet with increasing pressureand temperature.2

2Mineral abbreviations are after Kretz (1983); we also use Ca-Napyroxene (Ca-Na px), aegirine (Ae), Na amphibole (Na-amp),phengite (Phe), and stilpnomelane (Stp) throughout this paper.

1Corresponding author. Present address: Pheasant MemorialLaboratory for Geochemistry and Cosmochemistry, Institutefor Study of the Earth’s Interior, Okayama University, Misasa,Tottori 682-0193 Japan; email: [email protected]

0020-6814/06/931/416-15 $25.00 416

BLUESCHIST-FACIES METABASALTS 417

Geologic Outline

In the Chugoku Mountains, Pre-Triassic rocksoccur in the Oki, Oeyama, Akiyoshi, Maizuru, andUltra-Tamba belts. These geotectonic units tectoni-cally underlie nearly E-W–trending Paleozoic ophi-olites and accretionary complexes (e.g., Isozaki,1997; Ishiwatari and Tsujimori, 2003; Fig. 1). A fewblueschist localities have been identified in serpen-tinized peridotite bodies of the early PaleozoicOeyama belt; phengites from blueschist-faciesmetapelite yield K-Ar ages of 330–280 Ma (Nish-imura, 1998; Tsujimori and Itaya, 1999). In thisstudy, several lawsonite-bearing metabasalts werecollected at an outcrop (6 × 5 m) of a blueschist-bearing HP block exposed in the Paleozoic schistlocality along the southwestern margin of the Seki-nomiya serpentinite body (SSB) in the easternChugoku Mountains (Fig. 1); the SSB is the largest(20 × 5 km) serpentinized peridotite body of theOeyama belt (Ishiwatari and Hayasaka, 1992). Law-sonite blueschist and rare jadeitite occur as tectonicblocks along the southern margin of the SSB (Hashi-moto and Igi, 1970; Tazaki and Ishiuchi, 1976); theblueschist-and jadeitite-bearing serpentinitemélange unit of the SSB is similar to the Osayamaserpentinite mélange (OSM in Fig. 1) located about150 km west of the studied area (Tsujimori, 1997,1998; Tsujimori and Itaya, 1999; Tsujimori andLiou, 2004, 2005a, 2005b; Tsujimori et al., 2005).

In outcrop, lawsonite-bearing metabasalts exhib-iting rare flattened pillow structures are interlayered

with metabasaltic breccia, meta-chert, and marblelayers < 1.5 m thick. Meta-pillow basalt is weaklyfoliated and consists of rare Na-amphibole–bearingbluish rims (1 to 5 cm wide) and dark greenish coreswith a Ca-Na pyroxene-lawsonite-chlorite assem-blage. In contrast, basaltic breccia is moderatelyfoliated fine-grained blueschist.

Petrography

Meta-pillow basalt core (PBC)

The PBC is fine-grained and well-recrystallizedexcept for minor relict igneous augite; rare lawsonitepseudomorphs after plagioclase are present (Figs.2A and 2B). The mineral assemblage consistsmainly of Ca–Na pyroxene, lawsonite, and chlorite,with minor titanite, relict igneous augite, and quartz.Some quartz-lawsonite veins (2–15 mm) (Fig. 3)cross-cut the matrix; the matrix contains Ca–Na-px+ Lws + Chl and minor quartz and titanite (Fig. 2C).Yellow to pale green pleochroic Ca-Na pyroxeneoccurs as irregularly shaped fibrous aggregates(<0.2 mm) of tiny crystals, discrete prismatic crys-tals (<0.05 mm), and replacements of relict igneousaugite. Augite occurs as colorless subhedral toeuhedral crystals (0.3–0.5 mm). Except for thereplacements of relict augite, Ca-Na pyroxenecommonly coexists with quartz. Lawsonite occurs aselongate prisms (<0.1 mm) or as aggregates in thematrix; relatively large euhedral crystals (maximum0.5 mm) occur in quartz-lawsonite veins. Somelawsonite contains submicron iron oxide inclusions,

FIG. 1. Simplified map of the Chugoku Mountains, showing various petrotectonic units and the sampling locality inthe Sekinomiya serpentinite body (SSB). Modified after Tsujimori and Liou (2005a).

418 TSUJIMORI AND LIOU

FIG

. 2. P

hoto

mic

rogr

aphs

of m

eta-

pillo

w b

asal

ts fr

om S

ekin

omiy

a se

rpen

tinite

bod

y. A

. A p

seud

omor

ph a

fter

igne

ous

plag

iocl

ase

cons

istin

g of

law

soni

te a

nd c

hlor

ite in

met

a-pi

llow

bas

alt c

ore.

PPL

= p

lane

pol

ariz

ed li

ght.

B. E

nlar

ged

view

of a

par

t of A

sho

win

g a

boun

dary

bet

wee

n th

e ps

eudo

mor

ph a

nd fi

ne-g

rain

ed m

atri

x, P

PL. C

. Ca-

Na

pyro

xene

–be

arin

g fin

e-gr

aine

d m

atri

x of

a P

BC

. XPL

= c

ross

ed p

olar

ized

ligh

t. D

. Na-

amph

ibol

e–be

arin

g m

atri

x of

met

a-pi

llow

bas

alt r

im, P

PL. E

. Enl

arge

d vi

ew o

f a p

art o

f D. F

. Rel

ict

igne

ous

augi

te in

PB

R, P

PL.


probably magnetite. Polycrystalline chlorite coexistswith all other minerals.

Meta-pillow basalt rim (PBR)

The PBR occurs in 1–5 cm thick pillow rims, andconsists mainly of pumpellyite and Na-amphibole,with minor titanite, stilpnomelane, quartz and albite(Figs. 2D and 2E), rare lawsonite, Ca-Na pyroxene,and relict augite (Fig. 2F). Na-amphibole occurs asdiscrete prisms (< 0.2 mm in length) and showsslight color zonation from deep blue at the cores topale blue at the rims. Pumpellyite occurs as irregu-lar aggregates of tiny crystals or radial aggregates ofacicular crystals. In the PBR, Ca-Na pyroxenereplacing relict augite does not coexist with quartz.

Meta-basaltic breccia (BB)

The BB consists of heterogeneous, moderatelyfoliated rocks. The mineral assemblage consistsmainly of Na-amphibole, lawsonite, pumpellyite,chlorite, and albite, with minor quartz and titanite;a penetrative schistosity is defined by preferredorientation of Na-amphibole and lawsonite. Rarephengite, stilpnomelane, and K-feldspar occur asaccessories. A few relict clastic augite grains havepersisted. Na-amphibole occurs as fine-grainedbundle aggregates, lawsonite as elongated prisms(<0.05 mm in length), and pumpellyite as aggregatesof tiny crystals. BB is characterized by the assem-blage Na-amp + Lws + Pmp + Chl, with albite,quartz, and titanite in excess.

Bulk-Rock Compositions

Concentrations of major (Si, Ti, Al, Fe, Mn, Mg,Ca, Na, K, and P) and trace elements (Ni, Cu, Zn,Pb, Y, and V) were analyzed with a Rigaku System3270 X-ray fluorescence spectrometer at KanazawaUniversity. The operating conditions for both majorand trace elements were 50 kV accelerating voltageand 20 mA beam current. Other trace elements (Sc,Cr, Co, Rb, Sr, Zr, Nb, Ba, La, Ce, Pb, Sm, Eu, Yb,Lu, Hf, and Th) were determined by instrumentalneutron activation analysis (INAA). The INAAsamples were activated at Kyoto University Reactor,and the gamma-ray spectroscopic analyses weredone at the Radioisotope Laboratory of KanazawaUniversity.

Representative PBC, PBR, and BB bulk-rockanalyses are listed in Table 1. The three rock typeshave mafic compositions with SiO2 = 47.0–52.7wt%, Al2O3= 14.2–15.6 wt%, FeO* = 11.3–12.6

and TiO2 = 1.6–1.8 wt%, and variable water contentbased on the LOI (loss-on-ignition): PBC (5.0 wt%),PBR (2.7 wt%), and BB (3.3 wt%). Water contentsup to 5 wt% are comparable with calculated H2Ocontent in blueschist-facies MORB (Hacker et al.,2003). PBC is richer in MgO (6.5 wt%) than thePBR (4.0 wt.%) and BB (4.2 wt%). The PBR hashigher CaO (12.7 wt%) than the PMC (8.8 wt%) andBB (5.2 wt%); this suggests Ca-metasomatism dur-ing hydrothermal alteration was more severe at thepillow rim. The BB contains higher Na2O (3.6 wt%)than the PMC (2.7 wt%) and PMR (2.8 wt%); theenrichment in Na suggests that the degree of sea-water alteration might have been higher in thebreccia. The pillow basalts are characterized bysignificantly higher Cr (109–131 ppm) and Ni (56–81 ppm), and lower Ba (78–124 ppm) than basalticbreccia (Table 1). In a spider diagram (Sun andMcDonough, 1989) N-MORB for normalization: Fig.4A), the PBC and BB show a trend similar to thoseof Sambagawa mafic schists (Okamoto et al., 1999).Nearly flat chondrite-normalized REE patterns (Sun

FIG. 3. X-ray images of Al concentration showing occur-rences and internal textures of Ca–Na pyroxenes in quartz-lawsonite veins of the meta-pillow basalt core; brightershades indicate higher Al content (jadeite component). A. Acrystal nearly parallel to the crystallographic c-axis. B. Crystalsoblique to the c-axis.


and McDonough, 1989; Fig. 4B) and relationships ofZr (111–136 ppm)–Zr/Yi (2–3) and Nb/Y (0.1–0.2)–Ti/Y (139–240) suggest a MORB-like affinity for the

pillow basalts (Pearce, 1982, 1983); new discrimi-nation schemes by Vermeesch (2006a, 2006b) andSnow (2006) also suggest a MORB origin.

Mineral Chemistry

Electron microprobe (EMP) analysis and X-rayelement mapping were carried out with a JEOLJXA-8800R EMP at Kanazawa University and aJEOL JXA-8900R EMP at Okayama University ofScience. Quantitative analyses were performed with15 kV accelerating voltage, 12 nA beam current,and 3–5 µm beam size. Natural and synthetic sili-cates and oxides were used for standards. The ZAFmethod (oxide basis) was employed for matrixcorrections. In this study, the presence of quartz wasidentified by X-ray mapping. Representative EMPanalyses are listed in Table 2; Fe was assumed to beFe2+ unless otherwise noted.

Metamorphic clinopyroxenes

Figure 5 shows compositions of metamorphicclinopyroxenes from each rock type; the Fe2+/Fe3+

ratio and end-member components were calculatedfollowing Harlow (1999). Two metamorphicpyroxenes, aegirine-augite and diopsidic pyroxene,are common in the PBC. Aegirine-augite is jd7–29di +hd21–54ae40–56 with XMg = 0.63–0.95, and diopsidicpyroxene is jd0–7di + hd>94ae0-20 with XMg = 0.77–0.88. No apparent difference in composition existsamong different textural types of metamorphicpyroxenes. As shown in Figure 3, a single pyroxenecrystal is chemically heterogeneous and does notshow regular chemical zoning. The X-ray image ofAl distribution shows neither systematic composi-tional zoning nor sector zoning. A heterogeneity—inthat the Al-rich, lighter colored part (jadeite-rich)parallel to c-axis is intergrown with Al-poor darkerportion—is recognized in the elongate grain thatwas cut nearly parallel to the crystallographic c-axis(Fig. 3A). In the crystals oblique to the c-axis, thecompositional heterogeneity is irregular and morecomplex (Fig. 3B). Most Ca-Na pyroxenes in PBRshow a wide compositional range within the aegir-ine-augite field (jd0–24di + hd26–80ae20–67; XMg =0.65–0.95) and overlap with those of PBC.

Lawsonite

The compositions of lawsonite are plotted inFigure 6; all Fe was assumed to be Fe3+. Lawsonitein the PBC and PBR has 0.06–0.12 Fe3+, 1.82–1.91Al, and 0.94–1.01 Ca p.f.u. of 8 oxygen; there was

TABLE 1. Bulk-Rock Compositions of Representative Rock Types of the Investigated Samples

Rock type PBC PBR BB

Major-element compositions wt%

SiO2 46.98 49.10 52.71

TiO2 1.68 1.58 1.75

Al2O3 14.38 14.23 15.63

FeO1 12.78 11.64 11.29

MnO 0.18 0.25 0.18

MgO 6.50 4.00 4.17

CaO 8.82 12.70 5.22

Na2O 2.66 2.79 3.62

K2O 0.92 0.27 1.82

P2O5 0.19 0.26 0.22

L.O.I. (H2O) 5.03 2.70 3.30

Total 100.12 99.52 99.91

Trace-element compositions, ppm

Sc 43.6 34.2

Cr 109 131 25

Co 52 57 41

Ni 56 81 25

Cu 78 27 34

Zn 118 93 123

Rb 23 7 45

Sr 120 65 129

Y 42 68 53

V 332 301 335

Zr 111 136 162

Nb 8 10 3

Pb 4 7

Ba 124 78 251

La 5.0 10.4

Ce 20.9 27.8

Sm 3.8 6.2

Eu 1.4 1.9

Yb 4.8 6.7

Lu 0.5 0.7

Hf 1.3 3.1

Th 0.6

1Total Fe as FeO.


no systematic chemical zoning. In contrast, lawso-nite in the BB contains a significantly lower Fe3+

(0.02–0.06 Fe3+ p.f.u.). Overall, lawsonite areslightly less aluminous than in lawsonite eclogites(Tsujimori et al., 2006a).

Chlorite

Chlorite in the PBC and PBR has 5.6–6.0 Si,4.3–4.7 Al, and 0.1 Mn p.f.u. of 28 oxygen. The XMgvalues increase from PBR (0.43–0.46) to PBC(0.48–0.63) (Fig. 7).

Na-amphibole

Compositions of sodic amphibole are plotted inFigure 8; the structural formulae of amphiboles werecalculated based on O = 23, and the Fe2+/Fe3+ ratiowas estimated with total cation = 13 excluding Ca,Na, and K. Na-amphibole in the PBR is stronglyzoned; the cores are Fe3+-rich with Fe3+/(Fe3+ +[6]Al) = 0.61–0.74 and XMg = 0.30–0.74; in contrast,the rims are mostly ferroglaucophane with XMg =0.34–0.53, and the Fe3+/(Fe3+ + [6]Al) ratioincreases from the mantle to the rims (0.10–0.44).

This chemical zoning suggests Na-amphibolegrowth during prograde metamorphism (Liou andMaruyama, 1987). Na-amphiboles in the BB areferroglaucophane with XMg= 0.33–0.45 and Fe3+/(Fe3+ + [6]Al) = 0.05–0.37; the XMg value overlapswith that of the PBR, but the minimum Fe3+/(Fe3+ +[6]Al) ratio (0.05) is significantly lower than rimcompositions of the PBR.

Pumpellyite

Pumpellyite in PBR and BB is Al-rich, with Al/(Al+Fe+Mg) = 0.65–0.86 (Fig. 9). Such Al-richpumpellyite is common among blueschist-faciesrocks (e.g., Maruyama and Liou, 1988). Pumpellyitein PBR is less magnesian (XMg = 0.15–0.16) than inthe BB (XMg = 0.51–0.55).

Other minerals

Phengite is Si-rich, containing 3.55–3.65 p.f.u.for O = 11. Relict igneous augite contains 0.4–0.9wt% TiO2, 2.4–3.6 wt% Al2O3, and 0.3–0.6 wt%Cr2O3; the XMg ranges from 0.78 to 0.91.

FIG. 4. Bulk-rock chemistry of the investigated samples from the Sekinomiya serpentinite body. A. N-MORB–normalized incompatible element patterns for analyzed samples of PBC, PBR, and BB. For comparison, the elementabundance patterns of the mafic schist of the Sambagawa metamorphic belt, SW Japan (Okamoto and Maruyama, 1999)are shown. B. Chondrite-normalized rare-earth element (REE) patterns for PBC and PBR. Normalizing values are fromSun and McDonough (1989).


TAB

LE 2

. Rep

rese

ntat

ive

Ele

ctro

n-M

icro

prob

e A

naly

ses

of R

ock-

Form

ing

Min

eral

s in

the

Inve

stig

ated

Sam

ples

Met

a-pi

llow

bas

alt c

ore

(PB

C)

Met

a-pi

llow

bas

alt r

im (P

BR

)M

eta-

basa

ltic

brec

cia

(BB

)

Ca-

Na-

pxLw

sC

hlr.

Cpx

Na-

amp

Pmp

Lws

Na-

amp

Pmp

Lws

Chl

Phe

r. C

px

core

rim

SiO

253

.85

53.6

038

.14

28.0

552

.77

56.8

855

.88

36.6

938

.61

56.6

636

.55

38.7

326

.72

53.3

551

.89

TiO

20.

110.

130.

200.

000.

520.

000.

050.

150.

090.

030.

030.

210.

090.

010.

56

Al 2O

35.

423.

8229

.99

17.8

92.

7810

.13

4.34

21.9

930

.20

9.50

22.6

230

.90

17.9

822

.74

2.24

Cr 2O

30.

010.

000.

040.

000.

300.

000.

020.

040.

000.

010.

000.

000.

000.

000.

21

Fe2O

312.

652.

670.

7229

.80

7.10

FeO

217

.78

18.1

626

.62

6.56

17.1

123

.70

6.45

18.1

78.

023.

80

MnO

0.10

0.10

80.

030.

560.

190.

160.

100.

460.

100.

120.

540.

000.

520.

050.

28

MgO

3.54

4.58

0.05

16.1

816

.97

6.27

6.52

3.02

0.18

5.86

1.72

0.00

12.7

74.

1115

.94

CaO

6.84

8.35

17.0

20.

0719

.20

0.43

0.56

21.7

917

.46

0.44

21.9

317

.72

0.07

0.00

19.5

9

Na 2O

10.3

29.

420.

000.

000.

697.

036.

940.

120.

007.

210.

130.

010.

000.

040.

34

K2O

0.01

0.00

0.01

0.00

0.00

0.01

0.01

0.01

0.01

0.02

0.00

0.01

0.00

10.7

40.

00

Tota

l97

.99

98.1

688

.12

89.3

799

.97

98.0

298

.12

90.7

189

.30

98.0

091

.54

88.2

987

.94

94.8

598

.14

O=

66

828

1123

2324

.58

2324

.58

2811

6

Si1.

996

1.99

42.

021

5.83

31.

927

7.98

48.

012

6.14

52.

017

8.01

46.

113

2.03

55.

762

3.59

71.

945

Ti0.

003

0.00

30.

008

0.00

00.

014

0.00

00.

005

0.01

80.

003

0.00

30.

004

0.00

80.

014

0.00

10.

016

Al

0.23

70.

168

1.87

34.

385

0.11

91.

675

0.73

34.

341

1.85

91.

584

4.46

01.

914

4.57

01.

807

0.09

9

Cr

0.00

00.

000

0.00

20.

000

0.00

90.

000

0.00

20.

005

0.00

00.

001

0.00

00.

000

0.00

00.

000

0.00

6

Fe3+

0.50

70.

516

0.10

60.

000

0.31

31.

127

0.11

70.

269

0.02

9

Fe2+

0.04

50.

049

4.62

90.

201

1.69

51.

714

0.90

31.

880

1.12

25.

375

0.21

40.

223

Mn

0.00

30.

003

0.00

10.

099

0.00

60.

019

0.01

20.

066

0.00

40.

014

0.07

60.

000

0.09

60.

003

0.00

9

Mg

0.19

60.

254

0.00

45.

013

0.92

41.

313

1.39

40.

754

0.01

41.

235

0.42

80.

000

4.10

60.

413

0.89

0

Ca

0.27

20.

333

0.96

60.

016

0.75

10.

064

0.08

63.

911

0.97

70.

066

3.93

00.

998

0.01

50.

000

0.78

7

Na

0.74

20.

679

0.00

00.

000

0.04

91.

914

1.93

00.

038

0.00

01.

978

0.04

20.

001

0.00

00.

005

0.02

4

K0.

000

0.00

00.

001

0.00

00.

000

0.00

10.

002

0.00

30.

001

0.00

30.

000

0.00

00.

000

0.92

40.

000

Tota

l4.

000

4.00

04.

981

19.9

754.

000

14.9

7915

.018

16.1

844.

992

15.0

4716

.175

4.98

619

.939

6.96

43.

999

1 Tot

al F

e as

Fe 2O

3.2 T

otal

Fe

as F

eO.


FIG. 5. Compositional variations of clinopyroxene in PBC and PBR from the Sekinomiya serpentinite body. Abbre-viations: jd = jadeite; ae = aegirine; di + hd = diopside and hedenbergite; Quad = Ca-Mg-Fe pyroxenes.

FIG. 6. Compositional variations of lawsonite in PBC, PBR, and BB from the Sekinomiya serpentinite body. Thedashed line delineates compositional fields of lawsonite from Guatemalan lawsonite eclogites and related HP-LT rocks.


FIG. 7. Compositional variations of chlorite in PBC and BB from the Sekinomiya serpentinite body.

FIG. 8. Compositional variations of Na-amphibole in PBC and PBR from the Sekinomiya serpentinite body.


P-T Conditions of Metamorphism

Metamorphic conditions of the investigatedsamples were estimated based on available phaseequilibria. The presence of pumpellyite andabsence of epidote in the PBR and BB suggest thatthe maximum T of metamorphism is limited by theexperimentally determined reaction: Mg-Al Pmp =Czo + Grs + Chl + Qtz + H2O (Schiffman and Liou,1980). Furthermore, the generic petrogenetic gridsin the system Na2O–CaO–MgO–Al2O3–SiO2–H2Oimply that the Na-amp + Lws and Na-amp + Pmpassemblages in the PBR and BB are stable at higherP and lower T than the pumpellyite-actinolite facies;they can be described as lawsonite-blueschist facies(e.g., Evans, 1990; Frey et al., 1991; Banno, 1998;Katzir et al., 2000). The reaction: 17 Gln + 24 Lws =6 Pmp + 9 Chl + 19 Qtz + 34 Ab + 8 H2O (Katzir etal., 2000) constrains the approximate P = 0.7 GPa atT = ~300°C for the Na-amp-bearing mineral assem-blages of PBR and BB. The presence of Si-richphengite (up to 3.65 p.f.u.) also suggests high-P andlow-T conditions (Velde, 1967; Massonne andSchreyer, 1987). In the PBC, the Ca-Na px (maxi-mum Jd = 29%) + Qtz assemblage yields a P > 0.65GPa at T = 280 °C (Holland, 1983). In very low-Tblueschist-facies metamorphism, the Ca-Na px +Lws + Chl assemblage has been described inmetabasalts from several localities (e.g., Okay,1982; Hirajima, 1984; Sakakibara, 1991). Hirajima(1983) calculated a Schreinemakers’ grid showingthe stability field of diopside (Di) + Lws + Chl atblueschist-facies conditions. The Ca-Na px + Lws +Chl assemblage in our PBC may have formed at theP-T conditions of his Di + Chl + Lws field. Sakak-ibara (1991) described the Ca-Na px + Lws + Pmp +Chl assemblage from Tokoro metabasalts ofHokkaido, Japan, and estimated T = 200–230°C at

0.6 GPa. Because many petrologic similarities existbetween the investigated PBC and the “Lw-Napxzone” metabasalts of Sakakibara (1990), the investi-gated samples may record blueschist-facies condi-tions similar to those of Tokoro metabasalts.

Discussion

Na-Ca pyroxene–lawsonite–chlorite assemblage in low-grade metamorphism

In the investigated samples, the PBC differs fromthe PBR and BB in its lack of Na-amphibole.Although these rocks differ in mineral assemblages,they may have recrystallized at the same P-T condi-tions; the appearance of different assemblages maybe mainly controlled by bulk-rock compositions, asillustrated below.

The mineral parageneses of the investigatedmeta-mafic rocks can be discussed in terms of afour-component system Ca (CaO-TiO2)–2Al (Al2O3-Na2O-0.87K2O) – 2Fe3+ (Fe2O3) – R (FeO + MgO –0.345K2O) after Maruyama et al. (1985) (Fig. 10).We assume that: (1) K2O and TiO2 are present onlyin phengite [K2Al1.74R0.65Si3.57O10(OH)2] and titan-ite, respectively; (2) quartz and albite are excessphases; (3) the fluid phase consists of pure H2O andis treated as an excess phase; and (4) FeO and MgOare exchangeable. In this system, parageneses ofminerals are graphically represented as invariantpoints, univariant lines, divariant planes, andtrivariant tetrahedrons within a tetrahedron withCa-2Al-2Fe3+-R at its apices (Fig. 10A). Assumingalbite as an excess phase in the PBC, the Ca-Na px+ Lws + Chl assemblage plane can coexist within aNa-amp + Lws + Pmp + Chl tetrahedron. In otherwords, lack of Na-amphibole in the PBC is not dueto retrogression. Moreover, the Ca-Na px + Lws +

FIG. 9. Compositional variations of pumpellyite in PBR and BB from the Sekinomiya serpentinite body. Composi-tional fields of pumpellyite from the Ward Creek blueschist (Maruyama and Liou, 1988) and East Taiwan ophiolite(Liou, 1979) are also shown.


Chl (+ Mag or + Hem) assemblage requires a bulk-rock composition richer in Fe3+ than that of theNa-amp + Lws + Pmp + Chl assemblage. This sug-gests that a relatively high oxidation state is neces-sary to form the Ca-Na px + Lws + Chl assemblage.Moreover, the presence of Ca-Na pyroxene inquartz-lawsonite veins suggests fluid infiltrationpromoted the crystallization of Ca-Na px + Lws +Chl mineral assemblage.

de Roever (1972) proposed the lawsonite-albitesub-facies in a P-T field just below the blueschistfacies, in which Lws + Chl +Ab is stable instead ofNa-amp, also described as “lawsonite-chloritefacies” (e.g., Turner, 1981; Peacock, 1993). Thehigh-pressure limit of the lawsonite-chlorite faciesis defined by the reaction: 5 Gln + 2 Lws = Tr + 10Ab + 2 Chl. This reaction does not involve H2O, andits P-T equilibrium curve is thus independent offluid pressure.

According to this reaction, however, the appear-ance of Na-amphibole requires an initial mineralassemblage containing Ca-amphibole. Althoughthe Ca-Na px + Lws + Chl assemblage in PBC mighthave been in equilibrium with lawsonite-blue-schist–facies P-T conditions, the lack of Na-amphi-bole might be due to that the absence of Ca-amphibole from the initial mineral assemblage. Thisis consistent with the fact that actinolitic amphiboleis not common in hydrothermally altered pillowbasalts in the modern ocean floor (e.g., Alt andTeagle, 2000).

Significance of the Ca-Na px + Lws + Chlassemblage as a possible precursor of the lawsonite eclogitic mineral assemblage

The transformation from blueschist to eclogite isone of the most characteristic reactions in thesubduction-zone process (e.g., Peacock, 1993;Maruyama et al., 1996; Ernst, 2005). Minorprograde blueschist-facies minerals are preservedin glaucophane-epidote eclogites (e.g., Clarke et al.,1997; Gao et al., 1999; Tsujimori, 2002) as inclu-sions of porphyroblastic garnets. Considering thosenatural occurrences, it is likely that epidote-blue-schist or lawsonite-blueschist transforms to glauco-phane-epidote eclogite by various dehydrationreactions. In other words, blueschist is a majorprecursor of glaucophane-epidote eclogite. How-ever, the precursor assemblage for lawsonite eclo-gite is not well known because lawsonite eclogite isnot common.

Basaltic lawsonite eclogites contain a character-istic Grt + Omp + Lws assemblage (cf., Tsujimori etal., 2006b). In Guatemalan lawsonite eclogites,prograde glaucophane is less common withinporphyroblastic garnet; instead chlorite andpumpellyite occur as prograde inclusions (Tsujimoriet al., 2006a). Moreover, lawsonite xenoliths inkimberlitic pipes at Garnet Ridge in the ColoradoPlateau also lack Na-amphibole in prograde-zonedgarnet (Usui et al., 2006). If we consider that chloritedecomposes to produce garnet with increasing

FIG. 10. Phase relations observed in the investigated rocks in the Ca-2Al-2Fe3+-R tetrahedron. A. The Ca-2Al-2Fe3+-R tetrahedron showing blueschist-greenschist minerals. Filled circles represents observed mineral assemblages.Bulk-rock compositions of each sample are also drawn as small filled squares on the 2Al-2Fe3+-R plane. Abbreviations:C = meta-pillow basalt core; R = meta-pillow basalt rim; B = meta-basaltic breccia. B. Tetrahedrons showing observedmineral assemblages.


pressure and temperature, the Ca-Na px + Lws +Chl assemblage in low-T blueschist-facies meta-basalts appears to be the most suitable candidate forthe precursor mineral assemblage of lawsonite eclo-gite. A simple phase relationship among lawsonite,chlorite, diopside, pyrope, and grossular in the CaO-Al2O3-MgO-SiO2-H2O system is shown in Figure 11.In this Schreinemakers’ grid, the Prp (= Grt) + Di(= Omp) + Lws stability field represents lawsoniteeclogite. During subduction, incipiently metamor-phosed ocean-floor basalts containing the Di (= Ca-Na px) + Lws + Chl assemblage might continuouslytransform to the Grt + Omp + Lws mineral assem-blage with increasing P and T (Fig. 11). Okay (1982)described grossular-rich garnet in a Ca-Na pyrox-ene- and lawsonite-bearing metabasalt of theTavºanli region of northwestern Turkey. The invari-

ant reaction 5 Di + 4 Lws = 3 Grs + Chl + 6 Qtzsuggests that the grossular-rich garnet can coexistwith the Ca-Na px + Lws assemblage at lower valuesof P and T. Recently, Tsujimori et al. (2006a) sug-gested lawsonite eclogitization was initiated at T =~300°C and the chlorite-consuming reaction to formFe–Mg–Mn garnet is more effective than the lawso-nite-consuming reaction to form grossular compo-nent. During incipient eclogitization, Ca-Napyroxene might change compositions to omphaciteor jadeite by consuming albite. Furthermore, chlo-rite might be continuously consumed to form garnet,similar to the case of the garnet-forming reaction inthe pelitic system proposed by Inui and Toriumi(2004).

As mentioned above, the Ca-Na px + Lws + Chlassemblage is stable at a relatively high oxidation

FIG. 11. Schreinemakers’ grids in P-T space. The invariant points and univariant reactions are calculated by theBEYES version 1.1 program with an internally consistent thermodynamic dataset (Chatterjee et al., 1998). The greyarea represents the Di (= Ca-Na px) + Lws + Chl assemblage in low-T blueschist-facies metabasalts. The hatched arearepresents lawsonite eclogite mineral assemblage of Prp (= Grt) + Di (= Omp) + Lws. Arrow represents a possibleprograde P-T path of lawsonite eclogite.


state. In general, the degree of oxidation of oceaniccrust increases through reaction with seawater-derived hydrothermal fluids near the spreadingcenter and oxygenated deep-sea water (e.g., Alt,1995). Bach and Edwards (2003) and Edwards et al.(2004) suggested that the average oxidation state ofupper basaltic ocean crust increases with timethrough microbial Fe-oxidation; in other words,older oceanic crust is more intensely altered andoxidized. If so, the Ca-Na px + Lws + Chl assem-blage rather than Na-amphibole–bearing mineralassemblage may be favorable for such oxidizedoceanic crust. The subduction of old oceanic crust iscrucial to the formation of a “cold” subduction-zonegeotherm (e.g., Peacock, 1993; Peacock and Wang,1999). We propose that the subduction of oldoceanic crust first produces the Ca-Na px + Lws +Chl assemblage by fluid infiltration in a cold sub-duction zone at shallow depths, and then the fullyhydrated metabasalts with the Na-amphibole–freemineral assemblage continuously transform tolawsonite eclogite below 300°C at greater depths. Inessence, the Ca-Na px + Lws + Chl assemblage is alow-T equivalent of the lawsonite eclogite mineralassemblage.

Acknowledgments

This research was supported by the JapaneseSociety for the Promotion of Science, through aResearch Fellowship for Research Abroad for thefirst author (Tsujimori) and the National ScienceFoundation through NSF grant EAR-0510325(Liou). We thank A. Ishiwatari, H. Ishida, H.Shukuno, and M. Motoya for helping with the XRFand INNA analyses and T. Itaya for providing thefacility for the EMP analyses. We thank C. G.Mattinson and W. G. Ernst for critically reviewingthis manuscript.

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