tectonic and metasomatic mixing in a high-t, subduction ...mdbarton/mdb_papers_pdf...lina schist,...

Tectonic and metasomatic mixing in a high-T, subduction-zone

mélange—insights into the geochemical evolution of the

slab–mantle interface

Gray E. Bebout a,*, Mark D. Barton b

aThe Pheasant Memorial Laboratory, Institute for Study of the Earth’s Interior, Okayama University at Misasa, Misasa,

Tottori-ken 682-0193, JapanbCenter for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA

Accepted 8 February 2002

Abstract

The Catalina Schist (California) contains an amphibolite-grade (0.8–1.1 GPa; 640–750 jC) mélange unit consisting ofmafic and ultramafic blocks in high-Mg, schistose mélange matrix with varying modal proportions of talc, chlorite,

anthophyllite, calcic-amphibole, enstatite, and minor phases including zircon, rutile, apatite, spinel, and Fe–Ni sulfides. This

mélange unit is interpreted as a kilometer-scale zone of tectonic and metasomatic mixing formed within a juvenile subduction

zone, the study of which may yield insight into chemical mixing processes at greater depths in subduction zones. Relationships

among the major and trace element compositions of the mafic and ultramafic blocks in the mélange, the rinds developed at the

margins of these blocks, and the surrounding mélange matrix are compatible with the evolution of the mélange matrix through a

complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing. Simple, linear

mixing models are compatible with the development of the mélange matrix primarily through simple mixture of the ultramafic

and mafic rocks, with Cr/Al ratios serving as indicators of the approximate proportions of the two lithologies. This conclusion

regarding mafic–ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance

of large parts of the mélange matrix with rinds developed at the margins of mafic and ultramafic blocks. The overall process

involved development of metasomatic assemblages through complex fluid-mediated mixing of the blocks and matrix concurrent

with deformation of these relatively weak rind materials, which are rich in layer silicates and amphibole. This deformation was

sufficiently intense to transpose fabrics, progressively disaggregate more rigid, block-derived materials in weaker chorite- and

talc-rich mélange, and in some particularly weak lithologies (e.g., chlorite-, talc-, and amphibole-rich materials), intimately

juxtapose adjacent lithologies at the (sub-)cm scale (approaching grain scale) sampled by the whole-rock geochemical analyses.

Chemical systematics of various elements in the mélange matrix can be delineated based on the Cr/Al-based mixing model.

Simple mixing relationships exhibited by Al, Cr, Mg, Ni, Fe, and Zr provide a geochemical reference frame for considerations

of mass and volume loss and gain within the mélange matrix. The compositional patterns of many other elements are explained

by either redistribution (local stripping or enrichment) at varying scales within the mélange (Ca, Na, K, Ba, and Sr) or massive

addition from external sources (Si and H2O), the latter probably in infiltrating H2O-rich fluids that produced the dramatic O and

H isotopic shifts in the mélange. Mélange formation, resulting in the production of high-variance ultramafic assemblages with

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0009 -2541 (02 )00019 -0

* Corresponding author. Permanent address: Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015,

USA. Tel.: +1-610-758-5831; fax: +1-610-758-3677.

E-mail address: [email protected] (G.E. Bebout).

www.elsevier.com/locate/chemgeo

Chemical Geology 187 (2002) 79–106

high volatile contents, may aid retention of volatiles (in this case, H2O) to greater depths in subduction zones than in original

subducted mafic and sedimentary materials. The presence of such assemblages (i.e., containing minerals such as talc, chlorite,

and Mg-rich amphiboles) would impact the rheology of the slab–mantle interface and perhaps contribute to the low-velocity

seismic structure observed at/near the slab–mantle interface in some subduction zones. If operative along the slab–mantle

interface, complex mixing processes such as these, involving the interplay between fluid-mediated metasomatism and

deformation, also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas,

producing ‘‘fluids’’ with geochemical signatures inherited from interactions with hybridized rock compositions. D 2002

Elsevier Science B.V. All rights reserved.

Keywords: Subduction zone; Mélange; Slab–mantle interface

1. Introduction

Although geochemical studies of arc magmatism

commonly call upon metasomatic ‘‘fluids’’ (hydrous

fluids or silicate melts) as agents for transporting

‘‘slab signatures’’ to sites of arc magmatism (e.g.,

Gill, 1981; Pearce and Peate, 1995), little is known

about the structural and geochemical processes that

operate along the slab–mantle interface. At shallow

levels ( < 10 km), large volumes of dominantly sedi-

mentary material accumulate in accreted wedges,

occurring in many complexes as mélange-like mate-

rial between the slab and the hanging wall (see Shreve

and Cloos, 1986). The scarcity of appropriate expo-

sures has made it difficult to infer the nature of the

slab–mantle interface zone at greater depths. Recent

geophysical study of the slab–mantle interface has

identified zones of low seismic velocity at or near the

top of the subducting oceanic lithosphere (e.g., Fukao

et al., 1983; Hori et al., 1985; Matsuzawa et al., 1986;

Helffrich et al., 1989; Helffrich, 1996; Helffrich and

Abers, 1997). These low-velocity domains have gen-

erally been interpreted as containing extensively

hydrated assemblages in the subducting oceanic litho-

sphere; however, Abers et al. (1999; also see discus-

sion by Abers, 2000) have speculated that they might

also represent hydrated hanging-wall materials. The

generation of zones rich in layered hydrous minerals,

perhaps in part a mechanical mixing zone, could also

promote aseismic behavior at great depths in subduc-

tion zones (Peacock and Hyndman, 1999). The Cata-

lina Schist, exposed on Santa Catalina Island

(California), contains a kilometer-scale amphibolite-

grade, dominantly ultramafic, mélange (Fig. 1). Pet-

rological considerations indicate that the mélange

formed at 0.8–1.1 GPa and 640–750 jC (Sorensenand Barton, 1987; Sorensen, 1988). The mélange

contains mafic and ultramafic blocks showing varying

metasomatic alteration and, in the case of some mafic

blocks, migmatization (Sorensen, 1988). The unit is

thought to represent a zone of tectonic and metaso-

matic mixing near the slab–mantle interface (Bebout

and Barton, 1989), and may, thus, yield insights into

the complex structural and metasomatic processes

operating at depth in subduction zones.

The amphibolite mélange unit was first mapped by

Bailey (1941), who recognized that it consists predom-

inantly of schists containing assemblages of talcFanthophylliteF chloriteF actinoliteF enstatiteFquartz. Sorensen (1988) and Sorensen and Grossman

(1989), in their detailed petrologic and geochemical

studies, documented metasomatic exchange between

mafic blocks and the surrounding mélange matrix to

produce ‘‘rinds’’ on these blocks and suggested that

infiltrating aqueous fluids facilitated this exchange.

Sorensen and Barton (1987) and Sorensen (1988)

suggested that this high-T infiltration event also

resulted in varying degrees of hydration and metaso-

matism leading to migmatization of some mafic blocks

floating in the mélange. Detailed isotopic studies of the

mélange (Barton et al., 1987; Bebout, 1991) led to an

infiltration model whereby large amounts of aqueous

fluid, previously equilibrated with metasedimentary

rocks, entered themélange, leading to large-scale stable

isotope homogenization and producing abundant meta-

somatic features within the mélange unit. In this paper,

we combine petrological, field (including mapping),

petrographic, and geochemical (major and trace ele-

ment; isotopic) evidence relevant for consideration of

the larger-scale development of this kilometer-scale

G.E. Bebout, M.D. Barton / Chemical Geology 187 (2002) 79–10680

mélange unit. We argue that the rind-forming process

demonstrated by Sorensen (1988) and Sorensen and

Grossman (1989) for mafic blocks, and illustrated in

our study for ultramafic blocks, is representative of the

initial mixing processes involving combinations of

diffusive and infiltrative metasomatism and mechan-

ical (tectonic) mixing, which ultimately resulted in the

production of the far more voluminous mélange matrix

in this mélange unit. Finally, we discuss some impli-

cations of these mixing processes for rheology and

geochemical hybridization along the forearc and subarc

slab–mantle interface.

2. Analytical techniques and normalization of

mineral compositional data

X-ray fluorescence (XRF) analyses were per-

formed at the University of California, Los Angeles

(for major elements) on a Phillips–Norelco instru-

ment and at the University of Southern California (for

trace elements) on a Rigaku instrument. Samples were

disintegrated in a jaw crusher to a size range appro-

priate for use of tungsten carbide and steel shatter

boxes. Loss-on-ignition data were obtained by heating

of samples in crucibles to 900 jC for 1 to 1.5 h;

Fig. 1. Geologic map of a part of the amphibolite-facies mélange unit near the airport on Santa Catalina Island, illustrating the distributions and

proportions of contrasting mélange matrix compositional types and tectonic blocks. Large serpentinite zones are, in general, enveloped by, or

otherwise spatially related to, more siliceous ultramafic rocks similar in composition to metasomatic zones along shear zones and veins in

serpentine-rich zones (see Fig. 2b). Dark lines are roads.

G.E. Bebout, M.D. Barton / Chemical Geology 187 (2002) 79–106 81

results were reproducible to F 0.1 wt.%, and the LOIfor nearly monomineralic whole-rock samples (e.g.,

chlorite schists) agree to within 10% of the stoichio-

metric H2O contents of the appropriate hydrous min-

eral. Major element samples were fused with Li2B4O7at 1050 jC for 20 minutes. Samples for trace elementanalyses (for Ba, Sr, Cr, and Ni data) were prepared as

powdered samples, bound by mixing sample and

cellulose binder in a 4:1 weight ratio.

Electron microprobe analyses were performed on a

Cameca Camebax instrument at UCLA. All analyses

were obtained at a 15 kV accelerating voltage. Elec-

tron microprobe analyses were normalized using the

schemes of Laird and Albee (1981) for amphibole and

chlorite, and the nomenclature of Leake (1978) is used

for amphibole.

3. Geologic setting and field observations

The Catalina Schist consists of tectonometamor-

phic units ranging in grade from lawsonite–albite to

amphibolite facies and is believed to represent pro-

cesses of underplating at 15–45-km depths in an

Early Cretaceous accretionary complex (Platt, 1976;

Sorensen, 1986; Grove and Bebout, 1995). Each unit

contains metasedimentary, metamafic, metaultramafic

rocks, and mélange in varying proportions. The mél-

ange domains contain intact, variably metasomatized

blocks of mafic, ultramafic, and sedimentary litholo-

gies within heterogenous, penetratively deformed

matrices. In each unit, the mélange zones contain

mineral assemblages consistent with equilibration at

P–T conditions represented by the peak metamorphic

mineral assemblages in more coherent (less deformed)

sections of metasedimentary and metamafic rocks (see

Grove and Bebout, 1995). The mélange zones have

been identified as zones of preferential metamorphic

fluid infiltration based on their highly homogenized

stable isotope compositions (O, H, and N; see Bebout,

1991, 1997) and the abundance of metasomatic fea-

tures such as veins and layers with high-variance

mineral assemblages (Bebout and Barton, 1993).

The amphibolite-grade ultramafic mélange unit of

the Catalina Schist occupies an approximately 8 km2

area and is separated, along a sheared contact, from

more coherent exposures of metagabbroic and meta-

sedimentary schist and gneiss (see map of a part of this

unit in Fig. 1). The mélange contains variably foliated

assemblages of talc, chlorite, anthophyllite, clinoam-

phibole (ranging in composition from actinolite to

magnesiohornblende), enstatite, and (rarely) quartz.

Trace phases include Ni–Fe sulfides, chromite–mag-

netite, ilmenite, rutile, zircon, apatite, ankeritic dolo-

mite, and clinopyroxene. On a large (tens of meters to

kilometer) scale, the mélange consists of domains of

more aluminous, dominantly chlorite-schist containing

both mafic and ultramafic blocks (shown as ‘‘alumi-

nous matrix’’ on Fig. 1) and more siliceous domains of

dominantly amphibole (clinoamphibole and/or

orthoamphibole)- or talc-schist which contain far fewer

blocks of aluminous mafic material (shown as ‘‘sili-

ceous matrix’’ on Fig. 1). Blocks of metasedimentary

materials are conspicuous by their scarcity in the

mélange unit.

Fig. 2a demonstrates schematically the textural

and mineralogical relationships between more alumi-

nous parts of the ultramafic mélange matrix and

blocks floating in the mélange, based in part on the

work by Sorensen (1988) on the mafic blocks. Ultra-

mafic blocks, likely to have been serpentinized dur-

ing retrogradation of the Catalina Schist (see Grove

and Bebout, 1995), are relatively homogeneous

bodies (of up to kilometer scales) containing no mafic

material and with whole-rock compositions suggest-

ing dunite to harzburgite protoliths (see Sorensen,

1988; Sorensen and Grossman, 1989; Bebout and

Barton, 1993). Foliation in the mélange matrix com-

monly wraps around blocks. The most common block

types are the retrograded ultramafic blocks, variably

metasomatized blocks of garnet + omphacitic clino-

pyroxene (the latter described by Sorensen, 1988),

and to a far lesser extent, metagabbroic gneiss blocks

that lithologically resemble the coherent metagab-

broic section of the amphibolite unit. The mafic

and ultramafic blocks show a continuum of variable

metasomatic exchange with the surrounding mélange

matrix, evident by the presence of metasomatic

‘‘rinds’’ at the block margins, and the metasomatized

lithologies show varying degrees of incorporation

into the mélange foliation (see field photograph in

Fig. 3 demonstrating the strong deformation of mate-

rials interpreted as highly metasomatized block mate-

rial). Sorensen (1988) described in detail the

metasomatic assemblages in rinds developed on

mafic blocks (see Fig. 2a), inferring fluid-mediated


additions to the blocks of elements such as Rb +

Th +BaF SrFREE. The less metasomatized maficblocks have spherical to discoidal shapes suggesting

that they behaved as rigid objects within the mélange

until they were extensively metasomatized to more

easily deformed assemblages containing chloriteFamphibole. Discoidal masses dominantly of clinoam-

phibole-rich material, strongly resembling (in miner-

alogy and mineral chemistry) the rind material

developed at the margins of mafic blocks, are com-

mon in the more aluminous parts of the mélange (see

Fig. 1). These discoids (e.g., samples 7-2-26b1 and 7-

2-26b6, for which electron microprobe data are

presented in Fig. 4 and Table 2) range in size from

several mm’s to several m’s in diameter (see Fig. 3).

At the thin section scale, some of the more aluminous

Fig. 2. Sketches illustrating field relations (at scales of cm to m) in the mélange unit. (a) Cartoon demonstrates schematically the textural and

mineralogical relationships between the siliceous ultramafic mélange matrix and mafic and ultramafic blocks floating in the mélange. The rinds

developed on mafic blocks were studied in detail by Sorensen and Barton (1987), Sorensen (1988), and Sorensen and Grossman (1989). (b)

Sketch illustrating the complex mineralogical zonations at the rims of ultramafic blocks in the mélange, indicating additions of Si and other

components. A conspicuous feature of the zonations is the common presence of fibrous zones of nearly monomineralic clinoamphibole -

F orthoamphibole or enstatite (up to 0.25 m thick; e.g., sample 6-4-100FR used on many of the geochemical diagrams in this paper).


samples contain interlayered domains (0.1 mm to

several millimeter thick across the foliation) rich in

either chlorite or amphibole.

Fig. 2b illustrates schematically the mineralogical

zonations that occur at rims of ultramafic blocks and

the textural and mineralogical relations of veins and

shear zones which cross-cut the blocks. The general-

ized progression from contacts with sheared zones

inward toward cores of blocks is chlorite + clinoam-

phiboleF talc! orthoamphibole! clinoamphibole +orthoamphiboleFenstatite! orthoamphiboleF ensta-tite! serpentiniteF orthoamphiboleF enstatite. Thezonations are similar to those reported for metaso-

matic zones in other amphibolite-grade ultramafic

suites and interpreted as reflecting metasomatic trans-

fer of silica and other components (e.g., Carswell et al.,

1974; Sanford, 1982; Pfeifer, 1987).

Meter- to kilometer-scale compositional domains in

the mélange closely resemble in mineralogy the lith-

ologies developed in metasomatic zones (rinds) on

either mafic or ultramafic blocks (see Fig. 1). Large

parts of the more aluminous matrix (aluminous matrix

unit on Fig. 1) containing clinoamphiboleF chloriteassemblages are similar mineralogically to parts of

metasomatic rinds on mafic blocks showing varying

degrees of deformation and incorporation into the

mélange (see Fig. 2a). In many outcrops mapped as

Fig. 4. Mineral compositions of the mélange matrix. (a) Clinoam-

phibole compositions, demonstrating varying tschermak substitution

and more aluminous compositions in more Al-rich whole rocks.

Note that the compositions of clinoamphiboles in the small discoids

from chlorite schists (samples 7-2-26b1 and 7-2-26b6) and in

extremely aluminous mélange matrix sample 6-4-9 overlap those for

clinoamphibole in rinds developed on mafic blocks (data for rinds

from Sorensen, 1988). (b) Chlorite compositions, demonstrating

varying tschermak substitution and relationship with whole-rock Al

contents (see text). Numbers below some sample numbers are

whole-rock wt.% Al2O3. Representative mineral chemical compo-

sitions, obtained by electron microprobe techniques, are provided in

Table 2. On (b), note data (for sample 6-4-7) for a single chlorite

grain with zoned chemical composition toward a contact with an

adjacent amphibole grain (indicated by large arrow).

Fig. 3. Field photograph of aluminous mélange, demonstrating its

makeup of highly deformed domains (lenticular in two dimensions)

containing either chloriteF clinoamphibole schist (darker regions)or talcF orthoamphibole regions (lighter regons), the two litholo-gies similar mineralogically to rinds on mafic and ultramafic rocks,

respectively. Note handle of geologic rock hammer for scale. The

horizontal dimension of the photograph represents approximately

1.25 m.


Table 1

Summary of mineral assemblages of samples for which whole– rock geochemical data (and/or electron microprobe data) are presented in this

paper

Sample Tca Oam Cam Opx Chl Qtz Op/Rut Other Retrograde

6-3-7b X X trc X btt

6-3-9 X X X X X rut mg, tc

6-3-21 X X X rut ap tc

6-3-22 X tr btt chl

6-3-31Cb X+ d X+ X+ X� e X yesf6-3-32 tr + X+ X� X tc6-3-33 tr X X X

6-3-35a X X X

6-4-3 X X

6-4-6B X X yes tc, serp

6-4-7b X X ilm ap

6-4-9b X X ilm ap, zirc, btt chl

6-4-11 X X X tr tc

6-4-12b X X tr X ilm, mt ap, zirc

6-4-19 X X X

6-4-23C tr X tr tc, serp

6-4-25b X X carb (dolo) tc, serp

6-4-28b X X tc, serp

6-4-64 X X yes chl

6-4-65 X X yes

6-4-94 X X X

block X X X

6-4-103a X X X yes tc

6-4-103b tr X

8-3-49Vb X tr X8-4-10ab X ilm zirc, ap

8-4-10b X ilm ap

8-4-11b X X X X spinel chl, tc

8-4-13b X + X X� tr8-4-15ab X X X tr rut, ilm zirc tc, serp

8-4-15bb X + X� tr8-4-16b X X rut! ilmg chl8-4-19ab X X X tr ilm

8-4-19bb X X X X ilm

8-4-19cb X X ilm tc

Discoids

7-2-26b1b X X X X rut ap, zirc

7-2-26b6b X X tr X rut sulfide, btt chl

Ultramafic rind

6-4-100FR X X tr tc, serp

a Mineral abbreviations are as follows: Tc = talc; Oam= orthoamphibole; Cam= clinoamphibole; Opx= orthopyroxene; Chl = chlorite;

Qtz = quartz; Op/Rut = unidentified opaque phase or rutile; ilm = ilmenite; carb = prograde carbonate (dolomite); zirc = zircon; ap = apatite;

btt = biotite; mt =magnetite; mag =magnesite.b Probe data exist for these samples.c Present in trace amounts.d Replaces other minerals.e Is replaced by other minerals.f Unidentified opaque mineral present.g Replacement of rutile by ilmenite at rutile rims.


Table 2

Representative mineral compositions for mélange matrix samples, ultramafic rinds, and discoids in chlorite schist

Sample 6-3-7 6-4-7 6-4-9 Discoids Coexisting amphiboles

7-2-26b1 7-2-26b6 8-4-19a 8-4-19a 6-4-12

Mineral Clinoamph Clinoamph Clinoamph Clinoamph Clinoamph Clinoamph Orthoamph Orthoamph

!More aluminous whole rocksSiO2 50.58 48.51 46.71 49.42 50.61 56.84 57.32 56.17

MgO 17.61 18.27 17.43 16.51 17.4 22.65 25.15 24.81

Na2O 1.98 1.89 2.46 2.1 1.65 0.15 0.00 0.12

Al2O3 9.00 7.90 9.50 10.92 9.48 0.40 0.18 0.89

FeO* 6.63 7.52 8.90 6.29 5.86 4.65 12.82 13.17

MnO(NiO) 0.16 0.20 0.19 (0.06) 0.15 0.22 0.39 0.46

Cr2O3 0.22 0.10 0.00 0.25 0.84 0.00 0.03 0.18

K2O 0.09 0.39 0.42 0.11 0.11 0.04 0.00 0.00

CaO 10.84 11.99 11.40 10.92 11.13 11.95 0.86 0.45

TiO2 0.43 0.30 0.43 0.47 0.30 0.04 0.03 0.01

Total 97.54 97.07 97.44 97.05 97.53 96.94 96.78 96.26

Normalizations (Laird and Albee, 1981)

Si 7.033 6.872 6.613 6.936 7.060 7.875 8.034 7.922

AlIV 0.967 1.128 1.387 1.064 0.940 0.065 0.000 0.078

AlVI 0.507 0.192 0.199 0.742 0.619 0.000 0.030 0.069

Ti 0.045 0.032 0.046 0.049 0.031 0.004 0.003 0.001

Fe +3 0.583 0.639 0.887 0.339 0.434 0.211 0.000 0.034

Cr 0.008 0.004 0.000 0.009 0.031 0.000 0.001 0.007

Fe +2 0.189 0.252 0.167 0.398 0.249 0.328 1.503 1.519

Mn 0.019 0.024 0.022 0.007 0.018 0.025 0.046 0.055

Mg 3.650 3.857 3.679 3.454 3.617 4.676 5.254 5.214

Ca 1.614 1.820 1.729 1.641 1.664 1.774 0.128 0.067

NaM4 0.386 0.180 0.271 0.359 0.336 0.041 0.000 0.033

NaA 0.149 0.338 0.404 0.214 0.110 0.000 0.000 0.000

K 0.017 0.070 0.076 0.019 0.019 0.007 0.000 0.000

Sum 15.166 15.409 15.480 15.233 15.129 15.007 15.000 15.000

Sample Rind on ultramafic block Chlorites

6-4-100FR 6-4-100FR Chlorite-rich Talc-rich Discoids UM rind

6-4-9 6-4-12 7-2-26b1 7-2-26b6 6-5-56A

Mineral Clinoamph

fibrous zone

Orthoamph

fibrous zone

Chlorite Chlorite Chlorite Chlorite

SiO2 56.97 57.81 30.79 28.89 27.86 28.26 31.85

MgO 23.86 28.02 28.94 28.07 23.93 24.21 33.03

Na2O 0.37 0.13 0.00 0.00 0.00 0.00 0.00

Al2O3 1.03 0.71 18.37 20.24 22.02 21.48 14.54

FeO* 5.06 9.12 9.64 7.51 12.72 12.83 3.02

MnO(NiO) (0.06) (0.14) 0.06 0.08 0.00 0.10 0.00

Cr2O3 0.06 0.09 0.03 1.52 0.16 0.44 3.13

K2O 0.04 0.00 0.00 0.00 0.00 0.00 0.03

CaO 9.10 0.54 0.02 0.02 0.01 0.01 0.00

TiO2 0.06 0.05 0.02 0.05 0.11 0.06 0.04

Total 96.61 96.61 87.87 86.38 86.81 87.39 85.64

Normalizations (Laird and Albee, 1981))

Si 7.872 7.953 SiIV 2.967 2.850 2.776 2.808 3.134

AlIV 0.128 0.047 AlIV 1.033 1.150 1.224 1.192 0.866


more aluminous mélange matrix (see Fig. 1), highly

deformed domains contain either clinoamphibole + ch-

lorite-rich assemblages resembling parts of mafic rinds

(darker regions on Fig. 3) or talc + orthoamphibole +

clinoamphibole assemblages resembling ultramafic

rinds (lighter regions on Fig. 3). Larger expanses of

the mélange mapped on Fig. 1 as siliceous matrix and

containing mineral assemblages rich in talc and

orthoamphibole resemble assemblages observed in

metasomatic zones on rims of ultramafic blocks (see

Fig. 2b). These large domains of siliceous mélange

matrix contain few or no mafic blocks and, in general,

envelope large bodies of ultramafic rocks (see Fig. 1),

consistent with their generation through metasomatism

of ultramafic blocks.

4. Mineral assemblages, textures, and compositions

in the mélange

Table 1 summarizes mineral assemblage data for the

samples of mélange matrix for which whole-rock geo-

chemical data are presented and discussed in this paper,

and for two samples of amphibole-rich discoids col-

lected from chlorite schist in one part of the mélange

unit (samples 7-2-26b1, 7-2-26b6) and one rind devel-

oped on an ultramafic block (sample 6-4-100FR).

Sorensen and Grossman (1989) provide the mineral

assemblages of the rinds developed on mafic blocks.

Also contained in Table 1 is information regarding

retrograde assemblages, deduced using petrographic

observations, and inferred replacement textures among

high-T phases. Samples selected for whole-rock geo-

chemistry show relatively little retrograde overprinting.

Retrograde alteration of the high-T parageneses in-

cludes minor replacement of high-temperature miner-

als by chloriteF talcF serpentineFmagnesite at theirrims and cross-cutting of the high-T assemblages by

serpentineF talc-bearing vein networks to variable andsometimes pervasive replacement of the earlier assem-

blages by chloriteF talcF serpentineFmagnesite.Table 2 presents representative mineral chemical

analyses for the clinoamphiboles and chlorite. The

compositions of these minerals covary with whole

rock compositions to a greater degree than the com-

positions of the other major phases of the mélange

matrix (talc, orthoamphibole, enstatite). With the

exception of the extremely talc- and enstatite-rich

schists, all samples contain amphibole, in some cases

coexisting ortho- and clinoamphiboles. Orthoamphi-

boles are all Al-poor, magnesian anthophyllite with

limited Mg–Fe + 2 variations. Clinoamphiboles range

from actinolite to magnesiohornblende in composition

(Fig. 4a; cf. Sorensen, 1988) and are generally the

only Ca-rich mineral present, with the local exception

of scarce ankeritic dolomite and apatite. Clinoamphi-

bole compositions correlate with whole-rock compo-

sitions in more aluminous samples, clinoamphiboles

are more pargasitic (see data for sample 6-4-9, which

contains 13.7 wt.% Al2O3, and ‘‘discoids’’ on Fig.

Table 2 (continued )

Sample Rind on ultramafic block Chlorites

6-4-100FR 6-4-100FR Chlorite-rich Talc-rich Discoids UM rind

6-4-9 6-4-12 7-2-26b1 7-2-26b6 6-5-56A

Normalizations (Laird and Albee, 1981)

AlVI 0.040 0.068 AlVI 1.055 1.203 1.363 1.323 0.820

Ti 0.006 0.005 Ti 0.001 0.003 0.009 0.004 0.003

Fe +3 0.165 0.000 Fe +3 0.000 0.000 0.000 0.000 0.000

Cr 0.002 0.003 Cr 0.001 0.040 0.004 0.011 0.081

Fe +2 0.419 1.049 Fe +2 0.777 0.619 1.060 1.066 0.248

Mn(Ni) (0.007) (0.007) Mn(Ni) 0.005 0.006 (0.011) 0.009 0.000

Mg 4.914 5.745 Mg 4.157 4.126 3.554 3.585 4.844

Ca 1.348 0.079 Ca 0.002 0.002 0.001 0.002 0.000

NaM4 0.098 0.033 Na 0.002 0.000 0.000 0.000 0.000

NaA 0.000 0.000 K 0.000 0.000 0.000 0.000 0.004

K 0.006 0.000 Sum 10.000 9.999 9.980 10.000 10.000

Sum 15.006 15.000

*All Fe reported as FeO.


4a). Clinoamphiboles in aluminous samples com-

monly are zoned from aluminous cores to lower Al

rims. Some samples contain two clinoamphibole

compositional types; coarse-grained aluminous

amphibole cross-cut by fibrous low-aluminum (acti-

nolitic) amphibole (e.g., sample 8-3-49V; see Fig. 4a).Compositions of many clinoamphiboles in relatively

aluminous samples overlap with those reported by

Sorensen (1988) for rinds on mafic blocks (see data

on Fig. 4a for ‘‘discoids’’, samples 7-2-26b1 and 7-2-

26b6, and chlorite-rich mélange matrix sample 6-4-9;

representative analyses of clinoamphibole and chlorite

in these lithologies are provided in Table 2).

Chlorite is abundant in the mélange, and some

samples are virtually pure chlorite (e.g., sample 6-4-

9). Like the clinoamphiboles, the chlorites generally

have higher Al contents in the more aluminous rocks.

Chlorite shows strong tschermakite substitution com-

positional trends, as demonstrated in Fig. 4b. Chro-

mium content varies widely (0.1–1.6 wt.% Cr2O3),

with the highest contents found in the less aluminous,

low-calcium, more siliceous samples (e.g., sample 6-

4-12 in Table 2).

Chlorite and amphibole contain less Al and have

higher Mg/Fe and Cr contents in mineralogical zona-

tions at rims of ultramafic blocks (ultramafic rinds;

see Fig. 2b). Clinoamphiboles in these zones tend to

be more actinolitic or even tremolitic than clinoam-

phiboles in more aluminous mélange matrix away

from ultramafic blocks (Fig. 4a). Chlorites are more

magnesian and more Cr-rich than any measured in

mélange matrix (for ultramafic rind chlorites, up to

3.13 wt.% Cr2O3; e.g., 6-5-56A in Table 2), and show

decrease in tschermakite substitution toward the

blocks (e.g., C to B to A in sample 6-5-56, Fig. 4b).

5. Major and trace element evidence for the

evolution of the mélange matrix

Major and trace element compositions were used to

delineate the behavior of various elements during

mélange matrix formation and to compare the mél-

ange matrix compositions with the compositions of

‘‘rinds’’ developed on mafic and ultramafic blocks.

The plots of Cr, Al2O3, and SiO2 compositions in Fig.

5a and b introduce the approach used in the following

discussions to document and interpret the composi-

tional variations. On both plots, the mélange matrix

samples show an extremely wide range in composi-

tion that extends toward, but does not overlap with the

compositional ranges for the mafic and ultramafic

blocks. Data for a clinoamphibole + orthoamphibole

rind (fringe; sample 6-4-100FR) on an ultramafic

block are plotted on these and later diagrams, as are

Fig. 5. Chemical discrimination diagrams, demonstrating the

conceptual model for mixing in the mélange unit. (a) Cr vs.

Al2O3, (b) SiO2 vs. Al2O3. On (a), the darker-shaded lines

emanating from the origin represent the Cr–Al compositions

resulting from varying ultramafic:mafic proportions (labelled for

each). The envelopes of simple mixing of ultramafic and mafic

compositions are schematically indicated on Fig. 5a and b by the

regions between the two subparallel, hand-drawn, solid lines. The

two small, unfilled circles on (a) are the mean mafic and ultramafic

end-member compositions, with the dashed line connected the two

circles representing linear mixtures of the two end members. Large,

shaded arrows on (a) and (b) indicate the trends from mafic and

ultramafic end members toward the compositions of rinds

developed on each block type.


data for rinds (including ‘‘inner rinds’’, ‘‘outer rinds’’,

and ‘‘other rinds’’) developed on mafic blocks (from

Sorensen, 1988; Sorensen and Grossman, 1989). The

comparisons of the mélange matrix compositions with

those of the rinds are key, given that the rinds on

mafic blocks are clearly the result of mafic:ultramafic

mixing (Sorensen and Grossman, 1989) and, further-

more, the rinds on ultramafic blocks appear to have

resulted from infiltrative–diffusive metasomatic alter-

ation of the dunite or harzburgite. Whole rock anal-

yses for 37 samples of the mélange matrix thought to

be representative of the present exposures are given in

Table 3; (cf. Table 1). These data were used to derive

an average mélange matrix composition (Table 3,

Figs. 5–11). The average composition is useful in

evaluating the overall open- and closed-system behav-

ior of various elements within the ultramafic mélange

unit as a whole. In the following considerations of

mafic–ultramafic mixing, mafic block compositions

are taken to be compositions of the ‘‘non-migmatitic

blocks’’ of Sorensen and Grossman (1989). Field

observations demonstrate that the metagabbroic

gneiss, which is another candidate for a mafic end

member, is far less abundant as block material in the

mélange.

Fig. 5a shows the covariation of Al2O3 and Cr

content of mélange matrix and demonstrates that a

significant number of the matrix samples fall in an

envelope that represents possible simple mixtures of

these components from mafic and ultramafic blocks.

This mixing envelope is indicated schematically on

Fig. 5a and b by the regions between the two subpar-

allel, hand-drawn, solid lines; on Fig. 5a, the dotted

line within the mixing envelope is a mixing line

between the mean compositions of the two block types

(see data for end members in Table 4 and discussion in

Section 5.1). On Fig. 5a, significant loss or gain of

elements other than Cr and Al2O3 would be expected

to shift compositions away from or toward the origin,

respectively, without changing the Cr/Al2O3 ratios of

the samples. In fact, the samples that fall outside this

envelope are either (1) extremely siliceous schists

(labeled on Fig. 5a as Amphibole and Talc Schists),

most of which are regarded as having been ultramafic

rocks with significant amounts of added Si (and

possibly other components), or (2) nearly monominer-

alic chlorite schists (labeled on Fig. 5a, b) regarded as

having experienced significant mass loss.

The massive addition of SiO2, at least at the local

scale, is indicated by comparison of Fig. 5a and b. The

talc- and amphibole-rich, chlorite-poor schists that are

common in the ultramafic mélange unit have Cr/

Al2O3 ratios that would indicate high ultramafic:mafic

mixing ratios greater than f1:1, and mostly > 3:1, onFig. 5a, but their compositions on this plot are shifted

significantly toward the origin from the envelope of

simple ultramafic:mafic mixtures. This shift is con-

sistent with the siliceous character of the samples

(talc- or amphibole-rich schists), indicating possible

bulk metasomatic addition of SiO2 to these samples

and dilution of both Cr and Al2O3 in a constant ratio.

On Fig. 5b, these high-Cr, low-Al2O3 samples plot to

the right of, and at slightly higher Al2O3 than most of

the ultramafic data. Some shift to SiO2 concentrations

exceeding 60 wt.%. Chlorite-rich schists, which on

Fig. 5a, fall to the right side of the simple mixing

zone, plot as the high-Al2O3, low-SiO2 samples far to

the left of the simple mixing envelope in Fig. 5b.

Perhaps most important for the overall interpreta-

tion of the mélange chemical evolution, the compo-

sitions of many of the mélange matrix samples

overlap with those of the rinds formed on mafic and

ultramafic blocks (see data for rinds on Fig. 5a, b).

This demonstrates that rind-forming processes (e.g.,

for mafic blocks, described by Sorensen and Gross-

man, 1989) could have produced many of the mél-

ange matrix compositions. Data for the inner and

outer rinds of Sorensen and Grossman (1989) plotted

on these diagrams show mixing trends at the margins

of the mafic blocks. These trends, which are indicated

by the large, shaded arrows on Fig. 5, encompass

many of the mélange matrix compositions. Inner rinds

come close to the mafic block compositions. In

contrast, the outer rinds plot toward the ultramafic

block compositions, but only partly overlap the enve-

lope for simple mixtures. Some approach the average

mélange matrix composition (Fig. 5). The metaso-

matic rind from the ultramafic block (indicated by the

filled circle) is shifted from the ultramafic block data

toward lower Cr and higher SiO2 and is as siliceous

as the most siliceous mélange matrix sample (see Fig.

5b).

Fig. 6 shows the compositional variability of mél-

ange matrix samples, relative to the compositions of

tectonic blocks and rinds, for elements that show

overall compositional variation similar in style to that


of Cr and Al2O3; that is, showing variability in con-

centration largely explained by simple mixing, but with

superimposed effects of mass loss or, more commonly,

mass gain. The plot of MgO vs. Al2O3 (Fig. 6a)

resembles that of Cr vs. Al2O3, whereas the plot of

Ni vs. Al2O3 (Fig. 6b) differs in that a larger number of

the mélange matrix samples plot to the low Ni–Al side

of the simple mixing envelope (see later discussion). In

the plot of Fe2O3 vs. Al2O3 (Fig. 6c), the mélange

matrix samples also plot either in the envelope of

Table 3

Major and trace element compositions of mélange matrix samples and the average mélange matrix (in wt.%, unless otherwise specified)

Sample 6-3-7 6-3-9 6-3-21 6-3-22 6-3-31c 6-3-32 6-3-33 6-3-35a 6-3-60 6-4-3

SiO2 55.17 40.24 53.04 50.30 55.95 56.01 47.97 60.18 48.37 60.31

TiO2 0.20 0.07 0.56 0.37 0.03 0.05 0.09 0.05 0.04 0.07

Al2O3 5.59 7.57 7.97 8.89 2.58 2.89 6.55 1.15 3.37 1.18

Fe2O3 6.37 5.62 7.64 6.07 5.74 7.33 8.36 6.07 8.81 2.33

MnO 0.10 0.07 0.13 0.13 0.16 0.23 0.19 0.14 0.15 0.08

MgO 22.30 35.82 19.74 20.72 32.22 29.28 27.95 27.89 32.52 25.39

CaO 7.17 0.53 3.97 7.40 1.10 0.32 3.82 0.06 0.09 9.38

Na2O 0.89 0.03 0.33 0.96 0.02 0.01 0.00 0.01 0.07 0.03

K2O 0.15 0.01 0.44 0.52 0.02 0.03 0.04 0.01 0.02 0.07

Cr (ppm) 1324 5672 1259 817 2356 2559 1603 939 2626 584

Ni (ppm) 893 1498 800 513 1769 1621 1213 1116 1525 872

Sr (ppm) – 10 – 0 16 10 – – 12 93

Zr (ppm) – 5 – 0 5 5 – 90 5 34

Ba (ppm) – 4 – 0 131 58 – – 180 17

LOI 1.41 9.5 5.9 2.9 2.1 4.1 4.4 3.9 5.9 0.6

Total 99.56 100.17 99.89 98.41 100.34 100.66 99.68 99.69 99.75 99.56

UM/M (Cr/Al) 1.06 3.90 0.67 0.35 4.95 4.78 1.10 4.32 4.10 2.41

Mass change 32 � 55 13 24 13 3 11 177 � 2 295Volume Change � 42Density dataa

6-4-65 6-4-94 6-4-103a 6-4-103b block 6-5-53 8-3-49’ 8-4-10a 8-4-10b 8-4-11

SiO2 61.38 52.68 51.24 41.31 58.87 59.02 57.07 27.28 28.08 53.23

TiO2 0.01 0.44 0.54 0.36 0.37 0.42 0.22 1.30 0.58 0.04

Al2O3 0.69 6.46 4.35 8.05 6.34 6.34 4.93 18.14 18.67 2.64

Fe2O3 5.10 7.07 10.67 11.63 7.33 11.43 7.28 9.28 11.88 5.10

MnO 0.09 0.15 0.22 0.13 0.17 0.22 0.20 0.07 0.12 0.11

MgO 28.31 18.57 26.37 29.68 16.48 14.31 16.96 33.11 29.72 28.48

CaO 0.09 9.24 1.00 0.28 7.31 5.34 10.37 0.07 0.26 7.17

Na2O 0.01 1.44 0.05 0.06 1.09 1.12 1.30 0.00 0.00 0.38

K2O 0.03 0.20 0.01 0.01 0.11 0.09 0.39 0.01 0.03 0.05

Cr (ppm) 1493 1437 1783 1742 1426 920 1079 187 167 1701

Ni (ppm) 1389 654 999 660 659 501 810 209 173 1060

Sr (ppm) 12 68 12 15 31 – 33 7 7 33

Zr (ppm) 6 22 58 78 41 – 18 375 65 7

Ba (ppm) 45 16 14 18 32 – 152 19 8 23

LOI 3.62 3.02 5.06 8.51 1.29 1.11 0.83 11.48 11.37 2.39

Total 99.61 99.47 99.79 100.26 99.55 99.54 99.75 100.81 100.74 99.86

UM/M (Cr/Al) 16.24 0.99 1.96 0.96 1.00 0.61 0.97 0.02 0.01 3.29

Mass Change 100 18 22 –4 20 47 56 –21 –22 45

Volume change 147 24 54

Density dataa talc chlorite clamp

a Density for this mineral used in calculation of volume change in this nearly monomineralic sample.b Average mélange matrix composition.


simple mixing, or to the left of it; this relationship is

also apparent on the plot of Zr vs. Cr (Fig. 6d).

Other alkalis and alkaline earths (CaO, Na2O, K2O,

Ba, and Sr) show erratic concentration variations

within the mélange matrix and rinds, seemingly

requiring considerable mobility of these elements

(Fig. 7). Na2O, K2O, and Ba are locally higher in

some rinds of mafic blocks than in the blocks them-

selves (Sorensen and Grossman, 1989); however,

along with CaO and Sr, they are typically depleted

6-4-6b 6-4-7 6-4-9 6-4-11 6-4-12 6-4-19 6-4-23c 6-4-25 6-4-28 6-4-64

39.78 41.07 34.52 60.75 58.89 63.26 55.94 56.16 56.19 54.33

0.04 1.11 0.29 0.05 0.04 0.55 0.08 0.01 0.06 0.30

7.86 9.88 12.40 1.24 1.08 10.39 3.25 0.79 1.03 5.72

5.04 9.13 13.20 4.86 8.68 7.78 5.30 7.63 6.24 12.99

0.07 0.10 0.09 0.08 0.21 0.08 0.08 0.07 0.09 0.20

32.97 25.27 27.20 26.50 27.38 7.88 22.08 28.06 29.82 18.57

6.15 6.18 1.47 2.53 0.17 4.27 7.69 0.60 0.04 4.86

0.03 0.42 0.20 0.04 0.01 0.65 0.60 0.00 0.01 0.62

0.02 0.12 0.03 0.02 0.01 2.73 0.12 0.01 0.01 0.12

2178 878 3054 1937 1828 334 1585 1433 1504 1689

1804 781 2428 1837 1491 187 1402 1033 1348 811

50 67 33 14 – 59 83 21 9 36

8 120 57 6 – 98 8 4 5 22

11 – 16 15 – 948 7 7 11 497

7.7 6.4 10.4 4.3 2.9 2.5 4.3 7.5 7.1 1.7

100.03 99.85 100.33 100.75 99.66 100.24 99.74 101.06 100.85 99.66

1.27 0.33 1.11 9.87 11.05 0.06 2.39 12.16 9.00 1.36

� 14 13 � 42 49 59 33 45 105 91 1410 � 25 85 97 46 59 153 136chloritea chlorite talc talc clamp clamp talc talc

8-4-13 8-4-15a 8-4-15b 8-4-16 8-4-19a 8-4-19b 8-4-19c Melmataveb 6-4-100FR

53.37 57.47 53.69 55.55 52.81 46.16 29.13 50.75 61.17

0.05 0.12 0.07 0.51 0.12 0.27 0.53 0.27 0.03

2.18 1.59 2.14 0.69 2.66 4.84 12.99 5.63 1.21

7.11 4.40 11.74 12.59 7.71 8.94 14.88 8.12 6.07

0.16 0.15 0.19 0.22 0.22 0.22 0.11 0.14 0.10

30.86 23.62 28.43 28.53 25.07 27.94 32.08 25.99 21.36

2.58 11.57 0.28 0.42 9.05 7.44 0.10 3.73 7.28

0.31 0.00 0.00 0.00 0.00 0.17 0.19 0.29 0.01

0.03 0.06 0.01 0.04 0.07 0.07 0.02 0.15 0.10

1479 216 1944 359 490 1358 332 1474 825

1195 571 1535 842 669 1096 404 1038 956

19 66 11 8 46 37 8 31 49

7 12 5 89 13 36 158 48 6

32 68 145 44 18 21 11 84 7

2.94 0.51 3.06 1.06 1.85 4.24 11.08 4.56 2.55

99.86 99.58 99.98 99.74 99.68 100.55 101.21 99.91 100.05

3.48 0.56 4.88 2.56 0.80 1.28 0.03 1.18 3.50

70 503 37 551 216 39 9 24 203

289

clamp


compared to the simple mixing lines in the mélange

matrix.

5.1. A mixing model

Simple mixing calculations allow partial quantifi-

cation of the contrasting compositional systematics in

the mélange matrix. In these calculations, the mafic

and ultramafic blocks were taken as the mixing com-

ponents, using the mean concentrations provided in

Table 4. The distinctive characteristics of these two

components are: (1) the serpentinites, derived from

dunitic/harzburgite protoliths, have low concentrations

of Al, Zr, Ca, Na, and the LILE (K, Sr, Ba), and high

concentrations of Mg, Cr, and Ni; (2) mafic blocks

(‘‘non-migmatitic clinopyroxene-bearing blocks’’ and

‘‘non-migmatitic blocks with rinds’’ of Sorensen and

Grossman, 1989) have higher concentrations of Al, Zr,

Ca, Na, and the LILE, and lower concentrations of Mg,

Cr, Ni. Excluded are metasedimentary rocks which

have low Mg and Ca and high Al, Si, Na, and the LILE

(documented in Bebout et al., 1999) because they are

virtually absent as tectonic blocks in the mélange unit,

and not needed to explain the observed variations.

Given that the metasedimentary rocks are easily dis-

aggregated and, thus, might have been fully incorpo-

rated into the mélange matrix, we later explore this

possibility from a geochemical perspective.

Aluminum and Cr were selected as the initial test

‘‘geochemical reference frame’’ because of their per-

ceived relative immobility in fluids in previous studies

of metasomatic alteration (see discussions by Ague,

1994; Baumgartner and Olsen, 1995; Ague and van

Haren, 1996; Yang et al., 1998) and because of their

starkly contrasting concentrations in the two mixing

end members (Figs. 5 and 6). Sorensen and Grossman

(1989) similarly suggested that the concentrations of Cr

and Al in rinds onmafic blocks reflect simple mixing of

mafic and ultramafic components, and contrasted their

behavior with that of elements such as Ba, Rb, and Th

that appear to have been added to rinds by metasomatic

fluids. Mean concentrations of Cr and Al2O3 in the

mafic and ultramafic end members (see Table 4) were

used to calculate the Cr/Al2O3 that would result from

mixing the end members in varying proportions.

Employing this calculated relationship, a model ultra-

mafic:mafic mixture for each of the samples was

obtained using its observed Cr/Al2O3 (see the mixing

line and labeled lines of constant ultramafic:mafic

mixing on Fig. 5a). On Fig. 5a, the several lines repre-

senting constant ultramafic:mafic proportions (using

themean compositions of the two block types; Table 4),

demonstrate that the ultramafic/mafic ratios vary

between >10:1 to < 1:1 for domains within the mél-

ange. The average matrix composition corresponds to a

ultramafic:mafic ratio of f 1.2:1, suggesting this ratioas a approximation of the time-integrated ultramafic–

mafic mixing proportions for the mélange matrix.

Given the mean Cr/Al2O3 for the two end-member

block types (Table 4), the nominal concentrations of

the other major and trace elements were calculated

based on simple mixing. Comparisons of the observed

compositions with those calculated from simple mix-

ing (hereafter referred to as ‘‘observed/calculated’’)

affords an evaluation of the degree to which simple

mixing explains the contents of individual compo-

nents and, conversely, the need for metasomatic trans-

fer. This analysis, although limited by the uncertainty

in the mafic and ultramafic end members, allows some

broad delineations to be made regarding the composi-

tional behavior of the different major and trace ele-

ments during mélange matrix formation.

5.2. Relative mobilities of elements within the mélange

Using the method described above, and even with

the scatter shown, it is possible to place the composi-

tional variations of the elements into one of several

Table 4

Compositions of the ultramafic and mafix mixing components

(wt.% unless otherwise specified)

Ultramafic Mafic

Mean 1r Mean 1r

SiO2 48.58 2.88 44.64 2.05

TiO2 0.02 0.03 2.00 0.71

Al2O3 0.55 0.21 14.64 1.24

Fe2O3 9.02 1.27 15.00 2.73

MgO 40.00 2.75 9.30 2.01

CaO 0.10 0.10 11.61 3.75

Na2O 0.02 0.01 1.26 0.21

K2O 0.04 0.07 0.27 0.1

Cr (ppm) 3146 688 271 158

Ni (ppm) 2765 422 102 17.8

Sr (ppm) 8 3 228 259

Zr (ppm) 5 0.4 151 41

Ba (ppm) 11 2 65.5 33.8

LOI 0 0 0.63 0.37


groups (see Fig. 8). The first group, consisting of Ni,

Fe2O3, MgO, and perhaps also Zr, behaves coherently

with Cr and Al2O3 and, thus, is consistent with simple

mixing and relative fluid immobility. On Fig. 8a, these

elements show similarity in both the overall range of

observed/calculated for mélange matrix samples (ver-

tical lines) and the average mélange matrix observed/

calculated (filled circles), the latter falling at or just

below the 1:1 line. The observed/calculated values for

Mg, shown in order of increasing calculated ultra-

mafic block proportions [expressed as ultramafic/

(ultramafic +mafic)] to the right-hand side on Fig.

8b, support its use as one of the geochemical reference

frame elements, as all but three samples straddle the

1:1 observed/calculated line, with a slightly greater

number of samples falling below this line. The three

anomalous samples exert a significant effect on the

average observed/calculated shown on Fig. 8a; a more

realistic value may be represented by the unfilled

circle which excludes these three samples from the

average. These relationships stand out in isocon plots

(after Grant, 1986) in Fig. 9a–f for each of the

elements proposed for the geochemical reference

frame. In these plots, the observed compositions of

the mélange matrix and rind samples are compared

with the compositions calculated as reflecting simple

Fig. 6. Chemical discrimination diagrams, comparing the compositions of mélange matrix samples with those of mafic and ultramafic blocks

and metasomatic rinds developed on each of the two block lithologies. (a) MgO–Al2O3, (b) Ni–Al2O3, (c) Fe2O3–Al2O3, and (d) Zr–Cr. On

(d), one mélange matrix samples plots at higher Zr concentrations at Zr = 375 ppm, 18.14 =Al2O3 wt.%, and one inner rind sample plots at

Zr = 531 ppm, 13.40 =Al2O3 wt.%. On each diagram, the envelope of simple mixing of ultramafic and mafic compositions is crudely indicated

by the regions between the two hand-drawn, solid lines.


mixing between the mafic and ultramafic end mem-

bers in proportions estimated by the Cr/Al2O3 ratios

(see Fig. 5a and discussion above; see similar

approach by Yang et al., 1998). From these plots, it

is evident that Fe and Zr (Fig. 9e and f) scatter more

than Al, Cr, and Mg (see Fig. 9a, b, and c), whereas

the Ni data (see Fig. 9d) fall systematically below the

1:1 line when compared with the Al, Cr, Mg, and Zr

compositions. In each plot, nearly all of the rind data

plot within the range of mélange matrix compositions.

Also in each plot, the samples that fall above the 1:1

line tend to be the more chlorite-rich schists which are

depleted in Si, Ca, Na, and the LILE presumably by

significant mass loss with the result that the less

mobile elements are concentrated (see Figs. 5a and

7a–d). Although TiO2 is probably relatively immobile

(Ague, 1994; Baumgartner and Olsen, 1995), it does

not exhibit simple mixing relationships. This likely

reflects the large original spread in values in the mafic

end member ( < 1.0 to 3.5 wt.%; Sorensen and Gross-

man, 1989; this study), perhaps with a contribution

from sampling issues caused by the rather coarse size

(up to 5 mm) and spotty distribution of rutile aggre-

gates in the mélange matrix.

On Fig. 8a, Si shows a relatively tight range in

observed/calculated and an average mélange matrix

Fig. 7. Chemical discrimination diagrams, comparing the compositions of mélange matrix samples with those of mafic and ultramafic blocks and

metasomatic rinds developed on the two block lithologies for oxides and trace elements showing more erratic variation relative to apparent simple

mafic–ultramafic mixing envelopes. (a) CaO–Al2O3, (b) Na2O–Al2O3, (c) K2O–Al2O3, and (d) Ba–Sr. On (a), (b), and (c), the envelopes of

simple mixing of ultramafic and mafic compositions are indicated schematically by the regions between the two hand-drawn, solid lines.


observed/calculated somewhat higher than 1:1. The

significant degrees of enrichment of mélange matrix

samples in SiO2 relative to the calculated composi-

tions for simple mafic–ultramafic mixing are empha-

sized in Fig. 10a (in an isocon diagram) and in Fig.

10b, the latter a plot of the observed/calculated SiO2vs. the observed/calculated Al2O3. One of the princi-

pal conclusions derived from the mixing models is

that mechanical mixtures of mafic blocks with the

ultramafic rocks cannot account for the present, sili-

ceous state of the mélange (see Fig. 5b and the

discussions above). The evidence for SiO2 additions

in mineralogical zonations at rims of mafic blocks

(Sorensen, 1988) and ultramafic blocks (this study;

Fig. 2b) is compatible with contributions of SiO2mobilized in fluids. Overall addition to the mélange

unit is also evident for H2O (here represented by the

LOI data), as nearly every mélange matrix samples is

significantly enriched in H2O relative to the calculated

ultramafic–mafic mixtures (see Figs. 8a and 10c).

Other elements, such as Ca, Na, and Sr, show

scatter in observed/calculated values (see the vertical

lines on Fig. 8a, indicating overall ranges in observed/

calculated for mélange matrix samples), and Na, and

particularly Sr, have average mélange matrix ob-

served/calculated significantly lower than that of the

geochemical reference frame elements and the 1:1 line

(the latter line indicating the results of simple mixing

of the mafic and ultramafic end members; see Fig.

10d–f). Potassium and Ba also show much larger

ranges in observed/calculated concentrations, but

(particularly for Ba) have average mélange matrix

observed/calculated higher than that of the geochem-

ical reference frame elements and the 1:1 line indicat-

ing addition. For Ca, Na, and K, a large number of the

mélange matrix samples have observed/calculated

significantly lower than the observed/calculated for

the mafic rind samples. For Ca, K, and Sr, the

mélange matrix observed/calculated values are lower

than that of the ultramafic rind.

6. Discussion

6.1. The nature of the tectonic mixing

A key inference from the data is that some elements

(Al, Cr, Fe, Ni, Mg, and Zr) are best explained by

Fig. 8. Diagram illustrating the differing compositional systematics

of selected major and trace elements. (a) Diagram showing the

overall range in observed/calculated for individual mélange matrix

samples (broad vertical lines) and the observed/calculated value for

the averaged mélange matrix composition (filled circle for each

element or LOI). (b) Plot of ultramafic/(ultramafic +mafic) vs. Mg

observed/calculated for mélange matrix samples and rinds on mafic

and ultramafic blocks, demonstrating that Mg observed/calculated

shows similar variation to that of the other geochemical reference

frame elements. Three samples with extremely high observed/

calculated (>3) strongly influence the average mélange matrix

composition observed/calculated for Mg, as shown on (a), but do

not as strongly affect the average mélange matrix composition

observed/calculated for other elements. On (b), the symbols are the

same as those used in Figs. 5–7. Note that, for CaO, Na2O, K2O, Sr,

and Ba, some of the extremely chlorite- and talc-rich schists have

concentrations below the analytical detection limits (concentrations

of 0 wt.% or ppm on Table 3), or (for the trace elements) were not

analyzed (denoted by ‘‘– ’’ on Table 3).


simple mixing (cf. similar conclusion reached by

Sorensen and Grossman, 1989, for the mafic rind

compositions). Field observations in the Catalina

Schist amphibolite-grade mélange unit clearly show

that the mélange matrix developed in mafic-block-

bearing shear zones between domains of low-Al ultra-


mafic materials. Progressive hydration and metaso-

matic alteration of both mafic and ultramafic blocks

produced weak rinds rich in sheet silicates and amphib-

oles. The similarity of the rinds and the mélange matrix

in mineralogy and chemical compositions is consistent

with production of the matrix through progressive

incorporation of block rinds. Rind-like materials can

be seen to be progressively incorporated into the

mélange matrix foliation (see Fig. 3), resulting in

adjacent, mineralogically/chemically disparate litholo-

gies at scales of meters down to millimeters. At the

lower size limit, this process is represented by small

discoids of clinoamphibole-rich material within the

more aluminous schists, and by the existence of layers

with contrasting mineralogy at thin-section scale (e.g.,

layers of amphibole and talc in chlorite-dominated

schist). Deformation was sufficiently intense to trans-

pose fabrics, disaggregate rigid block-derived materi-

als in weaker chorite- and talc-rich mélange, and in

some particularly weak lithologies (e.g., chlorite-, talc-,

and amphibole-rich materials), intimately juxtapose

disparate lithologies at the (sub-)cm scales sampled

by the whole-rock geochemical analyses.

Yang et al. (1998) similarly argued for the com-

bined effects of tectonic mixing and metasomatism to

produce an upper-amphibolite facies, ductile shear

zone, concluding that certain elements (in that case,

HREEs, Ti, V, and Sc) fit a simple mixing model

(representing ‘‘mechanical mixing due to shearing’’),

whereas other elements exhibited mobile behavior.

Deformation in the Catalina Schist amphibolite-facies

mélange unit should have enhanced chemical

exchange, particularly by increasing chemical poten-

tial gradients through juxtaposing contrasting litholo-

gies (cf. Wintsch, 1985), and also enhancing the

permeability of the mélange (cf. Rutter and Brodie,

1985) facilitating infiltration-driven metasomatism.

Deformation-enhanced metasomatism could explain

the apparently greater alteration of some mélange

matrix samples relative to rind samples, notably the

chlorite-rich schists with that have exceptionally low

concentrations of Si, Ca, Na, K, Sr, and Ba.

6.2. Estimating mass gains and losses in the mélange

matrix

Use of the observed/calculated for the geochemical

reference frame elements affords estimates of mass

gains and losses during mélange formation. We

emphasize that these calculations serve to qualita-

tively delineate the contrasting evolution of the con-

trasting compositional domains in the mélange matrix,

although they do not fully treat the continuum of

coupled tectonic and metasomatic mixing proposed

based on the field and petrographic observations (as

discussed above). Total mass changes (Table 3; after

Gresens, 1967; Grant, 1986) were calculated using the

ratio of the concentrations of Cr or Al2O3 predicted

from simple mixing (Ci; normalized using Cr/Al2O3)

to the observed concentrations (Cf):

% change in mass ¼ ½Ci=Cf � 1� � 100The results indicate a wide range in mass loss and

gain, ranging from f 55% mass loss, in the mostchlorite-rich schists, to >100% mass gain in the most

siliceous schists. For the rinds on mafic blocks,

calculated mass gains range from small in the inner

rinds (f 8–10% gain) to gains approaching those ofmany of the mélange matrix samples in the outer rinds

(2–44% mass gain; see Table 5). An approximately

200% mass gain is calculated for the rind on the

ultramafic block (sample 6-4-100FR). For the average

mélange matrix composition, a mass gain of f 24%is calculated, and based on an analogous calculation

for changes in individual components, we suggest that

the majority of the mass changes are in SiO2 and H2O

(see Figs. 5b and 10a–c).

A similar calculation for volume changes illumi-

nates the differing evolution of the contrasting com-

positional domains in the mélange matrix. Selected

Fig. 9. Isocon diagrams (after Grant, 1986) presenting results of the Cr–Al-based mixing calculations for elements which show overall

systematics similar to those of Cr and Al. This plot compares the observed concentrations with those calculated as the result of simple mixing of

the two end-member lithologies, using the mixing proportions based on Cr/Al2O3. These elements, together with Cr and Al, constitute the best

‘‘geochemical reference frame’’ with which assessments of mass loss and gain can be made. (a) Al2O3 isocon plot, (b) Cr isocon plot, (c) MgO

isocon plot, (d) Ni isocon plot, (e) Fe2O3 isocon plot), and (f) Zr isocon plot. Note that, for each of these elements or oxides, the data straddle the

1:1 line on these plots, and that the point representing the average mélange matrix falls on or slightly below the 1:1 line (shaded diagonal line).


values of volume change were calculated for samples

with simple mineralogy by using mineral densities as

a proxy for whole rock values (Table 3), specifically,

the nearly monomineralic chlorite, talc, or amphibole

schists. Densities used were 3.4 g/cm3 for the anhy-

drous ultramafic and eclogitic end members and 2.65,

2.75, and 3.1 g/cm3 for clinochlore, talc, and amphib-

ole, respectively. Volume changes come from the

following relationship where qi and qf represent theinitial (calculated) and final (observed) densities,

respectively.

% change in volume ¼ ½ðCi=Cf Þðqi=qf Þ � 1� � 100

Chlorite schists show volume changes of � 42%to + 10%, talc schists show volume changes of + 85%

to + 153%, and the three clino- and orthoamphibole-

rich schists show volume changes of + 46% to + 59%.

The negative to low positive volume changes for the

chlorite schists reflect their loss of mobile elements as

indicated by their enrichment in the geochemical

reference frame elements. Volumes still increase

slightly in some chlorite schists because of the much

lower density of chlorite relative to the mixing end

members. Calculated volume change for the rinds on

mafic blocks, using the specific gravity data of Sor-

ensen (1988) for these samples, demonstrate that, on

the average, the outer rinds gained more mass and

volume than the inner rinds (see Table 5), and that all

rinds have volume changes that fall on the low end of

the positive volume gains calculated for mélange

matrix samples.

The talc- and amphibole-rich mélange matrix sam-

ples that show the largest gains in mass are those that

contain the smallest concentrations of Al2O3 ( < 3

wt.%, cf. Table 1, Table 3). These resemble ultramafic

rind sample 6-4-100FR (with 1.21 wt.% Al2O3), for

which a f 200% mass gain was calculated (see Table5). These samples with the largest gain also show the

greatest variation among the observed/calculated val-

ues for the geochemical reference frame elements (Cr,

Al, Mg, Ni, Fe, and Zr). The most likely interpretation

is that these samples represent significant metasomatic

additions of Si, Mg, FCa to the other reference frameelements. The ultramafic rind sample (6-4-100FR) is

fibrous and undeformed (see sketch in Fig. 2b) and

appears to have formed via metasomatic processes at

the rim of an ultramafic body (with little or no

deformation). Alternative interpretations, for example,

a protolith depleted in the reference frame elements,

are possible but seem less likely given the field

relationships and restricted Cr and Al concentration

ranges for the serpentinites.

6.3. Implications of mixing for stabilities of accessory

mineral trace element hosts

The work by Sorensen and Grossman (1989, 1993)

on variably migmatitized mafic blocks and their

metasomatic rinds, together with the mineral assem-

Fig. 10. Diagrams presenting results of the Cr–Al-based mixing calculations for elements which show overall systematics dissimilar to those of

the elements belonging to the ‘‘geochemical reference frame’’ (see Figs. 8 and 9). (a) SiO2 isocon plot, (b) Si (observed/calculated) vs. Al

(observed/calculated), (c) LOI (observed/calculated) vs. Si (observed/calculated), (d) CaO (observed/calculated) vs. Na2O (observed/calculated),

and (e) K (observed/calculated) vs. Ba (observed/calculated), and (f) K (observed/calculated) vs. Sr (observed/calculated). On these plots, the

1:1 lines represent compatibility with simple mixing, without depletion or enrichment relative to the simple mixtures of mafic and ultramafic

rocks. Note the compatibility (and even resemblance) of (b) with Fig. 5b, the two emphasizing the significance of Si losses and gains in the

mélange matrix.

Table 5

Calculated mass and volume loss for rinds (data for mafic rinds

from Sorensen and Grossman, 1989)

Sample UM/M DMa DVb

Inner rinds

CA866A 0.01 8 16

CA866B 0.12 7 19

L11786 0.05 10

Outer rinds

CA865 0.40 2 15

CA864 0.92 16 31

L11781 0.99 44

Other rinds

714822 0.63 13 30

1113842 0.15 23 39

712847 0.35 2 15

Ultramafic rindc

6-4-100FR 3.50 203 289

a Change in mass relative to calculated compositions.b Change in volume, using densities from Sorensen (1988).c Ultramafic rind from thi s study (composition in Table 3).


blage data presented in this paper (Table 1), allows

consideration of the role of accessory minerals on

trace element behavior during mélange formation.

One consequence of tectonic (mechanical) mixing is

that it shifts bulk compositions such that accessory

trace element mineral hosts can be destabilized. For

example, mechanical mixing alone or combinations of

mechanical mixing and fluid addition can destabilize

titanite, a significant trace element host in metamafic

rocks (cf. Green, 1981; Sorensen and Grossman,

1989). In the Catalina Schist amphibolite-grade mél-

ange, titanite occurs in mafic blocks and in rinds

developed at the margins of mafic blocks, but is

replaced by rutile and ilmenite as the Ti-rich phases

in the mélange matrix samples. Sorensen and Gross-

man (1989) demonstrated the importance of phases

such as titanite in the trace element budget of the

metasomatized blocks (particularly for elements such

as the LREE, U, and Th). Release of these trace

elements due to breakdown of titanite to rutile or

ilmenite could result in the removal/redistribution of

the elements by fluids, depending on the evolving

mélange mineral assemblage during mechanical and

metasomatic mixing. Apatite, common in trace

amounts in the mélange (Table 1), concentrates LREE

in the mafic blocks (Sorensen and Grossman, 1989)

and should also play a role in governing the behavior

of the REE and U.

Rutile may play a similar role, incorporating

instead HFSE (e.g., Nb, Ta; see recent discussion by

Johnson and Plank, 1999). Rutile, commonly in

various stages of replacement by ilmenite (based on

petrographic evidence), occurs in numerous samples

of the mélange matrix (see Table 1). Sorensen and

Grossman (1989) reported that rutile from migmatitic

mafic blocks in the mélange contains 20 to 160 ppm

Ta. The common replacement of rutile by ilmenite is

suggestive that increases in FeO activity (e.g., varia-

tions in FeO/MgO) could result in its destabilization.

This is consistent with addition of mafic rocks which

have higher FeO/MgO through mechanical mixing.

6.4. Degree of involvement of metasedimentary rocks

Field relations suggest that sedimentary rocks were

not important in formation of the mélange matrix.

Metasedimentary blocks, mostly chert, are rare within

the mélange, and semipelitic to pelitic schist occurs as

blocks in only one or two exposures near the contact of

the mélange unit with the coherent metasedimentary

and metagabbroic section. In contrast, field relations

indicating mechanical and metasomatic interactions

involving mafic and ultramafic rocks are ubiquitous

within the complex.

Unequivocal geochemical evidence to preclude

involvement of sediment is difficult to produce. The

mélange matrix compositions could accommodate a

moderate, though unrecognized amount of sediment.

One of the largest differences between mafic and

sedimentary compositions is the higher content of

LILE and Na in the latter (Bebout et al., 1999; Fig.

11a). Stripping of LILE (e.g., K2O) and Na would be

required if sediments were involved in mélange for-

mation (see data for the Catalina Schist metasedimen-

tary rocks relative to the mélange matrix data and the

mafic–ultramafic simple mixing region in Fig. 11a).

Also, because of the low CaO content of the meta-

sedimentary rocks compared to the CaO content of the

similarly aluminous, somewhat more Mg-rich mafic

end member (Fig. 11b), the production of mélange

matrix by large amounts of sedimentary-ultramafic

mixing would likely require substantial addition of

CaO from external sources. The relatively good geo-

chemical fit obtained from mafic–ultramafic mixtures

alone (see Fig. 11b) and the clear field evidence for

their mutual interaction also argues against a major

sedimentary component (on Fig. 11b, see the large

arrows indicating rind-formation trends). Nonetheless,

incorporation of small amounts of sediment, partic-

ularly the more SiO2-rich material, cannot be pre-

cluded.

6.5. Integrated model for the evolution of the mélange

Field and petrographic evidence suggest that the

mélange matrix was developed through metasomatic

interactions and mechanical mixing of mostly ultra-

mafic material and mafic lithologies. In the field,

intermediate stages of this process are clear. Large

parts of the mélange matrix compositionally overlap

with rinds on the mafic (hornblende-rich) or ultramafic

(actinolitic clinoamphiboleF orthoamphiboleF ensta-tite) blocks. Nowhere in the mélange can similar

relationships be found related to minor metasedimen-

tary (metagreywacke and metachert) blocks. Intense

veining by high-T assemblages indicates fracturing of


blocks as they were entrained in the mélange. Within

larger expanses of serpentinite, schistose zones con-

taining Al-rich, high-T mélange matrix and showing

development of metasomatic mineralogical zonations

in ultramafic materials at their contacts, represent

localized shear zones developed during block incorpo-

ration (Fig. 2b). The mineralogical zonations evidence

substantial fluid movement along these structures with

concomitant metasomatic transfer of soluble constitu-

ents (e.g., Ca, Si, in addition to H2O). Although the

contents of most elements in the mélange matrix fit a

mixing model based on reaction of mafic and ultra-

mafic blocks and their rinds, some elements (e.g., Na,

K, Ba, Sr) appear to require fluid-mediated metaso-

matic redistribution or stripping after mixing (Fig. 10).

Our earlier stable isotope studies of the Catalina

Schist, including the amphibolite unit mélange, con-

cluded that the mélange unit equilibrated with an

aqueous fluid that had an O-isotopic signature derived

from a lower-temperature sedimentary source, and

that this fluid homogenized O and H isotopic compo-

sitions in the mélange matrix over kilometer-scale

distances (Barton et al., 1987; Bebout, 1991; Bebout

and Barton, 1989, 1993). Furthermore, we inferred

that these sediment-sourced fluids would have been

capable of transporting large amounts of SiO2 into the

mélange unit to the point of creating talc-bearing

assemblages (Bebout and Barton, 1989, 1993; cf.

Manning, 1995).

Felsic pegmatites in the mélange formed during the

high-T evolution of the mélange, as they are observed

to both cross-cut the mélange fabric and to be

deformed into the mélange fabric (Fig. 2a; Sorensen

and Barton, 1987). Sorensen and Barton (1987) pro-

posed that the pegmatites in the mélange represent

melts derived from migmatization of mafic blocks in

the mélange unit, a hypothesis developed in detail by

Sorensen and Grossman (1989). Our further work has

indicated that the pegmatites show a wide range of

trace element compositions which fall between those

observed for pegmatites obviously derived from the

coherent metasedimentary sections and those in the

coherent metamafic sections in the Catalina Schist

amphibolite-grade unit (Bebout and Barton, 1993,

unpublished data). Thus, the mélange unit appears to

preserve effects of the transfer of both H2O-rich C–

O–H–S–N fluids, which homogenized the O and H

isotopes, and water-rich felsic melt, the latter sourced

from diverse mixtures of mafic and sedimentary

components, some also possibly outside the mélange

zone.

Fig. 12 illustrates some possible mineralogical paths

for mechanical mixing and metasomatic silica. On

these activity–activity diagrams, a trajectory between

Fig. 11. Chemical discrimination diagrams comparing the compo-

sitions of mélange matrix samples with those of mafic, ultramafic,

and sedimentary rocks in the Catalina Schist (data for metasedi-

mentary rocks from Bebout et al., 1999). (a) Plot of K2O vs. Na2O,

demonstrating the far higher concentrations in the metasedimentary

rocks relative to the mafic mixing tectonic blocks, and (b) plot of

MgO vs. Cao, demonstrating the low Mg, low Ca character of the

metasedimentary rocks in the Catalina Schist. Data for metasedi-

mentary rocks are for samples from all units of the Catalina Schist,

as detailed geochemical study has indicated no detectable variations

in the Si–Al–Mg–Ca compositions as functions of differing

metamorphic grade. The two large, shaded arrows on (b) are the

trends in composition from the mafic and ultramafic blocks toward/

through the data for rinds developed on each of the block types. On

(a) and (b), the extent of the envelope of simple mixing of

ultramafic and mafic compositions is indicated by the hand-drawn,

dark, thin lines.


mafic rocks (garnet–pyroxene–amphibole) and oli-

vine-rich rocks broadly reproduces the patterns

observed. Mechanical mixing can stabilize chlorite +

Al-rich clinoamphibole assemblages, but cannot pro-

duce the highly siliceous, nearly monomineralic talc

schists that are abundant in the mélange (Figs. 5b and

10a and b). The latter require silica introduction, and on

the diagram, this is shown as fluid-mediated addition.

These fluids, which the evidence noted above shows

must be external to the unit, would also redistribute or

leach alkalis and other soluble components to yield the

diverse mélange matrix compositions. Also illustrated

is the destabilization of titanite along the same path.

Silica solubility relationships with olivine and talc-

bearing assemblages require that a minimum of several

rock masses of water must have passed through the

ultramafic rocks to produce the talc-rich assemblages

(cf. Bebout and Barton, 1989, 1993; Manning, 1995;

Newton and Manning, 2000). Multiple paths are pos-

sible; thus, there is a large uncertainty in any fluid–

rock estimate. The most favorable circumstances for

the silicification would be for waters equilibrated with

sedimentary rocks in the amphibolite unit of the

Catalina Schist to move into the ultramafic mélange.

Least favorable would be significant up-temperature

flow from lower-grade units, which would require

Fig. 12. Calculated activity–activity diagrams (in a–c for systems labeled on each diagram; all for 650 jC, 1.0 GPa) illustrating possiblemineralogical consequences of mechanical mixing and fluid-mediated SiO2 additions. Activities are those of the oxides (lime, periclase and

quartz) at 650 jC and 1.0 GPa. In (d), labeled ‘‘Mass Exchange Processes’’, the various lithologies regarded as mixing components (‘‘Eclogite’’,‘‘Garnet Amphibolite’’, and ‘‘Peridotite’’) and a generalized compositional range for mélange matrix are indicated. Thermodynamic data for

most species are from Johnson et al. (1992) and adapted from Robie et al. (1978) for sphene (titanite) and rutile.


considerable layer-parallel flow before silicification

could take place under amphibolite-facies conditions

(Barton et al., 1987; M.D. Barton, G.E. Bebout, in

preparation). These volumes are consistent with the O

isotopic shifts described above, which also require an

external source of sediment-equilibrated fluid.

Fig. 13 illustrates schematically the inferred tec-

tonic setting of formation of the amphibolite-facies

mélange unit (also see Fig. 3 in Bebout and Barton,

1989). The mélange is possibly a fragment of a zone of

complex mechanical and metasomatic mixing of mafic

(possibly slab-derived; see discussion by Sorensen and

Grossman, 1989) components with ultramafic materi-

als from the hanging wall. Mafic and ultramafic blocks

in the mélange represent variably but incompletely

incorporated mafic and hanging-wall materials that

were entrained in the evolving shear zone. Some of

the blocks were eventually extensively metasomatized

and became tectonically (mechanically) incorporated

into the mélange matrix (at various scales). During this

development, the mélange was infiltrated by hydrous

C–O–H fluid enriched in SiO2 and other dissolved

components. The H2O-rich fluids stabilized the present

hydrous, siliceous assemblages in the mélange and

locally stabilized ankeritic dolomite and magnesite.

The model involving infiltration of the mélange by

fluids from subducted, dominantly sedimentary sour-

ces is supported by evidence for extensive SiO2mobility in the lower-grade units of the Catalina Schist

and isotopic systematics (Barton et al, 1987; Bebout,

1991; Bebout and Barton, 1989, 1993). A model of

SiO2 addition to the mélange via infiltrating aqueous

fluids from the slab and sediments is also supported by

experimentally and theoretically inferred high solubil-

Fig. 13. Cartoon showing inferred tectonic setting of mélange formation along the slab–mantle interface. The mixing zone develops between

the downgoing slab and sediments and the hanging wall, and probably grades into highly deformed, low-Al, hydrated ultramafic rocks (in the

Catalina Schist, initially dunites and harzburgites, based on major element chemistry) upward into the hanging wall. Fluids (hydrous fluids and

siliceous silicate melts) derived in the slab and sediment enter the mixing zone and may either be transported along the mélange or, after

experiencing fluid– rock interactions in the mélange, rise into the hydrated and deforming hanging wall. Beneath arcs, some of the chemically

evolved fluid could ultimately ascend into and chemically interact with arc source regions in the mantle wedge.


ities of quartz in aqueous fluids under subduction-zone

P–T conditions (e.g., Manning, 1996). Calculated

mineral–fluid solubility relations (Manning, 1998)

indicate that other components (Al, Na, in particular)

may well be mobile in metamorphic fluids at blues-

chist- to eclogite-facies conditions.

7. Implications of mélange formation deep in

subduction zones

The inferred depths of peak metamorphism for the

Catalina Schist range from 25 to 45 km, obviously

shallower than those inferred for the slab–mantle inter-

face beneath arcs. Inferred metasomatic and structural

processes for the Santa Catalina rocks must, therefore,

be considered as analogues to those which operate at

greater depths, or at least as an indication of the

complexity of such processes which may affect such

deeper zones. The Santa Catalina rocks probably rep-

resent early stages of subduction (Platt, 1976; Grove

and Bebout, 1995), but the rocks of the highest-grade

amphibolite unit have experienced 650–750 jC tem-peratures not unlike those that may affect deeper parts

of subduction zones at later stages (Peacock, 1993).

The lower-grade mélange matrices in the Catalina

Schist may be considered analogues to the ultramafic

mélange in the amphibolite unit but formed through

intense mechanical mixing and with coeval metaso-

matic alteration at significantly lower temperatures

more similar to those predicted for these depths by

thermal models of mature subduction zones (350–450

jC; Grove and Bebout, 1995; cf. Peacock, 1993). Thelower-grade mélange exposures show many similar-

ities with the amphibolite-grade mélange, including

metasomatic rinds on tectonic blocks, field evidence

for progressive tectonic incorporation, and domains of

high-variance mineral assemblages, indicating meta-

somatic formation (see Bebout and Barton, 1993;

Bebout, 1997).

Geochemical models invoking additions from the

subducting slab and sediments to explain complex

isotopic and trace element systematics generally call

upon discrete hydrous fluid or melt phases derived

from one or the other of the subducting lithologies.

Interestingly, the Catalina Schist amphibolite-facies

mélange contains evidence for the migration of both a

hydrous phase and felsic silicate melts during its

deformation, perhaps providing a field expression of

the generation and mobility of multiple slab/sediment-

derived fluid phases. Presumably, these fluid phases

must traverse any complex mechanical mixing zone at

the slab–mantle interface in order to enter the mantle

wedge, and the potential exists for extensive fluid–

melt–rock interactions resulting in modification of the

slab fluid phases prior to their migration into the

overlying mantle wedge. It is difficult to explicitly

evaluate such fluid–rock interactions in studies of arc

geochemistry; however, we suggest that further stud-

ies could consider the possibility that single, mixed

(perhaps even homogenized at some large scale; cf.

Morris et al., 1990; Bebout, 1991) fluid phases

possessing chemical signatures of mechanically hybri-

dized lithologies could produce many of the arc geo-

chemical systematics. It is notable that, in the Catalina

Schist ultramafic mélange, LILE are highly mobile as

has been inferred in studies of arc magma sources

(e.g., Gill, 1981; Pearce and Peate, 1995). In the

voluminous chlorite-dominated parts of the mélange,

LILE were stripped but only locally enriched else-

where as in the rinds on the mafic blocks.

Mélange formation, resulting in the production of

high-variance assemblages with high water content

may also be an important process in facilitating reten-

tion and storage of H2O at depth in subduction zones

(Bebout, 1991). Whereas chlorite is unstable in mafic

and sedimentary bulk compositions at amphibolite and

greater metamorphic grades, chlorite (containing f 13wt.% H2O) in rocks of bulk compositions near that of

its own composition may be stable to far greater

temperatures and pressures (cf. Jenkins and Chernosky,

1986). Talc (containing f 5 wt.% H2

tectonic and metasomatic mixing in a high-t, subduction ...mdbarton/mdb_papers_pdf...lina schist,...

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