Chapter 3: Calculated phase relations in amphibolites

FMASH [Als] Cfd + Oam,Qtz,H2O

Chapter 3: Chapter 3: Calculated phase relations in amphibolites

location of the continuous reactions which are

likely

to control the appearance of the relevant assemblages.

In contrast to ^the

FMASH

system, in which the natural ^phase relationships ^are

relatively well documented, aluminous amphibolites ^are relatively poorly described and poorly understood. ThuS, ^a theoretical approach is more appropriate to ^these rock-types, and

equilibrium thermodynamics calculations

will

be used to develop ^a P-T projection for the amphibolites in the model system

CFMASH.

^In order to evaluate the applicability of ^the calculated grid to natural amphibolites, ^the predicted ^phase relations

will

^be compared to the reaction textures and assemblages which have been described in the literature. The

final

secúons discuss the continuous reactions discerned from the calculated CFMASH ^phase diagrams ^and the effect

of

sodium on the equilibria. Later chapters discuss the metamorphic history preserved in reaction textures in newly described amphibolites from central Australia (Chapter 4), the

ZillertalAlps

in Austria (Chapter 5), ^and the ^phase relations in world wide amphibolite occuffences (Chapter 6) from the point of view of the phase diagrams constructed here.

3.2 An appropriate ^{model system} for calculations

The ^phases involved in the aluminous amphibolites reported in the literature include

"typical"

mafic meta-igneous phases, such ^as clinopyroxene, orthopyroxene, hornblende, plagioclase, galnet, chlorite, quartz, spinel, biotite, epidote, rutile, ilmenite, magnetite ^and titanite

with

aluminosilicates (mainly kyanite), staurolite, cordierite, orthoamphibole (both gedrite and anthophyllite), cummingtonite, corundum, calcite ^and

dolomite.

^{The primary} motivation of this study was to address the metamorphic record of aluminous amphibolites from the Harts Range ^(see Chapter 4) and the Zillertaler Alpen in Austria ^(Chapter

5).

^These

rocks ^are dominantly quartz-saturated, precluding the stability of Mg-Fe-spinel or corundum and do not contain equilibrium clinopyroxene or orthopyroxene. Although chlorite is observed in the Zillertalrocks,

it

is absent from the Harts Range amphibolites and ^so is neglected in the

first

instance. Cummingtonite ^does not occttr

with

the aluminous ^phases kyanite, staurolite or cordierite

in

the Harts Range amphibolites and ^so has also been neglected. Several of the observed ^phases (e.g. ilmenite, magnetite, biotite, epidote, calcite ^and dolomite) contain considerable amounts

of

an otherwise minor components (such ^as,

Ti,

^Fe3+,

K,

CO2) and these are thus excluded from the analysis presented here. Epidote typically contains ^a significant proportion of the pistacite molecule (CaZFe3+A1ZOrZ(OH)) and, ^{as such,} is

stabilised by the presence of ferric

iron.

The major element chemisüy of the remaining ^phases can be described in the model system NazO-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH)'

In view of the complexity ^and fundamental uncertainties in of the activity-composition (a-X) relations in plagioclase feldspar (e.g. Holland

&

Powell,1992), there seems

little

point in making quantitarive thermodynamic calculations in the fuII

NCFMASH

system and

I will

begin by neglecting Na2O. Additionat justification for neglecting the effect of Na on the phase relations

in

CFMASH is provided

in

Chapter 2 where the addition

of

sodium to ^the

FMASH

Chapter3-CFMASH- ²⁶

system caused negligible change (of the order of 0.2 kbar) in the P-T positions of invariant equilibria in the

FMASH (NFMASH)

system. Moreover,

in

the Harts Range aluminous amphibotites, Na2O is a relatively minor component (e.g. coronas of staurolite ^and plagioclase around

þanite

involve calcium-rich bytownite-anorthite plagioclase, Chapter 4) ^{and thus} many

of

the important equilibria should be evident in the

CFMASH

system' Thus' ^the CFMASH compositional system ^seems an appropriate model for calculating the phase relations of the aluminous assemblages

in

amphibolites.

A

qualitative extension into NCFMASH ^is considered

in

a later section.

As mentioned above, ^the rocks of interest here ^are quartz-saturated, medium ^grade samples which are generally considered to have equilibrated

with

aqueous vapour.

Considering quartz, anorthite and ^aqueous vapour to ^be in excess ^(a11rO =1), the components required to graphically represent the compositional relationships between the phases in the

amphibolites can ^be reduced to the ternary system Al2O3-FeO-MgO

(AFM)'

The generalised chemographic relations of the ^phases in

AFM

^{¿ìre pfesented in}

^Fig' 3'1'

The relative

compositions in the QFMASH system are similar to those in

FMASH

^(See Chapter 2),

with

the exception that hornblende (projected from quartz, anorthite and ^aqueous vapour) is also present. In natural occulrences, homblende typicalty ^has Xps intermediate between

orthoamphibole and cordierite (Hietanen, 1959; ^James et al, 1978; Spear,1982; Froese

&

Hall,

1983; Schumacher

&

Robinson, 1987; Helms

etal.,1987)

^{and plots at a negative}

A/AFM

value due to the projection from anorthite. Hornblende ^and orthoamphibole both show significant tschermakite substitution. Depending on ^the extent of the MglFe-1' tschermakite and

Mglca-1

substitutions

in

the relevant ^phases, orthoamphibole may occur on either ^side

of

the staurolite-hornblende tie-line (Frg.

3.1).

As mentioned

in

Chapter 2,the relative Xps

of

staurolite and garnet may comply

with

the usual trend, that is XF",Grr

)

XFe,st (e.g' Spear, 1982;Froese

& Hall,

1983; Selverstone et al., 1984, Chapter 6) or may be "reversed" (e'g' Purtscheller

&

Mogessi

e,1984;Grew &

Sandiford, 1985;

Arnold

et

al',

1994; Chapter 4

)'

These variations

in

the chemographic relations allow the possibility of two degenerate reactions (co-linearities in

AFM

⁺

An

_{+ Qtz,} ^{+ aqueous vapour):}

orthoamphibole

+

staurolite + hornblende and

staurolite

+

garnet + aluminosilicate

which may resulr

in

singularities

in

the P-T projection (e.g.

Fig.

3'2).

3.3 A CFMASH _Phase diagram

As a starting point from which to develop the CFMASH ^phase diagram,

I

consider the effect

of

a small amount

of

CaO on the

FMASH

equilibria discussed

in

the previous ^chapter.

The

FMASH

invariant point, Ip1 ^(see Chapter 2) involves many of the phases of interest in the

CFMASH

system and

will

form the basis of this discussion. Qualitatively incorporating ^a small amount

of

a new component into existing ^phases increases ^the variance of all of the

Chaptcr3-CFMASH- ²⁷

Grt

Oam

AlzOs Als

St

+

An, Qtz,

^H2O

Hbl

FeO Mgo

Fisure 3.1. AFM diagram illusrating the gerreralised compositions of the phases of

ilig;;r ilïÉ"

CFMÃ'SH;od;l rytt""*, pr-o;ecteO from anórthite, quartz and aqueous vapour onto ^the AFM _Plane'

Chopter3-CFMASII- ²⁸

Grt

Grt Hbr

(sr) [Crd, Als]

Grt

(Hbr) Hbr

AI

St +

AI

Grt

SI Grt

St

^Grt

Oam

Oam

Grt

^Hbl

Oam

Oam

CFASH