T'esting the calculated phase relations

CFMASH +Qtz,An,H20

3.4 T'esting the calculated phase relations

Although they are internally consistent, the uncertainties

in

the thermodynamic

calcuiations outlined above imply that caution is ^needed in the application of the phase relations determined from equilibrium thermodynamics calculations to amphibolites. The relevance

of

the calculated ^phase diagrams ^{can be} determined from comparison of the predicted ^phase relations

with

those determined in natural assemblages. The comparison here comprises three parts: identifying the

likely

stable CFMASH invariant points a¡d univariant reactions

from

natural low-variance assemblages; consÍaining the location of the invariants ^and univariants

in p-T

space; and comparing the calculated ^phase diagram to those determined by previous authors.

3.4.1 CFMASH invariant points and univariant ^reactions

There are several reported instances of low-variance amphibolites and although ^these correspond more closely to the

NCFMASH

system, they ^are closely related to CFMASH ^and

FMASH

sub-system

equilibria.

These assemblages may be used to infer, independent of the calculations, which of the six CFMASH invaria¡rt points ^are

likely

to be relevant to real rocks i.e. which CFMASH invariant points are stable.

(A fuller

account of the relevant occulrences is given

in

Chapter 6.)

As shown in the previous chapter, rocks

with low

Ca-,

K-

and Na-contents can ^be approximated by the

FMASH

model sysrem. Exampies

of

^{these reported from} the literature include the assemblages (with co-existing quartz):

sillimanite/kyanite-staurolite-garnet-orthoamphibole(F{bl,Pl,Crd)

Robinson

&

Jaffe (1969b), sillimanite/kyanite-staurolite-<ordierite--orthoamphibole (Hbl, ^Pl, Grt)

Schumacher

&

Robinson (1987)' Chapter3-CFMASII- ³⁴

sillimani te-staurol ite-cordieri

te-garnet

^(Hbl, Pl, O am)

Sharma

&

MacRae (1981), sillimanite-cordierite-game t--orthoamphibole (Hbl, ^{Pl, S Ð}

Sharma

&

MacRae (1981).

These univariant assemblages intersect

in

an

FMASH

^invariant assemblage involving aluminosilicate, staurolite, cordierite, garnet and orthoamphibole

with

quartz, which suggests that the

[tlbl, ^Pl]

^inva¡iant

point

is stable in the

FMASH

^system.

In

Ca-rich rocks hornblende ^and plagioclase a¡e commonly equilibrium ^phases.

Examples of four-phase assemblages

involving

hornblende include (with plagioclase ^and quartz):

staurolite-g arnet-orth ^{o amphib} ole-hornb

lende

^{(Crd, Al s)}

(Spear, 1971

;

1982; ChaPter 4) kyanite-staurolite-garnet-hornblende ^(Crd,Oam)

(Purtscheller

&

Mogessie, 1984; Selverstone et ai., 1984; Chapters 4 &.5), kyanite-staurolite--ortho ^amphib ole-hornblende ^(Crd, Grt)

(Chapter 4),

sillimanite-staurolite-cordierite-hornblende (Grt, Oam)

(Schumacher

&

^{Robinson, 1987),} kyanite--cordierite--orthoamphibole-hornblende ^{(Grt, S} t)

(Chapter 4) and cordierite-garnet--orthoamp hibole-hornblen

de ^(Als'

^{S t)}

(Humphreys, 1993).

Apart from the last, all of these four phase assemblages ^are consistent

with

the stability

of

the

CFMASH

invariant points

[Crd]

^and

tcrtl.

^Thus the phirse relations observed in real rocks suggest that the three invariant points,

[Hbl], [Crd]

^and

[Grt]

^are potentially stable (cf.

Fig. 3.4).

However, the occurrence of cordierite-garnet--orthoamphibole-hornblende- plagioclase-quartz (Als, St) is inconsistent

with

the

CFMASH

^{grid and seems} to indicate that either

[Als] or

[St] is ^stable.

3.4.2 P-T constraints

Information from natural amphibolite assemblages and ^the rocks

with

which they are associated can also provide

tn

independent constraint on ^the pressures and temperatures of the

CFMASH equilibria.

^In particular, the aluminosilicate polymo¡phs associated with individual assemblages might be expected to provide broad constraints on ^the location of the stability

Chapter3-CFMASII- ³⁵

fields of the relevant aluminosilicate-bea¡ing assemblages and here

I

briefly describe examples

of

these.

The calculated ^phase diagram predics that the assemblages aluminosilicate-homblende and aluminosilicate-orthoamphibole

will

be restricted to elevated ^pressures in the kyanite and kyanite and sillimanite stability fields, respectively

Fig. ^3.5).

^{This is}

ⁱⁿ

^accord

^with

^most

recorded assemblages. For example, hornblende is generally observed in equilibrium

with

kyanite rather than sillimanite or andalusite, whereas orthoamphibole is

typically

obsewed with either sillimanite or kyanite but only rarely

with

andalusite.

As discussed

in

Chapter

2,themre

examples of andalusite--orthoamphibole rocks reported

in

the

literatue

(e.g. Seki

&

Yamasaki, ¹⁹⁵⁷⁾ probably reflect problems associated

with very

small ^scale equilibrium volumes, rather than equitibrium associations, however the discrepancies involving aluminosilicates

with

hornblende a¡e more problematic. There are two instairces of hornblende reported in association

with

sillimanite, neither of which can be easily dismissed. Schumacher and Robinson (1987) report corroded sillimanite in the cores

of

aluminous enclaves separating sillimanite from the hornblende--orthoamphibole-plagioclase

matrix.

They suggest rhar sillimanite-hornblende-gedrite-plagioclase ^{dehned a} relatively early equilibrium assemblage which later became metastable with respect to cordierite-plagioclase

*

other aluminous phases. Similarly, Humphreys (1993) reports equilibrium assemblages including sillimanite-staurolite-cordierite-hornblende-plagioclase-qua¡tz. Since, there is no indication that either of ^these authors are in error this discrepancy ^be¡,veen the calculated ^phase relations and those obsewed in natural amphibolites probably reflects ^the relatively large uncertainties attached to the thermodynamic data from which the phase diagram was calculated.

The calculated uncertainties on ^the reactions bounding the hornblende-aluminosilicate held (Grt, St), (Grt, Oam) ^are

of

the order

of

^{150'C and thus allow the} possibility that the aluminosilicate-hornblende stability field may extend as far as 100oC into the sillimanite stabiliry held.

Further constraints on ^the relevance of the calculated CFMASH ^phase diagram to natural amphibolites might be expected from the evolving compatibility ^relations

of

amphibolites in a prograde metamorphic sequence. The only well-documented example of this type of

field

setting involves four separate outcrops of the Post Pond ^and Ammonoosuc

Volcanics (Spear, 1978). However, the compatibiliry diagrams constructed

for

each outcrop do not detail the

full

^sequence of reactions and the assemblages are thought to have equilibrated

with fluids

of variable composition (Spear, 1982). Thus even these relatively

well

described amphibolites ^are unlikely to provide ^a complete enough record to enable comparison with the calculated ^phase relations.

Similarly, reaction textures might be expected to yield information about the

applicability of the calculated grid to the phase relations in natural

rocks.

Later in this thesis

I

discuss a set of low-variance rerction textures

from

amphibolites in the Harts Range ^area

of cen¡al

Australia ^(Chapter

4).

These rocks provide evidence of the reactions:

kyanite + hornblende

=

staurolite + anorthite,

Chapter3-CFMASII- ³⁶

and

þanite

+ orthoamphibole

=

cordierite + plagiocl ^ase' kyanite + hornblende

=

cordierite + plagioclase

staurolite + hornblende

=

garnet + orthoamphibole'

However, the multi-phase metamorphic history and the complexiry of the structural relations between the compositional units in the area mean that ^the interpretation of the pressure- temperature history of these rocks is by no means

trivial.

^{Later in} this thesis the calculated

CFMASH

^grid

will

be used to interpret ^these reaction textules ^and thus the P-T history of the area (Chapter 4).

3.4.3 Phase diagrams developed by other workers

Anappealingaspectofthecalculatedgridisthatitisessentiallytopologicallyequivalent

ro rhat of Froese and

Hall

^{(1983) which} was developed

for

aluminous amphibolites' ^The Froese ^and

Hall

^{(1983) grid} was constnrcted

from

^a combination of published pressure-

temperature information

for

stable invariant points in potassium-poor pelites ^and in mafic rocks which were then linked by GFMASH ^{reactions and} invariant points using Schreinemakers

anaiysis.

Stight inco;tsistencies between the two grids stem from Froese and

Hall's

⁽¹⁹⁸³⁾ assumptions that the ^phases involved have fixed compositions, that garnet is more Fe-rich than co_existing staurolite and that orthoamphibore is more Fe-rich than the staurolite-hornblende

tie-line.

These assumptions ale

confary

^{to the phase relaúons}

in

some observed assembiages (e.g. Purtscheller ^and Mogessie, 1984; Schreyer et

al''

1984)'

Spear and Rumble ^{(1986; see} also Spear, 197S) constructed ^an

NCFMASH

^P-T Schreinemakers net for the ^phases observed

in

the Post Pond ^and Ammonoosuc Volcanics' They used fixed composition phases and the AV and AS of the reactions to determine the

slopes

of

the discontinuous reactions. The invariant points were positioned by comparison

with

peak metamorphic conditions determined from natural assemblages (Spear

&

Rumble'

1936). Because the grid calculated here and the

NCFMASH

^{gdd of Spear and} Rumble (1986) involve different compositional systems, they ^are not directly comparable (however' ^see

section 3.6

for

a qualitative extension into

NCFMASH)'

A

problem encountered in the comparison

of

the calculated ^phase relations ^{and those} determined from natural amphiborites is the continued assertion that staurolite-hornblende assemblages occur under high pressure conditions of

)

⁶ kba¡ (e'g' Selverstone et

al''

^1984;

Grew

&

Sandiford, 1gg5). The calculated ^phase diagram (Fig. 3.a) suggests that staurolite- hornblende may be stable under high

PII

ratios, but only at limited pressures' up to

approximately ⁷

kbar.

The P-T grids of Froese and

Hall

(1983) and Spear ^and Rumble (1986) also predict that hornblende-staurolite assembrages

will

^{be limited to a high}

Pft

stability freld which is closed to higher pressure by a reaction

involving

staurolite, gafnet' hornblende'

aluminosilicate(+chloriteororthoamphiboleforSpear&Rumble,1986)withanorthite,

quütz

and aqueous vapour in excess. However, there is ^a large discrepancy in the maximum ChaPter3-CFMASII- ³⁷

pressure at which staurolite-hornblende is stable. spear ^and Rumble (1986) predict that staurolite-hornblende

will

be a stable assemblage ^at plessures up to

-14

kbar, whereas the grid calculated here predicts ^a maximum pressure of

-

^{7 kbar (Fig'}

^3'a)'

^This problem is ^add¡essed in later sections.

3.4.4 ^SummarY

The comparison between calculated ^phase equilibria and those observed in real rocks or predicted by other methods is largely encouraging, suggesting that, topologically' ^the

calculated grid provides ^a reasonable approximation of ^natural rock systems' However' ^Some problems ^are

difficult

^to reconcile. The estimates

of

equilibration conditions from natural aluminous amphibolites ^are

in

general higher than pred'icted by the calculated P-T projection (e.g. for hornblende-staurolite stability) and the occ1¡rence

of

sillimanite-hornblende assemblages in natural amphibolites is also in contrast to the calculated ^phase relations' Another notabie problem relates to the existence of cordierite-garnet-hornblende assemblages

from

South

Africa.

Such discrepancies ^are probably not surprising in view of the uncertainties attached to the positions of the calculated reactions