Pseudosections and continuous reactions in the CFMASH system

CFMASH +An,Qtz,HzO

3.6 Pseudosections and continuous reactions in the CFMASH system

hornblende. Precise resolution of this problem is impossible given the poor constraints on currently available thermodynamic data. For example, the combined effects of poorly

constrained thermodynamic ^data

for

staurolite ^and hornblende and non-idealiry in garnet, with or witlrout rhe presenc e of ZnO or Fe2O3 might allow ^a P-T projection which is an adequate approximation

of

the natural amphibolites. As ^an example, Fig. 3.16 shows the effect

of

the uncertainry

in

the enthalpy of hornblende on the CFMASH grtd presented earlier (in Fig.

3.15).

Decreasing the enthalpy of hornblende (e.g. by specifying that ÀlInU = -10 kJmol-1or ÂH6 =

AII¡6 -

^-10

^kJmol-l)

expands the stability

field

of staurolite-hornblende. ^Neither

of

these adjustments is sufficient to alter the topology of the phase diagram'

Although ^the problems of resolving the inconsistencies

in

the chlorite-bearing

CFMASH

^{grid are} insurmountable, there is relatively good correspondence beween the calculated, adusted grid and the higher temperature (chlorite-free) natural assemblages' The simila¡ities between the chlorite-free grid and the adjusted chlorite-bearing grld ^are

strfüng

and consequently, except for relatively low-temperature chlorite bearing assemblages, either grid may provide a basis for interpreting natural amphibolites ^and their reaction textures. In order to facilitate this, the discussion below concerns the divariant (continuous) reactions appropriate to both

of

these gtids.

10

o

8 t-(d _o

Y È

7

6

400 500 600 ^{700 800}

T ('C)

Figure 3, staurolite field of s

lighær ^region respectivelY.

the ^enthalpie

Ky

/siil

[Crd, Oam]

i chrl

/

/

[Crd,

^Grt]

i

[Crd, Als] [Hbl,

^{Grt, An]}

/ /

Chaptcr3-Ct;MASII-

⁵⁶

(powell, 1991).

The way the mineralogy changes

with

physical conditions is of primary interest here. Thus the most petrologically useful ^phase diagram is one that is sectioned

witlt

respect to a compositional

variable.

However, bulk composition terms such ^as Xps are

extensive, not intensive, variables ^as they do not have the ^same value for all ^phases in an

equilibrium

assemblage. These ^phase diagrams which are sectioned for bulk composition are

known

as pseudosections.

The divariant and trivariant fields which dominate the CFMASH pseudosections are related to the subsystem CFASH a¡rd

CMASH

univariant reactions

in

a similar way to which

CFMASH

^{diva¡iant reactions are} related to the

FMASH

subsystem reactions in Figs. 3.3 and

3.4).

The orientations of the divariant reactions can be estimated from those of the

corresponding subsystem reactions. The continuous nature of G-X relations means that the compositions of phases

will

change consistently along the univariant lines and ^so can be calculated using equilibrium thermodynamics (e.g. THERMOCALC, version 2.2b7, Powell

&

Holland,

1988). However, ^as the adjustments made to the thermodynamic data have ^a significant effect on the compositions of the phases at a given point on ^a univa¡iant curve or at an invariant point, the continuous reactions ^seen by a given bulk composition

will

also vary

with

the thermodynamic data. As a result of this, schematic pseudosections ^are presented here.

Two

series

of

schematic pseudosections ^are presented here; one for the chlorite-absent

CFMASH

sysrem (Fig. 3.17) and one

for

the chlorite-bearing equilibria

ffig. ^3.18).

^They

were both constmcted

for

a range of compositions (Xps)

for

the CFMASH system

with

hornblende, anorthite, quartz and ^aqueous vapour

in

excess. The major difference in the topology

of

the pseudosections developed

for

the chlorite-absent ^and the chloite-bearing ^phase relations lies

in

the low temperature

equilibria. In

the chiorite-absent system,

kyanite-

hornblende appears to be stabie over ^a wide range of compositions down to very low pressures and temperatures. In contrast, the chlorite-bearing equilibria restrict kyanite-hornblende to ^a small

p-T

_{window and the}

_low

temperatue-low pressure field is dominated by the trivariant

field,

chlorite-hornblende. The assemblages predicted for other P-T ranges are similar except that the complex

cenÍal

portion of the chlorite-bearing CFMASH system (enlarged in Fig.

3.19) occurs about 3 kbar higher than that in the chlorite-absent system, ^{as a} result

of

the different locations

of

the invariant equilibria

in

the

two

systems ^(See Figs. 3.4 and 3.15)" The d.iagrams predict that assemblages

involving

garnet-homblende

will

dominate the intermediate to high pressure and ternperature field for rocks

with

high Xps ^and

will

contract to successively higher conditions with increasing

Mg-content.

Cordierite-homblende assemblages are stable up to pressures

of

about 5 kbar for low

Xp"

rocks but refreat to very low pressures

with

increasing Fe-content. Orthoamphibole-hornblende is limited ^to relatively high temperatures and occur over a wide range of

Xps.

Staurolite-hornblende ^is only stable under high PÆ ratios, and is most stable

in

^a small P-T range in rocks of intermediate Xpg'

Chapter3-CFMASIt- ^:;7

Figure 3.17. Schematic pseudosections for the chlorite-absent CFMASH sysrem (Fig. 3.4) with homblende, anorthite, quartz and aqueous vapour in excess, showing the changing topology of the diva¡iant and trivariant fields with decreasing

X¡".

a) Fe-rich assemblages; b, c and dare for

successively lower Xp" bulk compositions; e) Mg-rich rocks; l) CMASH endmember system.

Ky Grt +

Fe = ^0.9 I

6

4

2 8

b

4

2 12

10

0

12

10

0 P (kbars) 12

10

12

10

0

12

'10

0

12

10

I

6

I

b

4

2 4

2OO 4oo 600 8oo

^{1 ooo}

2OO 400 600 800

^{1 000}

2

2OO 4OO 600 8OO

^{1 000}

2OO 4OO 600 8OO

^{1 ooo}

2OO 4OO 600 8OO

^{I 000}

8

I

6

4

2 6

4

2

0 0

1 000 T l"c'l

Grt + Oam

+ Oam

, OE, An,

Grt + ^Ky

+

Fe = 0.7

Oam

+ Hbl, Qu,An, V

Grr + Ky

Grt + Oam

+ Oam

= ^0.5

Oam

X

+ Crd

Grt + Oam An, V

Grt + ^Ky

Grt + St

+ Crd

= ^0.4 X

Grt + Oam Grt + ^Ky

+ Oam

e = 0.15 X

+ Hbl, Olz, An, ^V

Ky

f

r CFMASH

* CFASH

* CMASH

/ ^Grl

Crd

Oam

,<

XFe=o 2oo 400 600

⁸⁰⁰

Chaptcr3-CFMASIf

-

58

Figure 3.18. Schematic pseudosections for the chlorite-bearing CFMASH system (Fig.3.15) with hornblende, anorthite,

qlqn

alq aqueous vapour in excõss, showing ttre ctrangiîg topálogy of ttre divariant and t¡ivariant fields with decreasing-Xp". a) Fe-rich asserÙl"g"s; ¡, c]

¿ä¿

e are for successively lower X¡" bulk composirions; f) Mg-rich rocks.

10

8

L(ú _o

Y

o- 6

10

500 600

^{700 600}

T ("C)

600

600

600

CÚ

_ol¿

o-

I

6

600

T ^("C)

Chapter3-CFMASII- ⁵⁹

(d _o

Y

o- 10

7

10

7

t-(d _o

o-

600 600

T ('C)

St

600 600

T ('C)

Figure 3,1g. Blow-up ol rhe compiex cenEal portion of lhe chlorire-bearing CFMASH pseudosections in Fig' 3.18.

Chapter3-CFMASII- 60

3.7 ^ qualitative extension into NCFMASH

This section discusses the qualitative extension

of

the CFMASH phase diagrams into

NCFMASH.

The addition of Na ro the

CFMASH

equilibria is ^assumed to increase the variance

of

a given assemblage, without causing ^a new phase to be stabilised. The relative proportions of sodium in natural amphibolite phases gives ^an indication of the extent to which the different ^phases accommodate sodium; plagioclase is typically more Na-rich than

hornblende, which is more Na-rich than orthoamphibole, ^and the remaining ^phases contain negligible sodium. Thus, the addition of Na to the CFMASH equilibria is assumed to stabilise plagioclase over hornblende, hornblende over orthoamphibole, ^and orthoamphibole over all other phases. Hornblencl.e and anorthite generally occur on opposite sides of

CFMASH

equilibria, so expanding the stability

field

of plagioclase often reduces that of hornblende. As a

resulr, the

NCFMASH

equilibria emanating from the univariants in Fig. 3.15 a¡e located between the anorthite-absent (An) and orthoamphibole-absent (Oam) CFMASH univa¡iants,

effectively

expanding the stability fields of those phases. 'With homblende, plagioclase, quartz and aqueous vapour

in

excess, these univariants intersect in an

NCFMASH

^inva¡iant

point; [Crd] ^(Fig.

^3.20).

The topology,and reactions in Fig. 3.20 are broadly consistent

with

those

of

Spear's (197g; Spear

&

^Rumble, 1986) corresponding invariant

point.

However, ^as mentioned earlier, the

CFMASH

and thus

NCFMASH

^{phase relations are} not entirely consistent

with

the phase relations observed

in

natural amphibolites. In

NCFMASH,

the stability of only ^a single invariant point [Crd] in NCFMASH implies ^that cordierite-homblende

will

never be a stable

assemblage

in

amphibolites, however, this assemblage is observed from several localities

including

the Ha¡ts Range ^(see Chapter 4), South

Africa

(Humphreys (1993)' southwestern New Hampshire (Schumacher

&

Robinson, 1987) and the Pamirs in the former USSR (Grew et al., 1988). Thus the

NCFMASH

^{invariant and} univariant equilibria

will

be discussed further

in

Chapter 6, in terms

of

the phase relations

in

natu¡al amphibolites.

In the

following

chapters the calculated

CFMASH

grids and qualitative

NCFMASH

phase diagrams developed here

will

be used to interpret the signihcance of aluminous amphibolite assemblages

from

^a wide variety of localities, beginning with the Harts Range, central Australia.

Chapter3-CFMASII-

⁶¹

[Crd, ^Grtl

Sl Hbt4a

[Crd]

[Crd, Oam]

o₁ I

a

[Crd, Chl]

[Als, Crd]

(st)

(SD

(s0

oarn^An

NCFMASH schematic + Hbl, ^Pl, Qtz,

H2O

P

T

Chaptcr3-CFMASI{-

⁶²

Chapter ^4z Metamorphism in the Harts Range region,

eastern Arunta Inlier.