Aluminum silicates in the Mount Raleigh pendant, British Columbia

J. metamorphic Ceol., 1989, 7, 547-563

Aluminum silicates in the Mount Raleigh pendant, British Columbia D. M. KERRICK Demrtment of Geosciences, The Pennsylvania State University, University Park, Pehnsylvania 16802, USA G. 1. W O O D S W O R T H Geological Survey of Canada, 1 0 West Pender Street, Vancouver, British Columbia V68 1R8, Canada

ABSTRACT In regionally metamorphosed pelites of the Mount Raleigh pendant, the tibrolite isograd occurs 5 km downgrade ftom the silhanite isograd. Fibrolite formed from the decomposition of biotite, a reaction that probably resulted from the late-stage influx of acidic volatiles. In contrast, sillimanite formed by the direct, 'volume-for-volume' replacement of andalusite. Andalusite and sillimanite coexist in a 3 km-wide mne above the sillimanite isograd. Electron probe analyses of these phases reveal low minor element contents and yield KD[=Pwo,/X~iol] values close to unity; the low F%03 contents are compatible with reducing conditions implied by the ubiquity of graphite. Because K, = 1.0, the zone of coexisting andalusite + sillimanite Cannot be attributed to multivariancy resulting from partitioning of minor elements between these phases. Rather, the metastable persistence of andalusite into the sillimanite P-T stability field is suggested. The modal proportions of sillimanite versus andalusite imply that minimal (4%) andalusite+sillimanite reaction occurred in a zone 1.5km above the sillimanite isograd; in contrast, there was a marked increase in reaction progress immediately above this zone. With an estimated thermal gradient (in the plane of exposure) of approximately 20" C/km, the 1.5 km-wide zone of nil reaction suggests that the andalusite-sillimanite equilibrium boundary was overstepped by about 30 "C before significant reaction occurred. Inclusion-rich areas in andalusite provided favourable sites for sillimanite nucleation; however, the growth of sillimanite may have been impeded by 'pinning' of sillimanite grain boundaries by inclusions.

Key w o d x Andalusite; fibrolite; graphite; metapelite; metastability; sillimanite; solid solution.

List 0f.b~vistiOrs

a =activity Ab =albite Alm =ahandine An =anorthite And =andalusite Ann =annite Bt =biotite Chl =chlorite Crd = cordierite F, Fib = fibrolite Grs =grossularite Grt =garnet KD = distribution coefficient

INTRODUCTION

Because the aluminum silicates continue to be of paramount importance in metamorphic thermobarornetry, it is important to pursue detailed field studies bearing on the nature of reactions involving the AISiOs polymorphs and the stability relations of these phases. In particular, we need to assess further the validity of an equilibrium model for AlaiO, thermobarornetry. Recent studies of the senior author emphasize the importance of such investigations.

K-3 K-fS KY Ms Phl PI prp Qtz Sil Sps St X Y

= equilibrium constant = potassium feldspar = kyanite = muscovite = phlogopite = plagioclase = PYroPe = quartz = sillimanite = spessartine = staurolite = mole fraction = activity coefficient

For example, additional studies are needed to evaluate further whether fibrolite formed at equilibrium (Kemck, 1987) and to assess the role of minor element crystalline solution on the andalusite-sillirnanite equilibrium (Kemck & Speer, 1988). Because of the small AS,, and AVb,, the andalusite-sillimanite equilibrium is particularly susceptible to relatively small perturbations in the Gibbs free energies of these phases. In view of the sluggish kinetics of reactions with small AG- (Fisher & Lasaga, 1981), isograd reactions involving andalusite and silliman-

517

548 D. M. K E R R I C K & C. J . WOODSWORTH

ite (or fibrolite) could be plagued by kinetic problems. Thus, field studies related to the andalusite-sillimanite equilibrium are particularly needed to further our The Mount Raleigh area is situated in the Coast understanding of AI,SiO, thennobarometry in contact Mountains of south-western British Columbia, Canada, metamorphism and low-pressure regional metamorphism. about 200 km north-west of Vancouver. Rocks in this area The Mount Raleigh area provides a well-exposed example are part of a discontinuous belt of low-pressure regional of the latter paragenesis. metamorphism that extends for 8OOkm from the North

GEOLOGICAL SETTING

1 24'1 9' 124'1 6

50'

50

'54.

"56'

Fig. 1. Geological map of the Mount Weigh area (after Woodsworth. 1979). Abbreviations for the isograd index minerals are: And = andalusite, Crd = cordierite, Fib = fibrolite, Sil = sillirnanite, St = staurolite. The staurolite, cordierite and andalusite-out isograds are from Woodsworth (1979) whereas the fibrolite and sillimanite isograds were determined in this study. The fibrolite and staurolite isograds are virtually identical at the scale of this map. Mineral assemblages of the samples are given in Table 1.

A L U M I N U M SILICATES, M O U N T RALEIGH PENDANT 519

of Washinaon north-west along the east side of the coast plutonic Complex (Evans & Berti. 19%; Greenwood, Woodsworth, Reed & Ghent, in Press).

The Mount Raleigh pendant consists mainly of Mesozoic (?) intermediate to mafic volcanics and volcaniclastics, aluminous greywacke and minor conglomerate (Wood- sworth, 1979). The total structural thickness of the stratified rocks is about 4000m. The pelitic rocks are the subject of this study and are largely confined to the Lower Cretaceous Mount Eurydice and Styx formations (Fig. 1) that form the lower half of the structural succession.

Strata in the pendant have undergone one period of regional metamorphism (Woodsworth, 1979). Metamor- phic grade increases from northeast to south-west (Fig. 1). Metapelites range from sub-staurolite zone assemblages (typically andalusite + garnet + biotite + muscovite) to cordierite-bearing sillimanite-zone rocks that locally contain K-feldspar (Woodsworth, 1979).

The stratified rocks are completely surrounded by plutonic rocks. Plutons on the south side of the pendant were emplaced during metamorphism and deformation in mid to Late Cretaceous time; plutons on the north and east

'sides of the pendant were intruded in the early Tertiary and postdate regional metamorphism and deformation.

Several lines of evidence suggest that the metamorphism is related to regional thermal gradients rather than to the effects of nearby plutons. There are no obvious spatial relations between plutons, as presently exposed, and the isograds. The large (30 km diameter) pluton north of the pendant (Fig. 1) was intruded after metamorphism and produced a contact aureole only a few hundred meters wide (Woodsworth, 1979). Isograds strike roughly perpendicular to plutons west of the pendant; these plutons predate metamorphism and deformation and may be the basement to the pendant strata (Woodsworth, 1979). The increase in metamorphic grade from northeast to south-west might suggest an intrusive heat source south of the pendant. In this case we would expect the isograds to parallel the contact between the pendant and the intrusives to the south. However, contacts between the plutons and the pendant on the south are steeply dipping, whereas the staurolite isograd, which is exposed over a vertical distance of approximately 1OOO m between sample localities 72-94 and 72-107 (Fig. l), dips about 30" to the south-west. Many of the metamorphic rocks have a well-developed south-south-west-plunging stretching lineation (shown primarily by stretched pebbles and the long axes of aligned andalusite and hornblende) that formed synchronously with metamorphism (Woodsworth, 1979). As shown by alignment of hornblende crystals and long axes of inclusions, the plutons south of the pendant have a weak lineation parallel to that of the pendant rocks. This lineation parallels foliation but is oblique to the pluton-pendant contacts, thereby strongly suggesting that ductile deformation and mineral growth post-doted emplacement of these plutons. At the present level of exposure, metamorphism thus appears to be independent of nearby plutons. On a regional scale, metamorphic grade in pendants of

the Coast Plutonic Complex increases from sub-greenschist

facies on the south-west through greenschist facies to a broad zone of amphibolite facies rocks (including the Mount Raleigh pendant) in the east-central Coast Plutonic Complex (Greenwood et al., in press). This pattern is similar to that well documented in the Coast Plutonic Complex about 400km north-west (e.g. Crawford. Hollister & Woodsworth, 1987) where metamorphism also appears to be independent of plutonism. No detailed structural studies have been made on pendants south-west of Mount Raleigh. However, metamorphic rocks along the east flank of the Coast Plutonic Complex about 50km north-north-west of Mount Raleigh have the same south-west-plunging stretching lineations as those in the Mount Raleigh pendant (Rusmore & Woodsworth, 1988). Metamorphism in both areas is thought to be Late Cretaceous (Rusmore & Woodsworth, 1988); the same style of Late Cretaceous metamorphism and south-west- plunging stretching lineations are found elsewhere near the east edge of the Coast Plutonic Complex (E. D. Ghent & G. J. Woodsworth, unpublished data). The conclusion that metamorphism in the Mount Raleigh pendant is part of a large-scale regional pattern is thus supported by relation- ships both within the pendant and on a regional scale.

METAMORPHISM OF PELlTlC ROCKS: A N OVERVIEW

From petrographical examination of over 300 samples of pelitic rocks from the Mount Raleigh pendant, which was supplemented by X-ray, bulk-rock chemical, and electron probe analyses, Woodsworth (1979) provided a detailed description of metamorphic mineral assemblages and prograde metamorphic reactions in pelitic rocks within the Mount Raleigh pendant. Woodsworth's (1979) study is supplemented by mineral assemblage data from the present study (Table 1).

Woodsworth (1979) concluded that the staurolite isograd is represented by the net reaction:

chlorite + muscovite = staurolite + biotite + garnet + quartz + H,O.

In addition to the appearance of staurolite, this reaction is supported by a large decrease in the modal abundance of muscovite, coupled with the disappearance of primary chlorite. In the present study we have found that, at the scale of Fig. 1, the first appearance of fibrolite is essentially coincident with the first appearance of staurolite. Thus, we have added fibrolite to Woodsworth's (1979) staurolite isograd (Fig. 1).

The cordierite isograd (Fig. 1) is characterized by a marked decrease in the modal amount of staurolite coupled with a significant increase in the modal amount of garnet. Accordingly, Woodsworth (1979) concluded that the cordierite isograd is represented by the reaction:

staurolite + quartz = sillimanite

+ cordierite + garnet + H,O.

550 D. M. K E R R I C K & G. 1. WOODSWORTH

Sample Table 1. Mineralogy of metapehtes from number7 And Sil Fib Bt Grt St Crd chl Ms Qtz PI the Mount Raleigh pendant’

x x 70-429- 1 X x x 70-49&ElX x x x x 70631-El X X X x x x x 70-631-EZ X X x x X 71-60-2 x x x x x X x x x 71-60-3 x x x x x x x x 71-60-4 x x x x x 71-606 x x x x x x x 71-63-2 x x x x x x 71-68-1 x x x x x x X X 71-83-9A x x x x x x 71-131-1 X X x x x 71-1461 X x x x x x 72-1-1 x x x x x x 72-1-4 x x x x x x x 72-7-6 X x x X X 72-11-1 x x x x x X X 72-15-1 x x x x x x 72-26- 1 x x x x x x x x 7 2 - 5 2 x x x x x x x x X 72-38-2 x x x x x x x 72-384 x x x x x x x 72-94-3 X x x x x x x 72-107-7 X x x x x x 72-113-2 X x x x X x x

x x

x x x x

Samples in this table are those discussed in text, listed in succeeding tables,

t The numbers following the second hyphen refer to multiple samples taken and plotted in figures.

from a single locality (e.g. 7160-2, 71-60-3,71-60-6). ThC procedure for numbering samples is further discussed in Woodsworth (1979, p. 3).

Because of the relatively high zinc content of the staurolite, this reaction may be multivariant, thereby explaining why staurolite persists upgrade from the cordierite isograd (Woodsworth, 1979). However, downgrade from the wrdierite isograd, we find no petrographical evidence that staurolite or garnet were invoived in reactions forming the aluminum silicates.

Although a sillimanite + K-feldspar isograd was not mapped, Woodsworth (1979) found this subassemblage in several specimens within high-grade rocks. He noted that the assemblage sillimanite + K-feldspar first appears at sample locality 71-83 (Fig. 1) and that the assemblage muscovite + quartz occurs at lower grades. Consequently, this isograd would be located approximately 200m north of the andalusite-out isograd (Fig. 1).

All samples studied by Woodsworth (1979) contain ilmenite + graphite + apatite +zircon + pyrrhotite f rutile f tourmaline. Some samples contain significant amounts of graphite. X-ray analysis by G. J. Woodsworth (unpublished) shows that the graphite is well ordered [the (002) basal reflection is virtually identical to well-ordered graphite].

A L U M I N U M S I L I C A T E S : D I S T R I B U T I O N A N D T E X T U R A L R E L A T I O N S

Fibrolite, coarse prismatic sillimanite and chiastolitic andalusite are widespread in the Mount Raleigh pendant. The first appearance of fibrolite essentially coincides with

the staurolite isograd, whereas the sillimanite isograd is about 5 km upgrade from the fibrolite isograd (Fig. 1). In most samples, fibrolite and sillimanite are readily distinguished. In samples containing both phases, there is a distinct bimodal distribution in grain-size between fibrolite, with fibre diameters less than about 3microns. versus coarse, prismatic sillimanite, with minimum dimensions typically exceeding 100 microns. Hereafter, the term sillhanite will be used for coarse, prismatic sillimanite. In distinguishing between fibrolite and sillimanite we have adopted Kemck & Speer’s (1988) subjective definition; i.e. crystals with minimum dimensions less than 10pm are considered to be fibrolite whereas those exceeding 10 pm are sillimanite.

Andalusite occurs as chiastolite porphyroblasts. In some cases, chiastolite contains inclusion-rich cores (Fig. 2a) whereas other crystals have inclusion-free cores (Fig. 2b). The chiastolite ‘cross’ vanes from relatively thick bands (Fig. 2a) to narrow, inclusion-filled zones (Fig. 2b). Many of the chiastolite crystals have thin, elongated inclusions of quartz (Fig. 2a and b) that are consistently oriented within each of the four sectors. Within each sector, the long direction of the quartz inclusions bisects the angle formed by the bounding inclusion-filled borders of the sector (Fig. 2b). Assuming the quartz inclusions to have a tabular form in three dimensions, their planar dimensions would be parallel to (110) of the host andalusite.

Sillimanite occurs as relatively inclusion-free, subhedral, prismatic crystals. In most cases, sillimanite occurs as

A L U M I N U M S I L I C A T E S , MOUNT R A L E I G H P E N D A N T 551

m. 2. Photomicrographs of metapclites from the Mount Raleigh area. (a) Chiastolite porphyroblast with inclusion-rich core. Note elongated quartz inclusions within each of the four sectors. Sample 71-63-2. Width of photo (long dimension) corresponds to 2.74 mm. (b) Chiastolite porphyroblast with inclusion-free core (centre of photo) and elongated quartz inclusions in each of the four sectors. The thin, indusion-rich borders of the four sectors extend vertically and horizontally from the core. Sample 72-113-2. Width of photo corresponds to 2.74 mm. (c) Crystals of prismatic sillimanite (light areas showing cleavage) enclosed in a single crystal of andalusite. Sample 71-60-4. Width of photo corresponds to 2.74 mm. (d) Prismatic sillimanite (dark areas) enclosed in a single crystal of andalusite (light matrix). Sample 7 1 W . Width of photo corresponds to 2.74 mm. (e) Single crystal of biotite (centre of photo) rimmed by fibrolite (light). This represents the incipient fibrolitization of biotite. Sample 71-83-9A. Width of photo corresponds to 0.685 mm. (9 Biotite (dark) replaced by Ebbrolite (light). This represents an advanced stage in the fibrolitization of biotite. Sample 72-38-2. Width of photo corresponds to 1.37 mm.

5sL D. M. K E R R I C K & C . J . WOODSWORTH

Fig. 2. (conrinued.) (g) Unaltered andahsite porphyroblast (left half of photo) adjacent to fibrolitized biotite (right half of photo). Most of the dark area in the right half of the photo is biotite: the light fibrolite patches within biotite are noted with white arrows. Sample 7146-1. Width of photo corresponds to 1.37 mm. (h) Unaltered prismatic sillimanite (S) with adjacent fibrolitized biotite [the light areas surrounded by biotite (dark) are fibrolite]. Sample 71-606. Width of photo corresponds to 1.37 mm. (i) Sillimanite crystals (dark areas noted with S) within an inclusiou-rich core of chiastolite. Sillimanite is absent in the surrounding inclusion-poor portions of the chiastolite. Sample 71-63-2. Width of photo corresponds to 2.74 mm. (j) Sillimanite (dark) within a single chiastolite crystal. Note that the sillimanite crystals within the inclusion-rich core of the chiastolite (left half of photo) are considerably smaller than those in the inclusion-poor area (right half of photo). Sample 71-60-3. Width of photo corresponds to 2.74 mm.

oriented crystals enclosed in andalusite (Fig 2c and d), thus providing cogent evidence that sillimanite directly replaced preexisting andalusite. As discussed by Woodsworth (1979, p. 36), the e-axes of these phases are nearly parallel, and (010) of the sillimanite virtually parallels (100) of andalusite. The molar volumes of these phases differ by about 3%; thus, this transformation closely approximates a ‘volume-for-volume’ replacement reaction.

In fifteen of the thirty fibrolite-bearing thin sections examined in this study, fibrolite is intimately intergrown with biotite (Fig. 2e, f , g and h). Incipiently fibrolitized biotite is characterized by thin fibrolite rims and/or splays of fibrolite penetrating biotite. Samples with extensively fibrolitized biotite contain small, isolated remnants of biotite in the same optical orientation. Figure 2 parts (e) and (0 respectively record incipient and advanced stages of the replacement of biotite by fibrolite. In most specimens, fibrolite is not intergrown with the coexisting

aluminum silicate(s) (Fig. 2g and h). Thus, as in the study of Kemck (1987). textural evidence provides compelling evidence that fibrolite replaced biotite. In three of the highest grade samples (72-1-1, 72-11-1 and 72-6@2), prismatic sillimanite is surrounded by fibrolite. IR these samples, there is no textural evidence suggesting whether there was a direct reaction relationship between these phases or the relative timing of growth of sillimanite versus fibrolite.

We have found no mineralogical or textural evidence suggesting that other phases were involved in the transformation of biotite to fibrolite. Carmichael (1969) and Foster (1978) suggested that polymorphic isograd reactions involving the aluminum silicates involved coupled exchange reactions between subdomains. One of their subdomain reactians involves replacement of the reactant aluminum silicate by muscovite. However, in most fibrolite-bearing specimens from the Mount Raleigh

A L U M I N U M SILICATES, MOUNT RALEIGH P E N D A N T 553

pendant, andalusite and/or sillimanite lack intergrowths or rims of muscovite. Furthermore, fibrolite is absent in those specimens displaying musmvitization of andalusite. Anda- lusite in samples containing abundant fibrolite typically shows no textural evidence of resorbed grain boundaries; thus, we contend that andalusite was not a reactant phase in the fibrolite-forming reaction. Numerous studies have proposed that staurolite and garnet were reactants in fibrolite-forming prograde metamorphic reactions (e.g. Thompson, 1976; Pigage, 1982; Holdaway, Guidotti, Novak & Henry, 1982; McLellm, 1985; Hollocher, 1987). However, in the Mount Raleigh area there is no textural evidence indicating that staurolite and/or garnet were reactant phases. In rocks with abundant staurolite and/or garnet coexisting with fibrolite, these ferromagnesian phases show no evidence of resorption or replacement by other phases.

ANALYTICAL M E T H O D S

Electron probe analyses were carried out with the Etec Autoprobe at the Pennsylvania State University using mineral standards and a ZAF data reduction procedure. A kyanite standard was used for A1 and Si, whereas other elements were standardized using a variety of other silicate standards. Each of the mineral analyses given in Tables 2-5 represents an average of five to ten individual (point) analyses.

For fibrolite, electron probe analyses were camed out in optically phase-pure regions consisting of myriads of interlocking fibrolite crystals. The low K20 content of the fibrolite suggests that the analysed areas contain virtually no biotite. This compositional data, coupled with the lack of opaque minerals (as determined by petrographical examination with both transmitted and reflected light) suggests that F%O3, the dominant minor element, is present in crystalline solution rather than in other phases.

ANALYSIS A N D INTERPRETATION OF DATA

The Mount Raleigh pendant contains broad zones of coexisting aluminum silicates: andalusite - and sillimanite coexist over a zone about 3.5 km wide (measured normal to isograds), whereas andalusite + fibrolite coexist over a zone about 8 km wide.

To evaluate whether the zones of coexisting aluminum silicates reflect multivariance due to minor element solid solution, electron probe analyses were carried out on coexisting aluminum silicates. Following the approach of Kemck & Speer (1988), it is assumed that minor elements are confined to the octahedral site in andalusite, sillimanite and fibrolite. Accordingly, these crystalline solutions are assumed to be binary mixtures of the components: MAISiOs (where M = 'impurity' cations) and Al$i05.

For the andalusite-sillimanite equilibrium with impure phases, the equilibrium constant (K,) is;

Adopting the standard state as unit activity at P and T, we have

Kcq = [X&x,,/x~$o, ] [ Y S s / Y S O J or:

The KD[ = ~ w s / X ~ i o , ] values for andalusite- sillimanite pairs range from 0.997 to 1.007 (Table 2). If we make the reasonable assumption of ideal mixing (Kemck & Speer, 1988), K, (the equilibrium constant) = KD (the distribution coefficient). Considering that the XAW, values in Table 2 were computed as 1 - XMui0,, rehtiue analytical errors of f3%, which we consider to be reasonable for electron probe analyses of minor elements, will introduce negligible errors in our computed KD values. Figure 3 shows that, compared to the andalusite- sillimanite equilibrium involving the pure phases ( KD = l.O), KD values in the range 0.997-1.007 would result in a relatively small shift (less than about l5OC) of this equilibrium. In a separate investigation of boron in the aluminum silicates (D. M. Kemck & C. A. Houser, unpublished data), andalusite and sillimanite from the Mount Raleigh pendant have negligible boron contents. Thus, boron introduces insignificant bivariancy to the andalusite-sillimanite equilibrium in this pendant. Elec- tron probe traverses across andalusite and sillimanite crystals in several samples show that these phases are essentially unzoned. Furthermore, the small, inclusion-free cores that are present in numerous andalusite porphyroblasts (Fig. 2b) have minor element concentrations identical to the exterior portions. Thus, andalusite and sillimanite crystals underwent no significant change in composition during their growth. These compositional data together with textural data showing that andalusite was replaced by sillimanite (Fig. 2c and d), strongly suggest that the sillimanite isograd corresponds to the univariant andalusite = sillimanite equilibrium. Andalusite immediately below the sillimanite isograd is close to end member composition. Considering a model of progressive metamorphism, the low minor element content of such 'precursor' andalusite also supports the contention that the sillimanite isograd correlates with the univariant andalusite-sillimanite equilibrium.

In contrast, textural evidence strongly suggests that andalusite was not a reactant in the fibrolitization reaction. In specimens containing intensely fibrolitized biotite there is no textural evidence that andalusite is corroded. Thus, we conclude that the fibrolite-forming reaction cannot be attributed to the andahsite-+ fibrolite reaction, such that fibrolite formation cannot be correlated with an AI,SiO, polymorphic reaction.

"he KD values of andalusite-fibrolite pairs (Table 2 ) display more variation than those of coexisting andalusite and sillimanite. For example, samples 70-631-El and 70-631-E2 were collected at the same locality but have distinctly different KD values (OM75 and 1.002) for andalusite-fibrolite pairs (Table 2). The 75" C temperature difference implied by these KD values (Fig. 3) is unreasonable for samples collected at the same locality.

K,= KD X K,.

551 D. M. K E R R I C K & G. J . W O O D S W O R T H

Sample: Phase:

SiO,

XO, MgO Fez03

VZO,

G O

"Z03

cr203

h4nO

SUm

Si Al Ti Fe Y+ Cr V h4n K Sum

x,,io, Xwios

70-429-1 And

36.20 63.33

0.04 0.16 0.03 0.W 0.01

99.81

-

-

0.980 2.021

0.002 0.003 0.001 0.001

-

- -

3.007

0.006 0.994

And

36.14 63.37

0.02 0.19 0.01 0.07 0.02

99.83

-

-

0.978 2.002

0.001 0.004

0.001 0.001

3.007

0.007 0.993

-

-

-

And 36.73 63.07 - - 0.29 0.03 0.06 0.00

100.19 -

0.990 2.005 - -

0.006 0.001 0.001 - -

3.003

0.008 0.992

Tabk 2. Electron microprobe analyses of aluminum silicates

70-498-E2 70-631-El 70-631-El 70-631-El 70631-E2 70-631-E2 70-631-E2 71-63-2 71-63-2 Fib Sil And Fib sil Fib Sil 38.25 61.49

0.00 0.17 0.02 0.07 0.01 0.01

100.02

-

1 .m 1.953 - -

0.003

0.001 - - -

2.989

36.00 63.49

0.00 0.34 0.02 0.07 0.01

100.34

-

-

36.55 63.33

0.03 0.42 0.07 0.01

-

0.02

100.44 -

35.79 60.79

0.17 1.25 0.07 0.05 0.05 0.01

98.17

-

Cations based on 5 oxygens 0.981 0.984 0,989 2.016 2.010 1.980

- 0.001 0.007 0.007 0.009 0.026

0.001 0.001 0.001 - 0.001

0.001 0.001

3.006 3.006 3.006

- - -

-

- - - -

0.006 0.009 0.012 0.037 0.994 0.991 0.988 0.963

,36.21 62.92

0.02 0.17 0.01 0.03 0.02

99.38

-

-

0.984 2.016

0.001 0.004

-

- - - -

3.005 0.005 0.995

35.29 62.90

0.05 0.25 0.06 0.01 0.02 0.09

98.67

-

0.968 2.034

0.002 0.005 0.001

0.001 0.003 3.014

0.012 0.988

-

-

36.97 62.62

0.02 0.21 0.03 0.02

-

- -

99.88

0.999 1.995

0.001 0.004 0.001

-

- - -

3.000

0.006 0.994

Tabk 2. (Continued)

Sample: 71-68-1 Phase:' And

AlzO3 62.73 Si02 36.57

XO, - Mgo Fez03 cr203 0.10

KZO -

- 0.23

0.05 - v2°5 MnO

Sum 99.70

Si 0.991 Al 2.004 rl -

Cr 0.002 V 0.001 Mn - K - Sum

- 3+ 0.005

3.002

71-68-1 Fib

37.36 62.49

0.06 0.25 0.07 0.11 0.01

100.35

-

-

1.005 1.982

0.002 0.005 0.002 0.002

-

- -

2.998 0.011 0.989

71-68-1 Sil

37.03 62.49 0.01

0.35 0.07 0.01 0.07

100.03

-

-

1 .Ooo 1.990 - -

0.007 0.002

0.002

3.000 0.011 0.989

-

-

71-131-1 71-131-1 72-26-1 72-26-1 And Fib And Fib

36.39 63.17

0.02 0.24 0.02 0.06 0.01

-

-

36.93 64.12 - - 0.11 0.03 - - -

36.85 63.29

0.02 0.19 0.04 0.03 0.01

-

-

36.87 62.69

0.42 0.23 0.05 0.05 0.01 0.03

-

99.91 101.19 100.43 100.35

Cations based on 5 oxygens

0.984 0.985 0.991 0.993 2.014 2.017 2.006 1.990

0.001 - 0.001 0.017 0.005 0.002 0.004 0.005 - 0.001 0.001 0.001

0.001 - - 0.001

- - - 0.001 3.005 3.005 3.003 3.008

0.007 0.003 0.006 0.025 0.993 0.997 0.994 0.975

- - - -

- - - - --- K r = 0.997 K$ = 0.997 K F = 1.004 KCF = 0.981

72-26-1 Sil

36.72 63.29

0.01 0.18 0.02 0.02 0.05

100.29

-

-

0.989 2.009

0.001 0.004

-

- -

0.001

3.004 0.006 0.994

-

71-60-2 And

36.39 62.97

0.03 0.24 0.01 0.01 0.01

99.67

-

-

0.986 2.012

0.001 0.005

-

- - - -

3.005

71-60-2 Fib

36.12 60.98

0.04 0.30 0.02 0.01

0.02 97.49

-

-

1 .OOo 1.991

0.002 0.006

-

- - -

0.001 3.001

71-60-2 Si

36.31 62.88

0.01 0.19

0.01 0.01

99.41

-

-

-

0.986 2.014 - -

0.004 - - - -

3.005 0.007 0.009 0.993 0.991 i::: c

A L U M I N U M SILICATES, MOUNT R A L E I G H P E N D A N T 555

T.#C 2. (Co-d)

Sample: 72-74 Phase:. And

SiO, 36.17 62.58

MRO 0.07 -

Si 0.m Al 1.993 Ti

0.003 0.033 0.001 Cr

V - Mn 0.001 K SUm 3.009

XUA~+IO, 0.038 X ~ I & ~ 0.962

-

3,

-

72-74 And

38.97 60.53

0.10 0.71 0.02

0.02 0.05

100.41

-

-

1.048 1.918

0.004 0.014

-

- -

0.001 0.002 2.987 0.021 0.979

72-11-1 And 36.35 63.22

0.04 0.19 0.01

-

- - -

99.82

0.984 2.017

0.002 0.004

-

- - - -

3.006 0.006 0 . M

72-11-1 72-11-1 72-15-1 72-15-1 Fib Sil A d Fib

36.27 60.85

0.53

0.05 0.04

0.02 98.52

- o.n

-

0.W 1.972

0.022 0.016 0.001 0.001

0.001 3.008

-

-

36.33 63.06

0.01 0.17 0.03 0.01 0.02

99.64

-

-

36.85 63.34 - - 0.24 0.01 0.01 0.01

100.49 -

36.67 63.05 - - 0.13 0.06 0.01 0.01 0.03 99.97

Cations based on 5 oxygens 0.985 2.015 - -

0.004 0.001 - - -

3.005

0.990 2.007 - -

0.005 - - - -

3.003

0.990 2.008 - - 0.003 0.001 - -

0.001 3.004

0.040 0.005 0.006 0.006 0.960 0.995 0.994 0.994

72-15-1 Sil 37.16 62.82 - - 0.20

0.04 0.01

100.23

-

-

Loo0 1 .m - - 0.004

0.001 -

- -

2.999 0.005 0.995 --

K F = 0.966 KF = 0.998 K F = 1.001

Phase abbreviations: And = andalusite, Fib = fibrolite, Si = sillimanite.

KEF = ~ ~ & / X ~ m . Kf Pi-IX,+eo5. And

T.Me 3. Electron microprobe analyses of garnet

Sample: 71-161 SiO, 38.67

2.16 FeO 30.58 Cr203 0.04 NiO 0.01 MnO 6.08 CaO 1.84 sum 100.33

Si 3.086 Al 1.971 Ti -

0.257 2.041 0.003 Cr

Ni 0.001 Mn 0.411 ca 0.158 Sum 7.927

3+

X A h 0.712 0.090 0.055

XSDa 0.143

x, XG,

7140-2 37.99 20.87

1.85 34.42 0.03 0.02 4.30 0.82

100.30

-

3.059 1.981

0.222 2.318 0.002 0.001 0.293 0.071 7.949 0.798 0.077 0.024 0.101

-

71-603 38.53 21.64

2.19 30.33 0.04 0.01 7.05 0.88

100.67

-

3.064 2.028

0.260 2.017 0.m 0.001 0.475 0.075 7.921 0.714 0.092 0.m 0.168

-

7143-2 71-68-1 72-1-4 72-36-2 37.74 20.98

2.22 33.93 0.03 0.04 4.38 0.78

100.10

- 37.65 20.99

1.85 38.56 0.02 0.01 0.82 0.28

100.18

-

Cations based

3.042 3.043 1.994 2.000

0.266 0.223 2.287 2.607 0.002 0.001 0.003 0.001 0.299 0.056 0.067 0.024 7.960 7.956

- -

37.93 22.01

2.03 31.98 0.03 0.04 5.30 1.37

100.70

- 38.20 20.57

2.73 36.24 0.02 0.01 0.65 0.94 99.37

-

on 12 oxygens

3.023 3.081 2.067 1.9%

0.241 0.32% 2.131 2.445 0.002 0.001 0.003 0.001 0.358 0.045 0.117 0.081 7.942 7.940

- -

0.783 0.896 0.749 0.843 0.091 0.077 0.085 0.113 0.023 0.008 0.041 0.028 0.102 0.019 0.1% 0.015

72-38-4 37.87 20.61

1.87 35.88 0.04 0.02 3.21 0.26 99.75

-

3.070 1.970

0.225 2.432 0.003 0.001 0.220 0.022 7.944 0.839 0.078 0.008 0.076

-

72-94-3 37.% 20.91

1.98 32.97 0.06 0.02 4.07 1.98 99.93

-

3.0% 1.984

0.237 2.220 0.004 0.001

0.170 7.99 0.764 0.082 0.059 0.095

-

o m

72-107-7 37.67 20.79 - 1.73 31.01 0.07 0.01 6.87 1.66 99.81

3.048 1.983

0.208 2.099 0.004 0.001 0.471 0.144 7.958 0.718 0.071 0.049 0.161

-

72-11-1 37.92 21.16

1.43 30.75 0.07 0.05 8.18 1 .a

101.23

-

3.034 1.9%

0.171 2.058 0.004 0.003 0.555 0.144 7.5% 0.703 0.058 0.049 0.189

-

Alm = ahandine, Prp = pyrope, Grs = grossular. Sps = sptssartine.

55c D. M. KERRICK & G. J . W O O D S W O R T H

Tabk 4. Electron microprobc analyses of biotite

Sample: 71-146-1 71-60-2 71-60-3 71-63-2

SiO, 37.01 35.11 35.66 34.84 A,03 19.11 19.65 19.14 19.94 TiO, 1.45 2.63 2.74 2.50

8.n 6.% 7.09 6.97 20.4 23.51 21.62 23.43

MI30 FeO MnO 0.13 0.11 0.28 0.23 G O - - 0.01 0.08 NaZO 0.18 0.31 0.16 0.20

Sum 95.55 97.46 95.76 97.41 KZO 8.46 9.17 9.07 9.22

71-68-1 34.27 19.35

6.16

0.06

0.19

95.91

1.78

25.58

-

8.52

72-1-4 35.03 19.39 2.97 6.71

22.98 0.19

0.19 9.13 96.60

-

72-36-2 34.40 20.37

1.55 6.66

24.01 0.05

0.26 9.39

%.69

-

72-38-4 34.68 19.27 2.63 6.34

24.25 0.12 0.01 0.31

%.44 8.84

72-94-3 35.15

1.34

21.71 0.15 0.06 0.22

96.35

20.31

8.43

8.97

72-%3

35.60 20.24

1.47

21.76 8.40

0. 18 0.06 0.22 9.33

97.26

~ ~-

72-107-7 35.19 21.00 1.59

22.18 0.21 0.09 0.25 9.19

97.72

8.02

72-11-1 35.28 19.74 1.95 6.84

23.40 0.15

0.23 9.57

97.16

-


Si 5.583 5.320 5.444 5.285 5.322 5.344 5.276 5.329 5.330 5.354 5.274 5.367 Atv 2.417 2.680 2.556 2.715 2.678 2.656 2.724 2.671 2.670 2.646 2.726 2.633 AIV' 0.981 0.829 0.888 0.850 0.864 0.832 0.959 0.821 0.960 0.942 0.985 0.908 Ti 0.164 0.300 0.314 0.285 0.208 0.340 0.179 0.304 0.153 0.166 0.179 0.223

1.972 1.571 1.613 1.575 1.426 1.527 1.521 1.451 1.906 1.883 1.791 1.59 2.578 2.979 2.761 2.973 3.321 2.931 3.079 3.116 2.752 2.736 2,780 2.977

Mg Fe Mn 0.017 0.015 0.036 0.029 0.007 0.025 0.007 0.015 0.019 0.023 0.026 0.020 Ca - 0.001 0.001 0.013 - - 0.001 0.001 0.011 0.010 0.014 - Na 0.054 0.090 0.048 0.059 0.057 0.056 0.m 0.091 0.064 0.064 0.072 0.069 K 1.628 1.773 1.766 1.784 1.688 1.776 1.838 1.733 1.736 1.789 1.758 1.857 Sum 7.395 7.558 7.427 7.569 7.571 7.488 7.661 7.553 7.602 7.613 7.606 7.603

X A M 0.453 0.525 0.495 0.523 0.571 0.521 0.537 0.547 0.471 0.478 0.485 0.526 xm, 0.346 0.277 0.289 0.277 0.245 0.271 0.265 0.255 0.330 0.329 0.312 0.274

Phl = phlogopite, Ann = annite.

Providing that ideal mixing is valid for andalusite and fibrolite in these samples, we conclude that the minor element distribution between andalusite and fibrolite does not reflect equilibrium partitioning. For thermobarometry, electron probe analyses were

carried out on garnet (Table 3), biotite (Table 4), and plagioclase (Table 5) . For garnet-biotite thermometry, the garnet compositions given in Table 3 represent rim compositions; i.e. averages of point analyses taken within

15 p n of the edges of garnet crystals. The biotite analyses were obtained on crystals in contact with anaiysed garnets.

T.Me 5. Electron microprobe analyses of plaaioclase

Sample: 72-384 7160-3 71-63-2 7 1 4 - 1 72-14 72-%2

SiO, 65.80 61.47 63.84 66.36 58.87 61.42 4 0 3 21.62 23.72 23.04 21.28 26.22 24.38 G O 2.23 5.64 4.03 1.64 7.83 5.76 N?@ 9.94 8.21 9.38 10.44 6.93 8.37

Sum 99.63 99.63 100.33 99.74 99.89 99.47 $0 0.04 0.09 0.04 0.01 0.04 0.04


Si 2 . 8 ~ 4 2.755 2.808 2.913 2.627 2.726 Al 1.121 1.243 1.195 1.101 1.380 1.276 ca 0.105 0.269 0.190 0.m 0.374 0.274 Na 0.848 0.708 0.800 0.889 0.600 0.720 K 0.002 0.005 0.002 0.001 0.002 0.002 Sum 4.970 4.980 4.995 4.981 4.984 4.998

XA" 0.110 0.274 0.191 0.080 0.383 0.275 x.4, 0.887 0.721 0.m 0.919 0.614 0.723 xch 0.002 0.005 0.002 0.001 0.002 0.002

An = anorthite, Ab =albite, Or = KAISi30,.

a0 500 ma T ('C)

Fig. 3. P- T diagram for the Al$i05 polymorphs. The numbered Lines (calculated using the Vertex computer program of Connolly & Kemck, 1987) correspond to the andalusite-sillimanite equilibrium for merent values of the equilibrium constant

the equilibrium involvtng stoichiometrically pure andalusite and sillimanite. The shaded band represents the garnet-biotite equilibrium (computed €room t k equation of Feny & Spear, 1978) for sample 71-68-1, collected at the sillimanite isograd.

(K, = o~ios/o,+io& And the heavy line (K- = 1.0) cornsponds to

A L U M I N U M SILICATES, M O U N T RALEIGH PENDANT 557

Because our garnet analyses represent rim compositions adjacent to biotite, we checked for the possibility of l d Emgrade exchange of garnet and biotite in selected garnet crystals within several samples. For each garnet

and+ for retrogression, the rim compositions in with adjacent biotite were compared to those with

adjacent quartz. Because the garnet rim compositions in these contrasting areas are virtually identical, we assume that the garnet rims in areas with adjacent biotite have not been affected by retrograde exchange with the biotite. For selected pressures, and using electron probe

analytical data for garnet-biotite pairs in the samples analysed, Table 6 lists temperatures obtained using the garnet-biotite thermometers of Ferry & Spear (1978), Hodges & Spear (1982) and Ganguly & Saxena (1984, 1985). At a fixed pressure, the temperatures derived from the Ferry & Spear (1978) thermometer are close to those of Hodges & Spear (1982). In contrast, the Ganguly & Saxena (1984, 1985) thermometer yields significantly higher temperatures for most samples.

The plagioclase analyses, which represent averages of rim compositions, were obtained for the plagioclase- garnet-quartz-Al$iO5 barometer that was originally proposed by Ghent (1976). For several samples from the sillimanite zone, simultaneous solution of the various formulations of the garnet-biotite thermometer and the plagioclase-sillimanite-garnet-quartz barometer yields a wide range of pressures (Table 7). Using this barometer, Ghent & Stout (1981) also obtained a wide variation in pressure (2.5-5 kbar) for three metapelite samples from the andalusite zone of the Mount Raleigh pendant. Because of the absence of kyanite in the pendant and the presence of an isograd marking the transformation of andalusite to sillimanite, pressures during metamorphism are considered to have been below that of the AllSiOs triple point. For all samples in Table 7, the Ferry & Spear (1978) garnet-biotite thermometer coupled with the Newton & Haselton (1981) plagioclase-garnet- sillimanite-quartz barometer, and the Hodges & Spear (1982) version of the garnet-biotite thermometer and plagioclase-garnet-sillimanite-quartz barometer, yield pressures markedly exceeding Holdaway’s (1971) triple point. Furthermore, none of the versions of the plagioclase-garnet-sillimanite-quartz barometer yield pressures below Holdaway’s triple point for all samples (Table 7). In view of our preference for Holdaway’s (1971) triple point at about 3.8 kbar (Woodsworth, 1979; Kerrick & Speer, 1988), we consider the plagioclase-garnet- Al$iO,quartz barometry of the Mount Raleigh samples to be inconclusive. Considerable scatter in pressures computed with the garnet-plagioclase-sillimanite-q~ barometer was also encountered by Kemck (1987) in a study of the Ardara aureole, Ireland. It is possible that such scatter reflects a large variation in yh in the compositional region near the peristerite solvus (Ashworth & Evirgen, 1984).

Following the approach of Kerrick (1987), pressures can be determined by garnet-biotite thermometry at the sillimanite isograd. This method takes advantage of the

Table 6. Garnet-biotite thermometry (“C) of samples from the Mount Raleigh pendant’

sample no. P(kbar) TFs +2~, -20, THS T’,

71-3

71-63-2

71-68-1

71-146-1

72-11-1

72-36-2

72-38-4

72-94-3

72-107-7

1 2 2.5 3 4 5

1 2 2.5 3 4 5 1 2 2.5 3 4 5 1 2 2.5 3 4 5 1 2 2.5 3 4 5

1 2 2.5 3 4 5 1 2 2.5 3 4 5 1 2 2.5 3 4 5 1 2 2.5 3 4 5

642 646 648 650 655 659

640 644 646 648 652 656

605 609 61 1 613 617 621

540 544 546 547 551 555

526 530 532 534 537 541

733 738 740 742 741 752

603 607 609 61 1 615 619 520 524 526 527 531 535 517 521 523 w 528 532

66s 669 671 673 678 682 662 667 669 671 675 679 626 630 632 634 638 642

558 562 564 565 569 573

544 547 549 55 1 555 559

761 765 768 no n s n g

624 628 630 632 636 640 537 541 543 544 548 552 534 538 540 542 545 549

616 621 623 625 629 633

614 618 620 622 626 630 581 585 587 589 593 597

523 525 527 531 534 M7 510 512 514 517 521

702 707 709 711 716 720 580 584 586 588 59 1 595 501 504 506 508 511 515 498 502 503 505 509 512

520

646 677 650 681 652 683 6 5 4 6 8 5 658 690 663 694

643 667 647 671 649 673 651 675 656 679 6 6 0 6 8 3 606 637 610 641 612 643 614 645 618 649 622 653 547 572 551 576

555 579 559 583 562 587 533 605 537 609 538 610 540 612 544 616 648 619

737 718 742 723 744 725 746 727 751 732 756 737 605 601 6 0 9 6 0 5 610 607 612 608 616 612 620 616 528 556 532 560 533 561 535 563 539 567 542 570 524 574 528 577 529 579 531 581 535 585 538 588

553 s n

TFs = temperatures determined with Ferry & Spear’s (1978) garnet-biotite thermometer. 2a, = two standard deviations for temperatures derived with Ferry & Spear’s (1978) garnet-biotite thermometer using a standard error for the equilibrium constant computed as in Hodges & Spear (1982, p. 1126). THs = temperatures determined with Hodges & Spear’s (1982) gamet- biotite thermometer. TGs = temperatures determined with Gan- guly & Saxena’s (1984,1985) garnet-biotite thermometer.

5!j8 0 . M. K E R R I C K & C. 1. WOODSWORTH

Tabk 7. Pressures and temperatures (" C ) determined from simultaneous solution of various formulations of the garnet-biotite thermometer and garnet-plagiodase-sillimanite-quartz barometer*

I I I I

FlBROLlTE ISOGRAD -

Sample: 7140-3

FS-GRS FS-NH HS GS

-

Sample: 71-63-2

FS-GRS FS-NH HS GS

Sample; 71-68-1

FS-GRS FS-NH HS GS

Sample: 72-36-2

FS-GRS FS-NH HS GS

P (kbar)

3.2 5.3 7.7 3.7

P (kbar)

4.1 6.3 8.9 3.2

P (kbar) 2.8 5.9 8.2

-0.6

P (kbar)

4.7 7.3 9.5 5.1

TVC) 651 660 663 653

T("C) 653 662 660 649

T r C ) 612 624 622 598

T("C) 750 762 756 752

FS-GRS = Ferry & Spear (1978) garnet-biotite thermometer and Ghent, Robbins & Stout (1979) garnet-plagioclase-sillimanite-quartz barometer. FS- NH = Ferry & Spear (1978) garnet-biotite thermometer and Newton & Haselton (1981) garnet- plagioclase-sillimanite-quartz barometer. HS = Hodges & Spear (1982) garnet-biotite thermometer and garnet-plagiodase-sillimanite-quartz barometer. GS = Ganguly & Saxena (1984,1985) garnet-biotite thermometer and garnet-plagioclase-sillimanite- quartz barometer.

I I I I I 1 I1

small pressure dependence of the garnet-biotite exchange equilibrium. Because of textural evidence (Fig. 2c and d) and KD values close to unity, it is reasonable to assume that the sillimanite isograd corresponds to the andalusite- sillimanite equilibrium. Using the Ferry & Spear (1978) garnet-biotite thermometer, the P-T band for sample 71-68-1, which represents the first appearance of sillimanite, crosses the andalusite-sillimanite equilibrium at a pressure of approximately 2.5kbar (Fig. 3). In contrast, the P-T band for the Ganguly & Saxena (1984, 1985) garnet-biotite thermometer crosses the andalusite- sillimanite equilibrium at approximately 2 kbar. Because of the relatively large error in garnet-biotite thermometry (Hodges & Crowley, 1985), these pressure estimates may have a large uncertainty. Furthermore, we caution that these pressure estimates are based on a single sample.

Using simultaneous solutions for three solid-solid equilibria involving components of the assemblage plagioclase +biotite + garnet + muscovite, Ghent & Stout (1981) derived pressures in the range 2.9-3.6 kbar for four samples of andalusite zone metapelites from the Mount Raleigh pendant. Taking into account all of the barometric data except

that of the garnet-plagioclase-sillimanite-quartz barometer, we consider that pressures between 2 and 3 kbar are most likely.

For comparative thermometry of various samples, we assumed a pressure of 2.5 kbar and derived temperatures from the Ferry & Spear (1978) and Ganguly & Saxena (1984, 1985) formulations of the garnet-biotite thenno- meter. As shown in Fig. 4 there is considerable variation in garnet-biotite temperatures for sillimanite-zone samples. The temperature data for sample 72-11-1 are suspect because of the large spessartine content of the garnets

I I SlLLMANfTE ISOGAAD

I

I FlBROLlTE ISOGRAD

I I I I I I

A s ,

71-68-1

4 I I I d'

71-146-1 72-94-3

I I I I

I I I I I 0 2 4

Distonce (km)

Fig. 4. Garnet-biotite thermometric data (for P = 2.5 kbar) as a function of distance upgrade from the sillimanite isograd. The filled circles correspond to temperatures computed with the garnet-biotite thermometer of Ferry & Spear (1978) whereas the open symbols were computed with the garnet-biotite thermometer of Ganguly & Saxena (1984,1985). The distance values for the samples were determined from Fig. 1 by an orthographic projection onto a line trending N 25" E.

A L U M I N U M SILICATES, M O U N T R A L E I G H P E N D A N T 559

(16--18mol.%). The garnet zoning profiles of samples 7160-3, 72-11-1 and 72-38-4 (Fig. 5 ) are characterized by an increase in spessartine near the rims. In a detailed study of garnet zoning in metapelites of the Mount Raleigh area, Woodsworth (197'7) concluded that Mn-enriched rims formed during retrograde chloritization. Thus, the thermometry data for these three samples may not record the thermal maxima. In contrast, samples 71-63-2 and 72-362 have unzoned Mn profiles (Fig. 5). This observation, coupled with the fact that the garnets in these samples approach binary almandine-pyrope solid solutions, lends credence to the garnet-biotite thermometric data for these two samples as representing that of the thermal maxima.

The zoning profile for garnet in sample 72-94-3, which was collected about 100m upgrade from the staurolite isograd. shows a marked decrease in Mn from core to rim (Fig. 5). As with Woodsworth's (1977) zoning profiles of garnets from the staurolite zone, such zoning probably

Distance (pm)

reflects prograde metamorphism. Therefore, the garnet zoning supports the argument that garnet-biotite thermometry of sample 72-94-3 records the thermal maximum.

Excluding samples 71&3,72-11-1 and 72-38-4, the data from the Ferry & Spear (1978) garnet-biotite thermometer yields a thermal gradient of approximately 20" C km in the plane of exposure. The thermometry data computed with Ganguly & Saxena's (1984, 1985) formulation of this thermometer yields a similar thermal gradient. Consider- ing that andalusite occufs over a 5.5 km-wide zone below the sillimanite isograd, a linear 20"C/km gradient would yield a corresponding temperature range of 110°C over this zone. According to Holdaway's (1971) AI,SiO, phase diagram, stable formation of andalusite over an isobaric temperature interval of 110°C would imply a muximum pressure of 3kbar, thereby supporting the pressure estimate between 2 and 2.5kbar derived from garnet- biotite thermometry at the sillimanite isograd.

I The Ferry & Spear (1.978) and Hodges & Spear (1982)

171-63-2] Al

80

I I I I I J 50 100 150 200

Distance (p)

I .

100 200

73 172-1 1-11 I

74 Gr I -I

3 - 0 . . 0 0 0

3-

10-

17: 0 . a m 0 .

15-

I I I I

# ' 0 0 0 SP

I I I I

50 100 150 200 Distance (pm)

. "" a"" a""

Distance (pm) Distance (pm) Fig. 5. Mole percentages of components (A1 = almandine, Py = pyrope, Sp = spessartine, Gr = grossdar) calculated from electron probe traverses (rim- core- rim) across selected garnets in samples from the Mount Raleigh area.

!3% D. M. KERRICK i? G. J . WOODSWORTH -

garnet-biotite thermometry at the fibrolite isograd (Fig. 4) yields temperatures below 550" C, whereas that of Ganguly & Saxena (1984, 1985) yields maximum temperatures of 580°C. Comparison of the garnet-biotite thermometry data for samples at or near the fibrolite isograd with that of sample 71-68-1 at the sillimanite isograd suggests a temperature difference of about 70°C between these isograds. Assuming that the sillimanite isograd corresponds to the andalusite-sillimanite equilibrium, this implies that fibrolite first appeared at temperatures more than 70" C below the sillimanite stability field at a pressure of 2.5 kbar. This conclusion is valid even if there are significant systematic errors in the calibration of the garnet-biotite thermometer. For P = 2 kbar, as obtained from the Ganguly & Saxena (1984, 1985) garnet-biotite thermometer for sample 71-68-1 at the sillimanite isograd, the thermobarometric data for the fibrolite isograd would also imply that fibrolite formed about 70°C below the andalusite-sillimanite equilibrium. In the Mount Raleigh area, the spatial distribution and apparent phase-forming reactions of fibrolite and sillimanite are similar to those described by Kemck (1987) in contact aureoles of Donegal, Ireland. Moreover, we suggest that the genetic model for fibrolite presented by Kemck (1987) is relevant for fibrolite in the Mount Raleigh area; i.e. fibrolite formed metastably within the andalusite stability field by the late-stage replacement of biotite. The influx of late-stage acidic volatiles is implicit in the reaction (Kemck, 1987):

2K(M&Fe, -x)3AISi3010(OH)2 + 14HCl = AlzSiOS + 5Si02 + 2KC1+ 6(M&Fel_,)C1, + 9H20.

As in Kemck's study, the lack of opaques in fibrolite aggregates within biotite supports the transfer of Fe into the fluid phase. As metamorphism in the Mount Raleigh pendant apparently resulted from a low-pressure regional thermal event, our study supports Ferry's (1983) contention that acid metasomatism may accompany regional metamorphism.

To examine the extent of the andalusite-, sillimanite transformation as a function of metamorphic grade, modal analyses were carried out on selected specimens collected from the sillimanite zone. Transparency photocopies were prepared from enlarged (16 x 24 cm) prints of photomicrographs of andalusite containing inclusions of coarse sillimanite (e.g. Fig 2c and d). The field of view covered by each photomicrograph encompasses an entire andahsite porphyroblast. For each transparency, the areas containing andalusite and sillimanite were cut out and then weighed. Triplicate measurements made on two different photomicrographs showed that this method yielded excellent reproducibility: for both sets of replicate measurements modal percentages varied by less than 2%. This technique proved to be considerably more reproducible than modal analyses determined by a Ziess Zidas digitizing system.

There is a general progression of reaction progress as a function of metamorphic grade (Fig. 6). Nevertheless, in samples 71-60-2 and (especially) ' 72-15-1, there are

considerable differences in the extent of this readon between andalusite porphvoblasts. The heterogeneity of the andalUSite-'Sillimanite reaction progress above the sillimanite isograd is emphasized by the fact that samples Containing sillbanite and no andalusite occur at the Same metamorphic grade as those with andalusite + sillimanite. In P ~ ~ l ~ , sample 72-15-1 (andalusite + sillimanite) and samples 72-38-2 and 71-83-9A (which contain only sillimanite) are equidistant from the sillimanite isograd.

Because KD values are very close to unity for andalusite-sillimanite pairs throughout the sillimanite zone (Table 2), multivariancy due to minor element partitioning cannot be invoked for the 3 km-wide zone of coexisting andalusite and sillimanite. Rather, we conclude that andalusite metastably persisted 'into the sillimanite P-T stability field due to sluggish reaction kinetics. Assuming that the sillimanite isograd is correlative with the andalusite-sillimanite equilibrium, a thermal gradient of W C / h in the plane of exposure would imply that andalusite in sample 72-15-1 metastably persisted to temperatures approximately 60' C above this equilibrium. Figure 6 suggests that relatively little andalusite-, sillimanite reaction occurred within a 1.5km-wide zone above the sillimanite isograd; however, there is a significant increase in the modal amount of sitlimanite above this zone. Proceeding upgrade, the first samples with sillimanite and no andalusite (72-36-2) occur about 2.7km above the sillimanite isograd, and most samples beyond 2.75 km upgrade from this isograd have sillimanite and no andalusite. With a 20"C/km thermal gradient in the plane of exposure, as deduced from garnet-biotite thermometry of lower grade rocks, the 1.5 km-thick zone of little (<5%) reaction immediately upgrade from the sillimanite isograd suggests that the andalusite-sillimanite equilibrium was 'overstepped' by about 30°C before significant (>5%) reaction occurred.

Because the incipient stages of the andalusite-, sillimanite reaction are preserved in a 1.5km-wide wne immediately above the sillimanite isograd, specimens from this zone afford an opportunity to examine the nature of sillimanite nucleation within andalusite. In samples 71-63-2 and 72-261, in which the andalusite-t sillimanite reaction progress was less than 5% (Fig. 6) , sillimanite crystals are confined to areas within the host andalusite that contain a large proportion of very fine-grained inclusions (Fig. 2i). Within the inclusion-rich areas, andalusite has a relatively large surface area. It is possible that the larger andalusite surface free energy in these areas compared to inclusion-free regions provides an explanation for the preferred nucleation of sillimanite. In essence, the free energy barrier for sillimanite nucleation in the inclusion- rich areas would have been less than that of inclusion-poor areas. Alternatively, the abundant grain boundaries in the inclusion-rich areas may have provided preferred sites for nucleation of sillimanite. Because there are numerous sillimanite crystals within samples 71-63-2 and 72-26-1, it appears that the limited andalusite + sillimanite reaction progress in these samples does not reflect sluggish nucleation of sillimanite; rather, impeded growth kinetics

A L U M I N U M SILICATES, M O U N T R A L E I G H P E N D A N T 561

I 1 I

' ' 1 : 60 - 72-15-1

50 -

4 0 - a .- v,

s? 30 -

a 70-631-€2 I

71.60-2

pb. 6. Modal penrntage of sillimanite

distance from the sillimanite isograd. The modal percentage for sample 71-68-1 was obtained by a visual estimate during petrographical examination, whereas the modal data for other samples were obtained by the method disarssed in the text. For

[=lo0 x (si/si + And)] as a function of 20 -

- 72-26-1 10 - '1 /71-63-2

I a I I threc samples, the bars encompass the 0

&mite porphyroblasts.

1 2

Distance (km)

3

are implied. We suggest that, although nucleation of sillimanite was favoured within inclusion-rich areas, the growth of sillimanite was impeded in such areas. Because coarse, prismatic sillimanite crystals are virtually free of inclusions (Fig. 2, d and h), growth of sillimanite crystals in inclusion-rich areas requires displacement of the inclusions. Impedance of grain growth due to 'pinning' of grain boundaries by foreign phases is a well-known phenomenon (Spry, 1969; Vernon, 1975). In contrast, sillimanite within inclusion-poor areas of the chiastolite crystals in higher grade samples are larger than those within inclusion-rich areas (Fig. 2j). Thus, relative to the inclusion-rich areas, growth of sillimanite within inclusion- poor areas of the chiastolite crystals at higher grade was unimpeded by grain boundary pinning.

We emphasize that the preceding interpretations of the andalusite-,sillimanite reaction are based on the textures illustrating that sillimanite nucleated within, and replaced, andalusite porphyroblasts. Accordingly, phases other than andalusite were not involved in producing the sillimanite that was measured in our modal analyses.

Above the sillimanite &grad, sillimanite is lacking in only one aluminum silicate-bearing sample (72-76). Compared to other specimens, the andalusite in this sample is unique in that it contains a high FsO, content (1.64wt%) and displays pink pleochroism (in contrast, the andalusite in all other specimens is colourless in plain polarized light). The lack of sillimanite in sample 72-74 is attributed to the high Fe content of andalusite. Considered on an isobaric T-XkAnio, diagram (Fig. 7), this sample would plot in the andalusite-only stability field. Because of uncertainties in the barometry, it could be

argued that the pressure during metamorphism was higher than that estimated herein, thereby weakening the argument that fibrolite formed metastably. In fact, if the

pressure was in the range 3.5-4kbar. the temperatures derived from garnet-biotite thermometry for samples at the fibrolite isograd (c. 550 "C) would imply that the lowest grade fibrolite formed within the sillimanite P-T stability field. If this were the case, our estimated thermal gradient of 20"C/km, coupled with the 5km separation of the fibrolite and sillimanite isograds, would imply that the

f r

~~~ ~~~~~ ~~

X F I A I S ~ ~ - Fig. 7. Schematic, isobaric partitioning loop for Fe-bearing andalusite and sillimanite solid solutions (see Kenick & Specr, 1988). T, is the equilibrium temperature for the pure, Fe-free system (X- = 1 .O). The shaded area represents the andalusite-onty field above TI. A = andalusite, S = sillimanite.

562 D. M. K E R R I C K & G. I . W O O D S W O R T H

sillimanite isograd corresponds to overstepping of the andalusite-sillimanite equilibrium boundary by about loo" C. However, in view of the petrological studies of the andalusite-sillimanite isograd in other areas of contact and regional metamorphism (Kemck & Speer, 1988), we consider that, in parageneses where andalusite has undergone a volume-for-volume replacement by sillimanite, it is unlikely that the first appearance of sillimanite reflects significant overstepping. For example, Kemck's (1987) analysis implies that the andalusite-t sillimanite isograd in the Ardara aureole correlates with the andalusite-sillimanite equilibrium at the prevailing pressure. This conclusion is confinned by conduction thermal modeling of this aureole (Kemck, 1987, Fig. 4; Bowers, Kemck & Lasaga, 1988).

Our interpretations are based on the assumption that metamorphism was isobaric. Non-isobaric metamorphism could considerably alter the conclusions. For example, if during metamorphism there was a decrease in pressure from northeast to south-west, it is feasible that the prograde P-T gradient was subparallel to the andalusite- sillimanite equilibrium boundary. This pressure decrease could provide an explanation for the extensive zone of coexisting andalusite + sillimanite. However, subparallel- ism of the prograde P-T path with the andalusite- sillimanite equilibrium boundary is unlikely in view of the prograde increase in the progress of the andalusite + sillimanite reaction (Fig. 6). Further support for isobaric metamorphism comes from the distribution of the sillimanite isograd in relation to the sillimanite-K-feldspar isograd. Let us assume that the sillimanite-K-feldspar isograd corresponds to the equilibrium dehydration of muscovite + quartz (Woodsworth, 1979). The 2.5 km separation between the sillimanite and si1limanite-K- feldspar isograds, coupled with a prograde thermal gradient of 20"C/km, would be compatible with an isobaric prograde path at 3kbar, first intersecting the andalusite-sillimanite equilibrium and then the muscovite + quartz = sillimanite + K-feldspar + H,O equilibrium at uHzo values for fluids in equilibrium with graphite (Kemck, 1972; Ohmoto & Kemck, 1977). Temperatures of approximately 650°C in the upper sillimanite zone, as suggested by our geothermometric analysis, are compatible with the Occurrence of sillimanite + K-feldspar in this zone (see Kerrick, 1m).

CONCLUSIONS

Graphite is an abundant phase in metapelites of the Mount Raleigh pendant. The low Fe3+ contents of andalusite within this pendant are compatible with the reducing conditions implied by the presence of graphite. As with the study of Kemck & Speer (1988). multivariancy in the andalusite-sillimanite equilibrium arising from minor element partitioning appears to be insignificant in graphitic assemblages.

In addition to implications regarding reaction kinetics, the 1.5 km-wide zone of limited andalusite-silliianite reaction immediately above the sillimanite isograd suggests

that very careful petrographical examination for small amounts of sillimanite is necessary in order to accurately locate the sillimanite isograd in similar metamorphic terranes. The sample collected at the sillimanite isograd (71-68-1) contains less than 1% (by volume) of sillimanite (Fig. 6); in fact, most of the chiastolite crystals in this specimen lack sillimanite. Had sillimanite been overlooked in this sample, the sillimanite isograd would have been misplaced by 1.5km. In view of the importance of the andalusite-sillimanite equilibrium as a primary thenno- barometer, such an error would have introduced significant uncertainty in the P-T analysis of the Mount Raleigh pendant.

Walther & Wood (1984) predicted temperature overstepping, which they define as the temperature interval over which reactants and products coexist, between 19°C (for reaction control) and 40" C (for transport control) for the andalusite+sillimanite reaction in contact metamorphism. Our results suggest that significant overstepping can also occur in low-pressure regionally metamorphosed regimes. Such studies emphasize the need for additional quantitative modeling of the nucleation and growth kinetics of the andalusite-, sillimanite transformation.

ACKNOWLEDGMENTS

We are very grateful to H. W. Day, M. J. Holdaway and C. T. Foster for critically reviewing this paper. We thank L. B. Eminhizer for the electron probe analyses, J. R. Bowers for computer processing of the probe data, R. E. Kohler for assistance in the modal analyses and S. J. Mackwell for invaluable assistance to the senior author in word processing. Geological Survey of Canada contribu- tion 42087.

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Received 1 March 1988; revision accepted 19 December 1988.

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Aluminum silicates in the Mount Raleigh pendant, British Columbia