t22l

The C anadian M ine ralo gistVol. 35, pp. 122l-1236(1997)

PETROLOGY OF THE FLINTON CREEK METAPERIDOTITES:ENSTATITE. MAGNESITE AND ANTHOPHYLLITE - MAGNESITE ASSEMBLAGES

FROM THE GRENVILLE PROVINCE

FREDERICK D. FORD* AND GEORGE B. SKIPPEN

Ottawa - Carleton Geoscience Center, Deparfinent of Geology, Carleton University, Oftawa, Owarto KIS 586

Ansrnacr

TVvo types of metaperidotite, enstatite - magnesite rock and anthophyllite - magnesite - dolomite rock, are present inultramafic lenses from the southern Flinton Synclinorium, Central Metasedimentary Belt, Grenville Province, in southeasternOntario. A mantle of the anthophyllite - carbonate rock always surrounds the primary enstatite - magnesite rock, which ispreserved only in the interior of the larger ultramafic lenses. The ensta66 - magnesite rock contains enstatite (Mges), magnesite(Mg*), dolomite (Mg+g), as well as talc (Mgg?), anthophyllite (Mgr6), forsterite (Mg*), and relict chromian magletite. Theassemblage enstatite + maglesite requires the presence of a relatively COlrich fluid phase. The presence of late-stage forsteriteveins in the enstatite - magnesite rock may be explained by infiltration of comparatively HrO-rich fluid at near-thermal-peakmetamorphic conditions. The anthophyllite - carbonate rock is composed of spherically radiuing anthophyllite intergrown withmagresite, dolomite and chromian mametite; the compositioas ef minerals ale similar in the two rock types. The presence ofonly one silicate in addition to the carbonates suggests that the anthophyllite - carbonate assemblage fomred as a metasomaticmantle duri-ng the early retrograde history of the ultramafic lenses. Kyanite - biotite assemblages in adjacent pelitic rocksindicate a regional metrmorphic grade in the middle amphibolite facies. Themrobarometry records metamorphic temperatuesof 620o to 680oC at pressures of 6.5 to 7.5 kbar.

Keywords; metaperidotite, ultramafic, anthophyllite, enstatite, nagnesite, Grenville Province, Flinton Group, Ontario.

SonaMan_E

Nous ddcrivons deux sortes de mdtapdridotite, soit une roche d enstatite + map6site et une autre a anthophyllite + magndsite+ dolomite, dans des lentilles ultramafiques affleurant dens le synclinorium de Flinton, de la ceinture m6tas6dimentaire centralede la Province du Grenville, au sud-est de I'Ontario. Un liser6 d anthophyllite + carbonate entoure dans tous les cas Ia rocheprimaire e enstatite + magn6site, pr6serv6e seulement d I'iltfriew des plus grosses lentilles ultramafiques. [a roche ] enstatite+ magn6site contient enstatite (Mg*), magndsite (Mg*), dolomite (Mg"p), de meme que talc (Mge), anthophyllite (Mg88),fors€rite (Mg*), et magndtite cbromiftsre r6siduelle. Llassemblage ensglilB a magn6site implique la pr6sence d'une phase fluiderelativement riche en COr. [a pr6sence de veines tardives I forstdrite dans la roche ] enstatite + magndsite resulterait peut-Otrede I'infiltration d'une phase fluide comparativement plus riche en H2O b des conditions de temp6rature proches de celles duparoxysme m6t,morphique. La roche i anthophyllite + carbonate contient des cristaux d'anthophyllite fibroradi6s enintercroissance avec magn6site, dolonite et magndtite chromifdre. Les compositions des mindraux sont semblables dans lesdeux sortes de roche. Vu Ia pr6sence d'un seul silicate avec ces carbonates, I'association anthophyllite + carbonate repr6senteraitun liserd m6tasomatique form6 lors de la r6trogression pr6coce de ces lentilles. l,es assemblages i kyanite + biotite des rochesp6litiques adjacentes indiquent un degr6 de m6tamorphisme r6gional dans le facids amphibolite moyen. La thermobarom6trieindique une temp6rature de mdtamorphisme de 620' d 680oC d une pression entre 6.5 et 7.5 kbar.

(Iraduit par la R6daction)

Mots-cVs: mdtaffridotite, ultramafique, anthophyllite, enstatite, magn6site, Province du Grenville, Groupe de Flinton, Ontario.

IxrnopucrroN

Metaperidotitic rocks are exposed in a series ofdiscontinuous boudins along the eastern marginof the Flinton Synclinorium, located in the CentralMetasedimentary Belt (Wynne-Edwards 1972) of the

Grenville Structural Province, in southeastern Ontario(Fig. l). The ultramafic boudins, herein termed theFlinton Creek Metaperidotites, are associated with asuite of metamolphosed mafic rocks ranging frommetagabbro to amphibolite. The metaperidotites areinteresting in that they contain significant proportions

* Present address: Geoscience Laboratorieso 933 Ramsey Lake Road, Sudbury, Ontario P3E 685.E-mnil addre ss: ford_f@torvO5.ndm.gov.on.ca

t222 THE CANADIAN MINERALOGIST

Ftc. l. Geology ofthe southern Flinton Synclinorium and surounding area; asterisk inhset map indicates geographic location. Open dots show the location of samples ofultrama.fic rock mentioned in the text. Diamonds indicate location of samples used forthermobarometry. The main !s'dy qgthe Flinton Creek Complex (black) occurs withinthe Flinton Group, in the core of the synclinorium, with a long teil extendingnorthward to the contact with the Northbrook Tonalite. Sporadic lenses of ultramaficrocks occur north ofthis point, in the contact zone between the tonalite and the FlintonGroup. At least two ulffamafic lenses occur entirely within the Northbrook, severalhundred meters away from the contact (semple 11M88). The Flinton Synclinorium isbounded on the east and west sides by shear zones that have not been indicated. Thecompilation was produced from the observations of Thompson (1972), Chappell(1978), and Bright (1986).

PETROLOGY OF THE FLINTON CREEK METAPERIDOTITES 1223

of magnesite and dolomite, in addition to silicateminerals more commonly associated with meta-morphosed ultramafic rocks, such as enstatite,anthophyllite, tremolite and forsterite (Trommsdorff &Evans 1974). In this paper, we discuss the petrology ofthese carbonate-bearing ultramafic rocks and describetheir petrogenesis on the basis of observed assemblagesand textures of the minerals, thermobarometry, andcalculated phase-relations.

FrupRlx.arnoNs

The metaperidotites generally occur as lenses in anelongate belt of deformed mafic rocks. Together, themafic and ultramafic rocks form what is herein termedthe Flinton Creek Complex. Individual ultramaficlenses vary in size from 1 m to over 50 m in longestdimension. Larger lenses contain a core of enstatite -

magnesite rock surrounded by an envelope of

/|lrild*'r \\ ,,\r

\\\ ilil+ + + + + +

t-+-{0m 5m lOm

ffil Enstatite-Magnesite Rock

ffi A"thophyllite-Carbonate Rock

WNNtr

Black Wall Alteration Zone

Gabbroic Rocks

Flinton CreekComplex

Metasedimentary Gneisses

Northbrook Tonalite

Flc. 2. Hypothetical cross-section of several ultramafic lenses looking along strike.Foliation in the host rock is subparallel to the long axis of the ultramafic lenses asdrawn; however, the direction of maximum elongation is perpendicular to the diagram-Note that the fwo ultramafic rock-types form concentric zones, with enstatite -

magnesite rock in the core of the larger lenses, surrounded by an envelope ofanthophyllite - carbonate rock. Smaller ultramafic lenses consist entirely of theanthophyllite - carbonate assemblage.

1224 TIIB CANADIAN MINERALMIST

anthophyllite - carbonate rock. Smaller lenses arecomposed enfuely of anthophyllite - carbonate rock.Sample locations are shown on Figure 1. An idealizedcross-section through the Flinton Creek Complex isshown as Figure 2.

A metasomatic reaction-zone or "blackwall" l0 to20 cm thick is developed where the ultramafic lensesare in contact with the surrounding metamorphosedmafic rocks. Similar reaction-zones have beendescribed from otler localities of metamorphosedultramaJic rocks around the world, includingPaddy-Go-Easy Pass (Frost 1975) and the northernAppalachians (Jahns 1967, Sanford 1982). Thefollowing mineral zones progress outward from theultramafic lens into the surrounding mafic rocks:monomi neralic tremolite zone, monomineralic biotitezone, chlorite zone. This sequence of mineral zones issimilal to the amphibolite-facies blackwall zonesdescribed by Sanford (1982) at Blandford, Massachu-setts and Gra.fton, Vermont. The presence of thismetasomatic reaction-zone between the ultramaficlenses and the surounding host-rocks indicates that thelws fissimilar rock-types were in contact with eachother during regional metamorphism, eliminating thepossibility of post-metamorphic tectonic emplacementfor the metaperidotites.

Mrtauonprusvr

Pelitic schist from the Bishop Corners Formation,containing the assemblage kyanite + biotite + staurolite+ muscovite + quartz (Ford & Skippen L994), iscoincident with the nortlernmost occurrence of theultramafic rocks at locality 93-43 (Fig. 1). An increasein regional metamorphic grade southward fromthis locality has been demonstrated in calcareoussedimentary rocks of the Lessard Formation(Thompson 1973), indicating that the assemblagekyanite - biotite gives a minimum estrmate ofmetamorphic grade for the Flinton Creek Complex.Mafic rocks within the gemFlex contrin assemblagescompatible with this estimate (amphibole - plagioclase- epidote - garnet). The increase in metamorphic gradesouthward within the Flinton Synclinorium issupported by the appearance of two-mica gfaniticpegmatite (with or without garnet) in Flinton Grouppsammitic rocks that are spatially related to themetaperidotites southward from localify 101788 onFigure 1; Thompson (1973) interpreted the bodies ofgranitic pegmatite to be produced by partial melting ofthe Flinton Group psammilsg.

Samples for geothermobarometry were collectedfrom several rock-types, including: pelitic schist,garnet-bearing amphibolite, and semipettic gneisscontaining garnet, amphibole, and biotite. Thestratigraphic position of ttris last-named rock-type is'nknowtr, as it is exposed only intermittently betweenthe Flinton Creek Complex and the Northbrook

714685

(l) Ganc-bioeetsnmrtyutogtlselibrdim ofHodgps & Spqr (19t2) bra pre of 7 kbo. (2) Aqhibol8 - plagi@le ttmorety uiag the elibrdi@of Honrd & Bhndy (194) tur a pm of 7 tbr. (3) INTERSX rult8 "iingTwQ (mim 1.02; B@ l9l). Th€ + nlus rspsdt I o Bcdd diocrercim (4) Aqhlole - gqnd - plagi@le bqome|try .dng ths elibmti@of KolE & Sps (t90) at T(l). lvfudal @blage: Gn - Bt - Mr - Ky - Pl -

Qtz (93-43b), Cft - Bt - IIbl - Pl - Qtr (93-IIA3, 93-368). Nfnml @mpositi@u8€d to obt8irl the P-T milts m srdablo upon r€qued ft@ the 6I$ qtrhtr, edftm the Dspository of Uryublish€d De, CISTI, Ncional Rerch Corcil ofCmada, Ottawa, Odrio KIA 0S2.

Tonalite. Thermobarometric results are summarized inTable 1. Temperafure estimates range from 620" to680'C. Estimated pressrues are 6.5 to 7.5 kbar.Thermobarometric results for 9343b (staurotte -kyanite - biotite) are consistent with published grids formetapelite (e.9., Powell & Holland 1990), whichindicate the stabitity of this assemblage at the estimatedtemperature and pressure. The tlermobarometry forsample 9343b indicates metamorphic conditionsapproaching the stability field of sillimaaite; this is inagreement with the regional location of the kyanite-to-silf imanite isograd (Carmichael et al. 1978) that liesbetween outcrop 93-43 (staurolite - kyanite - biotite)614 sillimanite-bearing pettic schists 4 km 16 the east.

Perr.ocnepnv AND MtrIRAL CHEMISTRy

Approximately 250 samples distributed ttrroughoutthe Flinton Creek Complex were selected for detailedpetrographic examination. The locations ef 5nmplesinvestigated by electron-microprobe analysis (fables 2,3) are shown on Figure 1.

Whole-rock chemisny

Sixteen 5amples of enstatite - magnesite andantlophyllite - carbonate rock were collected forwhole-rock geochemistry throughout the Flinton CreekComplex; representative compositions are given inTable 4. Low analytical totals, due to the presence ofcarbnates and hydrous mine6ls, preclude an extensivediscussion of tle chemical petrogenesis of these rocks;however, it is worth noting that both rock types havehigh levels of Cr (2000-7500 ppm) and Ni (1@0-2300ppm) contents, which is compatible with an ultramaficprotolith. The proportions of Si, Fe and Mg remainwithin the range expected for ultramafic rocks,indicating that variations in bulk composition are notresponsible for the different mineral assemblages

TABLE I

ThmoEEtry("C)

o) Q) (3)Bmnoty (brr)(3) (4)

9343bct-HAl93-36s

75@62@

6t4638737

616+20 64@+ 1280654+7 6765 + 155

750+ lE 7425*675

PETROLOGY OF THE FLINTON CREEK METAPERIDOTITES

TABLE Z AVEMGE COMPOSMON OF ENSTATME OLIVINE, ANTHOP}TYLUTE AND TREMOLITE *

1225

Saaple

Rk Tlpe

ENtaliteI I l5E7-lc I 10.18&c2 101788-lb

(e) (r4) (41)EM EM EM

OlMreI I I 5E7- I c I I 0.lss{2 I O I 7E8-lb 060489-2b-T$ffi ]d

(4) (46) (26) (4E) (26)EM EM EM EM EM

I ll5t7-lc I l0l88a2 lo3lEE{l(6) (46) (18)EM EM AC

Tremolltell lstT-lc l lo.ltt{2

(2e) aEM EM

SiO2 sLo/o 57.65Tio2 0.07AI203 0.08cao3 0.10FeO 6.05MnO 0.14MEO 3,1.99NioCaO 0.09K20

Total 99.17

O^f'gm abm 6si 1.998Ti 0.002Al 0.003Cr 0.003Fe 0.175Mn 0.00{Mg 1.808NiCa 0.003K -

Tota.l 3.96

m# t 0.9r

51.46 57.930.10 0 . l l0.@ 0.140.10 0 .147.25 6.190.24 0.1531.57 35.0E

0. l l 0 .10

,,, ,;6 6

t.992 t.9960.003 0.003

0.000 0.0060.003 0.0040.210 0 .1790.008 0.00.11.786 t.791

0.@t 0.00,1

..; ,,,,0.89 0.91

nt:on Y

E.09 9.320.16 0 .1650.98 8.17':' ':'

101.46 99.30

' : ' : '

0.t62 0.1920.@3 0.@3l.8x t.792

":' ":'

3.002 2.97

0.91 0.90

*:' 'j'

8.67 9.300.14 0 .1049.36 49.37,:, t:,

99.76 l@.lr

' : ' ' : '

0.178 0.1900.@3 0.0021.802 1.799

":'

2.99E 3.000

0.90 0.90

59.48 59.120.0t 0.0E0.14 0.360.t0 0.066.E8 6.980.35 0.2530.72 30.7t

0.70 0.360.12 0 . t2

9E.57 98. I I

23 ?31.917 7.9t70.@8 0.@t0.021 0.0570.011 0.0070.169 0.7E20.040 0.0296.llE 6.t44

0.100 0.0500.020 0.021

15.034 15.015

0.88 0.E8

57.33 v.740.18 o.lt0.67 2.U0.33 0.451.93 2.640.12 0.0923.64 22.62

13.20 12.E50.09 0.09

n.49 96.t4

23 237.A$ 7.920.01E 0.0190.109 0.4020.036 0.049o.22t 0.3130.014 0.0104.E34 4.76

1.9{0 t.9230.015 0.016

15.050 15.0E0

0.95 0.94

*:'

E.540.20s0.07t.:,

100.1 I

o1'

0.r740.@f

l .E2l

":'

3.@5

0.91

58.330.070.22o.o17.8t0 .1629.85

0.340.13

96.98

7.9M0.u}70.0350.@80.E900.0186.058

0.0500.022

t5.032

0.&7

f Mgi(FerMdMglNi)

{ detemi$d by etestm-mimprohe &alysis. The nuuber of amlys is gim itr bmceeb. Asltsis m Csd€ton Unimity Cmbndge Mim V etecrrol miqoprobeequipD€d with m f,erry dispeGire A@ronets. Operafing onditiom w 20KV ald I sp6im @rEtrt of 9 Emp. A@ge @[€ti@ tine ru l@s live tire. Datareductiotr M @mpleEd on a p€mDrl 6mpds Ning the EDDI prcgro of Prhgle (19t9).

present in the enstatite - magnesite and anthophyllite -carbonate rocks. The analytical data of Table 4 arecompared with standard ultramafic compositions onFigure 3.

Petrography of the enstatite - mngnesite rock

The enstatite - nagnesite rock is composed primarilyof coarse-grained, euhedral enstatite and medium- tofine-grained magnesite (Fig. 4a). Pettersen (1883)applied the name sagvandite to similar carbonate-orthopyroxenites from the Troms region of northernNorway. Carbonate-bearing orthopyroxenites have alsobeen described from Aaheim, western Norway (Kendel1970) and the Central Alps (Evans & Trommsdorff1970,1974). Additional work on the Troms locality hasbeen presented by Schreyer er al. (1972) and Ohnmacht(1974). A review of Norwegian metamorphosedultramafic rocks is provided by Bucher-Nurminen(1988).

Textures in this rock type are complex, with obviousreplacement and overgrowth of early minel4ls compli-cated by the later development of vein-related mineralssuch as magnesite and olivine. The driving force for thedevelopment of these Fxfures is believed to be fluidinflux. As such, the concept of "local eqrili!fig6", agdefined by Thompson (1959), is believed to apply

to the Flinton Creek Metaperidotites, whereby"equilibrium tends to be maintained locally within areacting system even though the system as a whole maybe distinctly out of equilibrium."

Enstatite in the enstatite - magnesite rock is coarse-

$ained (10 - 30 mm) and subhedral, and forms anetwork of mutually interfering crystals. Chlorite,opaque minerals and minor amounts of carbonate occuras randomly oriented inclusions in enstatite. Severalelectron-microprobe traverses across the larger grainsof enstatite revealed no compositional zonation.Enstatite crystals from different localities throughoutthe complex have similal compositions (TabIe2).

The rock is composed of 20 to 3OVo carbonateminerals. Magnesite commonly occurs as fine- tomedium-grained, anhedral crystals in magnesite-richdomains found interstitially among, and surrounding,the coarser-grained crystals of enstatite; however, asmall number 6f samples have large, single crystalsof coarse-grained (1 - 5 cm), euhedral magnesite intextural equilibrium with the adjacent enstatite (Fig.4a). Anhedral magnesite is commoDly associated withanthophyllite. Enclaves of magnesite - anthophyllitecan be found within large grains of enstatite, or ongrain boundaries, where the two minerals seem to begrowing into the larger crystal (Fig. ab). The followingreaction describes the relationship among the three

1226 THE CANADIAN MINBRALOGIST

TABLE & AVERAGE COMPOSMON OF CARBONATES. TAI,C AND CHLORITE T

Magncdb DolonlE Talc chloilES@plo lllsS? ll04E8 101788 lllsaT 110488 l0l78t I03lt8 lll5&7

-lc 43 -Ib -lc +3 -la 42 -lc(10) (ll) (9) (t2) (9) (10) (21) (4\

RL.IYF EM EM EM EM EM EM AC BM

enstatite - magnesite assemblage. Each veinlet hasa core of irregular, anhedral olivine that passestransitionally into the enstatite - magnesite rock.In this transition zone, olivine forms pseudomorphsof the original magnesite inclusions in enstatite.The occurrence of olivine in veins suggests that thepresence of a fluid phase may have contributed toits formation. The following reaction applies to thecoexistence of enstatite, magnesite and olivine inthe presence of a fluid phase:

Mg2SirO6 + 2V IgCO 3 = 2Mg2SiO4 + 2CO2enstatite magnesite forsterite (3)

Tbree amphibole minerals, anthophyllite, tremoliteand magnesio-cummingtonite, are found in the enstatite- magnesite rock. Anthophyllite replaces enstatite andtalc @g. 4d), and in some cases overgrows olivine,

TABLE4. BUIXCOMPOSMONOF THE FLINTON CREEK METAPERIDOTIIES t

ll04E8{2 ll04{5 103188{1 110,188dEM EM AC AC

SiO2 { %'1102

AI203Cr203FeOMtrOM!oNioCaOK20

TOEI

O,ry.almsiTi

Cr

MnMgNiC!K

TotrI

mg# i

8.94 49.39 94.$ 93.t9

3.56 4.6 4.29 l .16 1.45023 0.23 o.lt 0.8 0.08M.a2 43.6 44.4E 2t.25 20.n

0.44 0.39 0.44 2925 29.23

0.056 0.0sr 0.0300.003 0.m2 0.@20.935 0.940 0.989

o.ffi 0 ffi oqg

62.6 61.t6 32.a90.14 0.12 0.100 . t 5 L l 3 1 6 . 3 r0 1 2 0 . 1 6 l . 3 ll .16 t2t 3.u0.06 0.06 0.ll30.45 30.5 35.010.35 0.29 0.41

49.08

"0.0430.@30.94r

0.006

0.999

0.95

51.14

,_

2.@0

o.49

52.t1I

:0.0380.@20.95

o.9n

2.0@ t

0.49

89.58

t43.0330.d)7|.ns0.0960.2650.0094.8t3

10.026

o.94

l t u3.943 3.9420.@7 0.660.0t2 0.0E50.006 0.@E0.62 0.0650.@4 0.0032.912 2.884

":" ":'

7.003 7.W

0.n o.cl

f Mg/edN,tftMgJM) t - ToEl ircludE 0.44610 SrO = 0.@E Sr pe! 6 O,(yE@

. d€@i&dbydffin-Elqwrcbaml'sic Thenmrbqof alabs is gi@ itrbtuksAlrl'del @nditim indi€ad @ Table 2,

minslalso if one assumes that chemical equilibrium wasachieved:

4Mg2Si2O6 + HrO + CO, = Mg.tSisO22(OH)2 + MgCO,enstatite anthophyllite magnesite

( 1 )

Table 2 shows that both textural varieties ofmagnesite have the same chemical composition(101788-lb is the coarse-grained type, and 1 10488--c2is the anhedral variety).

Dolomite occurs as fine- to medium-grained,anhedral crystals in textural eqrilifdum with magnesite;dolomite - magnesite slmplectitic intergrowths wereobserved in fwo thin sections. Most silicate minerals,including enstatite and anthophyllite, seem to be instable coexistence with dolomite; however, in severalthin sections, dolomite appears to be a product of thereplacement of tremolite, suggesting that the followingsolid-phase reaction proceeded from left to right.

Ca2Mg5SisO2(OtI), + 4Mg(COs) = 2CaMg(CO.)2 +tremolite magnesite dolomiteMgrSirO,,(OH),anthophyllite (2)

Olivine has been found in the enstatite - magnesiterock at all but the northernmost occurrence (500 msoutheast of 93-43b), which is also the lowest-gradelocality. Olivine occurs as texturally late crystalsin veinlets that cross-cut and replace enstatite andmagnesite (Fig. 4c). At the microscopic scale, the olivineveinlets are not in sharp contact with the surroundins

f Total ircn ro FezOr

I AD.lys Mde at the Univmity of Otam X-my Fluorcne tucility

which also replaces enstatite. It has not been possibleto distinguish antlophyllite that developed alongthe prograde metamorphic path ftom retrogradeanthophyllite that developed at the same time as theanthophyllite - carbonate mantles that surround tleenstatite - magnesite rock.

The presence of embayed grains of tremofte incontact with carbonate suggests that tremolite becamemetastable prior to or during the growth ofanthophyllite. Most tremolite crystals have beenovergrown by later anthophyllite. Tremolite grains incontact witl magnesite are corroded, indicating thatprograde metamorphism exceeded the conditions ofsolid-phase reaction (2) above.

1.000 0.99

0.93 0.94SampleRL Type

SiO2 wLYoTi02A1203Fe2O3tMnOMgoCaONa2OK:toP205s

BaCrZaNi

Total

4t.3E 44.240.01 0.05o.5'7 1.507.t5 9.050 . 1 I 0 . 1 2

32.64 2a.453.25 4.470.16 0.290.r0 0.0r0.0r 0.0t0.09 0.05

61 6s209t 3333

94 lot1877 t541

20 2a

E6.04 92.94

5t.790.061.68

10.410.15

0,430.000.010.0r0.t2

T]6255

1092302

99.1 I

39.120.062.07

I 1.540. l4

33.501.64o.240.010.010.0s

705t56

75t949

56

E9.4t

Talc is found both as a prograde metamorphicmineral, and as a product of late-stage alteration.Prograde talc occurs as medium- to fine-grained,subhedral laths within enstatite crystals and alongenstatite grain-boundaries, indicating the early stabilityof talc and enstatite. The assemblage talc + enstatite,where not armored in large crystals of enstatite, isusually overgrown by anthophyllite (Fig. 4d). Thecoexistence of talc + enstatite with anthophyllite isgoverned by solid-phase reaction (4) below.

MgrSioOto(OII) 2 + 2MgrSirOu = MgzSieOzz(OH)ztalc enstatite anthophyllite (4)

t227

Penography of anthnphyllite - carbonate rock

This distinctive rock is composed of fine-grained,anhedral carbonate (magnesite and dolomite) andmedium-grained, subhedral needles of anthophyllite,which grow in spherical, radiating rosettes with anaverage radius of approximately 1 cm. Other silicateminerals are present in the anthophyllite - carbonaterock in minor amounts including: magnesio-cummingtonite, chlorite, and late-stage, retrogradetalc. No compositional differences could be detectedbetween anthophyllite from the enstatite - magnesiteand anthophyllite - carbonate rock types (Table 2).

PETROLOGY OF THE FLINTON CREEK METAPERIDOTITES

O Mineral observed in the FlintonCreek Metaperidotites

O Mineral not observed in the FlintonCreek Metaperidotites

AnthophylliteEnstafite

AntigoriteChrysotile

Forsterite

FeO+MgO+MnO

Dolomite Magnesite

Ftc. 3. Minerals commonly observed in metamorphosed ultramafic rocks projected into thesystem CaO - SiO2 - (FeO + MgO + MnO). Dark circles indicate minerals observedin the Flinton Creek metaperidotites, unfilled circles designate other minsl4ls 966-monly associated with metamorphosed ulramafic rocks tlat were not observed atFlinton Creek. Diagram assumes that a mixed HrO - CO, fluid phase is present inexcess. Diamonds indicate the compositions of Flinton Creek metaperidotites list€d inTable 2 (open: althophyllite - magnesite rock, fiIled: enstatite - magnesite rock). Theaverage compositions of various ultramafic rock-types (Le Maitre 1976) are alsoshown: Py pyroxenite, W webrlite, L lherzolite, H harzburgite, D dunite.

%"o

9o

Tremolite f

-

Diopside Q pyp 6w

THE CANADIAN MINERALOGIST1228

q B { ;s g 9 #; g 10i i 6 6 'E .H - = Oi J o * !r - ' {oc

v o t r r o> F ^ F Tl eFq

J < - a

. = 6 * d= t r ' 6 B6 '9 g $F€ 9 ;: F ; 4d E g t r

E Z H S€ g a ' ud 5 " a F- ^ O '

FI-ts g'" p d @, - t l J < F l

e gsEHRTg=Fs 3* z x ; +THB!- a b R3 0 -

e5 {8h " 3 d Pp F F Eqld o -i-

i J o h n jE e . 6 ' F '- € E F

X - - I o

E E BE F E6 n - C

5 ! eF ? H;4.f; tro t ! 6e o o ss3:g6'H F'E ' ^ ^: o<X - v

H < - .= o 5r l 5 E iQ ! t x

E:s= = : F0 s H{s st t < -

PETROLOGY OF THE FLINTON CREEK METAPERIDOTITES 1229

Magnesite and dolomite are more abundant in theanthophyllite - carbonate rock than in the enstatite -

magnesite rock, and make up approximately 30 to sOEaof the total volume. Magnesite is generally more abun-dant; however, the ratio of dolomite to magnesite isdependent on proximity of the contact between theultramafic body and the surrounding host-rock.Although the total amount of carbonate present in therock remains constant, the proportion of dolomite tomagnesite increases outward toward the contact, wheredolomite comprises up to 90Vo of the carbonate present.This relationship suggests that there is a metasomaticcontrol on the amount of dolomite present in theanthophyllite - carbonate rock.

PHasp Eerm-rsnra Iwvolvwc Solros IN Tm SysrevC a O - M g O - S i O 2 - H 2 O - C O 2

The metamorphism of peridotites in the modelsystem CaO - MgO - SiO2 - HrO - CO has beendescribed by Greenwood (1967), Johannes (1969),Ohnmacht (1974), Trommsdorff & Evans (1972, 1974,7977a,b), Evans & Trommsdorff (1970,1974), andEvans (1977). Phase relations presented in this paperemphasize only those aspects of the model system thatapply to tle rocks at Flinton Creek. The sevenmajor mineral phases observed in the Flinton Creekultramafic rocks are shown on Figure 3 in theternary compositional space CaO - MgO - SiO2. Ttvoadditional components, ferrous iron and manganese,are treated as dilutants of the magnesium component.Aluminum occurs primarily in magnesian chloriteowhich is always present and does not seem to beinvolved in reactions with higher- or lower-gradealuminous minerals. Certain minerals observedelsewhere in metamorphosed peridotites do not occurat Flinton Creek; these include quartz, diopside andserpentine.

Three databases contain thermochemical data for allthe required phases: Helgeson et al. (1978), Berman(1988) and Holland & Powell (1990). The Berman(1988) database can be further subdivided, since itcontains two values for magnesite, the first derivedby Chernosky & Berman (1989), and a later value

TABLE 5, ACTTVTTY MODELS USED FOR CALCULATING EQTJILIBRIA

Componeft Fomula Activity Model CA*

obtained by Miider & Berman (199L). These data areused to examine the equilibrium conditions of the fwosolid-phase reactions present in the CaO - MgO - SiO2- HrO - CO2 system (reactions 2 and 4 above).

The locatiqhs of these two equilibria in P - T spacehave been plotted for tle various thermochemicaldatabases using a Microsoft EXCEL spreadsheetprogram that solves the following equation by iteration:

T . '

AGpr =0=Nrh"+ Jcpar-r[s." + l{cntr) ar) (5)8 . m

+ w " ' ' ( p - p , \ + ( P - P , ) [ - ^ v 1 + A v .

+ L \ ( T - T , ) + A v 4 g - T , ) ' ]

+(Lv\ I 2 - Av) (P'? * l )+(Au. /3) lP' -D + e*+ Kf lnK

where AH, S and AV are the enthalpy, entropy andvolume of reaction at reference pressure P, andtemperature T. (1 bar and 298K). Cp is the heat capacity,calculated with the equation of Berman & Brown( le8s).

The Avr - Av, terms in equation (5) account for thechange in volume of the sohd phases with increasingtemperature and pressure, using the following expres-sion from Berman (1988):

v "'' = | + r, (T - Tr) + vr(T - Tr)2 + v, ( P - P r) + v o(P - Prf (6)

vn.r,

Note that in equation (5), Avt=A1vr x DPr'rr/105),Lv r= 67y2 7 DP.'Ty' I o5), Avr=^1v. 21 r:n'r'/ 1 Os) andAvr=l11vo2q oPr'ry'lQ8), where vo is the coefficient fromequation (6), and o is the molar volume of each mineralin the reaction at reference temperature, Tr, andpressure, Pr. The Holland & Powell (1990) databasecan be made compatible with this expression by6aking the following substitutions for the coeffrcientsof isothermal expansion and compression: vr = o( andv g = - 0 . 1 B .

The G6i".6* term was used by Berman (1988) tomodel progressive disorder in dolomite with increasingtemperature from 298.15 K (completely ordereddolomite) to 1423 K (completely disordered). TheG6i"o6", term in equation 5 is calculated by:

LG; = NI;-TLS; Q)

where the AH* and ASa;. disorder terms are calculatedusing equations 16 and l7 from Berrnan (1988), with aslight modification (the newer versions of Berman'sdatabase supply da and d5 terms, which must be substi-tuted for d, and do in these earlier equations).

Figure 5 shows the results given by Equation 5using average measured compositions of the minerals

from the Flinton Creek metaperidotites. Activities,given in Table 4, were calculated using an ideal-solution

Arnhophyllite Mg?SLO2(OH,Tr@olite CaJvlgrSisOz(OH),Ensatite Mg?Si?O6Foretqhe MgrSiO4Tale M&SLOrc(OH),Dolomite CaMg(COJMagn6ite MC(CO3)

okJ'(3-ca-KxxJ'?Gd5

(x"J,(xelCxd'CXMJCxk)

0.390.7t0.79o.E20.910.980.94

' Compositiom used uo ttre avenge ofthre listed in Tabl6 2 atd 3, with theq@ptio of talq which is frm wple l0l7E8-l a only. CA @lolaled actMty a.

1230

Eo

o

?noE

(L

1 0000

9000

8000

7000

6000

5000

4000

3000

2000

1 000

n

400 500 600 700 800 900 1000

Temperature ( "C)

Ftc. 5. Calculated solid-phase equilibria in a portion of the system CaO - SiOz - MgO - HrO - CO, adjusted for the measuredcompositions of minerals (Tables 2, 3). Solid lines represent the reaction talc + 2 enstatite = anthophyllite. Dashed lines fortremolite + 4 magnesite = anthophyllite + dolomite; Dd indicates that the variable dolomite-disorder model of Berman(1988) was used to the locate the reaction, whereas nDd indicates that the dolomite-disorder model was not used. Thermo-dynamic datasets used in the calculations are indicated as follows: H78: Helgeson et al. (1978), CB89: Chemosky & Berman(1989), HP90: Holland & Powell (1990), MB9l: M[der & Berman (1991). All datasets predict anthophyllite stability forthe pressure - tempemure mnge of metarnorphism, indicated by black dots with enor bars (150'C, +1000 bars); the Berman(1988) database, rvith modified data for magnesite (Mdder & Bermar 1991) and variable degree of disorder i-n dolomite,predicts the stability of anthophyllite + dolomite wirhin enor of the estimated P - T range.

300200

model. Also shown in this figure are the thermo-barometry results (Table l), which are considered torepresent peak conditions of metamorphism for theFlinton Creek metaperidotites. The observed replace-ment of talc + enstatite by anthophyllite indicates thatthe solid-phase equilibrium, reaction (4), limi1i11g 15"stability of anthophyllite is expected to lie just belowthe three P - T estimates. The petrographic data alsopredict the stability of anthophyllite + dolomite relativeto tremolite + magnesite for the P - T conditionsand mineral compositions at zunton Creek. Figure 5demonsftates that the ttrermodynamic data of Bennan(1988), as modified by Miider & Berman (1991),correspond best with the thermobarometry andpetrography of Flinton Creek rocks; this database isused in subsequent calculations.

Ulrrvanren-r PuesE-RE-ATroNS

The ultramafic lenses at Flinton Creek aremineralogically similar to the ultramafic boudins atGuglia in the Swiss Alps, as described by Evans &

TABLE 6. FLIJII>BEARING ASSEMBI-A.GES COD(STING ATINVARIANT POINTS IN THE SYSTEM MsO-SiO,

Abs€d. PbseDsigndim

PhrgPrBd

(Arrr)(E!)(nc)

Go)(Ms8)(Fo,M$)

Mg!, ED, Fo, Tlc

Mgs, Atb, Fo, Tlc

Mgs AlbFo, En

Mg$ Arb, En, nc

Ab, En, Fo, Tlc

T]c, Eq Alh

MtrT'MAFT*MAFE"MATEFATETEA

i As deigaated by Ere & Tromsdorff(1974).

Trommsdorff (1974). At both localities, rocks containingmagnesite, talc, enstatite and forsterite are overgrown atsome later stage by anthophyllite. Although the twooccurrences 41s mi nelalegically si mi I s1, petrographicanalysis reveals substantial differences in the sequenceof mineral assemblages that developed.

Four mineral phases and a fluid phase form aunivariant assemblage in the system MgO - SiO2 -

I t n / t < / (L l 4 / i v / cvJ _ , . r * ' * . . *g" ./ .o" / A|/*c

l= gt .f- ,1" IlF'./ oE I

lF/ l-/

9000

8000cg zoooEa 6000q(I)

d sooo

4000

3000

2000

12000

1 1 0 0 0

't 0000

1 000

500 550 600 650 700 750 800

Temperature ('C)

Frc. 6. Pressure - temperature diagram for univariant pofurts with pure magnesianminerals. Symbols along curves and numbers refer to incremental values of the molefraction of CO, along the univariant points. Calculated with database of Berman(1988), as modified by Mdder & Berman (1991). MAFE, MAFT, MEFI, TEAindicateunivariant curves with M (magnesite), A (anthophyllite), F (forsterite), E (enstatite),T (talc). Solid curves are stable; dashed curves are metastable.

PETROLOGY OF TTIE FLINTON CREEK METAPERIDOTITES t23L

MAIE and FATE therefore project through the compo-sition dimension onto the P - T path of TEA. TEA andMEFT do not intersect on Figure 6, indicating that theinvariant point MEFTA is metastable for end-membermagnesian phases using the thermodynamic data ofBerman (1988) and Mader & Berman (L991).

Evans & ftemmsdorff (1974) pointed out that theaddition of iron to the end-member magnesian phasesresults in a substantial increase in the field of stabilityof anthophyllite relative to talc + enstatite, with acorresponding shift in the location of TEA to signifi-cantly lower temperatures. According to the database ofBerman (1988), the addition of iron causes TEA andthe other univariant assemblages indicated on Figure 6to intersect at a single point (the invariant pointMEFIA) for an anthophyllite composition of Athrr;TEA shifts to lower temperatures with further additionof irono splitting the point MEFIA into duplicateinvariant points, as implied on Figure 7 for themeasured compositions o1 1trs minerals at FlintonCreeko as reported in Tables 3 and 4. For thesecompositions, the invariant point MEFTA is stable andduplicated at ll,l00 bars and <500 bars. MAFT andMAFE are stable between the duplicate invariant

450400

H2O - CO2. Evans & Trommsdortf (1974) describedone such assemblage characteristic of the univariantconditioir MEFT (magnesite - enstatite - forsterite -talc - fluid; Table 6), at the Guglia location. Thepresence of forsterite in the Guglia rocks from thebeginning of their petrological evolution is essential tothe recognition of the univariant assemblage MEFT asa stable one. This situation contrasts with the rocks atFlinton Creek, where forsterite overprints the earlier

'magnesite - enstatite assemblage. The significance ofthese petrographic observations can be understoodby locating the various rrnivariant assemblages for thesystem MgO - SiO2 - H2O - CO2 in P - T space. Figure 6has been calculated with the program TWQ (Berman1991) using thermodynamic data for end-membermagnesian compositions. The figure shows theP-T trajectory of the compositionally degenerateunivariant curve, TEA (talc - enstatite - anthophylite),as well as that of three normal univariant curves,MAFE , MEFT and MAFT. TEA contains both MAIE(magnesite - anthophyllite - talc - enstatite) and FATE(forsterite - anthophyllite - talc - enstatite) in P-Tspace, since both assemblages contain the solid-phaseequilibrium, talc + 2 enstatite = anthophyllite. Both

1232 THE CANADIAN MINERALOGIST

2.(Eo(I,

oooo-

1 3000

12000

11 000't 0000

9000

8000

7000

6000

5000

4000

3000

2000

1 000

400 450 500 550 600 650 700 750

Temperature (oC)

800

Frc. 7. hessure - temperafife diagram for nnivariant assemblages, calculated on the basisof measured compositions of the minglals (fables 2, 3). Number and symbols refer toincremental values of the mole fraction of CO2 along the univariant curves. Mi-neralassemblages and thennodynamic data as for Figure 6.

points, and MEFT is stable beyond this range atpressures lower or higher tlan either of the MEFIAinvariant points. It therefore appezrs that mineralcompositions in the Guglia metaperidotites leadto the stability of the MEFT assemblage above thehigher-pressure duplicate of the MEFTA invariantpoint" at approximately l0 kbars.

Tlu IwruuBNce oF FLUrD ColposmoN

The influence of fluid composition on the mineralassemblage of the ultramafic rocks at Flinton Creek canbe examined on Figures 8 and 9, which consider tfiestability of mineral assemblages, including the calcicphases dolomite and tremolite, on ln fugacity (HrO) -In fugacity (CO) diagramso constructed uggslding tothe method described by Korzhinskii (1959). The freeenergy of disordering in dolomite (Berman 1988) hasbeen included in the calculation. Also shown on thesefigures is the mechanical equilibrinm surface alongwhich calculated pressure of fluid is equal to lithostaticpressure P(COr) + P(HrO) = Protar. This surface wascalculated using the nonideal fluid-mixing model ofJacobs & Kerrick (1981). The addition of calcium as acomponent results in the stability of two additionalinvariant points labelled DTRAFE and DTRAFT, and

TABLE 7. INVARIANT POINTS IN A PORTIONOF THE SYSTEM CaO - MgO - SiO,

Ab$nt PhN PhffiPred D6tgmtlon

(Ath, En)

(Ath, Mgs)

(Ath, Fo)

(Ath, Tlc)

(Mgs, Tlc)

(En, Fo)

(En, MeO

(En, Tlc)

(Fo, Meo

(Fo, Tlc)

Dol, Fo, Mgs, Tr, Tlc DTRMFT

Dol, En, Fo, Tr, Tlc DTREFT

Dol, En, Mgs, Tr, Tlc IITREMT

Dol, En, Fo, Mes, Tr DTREMF

Ath, Dol, En, Fo, Tr DTRAFE

Ath, Dol, Mgs, Tr, Tlc DTRAMT

Ath, Dol, Fo, Tr, Tlc DTRAFT

Ath, Dol, Fo, Mgs, Tr DTRAMF

Arh, Dol, En, Tlc, Tr DTRATE

Ath, Dol, En, Mgs, Tr DTRAME

listed in Table 7. The fugacity diagrams allow thepetrography of the calcic phases to be used as a furthercheck on the evolution of the fluid phase.

Figure 8 is calculated for 680oC, the thermal maxi-mrrm applicable to the southern Flinton Synclinorium,as indicated by thermobarometry of rock fypes otherthan peridotite. This figure demonstrates that enstatite

'ul.i.,l i

I tt l: )

i l

N

(J

bI)

PETROLOGY OF THE FLINTON CREEK METAPERIDOTTTES r233

12.0

l l . 0

8.0 9.0 10.0

log / H,O11 .0

Ftc. 8. Ln/(H2O) - Inf(COr) diagram at 7000 bars and 680oC. Chemography as shown on Figure 4. The mechanical equilibriumsurface, along which fluid partial pressures sum to total pressure, is shown as a curved lile, with solid circles and numbersindicating incremental values for the mole fraction of CO2. Phase relations are indicated for talc (Tlc), anthophyllite (Ath),enstatite (En), forsterite (Fo), rnagnesite (Mgs), tremolite (Tr), and dolomite (Do). Designations of the invariant pointsarel is tedinTables6aadT.React ionl :Fo+HrO+9COr=4h+9Mgs.React ion2: .32Fo+9Tr+4HzO+36COr=l3Ath+ 18Dol.

- magnesite assemblages require high X(CO2) valuesfor stability; this result is in agreement with earlier,lower-pressure (2000 bars) experiments by Greenwood(1967) and Johannes (1969).

Figure 8 provides a possible explanation for thereplacement of enstatite - magnesite by late-stageolivine, through the introduction of relatively H2O-richfluids [X(CO) < 0.45] into the enstatite - magnesiterock. Both enstatite and forsterite can be seen on thefugacity diagram to be in equilibrium with dolomiteaDd magnesite, as required by the petrography oftheFlinton Creek rocks; enstatite coexists with thecarbonates and high-CO2 fluids along the mechanicalequilibrium surface, whereas forsterite and twocarbonates coexist with a more HrO-rich fluid.

Figure 9 is calculated for a temperature of 650"C,corresponding to the development of the anthophyllite- carbonate rock in envelopes that completely replace

earlier assemblages in the enstatite - magnesite rock.Decreasing temperature between Figures 8 and 9reduces the separation between the pairs of invariantpoints MAFE-DTRAFE and MAFT-DTRAFT suchthat each pair is almost coincident on Figure 9. Thedevelopment ofthe anthophyllite - carbonate assemblageappears to be related to a metasomatic influx of fluidthat carries the bulk composition of the peridotite intothe stability field of anthophyllite - magnesite. Forma-tion of a metasomatic rind could not have carried on tomuch lower temperatures, as the mechanical equilibriumsurface would intersect the talc - magDesite stabilityfield at approximately 64O"C.Talc - magnesite rindshave not been observed surrounding the anthophyllite -

carbonate envelope at Flinton Creek.The stability field of anthophyllite - magnesite -

dolomite on Figure p is limited along the mechanicalequilibrium surface by enstatite - magnesite - dolomite

En+2Fo

H"ry"# DTRAFT

-o+s

N

Q

bI)

L234 THE CANADI,AN MINERALOGIST

I t . 5

I1 .0

10.0

log / HrO

Frc. 9. Ln(H2O) - lnf(Cot diagram at 7000 bars and 650'C. The curved line indicates the location of the mech:nicalequilibrium surface. Symbols and chemography follow the convention established in Figure 8.

assemblages in CO2-rich fluids and by forsterite -tremolite assemblages with either dolomite oranthophyllite in H2O-rich fluids. Beneattr themechanical eqrrilifdu6 surface, the assemblageanthophyllite - magnesite - dolomite is restricted tofluids that approach binary mixtures of CO2 and HrO.For example, the point MAFE occurs at fugacities onFigure 9 corresponding to a P(CO) of.2028 bars and aP(HrO) of 4243 bars, with p(COr) + p(HzO) = 6271bars using the mixing relationships for CO, and HrOfrom Kerrick & Jacobs (1981); only minor dilution ofCO2 plus H2O by salts or otler gas species is thereforepossible with a total fluid pressure of 7000 bars. Themetasomatic event that led to the development ofanthophyllite - carbonate envelopes around theprograde assemblages apparently took place in thepetrological environment represented by Figure 9.

The absence of a talc - magnesite alteration rind,combined with the fact that the mechanical equilibriumsurface fails to pass through the anthophyllite +magnesite stabilify field in Figure 8, indicate thatconditions for the formation of the anthophyllite -

carbonate rind lie between approximately 650' and680'C, if one assumes that estimates of metamorphicpressure of7 kbart I kbar are correct.

MsrAMoRPrtrcEvor,moN

The metamorphic evolution of the enstatite -magnesite and anthophyllite - carbonate rocks can besummarized using the pressure - temperature andlog/(HrO) - logf(Co) diagnms in conjunction withavailable petrographic evidence.

On the basis of existing dala, little can be said aboutthe origin of the Flinton Creek metaperidotites prior totheir metamorphism in the presence of COlrich fluidsand the forrnation of enstatite - maglesite assemblages.Mineral assemblages such as enstatite + talc andtremolite + magnesite appear to have been stable duringthe early stages of metamorphism; however, petro-graphic evidence suggests that they later becamemetastable relative to anthophyllite and anthophyllite +dolomite. We propose that vein-related, forsterite-bearing assemblages were produced at near-peak

| ?ooo bu'r I| 650"C I

n/ n \ l l c

t t t \t t t It t , \

I N \

h t w

-{U$*rr"

0.05

conditions of metamorphism (approximately 680'C),by infiltration of a relatively HrO-rich fluid alongdiscrete zones, resulting in the local replacement ofenstatite - magnesite by forsterite. The anthophyllite -carbonate is considered to have formed during the earlysteges of cooling (at approximately 650"C) by infiltrationof a relatively H2O-rich fluid phase, resulting in thecomplete replacement of the enstatite - magnesiteassemblage in the smaller ultramafic lenses. Rindformation is believed to have occurred at approximately650oC because Figure 9 indicates that at this tempera-ture, the anthophyllite - carbonate rind would formfrom the widest range of compositions of infiltratingfluid without the formation of an additional talc -carbonate rind.

The temperafure est:mates for the formation offorsterite veins (680"C) and anthophyllite - carbonaterind (650'C) are consistent with those derived fromttrermobarometry of rocks listed in Table 1. Calculationof additional loe/(HzO) - log/(Co, diagrams at lowerpressures suggests that the temperature estimates forthe metamorphism of the ultramafic rocks are relativelyindependent of the est:mated pressure derived fromresults in Table 1. These consistent est:mates of tem-perature support the suggestion that the ultramaficrocks of the Flinton Creek Complex were metamorphosedsimultaneously with the surrounding rocks.

We tentatively conclude that the important factors inaccounting for the differences in the petrography ofmetaperidofites at Flinton Creek and Guglia werehigher pressures along the P-T-t path at Guglia and thepresence of a CO2-rich fluid at Flinton Creek.

ConclusroNs

Ultramafic lenses along 7 kilometers of exposure inthe Central Metasedimentary Belt of the GrenvilleProvince were metamorphosed in the presence of aCO2-rich fluid to produce enstatite - magnesite assem-blages as well as accompanying talc, anthophyllite and,ultimately, forsterite. Petrographic evidence indicatesthe mantle of anthophyllite - magnesite - dolomiterock around cores of the prograde assemblages are theproduct of a metasomatic process that occurred duringretrograde metamorphism in the presence of a fluidphase with a significant content of both CO2 and HrO.

Field relationships, including the development ofblackwall alteration around the lenses, indicate thatmeramorphism took place in situ.Thermobarometry ofadjacent alnminous schists indicates that metamorphicconditions reached at least 680"C and 7000 kilobars.

AcrNowtpocswGr.ITs

Instruction on the use of the electron microprobewas provided by P. Jones. FDF acknowledges theassistance of an NSERC post-graduate scholarship. Theresearch was supported by a grant to GBS from

1235

NSERC. R.M. Easton reviewed an early version of themanuscript and encouraged its submission. RobBerman, Mike Easton, Robert F. Martin, K. Buchetand an anonymous reviewer provided comment andcriticism that considerably improved the manuscript.

RBrsnENcss

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Received March 23, 1996 , revised mnnuscript accepted June16. 1997.