1221
The Canadian Mineralogist Vol. 35. pp. 1221-1236(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. Departmentof Geology, Carleton University, Ottawa, Ontario K1S5B6
Abstract
Two types of mctapcridotite, enstatite - magnesite rockand anthophyllite - magnesite - dolomiterock,arepresent in ultramafic lenses fromthe southern Flinton Synclinorium, Central Mctasedimentary Belt,GrenvilleProvince, in southeastern Ontario. A mantleof the anthophyllite - carbonate rockalwayssurrounds the primary enstatite - magnesite rock, which is preserved onlyintheinterior of thelarger ultramafic lenses. Theenstatite - magnesite rock contains enstatite (Mg«). magnesite (Mgw), dolomite (Mg,,), as wellas talc(Mg„),anthophyllite (MgM), forstcritc (MgM), andrelictchromian magnetite. The assemblage enstatite +magnesite requires thepresence of a relatively C02-rich fluid phase. The presence of late-stage forstcritc veins in the enstatite- magnesiterock may be explainedby infiltration of comparatively HjO-rich fluid at ncar-thcimal-peak metamorphic conditions. The anthophyllite - carbonate rockiscomposed of spherically radiating anthophyllite intergrown with magncsitc, dolomiteandchromian magnetite; the compositions of minerals aresimilar in thetwo rocktypes.The presence of onlyone silicate in addition to thecarbonates suggests that the anthophyllite - carbonate assemblage formed asa mctasomatic mantle duringthe early retrograde history of the ultramafic lenses. Kyanite - biotitc assemblages in adjacent pclitic rocks indicatea regionalmetamorphic gradein the middle amphibolite facies. Thermobarometryrecordsmetamorphictemperatures of620° to 680°C at pressures of6.5 to 7.5 kbar.
Keywords: mctapcridotite, ultramafic,anthophyllite, enstatite, magnesite,Grenville Province,FlintonGroup, Ontario.
SOMMAIRE
Nous decrivonsdeux sortcsde mltapendotite, soil une rocheaenstatite+magnesiteet une autrea anthophyllite +magnc'site +dolomite, dans des lentilles ultramafiques affleurant dans le synclinorium de Flinton,de laccinture metase-imentaire centrale de la Province du Grenville, au sud-est de 1'Ontario. Un liscri a anthophyllite + carbonate entoure dans tous les cas la roche primaireaenstatite +magnesite, prcservdesculcmcnt a I'intcncur des plus grosseslentilles ultramafiques. La rocheaenstatite + magnesite conticnt enstatite (Mgw)< magnesite (MgM), dolomite (Mg,,), de mSme que talc (Mg,7), anthophyllite (Mgsa), forstcnte(Mgw),ct magndtitechromifereresiducllc. L'assemblage enstatite+magnesiteimpliquela presence d'une phasefluide relativcmcntrichc en C02. La presencede veines tardivesa forstcntedansla rocheaenstatite+magnesitercsultcrait pcut-etre de 1'infiltration d'une phase fluide comparativement plus riche en H,0 a des conditions de temperature proches de cellcs du paroxysme m£tamorphique. La roche a anthophyllite + carbonate contient des cristaux d'anthophyllitc fibroradic's en intercroissance avec magnesite, dolomite et magncute chromifere. Les compositions des mineraux sont semblables dans les deux sortcs de roche. Vu la presenced'un seul silicateavec ces carbonates,l'associationanthophyllite +carbonatereprcsenterait un liserc mctasomatiquc forme1 lorsde la retrogression precocede ces lentilles. Les assemblagesa kyanite +biotite des roches peiitiques adjacentes indiquent un degre de metamorphismc regionaldans le facics amphibolite moyen. La thcrmobaromemc indiquc une temperature de mdtamorphisme de 620° a 680°C a une prcssion entrc 6.5 et 7.5 kbar. (Traduit par la Redaction)
Mots-ele's: mdtapendotite, ultramafique, anthophyllite, enstatite, magnesite. Province du Grenville, Groupe de Flinton, Ontario.
Introduction Grenville Structural Province, in southeastern Ontario (Fig. 1). The ultramafic boudins, herein termed the Metaperidotitic rocks arc exposed in a scries of Flinton Creek Metaperidotites, are associated with a discontinuous boudins along the eastern margin suite of metamorphosed mafic rocks ranging from of the Flinton Synclinorium, located in the Central metagabbro to amphibolite. The metaperidotites are Mctasedimentary Belt (Wynne-Edwards 1972) of the interesting in that they contain significant proportions
* Present address: Geoscience Laboratories, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5. E-mail address: [email protected]
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C*Addington * C*»* Granite *
^sa^CZ^?^* * 0km "
Fig. I. Geology of the southern Flinton Synclinorium and surrounding area; asterisk in inset map indicates geographiclocation.Open dots show the location of samples of ultramafic rock mentioned in the text. Diamonds indicate location of samples used for thcrmobarometry. The main body ofthe FlintonCreek Complex (black) occurs within the Flinton Group, in the core of the synclinorium, with a long tail extending northward to the contact with the Northbrook Tonalite. Sporadic lenses ofultramafic rocks occur north ofthis point, in the contact zone between the tonalite and the Flinton Group. At least two ultramafic lenses occur entirely within the Northbrook, several hundred meters away from the contact (sample 110488). The Flinton Synclinorium is bounded on the cast and west sides by shear zones that have not been indicated. The compilation was produced from the observations of Thompson (1972), Chapped (1978), and Bright (1986).
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of magnesite and dolomite, in addition to silicate Field Relations minerals more commonly associated with meta morphosed ultramafic rocks, such as enstatite, The metaperidotites generallyoccur as lenses in an anthophyllite, trcmolitcand forsteritc (Trommsdorff& elongate belt of deformed mafic rocks. Together, the Evans 1974).In this paper, we discuss the petrologyof mafic and ultramafic rocks form what is herein termed these carbonate-bearing ultramafic rocks and describe the Flinton Creek Complex. Individual ultramafic their petrogenesis on the basisofobservedassemblages lenses vary in size from I m to over 50 m in longest and textures of the minerals, thcrmobarometry, and dimension. Largerlenses contain a core ofenstatite - calculated phase-relations. magnesite rock surrounded by an envelope of
p ® >\ © w II /\* ft . vik r- »♦♦♦♦♦i ww mr MVS'X! *♦***_.*! \^ s !• b7ti-
'V IS Enstatite-Magnesite Rock
%*| Anthophyllite-Carbonate Rock Flinton Creek 11 Black Wall Alteration Zone Complex ^ Gabbroic Rocks _ |s\ I Metasedimentary Gneisses \*'\ Northbrook Tonalite
Fig. 2. Hypothetical cross-section of several ultramafic lenses looking along strike. Foliation in the host rock is subparallcl to the long axis of the ultramafic lenses as drawn; however, the direction of maximum elongation is perpendicularto the diagram. Note that the two ultramafic rock-types form concentric zones, with enstatite - magncsitc rock in the core of the larger lenses, surrounded by an envelope of anthophyllite - carbonate rock. Smaller ultramafic lenses consist entirely of the anthophyllite - carbonate assemblage.
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anthophyllite - carbonate rock. Smaller lenses arc TABLE 1. GEOTHERMOBAROMETRY RESULTS composed entirely of anthophyllite - carbonate rock.
Sample locations are shown on Figure 1.An idealized TbamoraebyCC) Buometfy(ban) cross-section through the Flinton Creek Complex is (1) (2) (» (3) (4) shown as Figure 2.
A metasomatic reaction-zone or "blackwall" 10 to 93-43b 614 616*20 640011280 20 cm thick is developed where the ultramafic lenses 93-HA3 63S 714 654 ±7 6765*155 7500 are in contact with the surrounding metamorphosed 9J-361 737 685 750118 7425 t 675 6200 mafic rocks. Similar reaction-zones have been described from other localities of metamorphosed (I) GliM-b»tilethemnm^usingfecaEbruionofHalgci& Spar (1982)for ultramafic rocks around the world, including >pressure of7 ktar. (2) Amphibole- pUgioclue thermometryusingIhe calibration ofHolland ft Bluntly(1994) fora pressure of 7 ktnr. (3) 1NTERSXresultsusing Paddy-Go-Easy Pass (Frost 1975) and the northern TWQ (version 102; Bcrman 1991). The * values represent I o sctlter of Appalachians (Jahns 1967, Sanford 1982). The intersections. (4) Amphibole-gmiet-plagioduebtrometryusinf! IhectJibrilion of Kohn& Spear (1990)atT(l). Mineral assemblages: Grt- Bl - Ms- Ky- PI- following mineral zones progress outward from the Qti (93.43b). Grt-Bl-Hbl-PI-Qtz (93-HA3,93-36a). Mineral compositions ultramafic lens into the surrounding mafic rocks: used to obtain the P-T resulu areavailableupon request from Ihe firstauthor, and from the DepositoryofUnpublished Data, CISTI, National ResearchCouncilof monomineralic tremolite zone, monomincralic biotite Canada, Ottawa, Ontario KlA 0S2. zone, chlorite zone. This sequence of mineral zones is similar to the amphibolile-facics blackwall zones described by Sanford (1982) at Blandford, Massachu setts and Grafton, Vermont. The presence of this Tonalite. Thermobarometric results arc summarized in metasomatic reaction-zone between the ultramafic Table 1. Temperature estimates range from 620° to lenses and the surrounding host-rocks indicates that the 680°C. Estimated pressures are 6.5 to 7.5 kbar. two dissimilar rock-types were in contact with each Thermobarometric results for 93-43b (staurolite - other during regional metamorphism, eliminating the kyanite- biotite)areconsistentwith published grids for possibility of post-mctamorphic tectonic emplacement metapelite (e.g., Powell & Holland 1990), which for the metaperidotites. indicate the stability ofthis assemblage at the estimated temperature and pressure. The thermobaromctry for Metamorphism sample 93-43b indicates metamorphic conditions approaching the stability field ofsillimanite; this is in Pelitic schist from the Bishop Corners Formation, agreement with the regional location ofthe kyanitc-to- containing the assemblage kyanite +biotite +staurolite sillimanite isograd (Carmichael el al. 1978) that lies + muscovitc + quartz (Ford & Skippcn 1994), is between outcrop 93-43 (staurolite - kyanite - biotite) coincident with the northernmost occurrence of the and sillimanite-bearing pelitic schists 4 km to the east. ultramafic rocks at locality 93-43 (Fig. I). An increase in regional metamorphic grade southward from Petrography and Mineral Chemistry this locality has been demonstrated in calcareous sedimentary rocks of the Lessard Formation Approximately 250 samples distributed throughout (Thompson 1973), indicating that the assemblage the Flinton Creek Complex were selected for detailed kyanite - biotite gives a minimum estimate of petrographic examination. The locations of samples metamorphic grade for the Flinton Creek Complex. investigated by electron-microprobeanalysis (Tables2, Mafic rocks within the Complex contain assemblages 3) are shown on Figure 1. compatible with this estimate (amphibolc - plagioclase - epidote - garnet). The increase in metamorphic grade Whole-rock chemistry southward within the Flinton Synclinorium is supported by the appearance of two-mica granitic Sixteen samples of enstatite - magnesite and pegmatite (with or without garnet) in Flinton Group anthophyllite - carbonate rock were collected for psammitic rocks that are spatially related to the whole-rock geochemistry throughout the Flinton Creek metaperidotites southward from locality 101788 on Complex; representative compositions are given in Figure 1; Thompson (1973) interpreted the bodies of Table 4. Low analytical totals, due to the presence of granitic pegmatite to be produced by partial melting of carbonates and hydrous minerals, preclude an extensive the Flinton Group psammites. discussion ofthe chemical petrogenesis ofthese rocks; Samples for geothermobarometry were collected however, it is worth noting that both rock types have from several rock-types, including: pelitic schist, high levels ofCr (2000-7500 ppm) and Ni (1000-2300 garnet-bearing amphibolite, and semipclitic gneiss ppm) contents, which is compatible with an ultramafic containing garnet, amphibole, and biotite. The protolith. The proportions of Si, Fc and Mg remain stratigraphic position of this last-named rock-type is within the range expected for ultramafic rocks, unknown, as it is exposed only intermittently between indicating that variations in bulk composition are not the Flinton Creek Complex and the Northbrook responsible for the different mineral assemblages
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TABLE 2. AVERAGE COMPOSITION OF ENSTATITE. OLIVINE. ANTHOPHYLLITE AND TREMOLITE •
Enstatite Olivine Anthoph.llilc Trvmolile Sample 111587-lc 1I0488-C2 101788-lb 111587.1c 1I04SS<2 IOI788.|b 060489.2b 050888.2a 111587-lc II048K-C2 103188-cl 111587-lc II0488-C2 W
SitH »t.% 57.65 57.46 57.93 41.49 4068 40.97 40.95 40.79 59.48 59.12 58.33 57.33 54.74
Tr02 007 0.10 Oil - - - - - 0.08 0.08 0.07 0.18 0.18
A1203 008 0 00 0.14 ~ ~ - - _ 0.14 0.36 0.22 0.67 2.44
CrS03 0.10 0.10 0 14 - •• -- - .. 0.10 0.06 0.07 0.33 0.45 FcO 60S 725 619 8.09 9.32 8.67 9.30 8.54 6.88 6.98 7.81 1.93 2.68 MoO 0.14 0 24 0.15 0.16 0.16 0.14 0.10 0.20 0.35 0.25 0.16 0.12 009 MgO 34.99 3457 3508 50.98 48.77 49.36 49.37 50.07 30.72 30.78 29.85 23.64 2262
NrO - - - 0.74 0.37 0.62 0.39 0.52 - _ - _ _
CaO 009 0.11 0.10 ~ - ~ - - 0.70 0.36 0.34 13.20 1285 K20 ------0.12 012 0.13 0.09 0.09
Toal 99.17 99.83 99 84 101.46 99.30 99.76 100.11 100 II 9857 98.11 96.98 97.49 96.14
Oxygen atoms 6 6 6 4 4 4 4 4 23 23 23 23 23 Si 1.998 1.992 1.996 0.997 1.003 1.003 1001 0.995 7.947 7.917 7.944 7.863 7.642
Ti 0.00! 0.003 0003 ~ - - - - 0008 0.008 0.007 0.018 0019
Al 0.003 0.000 0006 - - - - - 0021 0.057 0.035 0.109 0.402
Cr 0.003 0.003 0.004 - - - - - 0.011 0.007 0.003 0.036 0.049 Fc 0.175 0.210 0.179 0.162 0.192 0.178 0.190 0.174 0.769 0.782 0.890 0221 0.313 Mn 0004 0.008 0004 0.003 0.001 0003 0.002 0.004 0040 0029 0.018 0.014 0.010 Me I80S 1.786 1.791 1.826 1.792 1802 1.799 1.821 6.118 6.144 6.058 4.834 4.706
Ni - - - 0.014 0007 0012 0.008 0.010 _ - - _ _
Ca 0003 0.004 0.004 - - - - - 0.100 0050 0050 1.940 1.923 K ------0020 0.021 0.022 0.015 0.016
Total 3996 4.006 3.987 3.002 2.997 2.998 3 000 3005 15034 15.015 15.032 15.050 15.080
mglf 0.91 089 0.91 091 090 0.90 090 091 0.88 0.88 0.87 0.95 0.94
t Mg/(Fc«Mn»Mg.Ni) •daerairxd byclecuon-flttcroprobc analysis Thenumber ofanalyses isgiven inbracken Analysis onCailcton Unntrsiry Cambridge Mitrostra Vdeoronmicioprobe eqiippcdKithancnrrfjdiipcrsirtspcctronielcr. Openlir^eorAtioiisotTe20KVardaipcarricncurrtraof9Eiriip. Average collection timewas100s livetime. Data reductionuaj completedon a persona!computet using the EDDIprogramofPringle(l9S9)
presentin the enstatite- magnesite and anthophyllite- to the Flinton Creek Metaperidotites, whereby carbonate rocks. The analytical data of Table 4 are "equilibrium tends to be maintained locally within a compared with standard ultramafic compositions on reacting systemeventhough thesystemasa wholemay Figure 3. be distinctly out ofequilibrium." Enstatite in the enstatite - magnesite rock is coarse Petrography of the enstatite - magnesite rock grained (10-30 mm) and subhedral, and forms a network of mutually interfering crystals. Chlorite, The enstatite - magnesite rockis composed primarily opaque minerals and minor amounts ofcarbonate occur of coarse-grained, euhedral enstatite and medium- to as randomly oriented inclusions in enstatite. Several fine-grained magnesite (Fig. 4a). Pettersen (1883) electron-microprobe traverses across the larger grains applied the name sagvandite to similar carbonate- of enstatite revealed no compositional zonation. orthopyroxenites from the Troms region of northern Enstatite crystals from different localities throughout Norway. Carbonate-bearing orthopyroxenites havealso the complex have similarcompositions(Table2). beendescribed from Aahcim,western Norway (Kendel The rock is composed of 20 to 30% carbonate 1970) and the Central Alps (Evans & Trommsdorff minerals. Magnesite commonly occurs as fine- to 1970, 1974). Additional workon theTroms locality has medium-grained, anhedral crystalsin magnesite-rich been presentedby Schreyer clal. (1972)andOhnmacht domains found intcrstitially among, andsurrounding, (1974). A review of Norwegian metamorphosed the coarser-grained crystals of enstatite; however, a ultramafic rocks is provided by Bucher-Nurminen small numberof samples have large, single crystals (1988). of coarse-grained (1-5 cm), euhedral magnesite in Textures in this rock type arecomplex, with obvious textural equilibrium with the adjacent enstatite (Fig. replacement and overgrowth of early minerals compli 4a).Anhedral magnesite is commonly associated with cated by the laterdevelopment ofvein-related minerals anthophyllite. Enclaves of magnesite - anthophyllite suchas magnesite andolivine. The driving force for the can be found within large grains of enstatite, or on development of these textures is believed to be fluid grain boundaries, where the two minerals seem to be influx. As such, theconcept of "local equilibrium", as growing intothelarger crystal (Fig. 4b).The following defined by Thompson (1959). is believed to apply reaction describes the relationship among the three
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TABLE 3. AVERAGE COMPOSITION OF CARBONATES. TALC AND CHLORITE • enstatite - magnesite assemblage. Each veinlet has
Maaoesire Dolomite Talc Chlorite a core of irregular, anhedral olivine that passes Sample 111587 110489 101781 111387 110488 loi7ss to)ib» III5S7 transitionally into the enstatite - magnesite rock. -Ic -c3 -lb -lc <) -la <1 •Ic (10) (11) « (12) (») (10) (27) (4) In this transition zone, olivine forms pseudomorphs Rk. Type EM EM EM EM EM EM AC EM of the original magnesite inclusions in enstatite. SrOJ M % - 6206 6186 32*9 The occurrence of olivine in veins suggests that the TK>2 0 14 0.12 0.10 AI203 0 13 1.13 16)1 presence of a fluid phase may have contributed to CrtOS 0 12 0.16 I II its formation. The following reaction applies to the FcO 3.56 4.66 4.29 lit 145 1 It 1.21 34* MnO 0.26 0 2) 0 18 008 00* 006 006 0.11 coexistence of enstatite, magnesite and olivine in MtO 44.82 43.66 4148 21.23 20 97 3049 30.36 39.01 the presence of a fluid phase: NiO 0 33 0.29 041 CaO 29.23 2923 K20 MgjSi30„ + 2MgC03 = 2Mg2Si04 + 2C02 Total 4*94 49.39 51.74 52.17: 9448 93.19 S9 38 enstatite magncsitc forstcritc (3) Oxy.at 3 J 6 6 II II 14 Si 3 9*3 3.942 3.033 Ti O0O7 0.006 O.0O7 Three amphibole minerals, anthophyllite, tremolite Al 0012 0.083 1.175 Ci 0 006 0.00* 0.096 and magnesio-cummingtonite, arc found in the enstatite Fe 0043 0.0S6 0051 0030 00)* 0062 0063 0.263 - magncsitc rock. Anthophyllite replaces enstatite and Mo 0.003 0003 0.002 0.002 0002 0.00* 0003 0009 Mj 0.948 0.935 0.940 0.9*9 0915 2912 2.8*4 4 SI) talc (Fig. 4d), and in some cases overgrows olivine, Ni 0017 0019 0.0)0 0006 0.006 0007 0.979 0 977 Ca - - - TABLR 4. BULK COMPOSITION
Total 0999 1000 0999 2.000 2000* 7003 7006 10026 OF THE FLINTON CREEK METAPERIDOTITES J
mj»t 095 0.93 0.94 049 0 49 0 97 0,97 094 Sample Il0488-c2 1104-cS 103188-cl I10488-C7 t Mj/IFetMB«Mg«Ni) ]-Total 0 44% SiO - 0 00* Si ptr 6 Osytcm Rk.Typc EM EM AC AC • daeiuiiuoj by dectnMwnicroprabe analynt. The numberof analvtetit arttn i&brackets. Anihtici] condilwns indicated on Tebie 2. Si02 V.I.V. 51.79 39.12 41.38 48.24 Ti02 0.06 0.06 0.01 0.05 AI203 1.68 2.07 0.57 1.50 Fc20Jt 10.41 1154 7.15 9.05 minerals, ifone assumes that chemical equilibrium was MnO 0.15 0.14 0.11 0.12 achieved: MgO 33.19 33.50 32.64 2845 CaO 0.43 1.64 325 4.47 4Mg2Si20* + HjO + C02 = Mg,Sig022(OH)2 + MgCO, Na20 0.00 0.24 0 16 0.29 K20 001 0.01 0.10 0 01 enstatite anthophyllite magnesite P20J 0.01 001 0.01 0.01 (1) S 0.12 0.05 0.09 0.05
Table 2 shows that both textural varieties of Ba ppml 77 70 61 65 magnesite have the same chemical composition Cr 6255 5156 2091 3333 (101788-lb is the coarse-grained type, and 110488-c2 Zn 109 75 94 101 Ni 2302 1949 1877 1541 is the anhedral variety). V 57 56 20 28 Dolomite occurs as fine- to medium-grained, anhedral crystalsin textural equilibriumwith magnesite; Total 99.11 89.41 86.04 92.98 dolomite - magnesite symplectitic intergrowths were observed in two thin sections. Most silicate minerals, t Total iron as Fc-Oi ( Analysesmadeat the UniversityofOttawa X-rayFluorescencefacility including enstatite and anthophyllite, seem to be in stable coexistence with dolomite; however, in several thin sections, dolomite appears to be a product of the replacementoftremolite, suggesting that the following which also replacesenstatite. It has not been possible solid-phase reaction proceeded from left to right. to distinguish anthophyllite that developed along the prograde metamorphic path from retrograde Ca2Mg5Si»022(OH)2 +4Mg(COj) =2CaMg(CO,)2 + anthophyllite that developed at the same time as the tremolite magncsitc dolomite anthophyllite - carbonate mantles that surround the MgjSi.O^OH^ enstatite - magnesite rock. anthophyllite (2) The presence of embayed grains of tremolite in Olivine has been found in the enstatite - magnesite contact with carbonate suggests that tremolite became rock at all but the northernmost occurrence (500 m metastable prior to or during the growth of southeast of 93-43b), which is also the lowest-grade anthophyllite. Most tremolite crystals have been locality. Olivine occurs as tcxturally late crystals overgrownby lateranthophyllite.Tremolite grains in in veinlets that cross-cut and replace enstatite and contact with magnesite are corroded, indicatingthat magnesite (Fig. 4c). At themicroscopic scale, theolivine prograde metamorphism exceeded the conditions of veinlets are not in sharpcontact with the surrounding solid-phase reaction (2) above.
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SiO„ Quartz
0 Mineral observed in the Flinton Creek Metaperidotites O Mineral not observed in the Flinton Creek Metaperidotites
Talc Anthophyllite Enstatite
Antigorite Chrysolite
Forsterite
FeO+MgO +MnO Dolomite Magnesite
Fig. 3. Minerals commonly observed inmetamorphosed ultramafic rocks projected intothe system CaO - Si02 - (FeO+ MgO + MnO). Darkcircles indicate mineralsobserved in the Flinton Creek metaperidotites, unfilledcircles designate otherminerals com monly associated with metamorphosed ultramafic rocks that were not observed at Flinton Creek. Diagram assumes that a mixedH20 - C02 fluid phase is present in excess. Diamonds indicate thecompositions of Flinton Creek metaperidotites listed in Table 2 (open: anthophyllite - magnesite rock, filled: enstatite - magnesite rock). The average compositions of various ultramafic rock-types (Le Maitre 1976) are also shown: Py pyroxenite. W wehrlite, L Ihcrzolite, H harzburgitc, Ddunite.
Talc is found both as a prograde metamorphic Petrography ofanthophyllite - carbonate rock mineral, and as a product of late-stage alteration. Prograde talc occurs as medium- to fine-grained, This distinctive rock is composed of fine-grained, subhedral laths within enstatite crystals and along anhedral carbonate (magnesite and dolomite) and enstatitegrain-boundaries, indicating the earlystability medium-grained, subhedral needles of anthophyllite, oftalc and enstatite. The assemblage talc +enstatite, which grow in spherical, radiating rosettes with an where not armored in large crystals of enstatite, is average radius of approximately 1 cm. Other silicate usually overgrown by anthophyllite (Fig. 4d). The minerals are present in the anthophyllite - carbonate coexistence of talc + enstatite with anthophyllite is rock in minor amounts including: magnesio- governed by solid-phase reaction (4) below. cummingtonite, chlorite, and late-stage, retrograde talc. No compositional differences could be detected Mg,Si4Ol0(OH)2 + 2Mg2Si20„ = Mg,Si8022(OH)2 between anthophyllite from the enstatite - magnesite talc enstatite anthophyllite (4) and anthophyllite - carbonate rock types (Table 2).
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z <
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Fio. 4. Photomicrographsof ullramafic assemblages. A. Ensialite - magncsitc rock with coarse-grainedenslatilc (E) andmagnesilc (M) in Icxiuralequilibrium.
Widlh of field:7 nun. B. Replacementof enstatite (top) by anthophyllite +carbonate.Width of field:7 mm.C. Late-stageolivine (O)veinrunningdiagonally 00 f-l acrosscenterof photo.Olivinehasreplacedenstatite (E)andmagnesite (M),leaving severaloptically continuousrelics of ensialite incenter of the photo.Widlh C4 of field: 3.5 mill. D. Anthophyllite (A) overgrowing talc (T) and enstatite. Width field: 3.5 mm. of
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Magncsitc and dolomite are more abundant in the obtained by Mader & Berman (1991). These data are anthophyllite - carbonate rock than in the enstatite - used to examine the equilibrium conditions of the two magnesite rock, and make up approximately 30 to 50% solid-phase reactions present in the CaO - MgO - Si02 ofthe total volume. Magnesite is generally more abun - H20 - C02 system (reactions 2 and 4 above). dant: however, the ratio of dolomite to magnesite is The locations ofthese two equilibria in P - T space dependent on proximity of the contact between the have been plotted for the various thermochemical ullramafic body and the surrounding host-rock. databases using a Microsoft EXCEL spreadsheet Although the total amount ofcarbonate present in the program that solves the following equation by iteration: rock remains constant, the proportion of dolomite to magnesite increases outward toward the contact, where dolomite comprises up to 90% ofthe carbonate present. AC" =0=AW"" +jcp dT-T[S"r' +j(CP/T) dT\ This relationship suggests that there is a metasomatic (5) control on the amount of dolomite present in the mv"'' (p-p,) +{P-pt) [-iv,+Av,»Ai-,(i--r )+Av4(r-r,)"'| anthophyllite - carbonate rock. t(Ai-,/2-Av,) (J>!-l)*(Ar,/3) <'>'-l)+C/.„. *RTtnK
Phase Equilibria Involving Solids In The System CaO - MgO - Si02 - H20 - C02 where AH, S and AV are the enthalpy, entropy and volume of reaction at reference pressure P, and The metamorphism of peridotites in the model temperatureT, (1 barand 298K). Cp is the heat capacity, system CaO - MgO - Si02 - H20 - CO has been calculated with the equation of Berman & Brown described by Greenwood (1967), Johannes (1969), (1985). Ohnmacht (1974), Trommsdorff & Evans (1972,1974, The Av, - Av4 terms in equation (5) account for the 1977a. b), Evans & Trommsdorff (1970, 1974), and change in volume of the solid phases with increasing Evans (1977). Phase relations presented in this paper temperature and pressure, using the following expres emphasize only those aspects of the model system that sion from Berman (1988): apply lo the rocks at Flinton Creek. The seven
major mineral phases observed in the Flinton Creek v.. ultramafic rocks are shown on Figure 3 in the -^-=l+vl(T-Tr)+r,(T-Tr)+v,(P-Pr)+\;(P-Pry (6) ternary compositional space CaO - MgO - Si02. Two additional components, ferrous iron and manganese, arc treated as dilutants of the magnesium component. Note that in equation (5), Av,=A(v, x v^t'/HP), Aluminum occurs primarily in magnesian chlorite, Av2=A(v2x u^-Tr/iO5), Av3=A(v3x u^T'/lO5) and which is always present and docs not seem to be Av4=A(v4x i^T'/lO8), where vn is the coefficient from involved in reactions with higher- or lower-grade equation (6), and u is the molar volume ofeach mineral aluminous minerals. Certain minerals observed in the reaction at reference temperature, Tr, and elsewhere in metamorphosed peridotites do not occur pressure, Pr. The Holland & Powell (1990) database at Flinton Creek; these include quartz, diopsidc and can be made compatible with this expression by serpentine. making the following substitutions for the coefficients Three databases contain thermochemical data for all of isothermal expansion and compression: v, = a and the required phases: Helgeson el al. (1978), Berman v,= -0.1B. (1988) and Holland & Powell (1990). The Berman The Gdisor(,cr term was used by Berman (1988) to (1988) database can be further subdivided, since it model progressive disorder in dolomite with increasing contains two values for magnesite, the first derived temperature from 298.15 K (completely ordered by Chernosky & Berman (1989), and a later value dolomite) to 1423 K (completely disordered). The Gdnontn term in equation 5 is calculated by: TABLE 5 ACTIVITY MODELS USED FORCALCULATING EQUILIBRIA AGl=AHZ-T&SZ (7) Component Formula Activity Model CA»
Arnrtophyllilc Mg.Si.O^OH), (X^)' 0 39 where the AH dl4 and AS drs disorder terms are calculated Tremolite Ca,Mg,Si,0K(OH), Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 1230 THE CANADIAN MINERALOGIST 1 t T | 1 I I 1/ I T 1 T T T I I • | | IJi'ljl I I I 1i iI !i fi 1| iI 1i iI iI iI iI iI i i |n i i i i i i i i( I' 7 7A c 7 I 7// ?/ 3 CA (A £ Q. • 8 f cf f 8 7 , • • * I • •• • 1 . , / • /i_i lA I_| ; i . 7i ;I i.i /i i i i i t i i i i i i i t l l I 200 300 400 500 600 700 800 900 1000 Temperature (°C) Fie. 5. Calculated solid-phase equilibria in a portion ofthe system CaO - SiO, - MgO - H,0 - CO. adjusted for the measured compositions ofminerals(Tables 2,3). Solid lines represent Ihereaction talc+2 enstatite=anthophyllite.Dashedlines for tremolite +4 magnesite = anthophyllite +dolomite: Dd indicates that the variabledolomite-disorder model of Berman (1988) was used to the locale the reaction, whereas nDd indicates that the dolomite-disorder model was not used. Thermo dynamic datascts used in Ihecalculations areindicated as follows: H78: Hclgcsonet al. (1978),CB89: Chernosky& Berman (1989), HP90: Holland & Powell (1990), MB9I: Mader & Berman (1991). All datasets predict anthophyllite stability for the pressure - temperature rangeof metamorphism,indicated by blackdots with errorbars(±50°C,±1000bars); the Berman (1988) database, with modified data for magncsitc (Mader & Berman 1991) and variabledegree ofdisorderin dolomite, predicts the stability ofanthophyllite + dolomite within errorofthe estimated P - T range. model. Also shown in this figure are the thermo- TABLE 6. FLUID-BEARING ASSEMBLAGES COEXISTING AT barometry results (Table 1), which are considered to INVARIANT POINTS IN THE SYSTEM MgO-SiO, represent peak conditions of metamorphism for the Absent Phase Phases Present Flinton Creek metaperidotites. The observed replace Designation ment oftalc + enstatite by anthophyllite indicates that the solid-phase equilibrium, reaction (4), limiting the (Ath) Mgs. En, Fo. Tic MEFT* stability of anthophyllite is expected to lie just below (En) Mgs. Ath, Fo. Tic MAFT* the three P-T estimates. The pctrographic data also (Tic) Mgs, Ath. Fo. En MAFE* predict the stability ofanthophyllite +dolomite relative (Fo) Mgs.Alh.En.Tlc MATE to tremolite + magnesite for the P - T conditions (Mgs) Ath. En, Fo. He FATE (Fo.Mgs) Tic, En, Ath TEA and mineral compositions at Flinton Creek. Figure 5 demonstrates that the thermodynamic data of Berman (1988), as modified by Mader & Berman (1991), • Aide9grttedbyEvaru&TroininsdottT(1974) correspond best with the thermobaromctry and petrography of Flinton Creek rocks; this database is Trommsdorff (1974). At both localities, rocks containing used in subsequent calculations. magnesite, talc, enstatite and forstcritcareovergrown at some later stage by anthophyllite. Although the two Univariant Phase-Relations occurrences are mineralogically similar, pctrographic analysis reveals substantial differences in the sequence The ultramafic lenses at Flinton Creek arc ofmineral assemblages that developed. mineralogically similar to Ihe ultramafic boudins at Four mineral phases and a fluid phase form a Guglia in the Swiss Alps, as described by Evans & univariant assemblage in the system MgO - Si02 - Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 PETROLOGYOFTHE FUNTON CREEK METAPERIDOTITES 1231 12000 0.01S\ 0.04 • A°<» t 11000 MAFE 0.025^00511 i \ A 0.1 0/ 10000 MEFT IvIAFT 9000 •w 8000 2 CO J3 7000 6000 2 Q. 5000 4000 3000 2000 1000 400 450 500 550 600 650 700 750 800 Temperature (°C) Fig. 6. Pressure - temperature diagram for univariant points with pure magnesian minerals. Symbols along curves and numbers refer to incremental values ofthe mole fraction of C02 along the univariant points. Calculated with database of Berman (1988), as modified by Mader & Berman (1991). MAFE, MAFT, MEFT, TEA indicate univariant curves with M (magnesite), A (anthophyllite), F (forslcrilc), E (enstatite), T (talc). Solid curves are stable; dashed curves are metastable. H20 - C02. Evans & Trommsdorff (1974) described MATE and FATE therefore project through the compo one such assemblage characteristic of the univariant sition dimension onto the P- T path ofTEA. TEA and condition MEFT (magnesite - enstatite - forsterite - MEFT do not intersect on Figure 6, indicating that the talc - fluid; Table 6), at the Guglia location. The invariant point MEFTA is metastable for end-member presence of forsterite in the Guglia rocks from the magnesian phases using the thermodynamic data of beginning oftheir petrological evolution is essential to Berman (1988) and Mader & Berman (1991). the recognition ofthe univariant assemblage MEFT as Evans & Trommsdorff (1974) pointed out that the a stable one. This situation contrasts with the rocks at addition of iron to the end-member magnesian phases Flinton Creek, where forsterite overprints the earlier results in a substantial increase in the field ofstability magnesite - enstatite assemblage. The significance of of anthophyllite relative to talc + enstatite, with a these petrographic observations can be understood corresponding shift in the location ofTEA to signifi by locating the various univariant assemblages for the cantly lower temperatures. According to the databaseof system MgO - Si02 - H20 - C02 in P- T space.Figure 6 Berman (1988), the addition of iron causes TEA and has been calculated with the program TWQ (Berman the other univariant assemblages indicated on Figure 6 1991) using thermodynamic data for end-member to intersect at a single point (the invariant point magnesian compositions. The figure shows the MEFTA) for an anthophyllite composition of AthM; P-T trajectory of the compositionally degenerate TEA shifts to lower temperatures with further addition univariant curve, TEA (talc - enstatite - anthophyllite), of iron, splitting the point MEFTA into duplicate as well as that of three normal univariant curves, invariant points, as implied on Figure 7 for the MAFE , MEFT and MAFT. TEA contains both MATE measured compositions of the minerals at Flinton (magnesite - anthophyllite - talc - enstatite) and FATE Creek, as reported in Tables 3 and 4. For these (forsterite - anthophyllite - talc - enstatite) in P-T compositions, the invariant point MEFTA is stable and space, since both assemblages contain the solid-phase duplicated at 11,100 bars and <500 bars. MAFT and equilibrium, talc + 2 enstatite = anthophyllite. Both MAFE are stable between the duplicate invariant Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 1232 THE CANADIAN MINERALOGIST T~ryi—r—+—i—i—i i t *'' vJ.yl5.02S 0.025 n MEFTA to jO 2 0. J—I—I—1-1. a,. X .1—1—1—4—(—I—I—1—I—I—J—*- 400 450 500 550 600 650 700 750 800 Temperature ("C) Fig. 7. Pressure - temperature diagram for univariant assemblages, calculated on the basis of measured compositions ofthe minerals (Tables 2. 3). Number and symbols refer lo incremental values of the mole fraction ofCO. along (he univariant curves. Mineral assemblages and thermodynamic data as for Figure 6. points, and MEFT is stable beyond this range at TAB! E 7 INVARIANT POINTS IN A PORTION pressures lower or higher than either of the MEFTA OF THE SYSTEM CaO - MgO - SiO, invariant points. It therefore appears that mineral compositions in the Guglia metaperidotites lead Absent Ftutsc Phases Present Designation to the stability of the MEFT assemblage above the (Alh. En) Do!. Fo. Mijs. Tr, Tic DtRMFT higher-pressure duplicate of the MEFTA invariant (Alh. Mgs) Dol. En. Fo. Tr. Tic DtREFT point, at approximately 10 kbars. (Ath. Fo) Dol. En. Mgs, Tr. Tic DtREMT The Influence of Fluid Composition (Alh. Tic) Dol, En. Fo. Mgs. Tr DtREMF (Mgs. Tic) Alh, Dol, En. Fo, Tr DtRAFE The influence of fluid composition on the mineral (En, Fo) Ath, Do!, Mgs, Tr. Tic DiRAMT assemblage ofthe ultramafic rocks at Flinton Creek can be examined on Figures 8 and 9. which consider the (En. Mgs) Alh. Dol. Fo. Tr. Tic DtRAFT stability of mineral assemblages, including the calcic (En. Tic) Alh, Dol. Fo. Mgs, Tr DtRAMF phases dolomite and tremolite, on In fugacity (HjO) - (Fo. Mgs) Alh. Dol, En. Tic, Tr DtRATE In fugacity (C02) diagrams, constructed according to (Fo. Tic) Alh. Dol, En, Mgs. Tr DtRAME the method described by Korzhinskii (1959). The free energy of disordering in dolomite (Berman 1988) has been included in the calculation. Also shown on these figures is the mechanical equilibrium surface along listed in Table 7. The fugacity diagrams allow the which calculated pressure of fluid is equal to lithostatic petrography of the calcic phases to be used as a further pressure P(C02) + P(H20) = Pl0,ai. This surface was check on the evolution of the fluid phase. calculated using the nonideal fluid-mixing model of Figure 8 is calculated for 680°C, the thermal maxi Jacobs & Kerrick (1981). The addition ofcalcium as a mum applicable to the southern Flinton Synclinorium, component results in the stability of two additional as indicated by thermobarometry of rock types other invariant points labelled DTRAFE and DTRAFT, and than pcridotitc. This figure demonstrates that enstatite Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 PETROLOGY ofthe flinton creek metaperidotites 1233 12.0- 7000 bars 680°C 1I.0-- O V o 10.0 9.0 11.0 log / H20 Fio. 8. Ln/(H20) - ln/(C02) diagram at7000 bars and680°C.Chcmography asshownon Figure 4. The mechanical equilibrium surface, alongwhich fluid partial pressures sum to total pressure, is shown as a curved line, with solid circles and numbers indicating incremental values forthe mole fraction of C02. Phase relations areindicated fortalc(Tic), anthophyllite (Ath). enstatite (En), forsterite (Fo), magncsitc(Mgs), tremolite (Tr), anddolomite (Do). Designations of Ihe invariant points arelistedinTables6 and7. Reaction 1: Fo+H20 4- 9CO,=Alh +9Mgs. Reaction 2:32Fo+9Tr +4H20 +36C02= 13Ath + ISDol. - magnesite assemblages require high X(C02) values earlier assemblages in the enstatite - magnesite rock. for stability; this result is in agreement with earlier, Decreasing temperature between Figures 8 and 9 lower-pressure (2000 bars) experiments by Greenwood reduces the separation between the pairs of invariant (1967) and Johannes (1969). points MAFE - DtRAFE and MAFT - DtRAFT such Figure 8 provides a possible explanation for the that each pair is almost coincident on Figure 9. The replacement of enstatite - magnesite by late-stage development of theanthophyllite - carbonate assemblage olivine, through the introduction ofrelatively H20-rich appears to be related to a metasomatic influx of fluid fluids IXXCO;) < 0.45) into the enstatite - magnesite that carriesthe bulk composition ofthe pcridotite into rock. Both enstatite and forsterite can be seen on the the stability field of anthophyllite - magnesite. Forma fugacity diagram to be in equilibrium with dolomite tion of a metasomatic rind could not have carried on to and magnesite, as required by the petrography ofthe much lower temperatures, as the mechanicalequilibrium Flinton Creek rocks; enstatite coexists with the surface would intersect the talc - magnesite stability carbonates and high-CO, fluids along the mechanical field at approximately 640°C. Talc - magnesite rinds equilibrium surface, whereas forsterite and two have not been observed surroundingthe anthophyllite - carbonates coexist with a more H20-rich fluid. carbonate envelope at Flinton Creek. Figure 9 is calculated for a temperature of 650'C, The stability field of anthophyllite - magnesite - corresponding to the development ofthe anthophyllite dolomite on Figure9 is limited along the mechanical - carbonate rock in envelopes that completely replace equilibriumsurfaceby enstatite- magnesite- dolomite Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 1234 THE CANADIAN MINERALOGIST o GO o 8.0 9.0 log/H20 Fig. 9. Ln(H20) - ln/(C02) diagram at 7000 bars and 650*C. The curved line indicates the location of the mechanical equilibrium surface. Symbols and chemography follow the convention established in Figure 8. assemblages in COrrich fluids and by forsterite - carbonate rind lie between approximately 650° and tremolite assemblages with either dolomite or 680°C, if one assumes that estimates of metamorphic anthophyllite in H20-rich fluids. Beneath the pressure of7 kbar ± 1 kbar arc correct. mechanical equilibrium surface, the assemblage anthophyllite - magnesite - dolomite is restricted to METAMORPHIC EVOLUTION fluids that approach binary mixtures ofC02 and H20. For example, the point MAFE occurs at fugacitics on The metamorphic evolution of the enstatite - Figure 9 corresponding to a P(C02) of2028 bars and a magnesite and anthophyllite - carbonate rocks can be P(H20) of 4243 bars, with P(C02) + P(H20) = 6271 summarized using the pressure - temperature and bars using the mixing relationships for C02 and H20 log/(H20) - log/(C02) diagrams in conjunction with from Kerrick & Jacobs (1981); only minor dilution of available pctrographic evidence. C02 plus H20 by salts or other gas species is therefore On the basis ofexisting data, little can be said about possible with a total fluid pressure of 7000 bars. The the origin ofthe Flinton Creek metaperidotites prior to metasomatic event that led to the development of their metamorphism in the presence ofC02-rich fluids anthophyllite - carbonate envelopes around the and the formation ofenstatite - magnesite assemblages. prograde assemblages apparently took place in the Mineral assemblages such as enstatite + talc and petrological environment represented by Figure 9. tremolite + magnesite appear to have been stable during The absence of a talc - magnesite alteration rind, the early stages of metamorphism; however, pctro combined with the fact that the mechanical equilibrium graphic evidence suggests that they later became surface fails to pass through the anthophyllite + melastable relative to anthophyllite and anthophyllite + magnesite stability field in Figure 8, indicate that dolomite. We propose that vein-related, forsterite- conditions for the formation of the anthophyllite - bearing assemblages were produced at near-peak Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 PETROLOGY OFTHE FLINTON CREEK METAPERIDOTITES 1235 conditions of metamorphism (approximately 680°C), NSERC. R.M. Easton reviewed an early version ofthe by infiltration of a relatively H20-rich fluid along manuscript and encouraged its submission. Rob discrete zones, resulting in the local replacement of Berman, Mike Easton, Robert F. Martin, K. Bucher enstatite - magnesite by forsterite. The anthophyllite - and an anonymous reviewer provided comment and carbonate is considered to have formed during the early criticism that considerably improved the manuscript. stages ofcooling (at approximately 650°C) by infiltration of a relatively H20-rich fluid phase, resulting in the References complete replacement of the enstatite - magnesite assemblage in the smaller ultramafic lenses. Rind Berman, R.G. (1988): Internally-consistent thermodynamic formation is believed to have occurred at approximately data for minerals in the system Na20 - K20 - CaO - MgO 650°C because Figure 9 indicates that at this tempera - FcO - Fej03 - Al203 - Si02 - Ti02 - H20 - C02. /. ture, the anthophyllite - carbonate rind would form Petrol. 29,445-522. from the widest range of compositions of infiltrating . (1991): Thermobarometry using multi-equilibrium fluid without the formation of an additional talc - calculations: a new technique, with petrological applica carbonate rind. tions. Can. Mineral. 29,833-855. The temperature estimates for the formation of forsterite veins (680°C) and anthophyllite - carbonate .& Brown, T.H. (1985): Heat capacity of minerals rind (650°C) are consistent with those derived from in the system Na20 - K20 - CaO - MgO - FeO - Fe20j - thermobarometry ofrocks listed in Table I. Calculation A120} - Si02 - Ti02 - H20 - C02: representation, ofadditional log/(H20) - log/(C02) diagrams at lower estimation, and high temperature extrapolation. Contrib. pressures suggests that the temperature estimates for Mineral. Petrol. 89, 168-183. the metamorphism ofthe ultramafic rocks are relatively independent of the estimated pressure derived from Bright, E.G. (1986): Geology of the Mellon Lake area. Ontario Geot. Surv., Open File Rep. 5S98. results in Table 1. These consistent estimates of tem perature support the suggestion that the ultramafic rocks ofthe FlintonCreek Complex were metamorphosed Bucher-Nurminen, K.(1988): Metamorphism of ultramafic rocks in the central Scandinavian Caledonides. In Progress simultaneously with the surrounding rocks. in Studies ofthe Lithosphere in Norway (Y. Kristoffcrsen, We tentatively conclude that the important factors in ed.). Nor. Geol. Unders., Spec. Publ. 3, 86-95. accounting for the differences in the petrography of metaperidotites at Flinton Creek and Guglia were Carmichael, D.M., Moore, J.M. & Skippen, G.B. (1978): higher pressures alongthe P-T-t pathat Gugliaand the Isograds around the Hastings metamorphic "low". In presence of a COrrich fluid at Flinton Creek. Toronto '78 Field Trips Guidebook (A.L. Currie & W.O. Mackasey, eds.). Geol. Assoc. Can. - Mineral. Assoc. CONCLUSIONS Can., 324-346. Ultramafic lenses along 7 kilometers ofexposure in Chappell, J.F. (1978): The Clare River Structure and its the Central Metasedimentary Belt of the Grenville Tectonic Setting. Ph.D. thesis, Carleton Univ., Ottawa, Ontario. Province were metamorphosed in the presence of a C02-rich fluid to produce enstatite - magnesite assem Chernosky, J.V. & Berman, R.G. (1989): Experimental blages as well as accompanying talc, anthophyllite and, reversal of the equilibrium: clinochlore + 2 magnesite = 3 ultimately, forsterite. Petrographic evidence indicates forstcritc +spinel+2 C02 +4 H20 andrevisedthermody the mantle of anthophyllite - magnesite - dolomite namic properties for magnesite. Am. J. Sci. 289,249-266. rock around cores ofthe prograde assemblages arethe product of a metasomatic processthat occurredduring Evans, B.W. (1977): Metamorphism ofalpine peridotiteand retrograde metamorphism in the presence of a fluid serpentinite. Annu. Rev. Earth Planet. Set. 5, 397-447. phase with a significant content ofboth C02 and H20. .& Trommsdorff, V. (1970): Regional metamor Field relationships, including the development of phism of ultramaficrocks in the CentralAlps: parageneses blackwall alteration around the lenses, indicate that in the system CaO - MgO - Si02 - H20. Schweiz, metamorphism took place insitu.Thermobarometry of Mineral. Petrogr. Mitt. 50,481-492. adjacent aluminous schists indicates that metamorphic conditions reached at least 680°C and 7000 kilobars. .&. _(1974): Stability of enstatite + talc and C02-metasomatism of mctapcridotite, Val d'Efra, Acknowledgements Lcpontine Alps. Am. J. Sci. 274, 274-296. Instruction on the use of the electron microprobe Ford, ED. & Skippen, G.B. (1994): Shear zone associated, was provided by P. Jones. FDF acknowledges the late-stage deformation and retrograde metamorphismon the western margin of the Flinton Synclinorium. Geol. assistanceof an NSERC post-graduate scholarship. The Assoc. Can. - Mineral. Assoc. Can., Program Abstr. research was supported by a grant to GBS from 19, A37. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 1236 THE CANADIAN MINERALOGIST Frost, B.R. (1975): Contact metamorphism of serpentinilc, Mader, U.K. & Berman. R.G. (1991): An equation of stale chloride blackwall and rodingile at Paddy-Go-Easy Pass, for carbon dioxide to high pressure and temperature. Am. central Cascades, Washington. J. Petrol. 16, 272-313. Mineral. 76. 1547-1559. Greenwood. H.J. (1967): Mineral equilibria in Ihe system Oiinmacht. W. (1974): Petrogenesis of carbonate- MgO - Si02 - H;0 - C02. In Researches in Geochemistry orthopyroxenites (sagvanditcs) and related rocks from 2 (P.H. Abelson. cd.). John Wiley. New York. N.Y. Troms. northern Norway. J. Petrol. 15, 303-323. (542-567). Pettersen. K. (1883): Sagvandit, en ny bergart. TromsoMus. Arsh. 6.81. Helgeson, H.C.. Delaney. J.M.. Nesbitt, H.W. & Bird. D.C. (1978): Summary and critique of the thermodynamic Powell, R. & Holland. T. (1990): Calculated mineral properties of rock forming minerals. Am. J. Sci. 278A. equilibria in the pclitc system, KFMASH (K.O - FeO - 1-229. MgO - AljO, - SiO, - H20). Am. Mineral. 75. 367-380. Hodges, K.V. & Spear. F.S. (1982): Geothcrmometry. Pringle. G.J. (1989): EDDI - a FORTRAN computer geobarometry and the Al.SiO, triple point al Mt. program to produce corrected microprobc analyses of Moosilaukc, New Hampshire. Am. Mineral. 67. 1118- minerals using an energy dispersive X-ray spectrometer. 1134. Geol. Sun: Can.. Open File Rep. 2127. Holland. T.J.B. & Blundy. J. (1994): Non-ideal interactions Sanford. R.F. (1982): Growth of ultramafic reaction zones in in calcic amphibolcs and their bearing on amphibole - grccnschisl to amphibolite facics metamorphism. Am. J. plagioclase thermometry. Contrih. Mineral. Petrol. 116. Sci. 282.543-616. 433-447. SciiREYi-R. W.. Oiinmacht. W. & Mannchen, J. (1972): .& Powell, R. 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