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Enstatite - Magnesite and Anthophyllite - Magnesite Assemblages from the Grenville Province

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Enstatite - Magnesite and Anthophyllite - Magnesite Assemblages from the Grenville Province 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] Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 1222 THE CANADIAN MINERALOGIST '^O^'^i/'^Vi^^ C*Addington * C*»* Granite * ^sa^CZ^?^* * 0km "<n> 2km ♦ / ^ -^ >. -^ ^^^ *»»,♦♦»♦♦»♦ y^ 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). Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 PETROLOGY OF THE FLINTON CREEK METAPERIDOTITES 1223 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-<r^ n \^fglsi fp .*» *mk A , • v « ♦ ♦ ♦ ♦ ♦ T^L* 1 111 £i\Vr< ^«mLj^<^Xi^jML~t\~" > . .1*4 .!"> s ^ B r*_v' * . » » » ♦_c<lsv»v>Tl Wflffcv -^•1^™ITB'?cjK\_!t1_z~ ..... *1te| ♦ K s s v_-wuSfr .*.*.*.*.*. yj . Vss su ♦ ♦ ♦ . t\ Vss ">r ["LSuAin ^ESSESi] ',*.*.*.'.*. ♦ .1 s s H . ♦ ♦ ♦ ♦ * *• \ ** ^ ^ i »* » » » < • *l s \ \ ........ ts ss \ ♦ ♦♦•♦♦♦♦I L^TQneHl]^ 1 1 1 Is S \ v ' 0m 5m 10m .L V s v ' . 'i . '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. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/5/1221/3435713/1221.pdf by guest on 25 September 2021 1224 THE CANADIAN MINERALOGIST 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,
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