sign in sign up
<<
Home , Biotite, Coupled substitution, Phlogopite

967

The Canadimt M iner alo gist Vol. 34, pp.967 -97 5 (1996)

BARIUM.AND -RICHBIOTITE AND FROMTHE WESTERN AND EASTERNGABBRO, COLDWELLALKALINE COMPLEX, NORTHWESTERN ONTARIO

CLIFF S.J.SHAWI Departmentof EarthSclences, University of WestemOntario, Inndou Onnio N6A 587

ROBERTS. PENCZAK Depamnentof GeologicalSciences, Quzms Universitt, Kingston Ontario K7L 3N6

ABSTRACT

Bmium- and titanium-emichedphlogopite and are found in the subalkalineEastem and Westsm Gabbrointrusions tltat are cut by syeniteand syenitepegmatite in the Coldwell alkaline s6rnplsx,northwestgm Onurio. The containup to 6.1 trttVoBaO and 8,1 wtEo TiO2, occu$ as a fringe on Fe-Ti oxides, an overgrowth on cumulus and intercumulus clinopyroxene,and poarsegrains that encloseearlier-fonned . Ba-Ti-rich micasare normally found in potassicigneous rocks like ,whereas the samplesin this sfirdy occur in subalkalinegabbro. In the WesternGabbro, the mica is richer in Ba. This suite showsa trend of increasiqgBa with decreasingmineral Mg#, not presentin the less Ba-rich samplesof the EastemGabbro. Most of the grains analyzedhave apparentdeficiencies in their tetrahedral,octahedral and interlayer sites. Deficiencix in the octahedralsites are due to a combinationof Ti-vacancyand Ti-Tschermaksubstitution. The interlayer-site deficienciesare due to the replacementof K by Ba. Coupledsubstitutions such as nBa + IvAl = xlK + wsi also accountfor the accommodatioaof Ba in the structure.Moderately Ba-emichedmicas in the Easternand Westem Gabbro are a result of crystallization of Ba-enrichedresidual liquids trapped in the cumulus framework of the gabbros.In addition, the latest Ba-Ti-rich mica in the Westem Gabbroformed in responseto infiltration of fluids derived from adjacentbodies of Ba-rich syenitepegmatite.

Keywords:phlogopite, biotite, gabbro,barium, Coldwell alkaline s6rnpl61,Ontario.

Sol,lr4alRg

Nous documentonsla pr6sencede phlogopiteet de biotile enrichiesen Ba (usqu'A 6.17ade BaO, poids) et Ti (usqu'i 8.17o de TiO, dans les unitds intrusives subalcalinesappeldes Eastertr et Westem Gabbro, du complexe alcalin de Coldwell, en Ontario. Des venuesde sy6nite,localemqnt pegmatitique, recoupent ces unit6s.Le mica se pr6senteen bordure de grains d'oxydes de Fe-Ti, en surcroissancesur les sristaux de clinopyroxbne, soit d'origine cumulative ou en intercumulats, et en grains gtossiersqui englobentles min6rauxprdcoces. De telles compositionsde mica sont typiques des rochesign6es potassiques,par exempleles lampro'ites,tandis que dansce cas,il s'agit de rochesgabbrolques subalcalines. Dans I'unitd dite Westem Gabbro, le mica est enrichi en Ba. Dans cett€ suite, le mica fait prpuve d'un enrichissementen Ba a mesureque diminue son paramatreMgf, tandis que ce n'est pas le cas dansl'unitd EastemGabbro, dont le mica est moins enrichi en Ba. La plupat des cristaux analys6sauraient des lacunes dans les sites tdtra6driques,octa6driques, et inter-feuillets. Dans les sites octa6driques,les lacunesseraient dues i une combinaisond'une ddficienceen Ti et de la substitutionTlTschermak. Dans les sites inter-feuillets, les lacunesseraient dues au remplacementde K par Ba. Des sch6masde substitutioncoupl6e sBa NAI comme + = )oIK + rvSi expliquent aussila prdsencede Ba dans h Jtructure.La formation de mica emichi en Ba r6sulterait de la cristallisation de liquide r€siduel pidg6 dons les intersticesdes cumulatsde ces massifsgabbroiques. Nous attribuonsla g6ndrationtardive de mica enrichi en Ba et en Ti dansle WestemGabbro h I'infiltration de fluide ddrivd desvenues de pegmatitesydnitique adjacenfes. (Traduit par la R6daction)

Mots-cl6s:phlogopite, biotite, gabbro,baryum, complexe alcalin de Coldwell, Ontario.

I E-mail address.'[email protected] 968 TIIE CANADIAN MINERALOGIST

INTR.oDUc"noN GsoLocYAND PE'rRocRAPnY

The Coldwell alkaline complex in northwestem The Eastern Gabbro (Ftg. 1) consists of three Ontario (Frg. 1) is an unmetamorphosedintrusive discreteintrusions (Shaw 1994),the largestof which is complex consistingof subalkalineand alkaline gabbro the Ba-mica-bearingLayered Gabbro Intrusion. This as well as a variety of (Walker et al. 1993, intrusion consists of variably massive to layered Mitchel & Platt 1994). The complex was emplaced gabbroic cumulatesranging in composition from early in the magmatic evolution of the middle wehrlite to anorthosite.The Western Gabbro is an Proterozoicmid-continent rift at 1108Ma (Heaman& isolated body of layered and heterogeneousgabbro Machado 199 . It consistsof three inrusive centers intruded and locally alteredby youngersyenites. It has (Fig. 1; Mitchell & Platt 1994). T\e oldest center, been divided into a massive and a layered series Center l, consistsof a partial ring dyke of layered to (PenczakL992) that vary in compositionfrom massivegabbroic rocks (EasternGabbro) intruded by gabbro to anorthosite.Similarities in types and -rich augite (Mitchell & Platt 1978). A compositionsindicate that the WestemGabbro is likely small body of layered and heterogeneousgabbro equivalentto the LayeredGabbro. (lVestern Gabbro) occurs in the western part of the The gabbroic rocks are unmetarhorphosed,layered complex and also is associatedwith iron-rich and to massive cumulates with ,olivine and locally pegmatitic and Ba-enriched augite syenite clinopyroxene t apatite t Fe-Ti oxide as the main (Wilkinson1983, Penczak 1992), indicating that it also cumulus phases. Clinopyroxene also occurs as belongsto center 1. poikilitic intercumulus grains, together with ortho- In this work, we describethe composition,mecha- , biotite, phlogopite, Fe-Ti oxides and, nisms of substitution and possible paragenesesof locally, and traces of granophyric inter- Ba- and Ti-bearing phlogopite and biotite from the growth of quartz and K-feldspar, which is Ba-bearing EastemGabbro and the WesternGabbro. intherocks oftheEastemGabbro (Shaw 1994).

Flc. 1. Generalgeology of the Coldwell alkaline complex, showing the distribution of lhe intrusive centersand the location of the EasternGabbro and the WestemGabbro (after Walker er al. 1993). BIOTITE AND PHLOGOPITE,COLDWELL ALKALINE COMPLEX 969

Both the EasternGabbro and the Western Gabbro PE"TRocRAPHY were intrudedby iron-rich augitesyenite and by large, irregular bodies of green syenite . The Biotite and phlogopite occur in the gabbros as contactbetween gabbro and syenitepegmatite is sharp; intercumuluscrystals. Mostly, the mica forms ragged macroscopicalteration is resficled to an aureolea few grains 0.1-{.25 mm acrossthat fringe earlier-formed cm wide, but saussuritizationof feldspar,serpentiniza- Fe-Ti oxides and overgrow cumulus clinopyroxene. tion of olivine and uralitization of clinopyroxenecan Large, poikilitic plates of mica enclosing olivine, be traced for up to 20 cm from fhe contact with the clinopyroxene, Fe-Ti oxides and apatite also are . present.The grains show variable pleochroismfrom The gabbroicrocks are evolved (Shaw 1994),with black-brown or red-brown to pale yellow or yellow- whole-rock Mg# [100 Mg(Mg + 0.9Fe,o),molar] of brown. Mica abundancesvary from trace to more 13 to 60 they areemiched in Rb (on average,26 ppm), than l2%o, being most abundant in orthocumulate Ba (1267 ppm), Sr (1234 Wm), Zt (156 ppm) and layers. rare-earth elements (REE: 9l ppm La). Systematic There is minor optical zonation in some of the changes in bulk-rock and composition, grains observed;however, zonation is rare. Many of indicative of simple fractionation,are absentfrom the the samples sontaining Ba-rich mica, particularly gabbros. those in the Western Gabbro, show a pattern of Petrographicand mineral chemical criteria indicate alteration similu to that observedin gabbros close that the parentmagma was subalkalinein composition to syenite pegmatites(see above). In these sampleso (Shaw 1994; in prep.). This makesthe occurrenceof intercumulus mica is partly altered to chlorite. Ba-Ti-enriched micas unusual, as they are generally Secondary,subhedral biotite with red to straw-yellow confined to potassicigneous rocks (e.9., Mirchell & commonly overgrows the altered Bergmanl99l,Zhang et al. 1993). minerals.

TABLE 1. SELECTEDFT.F.CTRON-MICROPROBE DATA ON Ba-Ti-BEARINGMCAS. COLDWELLCOMPLD(

salrpl€ RP68 91-26?9L-259 91-nZ 9F265 91-2839I-272 91-259 pJ|56 9l-272 92-276 Ry22 R373 RP64 RP28 RP29 RP29 RP29 Locatioa WG EG EG EG EG EG Ec EG Wc EG EG WG WG WG WG WG \vG WC siq 39.37 36,98 35.49 37.69 37.38 36.48 37.46 35.05 33.56 33.58 35.14 34.58 34.35 34.89 33.79 33.91 33.0e 32.46 Tio, 0.91 3.33 4.36 3.K 4.81 2.s7 4.89 4.73 3.99 4.09 4.28 6.00 5.39 5.2s 6.24 6.K 6.93 7.02 AlrO3 13.79 12.43 t2.31 12.90 13.59 16.33 i2.83 11.95 15.94 12.87 15.63 v.n M.1l 15.36 13.82 13.!b 13.87 14.14 Cr2q 0.00 0.00 0.05 0.00 0.00 0.03 0.02 0.06 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 MnO 0.00 0.21 O.24 0.06 0.06 0.03 0.12 0.16 0.05 0.ll 0.11 0.09 0.08 4.02 0.04 0.09 0.13 0.r5 FeO 1t.98 18.53 27.82 15.01 15.61 15.43 17.90 26.88 19.18 27.78 13.r5 19.69 18.45 14.24 t9.W $.19 11,42 20.27 MgO 18.99 12.63 6.21 16.00 14.37 t5.06 12.45 6.81 t3.& 5.32 16.06 9.78 11.50 14.42 9.74 9.44 9.84 E.29 CaO 0O3 0.02 0.03 0.00 0.00 0.03 0.00 0.07 0.03 0.00 0.00 0.04 0.02 0.00 0.03 0.02 0.04 0.02 BaO 0.11 0.33 0.45 0.68 0.75 0.89 0.99 1.00 1.16 1.49 1.80 225 2.@ 3.04 3.67 4.33 5.04 6.07 K?O 7J6 9.19 8.43 9.18 E.52 8.89 9.09 8.06 6.29 8.67 8.74 8.66 8.50 7.63 7.6 7.47 7.00 7.04 NazO l.M 0.17 0.14 0.15 0.44 0.40 0.11 0.t4 0.55 0.08 0.33 0.09 0.17 0.74 0.34 0.47 0.52 0.35 F 0.75 1.66 0.33 1.24 0.E4 0.77 1.15 0.4,0 0,21 0.45 1.26 0.55 0.43 0.56 0.m 0.91 0.99 0.91 cl 0.06 0.11 0.78 0.36 0.08 0.02 0.37 4.44 0.02 1.92 0.03 0.05 0.04 0.02 0.07 0.06 0.07 0.08 GF+CI 0.33 0.72 0.31 0.60 0.31 0.33 0.57 0.27 0.09 0.62 0.54 A.24 0.19 0.24 0.39 0./m 0.43 0.40 Tobl 94.86 94.87 96.33 %.13 %.08 96.90 96.81 95.54 94.29 95.74 es.99 95.83 96.05 95.93 94.71 95.31 94.51 96.40 si 5.756 5.636 5.617 5.588 5.553 5.378 5.605 s.sn 5.n2 5.438 5.223 5.357 5.2e2 5.2@ 5.314 5.306 5.234 5.1fi Al 2.2U 2.232 2.296 2.2v 2.379 2.622 22O 2.245 2.828 2.456 2.738 2.605 2.671 2.129 2.561 2.574 2.5E5 2.644 Fer 0.000 0.132 0.087 0.158 0.068 0.000 0.133 0.168 0.000 0.106 0.039 0.038 0.037 0.011 0.125 o.ln 0.lEl 0.206 >tst 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 3.000 8.0@ 8.000 8.0m E.000 8.000 8.0m 8.000 8.000 8.000 A1 0.132 0.000 0.m0 0.000 0.000 0.214 0.000 0.m0 0.067 0.000 0.@0 0.000 0.000 0.000 0.0m 0.000 0.000 0.000 Tf* 0.1@ 0.382 0.519 0.386 0.537 0.318 0.550 0.x1 0.462 0.498 0.478 0.699 0.624 0.595 0.738 0.807 0.824 0.838 Cr 0.000 0.000 0.006 0.000 0.000 0.003 0.@2 0.008 0.@ 0.000 0.000 0.001 0.000 0.@ 0.000 0.0@ 0.000 0.000 lvln 0.000 0.027 0.032 0.@8 0.008 0.004 0.015 0.022 0.N7 0.015 0.014 0.012 0.010 0.003 0.005 0.012 0.017 0.020 Fel 1.465 2.330 3.595 1.703 1.871 Ln2 2.107 3.415 2.472 3.656 1.596 2.5t3 Z.3N t.1V 2.374 2.2ffi 2.123 2.4U MC 4.133 2.869 1.465 3.536 3.182 3.309 2.776 t.632 3.078 1.284 3.558 2.2s8 2.641 3.2& 2.283 z.mt 2320 1.961 :-Oc1 5.835 5.608 5.617 5.633 5.598 5.750 5.450 5.644 6.086 5.453 5.tu6 5./183 5.615 5.622 5.m 5.280 5.284 5.303 C€ 0.005 0.003 0.@5 0.000 0.000 0.005 0.000 0.012 0.005 0.@o 0.000 0.007 0.003 0.000 0.00s 0.003 0.m7 0,003 Ba 0.@6 0.020 0.028 o.o40 0.044 0.051 0.058 0.062 0.070 0.095 0.105 0.137 0.157 0.180 0.226 0.265 0.312 0.377 K 1,447 1.787 1.702 1.136 t.6t4 t.672 1.735 t.639 1,236 1.79r 1.657 1.711 1.670 r.467 1.496 1.491 1.412 1.425 Na 0.ln8 0.050 0.043 0.043 0.127 0.114 0.032 0.043 0.164 0.025 0.095 0.027 0.05t 0.216 0.104 0.143 0.159 0.108 I.Inr 1.866 1.860 1.778 1.819 1.785 t.uz 1.825 1.756 1.475 1.911 1.E571.882 1.881 1.853 1.831 1.902 1.890 1.913 F 0.347 0.8@ 0.165 0.581 0.395 0.359 0.544 0.202 0.102 0.230 0.592 0.269 0.210 A.267 0.44 0.450 0.495 0.457 cl 0.015 0.028 0.209 0.090 0.0m 0.005 0.0% 0.119 0.005 0.527 0.008 0.013 0.010 0.005 0.019 0.016 0.019 0.022 Charge u.t55 44.739U.U6 U.493 44.872U.422 45.13044.819 43.617 44.99a 44.520 45.386 45.012 M.939 45.47645.8W 45.821 45.799 Mg# 73.90 53.80 28.50 65.50 62.t0 63.fi 55.30 31.30 55.50 25.40 68.s0 47.00 52.60 64.30 47.70 4A.W s0.20 42,20

EG- East€rlxCnbbro, WG - W€st€rn Gabbro. Fcmrlae calmlatod on the basis of 22 atoms of orygen Charge calorlated with t€t + ot = 14 apfu Fe3* caloulated as the aoount required to fiIl tetrahe&al site, rcmaind€r a: Fe2* 970 THE CANADLA.N MINERALOGIST

ANALyITcAL METTIoDS Ba-richmica (>1 wt7oBaO\is morecommon in the WesternGabbro. The grainsshow a trend of increasing All samples were analyzed by wavelength- Ti and decreasingMg# with increasing Ba (Fig. 3). dispersionspectrometry (WDS) on a JEOL IXA-8600 The mica from the EasternGabbro is only moderately electron microprobe at the University of Western enrichedin Ba; only 27Voof the grains analyzedhave Ontario.An acceleratingvoltage of 15 kV and a beam more than 1 wt%oBaO. Mica in the EasternGabbro crurent of 10 nA were used. Most sarnoles were is Ti-rich Gig. 3a), but doesnot showthe distinct trend analyzedwith a defocused,5-pm electronbeam to that characterizesmica from the Western Gabbro. minimize the migration of . The sameis true of Mg# (Fig. 3b). Ba con-tentsvary @aTiSi3Oe)was analyzedto assessthe within individual samples; for example, sample overlap between the Balcrl and TiKcrl lines. The RP29 contains mica with Ba contents ranging from benitoite gave values within +0.157o absolute of 1.66 to 6.07 wt%oBaO. In this sample,Ti increases the expectedvalue of 37.08VoBaO; the amount with increasing Ba, and Mg# decreasesas Ba of Ti was found to be within +0.177oof the known increases. value of L9.32VoTiO2, indicating that error due to overlapof Ba and Ti peaksis minimal. Strucnral formulae

Rrsur-rs Structural formulae calculated on the basis of 22 atoms of (Table l) show that most This studyis basedon resultsof about140 analyses of the mica grains analyzedare apparentlydeficient of biotite and phlogopitefrom the EasternGabbro and in their tetrahedral site (rvsi + IvAl < 8.00). They the WesternGabbro (Table 1). Micas wereclassified as also show deficiency in the octahedral sites, with biotite (887o)or phlogopite(l2vo) accordrngro the cation totals between5.4 and 5.9 atoms per formula criteria of Deer et al. (1966). Structural formulae are vnit (apfu), which is less than the 6.00 cations basedon 22 atomsof oxygen, and the cation charge in an ideal trioctahedral mica. The interlayer was calculated with octahedrally and tetrahedrally (nCa + nBa + mNa + xtrK) site in most of the coordinatedcations made equal to 14 (Table 1). None grainsalso is deficient (<2.00),varying from 1.7 to of the sam.J'leswas analyzedfor Fe3+. 1.95apfu. All of the grains analyzedlie within the eastonite- Calculation of structural formulae on the basis of - phlogopite- annitefield (Fig. 2). Mica a constant number of cations in the tetrahedraland compositionsfrom the WesternGabbro define a trend octahedralsite (6.00+ 8.00)results in excesscharges from phlogopitetoward the siderophylliteend-member, for the cations, suggesting either substitutions whereasthose from the EastemGabbro lie parallel to involving vacancies or cations of variable valence the phlogopite- annitejoin (Fig. 2). state.

AI

Mg F",

Ftc. 2. Mg-Al-Fe plot of Eastem Gabbro (A) and Western Gabbro (O) micas. Fields from Mitchell (1995). BIOTITE AND PHLOGOPITE.COLDWELL ALKALINE COMPLEX 971 {(

a+ ^aa-a"'a (\l i-rr o'ro +* o i+*':l i= AI^},* r i vY I ra s -.tt s o irAoo' 35 l:'$*' F2 lt a r *t

raX

34 FI M%BaO wt % BaO

A *[ *" 14.6o - 5. .b 4 +iA ?1. 5U x X* :tt 4*"* t ttt. It r ct) ot f?. r+ Y r + t -'| ;at) =50 a . 560 lprl-.-^^' " x I A 7l i" ', t.t{. .. 40 -{ . r Y' ' 4AA "o6i. 30 &" A A 20 ul234567 o24661012 wt % BaO wt % BaO FIc. 3. BaO versu* (a) TiO2 and O) Mg# for mica samples Ftc. 4. Comparisonof mica compositionsfrom the Eastern of the EasternGabbro and Westem Gabbro (symbolsas Gabbro (A) and Westem Gabbro (O) with those from in Figure 2). from China (+), Tipoor mica from lamproites (*), Romanhovince potassicrocks (x) and lower crustal gabbro,Italy (Y). Thesedata are compiled from Mirchell & Bergman (1991), Holm (1982), Thompson(1977), Zharg et al. (1993),and Bigi et al. (1993).

CoN4pARrsoNwrnr Ba-Ti-RrcH MrcA SussrrnmoNs FROM OTHER LOCEITTNS As noted by Zhang et al. (1993), it is difficult Ba-Ti-rich mica is common in potassic igneous to assign unique substitutions to micas because rocks (Mitchell & Bergman1991, Edgar 1992,Zhary (1) the complexity ofthe potentialsubstitutions, (2) the et al. L993),but is rare in gabbroicrocks. Ba-Ti-rich problems in determining valency of Fe and Ti, and mica is known from only one other gabbroicsuite, that (3) the possibility that Ti, Fe and Mg might occur in of lower crust origin exposedin the Iwea zone, Italy tetrahedralcoordination (Foley 1990). @igl et al. L993). The mica compositionsdescribed here are similar Incorporation of Ba to Ba-Ti phlogopite from the Ilrea zone (Big et al. 1993),the RomanProvince (Thompson L977,Holn Ba substitutesfor interlayer cations in the l2-fold 1982)and Montana(Wendlandt 1977) in termsof their site in micas. Replacementof K by Ba in the mica Ba, Ti, Fe and Mg contents(Frg. ). They show some structure involves a charge compensationthat can similarities with low-Ti mica from lamproites either be achievedby vacanciesin the structure: Qvlitchell& Bergman1991), but havemuch lower Mg# (Fie.4). ruBa+n=2roK (1) 972 T1IE CANADIAN MINERALOGIST

@litchell1981, Wagner & Velde 1986),or by complex (Dasgupta et al. 1989, Brigatti & Poppi 1993). coupled substitutionsinvolving cations from both the However, this substitution does not account for octahedraland intedayer site, suchas deficiencies in the 12-fold site, so that the simple substitution(1) also must be important.More complex nBa IvAl )qIK + = + rvsi Q) substitutions,such as the "tetraferriphlogopitel'- phlogopitesolid solutions (Mitchell & Bergman1991), (Wendlandt1977, Mansker et al. 1979).Most of the cannotbe testedwithout infonnation on the proportion mica compositions from the Eastern Gabbro and of Fe3+. the Western Gabbro have low interlayer-site occu- The substitutionssuggested here are the same as pancies(Fig. 5a), suggesting the presence ofvacancies; those proposedfor micas in larnproites (Mitchell & however, this substitutiondoes not explain all of the Bergman 1991). The mica found in lamproites compositionalvariation (Mitchell & Bergmanl99l), commonlyhas as little as L.8apfu in the interlayersite, and a more complexcoupled substitution is likely. The which compareswith the averageinterlayer occupancy most likely substitution(2) is shown in Figure 5b. It of 1.84observed in our samples.Other incidences of has been suggestedas the main substitutioninvolving Ba-bearing trioctahedral mica generally have the transition from phlogopite-biotite to kinoshitalite interlayer cations close to the ideal occupancy(2.00); however,mica compositionsin gabbrofrom the lower crust (Bigi et al. 1993) also show deficienciesin the interlayersite.

2.O Deficienciesin the tetrahedralsite More than 70Voof the grains analyzedhave defi- o 1.4 cienciesin their tetrahedralsite, i.e., rvSi + NAI < 8.00, O suggestingthat another cation is presentin this site. + 1.6 Farmer& Boettcher(1981) have suggestedthat both o Tie and Fe3* can occur there. Bol et al. (1989) z suggestedthat incorporationof Ti4 at the tetrahedral + 1.4 \< site is possiblebut unlikely, becauseofthe electrostatic repulsion of Ba2+and Ti#, which would increasethe 1.2 lattice energyof the crystal. Octahedrallycoordinated Ti is much more likely to occw, as the tetrahedralsites 1.O are closerto the interlayerthan the octahedralsites. o. The presenceof an excesscharge on the basis of Ba 14 tetrahedral+ octahedralcations (Table l) might be due to the presenceof tetrahedrallycoordinated Fe3+, althoughthis has not beenproven. Zkang et al. (1993) have suggestedthat Fe3+might be capableof residing 1A in the tetrahedralsite in Ba-rich micasin potassiclavas b from northeasternChina. However, Fe3+ is not IJ tetrahedrallycoordinated in Ba-rich mica from gabbros 3 in the Ivrea Znne, Itzly (Bigl et a1.,1993),which is + 12 i'i'li**:\ similar in compositionto the suite describedhere. In the presentstudy, tetrahedralsites have been filled to tr \";aa €.o. co oN "1&: . 8 cationsusing Fe3+ (Table 1); the rest of the iron is + assumedto be Fe2*in the octahedralsite. o 'x m 10 in:" Deficienciesin the octahedralsite

ol Ti- andBa-rich micasare commonly deficient in the 42 octahedralsite (Nditchell& Bergman I99L,Big1 et al. 2K+4Si +4(Fe+Mg) 1993, Zhang et al. 1993). Several schemes of substitutionhave beenproposed & Bergman Ftc. 5. a) Proportion of Ba interlayer cations mica [Mitchell versus in (1991) and referencesthereinl. In the mica from the samples of the Eastem Gabbro and Westem Gabbro showing Interlayer site = 2 and Interlayer site = 2 - Ba Eastern Gabbro and the Western Gabbro" the most trends. b) 2K + 4Si + 4@e,Mg) versusBa + 3Ti + 4Al likely substitutionsare: (after Guo & Green 1990), showing trend of coupled substitutionBa + Al = K + Si. Symbolsas in Figure 2. 2wMg= uTi + ;1 (3) BIOTITE AND PHLOGOPITE.COLDWELL ALKALINE COMPLEX 973

(Forbes& Flower 1974), and I

*Mg+ vITi 1.2 2rvSi= + 2rvAl @) a 1.0 (Robert1976). 0'8 Substitution (4) recalls the TlTschermak substitu- tr tion: 0.6 0'4 vI1Mg,Fe2+;+ 2 rvsi= vITi& + 2 vI(Al,Fe3+)(5)

(Zharrget al.1993). 0.0 i However,this schemeof substitutioncannot be fully assessedowing to the lack of data on the oxidation octahedralsite stateof iron. The mica samples define trends intermediate betweenthose for substitutions(3) and(4) (Figs.6a, b). 1.4 Further evidence for a Ti-vacancy substitution is 1.2 shown by the correlation of Ti versus Mg + Fe$1" (Fig. 6c). Other substitutions, such as the Ti-oxy 1.0 substitution(Arima & Edgar 1983),cannot be tested, as the H2Ocontents of the micas are not known. tr08 For their Ba-bearingmicas from the gabbroic suite 0.6 ' in the Ivrea Zone, Bigi et al. (1993) reportedthe data 0.4 :;';ii-t-+: with all Fe expressedas Fe2*, as well as analyzed *.5;,'a:aaed]|; proportionsofFe2+ and Fe3*. The analyseswith all Fe 0.2 as Fe2+show deficienciesin the octahedralsite similar to those describedhere; however, such deficiencies are not present in sampleswhere the proportion of Si + ivAl Fe2+and Fe3+is known. This finding suggeststhat the calculated site-deficienciesmight be due in part to the lack of analyticaldata for Fe3+. (\+ Onrcnl or Ba-Ti-RrcH Mrc.qs 0 N rrre CoLowE[ SunE L + o)s Although micas from the EasternGabbro and the E WesternGabbro show the samepattem of substifution, it is likely that they have different paragenesesoon the basisoftheir distinctcompositional trends (Figs. 2, 3). The Eastem Gabbro micas are only moderately enriched in Ba, and could have crystallized from an evolved intercumulus liquid trapped during the accumulationof the LayeredGabbro lntrusion. In most of thesecases, the mica is found in freshgabbro, Le., wherealteration is minimal. Frc. 6. Extent of substitutionin the octahedralsite in micas (symbols as in Figure 2). a) Ti versa.roccupancy of the Moderately Ba-enrichedmicas in the Western NAl Gabbro have compositions similar to those in the octahedralsites. b) Si + verszsTi. c) Ti rerszs Mg + (Fe3a from Fe., - Fe3+required to fill Eastern Gabbro and likely formed by the same Fe2t calculated tetrahedralsites). Substitutions in a) and b) from Mirchell mechanism of early incorporation of Ba into & Bergman(1991), in c) from Ztang et al. (1993). intercumulus mica. This mechanism does not. however,explain the origin of late-formed(low-Mg#) Ba-rich mica, which overgrowssecondary minerals in the gabbro. Ba is strongly compatible in mica crystallization. (Henderson1982) and shouldenter the structureduring The mica in the Western Gabbro commonly is found the earliest stage of crystallization. The observation in rocks with intense alteration, similar to that that early-formed(high-Mg#) micas are less enriched found around syenite pegmatites. These pegmatites in Ba than later-fonned(low-Mg#) micas suggeststhat have been shown to be at least locally enriched in Ba; Ba was addedto the WesternGabbro durins the mica's some samples contain up to 9761 ppm Ba @enczak 974

L992), and might be a source for Ba in the micas. AcKNowHDcElvIEl.{Ts Fluids emanatingfrom the augite syenite pegmatites could have metasomatizedthe gabbros and enriched Funding for this project was provided by a grant them in Ba while conditions were still suitable for from the Ontario Geological Survey to Dr. R.H. the crystallizationof Ba-Ti mica. The high density of Sutcliffe. Thanksto E.C. Walker, G. Shore,G. Dubuc syenitepegmatites in the WestemGabbro may explain and D. Bessererfor discussionsand assistancein the the prevalenceof Ba-rich mica in this area. field. Y. Thibault, R.L. Barnett and D.M. Kingston assisted with electron-microprobeanalyses. A.D. Conomoxs oF CRysrALLzATroN Edgarand Y. Thibault readan earlierdraft of this paper and made many useful suggestions.Constructive Titanium-bearingmicas are stableover a wide range comments by reviewers A.E. Lalonde, K.L Currie, of pressuresand temperatures(Mitchell & Bergman R.H. Mitchell and R.F Martin greatly improved the 1991).Most studieshave been carried out in potassic manuscript. systems,and as such, may not be directly relevant to the presentstudy. At moderatepressures, Ti solubility REFERENcES decreaseswith pressureand increaseswith increasing temperature(Flower 1971, Arima & Edgar 1983). ARtr\4A,M. & Encen, A.D. (1983): A high pressure On this basis, pressureestimates of 2 kbar for the experimentalstudy on a -richleucite- emplacementof the Coldwell alkaline complex from the West Kimberley area, Australia: petrogenetic (I\4itchell & Platt 1978) suggestthat pressurecondi- implications: Contrib. Mineral. Petrol. M, 228-234. tions were suitable for the formation of titaniferous G. mica. Brcr, S., Bnrcar-n, M.F., Mazzuccuruu, M. & Rrvarcrn, (1993): Crystal chemical variations in Ba-rich An estimateof the temperatureof crystallizationof from gabbroic rocks of lower crust (Ivrea Zone, NW the intercumulusBa-Ti mica is not possible.The range Italy). Contrib. Mineral. Petrol. 113,87-99. of temperaturesover which titaniferous mica can crystallize is large (Mitchell & Bergman 1991), Bor, L.C.G.M.,Bos, A., Sernnn,P.C.C. & JANSEN,J.B.H. and someof the mica may be subsolidus.This is most (1989): Barium-titanium-richphlogopites in marbles likely for the Ba-rich micas in the WesternGabbro, as from Rogaland southwest Norway. Am Mineral. 74, field relationsindicate that the gabbrowas solid at the 4394M. time of pegmatiteemplacement. Thus it is likely that both true igneous micas and metasomaticmicas are BRIcATn, M.F. & Poppl L. (1993): Crystal chemistry of presentin the samerocks. Ba-rich trioctahedral micas-lM. Eur. J. Mineral. 5. 857-87I. CoNclusroNs DAscuprA, S., CHAKRABoRTI,S., SENGUyTA,P., Bnarract,rnve, P.K., Baxmnrr, H. & Flxuora, M. l) UnusuallyBa-rich Ti-bearingbiotite andphlogopite (1989): Compositionalcharacteristics of kinoshitalite occur asintercumulus phases in subalkalinegabbros in from the SausarGroup, India. Arn"Minzral. 74,2M-202. theColdwell alkaline complex. These micas contain up to 6.07wt%o BaO and 8.1 wt%oTiO, and6s similsl tu DEER,W.A., Howrs, R.A. & Zussrraar,J. (1966): An micasfrom lower-crustgabbro and in potassicigneous Introduction to the Rock-Foming Minerak Longmm, rocks. London. U.K.

2) Incorporation of Ba into interlayer sites occurred EDGAR,A.D. (1992):Barium-rich phlogopiteand biotite from by coupled substitution of Ba + Al for K + Si, someQuatemary alkali ,West Eifel, Gennany. Eur. J. Mineral. 4. 321-330. with developmentof interlayer vacanciesto explain deficienciesin the l2-fold site. Fanvtm.,G.L. & BoEmcHR, A.L. (1981): Petologic and crystal-chemical significance of some deep-seated 3) Ti incorporationis explainedby a combinationof .An. Mineral.6, II5+LL63. Ti-vacancyand Ti-Tschermaksubstitutions. Frcwnn, M.F.J. (1971):Evidence for tle role of phlogopitein 4) Moderately Ba-enriched(l-2 wtTo BaO) micas in the genesisof alkali .Contrib. Mineral. Petrol.32, the EastemGabbro and the Western Gabbro seemrc r26-t37. have formed from Ba-rich intercumulusliquid durins (1990): phlogopite the final stages of crystallization gabbri Fot:ey, S.F. Experimentalconstraints on of the chemistryin lamproites.2. Effect of pressure-temperature Strongly Ba-enriched micas in the Westem Gabbro variations.Eur J. Mineral.2. 3n -34L. seem to have formed by metasomatismdue to infiltration of a Ba-rich fluid derived from adjacent FoRBEs,W.C. & FLowE& M.J.F. (1974): Phaserelations of bodiesof syenitepegmatite. titan-phlogopite, K2MgTiAl2Si6O26(OlI)a:a refraclory BIOTITE AND PHLOGOPITE.COLDWELL ALKALINE COMPLEX 975

phase in the upper mantle. Earth Planet. Sci. Ictt. ?2, PENcZAK,R.S. (1992):Petology and Mineral Chemistryof 60-66. the Middleton CopperOccunence of the WesternGabbro, Coldwell Alluline Comple6 Onnrio. B.Sc. thesis,Univ. Guo, J. & GnnsN,T.H. (1990):Experimental shrdy of barium of Waterloo,Waterloo, Ontario. partitioning between phlogopite and liquid at upper-mantlepressure and temperature.Lithos, ?4, 83-95. RoBERr,J.-L. (1976): Titanium solubility in synthetic phlogopitesolid solutions.Chem Geol.17, 2B-2n. HralreN, L.M. & Macuoo, N. (1992): Timing and origin of midcontinent rift alkaline magmatism, North America: Snew, C.S.J. (1994): Petogenesis of the Eastern Gabbro, evidencefrom the Coldwell Complex. Contrib. Mineral. Coldwell Alkaline Complex,Onmrio. Ph.D. thesis,Univ. Petrol.110,289-303. of WesternOntario, london, Ontario.

IIENDERSoN,P. (1982):bnrganic .Pergamon THo&psoN,R.N. (1977):Primary basaltsand magmagenesis. hess, Oxford,U.K. III. Alban Hills, Roman comagmaticprovince, central laly. Contrib. Mineral. Petrol. 60, 9l-108. Hor.nr,P.M. (1982):Mineral chemistryof perpotassiclavas of theVulsiniandistrict,theRomanProvtnce,Idy.Mineral. WecNrn, C. & VEIDE, D. (1986): The mineralogy of Mag.46,379-386. K-richterite-beaxinglamproites.ArnMineral.7l,17-37.

MANSKER,W.L., EwrNc, R.C. & Knn, K. (1979): War-rrn, E.C.,Surclnr.p, R.H., Snaw, C.S.J., SHons, G.T. & Barian-titanian biotites in nephelinites from Oahu, PnNczar, R.S. (1993): Precambriangeology of the Hawui. Am. MineraL@, 156-159. Coldwell Alkaline Complex. Ontario GeoI. Surv., Open- File Rep.5868. MrcHEIr, R.H. (1981):Titaniferous phlogopites from the lrucite lamproitesof the West Kimberley area Western WrNoleuor, R.F. (1977): Barium phlogopites from Australia. Contrib. Mineral. Petrol. 76. 243-251. Haystacks Butte, Highwood Mountains, Montana. Carnegie Inst. Washington,Year Book 76, 534-539. (1995): Kbnberlites, Orangeites, and Related Rocts. PlenumPress, New York, N.Y. WnrnrsoN, S.J.(1983): Geology anil SulphideMineralization of the Marginal Phases of the Coldwell Complex, - & Brnouar, S.C. (1991):Petrologl of Inmproites. Northwestem Ontario. M.Sc. thesis, Carleton Univ., Plenumhess, New York, N.Y. Ottawa-Ontario.

& Purr, R.G. (1978): 1461figminelnlegy of Zrallc, Mmc, Suooarv, P., THol'4PsoN,R.N. & Duqcar, ferroaugite syenite from the Coldwell alkaling gemplgl, M.A. (1993): Barian-titanian phlogopite from potassic Ontario,Canada. J. Petrol.19,6n-651. lavas in northeast China: chemisry, substitutions and paragenesis.Arz. Mineral. 78, 1056-1065. & - (1994):Aspects of the geologyof the Coldwell Complex. Geol. Assoc. Can. - Mineral. Assoc. ReceivedNovember 14, 1995, revised manuscript accepted Can., Guideboob FieldTrip 42. April 30, 1996.