967 The CanadimtM ineralo gist Vol. 34, pp.967-975 (1996)
BARIUM.AND TITANIUM-RICH BIOTITEAND PHLOGOPITE FROMTHEWESTERNAND EASTERNGABBRO, COLDWELLALKALINECOMPLEX,NORTHWESTERN ONTARIO CLIFF S.J.SHAWI Department of EarthSclences, University of Westem Ontario,Inndou Onnio N6A587 ROBERTS. PENCZAK Depamnent of Geological Sciences, QuzmsUniversitt,KingstonOntarioK7L 3N6
ABSTRACT Bmium- and titanium-emichedphlogopiteand biotite are found in the subalkalineEastemand WestsmGabbrointrusions tltat are cut by syeniteand syenitepegmatitein the Coldwell alkaline s6rnplsx,northwestgmOnurio. The micascontainup to 6.1 trttVoBaO and 8,1 wtEoTiO2, Mica occu$ as a fringe on Fe-Ti oxides, an overgrowth on cumulus and intercumulus clinopyroxene,andpoarsegrainsthat encloseearlier-fonnedminerals.Ba-Ti-rich micasarenormally found in potassicigneous rocks like lamproites,whereasthe 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 lessBa-rich samplesof the EastemGabbro.Most of the grains analyzedhave apparentdeficienciesin their tetrahedral,octahedraland interlayer sites. Deficiencix in the octahedralsitesare due to a combinationof Ti-vacancyand Ti-Tschermaksubstitution.The interlayer-site deficienciesare due to the replacementof K by Ba. Coupledsubstitutionssuchas 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 trappedin the cumulus framework of the gabbros.In addition, the latest Ba-Ti-rich mica in the WestemGabbroformed in responseto infiltration of fluids derived from adjacentbodies of Ba-rich syenitepegmatite. Keywords:phlogopite,biotite, gabbro,barium, Coldwell alkalines6rnpl61,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 subalcalinesappeldesEastertret Westem Gabbro, du complexe alcalin de Coldwell, en Ontario. Des venuesde sy6nite,localemqntpegmatitique,recoupentces unit6s.Le mica se pr6senteen bordurede 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 typiquesdes rochesign6es potassiques,par exempleles lampro'ites,tandisque dansce cas,il s'agit de rochesgabbrolquessubalcalines.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 sonparamatreMgf, tandisque ce n'est pasle casdansl'unitd EastemGabbro,dont le mica est moins enrichi en Ba. La plupat des cristaux analys6sauraientdes lacunesdans les sites tdtra6driques,octa6driques,et inter-feuillets. Dans les sites octa6driques,les lacunesseraientdues i une combinaisond'une ddficienceen Ti et de la substitutionTlTschermak. Dans les sites inter-feuillets, les lacunesseraientduesau remplacementde K par Ba. Des sch6masde substitutioncoupl6e commesBa + NAI = )oIK + rvSi expliquentaussila prdsencede Ba dansh Jtructure.La formation de mica emichi en Ba r6sulteraitde la cristallisation de liquide r€siduel pidg6 donsles intersticesdes cumulatsde ces massifsgabbroiques.Nous attribuonsla g6ndrationtardivedemica enrichi en Ba et enTi dansle WestemGabbroh I'infiltration defluide ddrivddesvenues de pegmatitesydnitiqueadjacenfes. (Traduit par la R6daction) Mots-cl6s:phlogopite,biotite, gabbro,baryum,complexealcalin de Coldwell, Ontario.
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968
TIIE CANADIAN MINERALOGIST
INTR.oDUc"noN The Coldwell alkaline complex in northwestem Ontario (Frg. 1) is an unmetamorphosedintrusive complexconsistingof subalkalineand alkaline gabbro as well as a variety of syenites(Walker et al. 1993, Mitchel & Platt 1994). The complex was emplaced early in the magmatic evolution of the middle Proterozoicmid-continentrift at 1108Ma (Heaman& Machado 199 . It consistsof three inrusive centers (Fig. 1; Mitchell & Platt 1994). T\e oldest center, Center l, consistsof a partial ring dyke of layeredto massivegabbroicrocks (EasternGabbro)intruded by iron-rich augite syenite (Mitchell & Platt 1978). A small body of layered and heterogeneousgabbro (lVestern Gabbro) occurs in the western part of the complex and also is associatedwith iron-rich and locally pegmatitic and Ba-enriched augite syenite (Wilkinson1983,Penczak1992),indicatingthatit also belongsto center 1. In this work, we describethe composition,mechanisms of substitution and possible paragenesesof Ba- and Ti-bearing phlogopite and biotite from the EastemGabbroand the WesternGabbro.
GsoLocYANDPE'rRocRAPnY The Eastern Gabbro (Ftg. 1) consists of three discreteintrusions(Shaw1994),the largestof which is the Ba-mica-bearingLayered Gabbro Intrusion. This intrusion consists of variably massive to layered gabbroic cumulatesranging in composition from wehrlite to anorthosite.The Western Gabbro is an isolated body of layered and heterogeneousgabbro intrudedandlocally alteredby youngersyenites.It has been divided into a massive and a layered series (PenczakL992) that vary in compositionfrom olivine gabbro to anorthosite.Similarities in rock types and compositionsindicatethat theWestemGabbrois likely equivalentto the LayeredGabbro. layered The gabbroicrocks are unmetarhorphosed, to massive cumulateswith plagioclase,olivine and clinopyroxenet apatite t Fe-Ti oxide as the main cumulus phases. Clinopyroxene also occurs as poikilitic intercumulus grains, together with orthopyroxene, biotite, phlogopite, Fe-Ti oxides and, locally, hornblendeand traces of granophyric intergrowth of quartz and K-feldspar,which is Ba-bearing intherocks oftheEastemGabbro(Shaw1994).
Flc. 1. Generalgeology of the Coldwell alkaline complex, showing the distribution of lhe intrusive centersand the location of the EasternGabbroand the WestemGabbro (after Walker er al. 1993).
969
BIOTITE AND PHLOGOPITE,COLDWELL ALKALINE COMPLEX
Both the EasternGabbro and the WesternGabbro were intrudedby iron-rich augitesyeniteandby large, irregular bodies of green syenite pegmatite. The contactbetweengabbroandsyenitepegmatiteis sharp; macroscopicalterationis resficled to an aureolea few cm wide, but saussuritizationof feldspar,serpentinization of olivine and uralitization of clinopyroxenecan be traced for up to 20 cm from fhe contact with the pegmatites. The gabbroicrocks are evolved (Shaw 1994),with whole-rock Mg# [100 Mg(Mg + 0.9Fe,o),molar] of 13to 60 they areemichedin Rb (on average,26 ppm), Ba (1267 ppm), Sr (1234 Wm), Zt (156 ppm) and rare-earth elements (REE: 9l ppm La). Systematic changes in bulk-rock and mineral composition, indicative of simple fractionation,are absentfrom the gabbros. Petrographicand mineral chemicalcriteria indicate that the parentmagmawas subalkalinein composition (Shaw 1994; in prep.). This makesthe occurrenceof Ba-Ti-enriched micas unusual,as they are generally confined to potassicigneousrocks (e.9., Mirchell & Bergmanl99l,Zhang et al. 1993).
PE"TRocRAPHY Biotite and phlogopite occur in the gabbros as intercumuluscrystals.Mostly, the mica forms ragged grains 0.1-{.25 mm acrossthat fringe earlier-formed Fe-Ti oxides and overgrow cumulus clinopyroxene. Large, poikilitic plates of mica enclosing olivine, clinopyroxene, Fe-Ti oxides and apatite also are present.The grains show variable pleochroismfrom black-brown or red-brown to pale yellow or yellowbrown. Mica abundancesvary from trace to more than l2%o, being most abundant in orthocumulate layers. There is minor optical zonation in some of the grains observed;however, zonation is rare. Many of the samples sontaining Ba-rich mica, particularly those in the Western Gabbro, show a pattern of alteration similu to that observedin gabbros close to syenite pegmatites(see above). In these sampleso intercumulus mica is partly altered to chlorite. Secondary,subhedralbiotite with red to straw-yellow pleochroism commonly overgrows the altered minerals.
FT.F.CTRON-MICROPROBE TABLE 1. SELECTED DATA ON Ba-Ti-BEARINGMCAS. COLDWELLCOMPLD( salrpl€ RP68 91-26?9L-25991-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 EG EG WG WG WG WG WG \vG WC Ec EG 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 Al Fer >tst
5.756 2.2U 0.000 8.000
A1 Tf* Cr lvln Fel MC :-Oc1
0.132 0.000 0.1@ 0.382 0.000 0.000 0.000 0.027 1.465 2.330 4.133 2.869 5.835 5.608
C€ Ba K Na
I.Inr F
cl Charge Mg#
5.636 2.232 0.132 8.000
5.617 2.296 0.087 8.000
5.588 2.2v 0.158 8.000
0.m0 0.000 0.519 0.386 0.006 0.000 0.032 0.@8 3.595 1.703 1.465 3.536 5.617 5.633
5.553 2.379 0.068 8.000
5.223 2.738 0.039 8.000
5.357 2.605 0.038 8.0m
5.2e2 2.671 0.037 E.000
5.378 2.622 0.000 8.000
5.605 22O 0.133 8.000
s.sn 2.245 0.168 8.000
5.n2 2.828 0.000 3.000
5.438 2.456 0.106 8.0@
0.000 0.214 0.537 0.318 0.000 0.003 0.008 0.004 1.871 Ln2 3.182 3.309 5.598 5.750
0.000 0.550 0.@2 0.015 2.107 2.776 5.450
0.m0 0.x1 0.008 0.022 3.415 t.632 5.644
0.067 0.462 0.@ 0.N7 2.472 3.078 6.086
0.000 0.@0 0.000 0.000 0.498 0.478 0.699 0.624 0.000 0.000 0.001 0.000 0.015 0.014 0.012 0.010 3.656 1.596 2.5t3 Z.3N 1.284 3.558 2.2s8 2.641 5.453 5.tu6 5./183 5.615
5.2@ 5.314 5.306 2.129 2.561 2.574 0.011 0.125 o.ln 8.000 8.0m 8.000
5.234 5.1fi 2.5E5 2.644 0.lEl 0.206 8.000 8.000
0.000 0.595 0.@ 0.003 t.1V 3.2& 5.622
0.000 0.824 0.000 0.017 2.123 2320 5.284
0.0m 0.738 0.000 0.005 2.374 2.283 5.m
0.000 0.807 0.0@ 0.012 2.2ffi z.mt 5.280
0.000 0.838 0.000 0.020 2.4U 1.961 5.303
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 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 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 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 1.866 1.860 1.778 1.819 1.785 t.uz 1.825 1.756 1.475 1.911 1.E57 1.882 1.881 1.853 1.831 1.902 1.890 1.913 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 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 u.t55 44.739U.U6 U.493 44.872U.422 45.13044.81943.61744.99a44.52045.38645.012M.939 45.47645.8W45.82145.799 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* caloulatedas the aoount required to fiIl tetrahe&al site, rcmaind€r a: Fe2*
970
THE CANADLA.N MINERALOGIST
ANALyITcAL
Ba-richmica (>1 wt7oBaO\is morecommonin the WesternGabbro.The grainsshowa trendof increasing Ti and decreasingMg# with increasingBa (Fig. 3). The mica from the EasternGabbrois only moderately enrichedin Ba; only 27Voof the grains analyzedhave more than 1 wt%oBaO. Mica in the EasternGabbro is Ti-rich Gig. 3a), but doesnot showthe distinct trend that characterizesmica from the Western Gabbro. vary The sameis true of Mg# (Fig. 3b). Ba con-tents within individual samples; for example, sample RP29 contains mica with Ba contentsranging from 1.66 to 6.07 wt%oBaO. In this sample,Ti increases with increasing Ba, and Mg# decreasesas Ba increases.
METTIoDS
All samples were analyzed by wavelengthdispersionspectrometry(WDS) on a JEOL IXA-8600 electron microprobe at the University of Western Ontario.An acceleratingvoltageof 15 kV and a beam crurent of 10 nA were used. Most sarnoleswere analyzedwith a defocused,5-pm electronbeam to minimize the migration of potassium. Benitoite @aTiSi3Oe)was analyzedto assessthe overlap between the Balcrl and TiKcrl lines. The benitoite gave values within +0.157o absolute of the expectedvalue of 37.08VoBaO; the amount of Ti was found to be within +0.177oof the known value of L9.32VoTiO2, indicating that error due to overlapof Ba andTi peaksis minimal. Strucnral formulae Rrsur-rs This studyis basedon resultsof about140analyses of biotite and phlogopitefrom the EasternGabbroand the WesternGabbro(Table 1). Micas wereclassifiedas biotite (887o)or phlogopite(l2vo) accordrngro the criteria of Deer et al. (1966). Structuralformulae are basedon 22 atomsof oxygen, and the cation charge was calculated with octahedrally and tetrahedrally coordinatedcationsmadeequal to 14 (Table 1). None of the sam.J'les was analyzedfor Fe3+. All of the grainsanalyzedlie within the eastonitesiderophyllite- phlogopite- annitefield (Fig. 2). Mica compositionsfrom the WesternGabbrodefine a trend from phlogopitetowardthe siderophylliteend-member, whereasthosefrom the EastemGabbrolie parallel to the phlogopite- annitejoin (Fig. 2).
Structural formulae calculated on the basis of 22 atoms of oxygen (Table l) show that most of the mica grains analyzedare apparentlydeficient in their tetrahedralsite (rvsi + IvAl < 8.00). They also show deficiency in the octahedral sites, with cation totals between5.4 and 5.9 atoms per formula vnit (apfu), which is less than the 6.00 cations in an ideal trioctahedral mica. The interlayer (nCa + nBa + mNa + xtrK) site in most of the grainsalso is deficient (
