967 The Canadimt M iner alo gist Vol. 34, pp.967 -97 5 (1996) BARIUM.AND TITANIUM-RICHBIOTITE AND PHLOGOPITE 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 biotite are found in the subalkalineEastem and Westsm Gabbrointrusions tltat are cut by syeniteand syenitepegmatite in the Coldwell alkaline s6rnplsx,northwestgm Onurio. The micas containup to 6.1 trttVoBaO and 8,1 wtEo TiO2, Mica occu$ as a fringe on Fe-Ti oxides, an overgrowth on cumulus and intercumulus clinopyroxene,and poarsegrains that encloseearlier-fonned minerals. Ba-Ti-rich micasare normally found in potassicigneous rocks like lamproites,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 syenites(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 olivine massivegabbroic rocks (EasternGabbro) intruded by gabbro to anorthosite.Similarities in rock types and iron-rich augite syenite (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 plagioclase,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- pyroxene, biotite, phlogopite, Fe-Ti oxides and, nisms of substitution and possible paragenesesof locally, hornblende 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 pegmatite. 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 pegmatites. 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 mineral 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 & pleochroism 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