Chemistry of Biotites and Muscovites in the Abas Granite, Northern Portugal
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AmericanMineralogist, Volume 73, pages 754-765, 1988 Chemistry of biotites and muscovitesin the Abas granite, northern Portugal Ruov J. M. KoNTNGST*Jnr N. Bor-ANor** SrvroN P. VnrcNo, J. BrN H. JansnN Departmentof Geochemistryand ExperimentalPetrology, State University of Utrecht, Budapestlaan4, 3585CD Utrecht,The Netherlands ABSTRACT The micas in the Abas granite (northern Portugal) have been examined by means of electronmicroprobe, scanningand transmissionelectron microscopy, and partial chemical analyseson mica separates.Four different types of postmagmatic muscovites are distin- guishedin thin sections,and their chemical variations are describedin terms of celadonite, paragonite,and pyrophyllite substitutions. The paragonite substitution indicates that the four muscovite types formed at successivelylower temperatures.Generally, the biotites exhibit texturally magmatic features,whereas they chemically have a deficient octahedral occupancy,suggesting considerable alteration to muscovite or zinnwaldite. Our observa- tions suggestthat the chemical variations in the biotite composition can more likely be explained by solid solution rather than by fine-scale intergrowths of the end-member minerals. Deficient interlayer occupancyofthe biotites is causedat least in part by fine- scaleinterlayering of retrogradechlorite. INrnooucrroN S.l.rvrpr,rNc AND TEcHNreuE Petrologic and geochemicalinvestigation of Hercynian The samples examined in this study were selectedfrom 54 granitic rocks in central northern Portugal (Konings et whole-rock samplescollected from the Abas granite, both inside al., 1988) indicated that large-scalemetasomatic altera- and outsidethe contact zone with the Viseu granite.The samples tion of the Abas granite resulted in important changesin represent the various stagesof alteration, which gradually in- the whole-rock chemistry. A zone of tin-tungsten deposits creasestoward the contactzone. Samples 04,05,07,37,and79 are heavily altered; 06, 25, and 80 are intermediate;20 and 77 is located near the contact with the younger Viseu granite are less altered; and 29 has the most magmatic features. The (278 Mq- H.N.A. Priem, pers. comm.). Ten Haven et al. sample locations and details of the petrology are described by (1986) part related ofthe alterationand the tin-tungsten Vriend, Konings, and Jansen(in prep.). mineralization in the Abas granite to the intrusion of the Most of the mineral analyseswere made with a TPD electron Viseu granite. microprobe at the University of Utrecht, operatingat I 5 kV and The chemistry of the micas in the Abas granite is sig- 4 nA, using energy-dispersiveX-ray spectroscopytechniques. nificant for the understanding of the whole-rock chem- The srrr,rand X-ray scanningimages were made with a Cam- istry and the alteration processes.Replacement of biotite bridge Scientific Instruments Microscan M-9 at the Free Uni- by muscovite has causedmobilization of refractory ele- versity of Amsterdam (VU), operatingat 20 kV and 25 nA, using ments from the octahedral sheetsand from the enclosed wavelength-dispersivespectroscopy techniques. Some addition- al analysesthat were made at the VU included fluorine. Natural accessoryminerals. In addition, the occurrence of sec- and synthetic minerals were used as standards,and ZAF online ondary muscovite seemsclosely related to the alteration full matrix corrections were applied according to the method processes that were induced by the intrusion of the Viseu used by Microscan. The resolution of the microprobe and the granite and its derivatives. sev analysesis 3-5 pm. The structural formulas were calculated In the presentstudy, the results of electron-microprobe on the basis of 22 oxygens. analysesare interpreted in terms of either substitutional Specimensfor rrrvr were prepared by Ar thinning of grains replacements or mica intergrowths. Supplementary selectedfrom thin sections.The specimenswere coated with C. chemical analysesof mica separatesand scanning elec- Eleotron microscopy was performed with a rpr'rzooc microscope tron microscope (serra)and transmission electron micro- operatedat 200 kV. Semiquantitative chemical analyses,with a (i.e., scope (rnu) analyseswere made to support these inter- spatial resolution of about 500 A the approximate beam pretations. size) were done with a Link X-ray analyzer using a C sample holder. Mica fractions were separatedfrom a selectionof whole-rock samplesby meansof a Fault table, using the 30- to 50-pm frac- The separationof biotite from muscovite was with * Present address: Netherlands Energy Research Foundation tion. done a ECN, Department of Chemistry, P.O. Box 1, 1755 ZG Petien, Frantz electromagneticseparater, operated in steps of 0.2 mA. The Netherlands. In general,several muscovite and biotite fractions were obtained ** Presentaddress: CSIRO, Division of Geomechanics,P.O. from each sample, and all fractions were analyzedif the yield Box 54, Mount Waverley, Victoria 3149, Australia. was large enough.Further purification was performed by means 0003-004x/88/0708-o754$02.00 754 KONINGS ET AL.: MICAS FROM THE ABAS GRANITE 755 of heavyliquids. No feldsparor quartzimpurities were observed but the concentrations of the trace elementsLi, Rb, and afterthis treatment,but contaminationof especiallythe biotite Cs also tend to be somewhat higher in the Fe-rich frac- fractionas a resultof fine-scaleintergrowths with muscoviteor tion, probably owing to minor biotite contamination. The chloritecould not be avoided.The separateswere analyzed by concentrationsof the trace elementsLi, Rb, and Cs (Ta- rcres(inductively coupled plasma-emission spectroscopy) after bte 1) clearly increasewith progressivealteration for mus- sampledecomposition with a HF solution. covite as well as biotite. Representativemicroprobe analysesof the four mus- Prrnor-ocrc oBSERvATToNS covite types are listed in Table 2. The variation diagrams The Abas granite is a two-mica granite that locally of Si with respectto A1, Fe, and Mg as analyzedby mi- grades into a muscovite granite. The bulk of the rock croprobe (Fig. l) show remarkable differencesbetween consists of quartz, plagioclasethat has been mostly re- the chemical compositions of the four muscovite types. placed by albite, and K-feldspar. Biotite is a primary Type I muscovite has relatively low Si and Mg contents magmatic constituent, frequently replacedby muscovite, and high Na and Al contents, whereastype 4 muscovite tourmaline, and chlorite. The accessoryminerals zircon, has high Si and Mg contentsand very low Na, (K + Na + xenotime, and monazite are of primary origin, whereas 2Ca), and Al contents.Types 2 and 3 have compositions sillimanite and rutile are secondary.Magmatic as well as intermediate betweenthose of types I and 4. The Ti con- postmagmatic apatite is presentin minor amounts. tent of all muscovite types is extremely low and points The biotite is dark brown and enclosesa large number toward postmagmatic origin (Speer, 1984), which is in of accessoryminerals, which are often obvious in thin agreementwith our textural observationson the thin sec- section as a result oftheir nearly black pleochroic halos. trons. During the muscovitization process,the pleochroic colors The variation of the Na content of the muscovites is of biotite become pale brown, and the number of inclu- attributed to solid solution between the end members sions gradually decreases.During ubiquitous retrograda- muscovite and paragonite (Yoder and Eugster, 1955a, tion, some biotite is altered to greenbiotite and chlorite. 1955b;Iijama, 1955).Recent studiesby Chatterjeeand Replacementtextures suggestthat muscovite is mainly Froese(1975) and EugstereL al. (1972) have provided of secondaryorigin. Four types of muscovite can be dis- useful information about the phase relations and their tinguished: (1) Large euhedral crystalsapparently replace temperature dependencein this system. Discussing the biotite and tourmaline. Partially resorbed inclusions of possibilities for the use of the muscovite-paragonitesys- zircon, xenotime, and monazite, originally surroundedby tem as a petrogeneticindicator in pelitic schists,Guidotti biotite, are occasionally observed in these muscovite and Sassi(1976) and Guidotti (1984)argued that its ap- grains,and weakly colored pleochroic halos are rarely still plicability dependson the mineral assemblage.In the Abas recognizablein the muscovite matrix around theserelicts. g,ranite,all muscovite types coexist with albite, qtrartz, (2) Fine-grained sericite occurs as an alteration product and K-feldspar, and sillimanite is locally present.The Na of both magmatic and postmagmatic feldspar. (3) Fine- content of the whole-rock samples increaseswith pro- grained crystals occur along grain boundaries of type I gressivealteration. This suggeststhat the altering fluids muscovites or as aggregatesin cracks. (4) Small crystals were rich in Na and were constantly bufferedby the gran- overgrow the other types of muscovite. itic mineral assemblage.It is concluded that the decreas- Muscovites of types I and 2 are ubiquitous and are ing Na contents of muscovite types I to 4 must suggest related to the early and main alteration stages.Type 3 a decreaseof formation temperature. The NarO concen- muscovites do not occur in the least altered zones, but trations in the muscovite fractions (Table l), which are are common in the areas where the alteration is more dominated by type I muscovite, are fairly constant with- advanced. Type 4 muscovites are restricted to some in- in each sample set representingrocks with the