Three Novel Isograds in Metamorphosed Siliceous

American Mineralogist, Volume 81, pages 485-494, 1996

Three novel isogradsin metamorphosedsiliceous dolomites from the Ballachulish aureole,Scotland JonN M. Frnnv

Department of Earth and Planetary Sciences,Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.

AssrRAcr Isogradswere mapped in siliceousdolomites from the Ballachulish aureole on the basis of the formation of geikielite (MgTiOr), baddeleyite (ZrOr), and qandilite (MgrTiO.) by the following model reactions: rutile + dolomite : geikielite + calcite + CO2, zircon + 2 dolomite : baddeleyite + forsterite + 2 calcite + 2 CO2,and geikielite * periclase: qandilite. The (I, X.o,) conditions of reaction inferred from (l) mineral equilibria in pelitic rocks, (2) calcite + dolomite thermometry, and (3) the diopside * dolomite + forsterite + calcite * tremolite and dolomite + periclase + calcite equilibria are: geikielite isograd (640-655 "C, 0.76-0.80);baddeleyite isograd (660-710 "C, 0.76-0.95);qandilite isograd (725-7 55 "C, <0.08). These I-X"o, conditions for the geikielite and baddeleyite isograds are the same within error of those independently estimated from rutile + dolomite + geikielite + calcite and zircon + dolomite + baddeleyite + forsterite * calcite equilibria using Berman's thermodynamic data base. Rutile, zircon, geikielite, and baddeleyite are common in dolomites from at least two other contact aureolesin Scotland and Montana. Although the minerals occur in concentrations <0.010/0,they appear to have been in local equilibrium during metamorphism both with each other and with coexisting carbonates and silicates and are potentially useful but currently unexploited records of the physical conditions of metamorphism.

INrnoougrroN Gror,ocrc sETTTNG Siliceous dolomitic limestones continue to play a key The Ballachulish igneous complex and its contact role in petrologists' efforts to understand mineral reac- metamorphic aureole was recently the subject of a com- tions and chemical processesduring metamorphism at prehensive, interdisciplinary investigation (Voll et al. least since the classicstudy of Bowen (1940). Bowen rec- l99l). The plutonic rocks are composedof monzodiorite, ognizedthat progressivemetamorphism under most con- quartz diorite, and granite. The plutonic rocks intrude ditions resulted in a sequenceof now well-known reac- Dalradian metasedimentaryunits that include the Appin tions that could be mapped in the field as tremolite, Phyllite, Ballachulish Slate, and Leven Schist. Siliceous forsterite, diopside, periclase,and wollastonite isograds. limestonesand dolomites are interbeddedwith the Appin The other members of Bowen's sequence,monticellite, Phyllite and Ballachulish Slateand form mappable units, akermanite, spurrite, merwinite, and larnite, develop less the Appin and Ballachulish Limestones.The sedimentary commonly during especially high-temperature or low- rocks were first recrystallized at -6 kbar and -500 "C pressuremetamorphism. Tilley (1948) later argued that during Dalradian regional metamorphism at 490-520 Ma talc forms prior to tremolite in some cases.This study and then contact metamorphosedat 412 + 28 Ma by the documents that there exists an additional, complemen- Ballachulish igneous complex. With increasing grade of tary sequenceof progrademineral reactionsin metamor- contact metamorphism, isoglads were mapped in pelitic phosed siliceousdolomites involving trace minerals that rocks on the basis ofthe appearanceof(1) cordierite + occur in concentrations <0.010/0, including geikielite, biotite, (2) cordierite * potassium feldspar or andalusite baddeleyite,and qandilite. Although the report describes + biotite + quartz (in different bulk compositions), (3) developmentof theseminerals in the Ballachulishcontact andalusite * potassium feldspar, (4) corundum + potas- aureole,Scotland, ongoing work in severalother aureoles sium feldspar, and (5) silicate melt (Pattison and Harte indicates that these minerals are more likely not recog- 1991). Pressureduring contact metamorphism was -3 nized than rare. Mineral equilibria involving geikielite, kbar and maximum temperatures were -750'C (Patti- baddeleyite,rutile, zircon, forsterite, and carbonatespro- son l99l). vide useful, unappreciated constraints on temperature and This study focuses on contact metamorphosed sili- fluid composition during contact metamorphism of siliceousdolomites mapped as Appin Limestone in the Allt ceousdolomites. Guibhsachain area located in the northeast corner ofthe 0003-o04x/9610304-0485$05.00 485 486 FERRY: NOVEL ISOGRADS IN SILICEOUS DOLOMITE

TABLE1. Modes of selected samples of metamorphosed siliceousdolomites

Sample 1A 7A 't9.77 Calcite 6105 21.08 46.09 50.51 48.97 10.91 Dolomite 6.00 59.95 18.71 17.06 46.82 8.25 78.82 Forsterite 16.65 2.70 0.25 U.CJ 4.03 3.49 0.79 Diopside 00 0 0 2.14 2.65 0 Brucite (Per)' 00 9.60 11.20 0 00 Brucite (Srp). 0.55 0.05 2.46 1.70 0 o 0.05 Mica'. 1.20 2.80 0 0 6.41 4.32 tr Spinel o.75 0.05 0.44 0.39 0 0 0.29 Tremolite 0.25 1.62 0 0 o 1e 1.62 tr Chlorite 0 0.34 0 0 0.10 0.05 0.29 Serpentine 13.30 11.4022.45 18.61 11.6130.26 e.80 Apatite 00 0 0 tr 0.10 0.05 Pyrrhotite 0.25 tr 0 IT 0 0.29 0 Sphalerite 0tr 0 0 0 00 Zircon 00 0 0 IT tr0 Rutile 00 0 0 IT 00 Geikielite tr tr 0 0 0 tr tr Baddeleyite tr tr tr tr 0 0tr Qandilite 00 tr tr 0 00 Nofe.'Valuesgiven in volumepercent; tr : <0.05%. Samplenumbers Frcunr 1. Geologic sketch map of the Allt Guibhsachain refer to Figure 1. Numbers in normal type identity exclusively prograde minerals; numbers in italics identify exclusively retrograde minerals; in un- area the northeastcorner ofthe Ballachulishaureole, Scotland derlinednumbers identify mineralsthat lormed during both prograde and (from Weiss1986 and Maschand Heuss-ABbichlerl99l). Sym- retrograde metamorphismin the same sample. bols are sample localities; numbers inside ' (Per),after perictase;(Srp), intergrownwith serpentineafter forsterite. border refer to sam- -. ples describedin text and tables; numbers outside border refer Mica in sample1A is kinoshitalite;mica in all othersamplesis phlog- ooite. to the National Grid ReferenceSystem. 61 : geikielite, Bad : baddeleyite,Qnd : qandilite; other mineral abbreviationsfollow Kretz (1983).Isograds labeled on hgh-grade side.Diopside (Dil out and periclase(Per) isogradsfrom Ferry (unpublishedmanu- scnpt). MrNBur,ocY AND MTNERALcHEMrsrRY Principal minerals Modes of sevenrepresentative siliceous dolomites are aureole(Fig. l). Maschand Heuss-ABbichler(1991) iden- listed in Table l. Compositions of peak and some retro- tified isograds based on the formation of forsterite and grade minerals appear in Table 2. The 14 samplescan be periclasein the dolomites, presentedsome mineral chem- divided into three groups based on their principal peak ical data, proposed prograde mineral reactionsinvolving minerals. Rocks 400-500 m from the contact contain calcite, dolomite, forsterite, tremolite, diopside, and per- forsterite + diopside * tremolite + dolomite + calcite iclase, and estimated peak PTXco, conditions of contact + phlogopite (open and solid circles,Fig. l). Assemblages metamorphism.Heuss-ABbichler and Masch (1991) de- in samples collected from this interval differ somewhat scribed mineral textures in the metacarbonaterocks and from those reported by Masch and Heuss-ABbichler speculatedon mineral-fluid reaction mechanisms. (1991) and are the subjectof anotherreport (Ferry, unpublished manuscript). Minerals observed in samples collectedcloser to the pluton are the same as reported by MnrHoos oF rNvEsrrcATroN Maschand Heuss-ABbichler(1991). Dolomites 0-200 m As part ofa broader study offluid flow during contact from the contact contain forsterite + dolomite + calcite metamorphism 14 samples of siliceous dolomite were + mica * spinel (open squares,Fig. 1). Location 3 is one collected at 12 locations in the Allt Guibhsachain area of several dolomite xenoliths in granite that contained (Fig. l). Mineral assemblageswere identified by optical periclase + forsterite + calcite * spinel at the peak of and scanning electron microscopy. Qualitative analyses metamorphism (solid square,Fig. l). of all minerals were obtained by EDS with the JEOL All siliceous dolomites contain an abundance of ret- JXA-8600 electron microprobe at Johns Hopkins Uni- rogrademinerals (seefootnote to Table l). Peak periclase versity. Minerals that exhibited detectabledeviation from has been hydrated to brucite. Dolomite in samplesfrom ideal compositions were analyzed quantitatively with locations I and 3 is retrograde,and retrograde dolomite WDS with the use of natural silicate and carbonatemin- occurs with peak dolomite in the other samples.Serpen- eral standardsand a ZAF corr.eclionscheme (Armstrong tine is the most abundant silicate mineral in all but one 1988).Modes of the 14 sampleswere measured by count- sample (lA) and makes up >300/oof some rocks (e.g., ing at least 2000 points in thin section with back-scat- 7C). Secondarychlorite was produced by retrograde re- tered electron imaging. Any uncertainty in the identifi- action of spinel with forsterite and calcite. Secondary cation ofa particular point was resolved by obtaining an tremolite in samples <200 m from the contact is the EDS X-ray spectrum. product of retrogradealteration of forsterite (e.g.,samples FERRY:NOVEL ISOGRADS IN SILICEOUS DOLOMITE 487 lA,2A,8A, Tables I and 2). More detaileddocumen- lite, baddeleyite, forsterite, calcite, and dolomite record tation of the retrogrademinerals and a model that quan- T-X.o, conditions consistentwith those of the peak con- titatively interrelatestemperature, fluid composition, flu- tact metamorphism independently determined by pro- id flow, and reaction progressduring their formation are grade mineral assemblagesin carbonateand pelitic rocks presentedby Ferry (unpublished manuscript). from the Allt Guibhsachain area (presentedbelow). The compositions of dolomite, forsterite, diopside, and Qualitative analysesof zircon, rutile, and baddeleyite spinel are close to Mg-rich Fe-Mg solutions with Fe/(Fe indicate that they are close to ZrSiOo, TiOr, and ZrOr, + Mg) = 0.07 (Table 2). Calcite compositions fall in two respectively.Geikielite is a (Mg,Fe,Mn)TiO, solution with $oups. Although both groups are practically Ca-Mg so- a quite variable Mg/(Mg * Fe * Mn) in the range 0.45- lutions, the most magnesiancalcite inclusions in forster- 0.86 (Table 2). Qandilite, a rare Mg-Ti-rich spinel, has ite are typically much more Mg-rich than matrix calcite. a composition in sample 3A almost the same as the type Calcite inclusions were analyzedfor Si to verify that their specimenfrom Iraq (Al-Hermezi 1985).Using Ti atoms more magnesiancompositions are not an artifact of con- per formula unit as a measure, qandilite in sample 3,A' tamination by host olivine. The most magnesiancalcite has a mole fraction of the MgrTiOo component I 0.6. inclusionsin forsterite are believed to preservepeak com- Qandilite in sample 38 is significantly richer in the FerOo positions or nearly so; compositionsof matrix calcitehave component. In both samplesqandilite coexists with spi- been partially reset during cooling by exsolution of do- nel sensustricto, demonstrating the existenceof a wide lomite. Of the principal peak minerals tremolite deviates miscibility gap betweenMgAlrOo-rich and MgrTiOo-rich most from an ideal composition (e.g., samples 7A and spinels at the conditions of contact metamorphism. 7C), largely by the edenite (NaAlSi_,) and tschermakite (AlrMg ,Si_,) substitutions.Retrograde tremolite is nearly rsocRADs an Fe-Mg solution with Fe/(Fe + Mg) = 0.03 (samples TnnBr NovEL lA,2A, and 8A). Mica in most samplesis too alteredfor Becausegeikielite, baddeleyite, and qandilite tend to meaningful determination of composition; a limited occur as isolated grains in carbonateand becauseof their number of analysesindicate that with the exception of very low concentrations,conventional considerationsof sample lA mica is close to ideal phlogopite with Fe/(Fe textures and modal changesare uselessin identifi,ing the + Mg) < 0.02. Micas have Cl contents below detection progradereactions responsible for formation of the min- by EDS. Valuesof F/(F + OH) are low (<0.03) in ana- erals. Neverthelessrutile, zircon, geikielite, baddeleyite, lyzed micas within 200 m of the contact; F/(F + OH) : and qandilite display a regular spatial distribution in the 0.09-0.10 in micasfrom locations5 and 6. Mica in sam- Allt Guibhsachain area. Model progradegeikielite-, bad- ple lA, located -2 m from the contact between granite deleyite-, and qandilite-forming reactions were inferred and dolomite, is the unusual barium potassium mica ki- from the distribution, and isogradswere mapped on the noshitalite. Becauseno Ba-bearingmineral was observed basis of those reactions. in dolomites at lower grades,the formation of kinoshitalite probably involved metasomatictransfer of Ba from Geikielite isograd the adjacent pluton. At the lowest gradessampled in the Allt Guibhsachain area (locations 5, 6, 7A, Fig. l) siliceous dolomites con- Trace minerals tain rutile, zircon, and dolomite but no geikielite, bad- In addition to the principal peak and retrograde min- deleyite, or qandilite. Geikielite and calcite are presentin erals, the metamorphosed siliceous dolomites contain a and rutile * dolomite are absent from sample 78 and suite of trace minerals that occur in concentrations<0.30/o (with one exception) all others from higher-grade loca- by volume, including apatite, pyrrhotite, sphalerite, zir- tions up to the contact between dolomite and granite at con, rutile, geikielite, baddeleyite, and qandilite (Table location l. A model reaction that accounts for the pro- l). The latter five minerals are the focus of this report. grade appearanceof geikielite * calcite and disappear- Zircon, rutile, geikielite, baddeleyite, and qandilite are ance of rutile + dolomite is present in concentrations -0.010/o or less becauseonly rurile * dolomite: geikielire+ calcite + CO, (l) one geikielite grain and none of the other four minerals Tio2 CaMg(COl)z MgTiO: CaCo: COz. wereencountered in the courseofcounting >28000 points over 14 samplesof dolomite. In spite of their small abun- A geikielite isograd basedon Reaction I can be mapped dance, the minerals are easily identified by their distinc- between locations 7A and 78 (Fig. l). Becausethe ex- tively bright appearancein back-scatteredelectron im- posure of siliceous dolomite is insufrcient to locate the ages.Zircon, rutile, geikielite, baddeleyite,and qandilite exact position of the isograd,it was drawn parallel to the are not likely products of retrograde metamorphism for strike ofisograds in pelitic rocks in the area(Pattison and two reasons.First, the five minerals form euhedral crys- Harte l99l). Reactantand products occur togetherin only tals typically isolated in carbonate. Retrograde minerals one sample (7D, Fig. l). Natural geikielite contains sig- are texturally different, replacing progrademinerals, e.g., nificant amounts of Fe and Mn becauseof the strong brucite replacespericlase, serpentine replacesforsterite, partitioning of Fe and Mn into geikielite relative to other chlorite replaces spinel, etc. (Ferry unpublished manu- minerals in siliceous dolomites (Table 2). The composi- script). Second,equilibria involving zircon, rutile, geikie- tion of geikielite in individual samples and the stabili- 488 FERRY: NOVEL ISOGRADS IN SILICEOUS DOLOMITE

Tlele 2. Compositions of minerals in selected samples of metamorphosedsiliceous dolomite

Sample 1A 2A 3A 3B 7A 8A Matrixcalcite Ca 0.924 0.948 0.96s 0.963 0.933 0.913 0.943 Mg 0.070 0.050 0.034 0.036 0.066 0.082 0.055 Fe 0.001 0.001 0.000 0.000 0.001 0.002 0.001 Mn 0.005 0.001 0.001 0.001 0.000 0.003 0.001 Oxide sum 56.06 56.28 55.64 55.66 56.02 55.86 s6.08 Inclusioncalcite Ca 0.862 0.855 0.860 0.909 0.905 Mg 0.129 0.141 0.138 0.090 0.087 Fe 0.003 0.002 0.000 0.001 0.004 Mn 0.006 0.002 0.002 0.000 0.004 Oxide sum 56.64 56.14 55.87 55.42 56.58 r(rc)' 735 755 755 650 645 Dolomite Ca 1.027 1.030 1.042 1.033 1.010 1.014 1.030 Mg 0.954 0.962 0.952 0.959 0.981 0.954 0.962 Fe 0.009 0.006 0.002 0.004 0.009 0.424 0.006 Mn 0.010 0.002 0.004 0.003 0.000 0.008 0.002 Oxidesum 52.15 52.35 52.50 52.39 52.13 51.96 52.25 Forsterite Mg 1.931 1.954 1.975 1.976 1.960 1.907 1.950 Fe 0.051 0.032 0.012 0.021 0.037 0.083 0.038 Mn 0.012 0.002 0.004 0.004 0.001 0.010 0.002 Ca 0.002 0.002 0.004 0.001 0.001 0.001 0.001 Si 1.001 1.003 1.001 0.996 0.998 0.997 1.004 Oxidesum 100.59 100.18 99.99 100.29 100.22 100.38 99.46 Fe/(Fe+ Mg) 0.026 0.016 0.006 0.011 0.019 o.042 0.019 Diopside Ca 0.986 0.963 Na 0.009 0.025 Mg 0.969 0.915 FE 0.013 0.068 Mn 0.000 0.009 Ti 0.002 0.002 AI 0.019 0.012 Si 1.998 2.005 Oxidesum 100.72 100.77 Fel(Fe + Mg) 0.013 0.070 Tremolite K 0.006 0.007 0.007 0.029 0.005 NA 0.000 0.000 o.251 0.690 0.007 Ca 1.899 1.972 1.926 1.555 1.979 Na 0.069 0.019 0.074 0.445 0.021 Mg 4.813 4.769 4.786 4.696 4.897 FE 0.124 0.062 0.056 0.211 0.o42 Mn 0.015 0.009 0.00s 0.016 0.002 Ti 0.007 0.010 o.022 0.015 0.008 16lAl 0.072 0.151 0.134 0.151 0.036 r4tAl 0.027 0.141 0.372 0.636 0.014 DI 7.973 7.859 7.628 7.364 7.986 F bd bd bd bd o.o27 Oxidesum 98.56 97.48 97.88 97.70 97.87 Fe/(Fe+ Mg) 0.025 0.013 0.012 0.043 0.009 Spinel Mg 0.935 0.946 0.985 o.972 0.957 Fe2+ 0.067 0.055 0.025 0.033 0.045 Mn 0.009 0.003 0.003 0.002 0.001 Ca 0.002 0.001 0.004 0.001 0.001 Ti 0.013 0.005 0.017 0.007 0.003 AI 1.884 1.975 1.884 1.932 1.987 Fe3* 0.089 0.015 0.081 0.053 0.006 Oxidesum 99.95 100.01 99.50 100.39 99.55 Mica K 0.426 0.922 Na 0.151 0.002 Ba 0.406 o.021 Mg 2.625 2.602 Fe 0.096 0.042 Mn 0.004 0.000 Ti 0.088 0.075 FERRY: NOVEL ISOGRADS IN SILICEOUS DOLOMITE 489

TABLE2.-Continued

Sample 1A 7A 16rAl 0.175 o.249 rarAl 1.719 1.308 Si 2.281 2.692 F 0.005 bd Oxide sum 96.97 s5.72 Fe/(Fe+ Mg) 0.035 0.016 Geikielite Mg 0.635 0.864 0.569 0.722 FE 0.278 0.141 0.392 0.278 Mn 0.103 0.007 0.052 0.014 tl 0.993 0.994 0.994 0.992 Oxide sum 100.18 100.38 99.46 100.15 Fe/(Fe+ Mg) 0.304 0.140 0.408 0.278 Qandilite Mg 1.215 0.965 Fe+ 0.205 0.324 Mn 0.159 0.131 Ca 0.003 0.008 Fe3+ 0.742 1.055 AI 0.093 0.092 Cr 0.000 0.000 Ti 0.583 0.426 Oxide sum 97.09 95.7 /Vote..Oxidesum refers to the sum of metal oxide weight percents with all Fe as FeO; bd : below detection limits. Sample numbers refer to Figure 1. Except tor calcite, analysesare averagesof 3-5 spot analysesof 3-5 grains in thin section. Calcite : cations per O atom (less COr). Matrix calcite : average of 10-20 calcite grains in matrix; inclusioncalcite: most magnesiananalyzed calcite inclusionin forsterite. Dolomite: cations per 2 O atoms(l6ssCOJ.Forsterite:cationsper4Oatoms.Diopside:cationsper6Oatoms.Tremolite:cationsper23Oatoms(lessHrO).Spineland qandilite:cationsper3totalcations.Mica:cationsperllOatoms(lessHrO).Geikielite:cationsper3Oatoms.Feinall butspinel andqandilite taken as Fe2+' Fe3+/Fe2+in spinel and qandilite adjusted to balance I negativecharges. 'Calcite + dolomitetemperature (Anovitz and Essene1987).

zation ofreactants and products ofReaction I therefore gradeswithin -400 m of the contact; qandilite is present depend sensitively on whole-rock Fe- and Mn-contents in and geikielite is absent from both samples collected as well as P, T, and Xco,. from the dolomite xenolith at location 3. In principle, qandilite could form by reaction ofgeikielite with either Baddeleyiteisograd dolomite or periclase.Siliceous dolomites from the Allt Zftcon * dolomite occur in sample 7D and all others Guibhsachain area are uninformative as to which is the at lower grades; baddeleyite + forsterite + calcite are casebecause none contains either qandilite + dolomite presentin and zircon + dolomite are absentfrom sample or geikielite * periclase.Unpublished studiesof siliceous 8A and all others at higJrergrades. A model reaction that dolomites in the Beinn an Dubhaich aureole, Scotland accounts for the prograde appearanceof baddeleyite + (Holness 1992), and the Silver Star aureole, Montana forsterite + calcite and disappearanceofzircon * dolo- (Foote 1986),however, reveal coexistinggeikielite + per- mite is iclase in 22 samples and no occurrencesof qandilite + periclase dol- zircon * 2 dolomite dolomite. Consequently forms in siliceous omites at gradeslower than the qandilite isograd.A mod- ZrSiOc 2 CaMg(CO3)2 el reaction that accountsfor the prograde appearanceof : baddeleyite + forsterite + 2 calcite + 2CO, (2) qandilite and disappearanceofgeikielite + periclaseis ZrO2 Mg:SiO+ 2 CaCQ3 2 COz. geikielite * periclase: qandilite (3) A baddeleyite isograd based on Reaction 2 can be mapped betweenlocations 7D and 8A (Fig. 1). Like the geikielite MgTiOr MgO Mg2TiOa isograd,the baddeleyiteisograd was drawn parallel to the mapped strike of isogradsin pelitic rocks for lack of better con- A qandilite isograd based on Reaction 3 can be was parallel to straints. The baddeleyito isograd in Figure I perfectly between locations I and 3, and it drawn pelitic rocks for lack of better sepafatesoccurrences of reactants from occurrencesof the strike of isograds in products ofReaction 2; reactantsand products are pres- constraints (Fig. l). Reaction 3 only describesthe for- qandilite; it does ent together in no sample. mation of the MgrTiOo component of not account either for the significant Fe-content of the Qandilite isograd mineral (Table 2) or for the oxidation of Fe that probably Geikielite occurs in sample 1A and all others at lower occurs during the formation of the mineral. 490 FERRY: NOVEL ISOGRADS IN SILICEOUS DOLOMITE

800 inclusion in forsterite (Table 2) in sevensamples are shown as open rectanglesin Figure 2. The size of each open O Dl+Dol+Fo+Cal+Tr+Fluld rectanglecorresponds to uncertainties of +5 m in R and 750 'C I carcne+aonmnethemometry +20 in Z (representing an uncertainty of approxi- mately 10.5 molo/oMgCO, in calcite (s) I bogECln pelltlcrocks composition). Cal- 700 cite + dolomite temperaturesare not reported for those o forsterite-bearing dolomites in which either forsterite grains lack calcite inclusions, the calcite inclusions have (4) exsolved dolomite, or the compositions of all analyzed 650 o (2) o calcite inclusions have been altered by communication (3) betweeninclusions and the rock matrix via fractures.In- clusionswith the maximum Mg-content were almost cer- 600 0'42291F (km) r ("c) = 250 * sfl)e tainly in equilibrium with dolomite when they formed becauseforsterite developed in dolomite-bearing rocks 550 either by a reaction between tremolite and dolomite or 0.0 0.2 0.4 0.6 0.8 between diopside and dolomite (Masch and Heuss- A8bichler 1991). Nevertheless,the temperaturesrecord- distancefrom diorite,F (km) ed by calcite * dolomite thermometry formally are min- Frcunn2. Temperatureprofile radial to thepluton in theAllt imum estimates becausethe calcite may not have been Guibhsachainarea of the Ballachulishaureole. Distance. R. is included in forsterite and isolated from dolomite at peak measuredfrom theoutermost exposrueof dioriterather than the metamorphic conditions. The averagedeviation of cal- outermostexposure ofplutonic rock. Temperatures and distanc- cite + dolomite temperaturesfrom the curve in Figure 2 esfor the peliticisograds are from Pattison(1991); other results is * I (range -21 to +28 'C). Although arefrom this study(see text). Isograds in peliticrocks are based "C the inclusions on the progradeappearance cordierite + potassiumfeldspar (2), may not have formed precisely at the peak of metamor- andalusite+ potassiumfeldspar (3), corundum+ potassium phism, they evidently record peak temperaturewithin er- feldspar(4), and silicatemelt (5).Equation is for the curveand ror of measurement.With one exception, calcite + do- representsa least-squares fit to the datafor peliticisograds. lomite temperaturesreported for the Allt Guibhsachain areaby Maschand Heuss-ABbichler(1991) fall below the curve (their results not plotted in Fig. 2). Although they Prr,lsn EeurLrBRrA AND prrysrcAr coNDrrroNs AT carefully sought the most Mg-rich calcite in each sample, THE ISOGRADS their slightly lower temperaturesprobably result from an- Pressureand temperatureduring metamorphisrnin alyzing matrix grains whose compositions were partially the Allt Guibhsachain area reset during cooling by dolomite exsolution. Mineral equilibria in both pelitic rocks and siliceous Temperatures recorded by equilibrium among diop- dolomites from the Ballachulish aureole record a peak side, dolomite, forsterite, calcite, tremolite, and fluid at lithostatic pressureduring contact metamorphism of 3.0 P"o,4 P.o,: P: 3.0 kbar were calculatedfor six sam- + 0.5 kbar (Pattison l99l; Masch and Heuss-ABbichler plesof siliceousdolomite with the useof Berman's(1988) l 99l). data base(updated August 1990);the Kerrick and Jacobs Positions of isograds in pelitic rocks in the northeast (198l) equationof statefor CO,-H,O fluid; and Skippen's corner of the Ballachulish aureoleand peak temperatures (1974) model for calcite solutionscoexisting with dolo- recordedby them are plotted in Figure 2 (from Table I 6.4 mite; measuredcompositions of diopside, forsterite, and ofPattison l99l). The size ofeach solid rectanglecor- tremolite; simple ideal ionic mixing models for diopside respondsto Pattison's estimatesof uncertainty in Z and and forsterite; and the expressionfor tremolite activity distance(R). Thermal models of the aureoleindicate that from Holland and Powell (1990)assuming random mix- the diorite was the principal heat sourcefor contact meta- ing of cations over the Ml, M2, and M3 sites.Results morphism (Buntebarthl99l). Followingthe precedentof appear as solid circles in Figure 2. The size ofcircles has Pattison (1991, 1992)and Masch and Heuss-ABbichler no meaning becauseuncertainties in the calculated tem- (1991) values of R in Figure 2 thereforeare measured peratures are unknown. The averagedeviation in tem- radially from the outermost exposureof diorite. Temper- perature of the six-phaseequilibrium from the curve is 'C atures for the pelitic isograds at R < 800 m along with + l0 (range+ 1 to +22'C). Buntebarth's best estimate for peak T at the contact (760 The good agreementbetween temperatures recorded by + l0'C) were fit by least-squaresto an exponential func- the three independent mineral equilibria and the model tion of R subject to the constraint that T : 250 .C far curve in Figure 2 is justification for using them to con- from the intrusion (Buntebarthl99l): strain temperature and fluid composition during formation of geikielite, baddeleyite,and qandilite. Z("C):250 + 500e-o422erx(km) (4) which correspondsto the curve in Figure 2. Baddeleyite isograd Temperaturesrecorded by the most magnesiancalcite Thermodynamic analysis of the baddeleyite isograd is FERRY:NOVEL ISOGRADS IN SILICEOUS DOLOMITE 491 the most straightforward of the three becausezircon and baddeleyite are not complicated by significant solid solution. Figure 3 illustrates phaseequilibria involving cal- P = 3000bars cite, dolomite, diopside, tremolite, forsterite, zircon, and baddeleyite calculated from the data base of Berman Di+Dol+Fo+Cal+Tr+Fluid equilibrium (1988; updated August 1990, by personalcommunica- tion) with unit activities for all mineral components ex- inferredconditions of baddeleyiteisograd cept calcite whosecomposition is adjustedfor coexistence with dolomite. Solid circles illustrate the I-X"o, conditions of equilibration of the six occuffencesof the diopside * dolomite + forsterite + calcite + tremolite + fluid equilibrium with reduced activity of the tremolite o component in amphibole labeled for each point. The compositions of dolomite, diopside, forsterite, and calcite l- in the samplesdeviate only slightly from those of ideal compoundsin the systemCaO-MgO-SiOr-COr-HrO. The six-phaseassemblage is stabilized in the aureole over a - 30 'C interval largely by a reduction in tremolite activity. The T-Xco,conditions recordedby the samplesthere- fore lie close to the diopside + dolomite + forsterite + Dd calcite equilibrium curve at temperaturesbetween the di- ::,='" opside * dolomite * forsterite * calcite + tremolite invariant point for unit activity of tremolite and X.o, : I (Fig. 3). 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 The baddeleyite isograd is mapped between locations x"o, 7D and 8A (Fig. l). Peak temperaturesat the locations, Frcunr 3. T-X"o,diagam depictingphase equilibria among basedon the model profile in Figure 2, are 660 and 710 (Fo), (Cal), oC, diopside(Di), dolomite(Dol), forsterite calcite trem- respectively. If fluid composition at the baddeleyite olite (Tr), zircon(Zrn), baddeleyite(Bad), and COr-HrO fluid at isograd was the sameas at the peak of metamorphism at 3 kbar calculatedfrom Berman's(1988) data base, updated Au- locations 7A-7C, X.o, was 0.76-0.95. The inferred gust 1990.All mineralsexcept Cal and Tr are assumedto be T-Xco, conditions for the isograd therefore are those of puresubstances. Cal compositions were adjusted for equilibrium the shaded rectanglein Figure 3. The T-Xco, curve for with Dol. Solidcircles represent I-X"o, conditionsestimated for baddeleyite-formingReaction 2, independently calculat- six occurrencesof the assemblageDi + Dol + Fo + Cal + Tr ed from Berman's data base, passesthrough the middle with reducedTr activity (a'.) as indicated;unit Tr activity as- ofthe rectangle. sumedalong calculated curves. Conditions of the baddeleyite isogradpredicted by the Zm + Dol + Bad + Fo + Cal equilibrium are consistentwith independentlyinferred conditions based Geikielite isograd on othermineral-fluid equilibria (shaded rectangle). The geikielite isograd is mapped betweenlocations 7A and 78 (Fig. l). The minimum peak temperature estimated for location 7A is that of the model temperature profile in Figure 2, 640'C. The maximum peak temper- currencesof the diopside + dolomite + forsterite + cal- ature estimated for location 78 is that recorded by the cite * tremolite + fluid equilibrium in the aureole(same diopside * dolomite + forsterite * calcite + tremolite as circles in Fig. 3). The number by each solid circle is + fluid equilibrium, 655 "C. The X.o, of fluid recorded the activity of geikielite in the sample (estimated from by the six-phaseequilibrium at locations 7A and 78 is measuredmineral compositions assumingideal mixing). 0.76 and 0.80, respectively. The inferred T-Xco, condi- The number by eachopen circle is the activity of MgTiO, tions for the geikielite isograd therefore are those of the in a fictive geikielite in Fe-Mg and Mn-Mg exchange shadedrectangle in Figure 4. equilibrium with forsterite that would have developed The T-X"o,curve for geikielite-forming Reaction I with should the T-X"o, conditions have been appropriate for unit activity for the geikielite component, calculatedfrom formation of the oxide. The composition of the fictive Berman's data base, is significantly removed from the geikielite (Gk) was calculated from measured forsterite shadedrectangle in Figure 4. Analysis of phaseequilibria (Fo) composition in each sample taking (Fe/Mg)"*/(Fe/ for Reaction l, however, must consider the deviation in MB).. : 19.2 and (Mn/Mg)"0/(Mn/Mg).. :22.2 (the av- composition of natural geikielite from MgTiO, as well as erageofKo values for sevenanalyzed geikielite-forsterite T and X"o,. Dashed curyes in Figure 4 illustrate how the pairs from the area). T-Xco, conditions for Reaction I are displaced by re- Figure 4 quantitatively explains the occurrenceand ab- duced geikielite activities in the range 0.6-1.0. Circles in senceof geikielite in the Allt Guibhsachain area in terms Figure 4 represent the T-X.o, conditions of the six oc- of T, X"o,, and mineral composition. Geikielite devel- 492 FERRY: NOVEL ISOGRADS IN SILICEOUS DOLOMITE

on or below the dashedack contour coffesponding to the composition of fictive geikrelite that would have devel- O O Di+Dol+Fo+Cal+Tr+Fluidequilibrium \9 :;' inferredconditions -.;a,r' oped had T-Xco, conditions been appropriate for for- ::i '"7 of geikieliteisograd s mation of the mineral by Reaction l. Development of O geikielitepresent geikielite by Reaction I near the isograd evidently was O geikielite absent limited to relatively Fe- and Mn-rich rocks (ackless than 78 sample number -0.7); ar T > 700 "C, however, formation of geikielite in rocks with rutile * dolomite was independent of whole-

laor rock Fe- and Mn-contents. The excellent agreementbe- geikielite-formation o tween the model for in Figure 4 and the observedpresence or absenceof geikielite in siliceous l- dolomites from the Allt Guibhsachain area suggeststhat the accuracyof thermodynamic values for rutile and geikielite in Berman's data base is sufficient for meaningful estimatesof I'and X.o, from equilibria involving the two minerals.

Qandilite isograd The qandilite isograd is mapped between locations I Tr + Dol and 3 (Fig. l). Becausesample lA did not contain peri- claseand probably did not contain dolomite at the peak 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 of metamorphism, the physical conditions of the qandi- X"o, Iite isogradare bracketedby those at location 3 and those of the next lower grade sample from lA that contains Frcunr 4. T-X"o,diagram depicting phaseequilibria among peak dolomite, 24. The minimum peak temperature es- diopside (Di), dolomite (Dol), forsterite (Fo), calcite (Cal), trem- timated for location 2 is that of the model temperature olite (Tr), rutile (Ru), geikielite (Gk), and COr-HrO fluid at 3 profile in Figure 2, 725 'C. The maximum peak temper- kbar calculatedfrom Berman's (1988) data base,updated August ature estimated for location 3 is that recorded by the 1990. All minerals except Cal and Gk are assumedto be pure composition of calcite inclusions in forsterite from sam- substances.Cal compositionswere adjustedfor 'C. equilibrium with ple 3A,, 755 Qandilite formed in the stability field of Dol. Dashed curves show how the Ru + Dol + Gk + Cal equi- periclasewhich lies at Xco, < 0.08 for Z < 755 'C. The librium is displacedby reducedactivity of MgTiO, (aou).Circles inferred T-Xco,conditions for the qandilite isogradthere- representZ-X-, conditions estimatedfor six occurrencesof the assemblageDi + Dol + Fo + Cal + Tr (from Fig. 3). Values of foreare T:725-755 "C and X"o,1 0.08.Because there 4cknext to solid circlesare basedon measuredGk compositions are no data for MgrTiOo in any of the current thermo- in the sample. Values ofao* in brackets next to open circles are dynamic data bases,the indirectly inferred conditions for fictive valuesestimated from Fe-Mg and Mn-Mg exchangeequi- its formation could not be compared to conditions di- libria between Fo and Gk and the composition of Fo in the rectly computed from equilibria involving qandilite and sample. Gk develops in rocks with Ru + Dol by Reaction I at other minerals in the siliceousdolomites. Z-X.o, conditions above the dashedack contour corresponding to Gk composition in that rock. Gk fails to form in rocks with Ru + Dol by Reaction I at T-X"o, conditions on or below the Drscussrox dashedao* contour correspondingto composition of fictive Gk Novel and conventionalisograds in siliceousdolornites that would occur in the sample (seetext). Isograds based on the formation of geikielite, baddeleyite, and qandilite in siliceous dolomites supplement conventional isograds. All three minerals developed in oped in those samples (solid circles) metamorphosed at forsterite-bearingrocks and therefore their isogradslie at T-Xco, conditions above the dashed 4ck contour corre- gradeshigher than the forsterite isograd.The baddeleyite sponding to geikielite composition in the sample. Al- isograd occurs in the vicinity ofthe diopside-out isograd though not plotted on Figure 4 because peak values of (which in Si-poor dolomites correspondsto completion X"o, are unconstrained, geikielite-bearing rocks between of the diopside + dolomite + forsterite + calcite reaction locations 8A and lA must plot likewise because estimat- in Figs. 3 and 4). Calculated phaseequilibria in Figure 3 ed peak Z is higher than Z for ao. : X"o,: l. The oc- and field occurrencesin Figure l, however, more narrow- currence of rutile + dolomite + geikielite + calcite in ly bracket the baddeleyite isograd between the diopside- sample 7D could not be evaluated because geikielite grains out and periclaseisograds. The qandilite isograd occurs in the rock are too small for analysis. Geikielite failed to in the vicinity ofthe periclaseisograd. Coexisting geikie- develop in those samples (open circles) metamorphosed lite + penclase,however, are commonly observedin the FERRY: NOVEL ISOGRADS IN SILICEOUS DOLOMITE 493

qandilite, Beinn an Dubhaich and Silver Star aureoles.The qandi- Lindsley provided the referenceto the type specimen of and gave advice on the occurrence and chemistry of ki- lite isograd in the Allt Guibhsachain area thereforeprob- Steve Guggenheim noshitalite. Thoughtful reviews by Martha Gerdesand Dave Pattison are gradeshigher than the periclaseisograd (Fig. Foun- ably lies at efatefully appreciated. Research support by the National Science l) but at gradeslower than the monticellite isograd in Si- dation, Division ofEanh Sciences,grant EAR-9404578. poor dolomites. The geikielite, baddeleyite,and qandilite isogradsoffer increasedresolution of metamorphic grade at high but not the highest temperaturesof crustal meta- Rrppnrxcns crrED morphism. Al-Hermezi, H.M. (1985) Qandilite, a new spinel end-member,MgrTiOa, from the Qala-Dizehregion, NE Iraq. MineralogicalMagazne,49'739- Equilibria involving trace minerals in metamorphosed 744. siliceousdolomites Anovitz, L.M., and Essene,E.J. (1987) Phase equilibria in the system 19' 389-414. The trace minerals rutile, zircon, geikielite, baddele- CaCOr-MgCOr-FeCO3.Journal of Petrology, (1988) analysis of silicate and oxide min- qandilite <0.010/oof siliceousdolo- Armstrong, J.T. Quantitative yite, and compose erals: Comparison of Monte Carlo, ZAF and phi-rho-z procedures'In mites from the Allt Guibhsachainarea. Nevertheless, they D.E. Newbury, Ed., Microbeam analysis,p. 239-246. San Francisco evidently reacted both with each other and with more Press,California. min- abundant coexisting carbonate and silicate minerals in a Berman, R.G. (1988) Internally-consistentthermodynamic data for systemNarO-KzO-CaO-MgO-FeO-FeOr-AlO,-SiO:-TiO:- regular fashion producing a systematic pattern ofisograds. erals in the HrO-COr. Journal of Petrology, 29, 445-522. Furthermore, physical conditions of metamorphism pre- Bowen, N.L. (1940) Progressivemetamorphism of siliceous limestone dicted independently from carbonate-silicate-fluidequi- and dolomite. Joumal of Geology, 48' 225'21 4' libria are the samewithin error as those predicted directly Buntebarth, G. (1991) Thermal models of cooling. In G. Voll, J. Tdpel, Equilibrium and kinetics in con- from equilibria involving rutile, geikielite, zircon, bad- D R.M. Pattison,and F. Seifert,Eds., metamorphism: The Ballachulish igneous complex and its aureole, fluid. The tact deleyite, calcite, dolomite, forsterite, and p. 379-402. Springer-Verlag,Berlin. agreementleads to two conclusions.First, in spite of their Foote, M.W. (1986) Contact metamorphism and skarn development of small abundanceand refractory character,the trace min- the precious and basemetal depositsat Silver Star, Madison County, erals appear to have been in local equilibrium during Montana. Ph.D. thesis,University of Wyoming' l-aramie S., and Masch, L. (199I) Microtextures and reaction metamorphism both with each other and with the prin- Heuss-A0bichler, mechanisms in carbonate rocks: A comparison between the thermoau- cipal minerals in eachspecimen. Second, because the trace reolesof Ballachulishand Monzoni (Italy). In G' Voll, J. Ttipel, D.R M' minerals appear to record equilibrium, they have the po- Pattison,and F. Seifert,Eds., Equilibrium and kineticsin contactmeta- tential to provide information about the PTX.., condi- morphism: The Ballachulish igneouscornplex and its aureole,p. 229- tions of metamorphism. Their usefulnessmay be general 249. Springer-Verlag,Berlin. Powell, R. (1990) An enlargedand updated inter- of ru- Holland, T.J.B, and and widespread. Trace amounts of combinations nally consistent thermodynamic dataset with uncertainties and corre- tile, zircon, geikielite, and baddeleyite are common in lations:The systemKrO-NarO-CzO-MgO-MnO-FeO-FerO:-A1:Or-SiOu- forsterite-bearingsiliceous dolomites from the Ballachu- TiOr-C-Hr-Or. Journal of Metamorphic Geology, 8, 89-124' lish, Beinn an Dubhaich, and Silver Star aureoles, and Holness, M.B. (1992) Metamorphism and fluid infrltration of the calc- of the Beinn an Dubhaich granite, Skye' Journal of minerals may have been overlooked at other loca- silicate aureole the Petrology, 33, 126l-1293. tions. Silicate minerals in many siliceous dolomites are Kemck. D M.. and Jacobs,G'K. (1981)A modified Redlich-Kwongequa- completely altered to retrogrademinerals such as serpen- tion for HrO, COr, and HrO-CO, mixtures at elevatedpressures and tine and chlorite. Occurrences of the more refractory temperatures.American Journal of Science,281' 735-76'7. minerals. American Miner- minerals rutile, zircon, geikielite, and baddeleyite,if pre- Kretz, R. (1983) Symbols for rock-forming 68, 277-279. the only us- alogist, served in such rocks, may in fact represent Masch, L., and Heuss-ABbichler,S. (1991) Decarbonation reactions in able mineralogical record of physical conditions at the siliceousdolomites and impure limestones.In G. Voll' J. T6pel' D'R.M' peak of contact metamorphism. Pattison,and F. Seifert,Eds., Equilibrium and kineticsin contactmeta- morphism: The Ballachulish igneouscomplex and its aureole,p. 211- Dating a prograde mineral reaction 227. Springer-Verlag,Berlin. Pattison, D.R.M. (1991) P-T-a(HrO) conditions in the aureole' In G' Measurementof the radiometric age of baddeleyite in Voll, J. Tdpel, D.R.M. Pattison, and F. Seifert, Eds., Equilibrium and siliceousdolomites from the Allt Guibhsachainarea (and kinetics in contact metamorphism: The Ballachulish igneouscomplex other aureoles)would determine the age of Reaction 2 and its aureole,p. 327-350' Springer-Verlag,Berlin. - and sillimanite and the Al,SiO5 triple and of prograde contact metamorphism itself, provided (1992)Stabiliry ofandalusite point: from the Ballachulish aureole,Scotland. Journal of greater -700 Constraints the closure I for baddeleyite is than "C. Geology, 100,423-446. Practically, however, dating of baddeleyite could be lim- Pattison, D.R.M., and Harte, B. (1991) Petroglaphy and mineral chem- ited by its very small abundancein the dolomites. istry of pelites. In G. Yoll, J. Tdpel, D.R.M. Pattison, and F. Seifert, Eds., Equilibrium and kinetics in contact metamorphism: The Balla- chulish igneouscomplex and its aureole,p. 135-179. Springer-Verlag, Berlin. AcxNowr,nocMENTs Skippen,G.B. (1974) An experimentalmodel for low pressuremetamor- ofScience,274, Ben Harte and Dave Pattison clarified many aspectsof contact meta- phism ofsiliceous dolomitic marble. American Journal momhism in the Ballachulish aureoleboth in and out of the field' Don 487-509. 494 FERRY: NOVEL ISOGRADS IN SILICEOUS DOLOMITE

Tilley, C.E ( 1948) Earlier stagesin the metamorphism of siliceous dolo- Westschottland: Kristallisationverlarif in einem zonierten Kaledonisch- mites. Mineralogical Magazins, 28,272-2'76. en pluton. Ph.D. thesis,Ludwig-Maximilians-Univenit?it, Munich. Voll, G., Tdpel, J., Pattison, D.R.M., and Seiferr;F., Eds, (1991) Equilib- rium and kinetics in contact metamorphism: The Ballachulish igneous complex and its aureole,414 p. sp.ringer-verlag,Berlin. Mexuscntr'nrcerveo JuNE 16, 1995 Weiss, S. (1986) Petrogenesedes intrusivkomplexes von Ballachulish, MANUscRrprAccEprED Noverrosn 16. 1995