American Mineralogist,Volume 67, pages l10l-1117, 1982

On the mechanismof contact aureoleformation in dolomitic country rock by the Adamello intrusion (northern ltaly)

Kunr Bucnen-NURMINEN Mine ralog isc he s Inst it ut der Univ e r sit rit B as e I Bernoullist. 30. 4056Basel. Switzerland

Abstract

Intrusion of the Adamello (Tertiary) in the southern Alps (northern Italy) caused a distinct mineralogical and chemical zonation within the country rock dolomites. The contact aureole can be divided into an inner aureole characterizedby the widespreadand abundant occulTenceof forsterite and spinel (-rclinohumite); and an outer aureole with widespread but modally subordinatetremolite. The forsterite isograde separatingthe two zones is a c\ear, mappable boundary in the field. The mineralogical changesacross the isograd are accompanied by a significant increase of modal and oxide towards the forsterite zone. Open pore spacesand cavities in of the outer aureole are progressively sealedwith silicate * oxide assemblagestowards the contact, resulting in a nodular, silicate-rich . It is concluded that the observed zonation may have resulted from infiltration and reaction of silica- and alumina-rich fluids, given of by the cooling pluton within the dolomitic country rock. This conclusion is further supportedby the microtextures of many of the forsterite-zone samples. The geochemical model proposed for the mechanism of aureole formation suggeststhat SiO2 dissolved in the inferred fluid was filtered out by a reaction of the type 2Do * SiO2uo: Fo + ZCc + 2CO2.Samples from roof pendantsshow evidence that CaO was not conserved in the above reaction, rather that dissolved congruently and was partially removed from the site of reaction. The forsterite isograd is interpreted as the main infiltration front during the formation of the metasomaticaureole.

Introduction Kerrick, 1976,Suzuki,1977). Other effectsof chem- (i.e., Various effects of high level igneousintrusions on ical transportinclude the formation of skarns vein carbonate country rocks have been discussedin a Taylor and O'Neil, 1977)and hydrothermal (Bucher, number of recently published field studies. It has formation 1981a). in been demonstrated in a number of cases that the The present study describesa contact aureole observedzonation of the aureole may be attributed dolomitic country rocks from the southern Alps. to thermal (Kerrick, 1970, Rice, The data from this particular field example suggest 1977,Masch, 1977,Melson, 1966).The composition that the observed mineralogical zoning of the aure- of the inferred metamorphic fluid has been shown to ole is the result of SiO2-metasomatismalong steep pluton. be controlled by the local assemblage temperature gradients imposed by the throughout the aureole. Prograde metamorphism geologY occurred along a definite "buffer path" (Green- General wood, 1975),given by the initial bulk composition The Adamello pluton is the largest Tertiary intru- of the carbonate rock and thermal regime of the sion in the Alps and is located north of Brescia aureole. In a number of other field examples it has (northern Italy). The pluton is a compositeintrusion been shown that the buffer capacity of the local consisting of interfingered stocks of granite and assemblageswas partially or completely exhausted granodiorite together with at least two types of by the introduction of H2O from the pluton. H2O- tonalite. Numerous small bodies of diorite, gabbro metasomatismin addition to the thermal effect of and gabbrediorite occur along the contact zone the intrusion was seen as a major control of the (Bianci and Dal Piaz, 1948). The whole pluton mineralogical zonation of the aureole (Moore and measuressome 60 km in length while the width is 0003-004)V82l1l l2-l l0l$02.00 ll0l tt02 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

between7 km and 22 km. The pluton intrudespre- tion as well as brecciated . In the outer Permiancrystalline basementrocks in the north and aureole, the dolomites are frequently very porous Permianand Mesozoic sedimentsin the south. In with cavities typically I to 2 mm in diameter (maxi- the area of Cima Uzza (southeastrim of the intru- mum cavity size 3 cm). sion) the intrusives are in contact with Mesozoic Figure la shows a cut surface of sample Ca 196. dolomitic sediments(Callegary, 1962). Two profiles The rock is generally very porous and the cavities have been studiedin detail: (l) a profile at Cima di can measureup to 2 cm in diameter. The pore space Agosta and (2) from Cima tJzza to the south (Fig. in many instances is interconnected in a three- 1). This secondprofile is characterized by intrusive dimensional network. The cavities are frequently rocks with numerous small carbonateroof pendants coatedwith . In sampleCa 196talc is also near the summit of Cima \Jzza, while south of the present as a wall coating. The matrix rock is an passo del Frato (Fig. l), non-dolomitic sediments extremely pure dolomite+alcite marble. Note that are intercalatedin the low-grade part of the aureole. the rock is cut by numerousveinlets (three visible The dolomitic sedimentsbelong to the Esino For- on the surfaceshown in Fig. 1a).These veinlets are mation of Triassic age (Ladin) and are either pure filled by white calcite, although some lenticular dolomites or dolomitic (Callegary, open spacesmay still be presentalong the veinlets. te62). These veinlets may represent the pathways for Apart from massivepure dolomite,well stratified hydrothermal fluids to pass through the country dolomitic limestonessequences occur in the forma- rock and may have also connected larger primary

Fig. l. Photographsof dolomitic marblesfrom the profile at Cima lJzza. (z) SampleCal96: porous dolomite from the outer aureole; (b) Sample Cal85: nodular spinel-forsterite-marble; (c) Sample Cal80: veinlets of spinel-forsterite-+linohumite-marble in pure dolomite matrix; and (d) Sample Ca180:weathered surface with larger irregular nodulesrich in silicate minerals surroundedby pure dolomitic marble. BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK I 103 cavities, making them accessibleto the presumed The lithological characteristics of the rocks ex- fluid. The deposition of tremolite and other silicate amined are summarizedin Table lb. The rock type minerals along the cavity walls and within the (A) correspondsto "marmo dolomitico saccaroide" veinlets suggeststhat the pore space was probably and "marmoi calcareo-dolomitici saccaroide" in primary and unlikely to be the result of postmeta- the terminology of Callegary (1962).Except for the morphic dissolution and removal of silicate nodules very rare quartzitic and marly layers, the bulk of the or siliceousfossils. Furthermore, it shouldbe noted carbonate sediments are extremely poor in non- that siliceousnodules or siliceousfossils have not carbonate minerals. The silicate minerals in many been found in unmetamorphosedEsino dolomites. of the samplesfrom the outer aureole were deter- The pore spaceof porous dolomite is progressively mined by X-ray diffraction techniques from resi- reduced towards its contact with the Adamello dues preparedby dissolution of the carbonatesin intrusion. cold acetic acid. The inner aureole is defined by the first appear- Callegary (1962)presented a number of bulk rock ance of forsterite. Many of the marbles display analyses of (A)-type marble ("marmo dolomitico nodular regions composed of forsterite-spinel-clin- saccaroide", Callegary, 1962: Table 4). He found ohumite-calcite. These nodules are interpreted as that unmetamorphic rocks and tremolite zone mar- filling former open cavities. Figure lb shows the bles had insoluble residues(non-carbonate miner- weatheredsurface of sampleCa 185with its charac- als) typically about lVo or less. Metasomatized teristic nodular texture. The nodules are very rich marbles from the forsterite zone, colresponding to in silicate, oxide and sulfide minerals, whereas the the nodular silicate-rich rocks (Fig. lb and lc) may matrix of the rock is pure dolomite with rare minute have residues as high as 40Vo. Unmetasomatized flakes of phlogopite. The bulk compositionof the marbles from the forsterite zone revealed composi- matrix is equivalent to that of the marbles of the tions similar to those of the low grade portions of outer aureole.Figures lc and ld show the textural the aureole. Figure 2 showsthe estimatedmaximum relationships on a cut and a fresh surface of sample modal abundanceof non-carbonateminerals in the Ca 180respectively. In this example,the minerals CimaUzza profile. In this context maximum modal of the assemblage forsterite--spinel- abundance means the modal abundance of non- calcite are all in contact with the dolomite of the carbonate minerals in the typically silica-rich nod- matrix, and some retrograde (?) chlorite. These ules such as those shown in Figures lb, lc and ld. phasesare concentrated in irregular nodules which Such silicate-rich nodules are unknown from the arealso interconnected by veinsof complexgeome- lower grade portions of the aureole. The term try. The sameminerals are found in the veins as in metasomatizedmarble introduced above refers to the nodules. The matrix rock is again a very pure, the nodular marbles of the forsterite zone and to the coarsegrained dolomitic marble. The texture of the porous tremolite rocks from the outer aureole and weathered surface (Fig. ld) suggeststhat the sili- follows from the interpretation of macroscopicrock cate rich nodules are of metasomatic origin. The textures. The term does not refer to the skarn rocks pure dolomite "finger" on the surface is suggestive of a residual dolomite domain resulting from the incomplete reaction between matrix dolomite and inferred fluid phase. sum of corbonqte- The forsterite zone samplesnot affectedby meta- minerols somatismhave similar bulk rock compositioncom- , paredto the unmetamorphosedrocks. Their miner- s o alogy is characteized by a coarse dolomite matrix with scarce, isolated grains of forsterite, spinel, P clinohumite and phlogopite. The modal abundance non corbonote-minerols of non-carbonateminerals is on the order of l%. I Relative increase of the silicate minerals due to carbonatebreakdown and volume reduction cannot Fig. 2. Maximum modal content of non-carbonate minerals account for the magnitude of systematicincrease of found in marble samples as a function of distance from the silicate and oxide minerals bevond the forsterite contact to the intrusion. Note that inside the forsterite zone, isograd. many of the marbles are still pure dolomites. I 104 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK found at the immediate contact to the intru- is consistentwith Callegary (1962),who has sug- sives. Xanthophyllitegarnet-fassaite-spinelskarns, gestedthat many of the marblesrich in silicateand along with other types are described by Callegary oxide mineralsmay haveformed by metasomatism. (1962).The usageof the term "metasomatic" here His conclusionwas not restrictedto clearlv metaso-

Table I' Observed mineral assemblageslisted in the order of increasingdistance from the intrusion. Abbreviations are given on Table la. For the definition of rock types see Table lb.

samplecarbonates ol ivine amphibolechlorite micas other opaque ref,rogr. rock number clinohumitetalc spinel minerals minerals rninerals type

169* CC+Do FO+CLH 170* CC+Do FO+CLH SP PHL PY+GAL c I 7l* CC+DO FO+CLH )r PY TR A 172* CC+Do FO+CLH SP PY+PEN B 173 CC+Do FO 174 CC+D0 FO SP DO+CHL D 175 CC+Do FO SP CHL E 176 CC FO+CLH SP CHL E 177* CC+Do FO PHL CHL+TR I 78* CC+Do CLH lr CHL 179 CC+Do BR lB0 CC+Do FO+CLH SP+CHL CHL

t8t cc FO SP CHL L .|84*182 CC FO SP CHL E CC+Do FO+CLH AM SP+CHL PY+MT+GA 185* CC+00 FO+CLH AM CHL PR+ST+cA l86* CC+Do FO+CLH AM SP+CHL AP PY+MT+GATR l87* CC+Do FO SP+CHL PHL PY+RU .l89*I 88 CC+Do FO AM ANT ANT E CC+Do FO+CLH AM MT TR A 190 CC+D0 AM CHL PHL ANT 206 CC+Do PHL 205 CC+Do AM 204 CC+Do AM ANT 203 CC PHL+MU PY F 202 CC+Do AM CHL MU Q7 ^ 201 Cc MU PKL D 200'l98 CC+00 AM+TC D CC+00 CHL MU Q7 197 CC+Do AM+TC CHL MU D 196* CC+Do AM+TC CHL PHL+MU PY+GA '194195 CC+Do MU CC+Do I 93 CC+Do CHL MU KFS+PKL+02 i 'l9l192 CC+Do MU PKL CC+Do MU KFS+QZ PY+HE 207 CC+Do AM CHL MU 208 Cc+Do AM MU ANT l 209 CC+Do CHL MU 212 CC+Do FO 213* CC+Do FO+CLH SP PHL DI RU+PY+GADO+CHL D ?14* CC+Do FO SP PHL CHL A 215 CC+Do FO SP PHL DO E 216* CC+Do FO SP PHL I L+MT+PR ANT D ?17 CC+D0 FO+CLH AM SP PHL DO+CHL D Z1B CC+Do FO SP DO+CHL+TRD 219 CC FO SP PHL DI+XA DO+CHL E 220 CC+Do FO+CLH SP CHL D 210* CC+Do fU AM PY+PR c 211* CC+DO FO+CLH AM SP+CHL PHL PY+MT+GA c

*denotesmicroprobe samples (opaque rninerais were determinedon probe samplesonly) BUCHER.NURMINEN : CONTACT AUREOLE IN DOLOMITIC ROCK I t05

Table la. Abbreviations of mineral names assemblagein the unmetamorphic part of the aure- ole is dolomite * calcite I * chlorite. Just AM amphibol e MU muscovite north of the outcrop gap, in the low gradeportion of ANT ant'igorite MT magneti te the aureole, and tremolite are present in the AP pentlandite apati te PEN marbles, however, the paucity of talc-bearing sam- BR brucite PHL phlogopite ples precludesthe location "talc CC calcite PKL plagioclase of a out" isograd. CHL chlorite PR pyrite The only isograd which could be mappedwas found CLH cl'i nohumite PY pyrroti te to be the first appearanceofthe forsterite (shown as DI diopsi de QZ quartz dashedline labeled "3", Fig. 3). The outer aureole D0 dol om'ite RU ruti Ie is consequently referred to as the tremolite zone, F0 forsterite SP spinel the inner aureole as the forsterite zone. Some GA galeni te ST sphalerite generalfeatures ofthe two zones and the separating HE TC talc forsterite-isograd are outlined below. IL i lmenite TR treno'l i te KFS K-fe'ldspar XA xanthophyllite Outer aureole(tremolite zone) Talc occurs as minute flakes (<0.5 mm) in the matic scarns at the immediate contacts but also outer portion of the tremolite zone. The textures included many of the nodular dolomitic marbles of involving talc are difficult to interpret. Parallel the forsterite zone as well. intergrowth with tremolite and even replacementof tremolite by talc were observed.However, talc is Mineralogical zonation clearly restricted to the outermost portion of the The mineral assemblagesfound in the aureole are aureole. Quartz-bearingsamples, widespread in the listed in Table 1, in order of increasingdistance outermost aureole and in nonmetamorphic dolo- from the intrusivecontact. Samples169 through 177 mites, persist distinctly closer to the contact than havebeen collected from roof pendantsenclosed in talc-bearingdolomites. Antigorite was detected in a the intrusiverocks. Samples178 through 196repre- sent a continuous profile from the contact to the outer portion of the aureole. Samples207-209 were collected on the easternflank of Cimauzza. where- as samples 195-l9l are from the "non-metamor- phosed" dolomitessouth of Passodel Frato. Sam- i$.""" MAUZZA ples 2lO-220 have been collected from a parallel profile at Cima di Agosta, east of CimaUzza. A clear, systematic mineralogical zonation is evident (Table 1) and the areal distribution of miner- al assemblagesin the system CaO-MgO-AlzOr- SiOTCOTH2O is presentedin Figure 3. The typical " Y:/ %"? Table lb. Definition of rock types ozo/6*"r^::S)

A) The matrix of the rock is a very pure calcite-dolomite marble, inner aureo'le: nodules rich in non carbonate minerals, outer aureo'le: open cavities or brecciated Oro*sp texture, Q F0* SP*CtL B) The matrix of the rock is a pure dolomite, contains very large E FO+SP+CHL+TR (ETR*SP*CHL (> locm) silicate-rich nodules, O TR*CHL C) Bandedsilicate-rich marble, C TR*CHL*TC e TR*CHL*OZ D) Finely bandedsilicate-poor narble, O TC*CHL A oz-cHL E) Patchy silicate-rich calcite marble (partly with si'licate veins or other evidence of metasomatism), Fig. 3. Distribution of mineral assemblagesalong the profile from Cima Uzza. The dashed line labeled 3 represents the F) Densepure calcite marble, forsterite isograd. r 106 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

number of samples from the tremolite zone. This in the characteristic pseudomorph texture after serpentine mineral occurs on surfaces or periclase(?). Diopside does not occur in the sam- together with tremolite in veinlets. It could be ples of the Cima Uzza profile. This has interpreted from the textures of antigorite-bearing been found in samples from the Agosta-profile samples that this mineral is of late (retrograde ?) where it coexists with dolomite, amphibole and origin and does not represent a prograde mineral forsterite. phase.However, it must be noted that the occur- renceis restrictedto the tremolite-zone.The zonal Mineral reactionsand isograds distribution of ophidolomite in the aureole is con- The systematicsuccession of observed mineral sistent with a prograde interpretation of the antigor- assemblagesas shown in Figures 3 and 4 and listed ite assemblages.Phlogopite and chlorite are present in Table 1 may be related to a sequenceof succes- throughout the tremolite zone, whereas muscovite sive mineral reactions linking the different assem- disappearsprior to the first appearanceofforsterite. blages.The equilibrium conditions of these reac- The disappearance of muscovite could not be tions in terms of the variables T and Xc6, Lt mappedas an isograd. constant Pt are shown on Figure 5 for the system CaO-MgGSiO2-COrHzO. On Figure 6 a number The isograd forsterite of additional relevant equilibria are shown in addi- The isograd is a distinct and mappable boundary tion to the equilibria presented on Figure 5. in the field. It is characterized by the abrupt and All phasediagrams (Figs. 5, 6, 11 and 12) were abundant appearance of coarse grained forsterite calculated making use of the programs REAcrroN and coincides with the first appearanceof clinohu- (Finger and Burt, 171),surcnr (Helgesonand oth- mite and spinel. ers, 1978)and pINox (Flowers, 1978)utilizing the thermodynamic data given by Helgeson and others The inner qureole (forsterite zone) (1978)and Walther and Helgeson(1977). The data Forsterite represents the predominant silicate for clinochlore were slightly adjusted in order to be mineral within this zone. It is commonly accompa- consistentwith dataof Widmark (1980)and Fawcett nied by clinohumite and spinel. Amphibole disap- and Yoder (1966). The following standard states pears from the assemblagesclearly inside the for- were used in all calculations:for solids and gases sterite zone. Textures of chlorite are commonly unit activity of the pure compoundsat P and Z. For ambiguous and the distinction of prograde growth aqueousspecies: unit activity of the speciesin a as opposed to late secondary chlorite growth may hypothetical I molal solution referenced to infinite be obscure. However, prograde chlorite probably dilution at P and Z. Ideal mixing of COz and H2O remainsin the rocks beyond the "amphibole-out" was assumed(i.e., aHra : X11r6)ignoring the effect boundary. Brucite-marbles are comparatively rare of dissolvedspecies other than COz. The composi- in the aureole and only one sampleis listed in Table 1. However, Callegary (1962) described brucite ,tr marbles from the Uzza areas as well. One sample .P" contains the assemblagecalcite{olomite-brucite. This essentially silica-free rock is similar to Cc 179. Another sample of brucite marble described by Callegary contains the assemblage 0962) calcite- SPINEL / / I dolomite-brucite-forsterite-spinel. Brucite occurs

tAll phase diagrams were calculated for p = I kbar Strati- graphic evidence (Callegary 1962),the occurrence of extremely fine grained hornfelsesin the area as well as the preservedopen pore spaces in the dolomites suggest that load pressure was probably low. On the other hand the occurrence of the assem- blage calcite + dolomite * antigorite + tremolite requires pressuresabove 750 bars, the inversion pressure for the two CALCI-TEAND DOLOMITEIN EXCESS alternative topologiesfor ophicarbonateequilibria (Trommsdorff Fig. 4. Distribution of minerals in calcitedolomite marblesof and Evans. 1977). the Cima Uzza area. BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK I 107 tion of the fluid phase was approximated by P66, * PRESSURE=1 KB, EXCESSCALCITE - DOLOMITE (exceot P]g^,o: &. The solvation of the aqueous speciei is for oericlose*brucite fields) formally taken into account employing the o-nota- tion accordingto Walther and Helgeson(1980). All phase diagrams were calculated for pure endmembercomponents (compositions defined by Helgesonand others, 1978).Displacements of the O equilibrium conditions could result from the pres- U E. ence of complex l solid solutions rather than end- F members in the natural samples. However, Rice t U (1977)and Bucher (1981b)presented estimates on L the magnitude of this effect from the same rock U types and comparable mineral compositions. They concluded that the displacementof the equilibrium curves of Figures 5 and 6 due to solid solution is not likely to exceed+ l0'. This assumptionis probably 02 0t 06 08 10 applicableto this study as well, becausethe compo- tao, sitions of the minerals of the Adamello aureole are generally closer to the endmembercompositions Fig. 5. Isobaric l-X6e, diagram showing equilibria for than those reported in Rice (1977) and Bucher siliceous dolomites with excess calcite and dolomite, and the (1e81b). upper stability limits of dolomite at high temperaturesand low Xcoz. The following discussion refers to Figure 4 and will proceed from the right (south, unmetamorphic) side to the left (north, roof pendants). The disap- This assemblagecorresponds to the equilibrium pearance of K-feldspar and the appearance of r'rr + 107Do. + tot"" phlogopite acrossthe outcrop gap may be related to iT#8,.:5Ant the reaction ,r, The equilibriumposition of this reactionis shown 3Do + lKfs + lH2O = 3Cc + lPhl + 3CO22 in Figure 5 as the lower boundary of the antigorite (l) field (prograde serpentinization of tremolite). One The lowest grade assemblageof the outer aureole sample contains forsterite in addition to the four is Cc * Do * Tc * Tr * Chl + Mu + Phl and mineral assemblageCc * Do * Tr * Ant. This five implies the simultaneousequilibria mineral assemblage is confined to the invariant point of Figure 5 terminating the antigorite field 2Tc + 3Cc : lTr + lDo + lCOz + lHzO (2) towards higher temperatures. (Fig. 5 upper Tc-field boundary) and The mineralogical changes at the forsterite-iso- grad are most adequately described by the reaction 49Do + 3Tr + 8Mu + 29H2O: 8Chl + SPhl + 55Cc + 43COz (3) lTr + llDo 3 8Fo + 13Cc+ 9CO2 + lH2o (6) (Fie. 6) Reaction (2) removes talc from the marbles The equilibrium conditionsof this reaction sepa- whereasreaction (3) causesmuscovite to disappear rate the tremolite field from the forsterite field on in the samplesunder discussion. Very rare quartz- Figure 5. Aluminium-bearing samples contain spi- bearing assemblagesimply the equilibrium nel and chlorite in addition to Cc + Do * Fo * Tr, indicating that the rocks may have reached condi- 8Qz + 5Do + 1H2O: lTr * 3Cc + 7CO2 tions which correspond to invariant point (IP2) on (Fie. 5) (4) Figure 6. The absenceof diopside from all rocks of Antigorite occurs together with Do * Cc * Tr. the Cima Uzza profile, furthermore suggeststhat invariant point (IP2) must be at the water rich side of invariant point (IP3) of Figure 6 and that the Xqs, did not exceedabout 0.5 during metamorphism. 2Abbreviationslisted in Table la. The formation of spinel in the Adamello aureole l 108 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

PRESSURE=1K8, CALCITE*DOLOMITE IN EXCESS

rP2tP3 o

Ltl E- :) F t Ltl o_ LU F

02 0t, 06 08 1.0 X^^vv2

Fig. 6. Isobaric T-Xco2 diagram showing the equilibria for siliceous dolomite as in Fig. 5. In addition, a number of equilibria involving chlorite, spinel, muscovite, phlogopiteand K-feldspar are imposedon the systemgiven in Fig. 5. Note: The positions of the equilibria on the CO2-richside have not beenevaluated. The labeledinvariant points are referred to in the text. Two possiblepaths of metamorphismare given schematicallyby the arrowed lines. can be related to the isobaric invariant reaction in the aureole does not conflict with the interpreta- occurring at invariant point (IP2) (Fig. 6). Excess tion of a progressivelymetamorphosed sequence chlorite was then removed from the rock possiblv along a partially buffered path through the phase by the reaction diagrams of Figures 5 and 6. The distribution of assemblagesas shown on Figures 3 and 4 are fully lChl + 2Do * lSp + 3Fo + ZCc + 2CO2+ 4H2O with the diagrams Figures (Fie.6). (7) consistent calculated 5 and 6. The occurrence of talc, antigorite and peri- The assemblageDo * Cc * Fo + Sp produced by clase (as brucite pseudomorphs) suggeststhat the the reactions (6) and (7) remains stable to the mineral assemblagescoexisted with a relatively innermost part of the aureole and in all samples HzO-rich fluid throughout portions of the aureole. from the roof pendants.However, as indicatedby As discussedearlier, sampleswhich correspondto the occurrence of marbles bearing brucite-pseudo- the conditions of invariant point (IP2) require fluid morphs after periclase, the stability conditions for compositions with the highest CO2 content (about dolomite were apparently exceeded(particularly in 50 moleVo).This would indicate that. becausemost the very pure dolomitic marbles) through the reac- of the reactions from (l) through (8) release CO2 tion and tend to drive the fluid compositionstowards the COz-rich side of the phasediagrams (Figs. 5 and 6), lDo * lPe + lCc * lCOz (Fig. 5). (8) the fluid was diluted with H2O from an external All periclase was subsequentlyreplaced by bru- source while the assemblagestravelled through the cite according to the reaction isobaric divariant fields on Figures 5 and 6. Two possible consistent paths of prograde metamor- lPe * 1H2Oi 1Br. (9) phism are schematically shown in Figure 6. The observedsuccession of mineral assemblages In contrast to the profile at Cima Uzza all sam- BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK I 109

ples (Nos. 2lO-220) from Cima di Agosta are within calcite equilibration with dolomite were calculated the forsterite zone. One samplecontains the miner- as shown in Figure 7. The scatter in the calculated al pair diopside * dolomite, indicating higher X66. temperaturesmay be attributed to retrograde dolo- in the metamorphic fluid (Fig. 5). The observed mite exsolution from Mg-calcite. The maximum assemblageDi + Do * Cc * Tr * Fo requires temperature determined for the roof pendants is in conditions of invariant point (IP3) for this sample reasonableagreement with estimatesgiven by Jae- (Fig. 6). The rock contains spinel in agreementwith eer (1957) for the temperature at the immediate Figure 6. contacts of granodiorite or tonalite intrusions. The temperature at the forsterite isograd was found to Mineral chemistry be near 530', a value in agreementwith the phase The composition of minerals and the distribution diagrams(Figs. 5 and 6). However, the temperature of exchangeableelements among them allow for a of 500"for the Do * Cc * Tc * Tr sample south of number of important conclusions on the petrogene- Passo del Frato appears to be distinctly above the sis of these rocks. The analyzed samples are upper limit of the talc field (Fig. 5). This discrepan- marked with an asterisk on Table 1 cy is partly due to erronously high temperatures All analyzed ferromagnesianminerals have low, calculatedfrom the equation given by Rice (1977)at to extremely low iron contents. However, other temperatures below about 550"C (Baumgartner, cations, particularly zinc and vanadium were de- 1982). tected in substantialamounts in spinel. The iron content of minerals from the CimaUzza profile are Clinohumite also extremely low (e.g., the maximum FeO con- The compositional characteristicsof the analyzed tent of clinohumiteis 0.86 wt.Vo\. clinohumites are given in Table 2. FeO is uniformly low, most of the samplesare low in TiO2as well. In Calcite contrast the fluorine content is generally high. Xfith Calcite is the only mineral whose composition near the forsterite isograd is typically around 0.6, shows a systematic variation across the aureole. whereas samples form the roof pendants and the The measuredcalcites are essentiallymagnesite- contact show Xfrh values near 0.4. Clinohumiteis calcite solutions.The magnesitecomponent in cal- always associatedwith Fo * Cc * Do. Thesefour cite is buffered by its coexistence with dolomite in minerals are related by the equilibrium all rocks examined.Employing the equationgiven 4Fo * lDo + 1H2O: lclh + 1CO2 (10) by Rice (1977)for the temperature dependenceof the calcite composition,minimum temperaturesof Equilibrium conditions for the reaction (10) are

summitof cimo Uzzo 169 170 @ 171 pendont lost roof U} 178 oL o E 184 @O 185 @ 186 a @(F 187 o. f orsterite isogrod- E o o posso del Froto o-.4n 196

I t I I 400 500 600 700 IEMPERATURE OC Fig.7. Temperaturescalculated from the compositionof Mg-calcitecoexisting with dolomite. lll0 BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Table 2. Microprobeanalyses of clinohumite

'186 't '189 l85 tB5 171 178 172 170 84 169 213

si02 38.sI 38.00 38.59 38.02 37.65 38.45 38.47 37.54 38.44 37.70 38.12 Ti02 0.10 0.72 1.64 0.20 1.12 0.85 0.21 - 0.20 I.64 Fe0* 0.86 0.44 0.25 0.54 - 0.46 0.65 0.45 0.17 0.55 0.20 MgO 56.25 56.22 56.05 56.64 57.12 56.95 57.47 56.89 58.l0 58.65 56.76 .l.20 H20*** 1.00 1.20 I .30 1.80 | .'10 2.?5 l 50 1.70 2.65 1.20 F 3.70 3.65 s.t4 2.60 3.77 1.56 2.55 3.55 2.75 0.67 3.19 .l00..l7.l00.42 Total** 98.80 98.11 98.76 100.17 98.29 98.38 100.03 100.14 99.82

Atomsper l8 (0,OH,F)

Si 4.05914.0213 4.0545 3.94453.9777 3.98013.9927 3.9705 3.9847 3.9tI7 3.9765 tl - 0.00790.05680.1279 0.0158 0.08710.0663 0.01 67 0.01 56 0.'1286 Fe 0.07580.0389 0.0219 0.0468 - 0.03980.0564 0.0398 0.0147 0.0477 0.0174 Mg 8.83758.8679 8.7778 8.7588 8.9951 8.78848.8875 8.9687 8.9771 9.0706 8.8285 F 1.23341.2216 1.0434 0.8531 1.2597 0.52050.8370 1.18750.9015 0.2198 1.0524 '| 'I H 0.70310.8471 0.91il 1.24570.7752 1.5536 .0384 0.8466 1..|755 .8341 0.8350

Y 0.64 0.59 0.53 0.41 0.62 0.25 0.45 0.58 0.43 0.ll 0.56 r *all Fe as Fe0, **total correctedfor F. Tracesof Ni and Zn. *** stoichimetric HrO.

presentedin Figure 8 for a fluid pressure of I kbar the other hand, amphiboles from the Cima Uzza and for a numberof clinohumitecompositions. The profile are uniformly low in Al and alkalies. No equilibria given in this figure have been calculated compositional dependenceupon the distance from using the procedure and the thermodynamic data the contact is apparent from the analyses. given in Rice (1980)(except for the heat capacity Chlorine was found in the forsterite isograd am- power function coefficientc for ,which is phibolesin small, but detectableamounts (0.1-0.2 in error in Table I (Rice, 1980)).In addition, the wt.Vo Cl). variation of the volume change of the solids in the reaction has been taken into account by means of the equation VCLH solid solution : Xr--ct.;1tV"E-cr-n * Xon-cr-HIfoH-cLH for each contour of constant clinohumitecomposition in Figure 8. o The observedcompositions of the Adamelloclin- Lrl E ohumites suggestthat the Xgs, dt the forsterite :l isogradwas near 0.5 (conditionsof point (A) in Fig. F E 8). This is in agreementwith the phase diagram LU Figure 6. L LrJ Marbles from the roof pendants and samples at F the contact characteristically coexisted with a more water-richfluid (X6e, : 0.1 to 0.4).

Amphibole 00 02 0r, 06 08 10 The amphibole analysesgiven in Table 3 can be tcoz separatedinto two chemically distinct groups: (l) temperatureversus Xg6, diagram showing the Amphiboles from the Agosta profile are Fig. 8. Isobaric essentially equilibrium conditions for the reaction 4Fo + lDo + lH2O = pargasites,a compositionfound by many authorsin lClh + lCOz for different compositionsof clinohumite (contours spinel-bearingdolomites (e.g., Rice, 1977).(2) On for Xp-"tn). BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK llll

Table 3. Microprobe analysesof amphibole vanadium contents were found in the roof pendants '186 near the summit of Cima Uzza. Spinels from near 189 169 ]85 2lI 210 the forsterite isograd are remarkably rich in zinc. 5i02 55.02 58.80 57.81 58.11 46.55 44.27 Spinel with about 7Vogahnite-component was found Ti02 0.13 0.23 0.52 1.33 '13.95 in sample 184. From the same locality, in sample A1203 3.77 0.20 I.43 15.40 185,abundant sphalerite was detected.The curious Feo 0.20 0.13 0.41 0.29 1.30 1.07 compositional variations found in spinel are likely MSo 23.11 25.?0 24.03 lJ.Ot 19.24 18.85

Cao 1?.84 12.47 t3.44 tJ.cf, 13.15 13.14 to reflect local variations in fluid-composition. The 'I Na20 0.94 .',13 0.23 0.13 2.01 2.53 heavy metal content of this fluid may be derived KzO 0.t6 0.16 0.05 0.47 0.66 from the pluton itself. The occurrenceof particular- r u-oc 0.42 0.i8 0.27 0.40 0.49 ly gahnite-rich spinel and sphalerite is restricted to Total* 98.58 100.47 99.73 99.43 99.98 99.71 the forsterite isograd.

* Total correctedfor F, including?% Hrl andtraces of V,Zn,C1 Other oxides and sulfides. The observed oxides in the microprobe samples include magnetite, il- Atomsper 24 (0,0H,F) menite and rutile (Table l). Of the sulfides, pyrrho- Si 7.5897 7.9402 7.9713 7.9237 6.4773 6.1908 tite is the most abundant. In some samplespyrite is - Ti 0.0132 0.0235 0.0544 0. I 398 also present. Galena occurs in some samples,for A'I 0.61290.0318 0.22940.2169 2.2877 2.5381 '1251 instance in the assemblagegalena-sphalerite-py- Fe 0.023 0.0]46 0.04660.0330 0.I 512 0. Mg 4.7517 5.0722 4.8768 4.81 08 3.99053.9291 rite. Pentlandite was found in one marble sample r.8962 1.80421.9607 | .9796 1.96051.9688 together with pyrite. This nickel mineral most prob- Na 0.25',I40.2958 0.06070.0343 0.5422 0.6859 ably was precipitated from fluids derived from the K 0.03810.0275 0.0086 0.08340.1 177 pluton. It was found that sphaleritecontains about 7 '164 F 0.2835 0. I836 0.0775 0. I 0.1760 0.?167 moleVoFeS. ilmenite has about 1.6 wt.VoMnO and 4Vo V2O3, pyrrothite typically contains ap to lVo wt.7oNiO, while rutile containedas much asSwt.Vo vanadiumexpressed as V2O5.The compositionof Sheetsilicates pentlanditeis 46 wt.VoNiS and 53% FeS. Sheetsilicate analyses are given in Table 4. Phlogopite. Phlogopite analyses show a correla- Petrographicobservations tion between composition and distance from the contact. Parallel to the inferred direction of tem- In the nodular marble the nodules are dramatical- perature increase, an enrichment in aluminium and ly enriched in silicatesand oxide minerals in the a correspondingdecrease in silica was detected. forsteritezone. Figure 9a showsthe assemblageFo Chlorite. The total Al per 36 anions varies be- + Sp + Chl in a pure calcite matrix. Note the tween4.52 and4.88 (excluding sample 184, possibly remarkable modal abundance of all minerals rela- late chlorite). This variation cannot be related to the tive to calcite. inferred temperature distribution in the aureole. Even more pronounced are the textural relations The observedexcess Al of 0.52 to 0.88 (relativeto of sample172 (Fie.9b). The assemblageis Cc * Do stoichiometric clinochlore) suggests that the ana- * Fo * Sp + Clh. The modal abundanceofFo + Sp lyzed chlorite in fact representschlorite in chemical + Clh exceeds60Vo in this sample. However, there equilibrium with the remaining minerals at the de- is still somemodal dolomite present.This observa- termined temperatures between 500-550" (Wid- tion suggeststhat the assemblageFo + Sp + Clh in mark, 1980). this rock is unlikely to be the result of an isochemi- cal metamorphism of an originally sedimentary sili- Oxides and sulfides cious dolomite (maximumFo-content of "siliceous Spinel. The chemicalanalyses of spinel,given in dolomite" cannot exceed 38 vol.Vo Fo (Bucher, Table 5, show remarkable variation. All samples 1981a). (except 216) are very low in iron, however, they It may still be argued that the observed concen- may contain as much as2.5 wt.VoV2O3 (single spot trations of silicate and oxide minerals in the nodules analysesrevealed maximum wt.VoY2O3 near 4.00, might be the result of a redistribution of chemical and indicated an inhomogeneous distribution of specieson a small scaleonly. The silicatenodules vanadium in spinel). The spinels with the highest could then represent metamorphic segregations. nl2 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Table 4. Microprobe analysesof sheet

Phlogopite Muscovite Talc ChI ori te 't96 187 i96 216 196 ]85 187 196 178 184

si02 39.48 43.17 36.85 47.25 6t.B9 30.02 3l.19 30.37 29.96 32.30 'l.00 Ti02 0.41 I .0] 0.16 0.0 At^0^ I6.00 I4.63 I 8.96 29,BB 0.25 20.61 20.37 21.84 21.82 19.70 L5 Fe0 0.t0 0.68 2.40 4.00 0.53 0.44 0.37 1.43 0.53 0.37

Mgo 25.31 25.29 25.78 1.77 JL. L5 36.96 34.75 34.15 34.15 34.65 'l'l.36 K"0 I0.0r 9.95 v.bl L 't2 Hzo* 4.0 4.0 4.2 4.0 4.15 12.20 12.20 .20 12.20 12.?0 F 0.83 1.07 0.04 0.l0 0.29

96.98 98.76 98.73 98.67 99.56 100.23 98.88 100.59 98.67 99.21

* stoichiometricH"0, ** sumcorrected for F. Tracesof V,Zn,Cr,Niand Na.

atomsper 24 (0,0H,F) atomsper 36 (0,0H,F)

Si 5.6r79 6.00745.2?35 6.4814 7.8978 5.6104 5.87?9 5.6639 5.6605 6.0539 Ti 0.1070 0.0429 0.1076 0.0165 0.0047 A1 2.7840 2.3994 3.1676 4.8307 0.0375 4.5397 4.5205 4.8004 4.8588 4.3517 Fe 0.01l9 0.0791 0.2845 0.4588 0.0554 0.0687 0.0582 0.2230 0.08s3 0.0579 Mg 5.3683 5.2456 5.4470 0. 3618 6. | 303 10.29599.7530 9.6598 9.6172 9.6800 K I.817t I.7663 1.7197 I .9878 H 3.79683.7130 3.9713 3.B43l 3.7879 15.209315.3237 15.177? 15.3758 15.2529 F 0. 3735 0. 4705 0.0l 79 0.0433 0.1 180

However, the unmetamorphoseddolomites and the throughall mineralsin a given assemblage(Fig. 9c). tremolite-zone marbles contain very small portions The absenceof alteration products such as chlorite of noncarbonateminerals. An extremely large vol- and serpentine along these solid + fluid inclusion ume of marble would have to be depleted in silicate trails suggeststhat they are probably related to the minerals in order to form the observed concentra- formation of the high grade assemblage. It is tions in the nodules. In addition, the matrix dolo- thought that these inclusion trails represent healed mite of the forsterite zone samplesis very similar in microcracks. A very unusual growth texture in bulk rock compositionto the tremolite zone mar- was observed in some samples,particularly bles and the unmetamorphoseddolomite (i.e., very 180(Fig. 9d). In theserocks, irregularcloudy areas low in silicate minerals) (Callegary, 1962). It is may be recognized in olivine. Frequently these necessaryto again stress that cherty nodules were brownish cloudy fields occupy the core regions of not found in the unmetamorphic source rock. an olivine grain and are overgrown by rims of clear In sample 171, domains containing swarms of olivine. The cloudy volume of the olivine is com- small Fo + Clh + Sp grains in a pure dolomite posed of a very dense cloud of fluid inclusion matrix can be observed. In many other samples bubbles and the texture is comparable to that of modal forsterite exceeds50Vo, in sample 181some pumiceor sponge.The texture may be the result of domains with modal spinel exceeding 50% (in pure very rapid olivine growth under the conditions of an carbonatematrix) have been observed. In sample extraordinarily high fluid/rock-ratio. 178, in the nodule domains, the sum of modal carbonatemay be as low as lIVo, however, the Discussionof a model for the aureole formation carbonate is still calcite and dolomite. Another The observationsand mineral compositional data striking feature of the forsterite zone samplesis the presented in the proceeding sections strongly sug- frequent occurrence of very closely spaced and gest that the mineral assemblagesformed by reac- dense fluid and solid inclusion trails which cut tion of a SiO2-rich fluid with pure dolomite. This BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK lll3

Table 5. Microprobe analysesof spinel

'170 '187 I78 17? t84 ?13 211 216 171 186 2't4 Ti0^ a 0.14 0.99 0.50 0.28 0.20 0.00 0.32 0.59 0.08 0.00 0.00 Al^0^ 69.99 66.55 13 66.00 69.01 69.35 69.78 63.47 67.18 70.57 70.71 69.46 \/^0^ 0.64 ?.53 4.5 ?.48 9.17 l 55 0.00 0.14 2.26 0.09 0.21 0.96 Cr^0^ 0.00 0.?1 0.38 0.08 0.12 0.00 0.17 0.17 0.08 0.00 0.11 '| 7n0 0.35 0.27 0.30 3.85 1.30 0.79 0.34 0.36 0.73 0.86 .55 Fe0* 2.92 I .8l ?.54 0.82 0.38 3.88 il.37 1.5? |.06 l.l8 0.36 Mgo ?6.87 27.43 27.17 24.84 26.37 24.64 23.42 27.17 26.79 27.37 26.65

Total** 100.90 99.86 99.47 99.04 99.27 99.18 99.24 99.35 99.48 I 00.50 99.60 Numberof cations per 4

Ti 0.00250.0179 0.00920.0052 0.00350.0000 0.00610.0107 0.00t4 0.0000 0.0000 .l.9705 't.9828 AI 1.9694 1.8984 I .8996 I .9914 2.0035l.B93l 1.9057I.9950 1.9717 V 0.0126 0.04910.04850.0030 0.03000.0000 0.00290.0438 0.0010 0.00380.0187 Cr 0.0000 0.00400.00730.0015 0.00220.0000 0.00340.0030 0.0015 0.00000.0020 7n 0.00600.0049 0.00530.0696 0.02000.0141 0.00630.0064 0.0129 0.0lsl 0.0275 0.0583 0.0367 0.05190.0168 0.00760.079i 0.24060.0307 0.02120.0243 0.0072 Mg 0.9562 0.98940.89000.9062 0.94760.8948 0.88480.9802 0.95780.97060.9567

* ail Fe expressedas Fe0, ** total includestraces of Ni. conclusion is further sustainedby the observed forsterite-rich areas(path C + D). The extreme case dependenceof the total rock composition on the of swarms of forsterite grains in a pure dolomite distance to the contact. A dramatic increase in the matrix may be represented by path (E) and the modal abundance of non-carbonatesis recognized correspondingreaction is: at the forsterite isograd. This observation implies 2Do + SiO2.q + 4 HCI € Fo * 2CO:-+ 2CaCl2 that the forsterite isograd represents, not only the + 2Il2O (12) reactionisograd lTr + 1lDo * SFo + 13Cc+ gCOz * 1H2O, but also appears to be the trace of a or in ionic form metasomaticfront surface. This metasomaticfront 2Do + SiO2.q + 4H* i Fo + 2CO2 + can most adequately be characterized by the reac- H2O tion 2Do * lSiO2,q i lFo + 2Cc + 2CO2. The forsterite isograd is not isochemical. The observed Reactionpaths A andB characterize,J" 11."1- increase of modal silicate minerals across the for- ite isograd, whereaswith decreasingdistance to the sterite isograd may be as large as 10 fold. contact, loss of CaO becomesincreasingly impor- Figure 10 shows 5 different paths of bulk chemis- tant. Paths D or E are characteristic of nodular try modification as deduced from the microtex- marbles from the roof pendants. tures. The reaction along the paths can be written as In Figure 11, a model for SiO2-metasomatismin dolomitic marbles is presented pressure. 2Do + SiO2,q€ Fo 1 2CO2+ 2Cc. (ll) for I kbar The occurrence of talc, serpentine and brucite, If CaO is conservedin the reaction,a Fo + Cc + together with the absence of diopside and the Do rock of composition(A) results,or if dolomiteis composition of clinohumite suggeststhat X6e, was totally consumed by the reaction a FeCc-rock of low throughout. The activity of H2O was-held composition (B) will be the end product. Many of constant at a value of 0.5. This correspondsto the the local textures suggest,however, that CaO was presumed conditions existing at the forsterite iso- not conserved during the reaction but was rather grad and the invariant point IP2 of Figure 6. For the removed from the rock, resulting in extremely rocks in the Cima Uzza avreole this representsthe BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

6"&ov T\\-:,4

Fig. 9. Microtextures of samples from the forsterite zone (figures drawn from photomicrographs, the frame of the drawings measures2 x 2 mrn). (a) Sample l8l: Assemblagecalcite (white) + forsterite (irregular )* spinel (black). Forsterite slightly elongatedparallel to c. (b) Sample 172:Assemblage calcite (white) + dolomite (stipled) + forsterite (irregular cleavage)+ clinohumite (not distinguishedfrom forsterite) f spinel (black). (c) Sample 176:Assemblage calcite (dashed)+ forsterite (irregular cleavage)+ clinohumite (not distinguishedfrom forsterite) t spinel (euhedralcrystal). Note the entrainmentof solid and fluid inclusions (stipled) cuttingacross the mineralsofthe assemblage.(d) Sample180: Assemblage carbonate (dashed) t forsterite(white) + spinel(black). Note the irregular cloudy areasin forsterite, partly with clear forsterite overgrowths. Spinel in this sampleoccurs also as clustered aggregatesof very small grains.

lower limit for the activity of water which was terms of csior.o/osior"qand temperature.A SiO2-rich probably much higher for most other portions of the fluid lost from the pluton will tend to drive the fluid aureole.The activity of water was chosento be 0.5 composition in the dolomitic marble rapidly to- in Figures 11 and 12 in order to be consistentwith wards the reaction boundary of the dolomite-for- the observedcoincidence of the forsterite and spi- sterite reaction when passingthrough cracks and nel isograd.Under thesegiven constraintsand with open pore space in the marble. The continued calcite in excess, the equilibrium conditions of precipitation of forsterite subsequentlybuffered the reactions such as reaction (11) can be shown in fluid composition along the dolomite-forsterite field BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK 1il5

modeled in an analogousmanner to the SiO2-meta- somatism(Fig. 12). Spinel-precipitationwill filter most of the aluminium out of a fluid introduced to the marbles from the pluton along a path section (A). This occurs within the forsterite zone. The observed coincidence of the forsterite isograd with the first and abundant occurrence of spinel further supports the interpretation that the two mineralogi- cal boundarieswhich appear to coincide in the field represent an infiltration front. Along path section (B) (Fig. 12) chlorite is expected to form from Al- metasomatismin the outer portion of the forsterite- zone. This zone, characterized by the assemblage Do * Fo * Cc * Chl is very narrow, if presentat all Mgo Dolomite in the Cima Uzza profile. In the outer aureole, chlorite may be formed path (C) Fig. 10. Path of bulk chemistry alreration by SiO2- along and result in metasomatismas inferred from local microtextures (Fig. 9). the observed characteristic assemblageCc * Do * Tr + Chl. Comments made on tremolite-formation by SiO2-metasomatismapply in an analogousway boundary (Fig. ll). This filtration mechanism to chlorite formation. would effectively absorb dissolved SiO2 from the fluid. The filtration path is essentially given by the Summary and conclusions equilibria involving dolomite (arrow path Fig. ll). The mineralogical and rock-compositional zona- The most significant mass transfer occurred along tion observed in dolomites in the contact aureole the dolomite-forsterite-boundary at high tempera- may have formed by silica- and aluminium metaso- tures and in rocks close to the pluton. SiO2-rich matism. This conclusionwas deducedfrom micro- fluid, upon reaching the outer aureole, could precip- textural characteristics and observed modal varia- itate tremolite, or in the low-temperature regime, tions in the rocks, as well as from textures on the talc or quzrtz, and providef, Xcoz was low enough, scaleof hand specimens,and the composition of the antigorite (not present under the conditions of Fig. minerals(e.g., clinohumite). l1). Although much of the tremolite of the tremo- lite-zone of the outer aureole such as vein-tremolite FORSTERITE or tremolite coatingsof open pore spacecould have formed by such a filtration mechanism.some of the tremolite must be attributed to tremolite-formation (J SATURATION o from Qz + Do initially present in the rocks. t.lJ With this interpretation of the aureole, the for- E l sterite isograd is equivalent to the outer limit of an infiltration E front. SiO2-metasomatismbeyond this t.rJ front was apparently restricted to highly permeable TREMOLITE ul fractures or very porous rocks. In addition, the F minor modal tremolite found in the tremolite-zone P=lkb,oHzo=05 of the aureole could be explained by the decreasing CALCITEPRESENT IN EXCESS likelihood that a SiO2-richsolution would have been -L-3-2_10 LoG(qsio2(oq)/osi021ool able to reach the outer aureole. As shown in Figure ) 11the precipitation of tremolite requires a fluid with Fig. I l. Isobaric temperaturevs. log (a5iqr*/o5;6r*)diagram at 4sio,.o/osio,,"100 times higher than what is required constant asr6. The path of SiO2 infiltration along the dolomite for forsterite production in the vicinity of the con- field boundary is given schematically by dashed arrows. The tact. Most of the SiO2.qgiven off by the pluton is arrow inside the divariant forsterite + calcite field showsa rarely path therefore expected to be filtered out of the fluid in realized in the forsterite zone. For this case, dolomite is totally consumed by the metasomatic rection. the forsterite zone of the inner The conditions aureole. necessaryfor the forsterite-diopside reaction apparently did not The inferred aluminium metasomatism mav be occur in the samplesexamined. lll6 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Volkmar Trommsdorff and Ezio Callegari for valuable discus- sions about various aspectsof the geology of the Adamello area. Martin Frey kindly supported the study by introducing the author to the analytical techniques necessaryfor dealing with A-J o DOLON4ITE- very fine-grained,low grade carbonaterocks. Assistanceduring FORSTE RITE sP+F0+cc CHL+ DO the field work by Heiko Oterdoom and Roli Oberhiinsli is tlJ gratefully acknowledged. I thank Susanne Tobler for typing E. l various versionsof this manuscript. The field work and a part of F the laboratory work was supported by the Schweizerischer E ul Nationalfonds, grant 2.615-0.75(Prof. V. Trommsdorff, ETH, o_ Zurich, Switzerland). LU F P=lkb , orro=0 5 CALCITEPRESENT IN EXCESS References

-15 -10 -5 0 5 Bianchi, A. and Dal Piaz, A. G. (1948)Ditrerenziazioni petrogra- fiche e metamorfismiselettivi di contatto nel Massiccio dell'A- LOG(o41:*/(o413+o311+ )) damello. Rendiconti della Societi Italiana di Mineralogia e Fig. 12.Isobaric temperaturevr. log(aa*/c61t,. di{+) diagram Petrologia,5,3-26. at constant dHro. Infiltration path of Al-metasomatismshown by Baumgartner,L. (1982)Petrographische Untersuchungen auf der dashed arrows. In the upper portion (>540") of the diagram, Alp Confin (Mesocco,Graubiinden). Ms. Thesis.Universidt calcite + forsterite are stable, below this temperaturetremolite Basel. * calcite axe present in excess. Bucher-Nurminen, K. (l98la) The formation of metasomatic reaction veins in dolomitic marble roof pendantsin the Bergell intrusion (Province Sondrio, Northern ltaly). American Jour- The field observations suggest that the metaso- nal of Science.281. 1197-1222. matic infiltration front did not advance through the Bucher-Nurminen, K. (l98lb) Petrology of chlorite-spinel mar- bulk volume of the dolomite, but that fluid flow was bles from northwestern Spitsbergen (Svalbard). Lithos, 14, 203-213. focused by the interconnected cavity space in the Callegari, E. (1962) La Cima Uzza (Adamello Sud-Orientale). dolomite. The infiltration front is manifested as a Parte I. Studio petrografico e petrogenetico delle formazioni mapableboundary in the field and characterizedby metamorfiched contatto. Memorie degli Istituti di Geologiae the first and abundant appearance of forsterite. Mineralogia dell' Universiti di Padova, 23, 116. However, minor evidenceof metasomatismcan be Carpenter,A. B. (1967)Mineralogy and petrology of the system (tremolite-coat- CaO-MgMOrHzO at Crestmore, California. American found in the outer aureole as well Mineralogist, 52, 134l-1363. ings,antigorite-veins). The absenceof diopside,the Fawcett, J. J. and Yoder, H. J., Jr. (1966)Phase relations of occurrence of talc, antigorite, brucite and the com- chlorites in the system MgO-AlzOr-SiOrH2O. American position of clinohumite indicate that ag,6 was Mineralogist,51, 353-380. above about 0.7 throughout the aureole except for Finger, L. W. and Burt, D. M. (1971)Reaction, a FoRTRANIv program to reactions. Camegie probably computer balance chemical rocks at the forsterite isograd, where it Intitution of Washington, Year Book, 71, 617-620. was as low as 0.5. Flowers, G. (1978)rrNox: Computer program library, theoreti- The coincidenceof the spinel and forsterite iso- cal geochemistry, H. C. Helgeson. University of California, grads is likely to be the result of a homogeneous Berkeley. sourcefluid composition from the large reservoir of Greenwood, H. J. (1975)Buffering of pore fluids by metamor- phic reactions.American Journal of Science,275,573-594. igneous rocks. The source fluid reacting with the Helgeson,H. C., Delany,J. M., Nesbitt, H. W. and Bird, D. K. dolomitic country rocks was initially in equilibrium (1978)Summary and critique of the thermodynamicproperties (or nearly so) with the sourcetonalite. of rock-forming minerals. American Journal of Science, 27E- It is conceivable. that metasomatic aureole for- A. mation was imposed on a sequence of initially Jaeger, J. C. (1957)The temperature in the neighborhood of a intrusive sheet. American Journal of Science, 255, prograde cooling isochemical metamorphism of low-silica 306-3I 8. dolomite, below the maximum temperature condi- Kerrick, D. M. (1970)Contact metamorphismin some areasof tions. This could at least partly explain the apparent The Sierra Nevada, California. GeologicalSociety of America discrepancies between the Cc-Do temperatures Bulletin, 81, 2913-2938. from the outer aureole and the calculated phase Masch, L. (1977)Die Aureole des Monzoni-Intrusivkomplexes (Norditalien)

Moore, J. W. and Kerrick, D. M. (1976)Equilibria in siliceous bonates:phase relations in a portion of the systemCaO-MgO- dolomites of the Alta Aureole, Utah. American Journal of SiO2-H2O{O2. Contributions to Mineralogy and Petrology, Science,276,502-524. 60. 39-56. Rice, J. M. (1977) Contact metamorphism of impure dolomitic Walther, J. V. and Helgeson, H. C. (1977) Calculation of the in the Boulder Aureole, Montana. Contributions to thermodynamicproperties of aqueoussilica and the solubility Mineralogy and Petrology, 59, 237-259. of quartz and its polymorphs at high pressuresand tempera- Rice, J. M. (19E0)Phase equilibria involving humite minerals in tures. American Journal of Science. 277. 1315-lJl5. impure dolomitic limestone. Part I: Calculated stability of Walther, J. V. and Helgeson, H. C. (1980) Description and clinohumite. Contributions to Mineralogy and Petrology, 71, interpretationofmetasomatic phaserelations at high pressures 219-235. and temperature. I. Equilibrium activities of ionic speciesin Suzuki, K. (1977)Local equilibrium during the contact metamor- non-ideal mixtures of COz and H2O. American Journal of phism of siliceousdolomites in Kasuga-mura,Gitu-ken, Japan. Science,280, 575-606. Contributions to Mineralogy and Petrology, 61,79-E9. Widmark, T. (1980)The reaction chlorite + dolomite = spinel + Taylor, B. E. and O'Neil, J. R. (1977)Stable isotope studies of forsterite + calcite + + water. Contributions metasomaticCa-Fe-Al-Si skarnsand associatedmetamorphic to Mineralogy and Petrology, 72, 175-179. and igneousrocks, OsgoodMountains, Nevada. Contributions to Mineralogy and Petrology, 63, 1-49. Manuscriptreceived, June 30, 1961; Tromsdortr, V., and Evans, B. W. (1977) Antigorite-ophicar- acceptedfor publication, June 17, 1982.