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

On the mechanism of contact aureole formation in dolomitic country rockby the Adamello intrusion (northern ltaly)

Kunr Bucnen-NURMINEN

Mine ralo g is c he s Ins t it ut de r U niv e r s it rit B a s e IBernoullist. 30. 4056 Basel. Switzerland

Abstract

Intrusion of the Adamello granite (Tertiary) in the southern Alps (northern Italy) causeda distinct mineralogical and chemical zonation within the country rock dolomites. Thecontact aureole can be divided into an inner aureole characterized by the widespread andabundant occulTence of forsterite and spinel (-rclinohumite); and an outer aureole withwidespread but modally subordinate tremolite. The forsterite isograde separating the twozones is a c\ear, mappable boundary in the field. The mineralogical changes across theisograd are accompanied by a significant increase of modal silicate and oxide mineralstowards the forsterite zone. Open pore spaces and cavities in dolomite of the outer aureoleare progressively sealed with silicate * oxide assemblages towards the contact, resulting ina nodular, silicate-rich marble.

It is concluded that the observed zonation may have resulted from infiltration andreaction of silica- and alumina-rich fluids, given of by the cooling pluton within thedolomitic country rock. This conclusion is further supported by the microtextures of manyof the forsterite-zone samples. The geochemical model proposed for the mechanism ofaureole formation suggests that SiO2 dissolved in the inferred fluid was filtered out by areaction of the type 2Do * SiO2uo : Fo + ZCc + 2CO2. Samples from roof pendants showevidence that CaO was not conserved in the above reaction, rather that calcite dissolvedcongruently and was partially removed from the site of reaction. The forsterite isograd isinterpreted as the main infiltration front during the formation of the metasomatic aureole.

Introduction

Various effects of high level igneous intrusions oncarbonate country rocks have been discussed in anumber of recently published field studies. It hasbeen demonstrated in a number of cases that theobserved zonation of the aureole may be attributedto thermal metamorphism (Kerrick, 1970, Rice,1977, Masch, 1977, Melson, 1966). The compositionof the inferred metamorphic fluid has been shown tobe controlled by the local mineral assemblagethroughout the aureole. Prograde metamorphismoccurred along a definite "buffer path" (Green-wood, 1975), given by the initial bulk compositionof the carbonate rock and thermal regime of theaureole. In a number of other field examples it hasbeen shown that the buffer capacity of the localassemblages was partially or completely exhaustedby the introduction of H2O from the pluton. H2O-metasomatism in addition to the thermal effect ofthe intrusion was seen as a major control of themineralogical zonation of the aureole (Moore and

Kerrick, 1976, Suzuki,1977). Other effects of chem-ical transport include the formation of skarns (i.e.,Taylor and O'Neil, 1977) and hydrothermal veinformation (Bucher, 1981a).

The present study describes a contact aureole indolomitic country rocks from the southern Alps.The data from this particular field example suggestthat the observed mineralogical zoning of the aure-ole is the result of SiO2-metasomatism along steeptemperature gradients imposed by the pluton.

General geologY

The Adamello pluton is the largest Tertiary intru-sion in the Alps and is located north of Brescia(northern Italy). The pluton is a composite intrusionconsisting of interfingered stocks of granite andgranodiorite together with at least two types oftonalite. Numerous small bodies of diorite, gabbroand gabbrediorite occur along the contact zone(Bianci and Dal Piaz, 1948). The whole plutonmeasures some 60 km in length while the width is

0003-004)V82l1 l l2-l l0l$02.00 l l 0 l

tt02

between 7 km and 22 km. The pluton intrudes pre-Permian crystalline basement rocks in the north andPermian and Mesozoic sediments in the south. Inthe area of Cima Uzza (southeast rim of the intru-sion) the intrusives are in contact with Mesozoicdolomitic sediments (Callegary, 1962). Two profileshave been studied in detail: (l) a profile at Cima diAgosta and (2) from Cima tJzza to the south (Fig.1). This second profile is characteri zed by intrusiverocks with numerous small carbonate roof pendantsnear the summit of Cima \Jzza, while south of thepasso del Frato (Fig. l), non-dolomitic sedimentsare intercalated in the low-grade part of the aureole.The dolomitic sediments belong to the Esino For-mation of Triassic age (Ladin) and are either puredolomites or dolomitic limestones (Callegary,te62).

Apart from massive pure dolomite, well stratifieddolomitic limestones sequences occur in the forma-

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

tion as well as brecciated marbles. In the outeraureole, the dolomites are frequently very porouswith cavities typically I to 2 mm in diameter (maxi-mum cavity size 3 cm).

Figure la shows a cut surface of sample Ca 196.The rock is generally very porous and the cavitiescan measure up to 2 cm in diameter. The pore spacein many instances is interconnected in a three-dimensional network. The cavities are frequentlycoated with tremolite. In sample Ca 196 talc is alsopresent as a wall coating. The matrix rock is anextremely pure dolomite+alcite marble. Note thatthe rock is cut by numerous veinlets (three visibleon the surface shown in Fig. 1a). These veinlets arefilled by white calcite, although some lenticularopen spaces may still be present along the veinlets.These veinlets may represent the pathways forhydrothermal fluids to pass through the countryrock and may have also connected larger primary

Fig. l. Photographs of dolomitic marbles from the profile at Cima lJzza. (z) Sample Cal96: porous dolomite from the outer aureole;(b) Sample Cal85: nodular spinel-forsterite-marble; (c) Sample Cal80: veinlets of spinel-forsterite-+linohumite-marble in puredolomite matrix; and (d) Sample Ca180: weathered surface with larger irregular nodules rich in silicate minerals surrounded by puredolomitic marble.

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK I 103

cavities, making them accessible to the presumedfluid. The deposition of tremolite and other silicateminerals along the cavity walls and within theveinlets suggests that the pore space was probablyprimary and unlikely to be the result of postmeta-morphic dissolution and removal of silicate nodulesor siliceous fossils. Furthermore, it should be notedthat siliceous nodules or siliceous fossils have notbeen found in unmetamorphosed Esino dolomites.The pore space of porous dolomite is progressivelyreduced towards its contact with the Adamellointrusion.

The inner aureole is defined by the first appear-ance of forsterite. Many of the marbles displaynodular regions composed of forsterite-spinel-clin-ohumite-calcite. These nodules are interpreted asfilling former open cavities. Figure lb shows theweathered surface of sample Ca 185 with its charac-teristic nodular texture. The nodules are very richin silicate, oxide and sulfide minerals, whereas thematrix of the rock is pure dolomite with rare minuteflakes of phlogopite. The bulk composition of thematrix is equivalent to that of the marbles of theouter aureole. Figures lc and ld show the texturalrelationships on a cut and a fresh surface of sampleCa 180 respectively. In this example, the mineralsof the assemblage forsterite-clinohumite-spinel-calcite are all in contact with the dolomite of thematrix, and some retrograde (?) chlorite. Thesephases are concentrated in irregular nodules whichare also interconnected by veins of complex geome-try. The same minerals are found in the veins as inthe nodules. The matrix rock is again a very pure,coarse grained dolomitic marble. The texture of theweathered surface (Fig. ld) suggests that the sili-cate rich nodules are of metasomatic origin. Thepure dolomite "finger" on the surface is suggestiveof a residual dolomite domain resulting from theincomplete reaction between matrix dolomite andinferred fluid phase.

The forsterite zone samples not affected by meta-somatism have similar bulk rock composition com-pared to the unmetamorphosed rocks. Their miner-alogy is characteized by a coarse dolomite matrixwith scarce, isolated grains of forsterite, spinel,clinohumite and phlogopite. The modal abundanceof non-carbonate minerals is on the order of l%.

Relative increase of the silicate minerals due tocarbonate breakdown and volume reduction cannotaccount for the magnitude of systematic increase ofsilicate and oxide minerals bevond the forsteriteisograd.

The lithological characteristics of the rocks ex-amined are summarizedin Table lb. The rock type(A) corresponds to "marmo dolomitico saccaroide"and "marmoi calcareo-dolomitici saccaroide" inthe terminology of Callegary (1962). Except for thevery rare quartzitic and marly layers, the bulk of thecarbonate sediments are extremely poor in non-carbonate minerals. The silicate minerals in manyof the samples from the outer aureole were deter-mined by X-ray diffraction techniques from resi-dues prepared by dissolution of the carbonates incold acetic acid.

Callegary (1962) presented a number of bulk rockanalyses of (A)-type marble ("marmo dolomiticosaccaroide", Callegary, 1962: Table 4). He foundthat unmetamorphic rocks and tremolite zone mar-bles had insoluble residues (non-carbonate miner-als) typically about lVo or less. Metasomatizedmarbles from the forsterite zone, colresponding tothe nodular silicate-rich rocks (Fig. lb and lc) mayhave residues as high as 40Vo. Unmetasomatizedmarbles from the forsterite zone revealed composi-tions similar to those of the low grade portions ofthe aureole. Figure 2 shows the estimated maximummodal abundance of non-carbonate minerals in theCimaUzza profile. In this context maximum modalabundance means the modal abundance of non-carbonate minerals in the typically silica-rich nod-ules such as those shown in Figures lb, lc and ld.Such silicate-rich nodules are unknown from thelower grade portions of the aureole. The termmetasomatized marble introduced above refers tothe nodular marbles of the forsterite zone and to theporous tremolite rocks from the outer aureole andfollows from the interpretation of macroscopic rocktextures. The term does not refer to the skarn rocks

Fig. 2. Maximum modal content of non-carbonate mineralsfound in marble samples as a function of distance from thecontact to the intrusion. Note that inside the forsterite zone,many of the marbles are still pure dolomites.

so

P

I

sum of corbonqte-, minerols

non corbonote-minerols

I 104

found at the immediate contact to the intru-sives. Xanthophyllitegarnet-fassaite-spinel skarns,along with other types are described by Callegary(1962). The usage of the term "metasomatic" here

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

is consistent with Callegary (1962), who has sug-gested that many of the marbles rich in silicate andoxide minerals may have formed by metasomatism.His conclusion was not restricted to clearlv metaso-

Table I' Observed mineral assemblages listed in the order of increasing distance from the intrusion. Abbreviations are given onTable la. For the definition of rock types see Table lb.

sample carbonates o l i v ine amphibo le ch lo r i te micas o ther opaque re f , rogr . rocknumber c l inohumi te ta lc sp ine l m inera ls minera ls rn inera ls type

169* CC+Do170* CC+DoI 7 l * CC+DO172* CC+Do173 CC+Do174 CC+D0175 CC+Do176 CC177* CC+DoI 78* CC+Do179 CC+DolB0 CC+Dot8 t cc182 CC.|84*

CC+Do185* CC+00l86* CC+Dol87* CC+DoI 88 CC+Do.l89*

CC+Do190 CC+D0206 CC+Do205 CC+Do204 CC+Do203 CC202 CC+Do201 Cc200 CC+00'l98

CC+00197 CC+Do196* CC+Do195 CC+Do'194

CC+DoI 93 CC+Do192 CC+Do'l9l

CC+Do207 CC+Do208 Cc+Do209 CC+Do

212 CC+Do213* CC+Do?14* CC+Do215 CC+Do216* CC+Do?17 CC+D0Z1B CC+Do219 CC220 CC+Do210* CC+Do211* CC+DO

FO+CLHFO+CLHFO+CLHFO+CLHFOFOFOFO+CLHFO

CLH

FO+CLHFOFOFO+CLHFO+CLHFO+CLHFOFOFO+CLH

FOFO+CLHFOFOFOFO+CLHFOFOFO+CLHf U

FO+CLH

SP+CHLS PS PSP+CHL

CHLSP+CHLSP+CHL PHL

S P) rS P

SPSPS P

l r

PHL+MUCHL MU

MU

SP+CHL PHL

PY+GALPYPY+PEN

PY+GA

KFS+PKL+02PKLKFS+QZ PY+HE

ANT

DI RU+PY+GA

I L+MT+PR

DI+XA

PY+PRPY+MT+GA

PHL

PHL

TR

DO+CHLCHLCHLCHL+TRCHL

CHLCHLCHL

cAB

DEE

L

E

EA

F^DD

D

i

l

DA

EDDDEDcc

CHL PHLPHL

AMAMAM

AMAMAM

AMAM

AM

CHL MUCHL MUCHL PHL+MU

MU

CHL MUMUMU

CHL MUMU

CHL MU

SP PHLSP PHLSP PHLSP PHLSP PHLSPSP PHLSP

BR

PY+MT+GAPR+ST+cA

AP PY+MT+GAPY+RU

ANTMT

ANT

TR

ANTTR

ANTPY

Q7PKL

Q7AM+TC

AM+TCAM+TC

AMAM

AMAM

AM

DO+CHLCHLDOANTDO+CHLDO+CHL+TRDO+CHLCHL

*denotes microprobe samples (opaque rninerais were determined on probe samples only)

Table la. Abbreviations of mineral names

AM amphi bol e

ANT ant ' igor i te

AP apat i te

BR bruc i te

C C c a l c i t e

CHL ch lo r i te

CLH c l ' i nohumi te

DI d iops i de

D0 dol om'i te

F0 forsterite

GA ga l en i te

HE hemati te

IL i lmen i te

KFS K- fe ' ldspar

MU muscovi te

MT magnet i te

PEN pent land i te

PHL ph logop i te

PKL p lag ioc lase

PR pyr i te

PY pyrroti te

QZ quar tz

RU ru t i Ie

S P s p i n e l

ST spha ler i te

TC ta lc

TR treno'l i te

XA xanthophy l l i te

matic scarns at the immediate contacts but alsoincluded many of the nodular dolomitic marbles ofthe forsterite zone as well.

Mineralogical zonation

The mineral assemblages found in the aureole arelisted in Table 1, in order of increasing distancefrom the intrusive contact. Samples 169 through 177have been collected from roof pendants enclosed inthe intrusive rocks. Samples 178 through 196 repre-sent a continuous profile from the contact to theouter portion of the aureole. Samples 207-209 werecollected on the eastern flank of Cimauzza. where-as samples 195-l9l are from the "non-metamor-phosed" dolomites south of Passo del Frato. Sam-ples 2lO-220 have been collected from a parallelprofile at Cima di Agosta, east of CimaUzza.

A clear, systematic mineralogical zonation isevident (Table 1) and the areal distribution of miner-al assemblages in the system CaO-MgO-AlzOr-SiOTCOTH2O is presented in Figure 3. The typical

Table lb. Definition of rock types

A) The mat r ix o f the rock is a very pure ca lc i te -do lomi te marb le ,

inner aureo ' le : nodu les r i ch in non carbonate minera ls ,

ou ter aureo ' le : open cav i t ies o r b recc ia ted tex tu re ,

B) The mat r ix o f the rock is a pure do lomi te , con ta ins very la rge

( > l o c m ) s i l i c a t e - r i c h n o d u l e s ,

C ) B a n d e d s i l i c a t e - r i c h m a r b l e ,

D) F ine ly banded s i l i ca te -poor narb le ,

E) Patchy s i l i ca te - r i ch ca lc i te marb le (par t l y w i th s i ' l i ca te ve ins

or o ther ev idence o f metasomat ism) ,

F) Dense pure ca lc i te marb le ,

I t05

assemblage in the unmetamorphic part of the aure-ole is dolomite * calcite I quartz * chlorite. Justnorth of the outcrop gap, in the low grade portion ofthe aureole, talc and tremolite are present in themarbles, however, the paucity of talc-bearing sam-ples precludes the location of a "talc out" isograd.The only isograd which could be mapped was foundto be the first appearance ofthe forsterite (shown asdashed line labeled "3", Fig. 3). The outer aureoleis consequently referred to as the tremolite zone,the inner aureole as the forsterite zone. Somegeneral features ofthe two zones and the separatingforsterite-isograd are outlined below.

Outer aureole (tremolite zone)

Talc occurs as minute flakes (<0.5 mm) in theouter portion of the tremolite zone. The texturesinvolving talc are difficult to interpret. Parallelintergrowth with tremolite and even replacement oftremolite by talc were observed. However, talc isclearly restricted to the outermost portion of theaureole. Quartz-bearing samples, widespread in theoutermost aureole and in nonmetamorphic dolo-mites, persist distinctly closer to the contact thantalc-bearing dolomites. Antigorite was detected in a

Fig. 3. Distribution of mineral assemblages along the profilefrom Cima Uzza. The dashed line labeled 3 represents theforsterite isograd.

BUCHER.NURMINEN : CONTACT AUREOLE IN DOLOMITIC ROCK

" Y:/ %"?ozo/6*"r^::S)

i$.""" MAUZZA

O r o * s pQ F0 * SP *CtLE F O + S P + C H L + T R( E T R * S P * C H LO T R * C H LC T R * C H L * T Ce T R * C H L * O ZO T C * C H LA oz-cHL

r 106 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

number of samples from the tremolite zone. Thisserpentine mineral occurs on fracture surfaces ortogether with tremolite in veinlets. It could beinterpreted from the textures of antigorite-bearingsamples that this mineral is of late (retrograde ?)origin and does not represent a prograde mineralphase. However, it must be noted that the occur-rence is restricted to the tremolite-zone. The zonaldistribution of ophidolomite in the aureole is con-sistent with a prograde interpretation of the antigor-ite assemblages. Phlogopite and chlorite are presentthroughout the tremolite zone, whereas muscovitedisappears prior to the first appearance offorsterite.The disappearance of muscovite could not bemapped as an isograd.

The forsterite isograd

The isograd is a distinct and mappable boundaryin the field. It is characterized by the abrupt andabundant appearance of coarse grained forsteriteand coincides with the first appearance of clinohu-mite and spinel.

The inner qureole (forsterite zone)Forsterite represents the predominant silicate

mineral within this zone. It is commonly accompa-nied by clinohumite and spinel. Amphibole disap-pears from the assemblages clearly inside the for-sterite zone. Textures of chlorite are commonlyambiguous and the distinction of prograde growthas opposed to late secondary chlorite growth maybe obscure. However, prograde chlorite probablyremains in the rocks beyond the "amphibole-out"boundary. Brucite-marbles are comparatively rarein the aureole and only one sample is listed in Table1. However, Callegary (1962) described brucitemarbles from the Uzza areas as well. One samplecontains the assemblage calcite{olomite-brucite.This essentially silica-free rock is similar to Cc 179.Another sample of brucite marble described byCallegary 0962) contains the assemblage calcite-dolomite-brucite-forsterite-spinel. Brucite occurs

tAll phase diagrams were calculated for p = I kbar Strati-graphic evidence (Callegary 1962), the occurrence of extremelyfine grained hornfelses in the area as well as the preserved openpore spaces in the dolomites suggest that load pressure wasprobably low. On the other hand the occurrence of the assem-blage calcite + dolomite * antigorite + tremolite requirespressures above 750 bars, the inversion pressure for the twoalternative topologies for ophicarbonate equilibria (Trommsdorffand Evans. 1977).

in the characteristic pseudomorph texture afterpericlase (?). Diopside does not occur in the sam-ples of the Cima Uzza profile. This pyroxene hasbeen found in samples from the Agosta-profilewhere it coexists with dolomite, amphibole andforsterite.

Mineral reactions and isograds

The systematic succession of observed mineralassemblages as shown in Figures 3 and 4 and listedin Table 1 may be related to a sequence of succes-sive mineral reactions linking the different assem-blages. The equilibrium conditions of these reac-tions in terms of the variables T and Xc6, Ltconstant Pt are shown on Figure 5 for the systemCaO-MgGSiO2-COrHzO. On Figure 6 a numberof additional relevant equilibria are shown in addi-tion to the equilibria presented on Figure 5.

All phase diagrams (Figs. 5, 6, 11 and 12) werecalculated making use of the programs REAcrroN(Finger and Burt, 171), surcnr (Helgeson and oth-ers, 1978) and pINox (Flowers, 1978) utilizing thethermodynamic data given by Helgeson and others(1978) and Walther and Helgeson (1977). The datafor clinochlore were slightly adjusted in order to beconsistent with data of Widmark (1980) and Fawcettand Yoder (1966). The following standard stateswere used in all calculations: for solids and gasesunit activity of the pure compounds at P and Z. Foraqueous species: unit activity of the species in ahypothetical I molal solution referenced to infinitedilution at P and Z. Ideal mixing of COz and H2Owas assumed (i.e., aHra : X11r6) ignoring the effectof dissolved species other than COz. The composi-

, t r.P"

Fig. 4. Distribution of minerals in calcitedolomite marbles ofthe Cima Uzza area.

SPINEL / / I

CALCI -TE AND DOLOMITE IN EXCESS

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK I 107

tion of the fluid phase was approximated by P66, *P]g^,o : &. The solvation of the aqueous speciei isformally taken into account employing the o-nota-tion according to Walther and Helgeson (1980).

All phase diagrams were calculated for pureendmember components (compositions defined byHelgeson and others, 1978). Displacements of theequilibrium conditions could result from the pres-ence of complex solid solutions rather than end-members in the natural samples. However, Rice(1977) and Bucher (1981b) presented estimates onthe magnitude of this effect from the same rocktypes and comparable mineral compositions. Theyconcluded that the displacement of the equilibriumcurves of Figures 5 and 6 due to solid solution is notlikely to exceed + l0'. This assumption is probablyapplicable to this study as well, because the compo-sitions of the minerals of the Adamello aureole aregenerally closer to the endmember compositionsthan those reported in Rice (1977) and Bucher(1e81b).

The following discussion refers to Figure 4 andwill proceed from the right (south, unmetamorphic)side to the left (north, roof pendants). The disap-pearance of K-feldspar and the appearance ofphlogopite across the outcrop gap may be related tothe reaction

3Do + lKfs + lH2O = 3Cc + lPhl + 3CO22( l )

The lowest grade assemblage of the outer aureoleis Cc * Do * Tc * Tr * Chl + Mu + Phl andimplies the simultaneous equilibria

2Tc + 3Cc : lTr + lDo + lCOz + lHzO (2)

(Fig. 5 upper Tc-field boundary) and

49Do + 3Tr + 8Mu + 29H2O: 8Chl + SPhl+ 55Cc + 43COz (3)

(Fie. 6)Reaction (2) removes talc from the marbles

whereas reaction (3) causes muscovite to disappearin the samples under discussion. Very rare quartz-bearing assemblages imply the equilibrium

8Qz + 5Do + 1H2O : lTr * 3Cc + 7CO2(Fie. 5) (4)

Antigorite occurs together with Do * Cc * Tr.

PRESSURE = 1 KB, EXCESS CALCITE - DOLOMITE(exceot for oericlose* brucite fields)

0 2 0 t 0 6 0 8 1 0

tao,

Fig. 5. Isobaric l-X6e, diagram showing equilibria forsiliceous dolomites with excess calcite and dolomite, and theupper stability limits of dolomite at high temperatures and lowXcoz.

This assemblage corresponds to the equilibrium

r'rr + 107Do . iT#8,.:5Ant

+ tot"" ,r,

The equilibrium position of this reaction is shownin Figure 5 as the lower boundary of the antigoritefield (prograde serpentinization of tremolite). Onesample contains forsterite in addition to the fourmineral assemblage Cc * Do * Tr * Ant. This fivemineral assemblage is confined to the invariantpoint of Figure 5 terminating the antigorite fieldtowards higher temperatures.

The mineralogical changes at the forsterite-iso-grad are most adequately described by the reaction

lTr + llDo 3 8Fo + 13Cc + 9CO2+ lH2o

The equilibrium conditions of this reaction sepa-rate the tremolite field from the forsterite field onFigure 5. Aluminium-bearing samples contain spi-nel and chlorite in addition to Cc + Do * Fo * Tr,indicating that the rocks may have reached condi-tions which correspond to invariant point (IP2) onFigure 6. The absence of diopside from all rocks ofthe Cima Uzza profile, furthermore suggests thatinvariant point (IP2) must be at the water rich sideof invariant point (IP3) of Figure 6 and that the Xqs,did not exceed about 0.5 during metamorphism.

The formation of spinel in the Adamello aureole

O

UE.lF

tUL

U

(6)

2Abbreviations listed in Table la.

l 108 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

P R E S S U R E = 1 K 8 , C A L C I T E * D O L O M I T E I N E X C E S S

X^^vv2

Fig. 6. Isobaric T-Xco2 diagram showing the equilibria for siliceous dolomite as in Fig. 5. In addition, a number of equilibriainvolving chlorite, spinel, muscovite, phlogopite and K-feldspar are imposed on the system given in Fig. 5. Note: The positions of theequilibria on the CO2-rich side have not been evaluated. The labeled invariant points are referred to in the text. Two possible paths ofmetamorphism are given schematically by the arrowed lines.

o

LtlE-:)F

tLtlo_

LUF

1 .008060 t ,02

can be related to the isobaric invariant reactionoccurring at invariant point (IP2) (Fig. 6). Excesschlorite was then removed from the rock possiblvby the reaction

lChl + 2Do * lSp + 3Fo + ZCc + 2CO2 + 4H2O(Fie.6). (7)

The assemblage Do * Cc * Fo + Sp produced bythe reactions (6) and (7) remains stable to theinnermost part of the aureole and in all samplesfrom the roof pendants. However, as indicated bythe occurrence of marbles bearing brucite-pseudo-morphs after periclase, the stability conditions fordolomite were apparently exceeded (particularly inthe very pure dolomitic marbles) through the reac-tion

lDo * lPe + lCc * lCOz (Fig. 5). (8)

All periclase was subsequently replaced by bru-cite according to the reaction

lPe * 1H2O i 1Br. (9)

The observed succession of mineral assemblages

in the aureole does not conflict with the interpreta-tion of a progressively metamorphosed sequencealong a partially buffered path through the phasediagrams of Figures 5 and 6. The distribution ofassemblages as shown on Figures 3 and 4 are fullyconsistent with the calculated diagrams Figures 5and 6. The occurrence of talc, antigorite and peri-clase (as brucite pseudomorphs) suggests that themineral assemblages coexisted with a relativelyHzO-rich fluid throughout portions of the aureole.As discussed earlier, samples which correspond tothe conditions of invariant point (IP2) require fluidcompositions with the highest CO2 content (about50 moleVo). This would indicate that. because mostof the reactions from (l) through (8) release CO2and tend to drive the fluid compositions towards theCOz-rich side of the phase diagrams (Figs. 5 and 6),the fluid was diluted with H2O from an externalsource while the assemblages travelled through theisobaric divariant fields on Figures 5 and 6. Twopossible consistent paths of prograde metamor-phism are schematically shown in Figure 6.

In contrast to the profile at Cima Uzza all sam-

r P 2 t P 3

BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK I 109

ples (Nos. 2lO-220) from Cima di Agosta are withinthe forsterite zone. One sample contains the miner-al pair diopside * dolomite, indicating higher X66.in the metamorphic fluid (Fig. 5). The observedassemblage Di + Do * Cc * Tr * Fo requiresconditions of invariant point (IP3) for this sample(Fig. 6). The rock contains spinel in agreement withFigure 6.

Mineral chemistry

The composition of minerals and the distributionof exchangeable elements among them allow for anumber of important conclusions on the petrogene-sis of these rocks. The analyzed samples aremarked with an asterisk on Table 1

All analyzed ferromagnesian minerals have low,to extremely low iron contents. However, othercations, particularly zinc and vanadium were de-tected in substantial amounts in spinel. The ironcontent of minerals from the CimaUzza profile arealso extremely low (e.g., the maximum FeO con-tent of clinohumite is 0.86 wt.Vo\.

Calcite

Calcite is the only mineral whose compositionshows a systematic variation across the aureole.The measured calcites are essentially magnesite-calcite solutions. The magnesite component in cal-cite is buffered by its coexistence with dolomite inall rocks examined. Employing the equation givenby Rice (1977) for the temperature dependence ofthe calcite composition, minimum temperatures of

summi t o f c imoUzzo

lost roof pendont

calcite equilibration with dolomite were calculatedas shown in Figure 7. The scatter in the calculatedtemperatures may be attributed to retrograde dolo-mite exsolution from Mg-calcite. The maximumtemperature determined for the roof pendants is inreasonable agreement with estimates given by Jae-eer (1957) for the temperature at the immediatecontacts of granodiorite or tonalite intrusions. Thetemperature at the forsterite isograd was found tobe near 530', a value in agreement with the phasediagrams (Figs. 5 and 6). However, the temperatureof 500" for the Do * Cc * Tc * Tr sample south ofPasso del Frato appears to be distinctly above theupper limit of the talc field (Fig. 5). This discrepan-cy is partly due to erronously high temperaturescalculated from the equation given by Rice (1977) attemperatures below about 550"C (Baumgartner,1982).

Clinohumite

The compositional characteristics of the analyzedclinohumites are given in Table 2. FeO is uniformlylow, most of the samples are low in TiO2 as well. Incontrast the fluorine content is generally high. Xfithnear the forsterite isograd is typically around 0.6,whereas samples form the roof pendants and thecontact show Xfrh values near 0.4. Clinohumite isalways associated with Fo * Cc * Do. These fourminerals are related by the equilibrium

4Fo * lDo + 1H2O : lclh + 1CO2 (10)

Equilibrium conditions for the reaction (10) are

169170171

178

isogrod -

F r o t o

I600

IEMPERATURE OC

Fig. 7. Temperatures calculated from the composition of Mg-calcite coexisting with dolomite.

@

@O

@

@(F

o-.4n

184185186187

U}L

ooE

ao.Eoo

f orster i te

posso del196

I700

t500

I

400

l l l 0 BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Table 2. Microprobe analyses of clinohumite

'189 2131 6 9't 84170172'186

178171t B 5l 8 5

s i02 38 . s I

T i 0 2

Fe0* 0.86

MgO 56.25

H20*** 1 .00

F 3 . 7 0

38 .00 38 .59

0 . 1 0 0 . 7 2

0 . 4 4 0 . 2 556 .22 56 .05

1 . 2 0 I . 3 03 . 6 5 s . t 4

38 .02 37 .65 38 .45

1 . 6 4 0 . 2 0 1 . 1 2

0 . 5 4 - 0 . 4 65 6 . 6 4 5 7 . 1 2 5 6 . 9 5

1 .80 | . ' 10 2 .?52 . 6 0 3 . 7 7 1 . 5 6

38.47 37 .54

0 . 8 5 0 . 2 1

0 . 6 5 0 . 4 5

5 7 . 4 7 5 6 . 8 9

l 5 0 . l . 2 0

2 . 5 5 3 . 5 5

38 .44 37 .70 38 .12- 0 . 2 0 I . 6 4

0 . 1 7 0 . 5 5 0 . 2 0

58 . l 0 58 .65 56 .76

1 .70 2 .65 1 . 202 . 7 5 0 . 6 7 3 . 1 9

T o t a l * * 9 8 . 8 0 9 8 . 1 1 9 8 . 7 6 1 0 0 . 1 7 9 8 . 2 9 . l 0 0 . . l 7 . l 0 0 . 4 2

9 8 . 3 8 1 0 0 . 0 3 1 0 0 . 1 4 9 9 . 8 2

Atoms per l8 (0 ,OH,F)

S i

t l

Fe

Mg

F

H

4 . 0 5 9 1 4 . 0 2 1 3- 0 .0079

0 .0758 0 .0389

8.8375 8.8679

1.2334 1.2216

0 .7031 0 .8471

4 . 0 5 4 5 3 . 9 4 4 5

0 . 0 5 6 8 0 . 1 2 7 9

0 . 0 2 1 9 0 . 0 4 6 8

8 . 7 7 7 8 8 . 7 5 8 8

1 . 0 4 3 4 0 . 8 5 3 1

0 . 9 1 i l 1 . 2 4 5 7

3 . 9 7 7 7 3 . 9 8 0 1

0 . 0 1 5 8 0 . 0 8 7 1- 0 . 0 3 9 8

8 . 9 9 5 1 8 . 7 8 8 4

1 . 2 5 9 7 0 . 5 2 0 5

0 . 7 7 5 2 1 . 5 5 3 6

3.9927 3 .9705 3 .9847

0 . 0 6 6 3 0 . 0 1 6 7

0 . 0 5 6 4 0 . 0 3 9 8 0 . 0 1 4 7

8.8875 8 .9687 8 .9771

0 . 8 3 7 0 1 . 1 8 7 5 0 . 9 0 1 5' |

.0384 0 .8466 1 . . |755

3 . 9 t I 7 3 . 9 7 6 5

0.0 1 56 0 . '1286

0 . 0 4 7 7 0 . 0 1 7 4

9.0706 8 .8285

0 . 2 1 9 8 1 . 0 5 2 4' I

.8341 0 .8350

Yr

0 . 6 4 0 . 5 9 0 . 5 3 0 . 4 1 0 . 6 2 0 . 2 5 0 . 4 5 0 . 5 8 0 . 4 3 0 . l l 0 . 5 6

*a l l Fe as Fe0, * * to ta l cor rec ted fo r F . Traces o f N i and Zn. * * * s to ich imet r ic HrO.

presented in Figure 8 for a fluid pressure of I kbarand for a number of clinohumite compositions. Theequilibria given in this figure have been calculatedusing the procedure and the thermodynamic datagiven in Rice (1980) (except for the heat capacitypower function coefficient c for magnesite, which isin error in Table I (Rice, 1980)). In addition, thevariation of the volume change of the solids in thereaction has been taken into account by means ofthe equation VCLH solid solution : Xr--ct.;1tV"E-cr-n *Xon-cr-HIfoH-cLH for each contour of constantclinohumite composition in Figure 8.

The observed compositions of the Adamello clin-ohumites suggest that the Xgs, dt the forsteriteisograd was near 0.5 (conditions of point (A) in Fig.8). This is in agreement with the phase diagramFigure 6.

Marbles from the roof pendants and samples atthe contact characteristically coexisted with a morewater-rich fluid (X6e, : 0.1 to 0.4).

Amphibole

The amphibole analyses given in Table 3 can beseparated into two chemically distinct groups: (l)Amphiboles from the Agosta profile are essentiallypargasites, a composition found by many authors inspinel-bearing dolomites (e.g., Rice, 1977). (2) On

the other hand, amphiboles from the Cima Uzzaprofile are uniformly low in Al and alkalies. Nocompositional dependence upon the distance fromthe contact is apparent from the analyses.

Chlorine was found in the forsterite isograd am-phiboles in small, but detectable amounts (0.1-0.2wt.Vo Cl).

0 0 0 2 0 r , 0 6 0 8 1 0

tcoz

Fig. 8. Isobaric temperature versus Xg6, diagram showing theequilibrium conditions for the reaction 4Fo + lDo + lH2O =

lClh + lCOz for different compositions of clinohumite (contoursfor Xp-"tn).

o

LrlE:lF

ELUL

LrJF

BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Table 3. Microprobe analyses of amphibole

'186 189 169 ]85 2 l I 2 1 0

l l l l

vanadium contents were found in the roof pendantsnear the summit of Cima Uzza. Spinels from nearthe forsterite isograd are remarkably rich in zinc.Spinel with about 7Vo gahnite-component was foundin sample 184. From the same locality, in sample185, abundant sphalerite was detected. The curiouscompositional variations found in spinel are likelyto reflect local variations in fluid-composition. Theheavy metal content of this fluid may be derivedfrom the pluton itself. The occurrence of particular-ly gahnite-rich spinel and sphalerite is restricted tothe forsterite isograd.

Other oxides and sulfides. The observed oxidesin the microprobe samples include magnetite, il-menite and rutile (Table l). Of the sulfides, pyrrho-tite is the most abundant. In some samples pyrite isalso present. Galena occurs in some samples, forinstance in the assemblage galena-sphalerite-py-rite. Pentlandite was found in one marble sampletogether with pyrite. This nickel mineral most prob-ably was precipitated from fluids derived from thepluton. It was found that sphalerite contains about 7moleVo FeS. ilmenite has about 1.6 wt.VoMnO and4Vo V2O3, pyrrothite typically contains ap to lVowt.7o NiO, while rutile contained as much asSwt.Vovanadium expressed as V2O5. The composition ofpentlandite is 46 wt.Vo NiS and 53% FeS.

Petrographic observations

In the nodular marble the nodules are dramatical-ly enriched in silicates and oxide minerals in theforsterite zone. Figure 9a shows the assemblage Fo+ Sp + Chl in a pure calcite matrix. Note theremarkable modal abundance of all minerals rela-tive to calcite.

Even more pronounced are the textural relationsof sample 172 (Fie.9b). The assemblage is Cc * Do* Fo * Sp + Clh. The modal abundance ofFo + Sp+ Clh exceeds 60Vo in this sample. However, thereis still some modal dolomite present. This observa-tion suggests that the assemblage Fo + Sp + Clh inthis rock is unlikely to be the result of an isochemi-cal metamorphism of an originally sedimentary sili-cious dolomite (maximum Fo-content of "siliceousdolomite" cannot exceed 38 vol.Vo Fo (Bucher,1981a).

It may still be argued that the observed concen-trations of silicate and oxide minerals in the nodulesmight be the result of a redistribution of chemicalspecies on a small scale only. The silicate nodulescould then represent metamorphic segregations.

5 i02 55 .02

Ti 02

A1203 3 .77

Feo 0 .20

M S o 2 3 . 1 1

Cao 1? .84

Na20 0 .94

KzO 0 . t 6

r u - o c

58 .80 57 .81

0 . 1 3 0 . 2 3

0 . 2 0 I . 4 3

0 . 1 3 0 . 4 1

25.?0 24.03

12 .47 t 3 . 44' I . ' ,13 0.23

0 . 1 6 0 . 0 5

0 . 4 2 0 . i 8

46 .55 44 .27

0 . 5 2 1 . 3 3' 13 .95

15 .40

1 . 3 0 1 . 0 7

19 .24 1 8 .85

1 3 . 1 5 1 3 . 1 4

2 . 0 1 2 . 5 3

0 . 4 7 0 . 6 6

0 . 4 0 0 . 4 9

5 8 . 1 1

0 .29

l J . O t

t J . c f ,

0 . 1 3

0 . 2 7

To ta l * 98 .58 100 .47 99 .73 99 .43 99 .98 99 .71

* Total corrected for F, inc luding ?% Hrl and t races of V,Zn,C1

Atoms per 24 (0,0H,F)

S i

T i

A'I

Fe

Mg

Na

K

F

6 .4773 6 .1908

0.0544 0. I 398

2.2877 2.5381

0 . I 512 0 . ' 1251

3 .9905 3 .9291

1 .9605 1 .9688

0 .5422 0 .6859

0 .0834 0 .1 177

0 .1760 0 .?167

Sheet silicates

Sheet silicate analyses are given in Table 4.Phlogopite. Phlogopite analyses show a correla-

tion between composition and distance from thecontact. Parallel to the inferred direction of tem-perature increase, an enrichment in aluminium anda corresponding decrease in silica was detected.

Chlorite. The total Al per 36 anions varies be-tween 4.52 and 4.88 (excluding sample 184, possiblylate chlorite). This variation cannot be related to theinferred temperature distribution in the aureole.The observed excess Al of 0.52 to 0.88 (relative tostoichiometric clinochlore) suggests that the ana-lyzed chlorite in fact represents chlorite in chemicalequilibrium with the remaining minerals at the de-termined temperatures between 500-550" (Wid-mark, 1980).

Oxides and sulfides

Spinel. The chemical analyses of spinel, given inTable 5, show remarkable variation. All samples(except 216) are very low in iron, however, theymay contain as much as2.5 wt.Vo V2O3 (single spotanalyses revealed maximum wt.Vo Y2O3 near 4.00,and indicated an inhomogeneous distribution ofvanadium in spinel). The spinels with the highest

7.5897 7.9402 7.9713- 0 .01 32 0 .0235

0 .6129 0 .0318 0 .2294

0 .023 0 .0 ] 46 0 .0466

4.7517 5.0722 4.8768

r .8962 1 .8042 1 .9607

0 .25 ' , I 4 0 .2958 0 .0607

0 .0381 0 .0275 0 .0086

0. 2835 0. I 836 0. 0775

7 .9237

0.2169

0 .0330

4. 81 08

| .9796

0. 0343

0. I '164

nl2 BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Table 4. Microprobe analyses of sheet silicates

Ph logop i te

1 8 7 i 9 6

Muscovi te

1 9 6

Ch I o r i te

]85 187 1841 7 81 9 6216

Ta l c' t96

s i 0 2Ti 02At 0^

L 5

Fe0

Mgo

K"0L

Hzo*

F

36. 85

I . 0 ]

I 8 . 9 6

2 . 4 0

25.78

v . b l

4 . 2

0 . 0 4

4 7 . 2 5

0 . 1 6

2 9 , B B

4 . 0 0

1 . 7 7' l ' l . 3 6

4 . 0

0 . l 0

6 t . B 90 . 0

0 . 2 5

0 . 5 3J L . L 5

4 . 1 5

0 . 2 9

3 9 . 4 8 4 3 . 1 7' l . 0 0

0 . 4 1

I 6 . 0 0 I 4 . 6 3

0 . t 0 0 . 6 8

25.31 25 .29

I 0 . 0 r 9 . 9 5

4 . 0 4 . 0

0 . 8 3 1 . 0 7

3 0 . 0 2 3 l . 1 9

2 0 . 6 1 2 0 . 3 7

0 . 4 4 0 . 3 7

3 6 . 9 6 3 4 . 7 5

1 2 . 2 0 1 2 . 2 0

30.37 29.96 32.30

21 .84 21 .82 19 .701 . 4 3 0 . 5 3 0 . 3 7

3 4 . 1 5 3 4 . 1 5 3 4 . 6 5

' t2 .20 12.20 12.?0

9 6 . 9 8 9 8 . 7 6 9 8 . 7 3 98.67 9 9 . 5 6 1 0 0 . 2 3 9 8 . 8 8 1 0 0 . 5 9 9 8 . 6 7 9 9 . 2 1

* s to ich iomet r ic H"0 , * * sum cor rec ted fo r F . Traces o f V ,Zn,Cr ,N i and Na.

a toms per 24 (0 ,0H,F) a toms per 36 (0 ,0H,F)

S i

T i

A1

Fe

Mg

K

H

F

5 . 6 r 7 9 6 . 0 0 7 4 5 . 2 ? 3 5

0 . 1 0 7 0 0 . 0 4 2 9 0 . 1 0 7 6

2.7840 2 .3994 3 .1676

0 . 0 1 l 9 0 . 0 7 9 1 0 . 2 8 4 5

5 . 3 6 8 3 5 . 2 4 5 6 5 . 4 4 7 0

I . 8 1 7 t I . 7 6 6 3 1 . 7 1 9 7

3 . 7 9 6 8 3 . 7 1 3 0 3 . 9 7 1 3

0. 3735 0 . 4705 0 . 0 l 79

6 . 4 8 1 4

0 . 0 1 6 5

4 . 8 3 0 7

0. 4588

0. 361 8

I .9878

3. B43 l

0 . 0 4 3 3

7.8978

0 .0047

0 . 0 3 7 5

0 . 0 5 5 4

6. | 303

3 . 7 8 7 9

0 . 1 1 8 0

5 . 6 1 0 4 5 . 8 7 ? 9 5 . 6 6 3 9 5 . 6 6 0 5 6 . 0 5 3 9

4.5397 4 .5205 4 .8004 4 .8588 4 .3517

0 . 0 6 8 7 0 . 0 5 8 2 0 . 2 2 3 0 0 . 0 8 s 3 0 . 0 5 7 9

1 0 . 2 9 5 9 9 . 7 5 3 0 9 . 6 5 9 8 9 . 6 1 7 2 9 . 6 8 0 0

1 5 . 2 0 9 3 1 5 . 3 2 3 7 1 5 . 1 7 7 ? 1 5 . 3 7 5 8 1 5 . 2 5 2 9

However, the unmetamorphosed dolomites and thetremolite-zone marbles contain very small portionsof noncarbonate minerals. An extremely large vol-ume of marble would have to be depleted in silicateminerals in order to form the observed concentra-tions in the nodules. In addition, the matrix dolo-mite of the forsterite zone samples is very similar inbulk rock composition to the tremolite zone mar-bles and the unmetamorphosed dolomite (i.e., verylow in silicate minerals) (Callegary, 1962). It isnecessary to again stress that cherty nodules werenot found in the unmetamorphic source rock.

In sample 171, domains containing swarms ofsmall Fo + Clh + Sp grains in a pure dolomitematrix can be observed. In many other samplesmodal forsterite exceeds 50Vo, in sample 181 somedomains with modal spinel exceeding 50% (in purecarbonate matrix) have been observed. In sample178, in the nodule domains, the sum of modalcarbonate may be as low as lIVo, however, thecarbonate is still calcite and dolomite. Anotherstriking feature of the forsterite zone samples is thefrequent occurrence of very closely spaced anddense fluid and solid inclusion trails which cut

through all minerals in a given assemblage (Fig. 9c).The absence of alteration products such as chloriteand serpentine along these solid + fluid inclusiontrails suggests that they are probably related to theformation of the high grade assemblage. It isthought that these inclusion trails represent healedmicrocracks. A very unusual growth texture inolivine was observed in some samples, particularly180 (Fig. 9d). In these rocks, irregular cloudy areasmay be recognized in olivine. Frequently thesebrownish cloudy fields occupy the core regions ofan olivine grain and are overgrown by rims of clearolivine. The cloudy volume of the olivine is com-posed of a very dense cloud of fluid inclusionbubbles and the texture is comparable to that ofpumice or sponge. The texture may be the result ofvery rapid olivine growth under the conditions of anextraordinarily high fluid/rock-ratio.

Discussion of a model for the aureole formation

The observations and mineral compositional datapresented in the proceeding sections strongly sug-gest that the mineral assemblages formed by reac-tion of a SiO2-rich fluid with pure dolomite. This

BUCHER.NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Table 5. Microprobe analyses of spinel

l l l 3

2' t4186'187171216211? 1 3t 8 4

'17017?I 7 8

T i 0a

Al 0^1 3

\/^0^4 . 5

Cr^0^

7n0

Fe0*

Mgo

0 .00

69.46

0 . 9 6

0 . 1 1' | .55

0 . 36

2 6 . 6 5

0 . 1 4 0 . 9 9 0 . 5 0

6 9 . 9 9 6 6 . 5 5 6 6 . 0 0

0 . 6 4 ? . 5 3 ? . 4 8

0 . 0 0 0 . ? 1 0 . 3 8

0 . 3 5 0 . 2 7 0 . 3 0

2 . 9 2 I . 8 l ? . 5 4

? 6 . 8 7 2 7 . 4 3 2 7 . 1 7

0 . 2 8 0 . 2 069 .01 69 . 359 . 1 7 l 5 50 . 0 8 0 . 1 23 . 8 5 1 . 3 00 . 8 2 0 . 3 8

24 .84 26 .37

0 . 00 0 . 3269.78 63.47

0 . 0 0 0 . 1 4

0 . 0 0 0 . 1 70 . 7 9 0 . 3 43 .88 i l . 37

2 4 . 6 4 2 3 . 4 2

0 . 5 9 0 . 0 8 0 . 0 067 .18 70 .57 70 .712 .26 0 .09 0 . 21

0 . 1 7 0 . 0 8 0 . 0 00 . 3 6 0 . 7 3 0 . 8 61 . 5 ? | . 0 6 l . l 8

2 7 . 1 7 2 6 . 7 9 2 7 . 3 7

Tota l * * 100.90 99 .86 99 .47

Number of cations per 4 oxygens

9 9 . 0 4 9 9 . 2 7 9 9 . 1 8 9 9 . 2 4 9 9 . 3 5 9 9 . 4 8 I 0 0 . 5 0 9 9 . 6 0

T iAI

VCr

7n

Mg

0 .0025 0 .01791 .9694 1 .89840 .01 26 0 .04910.0000 0.00400 .0060 0 .00490. 0583 0. 03670.9562 0.9894

0 .0092 0 .0052I .8996 I .991 40 .0485 0 .00300 .0073 0 .00150 .0053 0 .06960 . 0 5 1 9 0 . 0 1 6 80 .8900 0 .9062

0 .0035 0 .0000. l . 9705

2 .00350.0300 0.00000.0022 0.00000 .0200 0 .01410 .0076 0 .079 i0 .9476 0 .8948

0 .0061 0 .0107 0 .00 t4l . B 9 3 l 1 . 9 0 5 7 I . 9 9 5 00 .0029 0 .0438 0 .00100 .0034 0 .0030 0 .00150 .0063 0 .0064 0 .01290.2406 0.0307 0.02120.8848 0.9802 0.9578

0.0000 0.0000' t . 9828 1 .97170 .0038 0 .01870.0000 0.0020

0 .0 l s l 0 .0275

0.0243 0.00720 .9706 0 .9567

* a i l Fe expressed as Fe0, * * to ta l inc ludes t races o f N i .

conclusion is further sustained by the observeddependence of the total rock composition on thedistance to the contact. A dramatic increase in themodal abundance of non-carbonates is recognizedat the forsterite isograd. This observation impliesthat the forsterite isograd represents, not only thereaction isograd lTr + 1lDo * SFo + 13Cc + gCOz* 1H2O, but also appears to be the trace of ametasomatic front surface. This metasomatic frontcan most adequately be characterized by the reac-tion 2Do * lSiO2,q i lFo + 2Cc + 2CO2. Theforsterite isograd is not isochemical. The observedincrease of modal silicate minerals across the for-sterite isograd may be as large as 10 fold.

Figure 10 shows 5 different paths of bulk chemis-try modification as deduced from the microtex-tures. The reaction along the paths can be written as

2Do + SiO2,q € Fo 1 2CO2 + 2Cc. (ll)

If CaO is conserved in the reaction, a Fo + Cc +Do rock of composition (A) results, or if dolomite istotally consumed by the reaction a FeCc-rock ofcomposition (B) will be the end product. Many ofthe local textures suggest, however, that CaO wasnot conserved during the reaction but was ratherremoved from the rock, resulting in extremely

forsterite-rich areas (path C + D). The extreme caseof swarms of forsterite grains in a pure dolomitematrix may be represented by path (E) and thecorresponding reaction is:

2Do + SiO2.q + 4 HCI € Fo * 2CO:- + 2CaCl2+ 2Il2O (12)

or in ionic form

2Do + SiO2.q + 4H* i Fo + 2CO2 + H2O

Reaction paths A and B characterize ,J" 11."1-ite isograd, whereas with decreasing distance to thecontact, loss of CaO becomes increasingly impor-tant. Paths D or E are characteristic of nodularmarbles from the roof pendants.

In Figure 11, a model for SiO2-metasomatism indolomitic marbles is presented for I kbar pressure.The occurrence of talc, serpentine and brucite,together with the absence of diopside and thecomposition of clinohumite suggests that X6e, waslow throughout. The activity of H2O was-heldconstant at a value of 0.5. This corresponds to thepresumed conditions existing at the forsterite iso-grad and the invariant point IP2 of Figure 6. For therocks in the Cima Uzza avreole this represents the

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

Fig. 9. Microtextures of samples from the forsterite zone (figures drawn from photomicrographs, the frame of the drawingsmeasures 2 x 2 mrn). (a) Sample l8l: Assemblage calcite (white) + forsterite (irregular cleavage) * spinel (black). Forsterite slightlyelongated parallel to c. (b) Sample 172: Assemblage calcite (white) + dolomite (stipled) + forsterite (irregular cleavage) + clinohumite(not distinguished from forsterite) f spinel (black). (c) Sample 176: Assemblage calcite (dashed) + forsterite (irregular cleavage) +clinohumite (not distinguished from forsterite) t spinel (euhedral crystal). Note the entrainment of solid and fluid inclusions (stipled)cutting across the minerals ofthe assemblage. (d) Sample 180: Assemblage carbonate (dashed) t forsterite (white) + spinel (black).Note the irregular cloudy areas in forsterite, partly with clear forsterite overgrowths. Spinel in this sample occurs also as clusteredaggregates of very small grains.

lower limit for the activity of water which wasprobably much higher for most other portions of theaureole. The activity of water was chosen to be 0.5in Figures 11 and 12 in order to be consistent withthe observed coincidence of the forsterite and spi-nel isograd. Under these given constraints and withcalcite in excess, the equilibrium conditions ofreactions such as reaction (11) can be shown in

terms of csior.o/osior"q and temperature. A SiO2-richfluid lost from the pluton will tend to drive the fluidcomposition in the dolomitic marble rapidly to-wards the reaction boundary of the dolomite-for-sterite reaction when passing through cracks andopen pore space in the marble. The continuedprecipitation of forsterite subsequently buffered thefluid composition along the dolomite-forsterite field

6"&ov T\\- : ,4

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK 1i l5

Fig. 10. Path of bulk chemistry alreration by SiO2-metasomatism as inferred from local microtextures (Fig. 9).

boundary (Fig. ll). This filtration mechanismwould effectively absorb dissolved SiO2 from thefluid. The filtration path is essentially given by theequilibria involving dolomite (arrow path Fig. ll).The most significant mass transfer occurred alongthe dolomite-forsterite-boundary at high tempera-tures and in rocks close to the pluton. SiO2-richfluid, upon reaching the outer aureole, could precip-itate tremolite, or in the low-temperature regime,talc or quzrtz, and providef, Xcoz was low enough,antigorite (not present under the conditions of Fig.l1). Although much of the tremolite of the tremo-lite-zone of the outer aureole such as vein-tremoliteor tremolite coatings of open pore space could haveformed by such a filtration mechanism. some of thetremolite must be attributed to tremolite-formationfrom Qz + Do initially present in the rocks.

With this interpretation of the aureole, the for-sterite isograd is equivalent to the outer limit of aninfiltration front. SiO2-metasomatism beyond thisfront was apparently restricted to highly permeablefractures or very porous rocks. In addition, theminor modal tremolite found in the tremolite-zoneof the aureole could be explained by the decreasinglikelihood that a SiO2-rich solution would have beenable to reach the outer aureole. As shown in Figure11 the precipitation of tremolite requires a fluid with4sio,.o/osio,," 100 times higher than what is requiredfor forsterite production in the vicinity of the con-tact. Most of the SiO2.q given off by the pluton istherefore expected to be filtered out of the fluid inthe forsterite zone of the inner aureole.

The inferred aluminium metasomatism mav be

modeled in an analogous manner to the SiO2-meta-somatism (Fig. 12). Spinel-precipitation will filtermost of the aluminium out of a fluid introduced tothe marbles from the pluton along a path section(A). This occurs within the forsterite zone. Theobserved coincidence of the forsterite isograd withthe first and abundant occurrence of spinel furthersupports the interpretation that the two mineralogi-cal boundaries which appear to coincide in the fieldrepresent an infiltration front. Along path section(B) (Fig. 12) chlorite is expected to form from Al-metasomatism in the outer portion of the forsterite-zone. This zone, characterized by the assemblageDo * Fo * Cc * Chl is very narrow, if present at allin the Cima Uzza profile. In the outer aureole,chlorite may be formed along path (C) and result inthe observed characteristic assemblage Cc * Do *Tr + Chl. Comments made on tremolite-formationby SiO2-metasomatism apply in an analogous wayto chlorite formation.

Summary and conclusions

The mineralogical and rock-compositional zona-tion observed in dolomites in the contact aureolemay have formed by silica- and aluminium metaso-matism. This conclusion was deduced from micro-textural characteristics and observed modal varia-tions in the rocks, as well as from textures on thescale of hand specimens, and the composition of theminerals (e.g., clinohumite).

- L - 3 - 2 _ 1 0

LoG (qsio2(oq)/osi021ool )

Fig. I l. Isobaric temperature vs. log (a5iqr*/o5;6r*) diagram atconstant asr6. The path of SiO2 infiltration along the dolomitefield boundary is given schematically by dashed arrows. Thearrow inside the divariant forsterite + calcite field shows a rarelyrealized path in the forsterite zone. For this case, dolomite istotally consumed by the metasomatic rection. The conditionsnecessary for the forsterite-diopside reaction apparently did notoccur in the samples examined.

Mgo

(Jo

t.lJEl

Et.rJ

ulF

Dolomite

FORSTERITE

SATURATION

TREMOLITE

P = l k b , o H z o = 0 5CALCITE PRESENT IN EXCESS

AD O L O N 4 I T E - - J

F O R S T E R I T E s P + F 0 + c c

CHL + DO

P= lkb , o r ro= 0 5CALCITE PRESENT IN EXCESS

l l l 6

-15 -10 -5 0 5

LOG(o41:*/(o413+ o311+ ))

Fig. 12. Isobaric temperature vr. log(aa*/c61t,. di{+) diagramat constant dHro. Infiltration path of Al-metasomatism shown bydashed arrows. In the upper portion (>540") of the diagram,calcite + forsterite are stable, below this temperature tremolite* calcite axe present in excess.

The field observations suggest that the metaso-matic infiltration front did not advance through thebulk volume of the dolomite, but that fluid flow wasfocused by the interconnected cavity space in thedolomite. The infiltration front is manifested as amapable boundary in the field and characterized bythe first and abundant appearance of forsterite.However, minor evidence of metasomatism can befound in the outer aureole as well (tremolite-coat-ings, antigorite-veins). The absence of diopside, theoccurrence of talc, antigorite, brucite and the com-position of clinohumite indicate that ag,6 wasabove about 0.7 throughout the aureole except forrocks at the forsterite isograd, where it probablywas as low as 0.5.

The coincidence of the spinel and forsterite iso-grads is likely to be the result of a homogeneoussource fluid composition from the large reservoir ofigneous rocks. The source fluid reacting with thedolomitic country rocks was initially in equilibrium(or nearly so) with the source tonalite.

It is conceivable. that metasomatic aureole for-mation was imposed on a sequence of initiallyisochemical prograde metamorphism of low-silicadolomite, below the maximum temperature condi-tions. This could at least partly explain the apparentdiscrepancies between the Cc-Do temperaturesfrom the outer aureole and the calculated phasediagrams.

AcknowledgmentsI thank Ian Hutcheon and Jack Rice for their critical reviews

which substantially improved this contribution. I am indebted to

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK

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Volkmar Trommsdorff and Ezio Callegari for valuable discus-sions about various aspects of the geology of the Adamello area.Martin Frey kindly supported the study by introducing theauthor to the analytical techniques necessary for dealing withvery fine-grained, low grade carbonate rocks. Assistance duringthe field work by Heiko Oterdoom and Roli Oberhiinsli isgratefully acknowledged. I thank Susanne Tobler for typingvarious versions of this manuscript. The field work and a part ofthe laboratory work was supported by the SchweizerischerNationalfonds, grant 2.615-0.75 (Prof. V. Trommsdorff, ETH,Zurich, Switzerland).

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Manuscript received, June 30, 1961;acceptedfor publication, June 17, 1982.

BUCHER-NURMINEN: CONTACT AUREOLE IN DOLOMITIC ROCK