the largely untold story of welfare reform and the - researchgate

Canadian MineralogistYol.27, pp. 579-591 (1989)

ABSTRACT

This is the first mineralogical and geochemical study ofrodingites that occur within serpentinized ultramafic rocksof the Abitibi greenstone belt in Ontario. Remnants of theoriginal texture and mineralogy are locally preserved; themineralogical and textural changes associated with therodingitization process can thus be traced. Rodingite pro-toliths include diabase and lamprophyre dykes, mafic lithicinclusions, serpentinites and intermediate volcanic rocks.Clinozoisite and epidote represent the early stage of alter-ation, and are followed by the appearance of hydrogros-sular and prehnite, which form at the expense of epidote.The most advanced stage of alteration is represented by acompletely recrystallized, fine-grained, diopside-bearingrock. Mass-balance calculations, using data from a larn-prophyte for the frst two stages, suggest an isovolumereplacement during which Ca, Al and Fe were added,whereas Na, K and Mg were removed. Crystallization tem-peratures of 270-330'C at pressures of l-2 kbars wereobtained from primary fluid inclusions in late-stage diop-side. The fluid had a low salinity (-2.5 equiv. wt.9o NaCl).The rodingitization process was not different in Archeantimes than in the Phanerozoic, so that the difference in tec-tonic settings is not an important factor. Thus, deforma-tion is not an integral part of'the process, although it cer-tainly affects the final texture and distribution of the rocks.

Keywords: rodingite, rodingitization process, Abitibi green-stone belt, fluid inclusions, epidote, grossular, diopside,prehnite, vesuvianite, serpentinite.

Souvalng

Nous pr6sentons ici les r€sultats d'une premibre 6tudemin€ralogique et g6ochimique des rodingites associ6es auxroches ultramafique serpentinis6es de la ceinture de rochesvertes de l'Abitibi, en Ontario. Les rodingites contiennentquelques exemples de la texture et de la mindralogie origi-nelles, ce qui permet de reconstruire les changements sur-venus au cours de la m6tasomatose. Ont €t6 affect6s: dia-base, filons lamprophyriques, enclaves lithiques mafiques,serpentinites et roches volcaniques intermddiaires. Clino-zoisite et dpidote reprdsenteraient le stade pr6coce de l'alt6-ration; hydrogrossulaire et prehnite ont cristallis6 par lasuite, aux d6pens de l'€pidote. Pendant le stade le plusavanc6 de la transformation, la roche est devenue compld-tement recristallis6e, et riche en diopside d granulomdtriefine. Les bilans gdochimiques fondEs sur les donndes pourun lamprophyre font penser que pendant les deux premiers

579

RODINGITES IN SERPENTINIZED ULTRAMAFIC ROCKSOF THE ABITIBI GREENSTONE BELT, ONTARIO

EVA S. SCHANDL, DAVID S. O,HANLEY AND FREDERICK J. WICKSDepartment of Mineraloglt, Royal Ontario Museum, Toronto, Ontaio MSS 2C6and Department of Geologt, University of Toronto, Toronto, Ontarto MsS IAI

stades du remplacement, qui serait isovolumique, Ca, Alet Fe ont 6te ajout6s, tandis que Na, K et Mg ont €t6 lessiv6s. Les inclusions fluides primaires dans la diopside tar-dive indiquent une formation e 270-330'C et l-2 kbars dpartir d'une phase fluide de faible salinit€ (€quivalent Al-2.590 NaCl par poids). La rodingitisation dans l'archdenne semble pas diff6rer des exemples phan6rozoiques; les dif-fdrences entre les contextes tectoniques ne seraient donc pasimportants. La ddformation ne semble pas essentielle auprocessus, mais influence certainement la texture des rodin-gites, ainsi que leur distribution.

(Traduit par la R6daction)

Mots-cl*i rodingite, ceinture de roches vertes de I'Abitibi,m6tasomatose, inclusions fluides, 6pidote, grossulaire,diopside, prehnite, vesuvianite, serpentinite.

INTRoDUCTIoN

Rodingites are calcium-rich, silica-undersaturatedrocks that form by meta$omatism of rocks duringserpentinization (Thayer 1966; Coleman 1977). It isnot surpri$ing that rodingite$ are found associatedwith almost all types of serpentinite, as calcium-richwaters appfff to be a product of the low-temperaturealteration of ultramafic rocks (Barnes et ol. 1967,Wenner 1979). Rodingites are dredged from present-day ocean floors (Ilonnorez & Kirst 1975) and arefound in Archean serpentinites (Anhaeusser 1979).The serpentinized ultramafic rocks of the Abitibigreenstone belt are no exception to this rule; rodin-gites have been found in the Dundonald sill, the Bow-man, Munro, Reeves and United chrysotile asbes-fss minss, and the Garrison chrysotile asbestosdeposit, although no detailed studies have been cdr-ried out on any of these occurrences.

The purpose of this paper is threefold: firstly, wewish to determine, where possible, the characteris-tics ofthe rodingite protoliths; secondly, we describethe mineralogy and the textures of the rodingites, andfinally, \tre attempt to establish some of the inten-sive parameters relevant to their formation.

ANelvtlcnl Mprnous

The characterization of mineral phases was car-ried out using a Guinier - de Wolff X-ray camera

580 THE CANADIAN MINERALOGIST

(CuKa radiation) and electron microprobe. Indexeddiffraction patterns were refined using the programof Appleman & Evans (1973). Mineral compositionswere determined with a Materials Analysis CompanyModel 400 electron microprobe equipped with aKevex Model 50004 energy-dispersion spectrometer,automated to produce simultaneous multi-elementanalysis and data reduction @lant & Lachance 1973).Operating conditions were the same as described byWicks & Plant (1983). Whole-rock analyses were car-ried out by X-ray-fluorescence spectroscopy. Fusedbeads and powder pellets were prepared for majorand trace elements, respectively. The observed andaccepted values agree to approximately l9o. Fluid-inclusion measurements were obtained using aLinkam TH600 Fluid Inclusion Stage and the opera-ting conditions and calibration methods of MacDo-nald & Spooner (1981).

GEoLocIcAL Ssrrntc

The Archean metavolcanic rocks that conslitutethe Abitibi greenstone belt have been divided intoLower and Upper Supergroups (Jensen & Pyke1982). Both units display the sequence, from olderto younger, of komatiitic through tholeiitic to calc-alkaline volcanic rocks. Large sill-like bodies com-posed of komatiitic ultramafic and gabbroic rocksoccur within the Lower Supergroup and may repre-sent magmatic reservoirs for the komatiitic volca-nic rocks of the Upper Supergroup (Pyke 1982).

The basal sequence ofthe Upper Supergroup con-sists of both ultramafic and basaltic komatiites andMg- and Fe-rich tholeiitic basalts @yke 1982). Largesill-like bodies of ultramafic komatiite, pyroxeniteand gabbro differentiates also are found in the UpperSupergroup, but the relationships are more complexthan in the Lower Supergroup because these bod-ies, which are possible magma reservoirs for the over-lying tholeiitic volcanic rocks, occur in the same

TABLE 1. RODINCITE UALI1IES !N TTE AB1TIB1 CREENSTONI BELT

TO'ISHIP :mTICUPHIC STATUSPOSI1ION

stratigraphic level as komatiitic flows (Naldrett &Mason 1968, MacRae 1969, Arndt 1975).

Metamorphism in the Abitibi greenstone belt,based on mineral assemblages in the greenstones(Jolly 1982), occurred at pressures below 3 kbars andtemperatures less than 350"C. These temperatureswere exceeded in thermal aureoles developed aroundplutons, in which amphibolite- and greenschist-gradehornfels were produced (Jolly 1982).

RoDINGITE LOCALITIES

Each sample locality is listed in Table I, along withits relative setting within the stratigraphic sequencedescribed above. All samples were collected fromchrysotile asbestos mines or deposits (Vos l97l), withthe sole exception of the Dundonald sill. We con-tend that this situation is more a reflection of avail-able outcrop than of genetic significance. Firstly,several of the localities are within open-pit mines(Reeves, Munro, United) or exploration drifts (Gar-rison) and do not reflect the normal situation ofsparse outcrop in this part of the Abitibi belt.Secondly, a serpentinized ultramafic rock must bea certain thickness to host a chrysotile asbestosdeposit, and the same thick bodies, compared tothinner bodies, are more likely to host inclusions.This statement does not apply to dykes that maycross-cut any serpentinized ultramafic rocks. Finally,chrysotile asbestos develops within a specific set ofconditions, whereas any ultramafic rock exhibitssome degree of serpentinization. In Phanerozoic ter-ranes, rodingites. are associated with serpentinization,not neccessarily with chrysotile asbestos (Coleman1977).

Rodingites were found in large serpentinized ultra-mafic bodies in both Supergroups. The Bowman andUnited mines occur in what are interpreted to be largekbmatiitic sills dominated by thick serpentinizedultramafic layers, with minor pyroxenite and gab-bro (Pyke et al. 1978, Kretschmar & Kretschmar1986). Both bodies were emplaced into the LowerSupergroup volcanic rocks @yke 1982). The Munromine (Satterly 1951, Hendry & Conn 1957) and theGarrison deposit (Vos 1971) occur in the serpen-tinized ultramafic layer of large komatiitic sills orflows that also contain pyroxenite and gabbro layers.The Hedman mine occurs in a serpentinized ultra-mafic layer of a differentiated body having tholeiiticaffinity (Arndt 1975). The Dundonald sill is also adifferentiated layered intrusive body of tholeiiticaffinity that has a serpentinized ultramafic layer(Naldrett & Mason 1968). These bodies were intrudedinto or extruded as part of the basal sequence of theUpper Supergroup. The Reeves mine occurs in alarge serpentinized ultramafic intrusive body, but itsnature and position in the str4tigraphy are not wellestablished (Milne 1972).

LOCALITY

bman llne kloro

krri.son etne &r!13on

Eetun aloe fatdon

tundomtd slll hndonald

U.t!€d ntne Uidlolblan

lunro slne f,unto

Reeeos Xlne R€ev€a

L. Supargroup

U. supe.group

U. Supe€roup

U. supergroup

l. Sup€tgroup

U. SupelgtouP

1

chrysolll€ prod@er

chlysoiile dgpo3it

ligh! aggr€gatt

chrysotlle dePosit

chrysoille producer

chrysolil€ producer

The occurrences of rodingites within these bodiesvary both in geological setting and rock type. Smallgabbro xenoliths (<45 cm in diameter) were rodin-eitized in the Bowman mine @yke et al. L978), i\the Hedman mine, in the Garrison deposit, and inthe Dundonald sill. These xenoliths occur in theupper pafr ofthe ultramafic layers, near pyroxeniteand gabbroic layers. In the United mine, rodingitizedgabbro dykes occut at the contact between perido-tite and gabbro. Narrow diabase dykes (<30 cmwide) were rodingitized in the Munro mine, theUnited mine (Kretschmar & Kretschmar 1986), andthe Garrison deposit. Lamprophyre dykes (<2 mwide) were rodingitized in the Munro mine. Rodin-gitized xenoliths of uncertain parentage occur in theMunro mine. All Munro mine samples were collectedin and around the B pit (Wicks et al.1984). Rodin-gites also developed at the contact between serpen-tinite and volcanic rocks in the Reeves (Milne 1972)and United mines (Kretschmar & Kretschmar 1980.

MTNSRATocy AND TEXTURES OFRODINGITE PROTOLITHS

Five protoliths were recognized in partly rodingitized samples: coarse-grained gabbro, medium- tofine-grained diabase, lamprophyre, pyroxenite andfine-grained, foliated andesite. In addition, fine-grained rodingite assemblages were found dispersedthroughout the Munro serpentinite, occuning inter-stitially to and as replacements of serpentine pseu-domorphs after olivine. Unidentified rodingitizedxenoliths occur in the upper part of the ultramaficlayer at Munro.

RODINGITES IN THE ABITIBI BELT

The gabbros are coarse grained. In samples inwhich clinopyroxene dominates over plagioclase, thepyroxene is subhedral, equant to elongate, up to 0.3x 1.4 cm. In gabbros in which plagioclase isdominant, the clinopyroxene tends to be moreequant, anhedral and interstitial to the plagioclaselaths, which are up to 0.7 cm long. A coarse-grainedgabbro-norite was found in the Dundonald sill. It

, contains both clinopyroxene and polysuturedorthopyroxene. The primary textural features thatcan still be recognized resemble those of gabbro inthick differentiated bodies such as the Munro-BeatWkomatiite (Wicks et al. 1984) arrd Fred's Flow (Arndt1977).

Primary textures are better preserved in gabbrothan in diabase. The diabase contains phenocrystsof plagioclase and glomerophyric aggregates ofpyroxene, with some amphibole in a fine-grainedgroundmass. The plagioclase exhibits polysyntheticlwinning; in one sample, the plagioclase is labradorite(Anjo), based on the Michel-L6vy method.

Pigeonite, inverted pigeonite and harrisitic texturesare recognized in the pyroxenite inclusions. The lam-prophyre dyke in the Munro mine is rodingitized onlyat its margin. In the center, the lamprophyre dykecontains large phenocrysts of hornblende, pyroxene,minor orthoclase (Carlsbad twins, negative opticsign) and biotite in a groundmass of orthoclase,acicular amphibole and pyroxene.

MtxsRAlocv oF THE RoDINcITES

The minerals identified during this study are listedin Table 2. Results of representative microprobe ana-

581

TABLE 2. CEARACTERISTIC MINERALS IN RODINGITE FROM THE ABITIBI BELT

localtty Rodingite- , ^ F ^ 1 t * L

Dl+ ggr Ep ves Tte ChL Phl

Munro di.abaselmproph)reserpen!j-ni!e

Cari.son ddabasegabbro

Uliled diabasepyroxenj.tegabbro

Dundonald diabasegabbro

Boman gabbroHednan gabbro

Reeves andesite

xx xxx xxu xxx x

x x x g x a x x x xx x x x xx x x x x

xxx xxx

xxxxx

xx

x

xxx xxxxx gx

xx

xxx

x

x

xx

x x xx

t

t

g x

x

x xxxx

xx xxxxr xxx

xx xxxxx xxx

x u x

xxx irportant Bineral phases, xx co@oni x present,for detalLs)+ nlneral- abbrevi-atl"one afler (relz (1983) exceptand T te ( t i fan i te )

? uay be preseng (see lext

for hg! (hydrogroeeular)


1ABLE 3. REPRESENTATIVE COIiIPOSITIONS OF ITINERALS IN RODINCITE FROI.I ABITIBI BELT

DLopside

Munro Munro Reevegvl82 uaz v821l-6 L23A 307

phenocryst vej.n

Hydrogrossular Yesuvi.anj.te Phlogopj.te Chlorite

CarrLaon llunro llunf,onaz u82 l//,82L37 116A 1164

Carri.son Bomant u82

237

llunrottl82

r-16A

Reevesy,82307

sio2 54.20

A 1 ? 0 1 o . 2 8Ct rO, O.O8

P e O 1 . 4 6l.lno O.24HCo 17.58Cao 25,43

Na2O 0 .00Kzo 0.07

Toral 99.34

55. L4 52.7L0.00 0.00o . z5 0 .520 .o7 0 .10

L.77 LO,260 .18 0 .52

L7 .76 L2 ,4425.79 24,LO

o . @ o . @0 .06 0 .06

101 .02 t 00 .7 t -

39.370.00

z L . z t

0 . 0 5

L . Z 50 . 1 9o . 2 7

3 6 . 5 5

0 . @0 . 0 9

9 9 . @

39. 350.o0

2L .350 .06

o .660 .34o.29

36.86

0.000 .07

98. 98

38 .63 39 .481 .35 0 .74

16 .55 18 .960 .16 N .D .

6 .08 2 .830 . t 9 0 . 2 50 .40 r . 62

35.67 37.23

o .00 0 .270 .09 N .D .

99.L2 101.38

37 . r8 30.661 .56 0 .L2

14 .89 14 .89o.o2 0.o2

a .a2 8 .a2o .25 0 .13

30.30 24.940 .o1 0 .o1

0 . @ 0 . @4 .35 0 .O8

97 .68 83 .71

38 .56o .24

19 .590 .06

2 .790 .09o .27

36.92

0 . @0 .08

98 .60

r$peclmen donated by Dr. L Jensen. Yaluee reporied ln rt.Z. Smples analyzed uslng an !{AC electronnicroprobe (see lexl for detall-s). Total- j.ron j.s expressed as FeO.

lyses (10 out of 1@) are shown in Table 3. The majorminerals are diopside and hydrogrossular, with lesseramounts of epidote-group mineral$, prehnite, vesu-vianite and chlorite. In addition, relict phlogopite,amphibole and plagioclase were found in the chlorite-rich reaction zone between the serpentine and rodin-gite. Note that the amount and thus importance ofthe various minerals vary among localities and thatnot all minerals are present at each locality.

Plagioclase

In gabbros from the United mine, the anorthitecontent ranges between An50 and An2o. Chloritepseudomorphs of plagioclase were found in thechlorite-rich rims adjacent to rodingite. Theplagioclase grains were either chloritized or saussu-ritized prior to rodingitization.

Diopside

Diopside compositions (Table 3, Fig. l) rangefrom EnorFstwos0 to En26Fs2rWoar. In some in-stances, we could not establish whether individualgrains of diopside belong to the rodingite or to itsprotolith. Each locality appears to have a uniquecomposition. In the Munro mine, diopside inphenocrysts and that in late-stage veins from the dia-base dyke have similar composition (Fsr-Fsn andFs1-Fs3, respectively), with a constant 0.5 wt.9oAl2O3. In the Reeves mine, the compositions ofdiopside in the matrix and in the coarse-grained veinsfrom rodingitized andesite are Fs,3-Fs1, andFs15-Fs1s, with 2.4-3.1 and 0.5-2.1 fi.90 Al2O3,respectively. In the Garrison deposit, glomerophyric

diopside from rodingitized gabbro is Fslt-Fqr, witha variable l-3 wt.9o Al2O3. In one example fromthe Garrison deposit, a diopside characterized byEn rFsrWoae is enclosed by hydrogrossular. Twoobservations concerning diopside are noteworthy.Both the grain size and the Fe content of diopsidedecrease with increasing extent of rodingitization.

Hydrogrossular

Microprobe totals for hydrogrossular tend to beless than 10090 (Iable 3, Fig. l). The microprobeanalyses indicate that hydrogrossular contains lessthan 3 wt.Vo FeO and I wt.9o MgO, although somehydrogrossular contain up to 4190 andradite. Themicroprobe data and the cell parameters (Table 4)suggest that hydrogrossular contains water, but wedo not know the SiO4/(OH)4 value so that we usethe term hydrogrossular rather than hibschite @as-sa€ilia & Rinaldi 1984).

If it is anisotropic, hydrogrossular is difficult todistinguish from vesuvianite optically unless an opticsign can be determined. In our samples, hydrogros-sular is consistently corroded, so that this was notpossible. The presence of sector zoning in hydrogros-sular is diagnostic if present. In some ca$es, X-ray-diffraction studies were necessary to distinguishhydrogrossular from vesuvianite, $o that vesuvianitemay be more abundant than generally reported.

Epidote, prehnite ond vesuvionite

The epidote-group minerals epidote and clinozoi-site are important constituents of the rodingites.Clinozoisite occurs primarily after plagiocla$e but

RODINGITES IN THE ABITIBI BELT 583

EdnRrs0l{r d iabases vein. gahbroo in garnet

f l o 2 0 \- - / \Mg

te+MnFrc. l. Chemical composition of clinopyroxene and hydrogrossular in rodingites from the Abitibi greenstone belt.

may form after pyroxene as well. Clinozoisite alsooccurs in veins cross-cutting fresh diabase andlamprophyre. Prehnite occurs both as a matrixmineral and in veins. In the matrix, it is associatedwith clinozoisite, hydrogrossular and diopside. Vesu'vianite was identified by X-ray diffraction.Microprobe data for hydrogrossular and vesuvianitein the Garrison samples are similar but in the Reevesmine sample, vesuvieqite is poorer in Ca and richerin Al and Fe than hydrogrossular.

Tremolite, chlorite and phlogopite

Tremolite occurs in the reastion zones between ser-pentine and xenoliths of unidentified rocks in theMunro mine, and between serpentine and andesitein t}te Reeves mine. Chlorite occupies a position simi-lar to tremolite, but chlorite occurs in all reactionzones. No fresh phlogopite has been found, butpartly chloritized grains were observed within chlo-rite reaction zones at the Munro mine. Microprobeanalysis indicates that phlogopite replacement bychlorite was accompanied by the loss of Ti, K andSi, and an increase in Al and Fe.

TEXTURES oF THE RoDINGITES

Rodingitization of gabbro took place by the pseu-

MUilR0r d iabaset lusin

R t tutse andesi teA U E | N

domorphic replacement of plagioclase by hydrogros-sular + prehnite and chlorite. Coarse clinopyrox-ene is replaced by fine-grained diopside or chlorite,and in one sample from the Dundonald sill, byamphibole.

Rodingitized fine-grained diabase in the Munromine is an equigranular rock consisting of diopside ,hydrogrossular, prehnite, clinozoisite and vesu-vianite. Veins of coarser-grained diopside, prehnite,chlorite and carbonate cross-cut the matrix. In theUnited mine diabase, plagioclase alters to hydrogros-sular and chlorite, with clinozoisite needles penetrat-ing relict plagioclase. The matrix has altered topyroxene and amphibole. Prehnite, epidote andtitanite are minor phases in the matrix. Theplagioclase phenocrysts in the Garrison diabase havealtered to diopside and amphibole, whereas thematrix was altered to hydrogrossular, vesuvianite,diopside, prehnite and chlorite. Veins consisting ofequigranular diopside and hydrogrossular cross-cutthe matrix. Diopside dominales in the Munro rodin-gites after diabase, whereas hydrogrossulardominates in all the others.

Rodingitized pyroxenite from the United minecontains hydrogrossular t titanite after pigeonitelamellae, and diopside after the remaining portionof the pyroxene.

The least-altered portion of the lamprophyre of


IABIE 4. DIREM-CELL PANAilETER,S OF I,IINERAI.3 IN RODINCITE FROIiI ABXTIBI BELT

I ( ' ) StandardEroi 2E+t

N@be. ofPeaka Lrdgrod

!t82-136

182-137

tf82-134

182-158

ua2F236

f82-307

t{82-139

9 0 . 0 ( o )

90 .0 (0 )

90 .0(o)

90 .0(o)

90 .0(o)

90 .0(o)

HJrdrog!ossulsr

1 .1871(2)

1.1871 (1 )

1 .186s (1 )

1 .1870(3)

1 .1898 (2 )

1 .1857 (4 )

VesuvianLte

o,@22 13

0.m16 9

o.@27 9

0.0020

0.oo30

o.oo201.5615(53) 1 .5615(53) 1 .1833(65) 90 .0(O)

+Ce]-L pareat€ra are ln mn@atera.+REforelcad to qurtz standard.

Munro mine consists of pyroxene and amphibolephenocrysts within a matrix of needle-shaped amphi-bole, orthoclase, quartz, epidote and chlorite. It canbe classified as vogesite, based on its mineralogy.Toward the margin, which is the most extensivelyaltered part, pyroxene and zoisite-altered phenocrystsare set in a matrix of epidote-group minerals, pyrox-ene, titanite and minor carbonate. Approximately7590 of the rock consists of clinozoisite and epidote.Diopside is a minor phase, and ttre carbonate appearsto postdate rodingtization. Commonly, diopside andepidote veins cross-cut the alteration assemblage.Hydrogrossular was not observed in the altered lam-prophyre. Sample W83-112, which represents theoriginal compositions of the lamprophyre, contains6.22 wt .4/o CaO (Iable 5). Samples W83-65-C (1 7. 3wt.9o CaO), W83-65-11 Q0tloCaO) andW83-58-lQ4.7t/o CaO), in a sequence of increasing amountsof Ca, represent an increasing extent of alterationas defined by the mineral assemblages.

The least-rodingitized andesite at the Reeves mineconsists of clinozoisite and chlorite, which arereplaced by diopside and tremolite with increasingdegree of alteration. The most altered rocks have afine-grained granular texture consisting predom!nantly of diopside with minor clinozoisite,hydrogrossular and tremolite after epidote. Thehydrogrossular is cloudy and anhedral, and epidoteis cut by anastomosing microfissures delineated bytremolite and chlorite. A set of late diopside veinscross-cuts the alteration-

An unusual occurrence of rodingitized serpentinewas found in the Munro sill. Fine-grained pyroxene,garnet and epidote occur after interstitial pyroxenebetwee.n pseudomorphically serpentinized olivinegrains. In addition, some serpentinized olivine grainshave been altered to pyroxene. This type of altera-tion has been observed in the Jeffrey mine in

southeastern Quebec (Wares & Martin 1980) and inthe Alexo area of Ontario (N.T. Arndt, pers.comm.). The process appears to begin with the alter-ation ofthe pyroxene to diopside and epidote, andthen continues with the alteration of serpentine todiopside.

CHANcES rN WHoLE-RocK CHEMrsrRy

Coleman (1967) has used the ACF diagram todefine the rodingite field. Molar proportions ofappropriate oxides from our samples were calculatedfrom whole-rock compositions (Iable 5, Fig. 2). Oursamples plot in or near thg lodingifs field regardlessof the extent ofr-bdingitization. Given the wide rangein protolith composition, this fact suggests that anyrock type can be rodingitized in the presence of aCa-rich fluid.

Mineralogical and chemical changes can be quan-tified for the Munro lamprophyre, as the middle ofthe dyke still is unaltered. Losses and gains ofvari-ous elements were calculated by the method ofGresens (1967). This method utilizes the system ofequations:

oB

where/v is the volume factor, /, the protolith, B,the product, g, the density, Cn,the weight fractionof component n, anld Xn, the amount of componentn lost or gained during alteration. This system ofequations is underconstrained, so that either/v orXr must be known. Gresens (1967) described amethod using bulk-rock compositions, whichapproximates /v. A composition-volume diagram(Gresens 1967 , Fie. 3) is constructed by calculating


IABLE 5. TSOLE-R@K COI,'POSITIONS OI' RODINCITES, SERPENTINITES,AND TEEIR CONTACT ZONE FRO}' TFE ABITIB1 BELT

585

Ili.ne

S@ple LL6-R.

Munro

1L6-C 116-S

llunro

65C

Cafiigon

123-R L23-C L23-S 58trt LL3

Feo 1.08UnO O.zLMCO 8.15C8O 28.&

sro , 39 .86 33 .81 37 .87 39 .97r f i ; o .s7 0 .58 0 .10 0 .55A 1 2 O 1 1 1 . 6 8 1 0 . 4 1 2 ' & 1 0 . 4 9crlo\ 0.06 o.os 0.17 0.05Fe;o ; 5 .24 2 .9L 4 .81 4 .95

3.86 1 .83 1 .570 . 5 4 0 . 1 3 0 . 1 9

33.07 37 .92 9 .L91.23 0 .04 27 .87

NaZO O.2A 0 .16 0 .13 O.22

uo 0 .02 0 .03 0 .03 o .o2PrO. O,42 0 .41 0 .03 O.42L;O: r . 1 .76 12 .38 r .2 .88 1 .S3

Toral 98.19 99.47 98.80 9'r.32

35.43 35 .40 40 .90 48 .20 46 .40o.74 0 .31 0 .63 0 .66 0 .837.82 6 .90 14 .90 12 .& 14 .800.08 0 .25 0 .01 0 .o2 0 .022.L6 4 .26 8 .OO 8 .01 9 .10

1.56 2 .72 0 .90 <0.10 <0.010 . 6 9 0 . 3 1 0 . 1 8 0 . 1 8 0 . L 7

36.88 36 .21 5 .84 5 .93 A.20o.81 0 .24 24 .70 17 .30 6 .5 r .

0 .06 0 .320.01 0 .o90 . 5 4 0 . r 8

13.13 t2 ,41

99.97 99 .66

0.o7 0 .24 3 ,570.04 3 .18 2 .L t10 . 3 6 0 . 3 7 0 . 4 54 . 2 3 2 . L 6 6 . 7 0

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

tl.6ller j.s an abbrevj.atj.on of: Rpd{ngite (R): Contacl (C)i Serpenej.nj.te (S).t . the Last three enlr j .Ea raptea€rt least (U3), j .n iemedLate (65C), and rcst (581.)

aLtered laEprophyre. Values r€ported j"n rt. e L.O.I.: loss otr lgnleion. Cooposi.tlondelemLned by x-iay fl@rescenc€ speccroscopy.

Xrz for a set of/v. The intersection of the curves forthe assumed immobile components Zr and Ti withthe line Xn : 0 was used to obtain a value of /yof 1.05. This value indicates that a very small volumeincrease accompanied rodingitization. The results(Fig. 3) suggest that Ca, Si, Al, Fe, and P increased,whereas Mg, Na, K and volatiles were lost duringrodingitization. These results support petrqgraphicobservations that primary hornblende and fefdspar(s)alter to epidote and diopside.

A well-developed reaction rim in the Munro minewas chosen for detailed study of the metasomaticexchanges. The reaction zone ranges from 2 to 20cm in width, and there is a zonation from partlyaltered biotite near the serpentinite, to chlorite withfine-grained clinozoisite needles near the rodingite.Petrographic observation of chlorite pseudomorphsafter plagioclase laths and zoned plagioclase flndicatethat the chlorite-rich reaction zone is formed at theexpense of the dyke ratler than the serpentinite. Theboundary between chlorite and rodingite is quitesharp, whereas between chlorite and serpentinite itis more diffuse. The reaction rim is shown schemat-ically in Figure 4.

The edge of the dyke was defined by using Cr,/Tivalues (Sanford 1982). Values of 0.01-0.1[ in thedyke, 0.'l l-0. 14 in the reaction zone and 0.83-1.73in the serpentinite suggest that the reaction zoneformed at the expense of the dyke. Variations inmajor oxides are less clear but appear to be consis-tent with this choice. Concentrations of MgO andHrO are much lower, and that of CaO is muchgreater in the rodingite than in the serpentinite or

in the reaction zone (Ftg. 4). Al2O3 and SiO2 arelower in the reaction rim than in the rodingite, andappear to have increased in the serpentinite. Thereis a loss of Na and K from both the rodingite andthe reaction zone.

FLUID INCLUSIoNS

Primary fluid inclusions were found in diopsidefrom the gabbro in the Bowman mine. The freezingtemperature of 24 inclusions ranges between -40oCto -50"C, and the final melting temperature of ice,from -0.93 to + 0.4oC. These values, indicate thatthe fluid is a low-salinity HrO containing less than1.6 wt.Vo NaCl equiv. (Potter et al. 1978). Usingpressure corrections of I and 2 kbars (Jolly 1982)and the homogenization temperature (x:208 t 6oC),the formation temperature corresponds rcn0"C atone kbar and 330'C at two kbars.

DISCUSSION

The dominant minerals in the rodingitized gabbrosand diabase are diopside, hydrogrossular and preh-nite, whereas the dominant minerals in the rodingi-tized lamprophyre and andesite are epidote andclinozoisite. Prehnite, hydrogrossular and diopsideoccupy late-stage cross-cutting veins. The mineralassemblages observed in the rodingites from theAbitibi greenstone belt are similar to those observedin rodingites from Phanerozoic terranes. All of theseminerals are associated with subgreenschist-faciesmetamorphism (Rice 1983). Our fluid-inclusion

586

MUlln0A d i ahase lamprophyrs

v [ARns0[. HI| |MAHEJ BOwMAilI E t ru rso 0uN00 l {A t 0

cFlc. 2. ACF plot of Abitibi belt rodingites, showing the rodingite field as delineated

by Coleman (1977).

results from diopside agree with this assessment ofa low temperatlue. In addition, the presence ofCOlfree inclusions is consistent with the absence ofcarbonate phases in these assemblages.

As part of the lamprophyre at Munro mine wasnot rodingitized, this outcrop will be used to demon-strate mineralogical and chemical changes thataccompany rodingitization. The initial reaction iden-tified in the lamprophyre destabilizes plagioclase andproduces clinozoisite. The second reaction involvesthe breakdown of hornblende and clinozoisite toform epidote with minor diopside and tremolite. Thischange in mineralogy is accompanied by a changein bulk chemistry. These stages can be recognizedwithin the rodingitizrid andesite in the Reeves mineas well, in which diopside dominates at the contactbetween rodingite and serpentinite, and epidote andclinozoisite are found at a distance from the contact.

Coleman (1967, 1977) proposed several reactionsto describe the onset of rodingitization:

3CaAl2Si2Os + Ca2* + 2H2O :(anorthite)

2Ca2Al3Si3O12(OH) + 2H+(zoisite)

2Ca2Al3Si3O12(OH) + 5C*+ + 2H2O :(zoisite)

Ca3Al2Si2.rO1o(OH)z + 4H+(hydrogrossular)

and

l.sCaAl2Si2O, + 0.5Ca2+ * 1.5H2O =(anorthite)

Ca2Al2SirO,o(OlI)z + 0.5A12O3 + H+(prehnite)

2Ca2Al2Si3O1o(OH)z + 2C*+ =(prehnite)

2CarAl2Si2.rOro(OH)z + 0.5Si4*(hydrogrossular)

in which plagioclase alters first to zoisite or preh-nite, which then alter to grossular. This intermediatestep may not be necessary, however, as

CaAl2Si2Os + 2C*+ + 3H2O + 0.5SiO2 :(anorthite)

CarAlrSir.rO,o(OH)z + 4H+(hydrogrossular)


t 114 .O

I

SlO2 Al2Og Fe2O3 FeO MnO

x -5o

Volume Faclor: '1.O2

Frc. 3. Chemical changes that resulted from rodingitization of the Munro mine lam-prophyre. Magnitude ofvolume factor indicates that no volume change occurredas a result of rodingitization,

587

394.O

ft

3

ooo,

Eo

oog(t

E(,

We have observed plagioclase as well as zoisite andprehnite alterrng to hydrogrossular. In addition, thereplacement of zoisite with prehnite is described bythe reaction

2CarAl3SirOt2(OH) + 3SiO2 + 2C** + 4H2O =(zoisite)

3Ca2Al2Si3Ore(OH) + 4H*(prehnite)

All of these reactions are consistent with the appar-ent increase in Ca and water, and the loss of Na andK from the system with increasing rodingitization.

Laurent (1980) defined two mineral assemblagesin rodingites in the serpentinized harzburgites fromthe Thetford Mines ophiolite in Quebec. The earlierassemblage, which comprises the bulk of the rodin-gite, consists of diopside, hydrogrossular andclinozoisite. The later assemblage occupies cross-cutting veins and consists of zoisite, grossular, preh-nite, diopside, vesuvianite and wollastonite. Laurentassociated the formation pf the early assemblage withthe production of lizardite mesh-rim textures dur-ing serpentinization under the ocean floor. Heassociated the later assemblage with the productionof lizardite hourglass and chrysotile interlocking tex-tures and chrysotile asbestos veins that presumablyformed during emplacement of the ophiolite.

According to Laurent (1980), the different mineralassemblages reflect different regimes of serpentini-zation and rodingrtization. These regimes are defined

by thermodynamic parameters as well as by the tec-tonic environment. Using his model, we can definetwo episodes of rodingitization as well. Late-stageveins cross-cut most samples found in the chrysotiledeposits, but no veins were found in samples fromthi Dundonald sill. This finding is consistent withthe two episodes of serpentinization found in theasbestos deposits, and the single one recognized inthe Dundonald sill (Wicks et al. 1983).

Although we can define two episodes of rodingiti-zation based on cross-cutting relationships, we canalso subdivide the first episode into stages based onthe observations made on the lamprophyre: a firststage in which feldspar alters to clinozoisite, a secondstage in which hornblende and zoisite alter to formepidote and diopside, and a third stage in which allminerals alter to dioPside.

The consistently corroded margins of hydrogros-sular in the other rodingites indicate that it is an earlyphase. However, different stages are not as appar-ent in ttre other rodingites, primarily because of thepersistence of diopside in both the matrix and theveins. Vein diopside has a chemistry similar to thatof the matrix that it cross-cuts. In addition, the grainsize of the matrix diopside decreases with increasesin the extent of alteration, a trend that is accompa-nied by a loss of Fe.

Modal variations in mineralogy within rodingitesof the same protolith could be due to variation inthe rodingitization regime. For example, diopsidedominates in rodingitized Munro diabase, whereas


RODINGITE CONTACT SERPENTINE

Ftc. 4. Schematic representation of chemical changesacross the reaction zone between the rodingite protolithand the serpentinite in the Munro mine.

hydrogrossular dominates in all the others, althoughhydrogrossular is corroded in these diabase rocks.The Munro serpentinite contains antigorite in addi-tion to lizardite and chrysotile. We suggest that achange in the regime of serpentinization at Munro,reflected in a change from lizardite-chrysotile toantigorite, resulted in a change in rodingite miner-alogy (production of diopside), due to the liberationof Mg during recrystallization of the serpentine.

Using the observations and reactions given above,we can define three types of rodingites in the Abitibibelt, representing a different extent of, or regime of,rodingitization. Epidote-rich rodingites represent theinitial stage of rodingitization, in which eitherclinozoisite or zoisite may be the dominant modalphase. Minor phases include diopside and titanite.The Munro lamprophyre dyke and the less-rodingitized andesite of Reeves represent this stage.Grossular-ich rodingites, which formed bythe alter-ation of epidote-group minerals to hydrogrossular

and prehnite, represent a second stage of rodingiti-zalion. Hydrogrossular forms most commonly afterclinozoisite, prehnite or plagioclase, and uncom-monly after pyroxene. Prehnite predates hydrogros-sular in some rocks but occurs inter$titially tohydrogrossular in others. Minor diopside may bepresent as well. The Garrison and United diabasesand the Hedman gabbro are examples of this type.Diopside-rich rodingites represent a third $tage ofrodingitization, in which the epidote-group minerals,hydrogrossular, prehnite and relict pyroxenerecrystallize to fine-grained diopside. The Munro dia-base and the mele 1e<lingitized andcite at tle Reevesmine are examples of this type. Although stage Irepresents the initial phase of rodingitization, it canbe followed either by stage 2 or stage 3. Observa-tions made on Phanerozoic (Adib & Pamic 1980) andocean-floor rodingites (Honnorez & Kirst 1975) areconsistent with these stages, as well as with the reac-tions proposed by Coleman (1967).

The stages of rodingitization are not always relatedto different events of serpentinization that could bedefined by serpentine mineral assemblages. The bestexample of this is found in the Dundonald sill gab-bro. The rodingitized gabbro consists of diopside andhydrogrossular. Based on the stages outlined above,it is a highly rodingitized rock. The olivine in the sillhas been pseudomorphically replaced by lizarditewithout the formation of antigorite or chrysotile. Theserpentinized olivine indicates that only one serpen-tinization event occurred, whereas the gabbro hasexperienced two stages of rodingitization during thesame event. Furthermore, we have not been able tocharacterize rodingite textures in a manner similarto that used to describe serpentine textures. Forexample, Wicks & Whittaker (1977) classified ser-pentine textures into pseudomorphic and nonpseudo-morphic textures, which correspond to retrogradeand prograde serpentinization. Our observationsindicate that there are no specific rodingite texturesassociated with either retrograde or prograde serpen-tine. Thus one cannot predict in detail serpentinitemineralogy and texture from rodingite mineralogyand texture or vice versa. The correlation betweenserpentinization and rodingitization based on miner-alogy and texture is not straightforward.

Vesuvianite was observed in the rodingitized dia-base and andesite but not in the rodingitized gab-bros or lamprophyres. Presumably this pattern ofdistribution is due to the greater extent of rodingiti-zation of diabase and andesite, because vesuvianiterequires more calcium than zoisite, hydrogrossularor prehnite. Grain size is also a factor. The fine grain-size of diabase and andesite appears to enhance reac-tion rates and results in a more extensive rodingiti-zation than in similarly situated but coarse-grainedgabbro and lamprophyre.

The presence of relict phlogopite and tremolite in

RODINGITES IN THE ABITIBI BELT s89

the chlorite reaction rim adjacent to the diabase dykebut not adjacent to the lamprophyre dyke may bedue to the relative time of emplacement of the dykes.A dyke emplaced into contact with a hot peridotitein the presence of water may be a more favorableenvilonmeirt for the crystallization of phlogopite andtremolite, in contrast to the emplacement of a dykeinto a cold, dry serpentinite. A late-stage emplace-ment of the lamprophyre is consistent vrith the morepristine mineralogy observed in its center.

Coleman (1967) used an ACF diagram to deline-ate a field in which the chemical compositions ofthoroughly rodingitized rocks would plot. This fieldis characterized by the composition that representsvarious modal proportions of diopside and chloritewith hydrogrossular, prehnite or epidote. Colemanalso indicated the trend toward this field of partlyrodingitized greywackes and basalts and stated thatthe bulk chemistry of the protolith would have aminor effect on the outcome.

We believe that protolith chemistry should beemphasized because the type of chemical changesnecessary to produce a rodingite will vary with theprotolith. For example, basalts must lose alkalis andMg, increase in ferric iron or gain Ca. Lamprophyreshave high Fe3* /F&* values already, and the one inthe Munro mine has low Mg values. This rock needsonly to lose alkalis and a little Mg and to gain Cato plot in the rodingite field. For Ca-rich protoliths,the bulk chemistry of the rodingite will evidently notindicate the extent of rodingitization.

The primary definition of the extent of rodingiti-zation should be based on the minerals present andnot on the location of the sample in an ACF dia-gram (Coleman 1977). The data presented by Cole-man (1967), Wares & Martin (1980) and our paperbear this out. Our samples plot near the rodingitefield regardless of their extent of rodingitization.

The ultramajic rocks in the Abitibi belt are eitherflows or sills; there is no evidence of a tectonitefabric. The absence of deformation is a markeddifference between these rocks and those found inPhanerozoic mountain belts. In the laffer environ-ment, tectonite fabrics are cornmon and indicate thatthe rocks originated in the mantle (Coteman 1977).Much attention has been drawn to the role of defor-mation and tectonism in Phanerozoic rocks as wellas in samples dredged from the ocean floor. Cole-man (1967) has invoked pressures greater than 4kbars and temperatures of 200-500'C for the for-mation of rodingites containing aragonite. Thisregime contrasts sharply with that of present-dayweathering of ultramafic rocks, which producesxonotlite in California (Barnes et al. 1972). Stableisotope data reconcile these two contrasting regimes.The data suggest that rodingitization occurs over atemperature range from 400 to 25'C (Wenner 1979).

Antigorite serpentinitm host rodingites within the

Tananao schist of Taiwan (Yui el a/. 1988). Theirenrichment in light stable isotopes of hydrogen andoxygen suggests that antigorite-tremolite and diop-side are in isotopic equilibrium. Clinozoisite, whichis found furthest from the serpentinite and appar-ently represents the least-altered schist in their study'is not in isotopic equilibrium with the other minerals.Yur et al. (1988) also noted that hydrogrossular com-monly occurs with diopside; their observations andinterpretations are consistent with our proposed divi-sion of rodingites.

As the Abitibi ultramafic rocks are sills and flowsand occur interlayered with altered basalts that havenot undergone metamorphism at a grade higher than'low greenschist fasies (Jolly 1982), temperatures andpressures could not have been higher than 4@oC and3 kbars, respectively. On the other hand, we did notfind either xonotlite or pe'ctolite, which would ruleout rodingitization near 25"C.

The differences in tectonic style and degree ofmetamorphism of rodingite localities throughout theworld suggest that rodingitization is simply a chem-ical process characterized by the addition of calcium'and the removal of silica and the alkalis from theprotolith. If the rodingite protolith and the serpen-tinite are considere! as one system, as suggested byThayer (1966) andCernt (1968), rodingitization canbe viewed as an autometasomatic process once wateris present. In this view, the extent to which rodin-gitization progresses depends upon the chemical com-position of the serpentine protolith.

The apparent sporadic absence of rodingites fromsome serpentinites has been noticed by severalauthors (Coleman 1967, Leaming 1978, Yui et al.1988). We believe that the presence or absence ofrodingite is governed by the composition and by theamount of fluid present. Firstly, if present, CO2 willscavenge calcium to produce dispersed calcite ordolomite. Both minerals are common in $ome partlyserpentinized peridotites. Less commonly, small clotsof fine-grained diopside and garnet occur in the inter-stitial zones between serpentine pseudomorphs afterolivine, as observed in the Munro mine serpentinite.In both instances, as calcium-bearing minerals arenot concentrated but dispersed throughout the ser-pentinite, they would be difficult to recognize asrodingite in the field. Another factor that is impor-tant to the transport of cations and to the concen-tration of the calcium-bearing minerals is the amountof fluid flowing tlrouglr the serpentinile. A high flowcould flush calcium out of the serpentinite, a lowerflow could concentrate calcium within the serpen-tinite, within inclusions or at lithological contacts.

The investigations of Anhaeusser (1979) and ourown indicate that the rodingitization process has notchanged since the Archean, despite the possiblydifferent tectonic driving forces in Phanerozoic timesthan in Archean times. Therefore, the parameters


that control the formation of rodingites in thePhanerozoic rocks apply also to the Archean andviceuersa. Among these controls are protolith chemis-try and grain size, and the extent ofserpentinization.Factors that influencg ledingitization but are noxessential to its operation are multiple lectonic eventsand varying episodes of serpentinization.

CoNcLUsIoNs

The rodingites hosted by ultramafic sills in theAbitibi greenstone belt formed from several pro-tolitls including dykes, lithic inclusions and volcanicrocks at t}te serpentinile contact. Variations in miner-alogy and textural relationships allow us to definethree groups of rodingites, each representing $ucces-sive stages in alteration of the protolith: l) epidote-rich rocks: earliest alteration of the protolith; 2)grossular-prehnite-rich rocks: intermediate altera-tion, and 3) diopside-rich rocks: final stage of alter-ation. The initial bulk-rock chemistry affects themodal proportion of minerals.

Observations and chemical trends reported hereare similar to those found in Phanerozoic terranes,indicating that the rodingitization process has notchanged since the Archean, a conclusion also reachedby Anhaeusser (199). Mass-balance calculations andfluid-inclusion data indicate that rodingitization wasan isovolume process; in these examples, rodingiti-zation was mediated by a low-salinity water at tem-peratures of approximately 300oC and at I kbar.Although deformation commonly affects the serpen-tinite host, it is not an essential factor in the rodin-gptrzation process. The parameters that appear toeffect the process of rodingitization are: a) timingof emplacement of the dyke or intrusion into theultramafic rock with respect to the onset of serpen-tinization, b) size of the protolith compared to tharof the serpentinite, c) grain size of protolith, and d)extent of serpentinization.

AcKNowLEDGEMENTS

This work was supported in part by Ontario Geo-science Research Grant 138 and by an NSERC oper-ating grant to F.J. Wicks. Both grants are gratefullyacknowledged. We thank Prof. E.T.C. Spooner foraccess to the facilities of the F. Gordon Smith FluidInclusion Laboratory, University of Toronto, andfor his assistance with the fluid-inclusion study. Weare grateful to Dr. A.G. Plant and Mr. M. Bonardi,who provided us with access to and expertise on theelectron microprobe at the Geological Survey ofCanada. We thank the referees, as well as the editor, Dr. Robert F. Martin, for their diligence inreviewing our manuscript.

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Received September 22, 1988, revised manuscrtptaccepted March 31, 1989,

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