American Mineralogist, Volume 65, pages 654-669, 1980

Textures and mechanismsof metamorphicreactions in the CockeysvilleMarble near Texas, Maryland

RtcHnno F. SnNrono' The JohnsHoPkins UniversitY Baltimore, Maryland 2I 218

Abstract

The Cockeysville Marble at Texas, Maryland was regionally metamorphosed at about 600.C and 6 kbar. During metamorphismthe reactionsbetween dolomite, qvattz, and micro- cline to producephlogopite, tremolite, and diopsidecaused the formation of mineral segrega- tions and reaction bands. Various textures, including radiating spraysof tremolite mantled by calcite in dolomitic rocks and reaction zones of tremolite + calcite and phlogopite between metadolomiteand calcite marble, can be explained by local metasomaticreactions taking placenear equilibrium by masstransfer down chemicalpotential gradientsbetween local do- mains. Modal mineralogy and reaction texturesare used to infer the relative magnitude of fluxesfor the major components,viz. CaO> MgO > SiO2> KAlSi3O8.Experimental solubil- ity data support the generalization that the flux of a fluid speciesis proportional to its solubil- itv.

Introduction prominent dark brown phlogopite bands between white calcite marble and tan metadolomite, and Detailed study of mineral relations in metamor- striking clusters of radiating, bladed tremolite man- phosed carbonate and pelitic rocks has shown that tled by calcite within massive metadolomite. These chemical equilibrium is commonly not achieved on and related textures, formed during the growth of the scaleof a thin section.Thompson (1959)consid- metamorphicminerals, were clear$ causedby chem- ered the role of local equihbrium and transferof ma- ical reaction and transport during metamorphism. in the presenceof activity gradients.The con- terial It will be shown that assemblagesof minerals was successfully employed by Carmichael cept whose grains are in mutual contact reveal the equi- (1969),Fisher (1970), Eugster (1970), Loomis (1976), librium metamorphic conditions of particular sam- Weare et al. (1976),and Foster (1977,1978) to inter- ples, and that the sequenceand modal relations of pret in pelitic rocks;by Vidale (1969),Hew- textures minerals in a set of reaction zonesyield information itt (1973),Vidale and Hewitt (1973),Burt (1974), on the stoichiometryof local metasomaticreaction$ Joesten(1974, 1977),Thompson (1975)' Brady and the nature of the dissolved,transported species' (1977),and Hoersch(1979) to explain calc-silicatere- In most casesmobile and inert components can be bands; and by Brady (1977) and Sanford action distinguished unambiguously from observedreaction (1978)to interpret ultramafic reactionzones. Prelimi- textures. The method of determining relative mobil- nary results of the present study were presented ities consistsof proposing limiting, alternative com- orally by Sanford and Eugster(1974). binations of mobile and immobile components in This paper is a systematicanalysis of the mineral- each reaction and predicting the various possible re- ogy and textures of the Cockeysville Marble at action textures.These predicted textures are clearly Texas, Maryland. The Cockeysville Marble in the dislinguishablefrom one another, and the choice of Flintkote StoneProducts (formerly, Harry T. Camp- which one representsthe actual mechanismis usually bell Sons')quarry at Texas,Maryland, containssev- obvious by comparison with the natural rock. An im- varieties of mineral segregations,including eral portant check on the mass transfer can be obtained potential gradients of in- I Presentaddress: Mail Stop 959, U. S. GeologicalSurvey, Res- by calculating the chemical ton, Yirginia 22092. dividual speciesin the fluid. 0003-004x,/80/0708-0654$02.00 654 SANFORD: M ETAMORPHIC REACTI)N S

Geologicsetting Formation under$ing the Cockeysville Marble at Texas and much of Figure I showsthe location of the quarry and of the Wissahickon Formation, which overlies the samples. The Cockeysville Formation occurs the Cockeysville, are composed of kyanite-, near the baseof the Glenarm Series,a group of pre_ staurolite-, and garnet-bearing schists (Fisher, cambrian or Early Paleozoicmetasediments resting l97l). unconformably on the precambrian Baltimore Gneiss(Higgins, 1972;Crowley, 1976).The Cockeys_ Rock types ville Formation is underlain by the SettersForma- The principal rock types found in the Texas tion, which comprises muscovite-microcline quiartz_ Quarry are metadolomite,calcite marble, and calc- ite with subordinate mica gneiss and mica schist. schist(for definitions seeChoquette, 1960). Medium- and overlain by the thick Wissahickon Formation, grained, bluish-gray metadolomite, calcite marble, which is composedmainly of pelitic schists (Higgins and calc-schist,all interlayered on a fine scale and and Fisher,l97l; Crowley,1976).These three forma_ containing the assemblagedolornite * quartz, char- tions make up the Glenarm Series,which mantles a acteiae the rocks in the northwestend of the quarry. group of elongategneiss domes. The Texaseuarry is The main part of the quarry contains coarse-grained, located in the Cockeysville Formation on the west white calcite marble grading into medium- and fine- flank of the Texas Dome. A detailed study of the grained bluish-gray and tan varieties of metadolo- Cockeysville Formation was carried out by Cho_ mite. The tan color is imparted by phlogopite.In this quette(1957, 1960). part of the quarry, tremolite occursin place of the as- The quarry is located on the noseand SW timb of semblagedolomite + quartz. Rare thin (l-2 cm) lay- an anticline plunging gently NW (Choquette, l95Z). ers of microcline-quartz rock also occur. Modes for Compositional layering, interpreted as bedding, typical samplesof each of these rock types are pre- strikes approximatelyNW and dips from 20o to 40o sentedin Table l. SW with a generalsteepening toward the SW. Fisher (1971) has demonstratedthat the-CockeysvilleFor_ Effects of sedimentaryand diageneticprocesses mation exposed at Texas is right-side-up, as in- The Paleozoicorthoquartzite-carbonate sequence, dicated by a bore hole which terminatesin the Set_ of which the CockeysvilleMarble is a part, resting ters Formation. The upper beds of the Setters uncomformably on Precambrian gneisses,is typical

CONVEYOBBELT

Fig' l' Map of Flintkote StoneProducts quarry at Texas,Maryland with locationsof samplessited in text. SANFORD: METAMORPHIC REACTIONS

qvuttz, and feldspar' Table 1. Representative modes of rock types in Cockeysville of calcite, dolomite, phlogopite' Marble at Texas, Maryland Prominent color variations from white to bluish gray to tan on the scale of meters are causedby locally No. phlogopite' respec- Q 0thersx Polnts abundant calcite, dolomite, or tively. Dolomite and calcite marbles very comnonly (1) 14.0 71.3 0.0 6.2 0.0 2.2 0.2 1518 contain thin (l-2 mm thick) partings of calc-schist 12, 16.1 72.9 2.8 7,4 0.0 0.0 2.8 2230 (l) 1r.4 80,8 0.8 6.8 0.0 0. I 0. I 3023 spacedfrom 2 to l0 mrn apart. Phlogopitegives the the calc- (4) 0.0 0.0 0.4 4.8 9.1 0,4 963 prominent tan to dark brown coloration to (5) 94.5 0,0 0.0 4.3 0.4 0.6 0.2 4000 ichist layers,but microcline,q\attz, dolomite (in cal- (6) 11.1 0.0 21.0 19.5 2.1 1.3 967 cite marbles), and opaques are also concentrated in (7) 28.6 l.o 28.1 34.1 5.5 0.5 1.1 11 41 theselayers. (8) 5.t 66.0 19,9 6.2 0.l 0.1 Calc-schist layers are similar to seamsdescribed by (1964),and interpreted by him to be the re- *l4osily pyrite, rare tourmalIne' Barrett (1) Me+adolml+e,C27-20O sult of intrastratal solution moving along planespar- (2) l"le+adolomi+e,Cl-1 99 (f ) Me+adolml+e, TQ?-z and 1Q12-3 allel or subparallel to bedding, removing calcareous (4) Calcite marble C27-200 insoluble, non-car- (5) Calci+e na.ble, fQ2-22 constituents and concentrating (6) Calc-schist, TQB-2 minerals. Vertical spacing between layers, (7) Calc-schlsi, TQ2-4 bonate (8) Microcllne-quar+z rock, Cl-199 broad horizontal extent oflayers, closegeneral paral- lelism, the merging and diverging of layers, and a sim- of sedimentationat the edgeof a stablecraton having structures resembling cross-beddingall suggest Texas differs low relief (Crowley, 1976).Dolomitic rocks' which ilar origin. The Cockeysville Marble at coarser,and are the prirnary object of this study, probably repre- in that partings are thicker, grain sizeis of glauco- sentdeposition in a restrictedintertidal, marine basin phlogopite or microcline is presentinstead to re- or a supertidal sabka environment (Selley, 1970, nite and illite. These effectscan be attributed than thesethin chap. 8). Such conditions would be expectedto pro- gional metamorphism.Less abundant of quartzite and duce evidence of sea-level fluctuations, periodic partings are layers (l-2 cm thick) be relict flooding and evaporation,pore water replacementre- microcline-quartz rock. These layers may possibly meta- actions, and occasional deposition of land-derived sedimentarybeds of quartz and clay, (Bathhurst' clastic material. Many features of the Cockeysville morphosed glauconite-rich hardground due to Marble at Texascan be attributed to thesesedimen- 1975,p.3g44l2), they may be concentrations pressuresolution, or they may be due tary and diageneticProcesses. locally intense regional The minerals calcite, dolomite, quattz, and micro- to chemical differences generated during later' cline are consideredto be premetamorphic.Dolomite metamorphism.The last possibility is discussed probably parallel may have formed by direct precipitation from sea- Taken together,these layers are be- water in a lagoon-like basin or by replacementof or nearly parallel to original bedding surfaces contactselsewhere preexistingcalcite sediments.Quartz is probably de- cause(l) they parallel formation 1960);(b) trital or recrystallizedfrom diageneticchert. Micro- in the Cockeysvilleformation (Choquette, and quartz- cline may be detrital, authigenic, or a product of they wrap around boudinaged dolomite com- clay-pore fluid reaction. Detrital microcline is com- ose layers; and (c) they represent concordant magnitude, mon in the underlying Setters Formation, though positional variations of widely different Primary rare in most carbonaterocks. Authigenic feldspar re- from lessthan a millimeter to tens of meters. placing limestone is reported by Pettijohn (1975, p. compositionaldifferences have probably been accen- metamor- 325). Of the possible clay precursorsof -islssline' tuated by stylolitic pressuresolution and pres- glauconite is the closestin composition,particularly phic reactions.Studies of stylolites and other (reviews by with respectto its K/Al ratio. Other clay minerals sure solution features in limestones p. 468- (e.g. illite, kaolinite, montmorillonite) would require Pettijohn, 1975,p.340-342; Bathhurst, 1975, can be metasomaticaddition of K or removal of Al. The dis- 471)haveshown that non-carbonateminerals bedding due tribution of phlogopite and tremolite probably re- concentrated along planes subparallel to dia- flects the premetamorphicdistribution of microcline to non-hydrostatic pressureduring burial and and quartz, respectively. genesis. The dominant foliation at Texasis a compositional While calcite marble commonly occurs in con- of layering associatedwith variations in the proportions cordant beds, many irregular lensesand stringers SAN FO RD: METAM O RPHI C REACTI oN S

calcite marble clearly transect compositional layering mite is nearly stoichiometric.A typical analysis and grade laterally into metadolomite.Isolated frag- yields the formula: mentsof metadolomiteare commonly surroundedby calcite marble. Streaks or bands of .,impurities," Ca.,nFeo o,Mg, o(COr), graphite,pyrite, s1 foumaline, in the marble are par- Calcite compositionsvary from Ca.nnMgoo,CKO,to allel to the local bedding. These texturessuggest an Ca"noMgo,*COr.FeO and MnO are insignifi@a1. in situ reaction relationship between calcite and dolo_ Calcite coexisting with dolomite is typically more mite. Such calcite bodiesrepresent limestone masses Mg-rich than calcite without nearby dolomite. Diop- untouched when the original limestonewas dolomi- side is rare and was not analyzed. tized, or they are the result of incompletededolomiti- All the above-mentionedminerals except calcite zation (similar to that discussedby Shearmanet al., will be treated as having ideal end-membercomposi- 1961; DeGroot, 1967;Friedman and Sanders,1967; tions. Compositional variations in calcite vary sys- Bathhurst, 1975). In either case, the absenceof tematically with mineral assemblagesin the rocks. phlogopite in the calcite bodies indicates that they Chemical reactions are written with the abbrevi- existedprior to the metamorphic growth phlogo- of ations for componentsin their respectivestates: e, pite in dolomite-bearingrocks. If dolomite had been SiO, in qaartz; Cc, Ca CO, in calcite; Do, present wherethe calcite bodiesare now, then micro_ CaMg(CO,), in dolomite; Mc, KAlSi3Os in micro- cline would have reactedto form phlogopite accord- cline; Ph, KMg.AlSi,O,o(OH), in phlogopite; Tr, ing to reaction (l), discussedbelow. For this reason CarMgrSirOrrOH),in tremolite; and Di, CaMgSirOu the discordant bodies of calcite not associatedwith in diopside. Componentsdissolved in the fluid are abundant metamorphic tremolite and phlogopite are written without abbreviation, e.g. SiOr, for SiO, in considered to be a product of diagenesis.Calcite the fluid. lensesand segregationsassociated with the formation Stable mineral of phlogopite and tremolite during metamorphism assemblages are discussedbelow. Although many thin sections contain all of the minerals calcite, dolomite, microcline, quartz, phlogopite, and tremolite (a few also contain diop- Mineral chemistry side), only a limited number of possible combina- Minerals were analyzedwith the Harvard Univer- tions actually occur with the mineralsin mutual con- sity Department of Geological SciencesApplied Re- tact. Theseare: search Laboratories model EMx-sM electron micro- Dolomite * Quartz + Phlogopite+ Calcite probe, and data reducedby the method of Benceand ZoneI Albee (1968)and Albee and Ray (1970).Standards Tremolite * Quartz + Phlogopite+ Calcite were natural and syntheticminerals whose composi- ZoneII tions approachedthose ofthe unknowns. Tremolite * Quartz * Microcline + Calcite All analyzed minerals in the rocks studied have Zonelll nearly ideal, end-member compositions. Microcline, Tremolite * Diopside + Microcline + Calcite which shows characteristictwinning, has composi- ZoneIY tions in the rangeOr*Abn to OrnrAbr.Tremolite con- Furthermore, in any given thin section only tains very little FeO, but AtO, varies from 0.21 to one of the above assemblagesis observed.It will be shown 2.57 mole percent. A typical analysisyields the for- mula: that these assemblagesrepresent mutually exclusive fields in temperature-pressure-fluid composition Ko o' Nao orCa, ooMgo,rFeo .TAL oesi, e2O2r(OH), (X.",) space, and that the sequencefrom zone I through zoneIV representsstable equilibrium Phlogopitedeviates most from its end-membercom- assem- blages related by systematicvariations position. A typical phlogopite analysisyields the for- in X.o, or mula: metamorphicgrade. Metamorphic K, ,.Nao orCao-rMfu o,Mg, on textures in zoneI Feo ,rTio ,2lar,12o4sir rsoro(OH)n Disseminatedphlo gopite in metadolomite Excess AlrO, (over two Al per formula unit) plus Phlogopite,commonly occurring in metadololite, small amounts of FeO and TiO, are present. Dolo- is scattired randomly throughoutlhe rock (Fig. 2aj SANFORD: M ETAMORPHIC REACTION S

Calcite marble also in books 0.5-2 mm in diameter. Abundances are roughly parallel to the bedding. of all sizesenclosed in characteristically uniform (for modal analysessee o""orc as lensesor stringers typically sub-parallel Table 2), with phlogopite ranging from 6-8 percent. metadolomite,with the lenses follow the shapeof The texture of the rock is granular with interstitial to the bedding. Phlogopite rims may not be continuous' anhedral calcite and euhedral phlogopite between the marble lenses, but at the ends of subhedraldolomite grains. Phlogopite is commonly concentrated case,coarse calcite The presenceof microcline in dolomite-freerocks, the linies. In the most extreme the bedding and en- coupled with its absencein metadolomite, suggests marble bodies freely transect The phlogopite rims that phlogopite formed at the expenseof microcline close metadolomite blocks. interface' accordingto the reaction: again follow the marble-metadolomite commonly Quartz, pyrite, and tourmaline also are Mc + 3Do + H,O: Ph + 3Cc+ 3CO, (l) concentratedwith phlogopite in theserims' Where calcite marble containing disseminatedmi- Reaction(l) producescalcite as well as phlogopite. For metadolomites,which have an averageof 7 modal percent phlogopite, a maximum of 5 percent calcite can be produced by this reaction. This is about one third of the averagecalcite content of the metadolomites (see Table 1). The remainder was therefore originally present in the metadolomite. It also follows that the phlogopitereaction (1) produces a minor amount of calcite and cannot be responsible for the pure calcitemarble lensesand veins discussed type are shown in Table 2. above under sedimentary and diagenetic features. These textures are evidenceof reaction (l) which Since no disseminated microcline remains in the metadolomite, reaction (l) has evidently gone to completion.

Phlogopiterims betweenmetadolomite and calcite marble mite. Phlogopite also occurs in monomineralic rims up to severalcm thick separatingcoarse-grained calcite Textures in zoneII marble from metadolomite (Fig. 2a). The shape of Tremolit e- calcite segre gations these rims is determined by the geometry of the bundles l-2 marble-metadolomite bodies; the thickness of the Tremolite occurs as individual fiber much of rims appearsto be directly proportional to the sizeof mm in diameter disseminatedthroughout bladed knobs the bodiesand to the phlogopitecontent of the meta- the metadolomite(Fig. 2c), as radiating fibrous masses5- dolomite. In the simplestcase, thick beds of marble 5-10 mm long in calc-schist,and as lenses of calcite and metadolomite are separatedby planar rims 100 mm long within veins and marble (Fig. 2d). All of these tremolite massesare surrounded by pure calcite zones between metadolomite and partly or completely Table 2. Modes of reaction segregations' calcite marble, SamPle No. C27'20O Laloes,thus representingmetamorphic of these Quartz is typically absent.The calcite layer NO. is commonly surrounded by a phlogo- Rock Type Cc Do Tr Ph Mc Q ofhers' Pol nl-s sigregations piie rim. The overall net reaction responsiblefor Metadolomite 14.0 17.3 0.0 6.2 0.0 2,2 0.0 thesesegregations is ProbablY

Ouier Rim 66.7 0.0 0.0 23.9 0.0 8.7 Tr 598

lnner Rim 59.7 0.0 0.0 20.3 15.3 3.4 1.4 295 5Do+ 8Q + H,O: Tr * 3Cc+7COz Q) Calc iie l"larble 85.3 0.0 0.0 Tr 4.8 9.1 Tr 963 quartz re- Core With rare exceptions(those in which relict mains in the segregationcore) the reaction has gone r Mostly pyrite, rare tourmal lne. to completion. SAN FORD: M ETA MO RPHI C REACTI oN S 659

Fig. 2a. Phlogopite rims between calcite marble (white) and metadolomite (gray). Grid, 2 x 3 cm. Phlogopite * tremolite rims betweenmetadolomite and dolomite on the right. The dark, banded layer in be- calcite marble tween consists of quartz, microcline, and pyrite. The white layer next to it is a zone of tremolite and cal_ Thin (l-2 cm) layers of microcline-quartz rock oc_ cite. Microcline-quartz layers are commonly frac- cur in the Texas quarry, commonly at contacts be_ tured and folded. Layers in contact with metadolo- tween calcite marble and metadolomite; rarely with mite develop two reaction zones: a tremolite-calcite metadolomite on both sides of the layer. Figure 3a zone next to the metadolomite and a phlogopite zone shows white calcite marble on the left and buffmeta_ next to the microcline-quartz rock (Fig. 3b). Table 3

Fig' 2b Photomicrograph of phlogopite rims in calcite rnarble (gray) at contact with metadolomite (white grains with int€rstitial gray calcite). Sample No. C27-ZN. Length of bar, I cm. 660 SANFORD: METAMORPHI C REACTIONS

'*'" ,ll;:llfat :.rt- :a: ;K:Td,: .'i.r. r fibers (cenler, white) surrounded by calcitc rim Fig. 2c. photomicrograph of tremolite-calcite segregation with bundle of tremolite gray calcite). Length of bar, I cm' lgray; in metadolomite (white grains with interstitial

grains with interstititial gray calcite)' Grid' I Fig. 2d. Calcite vein (gray) containing fibrous tremolite (white) in metadolomite(white cm wide. SANFO RD: M ETAMORPHIC REACTIONS

Fig. 3a. Microcline-quartz layer (banded dark gray) with calcite marble (white) on left and metadolomite (gray) on right. Between thc microcline-quartz layer and metadolomite is a zone of tremolite and calcite (white) and a zone of phlogopite (dark gray edge on microcline-quaftz Iayer). Grid, I x 2 cm. gives modes for the four units. The tremolite-calcite Textures in zoneIII zone is equivalent to the tremolite segregations dis- cussed above. Phlogopite in the phlogopite zone con- Tremolite * microcline segregations tains small microcline inclusions. some of which ex- This classof segregationsresembles the tremolite- hibit optical continuity, indicating replacement of calcite segregationsin zoneII exceptthat the phlogo- microcline by phlogopite. pite crystals are cornmonly ragged and rimmed by The most significant features of the sequence are: clusters of small microcline grains. In these tremo- (a) the absence of dolomite in all zones except in the lite-microcline segregations,microcline is essentially metadolomite; (b) the presence of tremolite virtually absentin the host rock, which is either calc-schistor limited to the tremolite zone; and (c) the concentra- metadolomite. The tremolite-phlogopite-microcline tion of phlogopite next to the microcline-quartz aggregatesare commonly surrounded by a calcite zone. Two reactions apparently involved in the for- rim. mation of the mineral zones are (l) and (2). The growth of tremolite in the tremolite-micro- Less common than microcline-quartz layers are cline segregationprobably took place by the reac- angular quartzite fragments. These exhibit similar re- tions (2) and (3): action rims except that the phlogopite zone is only partially developed (Fig. 3c). 5Ph+ 6Cc + 24Q:3Tr + 5Mc + 21120+ 6CO, (3)

MQ TC MD Fig. 3b. Photomicrographof sampleshown in Fig. 3a. Zonesfrom left to right are microcline-quartz(MQ), pblogopite(P), tremolite- calcite (TC), and metadolomite(MD). Length of bar, I mm. 662 SANFORD: METAMORPHIC REACTIONS

Fig. 3c. Quartzite fragments (dark gray) in metadolomite, tremolite--calcite zone (white) at contact. Grid, 2 x 2 cm. Reaction (2) creates the initial tremolite-calcite seg- mon optical orientation. Associated with the diopside regation, while reaction (3) must have proceeded are fine reddish-brown particles, presumably iron subsequently when phlogopite included in the core oxides, and rare sphene, suggestive of alteration became unstable. products. Phlogopite in the neighborhood of the diopside is typically ragged, embayed, and sur- Textures in zone IV rounded by small microcline crystals. The phlogopite zone is discontinuous. A discontinuous zone of Textures betweenmetadolomite and microcline-quartz coarse-grained quartz between the phlogopite and rock tremolite zones and microfractures filled with quartz suggest late-stage production of quartz. In rare casessegregations like those previously de- Diopside probably formed by the prograde reac- scribed also contain diopside. Patches of small, tion deeply embayed diopside relicts occur in the tremo- lite zone (Fig. 3d). Individual grains exhibit a com- Tr + 2Q * 3Cc:5Di + 3CO, + H,O (4)

,3,:;J"$& ir.,\g,: ,l.;: ' **\j" ;, * .-r l.: MQ TC MD Fig. 3d. Whole thin section photograph showing zones (delineated by dots) from left to rigbt, calcite marble (C), microcline-qtaftz (MQ), tremolite-calcite (TC), and metadolomite (MD). Dark patches in tremolite-calcite zone are altered diopside grains. Phlogopite zone is absent. Length of bar, I cm. SA NFORD: METAMORPHI C REACTION S 663

Table 3. Modes of reaction zones between m€tadolomite and microcline-quartz rocks, Sample No. Cl-199

No. Rock Type Ph t'4c Q ofhers* Po I nts

Me+adolomite 16.7 72.9 2.8 1.4 0.0 0.0 0,2 2230

Tremol i+e- 14.7 1.9 45.1 0.0 588 Calclte Zone Phlogop ite 3,7 0.0 0.0 12.' 18.6 3.1 363 Zone Microc I i ne- 6.2 0.l 0.1 5.1 66,0 19.9 2,A | 681 Quar-fz Zone

*l,tost I y pyr i+e, rare tourma I l ne

The reaction 3Tr * Mc + 6Cc: Di + Ph + 6CO,+ 2H,O (5) may have occurredalso. Textures indicating production of quartz and mi- crocline with consumption of diopside and phlogo- pite can be explained by reactions(4) and (5) pro- ceedingto the left in a later stageof metamorphism. The assemblagephlogopite-quartz is clearly favored over the assemblagetremolite-microcline, which sug- ^ geststhat reaction (3) also proceededto the left. co, Fig. 4. T-Xg6, diagramcalculated for 6 kbar for equilibriain Equilibrium assemblagesand metamorphichistory the system CaO-MgO-SiO2-KAISi3O8-H2O-CO2with a fluid phase present with all assemblages.Data from The observedreactions can be explained by pro- and calcite Hoschek(1973), Skippen (1974), and' Puhan and Johannes(1974). gressivemetamorphism of rocks originally composed Arrows indicate possiblefluid compositionpaths. of calcite, dolomite, qrrartz,and microcline. Mineral assemblagescharacterizing zones I through IV repre- sent fields separatedby divariant reactionsin P-T- 610' to 690oC. The observed assemblages could also X"o, space.In Figure 4 thesereactions are shown on be producedisothermally. If the fluid compositionis a T-Xco, sectionat 6 kbar. The diagram is basedon buffered to varying degreesin different rocks due to the conditions that (l) P",o I Pco,: P,, where P, is variations in bulk compositionand fluid/rock ratio, total pressure;(2) the activities of MgCO, and the fluid compositionswould follow diferent paths, CaCO, are fixed by coexistingcalcite and dolomite; as illustrated by the dotted lines. All the observed and (3) H,O and CO, mix ideally. Thermochemical mineral assemblagescould be stableat a single tem- data for reactions in the system CaO-MgO-SiOr- perature, e.g. 6OO'C.Because some zone I samples H,O-CO, are from Skippen (1974).These data have are lessthan 200 m from somezone IV samples,the been combined with experimentaldata on reaction secondcase is more plausible. (3) from Hewitt (1970)and Hoschek(1973), with cor- Also shown in Figure 4 are possible retrograde rections for the activity of CaCO, and MgCO, from paths which could producethe observedzone [V tex- Skippen,to yield thermochemicaldata for reactions tures associatedwith the reactions(4) and (5). Com- (l) and (5). Data thus derivedfor reaction(l) agree positions of calcite coexisting with dolomite yield well with experimental reversalsof Puhan and Jo- temperaturesbetween 402" and 542"C, which are hannes(1974). also plotted in Figure 4. Temperaturesare basedon Alternative meansof producing the variety of ob- the data of Goldsmith and Newton (1969).An aver- served assemblagesare shown in Figure 4. The age interchange energy (A) of 2655 cal/mole was dashed line represents progressivemetamorphism used in the present calculations. Reaction textures with completebuffering of the fluid compositionby and calcite-dolomite temperaturesindicate that ret- the rock. The different assemblagescould be ex- rograde metamorphism caused minor alteration of plained by a gradient in temperature,from about zone IV silicate assemblagesand that Ca-Mg ex- SANFORD: METAMORPHI C REACTION S change in carbonate minerals proceeded to lower sure solution, but that they are related to progressive temperatures than did discontinuous reactions in- metamorphic reactions. These layers are thicker (l-2 volving silicates. cm) than the thin (l mm), stylolite-like partings of The zonation from I to IV is possible isothermally phlogopite, dolomite, quartz, and other sparingly sol- at a wide range of temperatures from 450 to 600"C. uble minerals in calcite marbles, and they are almost The assemblage staurolite-kyanite-biotite-mus- exclusively limited to metadolomite-calcite marble covite-quartz in pelitic schists of the Setters Forma- contacts in metamorphic zones II-IV. tion immediately below the Cockeysville Marble at A theoretical analysis of the progressive meta- Texas (Fisher, l97l) suggestsminimum temperatures morphism of adjacent dolomite-rich and dolomite- and pressures of 635"C and 6.4 kbar (Thompson, free carbonate layers shows that such layers of 1976; Holdaway, l97l). Therefore, actual peak pres- "insoluble" minerals can be expected. At the low sures and temperatures were probably slightly higher temperatures of diagenesis, equilibrium calcite dis- than those of Figure 4. solves little MgCO.. We can assume that any origi- Regardless of the actual temperature-pressure- nal, metastable high-magnesium calcite breaks down fluid composition path, it is probable that the se- to low-magnesium calcite and dolomite at relatively quence of reactions encountered was the same, t.e., low temperatures and pressures.Thus, at the onset of from reaction (l) consecutively through (5). regional metamorphism, calcite in the original dolo- In most thin sections adjacent minerals are in tex- mite rock is close to pure CaCO., like that in the pre- tural equilibrium and represent high-variance assem- dominantly calcite layers and bodies. With progres- blages. Thus equilibrium has been achieved on the sive metamorphism calcite grains adjacent to scale of the individual grains. However, in many dolomite grains become richer in MgCO. by reaction cases equilibrium was not reached over distances with dolomite. In dolomite-free calcite marble there larger than a few millimeters. These will now be ex- is no reaction, and calcite remains close to its end- amined in detail. member composition. Figure 5a is an idealized dia- gram of the initial configuration at the contact be- Reaction mechanisms tween metadolomite and calcite marble, each with a small uniform content of quartz and microcline. Dif- Equilibration of calcite compositions and ferences in calcite composition between metadolo- concentrations of silicates and pyrite mite and calcite marble are based on microprobe Contacts between metadolomite and calcite analyses (see above). Chemical potential gradients marble are characterized by depletion of calcite and are thus created, as shown schematically in Figure concentrations of phlogopite, microcline, quartz, py- 5b. The concentration of CaCO, is higher in calcite rite, and tourmaline in all metamorphic zones I of the calcite marble (point A, Fig. 5b) than in calcite through IV. Reaction (l), which produces phlogo- of the metadolomite (point B, Fig. 5b). For MgCO, pite, could conceivably cause the concentration of the opposite is true. Therefore, CaCO. tends to dif- this mineral. However, this mechanism cannot ex- fuse from calcite marble to metadolomite and plain the increased abundance of the other silicates MgCO, from metadolomite to calcite marble. and pyrite relative to the carbonate minerals. Nor is Let us consider two limiting, alternative cases, ei- it likely that the components of these silicates dif- ther (l) the flux of MgCO. is much greater than that fused from the neighboring rocks toward the contact, of CaCO,, or (2) the flux of CaCO, is much greater since the chemical potentials of these components are than that of MgCOr. In the first case MgCO, would fixed by the presence throughout of minerals with be removed from the dolomite and migrate to the constant compositions (e.9., SiO, in sample C27-2OO pure calcite with which it would react to form mag- has a constant chemical potential in both metadolo- nesian calcite. The metadolomite would show a local mite and calcite marble because quartz is present loss of volume and an increased concentration of in- throughout). A more plausible explanation involves soluble material; the calcite marble would show a lo- dissolution of calcite from the contact with concen- cal increase of volume and a diminished concentra- tration of all other mineral constituents of the calcite tion of insoluble material. marble. In the second case, calcite would dissolve and re- The geometry and distribution of microcline- lease CaCO. into solution. This CaCO, would mi- quzrtz layers suggest that they did not form by pres- grate to the metadolomite where magnesian calcite SANFORD: METAMORPHIC REACTIONS

would replace dolomite. The calcite marble would rm show a local decrease of volume and an increased

cALCrlEl-CaCO3 MODE concentration of insoluble material; the metadolo- Ph) mite would show a local increaseof volume and a de- crease in concentration of insoluble material. The fact that the microcline-quartz layers are +H2O-CO2 VAPOS found within the calcite marble rath€r than at the the metadolomite suggeststhat the second a CaO border of case describes the actual situation, i.e., that CaCO, flux exceeded that of MgCOr. Microcline, quartz, and other relatively insoluble minerals responded r0o passively to local volume changes. The result is MODT shown diagrammatically in Figure 5c, and is similar t%) to the effect described by Smigelskas and Kirkendall (1946). The greater solubility of CaCO, relative to MgCO. in aqueous solutions in equilibrium with cal- cite and dolomite (Rosenberg and Holland, 1964; Rosenberg et al., 1967) supports the hypothesis of t xarsps greater CaCO, flux. The end product of this process would be the com- plete dissolution of calcite from original calcite UX Br-M9O marble lenses and layers, with the result that the minerals qvartz, microcline, pyrite, and tourmaline, MODE initially disseminated in minor quantities, would be- t%l come major constituents.

Mechanism of reaction (I) Reaction (l) results in the formation, in zones I and II, of layers and rims of phlogopite and calcite rsio between metadolomite and microcline-bearing rock. Phlogopite produced by this reaction commonly is concentrated immediately adjacent to the microcline- s ^-Mgoq). ^ bearing calcite marble or the microcline-quartz lay- :ru\ "€s'or-"

MODE Fig. 5. Sequence of reaction zones between metadolomite and t%l calcite marble: progression from diagenesis to intermediate metamorphic grade. Modal mineralogy at each stage is plotted schematically against distance (approximate length, 6 cm). Fig. 5a shows the hypothetical initial confguration. Figs. 5c, 5e, 59, and 5i are idealized from observed mineralogy. For each metamorphic +H?o-co2vaPoR zone the appropriate chemical potential (p) diagram is shown. +CALCITE F s,o Diagrams are constructed from Gibbs-Duhem relations among solid phases and fluid. Reaction paths offluid are shown by dotted MrcEocLrNE IPHLOGOPTTE lines. (a) Initial con-figuration. (b) pCaO us. pMgO, saturation lltl xcacos PGo and surfaces of calcite (curved as a result of solid solution) and (c) Reaction zone mineralogy at onset of reaction (1). €MsOEA dolomite. (d) p.KAlSi3Os vs. pMgO projected along pCaO with calcite present. Topology in metamorphic zone I. (e) Reaction zone mineralogy in metamorphic zone I. (f) pSiO, vs. pMgO projected along pCaO and pKAlSi3Or with calcite present. Topology of metamorphic zone II. (g) Reaction zonc mineralogy in m€tamorphic zone II. (h) pSiO2 vs. trr.MgO as in Fig. 5f. Topology of metamorphic zone III. (i) Reaction zone mineralogy in metamorphic zone III. SANFORD: M ETAMORPHIC REACTIONS

ers, whereas calcite forms a zone adjacent to the calcite marble or microcline-quartz rock. The reac- metadolomite. The reaction mechanism can be deter- tion zone typically consistsof both tremolite and cal- mined by considering two alternative limiting cases. cite. but with tremolite more abundant next to the In Figure 5d chemical potentials in the system quartz-bearingrock and calcite more abundant next CaO-MgO-KAISi3O8-HrO-CO, at constant P,(: P,, to the metadolomite.As in the previous section we fluid pressure), Z, and X.o, are projected along the can considertwo alternativelimiting pairs of coupled pCaO axis onto the pMgO-pKAlSi,O, plane. Calcite reactions. is present in the area contoured for varying CaCO, In Figure 5f chemical potentials in the system concentration in the calcite (X.^.".). Point A repre- CaO-MgO-SiOr-KAlSirOr-H2O-CO2at constant sents the calcite marble assemblage Mc + Ph + Cc, P,(: P), T, and X.o,are projectedalong both trlCaO and B represents the metadolomite assemblage Do * and pKAlSi.O, axes onto the pMgO-pSiO, plane. Ph + cc. Calcite is present in the area contoured for varying Reaction (l) takes place by two conjugatereac- CaCO, concentration in calcite. One of these con- tions. One occurswhere microcline breaksdown (A), tours coincideswith the boundary betweenthe stabil- and the other where dolomite breaks down (B). As ity fields of microcline and phlogopite. This bound- one alternativewe can assumethat MgO is relatively ary is drawn so that microcline, phlogopite, and immobile, and the reaction occurssolely by the mass quartz coexist rather than microcline, phlogopite, transfer of KAlSirO.. Coupled reactions for micro- and tremolite (or dolomite), to be consistentwith ob- cline and dolomite reacting are, respectively: servedphase relations in zone II. Point A represents Mc: KAISi:O' (la) the calcite marble assemblageQ + Tr * Cc, and 3Do + KAlSi3Os+ H,O: Ph + 3Cc+ 3CO, (lb) point B the metadolomiteassemblage Do * Tr * Cc. If it is assumedthat MgO is relatively immobile In this hypothetical case microcline dissolves,and and SiO, is the diffusing component, we have the KAlSi3O* in solution (probably as a variety of sepa- coupled reactions: rate species)diffuses to dolomite, with which it reacts to produce phlogopite and calcite.The resulting tex- 8Q:8SiO, (2a) : ture would have phlogopite + calcite in I :3 molar 5Do + 8SiO,+ H,O Tr * 3Cc + 7CO2 Qb) proportions replacingdolomite. which occur next to the quartz-bearing rock and However,the observedtexture showsa zoneof cal- metadolomite,respectively. The predicted texture is cite replacing dolomite and another zone of phlogo- a single zone of tremolite and calcite in a I :3 molar pite replacingmicrocline. This texture would be pro- ratio replacing dolomite. ducedby the secondalternative, that is if KAlSirO* is The secondalternative is for SiO, to be immobile relatively immobile and MgO is the primary diffus- relative to MgO with the coupled reactions: ing component.In this casethe coupledreactions are: 3Do:3Cc * 3MgO+ 3CO, (lc) 5Do: 5Cc* 5MgO + 5CO (2c\ :Tr Mc +.3MgO* H,O : Ph (ld) 8Q + 5MgO 12Cc + H,O t 2COz Qd)

Dolomite reacts to calcite, releasing MgO and COr. In this casetwo monomineraliczones, one consisting The MgO migrates to the microcline-bearing rock, of calcite and the other tremolite, would replace where phlogopite forms. Two monomineralic zones dolomite and quartz respectively. of calcite and phlogopite, respectively, are formed. The observed textures are intermediate between Because these predicted textures closely resemble the the two predictedtextures, as illustrated in Figure 59. observed textures the second alternative is probably Tremolite and calcite occur together(as predictedby correct, that is, KAlSi3Os is relatively immobile and the first case),but tremolite tends to be concentrated reaction (l) occurs by MgO diffusion which couples near the quartz-bearing rock and calcite is more the local reactions (lc) and (ld). Figure 5e shows the abundant next to the metadolomite(as in the second effects of reaction (l) on the idealized modal cross case).Thus both MgO and SiO, are significant dif- section between metadolomite and calcite marble. fusing components.The relative abundancesof cal- These effects are superimposed on the effects illus- cite and tremolite suggestthat MgO diffusion pre- trated in Figure 5c. dominatesover SiO, diffusion. Mechanism of reaction (2) Mechanismof reaction (3) In zone II reaction 2 creates a zone of calcite + Texturesin zone III resemblethose in zone I[, ex- tremolite between metadolomite and quartz-bearing cept that phlogopite is ragged and embayedby mi- SANFORD: METAMORPHIC REACTIONS 667 crocline. Hypothetical limiting cases for the mecha- gradients. The fluxes determined from textural evi- nism of reaction (3) are for MgO, SiOr, or KAlSirO, dence are consistent with known solubility data. At to be mobile. elevated temperatures aqueous solutions in equilib- Figure 5h shows the chemical potential relations rium with calcite and dolomite dissolve more CaCO. with the same axes and constraints as Figure 5f. than MgCO. (Rosenberg and Holland, 1964; Rosen- Comparison with Figure 5f shows that reaction 3 berg et al., 1967). Therefore, greater transport of produces a shift in the relationship between chemical CaCO, is to be expected, all other factors being potentials defined by the assemblage Mc + Ph + Cc equal. The component KAlSi3O' probably exists in and those defined by Q + Tr + Cc. solution as a variety of species.On the scale of a thin Coupled local reactions which conserve MgO are: section there appears to be no gain or loss of KrO, Al2O3, or SiO, which would cause additional phases 5Ph + 6Cc * 24SiO, : 3Tr * sKAlSi"O" Thus the transport of KrO and SiO, is lirn- + 2IH2O+ 6CO' (3a) to appear. concentration and mobility of AlrO, in 5KAlSi3O8:5Mc (3b) ited by the of the known low solubility of 24Q: 24SiO2 (3c) the fluid. Because AlrO. even in corundum-saturated solutions (Ander- In this case phlogopite would be rimmed by tremo- son and Burnham, 1967; Roberson and Hem, 1969), lite as reaction 3a proceeds. Silica consumed by 3a it is not surprising that MgO is the major diffusing would be supplied by the dissolution of quartz (reac- component for reaction (l). tion 3c). Excess KAlSi3Os produced by 3a would dif- Sitica in aqueous solutions probably occurs as hy- fuse down its chemical potential gradient toward the drated complexes such as HoSiOo (Alexander et al., metadolomite and precipitate as microcline (reaction 1954; Engelhardt et al., 1975). [n carbonate rocks 3b). with an intergranular fluid rich in CO,, the activity of Local reactions which conserve SiO, are: HrO is much less than unity. Therefore, the solubility in equilibrium with qvartz is considerably 24Q +6Cc*l5MgO+3H,O:3Tr+6CO, (3d) of silica lower than in COr-free aqueous solutions (Shettel, 5Ph: 5KAlSi3O8+ 15MgO + 5H,O (3e) The solubility of MgO could be relatively 5KAlSi3O8: 5Mc (30 1974). higher in carbonate-rich fluids if it occurs as an asso- Tremolite would replace qtartz (reaction 3d), ciated carbonate species such as MgCOr. Therefore, phlogopite would dissolve congruently (reaction 3e), it is possible for MgO flux to exceed that of SiOr. and microcline would grow by precipitation (reaction The few experimental kinetic investigations rele- 3f). vant to reactions in the Cockeysville Marble tend to The third case, with KAlSi3O8 conserved, is illus- support these interpretations. In the presence of an trated by the reactions: aqueous fluid the reaction calcite * quartz : wollas- + proceeded by means of wollastonite rim 5Ph: 5Mc + l5MgO + 5H,O (3g) tonite CO, growth on calcite and dissolution of quartz (Gordon, 24SiO, * 6Cc + l5MgO + 3H,O : 3Tr * 6CO, (3h) l97l). In contrast, wollastonite rims grew on quartz Q: SiO, (3r) and calcite dissolved in experiments open to CO, and Phlogopite would be replaced by microcline (reac- with no water added (Kridelbaugh,1973). These data tion 3g), tremolite would grow by precipitation and are consistent with lower SiO' solubility relative to replacement of calcite (reaction 3h), and quafiz that of CaO in more CO,-rich fluids. Reaction zones would be dissolved (reaction 3i). This is clearly the between pressed pellets of various oxides and hy- actual case. Microcline replaces phlogopite, in- droxides were produced in experiments with water dicating KAlSi3Os is conserved. Quartz rriust be dis- present (Ildefonse and Gabis, 1976). Although not solved to supply SiO. for tremolite growth in the directly comparable because of differences in fluid tremolite-calcite zone. MgO is transported from the composition, these experiments illustrate a principle zone where microcline replaces phlogopite to the relevant to the present study. Quartz and brucite sep- zone of growing tremolite. The cumulative effect is arated by graphite reacted to talc and olivine. Quartz shown in Figure 5i. dissolved leaving a void on that side of the graphite. The reaction proceeded by means of silica dis- Discussion solution and diffusion through the graphite to the The relative transpoft or flux of a particular com- side originally containing brucite. Thus, the total vol- ponent is a function of concentration (in the inter- ume of solids decreased on the side with the more granular medium), diffusion rates, and concentration soluble component, and the volume of solids in- 668 SANFORD: METAMORPHIC REACTIONS

creasedon the side with the lesssoluble component. Acknowledgments In the rocks of the presentstudy, this principle ac- Harry T. Campbell Sons'Co., Inc., now Flintkote Stone Prod- countsfor the decreaseof volume (suggesteCby con- ucts Co., generouslyallowed this researchto be carried out in their centration of quartz, microcline, phlogopite, and ac- quarry and provided accessto their drill cores. Carol Geldmacher cessoryminerals) due to the preferential dissolution and Ed Simkin were particularly helpful. ProfessorsHans P. Eug- W. Fisher for of calcite, diffusion of from ster and George deserve special thanks assistance CaO calcite marble to with early versions of this paper while I was a student at Th€ metadolomite,and the relative immobility of MgO. Jobns Hopkins University. Later drafts were improved by reviews Unlike the experiments,the natural rocks contain no from Alan B. Thompson, Jobn B. Brady, Juergen Reinhardt, voids,because of lithostatic pressure.Instead, the ini- Bruce G. Aitken, and Timothy P. Loomis. tial contact,marked by relatively insoluble minerals, moveswith respectto a volume-fixedreference frame References in the direction oppositeto that of the major diffus- ing species(as if, in the experimentof Ildefonseand Albee, A. L. and L. Ray (1970) Correction factors for electron probe phosphates, Gabis, the growing talc microanalysisof silicates,oxides, carbonates, on the brucite side expanded and sulfates.Anal. Chem.,42, L408-1414. and pushed the graphite marker along the experi- Alexander,G. B., W. H. Hestonand R. K. Iler (1954)The solubil- mental capsuleto fill the void on the quartz side). ity of amorphoussilica in water. J. Phys.Chem., 58, 453455. Anderson,G. M. and C. W. Burnham (1967)Reactions of quartz and corundum with aqueous chloride and hydroxide solutions Conclusions at high temperaturesand pressur€s.Am. J. Sci.,265, 12-27. Barrett,P. J. (1964)Residual seamq and c€mentationin Oligocene Simple conceptsof progressivemetamolphism and shell calcarenites,Te Kuiti Group. "/. Sediment.Petrol., 34,52+ relative chemical mobility are sufficient to explain a 531. variety of complex reaction textures found in the Bathhurst,R. G. C. (1975)Carbonate Sediments and Their Diage- metamorphosedcarbonate rocks at Texas,Maryland. nesis,2r.d ed. Elsevier,Amsterdam. (1968) Alternative choices of "funmobile" and "diffusing" Bence,A. E. and A. L. Albee Empirical corection factors for the electron microanalysis of silicates and oxides. J. Geol., componentsin each reaction imply alternative pairs 76.382403. of coupled local reactionsand lead to predictionsof Brady, J. B. (1977)Metasomatic zones in metamorphicrocks. Geo- diagnostic textures. It has been shown that calcite- chim. Cosmochim.Acta, 41, ll3-125. bearing dolomite will react with pure limestone when Burt, D. M. (1974)Metasomatic zoning in Ca-Fe-Si exoskarns.In A. W. Hofman, B. J. Giletti, H. Yoder, each are metamorphosedto temperatureswhere cal- S. Jr. and R. A. Yund, Eds., GeochemicalTransport and Kinetics,p.287-293. Carnegie cite in equilibrium with dolomite acceptssignifisanl Inst. WashingtonPubl. 634. MgCO, in solid solution. The greater transport of Carmichael,D. M. (1969) On the mechanismof progrademeta- CaO from the calcite marble over that of MgO from morphic reactions in quartz-bearing pelitic rocks. Contrib. Min- the metadolomiteleads to concentrationsof silicates eral. Petrol., 20, 244-267. (1957) and pyrite in layers and rims at Choquette,P. W. Petrographyand Structureofthe Cockeys- the metadolomite- ville Formation near Bailimore, Maryland. Ph.D. Thesis, The calcite marble contacts.Layers of nearly pure quartz John Hopkins University, Baltimore, Maryland. and microcline may result. Textures of phlogopite - (1960)Petrology and structureof the CockeysvilleForma- formed by reaction (l) indicate that MgO transport tion (pre-Silurian) near Baltimore, Maryland. Geol. Soc. Am. greatly exceedsthat of KAlSi.O.. Textures of the Bull., 71, 1027-1052. (1976) geology tremolite in reaction (2) suggestthat both MgO and Crowley, W. P. The of the crystallinerocks near Baltimore and its bearing on the evolution of the eastern Mary- SiO, are important diffusing components,with MgO land piedmont. Maryland Geol. Sum., Rept. Invest. No. 27. possiblybeing the dominant one. Microcline forming DeGroot, K. (1967)Experimental dedolomitization. J. Sediment. at the expenseof phlogopite indicatesthat MgO and Petrol., 37, 1216-1220. SiO, are the dominant mobile componentsfor reac- Engelhardt, G. von, D. Zeigan, H. Janke, D. Hoebbel and Wieker (1975) Zttr Abhiingigkeit tion (3) and that KAlSirO* was relatively inert. W. der Struktur der Sili- katanionen in wiissrigenNatriumsilikatldsungen vom Na: Si- The availableevidence suggests that for carbonate Verhaltnis. Z. anorg.allg. Chem.,4l8, 17-28. rocks metamorphosedat moderate metamorphic Eugster,H. P. (1970)Thermal and ionic equilibria among mus- grade, the fluxes of the componentsdecrcase in the covite, K-feldspar and aluminosilicate assemblages.Fortschr. order CaO > MgO > SiO2> KAlSirO.. The relative Mineral., 47, 106-123. (1970) fluxes are consistent with available solubility Fisher, G. W. The application of ionic equilibria to meta- data, morphic diferentiation: an example. Contrib. Mineral. Petrol., that is, the magnitudeof the flux of a particular spe- 29,9r-r0f. ciesis proportional to its solubility in the fluid: - (1971)Kyanite-, staurolite-,and garnet-bearingschists in SANFORD: METAMORPHIC REACTIONS 669

the Setters Formation, Maryland Piedmont. Geol. Soc. Am. Puhan, D. and W. Johannes (1974) Experimentelle Untersuchung Bull., 82,229-232. der Reaktion Dolomit + Kalifeldspat + H2O = Phlogopit + Foster, C. T., Ir. (1977) Mass transfer in sillimanite-bearing pelitic Calcil + CO2. Contrib. Mineral. Petrol., 48,23-31. schists near Rangeley, Maine Am. Mineral., 62,727-'146. Roberson, C. E. and J. D. Hem (1969) Solubility of aluminum in - (1978) Thermodynamic models of aluminum silicate reac- the presence of hydroxide, fluoride, and sulfate. U. S. Geol. tion textures (abstr.). Geol. Soc. Am. Abstracts tf ith Protrams, Sum. Water Supply Pap. 1827-C. r0,403. Rosenberg, P. E. and H. D. Holland (1964) Calcite-dolomite- FriedmarU G. M. and J. E. Sanders (1967) Origin and occurrence magnesite stability relations in solutions at elevated temper- of dolostones. In G. V. Chilingar, H. J. Bissell and R. W. Fair- atvres. 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Calculation of diffusion during metasomatic reactions in the presence of water some P-7LX (Fe-Mg) phase relations. Am. J. Sci., 276,425- at 550"C and 1000 bars. Geochim. Cosmochim. Acta, 40, 297- 454. 303. Thompson, J. B., Jr. (1959) Local equilibrium in metasomatic Joesten,R. (1974)Local equilibrium and metasomatic growth of processes. In P. H. Abelson, Ed., Researchesin Geochemistry, p. zoned calc-silicate nodules from a contact aureole, Christmas 427457. Wiley, New York. Mountains, Big Bend region, Texas. Am. J. Sci., 274,876-901. Vidale, R. J. (1969) Metasomatism in a chemical gradient and the - (1977) Evolution of mineral assemblage zoning in diffu- formation of calc-silicate bands. Am. J. Sci., 267,85'l-8'14. sion metasomatism. Geochim. Cosmochim. A cta, 4 l, 6494'1 0. - and D. A. Hewitt (1973) "Mobile" components in the for- Kridelbaugh, S. J. (1973) The kinetics of the reaction: calcite + mation of calc-silicate bands. Am. 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