The eye of the petrographer, the mind of the petrologistThe eye of the petrographer, the mind of the petrologist Pnrpn RosrNsoN Department of Geology and Geography, University of Massachusetts,

American Mineralogist, Volume 76, pages 1781-1810' 1991

The eye of the petrographer, the mind of the petrologist

Pnrpn RosrNsoNDepartment of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003, U.S.A-

Ansrucr

To bring meaningful numbers out of metamorphic rocks, one must first understandwhat the numbers represent. An idealized objective in metamorphic studies is to dem-onstrate that coexisting minerals approached equilibrium. In a few rocks this can be donereasonably, for example, in coarse-grained nodules from kimberlite that were rapidlyquenched from mantle conditions. More typical, and more interesting, are rocks showingdisequilibrium by mineral zoning and reaction textures at various scales. These provide a

view, if correctly interpreted, into a sequence of events atd P-T conditions. In makingquantitative estimates, a problem is to decide correctly which minerals were in equilibriumwith which other minerals, and what types of reactions and processes actually took place.

These decisions depend on the integration of both petrography and phase equilibriumtheory. Failing this, a false interpretation of rock history can be made. Features in meta-morphic rocks in central Massachusetts, including schists bearing strongly zoned garnetsproduced by four different types ofreactions, and a newly described occulTence ofwollas-tonite marble provide opportunities to explore these ideas.

In many examples, rock history can only be charted through study of the completegeologic setting at outcrop or larger scale. Particularly spectacular are well-preserved py-

roxene granulites, gabbros, and diabases in the Scandinavian Caledonides that are locallytransformed along shear zones into eclogite and amphibolite. Lacking knowledge of the

surroundings, there would be no way to know that the original hlgh f rocks survivedeclogite-facies overprinting with few to no mineralogical traces. Their field setting dem-onstrates the importance of structural environment, kinetics, and fluids in producing dif-

ferent minerals from the same rock composition along the same P'T path.

INrnonucrroN

In petrologic studies, as in field geology, the observa-tions that are possible are conditioned by the experienceand training of the observer. Petrologic research is anattempt to define the sequence of events in the formationofthe rocks, the conditions under which the events hap-pened, and why they happened. Petrology can never betotally quantitative, i.e., "getting numbers out of rocks,"but the petrologist's familiarity and deftness with mea-surements, calculations, and the constraints imposed byphase equilibria are essential. Also essential is observa-tional and descriptive petrography from the atomicthrough the microscopic and macroscopic to the petro-logic map scale, providing the framework for interpreta-tions. Maps can only be made by integrating knowledgegained from submicroscopic and microscopic scales usingthe hand lens and unaided eye; thus the most truthfulpetrologic maps are done by those most skilled in bothpetrography and mapping. These ideas are illustrated byexamples where specialized knowledge or backgroundopened the eyes of the petrographer, leading to unex-pected interpretations. These ideas are explored in threeparts, each chosen to represent a different scale ofobser-vation and thinking but having in common the use ofphase relations to try to understand the sequence of meta-

morphic events. The first part concerns the textural andphase relations between garnet grains and their immedi-

ate surroundings. The second part deals with features at

the scale of a whole thin section in a wollastonite marblewhere variations in local bulk compositions are shown to

have a dramatic effect on phase relations. The third sec-

tion covers relations between rocks at the scale of a large

outcrop or a mountainside, where an understanding of

the total setting is vital to understanding rock history.

A theme of this paper is "getting numbers out of rocks"and concerns the need for careful and thorough petro-graphic observations in thin sections and in the field if

one is to make meaningful maps of igneous and meta-

morphic rocks and if one is to combine theory and ex-periment to make useful petrogenetic models' In the game

of getting numbers out of rocks, much of my attentionhas been focused on the central Massachusetts Acadian

metamorphic high, shown on a metamorphic map of New

England in Figure I and in closer detail in Figure 2, wilh

metamorphic zones ranging from chlorite zone (C) in the

extreme northwest through Zone I (kyanite-muscovite-

staurolite) through Zone YI (sillimanite-orthoclase-gar-net-cordierite). In this whole area I am familiar with only

two "direct" numerical determinations. The first (Fig. 3)

is a slightly flawed number 8 found in a granulite facies

0003-o04x/9 l / l l l 2-l 78 r $02.00 1 7 8 I

t{ilA',5t(\ r , / t ;

i":#' / - l!Y,y,yt-'.1,r/

1782 ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST

Sill iman ite-Orthoclase-Garnet-Cordieri le Zone

Sill imanite-Orthoclase Zone

Sill imanite-1,/uscovite Zone

Kyanite-Slaurolite Zone

Andalusite-Staurol t€ Zone

Garnel Zone

Bioti le and Chlorite Zon6s

Weakly Metamorphosed

"3 A p p r o x i m o t e S c o l e2OO Km

Fig. 1. Generalized map of Acadian and Taconian metamorphic zones in southwestern New England. Modified from Osberg etal. (1989) based on a compilation by Robinson (1983) and an unpublished compilation, Metamorphic Map of the U.S.-CanadianAppalachians, by G.W. Fisher and W.E. Trzcienski. Heavy dashed line represents the approximate boundary where peak meta-morphism is Acadian to the east and Taconian to the west.

pegmatite formed by spherical aggregates of graphiteplatelets enclosed in a crystal of orthoclase. The second(Fig. a) is a much larger number 2 discovered in a garnet-rich schist in the same area by Elaine Padovani. Inconsidering the question of geothermometry and geoba-rometry in such a region, it is natural to turn to the com-positions ofgarnets in pelites as well as to considerationof garnet stability and geothermobarometry in a newlydescribed wollastonite-bearing marble.

INrnnpnnr.q,TroN oF GARNET coMposrrroNs rNMETAMORPHIC ROCKS

Aside from its aesthetic values, garnet has captured theattention of quantitative metamorphic petrologists forthree reasons: (l) It holds very high concentrations ofFeand Mn relative to most other coexisting minerals, andhence distribution coefficients are sensitive to tempera-ture. (2) It has high density compared with most coexist-ing minerals, and hence reactions involving it are likelyto be sensitive to pressure changes. (3) Elemental difiil-sion is relatively slow in garnet compared to most otherminerals, except at the highest temperatures, so that dis-equilibrium features are commonly preserved, and theseprovide a key, if properly interpreted, to the changingconditions of metamorphism. These disequilibrium fea-tures are the subject ofthis part ofthe paper.

The mention of metamorphic garnets in central Mas-sachusetts opens a whole can of worms (Fig. 5). In thiscase the heads of the wonns are the cores of the gamets,

and the tails are the rims. The roman numerals corre-spond to the different metamorphic zones in central Mas-sachusetts, and the compositional trajectories are differ-ent in each of the metamorphic zones. In the subsectionsthat follow, each of the major types of garnet zoning foundin central Massachusetts is discussed, the importance ofcareful petrographic observations to deduce equilibria andreaction mechanisms are emphasized, and the particularpitfalls involved in making geothermometric interpreta-tions are pointed out.

In the discussion that follows, the terms "prograde"and "retrograde" appear frequently because they are partof the everyday language of metamorphic petrologists, yetthey are fraught with ambiguity in the context of complexP-7 paths. A prograde path is commonly thought of asone involving a temperature increase, a dehydration, orboth. Yet prograde metamorphism to a high-pressure as-semblage might involve mainly a pressure increase withlittle or no increase of temperature. To consider progradeas the direction in which minerals grow would be entirelyerroneous because it is quite possible for some mineralsto grow with decreasing temperature. The ambiguity maybe epitomized by considering a hypothetical equilibriumA : B with a positive P-Z slope. P-T trajectoies, alsowith positive slopes, may be drawn to cross this reactionin different up-temperature directions so that A goes toB or alternatively so that B goes to A. Both senses of thereaction could independently be described as prograde.Thus, the best that one can say is that prograde meta-morphism involves a movement more or less directly

Fig. 2. Generalized map of Acadian and younger metamor-phic zones in west-central Massachusetts and adjacent states.Modified from Schumacher et al. (1989). Zones I to VI afterTracy et al. (1976). Diamonds indicate locations of sillimanitepseudomorphs after andalusite; closed circles indicate orthopy-roxene in metamorphosed mafic and intermediate igneous rocks;open circles indicate locations of relict granulite-facies assem-blages in Pelham gneiss dome; diagonal shading indicates zonesof retrograde metamorphism at New Salem and Quabbin Hill.The area of zone I between the retrograde zones and the Meso-zoic Connecticut Valley fault appears to be an area of intenselate Paleozoic metamorphic and tectonic overprinting.

away from lhe P-T origin in a way that may or may notinvolve significantly increasing pressure.

Zoning produced during prograde garnet growth

In the lowest grade zones in central New England, thegarnets are commonly well formed, are nearly euhedral,and show a characteristic zoning pattern with Mn-richcores zoning to less Mn-rich rims with first an increasingMg/Fe ratio and then a decreasing ratio (Figs. 6 and 7).With some reservations, these may possibly be taken asexamples of garnet gpowth under the simplest conditions,where diffusion in garnet is minimal to nonexistent. Onlythe surface of the garnet is in equilibrium with the matrix.

I 783

Fig. 3. The number 8 formed by spherically arranged graph-ite platelets enclosed in orthoclase of a pegmatite in Zone VI,central Massachusetts. For location see Robinson et al. (1986),stop 7.

A sequence of garnet compositions can be studied thatrecords garnet growth. Unless inclusions are preserved,only garnet rim and matrix compositions can be used forgeothermometry. Inclusions may reequilibrate withingarnet, but this requires diffusion. Note that garnet growthneed not be prograde but may be retrograde, depending

Fig. 4. The number 2 formed by scattered garnet in silli-manite-orthoclase-garnet-cordierite-biotite schist from Zone Yl,central Massachusetts. For location see Robinson et al. (1986),stop I 1.

ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST

I \

At t(({ \\ \ l ' \ \rrt( t-ra-

\

z

ozoE.6

/

1784

Fe 'lo zo 30 Mg -

oo

Fig. 5. Summary plot of compositional trajectories of zonedgarnets from central Massachusetts in terms of Fe, Mn, and Mg.Numerals I-VI refer to metamorphic zones in central Massa-chusetts, and P refers to partially resorbed granulite-facies garnetfrom the Pelham dome. From Robinson et al. (1989).

critically on the P-7path and the reaction isopleths. Thisconcept of growth, under either a progade net-transferreaction or a retrograde net-transfer reaction, is illustrat-ed in Figure 8. In aluminous rocks, Mn is most stronglypartitioned into garnet as compared with any other phase.For this reason, it is the element that appears most abun-dantly in early formed garnet and is also the last to bedepleted from garnet in any garnet resorption reaction in

ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST

Mg+

Fig. 7. Compositional trajectories oftwo typical zoned gar-nets from zone I, central Massachusetts in terms of Fe, Mn, andMg. Core compositions are at most Mn-rich points next to spec-imen number. Specimen 908 is staurolite-garnet-biotite-mus-covite schist; specimen 4F5 is similar but also with kyanite.From Tracy et al. (1976).

which the garnet reequilibrates. (It is interesting that insome skarns containing mostly andradite garnet, how-ever, Mn is not partitioned into garnet but into pyrox-ene.)

What are the means by which examples of progradegrowth zoning in garnet are destroyed or altered to ob-scure their story? First of all, prograde or retrograde dif-fusional ion exchange between garnet and its matrix (Fig.9), here taken to be biotite, could destroy the near-rimrecord. A second possibility is a retrograde (or possiblyprograde) diffusional net-transfer reaction (Fig. l0) bywhich the outside of the garnet is partially resorbed whilethose components retained by garnet actively diffuse to-

Fig. 6. Compositional maps of a zoned garnet in sample 908from staurolite-garnet-biotite-muscovite schist of zone I, centralMassachusetts. Contours based on Fel(Fe + Mg + Mn + Ca),etc., are Fe 79-880/0, Mg 7-llo/o, Ca 2-5o/o, and Mn l-90l0.Adapted from Tracy et al. (1976).

PROGRADE. NET TRANSFER,GARNET GBOWTH REACTION

Fe

BETROGRADE, NET TRANSFER.GABNET GROWTH REACTION

Fe

Fig. 8. AFM muscovite projections and Fe-Mn-Mg plotsshowing the nature of prograde and retrograde garnet-growthreactions. In each example, reaction proceeds from the solid tothe dashed tie lines, and in each example there is a net increasein the modal amount of garnet. The prograde reaction involvesdehydration but may not involve significant temperature in-crease, as shown by the nearly parallel tie lines in the AFM view.The retrograde reaction does involve a significant temperaturedrop, as shown by the crossing tie lines in the AFM view. Mod-ified from Roll (1987).

Mg

ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST 1 785

t

t3t2t 1t6

Gamet

Gamett *- t ;

t 1

ts

DistanceFig. 10. Hypothetical composition profiles ofgarnet and bi-

otite for a retrogtade net-transfer garnet resorption reaction fortime lo through r-. Diffirsion through garnet is a strong functionof temperature, whereas biotite is assumed to remain homoge-neous throughout the period ofgarnet equilibration. Because ofresorption, the garnet interface retreats in favor ofbiotite. Mod-ified from Spear (1991).

10'C/]VIa

%w725 (initial)

At6

! r

Distance --+- Biotite

- 1 2

+ L ?

Fig. 9. Hypothetical composition profiles ofgarnet and bio-tite for retrograde ion exchange for time lo through l-. Diffusionin garnet is a strong function of temperature, whereas biotite isassumed to homogenize throughout the period ofequilibrationwith garnet. From Spear (1991).

ward the interior. A third possibility, recently exploredby Spear and Florence (1991), is internal diffusive mod-ification of the primary zoning profile, which would pro-duce a new and obviously false record oforiginal garnetgrowth compositions. In a thermal model (Fig. I l) ofdiffusional ion exchange between garnet and matrix bio-tite done by Spear and Florence (1991), using diffusiondata from Cygan and Lasaga (1985) and assuming a qua-si-infinite reservoir of biotite host, we can see the limitsof diffusional reequilibration at a cooling rate of l0 "Cper m.y. beginning al.725 t and tailing down to a limitclose to 500 'C.

Karabinos (1983) has described garnet-zone pelites in

1.00

.96

l 786

Fig. 12. Textural map of garnet from eastern Vermont,showing two cycles ofprograde growth. From Karabinos (1983).Line C-R is the compositional profile shown in Figure 13, andTU is the point oftextural unconformity supposed by Karabinosto be the outer limit of the original g:rnet at the end of the stageofretrogression. I argue that resorption proceeded slightly deeperinto the crystal.

Hollocher (1981, 1987) has describedjust such an ex-ample of surface dissolution of garnet to produce chlorite(Figs. l5 and 16) in the New Salem retrograde zone, cen-tral Massachusetts (see Fig. 2). In this case there are de-tailed analyses of the full spectrum of relict primary zon-


RzR3

l m m

R1

end of lstp.rogrodestoge

end ofretrogression

T

MnF"

c

[.,.-...T] oo,n.t

MnFe

o

E.ooE,

IR

. lo

, .o2o . l 2

i .o,M g

F e

2mm ITU

Dis tonceFig. 13. Composition profile C-R in the garnet shown in Fig-

ure 12, modified from Karabinos (1983). Karabinos suggests thatthe zone with high Mn/Fe ratio was produced during retrogradegarnet resorption; I believe it was produced early in the secondstage ofgarnet growth.

-G-_.t

\!- -a r

-a -

-A.AA'41- -o--o--q--o--a--a- ' -

t t l

I

cl - l orortr , minor mico

ffi cntorire, minor muscovife

lr.-Tf, stourotite E ur..ouite, minor biofireFig. 15. Variably resorbed garnets from the New Salem ret-

rogradezone studied by Hollocher (1987). Numbers indicate thevariable spessartine content obviously left over from the periodof initial prograde garnet growth.

c

end of 2ndp.rogrodesrqge

Fig. 14. History ofgarnet growth, resorption, and growth forthe sample illustrated in Figures 12 and 13, as portrayed byKarabinos (1983).

( R 2 ) U O | A(R3) 64-438

l l T t t t l +Somole NS 7 |Ret rogroded Gornet

Fig. 16. Drawing of partially resorbed garnet from New Sa-lem retrograde zone by Hollocher (1981). Much of garnet is re-placed by chlorite, but where it remains it retains the spessartinecontents from the much earlier stage ofprograde garnet $owth.

ing in garnets that are partially dissolved in a regionalhydration reaction. This demonstrates that the volumeof garnet that was in equilibrium with the matrix duringthe retrograde reaction was zero, but the surface of thegarnet was being actively corroded to produce new matrixminerals.

In a more complex situation, garnet is growing, butdiffusion is causing reequilibration within garnet and pro-gressively destroying the earlier tape recorder message.

1787

Fig. 18. Compositional trajectories oftwo typical zoned gar-nets from zones IV and V, central Massachusetts, in terms ofFe, Mn, and Mg. Core compositions are at most Mg-rich pointsnext to specimen number. Both specimens are sillimanite-ortho-clase-garnet-biotite schists in which garnet has been resorbed bythe reaction garnet + orthoclase + HrO : sillimanite + biotite.From Tracy et al. (1976).

Spear (1991) has recently explored this situation in detailand shown that the last grown part ofthe prograde zonedgarnet, which is likely to record the highest temperature,is also the first to be destroyed by retrograde ion exchangeor resorption reactions.

Zoning produced by retrograde garnet resorption

In regions of higher grade metamorphism, the recordof earlier history, if there was one, was destroyed by dif-fusion, and the garnets are largely unzoned. What hap-pens beyond the peak of metamorphism depends criti-cally on environmental conditions and whether garnetresorption reactions take place. The garnet illustrated inFigure 17 is an irregularly shaped one from the silliman-ite-orthoclase-muscovite zone in central Massachusetts.It appears to have a continuous exterior rim of Mn andFe enrichment and Mg depletion wherever the garnet istouching the matrix and regardless of what phase is di-rectly adjacent in the matrix. This is interpreted to meanthat the exterior part of the garnet was equilibrated witha pervasive phase, probably an aqueous fluid, that was in


Mg+

g l m m

Fig. 17. Composition maps of a garnet in sample 9338 from the sillimanite-orthoclase-muscovite zone (Zone IV), central

Massachusetts. Contours based on Fe/(Fe + Mg + Mn + Ca), etc., areFeT3-77Vo; Mg l2-180lo; Mn 5-80/0. Continuous rims of

higher Fe, lower Mg, and higher Mn around this irregular garnet are characteristic ofthis zone and indicate a continuous retrogfade,

net-transfer, hydration resorption reaction through the medium ofan interstitial fluid. Modified from Tracy et al. (1976)'

l 788 ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST

Fig. 19. A series of model sections at constant T, P, andhumidity showing equilibrium compositions of coexisting garnetand biotite in an Fe-Mn-Mg ternary. From Robilson et al. (1982).

easy communication with other matrix ferromagnesianminerals. The fact that the Mn-content increases outward(Fig. l8) suggests the action ofa net-transfer, fractional,garnet-resorption, retrograde hydration reaction, as illus-trated in Figure 10, such as GAR + KSP + H,O : SILL+ BIO. Commonly the amount of fluid available for sucha reaction would be limited and, because of limited dif-fusion, would involve only a limited volume of the gar-net, while involving virtually all of the matrix. The re-action is fractional because the garnet interior isprogressively removed from participation in the retro-grade reaction, so that a decreasing volume ofthe exterioris subjected to more and more extreme reactions. Theeffective volume of reequilibration of garnet continues tofall, until it approaches zero and the garnet ceases to be

Fig. 21. Fractional garnet resorption illustrated schematical-ly as a series of steps. At stage I bulk composition I consists ofgarnet Gl and biotite Bl. The inner part of garnet Gl ceases toequilibrate with matrix biotite and the effective bulk composi-tion moves to 2, where it now consists of garnet G2 and biotite82. Still more of the garnet ceases to equilibrate with matrix,moving the bulk composition to 3, where it consists of garnetG3 and biotite B3. The process continues until the efective bulkcomposition contains no garnet, i.e., that difrrsion in garnet isso slow that the mineral is incapable of reacting with biotite.From Robinson et al. (1982).

in equilibrium with the matrix minerals, although, aspointed out above, its surfaces may still be subject tosurface corrosion that could contribute to the bulk chem-istry of matrix assemblages.

In 1982 we modelled this process in a very simple wayin an Mn-Fe-Mg ternary (Robinson et al., 1982) that isnow being duplicated more elegantly in computer pro-grams by Frank Spear and his students. The first step wasto select a series ofequilibrium sections (Fig. 19) cone-sponding to a series of changing metamorphic conditionsof temperature, pressure, and humidity. The figure showsputative garnet-biotite tie lines at a series of temperaturesand a series of progressively more humid conditions withfalling temperature. A similar series of templates could

600"c6250C

650"C

675"C700"c

Fig. 20. Model equilibrium garnet composition Fe-Mn-Mg trajectories for two different initial garnet-biotite ratios duringprogress ofthe hydration resorption reaction garnet + orthoclase + H2O : sillimanite + biotite. In the upper diagram the hydrationtakes place with falling temperature from 700 to 600 "C; in the lower diagram at constant temperature of 625 'C. In each case theequilibrium garnet composition trajectory is longer and more curved for the more biotite-rich bulk composirion. Frorn Robinsonet al. (1982).

ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST 1 789

Fig. 22. Comparisons of model equilibrium composition trajectories and fractional crystallization trajectories during retrogadegarnet resorption for two paths of cooling and hydration starting with the same initial bulk composition. Note that the fractionalpaths are more strongly curved than the equilibrium ones. The actual zoning path for the garnet in 9338 (Fig. 18) is shown for

comparison. From Robinson et al. (1982).

be constructed for retrograde hydration at constant tem-perature. Using such templates, a series of equilibriumgarnet-biotite tie lines (Fig. 20) has been constructed fora series of changing conditions for two different bulkcompositions, both originally on the same gamet-biotitetie line at the peak of metamorphism. One can see thatthe equilibrium paths of garnet for the two bulk compo-sitions are quite different and that garnet compositionchanges more for bulk compositions that are poorer ingarnet. This simple geometrical concept was well under-stood by Ferry and Spear (1978) when they designed theirgeothermometric experiments. In order to avoid disequi-librium problems with zoned garnet, they designed theirexperiments with large ratios of garnet to biotite so thatthere would be little tendency for composition change ingarnet and they could study composition changes in bi-otite, which reached equilibrium easily. It should be em-phasized that the equilibrium paths illustrated in this fig-ure are only indirectly related to the zoning in naturalresorbed garnet, which is produced by a fractional dis-equilibrium process.

The fractional aspect is illustrated in Figure 2l in aseries ofsteps involving alternate equilibration and frac-tionation, in which progressively less and less garnet par-ticipates, thus decreasing the garnet to biotite ratio. Asthe fraction ofparticipating garnet decreases, the changein composition of the remaining portion becomes moreand more extreme, while changes in biotite compositiondecrease dramatically. For this reason the fractional crys-tallization paths (Fig. 22) arc more strongly curved thanthe equilibrium ones, but the exact details depend criti-cally on the extent of fractionation. Obviously, this type

ofreaction ceases when the diffttsion is so slow that garnetcannot participate at all.

A serious consequence ofthis type ofreaction, in caseswhere garnet and biotite start out in roughly equal pro-portions, is that none of the original biotite composition

I

RETROGRADECONTINUOUS

Fig. 23. Consequences for geothermometry of a diffirsionalretrograde continuous net-transfer garnet-resorption hydrationreaction, here wiewed in an orthoclase AFM projection. As theresorption reaction proceeds, equilibrium garnet and biotitecompositions both move toward Fe enrichment. The originalgarnet composition Gl is protected from reaction in the interior,whereas all or nearly all of the original biotite Bl in the matrixis destroyed, thus removing the possibility of directly determin-ing the original peak metamorphic temperature. Garnet G2 andmatrix biotite 82 may be used to estimate the temoerature underwhich the retrograde reaction ceased. When garnet Gl is pairedwith biotite 82, with which it was never in equilibrium, a com-pletely false temperature estimate is made, which can normallybe much higher, even hundreds ofdegrees, than the original peak

temperature. From Robinson et al. (1982).

i l

Fe

1790

1.00

.96


bo

T

( r .

v .88()

.84

^ 800o

E 7oo

F ooo

.80200 400 600

Radius (pm)

900

\.

;;;i;;;^;/

500 L0 2N 400 600 800 1000

Radius (pm)Fig.24. Profiles of garnet composition and calculated garnet-

biotite temperature at temperatures from 725 to 500 qC. In theparticular model produced, the bulk garnet/biotite ratio begannear l: l, and a diflirsional retrograde net-transfer garnet-resorp-tion reaction with a major biotite reservoir was assumed to takeplace between 725 and 675'C. This was followed by pure ionexchange from 675 and 500 qC. Note that although the interiorofthe garnet is hardly changed in composition, Z(calculated) forthe core rises dramatically from 725 to 900'C as a result of amajor increase in Fe/(Fe + Mg) of matrix biotite as the garnetrim is resorbed. This is followed by a decline from 900 to 875"C as the biotite decreases in Fe/(Fe + Mg) during ion exchange.From Spear and Florence (1991).

remains that was originally in equilibrium with the garnetcore, unless a $ain remained in some unusual protectedlocation. Thus, when an unaltered garnet core composi-tion, such as Gl in Figure 23, is paired to make a tem-perature estimate with a highly altered matrix composi-tion such as B2, the result is always higher than the originalpeak temperature. In some cases the result may be hun-dreds of degrees higher, as pointed out by Robinson etal. (1975) and illustrated in a recent calculated profile

Gie.2q by Spear and Florence (1991).The style of reaction just discussed is illustrated in a

spectacular way by zoned garnets studied in detail in thePelham gneiss dome, Massachusetts (Fig. 25), by Roll(1987). These rocks occur in the Mount Mineral For-mation within the area of Zone I (kvanite-muscovite-

Fig. 25. Map showing the Pelham dome relative to the re-gional geology and metamorphic zones of west-central Massa-chusetts. Narrow belt shown in black in the southern part ofthedome is the Mount Mineral Formation containing relict granu-lite-facies assemblages. Adapted from Roll (1987).

staurolite), and they retain relics of an older granulite-facies metamorphism shown by relict high orthoclase withsillimanite inclusions, garnet cores with pyrope contentup to 350/0, coarse sillimanite, and relict associations of

Fig. 26. Schist sample Q-P25c from the lower part of theMount Mineral Formation, east limb of Pelham dome, centralMassachusetts. Contains a sheared relic ofhigh orthoclase about2.5 cm long with tails of secondary plagioclase and muscoviteset in a matrix of schist rich in muscovite, biotite, kyanite, andgarnet.

800 1000

IN H I

I

o

I P E L H A MD O M E

Fig. 27. Photomicrograph of schisr sample W67B from theupper part of the Mount Mineral Formation, east limb of Pel-ham dome. To left is the margin of a high orthoclase megacrystenclosing quartz, plagioclase, and sillimanite (stout needles farleft). The immediately surrounding matrix (right) contains mus-covite, biotite, and coarse kyanite (large grain close to orthoclaseright center).

garnet with coarse rutile. These occur in a variety of rockswith a mylonitic character related to late shearing in thedome, and they commonly occur side by side with rocksthat have been so completely reconstituted that they showfew or no traces of the older metamorphism and onlyfeatures of late mineral growth.

The sample in Figure 26 contains a sheared relic ofhigh orthoclase with tails of secondary plagioclase andmuscovite set in a matrix of schist rich in muscovite,biotite, kyanite, and garnet. Figure 27 shows the marginof a high orthoclase megacryst enclosing sillimanite. Thematrix in the immediate area contains muscovite. biotite.and coarse kyanite. In Figure 28, large garnet megacrystssurrounded by orthoclase and sillimanite are enclosedby thick rims of biotite produced by retrograde hydrationreactions. One of the samples studied in detail was a my-

Fig. 28. Schist sample l60C originally collected by L.D. Ash-walin 1972 from the middle of the Mount Mineral Formation,east limb of the Pelham dome. Garnet megacrysts up to 1.5 cmlong, surrounded by orthoclase and sillimanite, are envelopedby thick rims ofbiotite produced by retrograde hydration reac-tions.

t79r

@Fig. 29. Garnet composition maps from sample l60X (see

previous caption) with analytical points indicated by dots. Topmap shows contours of pyrope content ranging from 34olo downto l2o/o at 2olo intervals. Bottorn map shows contours of spessar-tine content of 2o/o and 30/0. Vertical ruling indicates areas ofbrown biotite; dashed horizontal ruling indicates areas of palegreen biotite. Adapted from Roll (1987).

lonitic garnet gneiss, sample 160X, from which the garnetcomposition map in Figure 29 and the composition tra-jectory diagram in Figure 30 were produced. These gar-nets show extreme zoning in pyrope content and Fe/Mgratio and a slight marginal increase in Mn. The pyropecomposition map shows that the large garnet was beingresorbed along a series ofcracks that look like fiords onsome island off the Norwegian Coast. The relict cores insample l60X contain 350/o pyrope and l.l0lo spessartine.From the core to the rim, the pyrope content decreasescontinuously to l0-l lo/o and spessartine increasessmoothly to a maximum of about 3ol0. A second garnetfrom a similar rock (Fig. 3l) is a more regular egg-shapedporphyroclast showing an irregular pyrope-rich interiorand a thin pyrope-poor, spessartine-enriched rim and ismore amenable to diffirsional modelling.

Roll modeled the time required to produce such re-sorption rims at 580 lC, 5 kbar using three different setsof difusion coemcients (Fig. 32). Under such conditionsit would take 40 m.y. for diffirsional penetration of thegarnet to a depth of 0.05 cm, rather longer than appearedto be available during Acadian metamorphism. Based onthese results, she suggested that the retrograde resorptionzoning might have been more easily produced in two ep-isodes, one during cooling at the end ofan early, probably


t792

Fe 10 20 30 Mg_

40

Fig. 30. Compositional trajectories ofzoned garnets from theMount Mineral Formation. Pelham dome. central Massachu-setts, in terms of Fe, Mn, and Mg. C indicates core, R indicatesrim. Composition maps for these garnets are given in Figure 29(160X), Figure 31 (W67B), and Figure 33 (X43A). All comefrom porphyroclastic schists with other relics of original neargranulite-facies conditions, in which garnet has been resorbedby the reaction garnet + orthoclase + HrO : alurninum silicate+ biotite. From Roll (1987).

Proterozoic, high-grade metamorphism and a secondduring Acadian retrograde hydration. These views re-quire reinterpretation based on new detailed isotopicstudies by R. D. Tucker (Tucker et al., 1988; Tucker andRobinson, l99l; Robinson and Tucker, l99l). Zircon andmonzvite from a sillimanite-orthoclase pegmatite withinthe schist in question yielded nearly concordant U-Pbages of 357 and 367 Ma, respectively, and monazite fromthe schist gave 367 Ma, precluding an early metamor-


a O8Oo

I

l i o7sE

580oC, 5 kbar

for 40 Ma

o o o o 02 0,o4 005 0 0a o 10

Distonce (cm)Fig.32. Model penetration distances produced in hornoge-

neous garnet 2 mm in diameter of 350/o pyrope composition byretrograde reequilibration at 580 'C, 5 kbar for 40 m.y. Gradi-ents are based on three sets ofdiffusion coefrcients: F: Freer(1979);C: Cygan and Lasaga (1985); E : Elphick et al. (1985).Under such conditions it would take 40 m.y. for diffusional pen-etration of the garnet to a depth of 0.05 cm. The model does nottake into account the movement ofthe garnet interface producedby resorption. From Roll (1987).

phism much before 360 Ma. The quartzite at the top ofthe Mount Mineral Formation contains virtually concor-dant single detrital zircons of a range of ages includinggrains with ages of 458, 440, and 439 Ma, showing thatthe quartzite itselfcould be no older than Early Silurian.The same quartzite contains the extremely retrograde-zoned, Mn-rich garnet described below and has yielded

F

cE

Fig. 31. Composition maps of egg-shaped porphyroclastic garnet from sample W67B of the Mount Mineral Formation (see Fig.27). Contours based on Me/(Fe + Mg + Mn + Ca), etc., are Py 16-220/0 at 20lo intervals, Sp 4-690 at l0lo intervals,andGr 4-5o/oat 1olo intervals. Maps show rimward decrease in pyrope and increase in spessartine consistent with resorption ofgarnet by reactionwith matrix through the medium of interstitial fluid. Adapted from Roll (1987).

Fig. 33. Composition maps of small fragmented garnet fromsample X43A from the Mount Mineral Formation on the south-west limb of the Pelham dome. Contours based on Mg/(Fe +Mg + Mn + Ca), etc., arePy l4-34o/o at 20lo intervals, Sp l-40loat l0lo intervals,Gr l-4o/o at lYo intervals. Although only I x 3mm in size, it contains almost the same range of pyrope com-positions as the much larger gamet in Figure 29. Probably thegarnet is a sheared fragment from the interior of a once largergarnet that was only tectonically exposed to the matnx over arelatively short period near the end of the late reequilibration.Adapted from Roll (1987).

a recrystallization age of metamorphic monazite of 297Ma, consistent with metamorphic sphene ages (Tuckerand Robinson, 1990), and Rb-Sr and Nd-Sm mineral iso-chrons (Gromet and Robinson, 1990) nearby in the dome.Thus, it appears that the relict granulite-facies metamor-phism was Acadian and the kyanite-muscovite overprintwas late Paleozoic, not Acadian. The boundary betweenthis late overprint and true Acadian isograds farther eastis being actively investigated.

In sample X43 (Fig. 33) almost the same range of py-rope compositions was obtained (Fig. 30) in a grain I x3 mm in size. In this case it seems hardlv likely that such

Fe lO 20 30 40

M q +

Fig.34. Summary diagram of the zoning trajectories for gar-nets in seven specimens of the Mount Mineral Formation stud-ied by Roll (1987). X43A, 160X, and W67B are from the schistsdescribed above. M22A is from a sillimanite-orthoclase peg-matite. The Fe-Mn-rich garnet is from a nearby garnet quartziteshown with contours for spessartine and grossular in Figure 35.There is little to no zoning in pyrope content, and the trajectoryis consistent with a resorption reaction involving formation ofFe-rich biotite. Specimens l60M and, especially, Y33 belong torocks that appear to have been totally reconstituted, with garnetnewly grown during the late hydrous metamorphic overprint, asdiscussed in the text.

t793

Fig. 35. Composition maps of egg-shaped porphyroclasticgarnet from quartzite sample M2l near the upper contact of theMount Mineral Formation on the east limb of the Pelham dome.Contours at intervals of l9o based on Mn/(Fe + Mg + Mn +Ca), etc., Sp 3-140/0, Gr 6-80/0. Maps show rimward increase inspessartine consistent with resorption ofgarnet by reaction withthe matrix biotite through the medium of interstitial fluid. De-pression hachures are omitted from closely spaced contours nearedge ofgarnet. Adapted from Roll (1987).

a steep gradient could have been preserved through evena single retrograde event at 580 'C, and more probablythe garnet is a sheared fragment from the interior of aonce larger garnet that was only tectonically exposed tothe matrix over a relatively short period near the end ofthe late reequilibration.

Figure 34 summarizes the zoning trajectories fot gar-nets in seven diferent specimens in the Mount MineralFormation studied by Roll. Three are from the schistsalready described, and a fourth is similar in form but isfrom a sillimanite-orthoclase pegmatite. The Fe-Mn richgarnet, from the quartzite at the top of the formation, isshown in maps in Figure 35, with contours for grossularand spessartine. In this specimen there is little to no zon-ing in pyrope content and the trajectory is consistent witha resorption reaction involving formation of Fe-rich bi-otite.

In Figure 36 biotite compositions from all seven rocksare plotted in terms of Ti per I I O atoms and the ratioMg/(Mg+Fe). Ti content for a given Mg ratio has beenshown to be a function of metamorphic grade in rockssaturated with ilmenite or rutile as well as aluminum sil-icate (Robinson et al., 1982; Schumacher et al., 1990). Inthese rocks variable Ti content seems to be related in part

to extent of participation in retrograde resorption reac-tions and proximity to grains of Ti oxide minerals. Theextreme example is biotite from deep within the "fiords"of the garnet l60X in Figure 29, which are colorless topale green in thin section and contain less than 0.025 Tiper I I O atoms. Because these fiord biotite grains wereclearly produced by the reaction of a K-bearing matrixsolution upon the enclosing walls of garnet, it appears


@@ wet

wa

% to a o

s v ' o . ovrr,V_l 16ox

16oM y- v a

A

Ol O,2 O.3 04 05 S" OZ OB 09

25

20

15

10

o5

BIOTITE

aM 2 1

a

aoO a

$oa

o

o

t794

Mg/(Mg+Fe)Fig. 36. Biotite compositions from all seven rocks plotted in

terms of Ti per I I O atoms and the ratio Mg/(Mg + Fe). Samplesshown are W67 (squares), X43 (circles), l60X (rriangles), y33and 160M (inverted triangles), M22 (small diamonds), and M2l(large diamonds). Inclusion biotite (closed symbols) is moremagnesian than corresponding rim (open symbols) or matrix(ruled symbols) biotite. Dotted triangles indicate biotite in l60Xthat fill cracks in garnet. From Roll (1987).

+ M


Ti

1 lOx'

\ \ \ ,continuous

+ hydrationion"id"ng"

Fig. 37. Schematic AFM projection, showing the contrastingeffects on biotite compositions ofcontinuous retrograde hydra-tion on matrix biotite and of retrograde ion exchange on inclu-sion biotite. As hydration proceeds, equilibrium tie lines be-tween garnet rims and matrix biotite move to the left, whereasinclusion biotite moves to the right. The original peak biotitecomposition is no longer preserved but presumably had a com-position intermediate between matrix biotite and biotite includ-ed in garnet cores. From a diagram prepared by M.A. Roll (per-sonal communication, I 986).

Fig. 38. Composition maps of euhedral garnet in sample Y33from schist near the upper contact of the Mount Mineral For-mation on the southwest flank of the Pelham dome. This schistis believed to have been completely reconstituted during the latemetamorphic reequilibration. Maps show rimward decrease rnpyrope, spessartine, and grossular content consistenl with ret-rograde garnet growth. Adapted from Roll (1987).

that under the ambient retrograde conditions estimatedat about 550'C, the matrix solution had not dissolved ordid not contain enough Ti to produce a biotite with anormal Ti content typically found in matrix locations inclose physical contact with Ti-oxide grains.

In three of the samples, M2l, W67, and X43, com-positions were obtained both on matrix biotite and onbiotite included within garnets. Inclusion biotites (Fig.36) are consistently more magnesian than matrix biotites,in agreement with the two different processes by which

Fe 10 zo Mo*

ao

Fig. 39. Comparisons of composition trajectories of garnetin W67B and in Y33. In each example, garnet core has the mostMg-rich composition. W67B is consistent with a retrograde gar-net resorption reaction, whereas Y33 is consistent with retro-grade growth. Adapted from Roll (1987).

Fig. 40. P-T diagram with garnet and biotite compositionisopleths for the reaction garnet + muscovite : biotite + alu-minum silicate. Note that isopleths change orientation whencrossing aluminum silicate field boundaries and when crossingthe muscovite-out reaction in lower right. Quadrants of garnetproduction (P) and garnet consumption (C) are indicated for the

t795

kyanite and sillimanite zones. Heavy lines with arrows representthree retrograde P-T trajectories illustrated in quasi-binary sec-tions in Figure 41: one parallel to garnet isopleths (Fig. alA),one parallel to biotite isopleths (Fig. 1O, and one intermediate(Fig. 41B). Adapted from Spear and Selverstone (1983).


GAR+MUS=BlO+Als i l

ltI

a'

/ - - - z -

=ffi7='j=- ---

1 - - t - - t - '

r, oc Blo + Alsil = GAR + KSP + H2o

XFE GAR XpE BIO

their compositions were achieved. The matrix biotiteformed by continuous retrograde net-transfer garnet re-sorption in which biotite grew in amount and becameprogressively more Fe enriched. The inclusion biotite didnot change in amount but became Mg enriched as a resultoflocal ion exchange with the enclosing garnet, probablyproducing a small Mg-impoverished compositional wellin the garnet, such as that described by Spear et al. (l 990).

It is apparent that if Fe-enriched matrix biotite ismatched with the relict core garnet composition withwhich it was never in equilibrium, an artificially high-temperature estimate will be obtained (see also Fig.2q.In the case of sample l60X such a procedure gives anestimated temperature of 1000 oC. By contrast, matchingof inclusion biotites with immediately surrounding gar-nets yields estimates of temperature of final ion exchange

equilibration of about 450 qC. A major problem is tolocate and analyze, or to estimate, the composition ofbiotite that was originally present in the rock at the peakof granulite-facies conditions. Spear et al. (1990) man-aged to locate biotite inclusions within sillimanite thathad apparently experienced little of the retrograde re-equilibration described here. In the Pelham dome rocks,the best estimate of original biotite composition was madeby choosing one intermediate between matrix and inclu-sion extremes (Fig. 37) and also one that would yield anestimated temperature and pressure consistent with thepresence of sillimanite in the rock. On this basis, a biotitecomposition chosen for sample l60X with Mg/(Mg+Fe): 0.66 yields conditions of equilibration with sillimanite,garnet, and high orthoclase of 700 "C and 6.8 kbar (Roll,I 987) .

1796

Zoning produced during retrograde garnet growth

The two garnet grains with short zoning trajectories inFigure 34 are from schists of the Mount Mineral For-mation that appear to have been completely reconstitutedduring the second more hydrous metamorphism. Thesegarnets are typically euhedral (Fie. 38) with Mn- and Ca-enriched cores and might be supposed to have formed byprograde Efowth zoning. However, unlike such garnets,they do not show a typical increase in the Mg/(Mg + Fe)but rather a decrease, resembling in this way zoning pro-files produced by retrograde garnet-resorption reactions.Chemical trajectories of the two types of garnet are com-pared in Figure 39. The garnet in sample W67B showsthe typical outward increase in both Ca and Mn associ-ated with diffusive rim resorption of the Mg componentinto other ferromagnesian minerals. The garnet in sampleY33, on the other hand, shows rimward decrease of Caand Mn that can only be associated with garnet growthbecause there is no other mineral in the rock that couldbe proportionally absorbing more Mn than the garnet.

Peter J. Thompson (1985) has described interestinggarnets from the Mount Monadnock area, New Hamp-shire, in which there is a similar strong rimward decreasein Mg/(Mg * Fe) accompanied by a similar decrease inMn but with a striking rimward increase in Ca. In thisexample, it appears that the pyrope content ofthe garnetwas undergoing progressive resorption with respect to thecordierite in the rock, but at the same time the grossularcontent ofthe garnet was undergoing progressive growthwith respect to the anorthite component of plagioclasethat slightly overbalanced the resorption. In this example,as in all of the others discussed here, the Mn profile isthe most reliable predictor of growth vs. resorption be-cause Mn is more strongly fractionated into aluminousgarnet than any other phase.


Fig. 41. Binary representations in terms of ?"-P and XF" ofthe phase relations along the three retrograde trajectories illustratedin Figure 40. For details see text. Modified and expanded from Roll (1987).

r .o o.8 0.6 0.4 0.8 0.6 0.4X F e X r e

o.8 0.6 0.4Xre

It is a common habit of metamorphic petrologists tothink of garnet growth in terms of a prograde reactionand resorption as a retrograde (i.e. decreasing tempera-ture) phenomenon. It is further habitual to visualize suchreactions in terms of binary Z-X loops. Study of com-position isopleths for a reaction on a P-T diagram illus-trates the true complexity even in an oversimplified qua-si-binary situation. For this purpose, Ihe Xr. isopleths forgarnet and biotite derived by Spear and Selverstone (1983)for the reaction garnet + muscovite : aluminum silicate+ biotite serve as an admirable example, as shown inFigure 40. In the habitual view, one may cross the iso-pleths by increasing pressure and increasing temperatureslightly such that garnet is produced, biotite is consumed,and progressively lower X." isopleths for both mineralsare crossed. Similarly, one may cross isopleths by de-creasing pressure and temperature such that garnet is re-sorbed, biotite grows, and progressively higher Xr" iso-pleths for both minerals are crossed. These two directionstoward garnet production or consumption (P and C inFig. a0) He in only two of the possible four quadrantsbetween the isopleths.

Relations within the remaining two quadrants are morecomplex. In these quadrants the directions ofgarnet andbiotite composition change are opposite, and for eachP-I trajectory, garnet production or consumption willdepend critically on the garnet-biotite ratio in the rock.The relations in these quadrants are illustrated by PT-X-.sections from Figure 40 taken to follow three specific tra-jectories. Figure 4lA shows a trajectory constrained alongthe Garnet Xo. : 0.85 isopleth for the kyanite zone. ThisPZ-X section is oriented with increasing T at the top,which is also the direction of decreasing P. Within thekyanite zone, as specified, garnet composition is constant,but along the same trajectory at lower temperature in the

S i l l I r \ B i o

Fig.42. Pavement outcrop of typical migmatitic schist of thesillimanite-orthoclase-garnet-cordierite zone, central Massachu-setts. Large garnets and leucosomes are believed to be the prod-uct offluid-absent partial melting. See Robinson et al. (1986),stop 4.

garnet-chlorite zone and also at higher temperature in thesillimanite zone, garnet is more Fe-rich. Taking a typicalbulk composition along the dashed line and following itin Figure 4lA in the kyanite zone only, it can be seenthat garnet is consumed with increasing temperature (i.e.,the bulk composition on the garnet-biotite tie line liesprogressively closer to the biotite end) and is producedas temperature decreases.

Figure 4lC shows a trajectory constrained along thebiotite isopleth Xr.: 0.45 for the kyanite zone. This sec-tion is also oriented with increasing T at the top, but inthis case, this is also the direction of slightly increasingP. Taking the same typical bulk composition along the

Feo MsoFig. 43. AFM projection showing mineral compositions in

typical assemblages of pelitic schist of Zone VI, central Massa-chusetts. From Robinson et al. (1982).

t797

O.5mm

Ftg. 44. Composition maps of garnet in sample FW407 ofsillimanite-orthoclase-garnet-cordierite-biotite schist from ZoneVI, central Massachusetts. Garnet shows significant Fe enrich-ment and Mg depletion only where in direct contact with adja-cent biotite, permitting retrograde ion exchange. From Tracy etal. (1976). See Robinson et al. (1986), stop 6.

dashed line and foltowing it in Figure 4lC,it can be seenthat garnet is produced in an up-temperature direction(i.e., the bulk composition lies progressively closer to thegarnet ends of the tie lines) and is consumed in a down-temperature direction.

Figure 4lB is a PT-X*. section for a trajectory betweenthe two bounding examples in 4lA and 4lC. On thistrajectory, with increasing temperature, garnet Xo. de-creases while biotite Xo" increases, consistent with thedecreasing distribution coefficient between these twophases. Note that for Fe-poor compositions (X), garnet isconsumed with increasing temperature even though thegarnet itself decreases in X.. until the bulk compositionconsists only of biotite. Similarly for Fe-rich composi-tions (Y), garnet is produced with increasing temperature.With falling temperature along the same trajectory, gar-net is produced in Fe-poor compositions (X) and is con-sumed in Fe-rich compositions (I').

The garnet-biotite two-phase region in a profile such asthat in Figure 4lB is divided into regions of production

and consumption by a vertical dash-dot line, which ispositioned differently for each trajectory. To the left ofthis boundary, as in composition Y, garnet is producedwith increasing temperature and consumed with decreas-ing temperature. To the right of this boundary, as in com-position X, garnet is consumed with increasing temper-ature and produced with decreasing temperature. By


B i o :

r 798

Fig. 45. Compositional trajectories of zoned garnets fromZone Yl, central Massachusetts, in terms of Fe, Mn, and Mg.Typical compositions are the most Mg rich, and zoning to moreFe-rich compositions occurs only where garnet is in direct con-tact with biotite or cordierite. From Tracv et al. (1976).

comparing Figures 4l B and 4lC, it will be seen that themore nearly parallel the P-T trajectory is to the biotiteisopleths, the larger the region for retrograde gamet con-sumption and the smaller the region for retrograde pro-duction, until, in the extreme example parallel to the bi-otite isopleth, there is no region for retrograde production.

In the trajectory of Figure 4lB, the potential for ret-rograde garnet production is proportional to the distancefrom the production-consumption boundary. This is il-lustrated in two calculated examples showing the changefrom 600 to 550 'C, in which Xo. garnet changes from0.832 to 0.845. For a bulk composition Xo" : 0.593, themole percent garnet will increase from 30 to 33, whereasfor a bulk composition XF, : 0.507, the mole percentgarnet will increase from 5 to 10.5. In the case ofgarnetgrowth by fractional crystallization, the interior of thegarnet will be effectively locked away from the exteriorbulk assemblage so that the effective bulk compositionwill become progressively more biotite rich. This will drivethe effective bulk composition toward lower Xo., furtherinto the region of retrograde production of garnet withdecreasing temperature. If, in the above examples of cool-ing from 600 to 550 oC, there is fractional crystallizationduring retrograde $owth such that half of the garnet istaken out of reaction at 575 "C (see dotted lines in Fig.4lB), then the effective bulk composition will be even

Fig. 46. AFM orthoclase projection of garnet and biotitecompositions, illustrating the relatively simple interpretation re-quired in cases of retrograde ion exchange. From Robinson etal. (1982).


Ir/g

Al

MgFe

Fig. 47 . Photograph of even-grained white quartzose marblewith brightly reflecting elongate grains of white wollastonite.Specimen was collected from a gas pipeline trench on the eastside of Rattlesnake Mountain by Berry ( I 989), from the heart ofzone VI, central Massachusetts.

more biotite rich and still more capable of growing gar-net: in the two cases, from 30 to 33.7o/o or from 5 toll.7o/0.

In Figure 4lB, it will be seen that the best opportunityfor retrograde growth zoning with a substantial garnetcomposition change will be in a trajectory at a slight angleto biotite isopleths where a small biotite-rich composi-tion region will be subject to garnet growth. Fractionalcrystallization will continue to maintain the effective bulkcomposition in this biotite-rich region where garnet growthcan continue. Such a situation of garnet-growth zoningwith falling temperature appears to be the origin of thezoned garnets in completely reconstituted schists such assample Y33 (Fig. 39).

Zoning produced during retrograde ion exchange

The pelites of the sillimanite-orthoclase-garnet-cordi-erite zone (Zone VI) in central Massachusetts are typifiedby leucosomes of quartz, feldspar, and cordierite com-monly containing garnet up to 2 to 3 cm in diameter (Fig.42). Such textural features have been described as theproduct of fluid-absent melting (Tracy and Robinson,1983). Typical assemblages and mineral compositions areshown in an orthoclase projection in Figure 43. Garnetis typically about 5-100/o more magnesian than garnet inZones IV and V, indicating considerable progress ofthestrong dehydration reaction biotite + sillimanite : or-thoclase * garnet + HrO. Very locally there is evidenceof late retrograde hydration reactions between garnet orcordierite and orthoclase to produce sillimanite + biotite,but this is not typical. The common four-phase assem-blage in projection may indicate that HrO was behavingas an inert component in the absence ofan aqueous fluid.Typically both larger and smaller garnets are homoge-neous from core to rim except at points where the garnetsare in direct contact with another ferromagnesian min-eral, either cordierite or biotite (Fig. aq. This style ofzoning was first described in this region by Hess (1969,

.-------.s--4.4o7o-.?4.st.FWl54FWt22

pS76oroesoefilcARNEr- f_l wotlAsToNm- WV)PtAGtoctlsE+'Jy' GMINS Lj'^J BAARNG L-I BEARING I2-4 CNJCfiE

Eu REGoN REGIoN REGoN

Fig. 48. Thin-section sketch of wollastonite marble, showingsubdivision of the rock into regions. wollastonite-bearing regionshave a veinlike character, separated by areas with quartz-calcite.Plagioclase-calcite regions occur predominantly away from di-opside, whereas close to diopside, plagioclase and calcite areseparated by garnet. From Berry (1989).

l97l). In these locations, the Fe-enriched garnet rim isaccompanied by zoned biotite or cordierite that is en-riched in Mg as compared to biotite or cordierite that isin the matrix away from the garnets. The local chemicalzoning trajectories (Fig. a5) are virtually flat with respectto Mq strongly indicating that there has been neithergarnet resorption nor garnet $owth. This type of zoningis most easily explained as the product of local retrogradeion exchange between garnet and surrounding ferromag-nesian minerals where they are in direct contact and with-out participation ofan interstitial fluid. It is a strong ar-gument that at peak conditions pervasive fluids were notpresent in most zone VI rocks. If no aqueous fluid waspresent, then we must appeal to straight difusion throughthe matrix or localized ion exchange at grain-to-graincontacts. The latter was first observed and studied in cen-tral Massachusetts by P. C. Hess (1969, l97l), and hisobservations have been repeated countless times.

The interpretation of these kinds of garnets in terms ofgeothermometry is apparently much simpler. The ho-mogeneous garnet composition can be compared with thematrix biotite composition away from the garnet to es-timate peak conditions (Fig. 46). The garnet rim com-

1799

Fig. 49. Thin-section sketch showing garnet-bearing regionsclose to diopside and preservation of anorthite + calcite awayfrom diopside. From Berry (1989).

position and the adjacent biotite or cordierite composi-tion can be used to estimate the temperature of lastretrograde ion exchange. The only question might bewhether some other process intervened between the peakmetamorphism and the retrograde ion exchange that couldhave effectively changed the typical matrix biotite com-position without also producing concentric zoning in thegarnet. In my opinion this is unlikely.

Conclusion concerning garnet zoning

It seems fairly clear that at temperatures below about500 "C, most garnet zoning can be interpreted in termsof simple growth zoning. At higher temperatures, how-ever, all sorts ofcontradictions are produced by diffusion,which commonly makes it very difficult to select the cor-rect mineral compositions for quantitative geothermom-etry and geobarometry. If all of the conditions and con-siderations outlined here are correctly accounted for, andone happened to be confronted with appropriate rocks,then it may be possible to construct detailed P-7 pathssuch as those ofSpear et al. (1990).

Pnlsn RELATToNS rN AWOLLASTONITE.BEARING MARBLE

In the next section of this paper, I want to show howdetailed petrographic observations in a single thin sectionled to a new conception for a petrogenetic grid. This con-ception comes from the Ph.D. thesis of Henry Berry (1989,l99l) and concerns one aspect ofhis interpretation ofthefirst documented occurrence of regional metamorphicwollastonite from the granulite-facies in New England,near Sturbridge, in the center of Zone Yl of Figure 2.This section is based on a small highlight of a more com-prehensive study that Berry is preparing for separate pub-lication.

Petrography

The rock is an even-grained, foliated, white quartzosemarble with brightly reflecting elongate grains of white


o 5mm

r 800

Fig. 50. Photomicrograph showing abundant garnet coronasbetween anorthite and calcite close to large diopside grain inlower right and absence of such coronas between anorthite andcalcite in upper left. Dark grains are sphene. Width of view, 3mm approximately. Photograph provided by H.N. Berry.

wollastonite (Fig. a7). The rock consists mainly of quartz,calcite, anorthite (An98), wollastonite, calcic scapolite(-Me86), ferroan diopside [Fe/(Fe + Mg) : 0.40-0.45],grossular-rich garnet (Gross88-90), and sphene. Locallywollastonite appears to be concentrated along veinlikesurfaces that may have allowed concentration of earlymetamorphic fluids.

An unusually detailed petrographic study of a singlethin section by Berry indicated different local chemicalenvironments that require treatment in different chemicalsystems (Fig. a8). In the first place, the rock can be di-vided into regions containing qtartz + calcite (mainly

Fig. 51. Thin-section sketch in wollastonite-bearing regionwith abundant diopside, showing garnet rim between anorthiteand wollastonite. Sketch provided by H.N. Berry.

iL,


eN(Fe,Mg)O

s i

g,

* . .

_ t. d . '

Ar2o3

Fig. 52. The chemical tetrahedron SiO,-CaO-AlrOr-(Fe,Mg)O, showing the compositions of quartz (QZ), anorthite (AN),wollastonite (wO), diopside (DI), calcite (CC), and the garnetsolid solution grossular (GR) to almandine-pyrope (ALM, PY).From Berry (1989).

plagioclase + calcite regions of Fig. 48) and those con-taining qtartz + wollastonite (wollastonite regions of Fig.48). Clearly conditions of the calcite + qvartz: wollas-tonite reaction had been reached in some parts ofthe rockbut not in others, suggesting a very locally variable XCO,of the fluid may have obtained at the peak of metamor-phism.

Secondly, the rock can be divided into regions whereplagioclase and calcite are in equilibrium and there is nogarnet, and other regions where plagioclase and calciteare separated by abundant garnet. The regions of abun-dant garnet are invariably close to the widely scatteredgrains of ferroan diopside. This is illustrated in Figure 49and more dramatically in Figure 50 where garnet hasgrown at the expense of anorthite + calcite only in thevicinity of diopside. The necessary participation of fer-roan diopside in garnet-forming reactions is shown by theanalysed garnet composition: grossular 88-90, alman-dine 5, pyrope 0.5-0.7, spessartine 1.3-1.8, atdradite2.T-3.4. An analogous reaction has taken place in wollaston-ite-bearing regions near diopside, where wollastonite andanorthite are not in contact and anorthite is rimmed bygarnet (Fig. 5l). These critical textural observations em-phasized the role of the diopside component in all thegarnet-producing reactions in this rock and led to consid-eration of equilibria in a more complex system and thederivation ofa specific new petrogenetic grid.

Chemography

The consideration of calcite-quartz-wollastonite equi-libria requires SiO,-CaO-H,O-COr, the addition of an-orthite and grossular reactions requires the addition of

sio2

a

CaO

ALM. PY

'ff,

(Fe,Mg)O Azog

QZPROJECTION

@

CaO-AlrO,

ANORTHITE PROJECTIONFig. 53. Quartz projection of phase compositions onto the

base ofthe tetrahedron ofFigure 52 and anorthite projection ofthe same compositions onto the plane SiOr-CaO-(Fe, Mg)O. Inthe anorthite projection ALM, PY garnets plot at negative valuesofCaO. From Berry (1989).

AlrO., and the addition of ferroan diopside and the realgarnet compositions requires the addition of (Fe,Mg)O,here lumped for simplicity as one component. The re-sulting chemical tetrahedron is shown in Figure 52. Allthe phases plot at point compositions except garnet, whichhas a theoretically complete range from pure grossular topure almandine-pyrope. When projected from the quartzapex onto the base of the tetrahedron, the phase com-positions appear as shown in Figure 53 (top). A morehelpful projection is from anorthite onto the plane SiOr-CaO-(Fe,Mg)O (Fig. 53 bottom), which already containsthe phases qtrarlz, wollastonite, calcite, and diopside.Garnet projects as a line below the diopside-wollastonitejoin. The grossular end of the series projects positivelybetween calcite and wollastonite, but the almandine-py-rope end projects to negative values ofCaO well outsidethe triangle ofprojection. The big advantage ofthis pro-jection is that it allows direct observation of the specialgarnet compositions that are coplanar with other phasesin the tetrahedron, and it is used in all discussion thatfollows.

ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST t 8 0 l

QZ+Ct+GAR

QZ+CAR+CO2

DI+AN+qZ+CC AR+CO2

Fig. 54. Anorthite projections showing three chemographiesof the equilibrium DI + AN + QZ + CC + GAR + CO' (re-action 14 of Berry, 1989) depending on the equilibrium com-position ofgarnet. Other numbers indicate continuous reactionsdescribed in detail by H.N. Berry. From Berry (1989).

A new petrogenetic grid

In Figure 54 are illustrated the chemographic relationsof the equilibrium quartz + calcite * anorthite + diop-side * garnet (reaction l4), all obviously within the sta-bility of calcite + qtrartz. If garnet is less grossular thanthe quartz-diopside-anorthite plane, as in reaction l4A,then the join diopside-anorthite gives way to the assem-blage quartz + calcite + garnet, consuming CO', and isthus a garnet-forming reaction that would take place withdecreasing f. If the garnet composition is slightly moregrossular than the quartz-diopside-anorthite plane, as inreaction l4B, then the plane diopside-anorthite-calcitegives way to quartz + garnet * minor CO, and is a gar-net-forming reaction that would take place with increas-ing Z. If the garnet composition is more grossular thanthe diopside-calcite-anorthite plane, as in reaction l4C,then all the phases quartz-calcite-diopside-anorthite giveway to garnet + a large amount of COr. The limitingsituation would be the reaction where quartz-calcite-an-orthite gives way to grossular + CO, without participa-tion of diopside.

Of course all of these segments are part of the samecontinuous reaction, which must change its P-I slopedrastically in the presence of CO'-bearing fluid, and eachsegment is separated from the next by an invariant pointcharacterized by a particular coplanarity of phases. Be-tween reaction l4A and reaction l4B, invariant point

+AN

REACTION

DI+ AN +CP2

REACNON

DI+AN+CCsio2-2Al2o3

REACTION

GAR+ CO2

I 802

>q: r.r. ral.o

T '-bOc

Fig. 55. Plot ofreaction coefrcients versus XG ofgarnet forreaction 14 of H.N. Berry. Points I67c, I83, and Il00c representgarnet compositions at invariant points produced by collineari-ties ofgarnet composition with other phases. From Berry (1989).

I67c represents the copianarity of quartz-diopside-anor-thite-grossular 67. Between reaction l48 and reaction l4C,invariant point I83 represents the coplanarity of diop-side-calcite-anorthite. Invariant point I I 00 represents thelimiting coplanarity of quartz-calcite-anorthite with gros-sular. The varied stoichiometry of reaction 14 with vari-able grossular content ofgarnet is illustrated in Figure 55,showing how CO, changes dramatically from a reactant


for garnet compositions below grossular 67 to a productthat becomes increasingly voluminous to a maximum forpure grossular. In the vicinity of 167 , the CO, coemcientis vanishing and there is no AS of devolatilization, andhence the reaction is primarily pressure dependent.

From the above considerations, Berry constructed theP-?"loop shown in Figure 56 (left). At each of the threeinvariant points the curve of reaction 14 is tangent to asecond univariant curve. At 167 this is the fluid-indepen-dent reaction grossular 67 * quartz: diopside + anor-thite, which has a metastable extension to lower temper-ature. This reaction is critical in subsequent discussions.At I83 the second univariant curve is the decarbonationreaction diopside + calcite + anorthite: grossular 83 +qtrartz + COr, which has a metastable extension to higherpressure. At Il00 the second reaction is the decarbon-ation reaction calcite + anorthite + qvarlz: grossular100 + COr. In this case the reaction 14 curve comes intangent to the other reaction at an end point, and thereare no metastable extensions. To me, the resulting dia-gram far exceeds the worst nightmares encountered inJ. B. Thompson's phase equilibrium course thirty yearsago. My student Berry had only to experience this byproxy. Figure 56 (righ| shows the same equilibria butwith the addition ofgarnet composition isopleths for var-ious continuous reactions within the region of stability ofgarnet + quartz-calcite-anorthite.

QZ + CC + AN + Ca-poorer GAR > Ca-r icher GAR + CO2

iri;e/Gr (67) GAR + QZ

l 8l ol +l 6

6'g

/:

J /o l

N

+

o

r@

Az

+cc

+o

CC + D l + AN + GAR > Gr (83) - r i chor GAR + CO2

T +

Fig. 56. P-I diagrams in presence of COr-bearing fluid, showing relations of reaction 14 of H.N. Berry to other reactionsinvolving garnet in the chemical system of Figure 54. Diagram to left shows intersections of reaction 14 with other reactionsinvolving collinearities ofgarnet compositions with other phases. Hachures on reaction 14 indicate the three segments: l4A (triplehachures), 14B (double hachures), and l4C (single hachures) demarcated by invariant points I67c and I83. Diagram to right showsisopleths ofgarnet composition for various reactions within the srability field ofgarnet. Adapted from Berry (1989).

m\ /

D l + A N

REACTION 168:

Dl+ AN + WO GAR + QZ

Fig. 57. Anorthite projections showing two chemographiesof the equilibrium DI + AN + QZ + WO + GAR (reaction 16of Berry, 1989) depending on equilibrium composition of garnet.From Berry (1989).

An analogous but simpler reaction sequence in the fieldof stability of quartz-wollastonite instead of quartz-cal-cite is illustrated in Figure 57. Ifgarnet has less grossularthan the quartz-diopside-anorthite plane as in reaction16A, then the join diopside-anorthite gives way to theassemblage quartz + wollastonite + gamet, a fluid-in-dependent reaction that would take place with increasingZ. If the garnet composition is more grossular than thequartz-diopside-anorthite plane, as in reaction l68, thenthe plane diopside-wollastonite-anorthite gives way toqvartz-garnet, a fluid-independent reaction that wouldtake place with decreasing Z. The limiting situation wouldbe the reaction where wollastonite-anorthite gives way togrossular-quartz with decreasing Twithout participationof diopside.

Reactions I 64, and l6B are parts ofa single continuousreaction marked by two invariant points, as shown inFigure 58 (top). At each of the invariant points the curveof reaction I 6 is tangent to a second univariant curve. AtI67w this is the fluid-independent reaction grossular 67+ quartz: diopside + anorthite, which has a metastableextension to higher temperature. This reaction is the sameone that connects at lower temperature toI67c in Figure56. At Il00 the second reaction is the fluid-independentreaction wollastonite + anorthite : grossular + quartz.In this case the reaction 16 curve comes in tangent to theother reaction at an end point, and there are no metasta-ble extensions. Figure 58 (bottom) shows the same equi-libria but with the addition of garnet composition iso-pleths for two continuous equilibria within the region ofstability of garnet * quartz-wollastonite-anorthite. All ofthe equilibria in Figure 58 are fluid independent.

The equilibria in Figure 56, with quartz-calcite stable,and those in Figure 58, with wollastonite stable, are crudemirror images of each other, reflected across the fluid-

T +

GAR + QZ > WO +-AN + Ca-poorer GAR

Fig. 58. P-Z diagrams showing relations of reaction 16 ofH.N. Berry to other reactions involving garnet in the chemicalsystem of Figure 57. All equilibria are fluid independent. Dia-gram at top shows intersections ofreaction 16 with other reac-tions involving collinearities of garnet compositions with otherphases. Hachures on reaction 16 indicate the two segments: 16A(square hachures) and I 68 (triangular hachures) demarcated byinvariant points I67w. Diagram at bottom shows isopleths ofgarnet composition for reactions within the stability field ofgar-net. Adapted from Berry (1989).

dependent reaction quartz + calcite : wollastonite * COr.These are shown in Figure 59, in which fluid-dependentand fluid-independent equilibria are keyed differently'Most of the equilibria in Figure 56 are fluid dependent;those in Figure 58 are all fluid independent, but theyshare the two fluid-independent equilibria connectingl6Tcto I67w and ll00c to Il00w. At low xco, (Fig. 59A),the fluid-dependent equilibria are at low Z. As XCO, in-creases, they and the related invariant points slide tohigher temperature along the fl uid-independent equilibria(Fig. 59B, C, and D), progressively shrinking the stabilityfield of pure grossular in the center and the field of gros-

sular-rich garnet on either side. Eventually the two in-variant points Il00c and Il00w merge along the calcite+ quartz : wollastonite * CO, reaction (Fig. 59C), andat higher values of XCO, (Fig. 59D), progressively lessand less grossular garnet can be in equilibrium with quartz-

ROBINSON: EYE OF THE PETROGRAPHER, MIND OF THE PETROLOGIST 1803

REACTION 164:

W O + G A R + Q Z

No+

E

o

i

N

l ( ,

p l ,z

E

o

lozw

'l /

/

1804 ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST

G R O S S U L A R

:/ /

:/ "r**2

GROSSULAR

IroocIroow

Iroocw

/D . /

/Fig. 59. Composite P-7 diagrams of phase relations of DI-

AN-QZ-CC-WO-GAR-CO, in Figures 56 and 58 separated bythe equilibrium CC + QZ : WO + CO,. Fluid-dependent re-actions (dashed) slide toward higher temperature against fluidindependent reactions (solid) as X.o, increases from A to D.Region of end-member grossular stability is marked in the cen-ter, and areas of stability of less grossular garnet are indicatedby stipple. Adapted from Berry (1989).

calcite-wollastonite-anorthite-diopside. Further, as XCO,increases, the total length of the combined reactions 14and 16 that govern the stability ofgarnet with anorthite-diopside and either quartz-calcite or wollastonite de-creases.

Petrogenetic interpretation

Berry's petrogenetic interpretation of these rocks, basedon the petrographic features and the petrogenetic grid de-veloped above, is illustrated in Figure 60. The rocks at-tained the quartz + calcite: wollastonite reaction at rel-atively low XCOr, and once encountered, the XCO, ofthe fluid increased. Slightly later in this history, a fluid-independent reaction was encountered that produced

Fig. 60. P-?rdiagram to illustrate P-Z-fluid evolution ofwol-lastonite marble. Isopleths I illustrate CC + QZ: WO + CO,for various X.o,. Reaction 2 is AN + WO : GR. Reaction 4'is the reaction CC + PLAG : SCAP. Reaction 15 is the contin-uous fluid-independent reaction WO + AN * Ca-poorer GAR: Ca-richer GAR + QZ. Reaction l6b defines the limit of GARstability with DI + AN + QZ + WO as in Figures 58 and 59.From Berry (1989).

scapolite from plagioclase * calcite, which removedpractically all of the albite component from plagioclase.Following a curye of increasing T and P while maintain-ing an increasing XCO, consistent with the quartz * cal-cite : wollastonite equilibrium, the rock eventuallyreached a pressure where calcite + wollastonite + anor-thite + diopside reacted to form garnet + quartz. Thiswould have been for a composition of grossular 83 alongthe fluid independent reaction l68, which would in effectalso be at invariant point I83cw in the terminology ofFigures 59C and 59D. Further reaction allowed the garnetto become slightly more grossular-rich, to about grossular90, but beyond this it appears that either compressionand heating ceased or reaction rates due to fluid loss orslow diffusion did not permit garnet of higher grossularcontent to form. The necessary counterclockwise P-Zpathfor this sequence of events independently supports theearly part of the P-T path claimed for this region on thebasis of assemblages in aluminous rocks (Schumacher etal., 1989; Robinson et al., 1989).

Summary

In the example given above, the role of the diopsidecomponent in all garnet-producing reactions is empha-sized, and this leads to consideration of equilibria in amore complex system and the derivation of a specific newpetrogenetic grid. In the interpretation of this rock, onecan see that it is not just the phases present in the rock

14A

I

c.

K ,roo

- -- REACTION r5|SOP|.€THNUMSERS SI'loW GARNETcoMPosrIoN

- ---'- REACTION 1 CONTOURNUMBERS SHOW FLUIDCOMPOSITloN

I67c

ROBINSON: EYE OF THE PETROGRAPHER. MIND OF THE PETROLOGIST I 805

Fig. 62. Eclogite boudins (dark smudges) in metamorphosedsandstones ofthe Seve nappe, south slope ofGrapesvare, Swed-ish Caledonides. Photograph by the author.

tifully developed with the growth of coarse metamorphictextures. Nearby, however, neither the eclogite nor itscountry rock may show any remaining vestiges of its eclo-gite-facies history, being entirely overprinted by amphib-olite-facies assemblages.

Eclogitized diabase dikes in the Grapesvare area'Swedish Caledonides

My first example is from rocks of the Seve nappe inthe Grapesvare area of the Swedish Caledonides (Fig. 6l)described by Andr6asson et al. (1985), Santallier (1988),and Msrk et al. (1988) and visited personally by me inAugust 1990. The rocks in question are metamorphosedProterozoic sandstones and subordinate shales injectedbv diabase dikes. believed to have formed in an exten-

Fig. 61. Geologic index map of the Scandinavian Caledon-ides showing eclogite localities discussed in this paper. Adaptedfrom a figure provided by P.-G. Andr6asson.

and their compositions that are important but also thespatial and textural relations of the minerals that explainthe reactions involved and their probable sequence.

Mnr,luonpHrc HrsroRy UNDERSTooD oNLyIN TERMS OF FEATURES OF LARGER ROCK BODIES

In the last segment of this paper, I consider petrologyon a larger scale. In many places, rock history can onlybe charted through study of the complete geologic settingof a large outcrop or even an entire mountain. Particu-larly spectacular are well-preserved pyroxene granulites,gabbros, and diabases in the Scandinavian Caledonides(Fig. 6l) that are locally transformed along shear zonesinto eclogite and amphibolite. Here a host of rock com-positions all appear to have been subjected to eclogite-facies conditions, but the exact record they give of thiswas apparently severely dependent on very local stressand fluid availability. In some locations, the original pro-toliths, such as coarse-grained Proterozoic gabbros, py-roxene granulites, and diabases, appear to have been to-tally protected or only slightly affected by Caledonianeclogite-facies overprinting. Elsewhere, eclogite is beau-

Fig. 63. Photogeologic map of distribution of metamor-phosed diabase layers and boudins at Grapesvare. Boudins innorthern two-thirds of map area are eclogite; those to south (in-dicated D) are diabase with garnet coronas and omphacitic py-roxene rims. Modified from a figure provided by P.-G. Andr6as-son.

l 806

Fig. 64. Outcrop views of eclogite boudins at Grapesvare.Top: Lower contact of large boudin with enveloping countryrock below. This boudin could not be seen on August 7,1990,because ofsnow cover from the previous winter. Bottom: Partialviews of two small boudins. Light colored area in center of bou-din is garnet-rich eclogite; darker ends are amphibolitized. Pho-tographs provided by P.-G. Andr6asson.

sional environment along the margin of Baltica. In thisthey closely resemble the typical rocks of the structurallylower Siirv nappe and differ from the latter mainly inbeing involved in more intense metamorphism. A typicalview of the field area is shown in Figure 62.The exposureis about 800/0, intemrpted only by a few areas of moss,small bogs, and lakes. The lighter-colored areas are highlyfolded and lineated metamorphosed feldspathic sand-stone and minor shale with rare calc-silicate. The darkeroval areas are large boudins of metamorphosed diabaseup to about 15 m thick. The detailed distribution of maficboudins over an area of about 8 x l0 km is shown inFigure 63. Some of the boudins occur in rows and areinterpreted to be dismembered portions of individualdikes. Within the map area, there are no obvious strati-graphic or tectonic contacts, though, ofcourse, there is ahigh degree ofoverall strain.

In the northern two-thirds of the map area, all of themafic boudins are eclogite, usually with a sheath of ret-rograde amphibolite of variable thickness (Fig. 64). Eclo-gite is well preserved in some very small boudins (Fig.64, bottom), whereas other large boudins are extensively


Fig. 65. Photomicrograph of eclogite from Grapesvare,showing zoned garnets about I mm in diameter in a matrix ofquartz, omphacite, hornblende, and dark fine-grained symplec-tite of pyroxene and plagioclase after omphacite. Provided byP.-G. Andr6asson.

G A R N E T

E P I D O T E \

/ / I n c r e a s i n g P

Fig. 66. Petrographic detail ofGrapesvare eclogite. Top: De-tailed photomicrograph of gamet at extinction under crossednicols separated from fresh omphacite by a zone ofplagioclase-pyroxene symplectite. Bottom: Sketch of the same contact re-gion. Epidote from an original low-grade assemblage wasoyerg.rown by garnet, with progressively increasing Xr" and withomphacite inclusions Jd31 that were in contact with matrix om-phacite Jd35. During retrogression some omphacite broke downto pyroxene-plagioclase symplectite and was replaced locally orelsewhere completely by amphibole. Provided by P.-G. An-dr6asson.

\ .\ J d

i a "

,j

/ r . :

Fig. 67. Large boudin of metamorphosed diabase from thesouthern part ofFigure 63. Note two geologists for scale to left.Photograph by the author.

amphibolitized. The typical eclogite consists of small eu-hedral garnets in a matrix of well-preserved omphacite,of retrograde pyroxene-plagioclase symplectite, and ofsecondary amphibole (Fig. 65). There is no preservationof original igneous texture except for rare light-coloredpatches that may be relict plagioclase phenocrysts.

A detailed study area along a garnet-omphacite contactis shown in Figure 66. The core of the garnet contains

Fig. 68. Photomicrographs of metamorphosed diabase. Top:Ophitic texture of plagioclase laths with abundant coronas ofgarnet at extinction under crossed nicols. Bottom: Plagioclaselaths with epidote needles separated from fine-grained areas ofpyroxene and amphibole by coronas of high-relief garnet. Pro-vided by P.-G. Andr6asson.

1 807

Fig. 69. Photomicrograph of margin of original igneous py-roxene in metamorphosed diabase boudin. Rim area is suffirsedwith exsolved ilmenite, and remaining pyroxene in rim area con-tains up to 8o/o jadeite content. Provided by P.-G. Andr6asson.

epidote believed to have been included during progradegarnet growth. The garnet shows apparent prograde growthzoning from Xr, : 0.35 to 0.40, and the garnet containsomphacite inclusions with composition Jd3l as com-pared to Jd35 in the coarse matrix omphacite. Omphacitebreakdown to symplectite of less sodic pyroxene * pla-gioclase is localized near the garnet contact. Closest tothe garnet, this consists of pyroxene Jd25 and An20; atthe symplectite edge farthest from garnet, it consists ofpyroxene Jdl I and An25. Very commonly the pyroxeneof this reaction symplectite is replaced by hornblende.Santallier (1988) has estimated conditions of eclogite for-mation at about 700 "C and 19 kbar. Detailed studies ofNd-Sm mineral isochrons in the eclogites (Mork et al.,1988) suggest that the eclogite-facies metamorphism tookplace at 500 Ma in the so-called Finnmarkian event, wellbefore the Scandian final emplacement of the thrust sheets.

In approximately the southern one-third of the maparea all of the mafic boudins are recognizable texturallyas diabase, with a fine ophitic texture of former igneousplagioclase and, locally, plagioclase phenocrysts recogniz-able in hand specimen. The form of the boudins (Fig. 67)and the character of the enclosing rocks is the same. Inthin sections (Fig. 68) there seems to be evidence for twometamorphic events, an early greenschist-facies meta-morphism with formation of epidote in plagioclase anda later high-pressure event with the formation of delicategarnet coronas between plagioclase and mafic minerals inthe matrix. Some original Ti-rich calcic igneous pyroxeneis still preserved (Fig. 69) but has been attacked margin-ally with exsolution of Fe-Ti oxides and shows an in-crease in jadeite component up to 8o/0. It appears thatthese rocks were just beginning the process of eclogiteformation but had gone through very much less of theprocess as compared with similar former dikes just a ki-lometer or two away to the north on the same mountain.What was it that caused these dramatic differences inreaction progress within the confines of a small area ln asingle thrust sheet? Andr6asson and coworkers are now


I 808

Fig. 70. Pavement outcrop at Vinddoladalen, Trollheim,Norway, showing progressive development of eclogite along ashear zone in coarse ophitic gabbro (shown to left). Gray areasare dominated by garnet, which tends to form coronas aroundlarge remaining plagioclases. Photograph by the author.

striving to understand the meaning of these differencesby studying mafic boudins in a transitional position mid-way up the slope.

Eclogitized Proterozoic gabbro, Vinddoladalen,western Trollheim, Norway

My second example comes from exposures of Protero-zoic basement rock about 100 m below a basement-cov-er contact in Vinddsladalen, western Trollheim (Fig. 6l),studied by Tsrudbakken ( I 98 I ) and visited by me in I 98 l.Here, there is an extensive sequence ofCaledonide thrustsheets, and both basement and tectonic coverhave been very complexly refolded. Within this basementthere is a boudin of coarse gabbro and metamorphosedgabbro about I 80 m thick. Within the core of the boudin,the gabbro appears pristine, with large plagioclase lathscommonly retaining finely exsolved opaque dust. Locallywithin the gabbro, there are shear zones, several ofwhichare well exposed on polished pavements (Fig. 70). Alongthe margins of the shear zones the gabbro shows garnetcoronas around plagioclase, and inward the rock changesto a gneissic rock with some relict plagioclase and thento an eclogite gneiss. The outer margins of the gabbro andeclogite against Proterozoic gneissic country rock are ev-erywhere converted into gamet amphibolite or even gar-net-free amphibolite, in some locations apparently pro-duced by hydration directly from gabb

Documents

The eye of the petrographer, the mind of the petrologistThe eye of the petrographer, the mind of the petrologist Pnrpn RosrNsoN Department of Geology and Geography, University of Massachusetts,