American Mineralogist, Volume 76, pages 756-772, 1991

Coronal reaction textures in garnet amphibolites of the Llano Uplift

Wrr,r,ravr D. Clnr,soN. CaNrsRr,l, D. JoHNsoNDepartment of Geological Sciences, University of Texas at Austin, Austin, Texas 78713, U.S.A.

ABSTRACT

Almandine-rich garnets in mafic amphibolites of the Llano Uplift of central Texas dis-play remarkably well-organized coronal textures with characteristics diagnostic of diffu-sion-controlled reaction. The garnets have undergone static resorption reactions of threetypes: (l) reaction with a metamorphic fluid along fractures to produce symmetrical layersof magnetite and symplectitic labradorite * ferroan pargasite; (2) reaction with quartz toproduce coronal layers ofplagioclase (oligoclase-andesine) + magnetite and orthopyroxene+ augite; and (3) reaction with omphacite to produce coronal layers of symplectiticlabradorite + ferroan pargasite + magnetite and magnesio-hornblende * oligoclase +orthopyroxene. Material-balance calculations assuming volume conservation in an inert-marker reference frame demonstrate that none of these reactions can be considered iso-chemical. Instead, substantial amounts of Na, Ca, Fe, Mg, Si, and Al must have beentransported across the boundaries ofthe reaction bands.

An open-system diffusion model for the garnet-quartz and garnet-omphacite reactionbands establishes that each ofthe assemblage zonations observed is stable with respect todiffusion over a substantial range of possible ratios of the phenomenological coefficientsfor intergranular diffusion. Differences in the relative values for the phenomenologicalcoefrcients between the garnet-quartz and, garnet-omphacite reaction bands may reflectdifferences in the chemistry of the intergranular fluid, particularly in the degree of silicasaturation. Garnet-omphacite reaction bands exhibit petrographic evidence for the pro-gressive replacement of orthoplroxene by hornblende, indicating that the product assem-blage in this reaction band evolved over time. Calculations based on the open-systemdiffusion model suggest that this temporal evolution results from changes in the amountsof material transported across the boundaries of the reaction band in response to theprogress ofa reaction (external to the garnet-omphacite corona) in which omphacite breaksdown to sodian augite + oligoclase * magnesio-hornblende. To be petrologically valid,models of diftrsion-controlled reactions must therefore address rigorously the difficultquestions ofthe degree ofchemical exchange ofa reaction with its surroundings and theextent to which that exchange may vary as reaction progresses.

ft.crnonucrroN mineral segregations (Carlson, 1989), analysis of coronalstructures should produce insight into reaction mecha-

The textures of metamorphic rocks reflect in intricate nisms that also operate in metamorphic rocks with lessways the elaborate interplay between the kinetics of in- exotic textures.tergranular diftrsion and the kinetics of other processes In this work, therefore, we seek a quantitative under-essential to recrystallization, such as the rates of break- standing of the kinetics of intergranular diffusion in co-down of unstable phases, the rates of nucleation and ronal reaction bands around garnets in a metagabbro bodygrofih of stable phases, and the rates of change of inten- from the Llano Uplift of central Texas. We use a trans-sive parameters (cf. Ridley and Thompson, 1986; Carl- port model based in nonequilibrium thermodynamics toson, 1989). In some instances, the relative importance of analyze the petrographic and compositional features ofthese factors is evident for example, the kinetic domi- these coronal textures. In contrast to previous attemptsnance of the diffusion process is expressed with unusual by others to explain analogous textures in terms ofsteady-clarity in coronal reaction bands such as those between state closed-system models, we argue that these texturescrystals of olivine and plagioclase in metagabbroic rocks can be rigorously explained only as the products of dif-(e.g., Johnson and Carlson, 1990) and in some mineral fusion-controlled reactions in an open system in whichsegregations (e.g., those described by Foster, 1981). Be- the local product assemblages may change with time incause the kinetics of intergranular diffusion may limit response to the progress of reactions taking place beyondreaction rates even in rocks lacking reaction coronas or the confines ofany individual coronal reaction band.

0003-004x/9 l /0s06-0756$02.00 7 s6

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In this volume honoring J. B. Thompson, Jr., it is fit-ting to acknowledge that his articulation of the conceptof local equilibrium in metasomatic processes, publishedover 30 years ago (Thompson, 1959), has been the basisfor nearly all subsequent attempts to understand ditru-sion-controlled reaction textures. Thompson's recogni-tion that sufficiently small regions of a rock may approachinternal chemical equilibrium, despite the presence ofgradients in chemical potential from one such region toanother, provides exactly the perspective required to an-alyze metamorphic reactions for which the kinetics arecontrolled by rates of diffusion rather than by the ratesof dissolution or precipitation at mineral surfaces. Thisconcept of local equilibrium in metasomatic processesmakes it possible to link the mineral assemblage andcompositions at any point within a reaction band to thechemical potentials at that point, even though the reac-tion band as a whole is a nonequilibrium system char-acteized by gradients in the chemical potentials of com-ponents in the intergranular medium.

Fisher (1973,1975,1977) brought the rigor ofnon-equilibrium thermodynamics to bear on the quantitativeanalysis of diffusion-controlled metamorphic processes,and Brady (1977) described important consequences ofdiffusional processes for metasomatic zones in metamor-phic rocks. On this foundation, Joesten (1977) construct-ed an integrative mathematical formalism for the analysisof the zoning of mineral assemblages during diffusionmetasomatism. Joesten's elegant model allows one toevaluate the diffusional stability of a reaction band forany arbitrary set of intergranular diffusion coefficients,knowing only the compositions of the phases involved inthe reaction. Features of the reaction band that can becalculated from the model include the relative widths ofproduct layers, their modal proportions, the fluxes ofel-ements across layers in the reaction band, and the stoi-chiometry ofthe reactions occurring at each interface be-tween layers in the reaction band. A similar approach,using a somewhat diferent formalism, has been devel-oped by Foster (198 1) and applied to the analysis of min-eral segregations in metamorphic rocks (Foster, 198 l,1 9 8 3 , 1 9 8 6 ) .

Quantitative analysis of diftrsion in coronal textureshas been restricted so far to the study of reactions be-tween olivine and plagioclase in metagabbroic rocks (Ni-shiyama, 1983; Joesten, 1986a, 1986b; Grant, 1988;Johnson and Carlson, 1990). The results ofthese inves-tigations emphasize an important discrepancy: whereascoronal reaction textures observed in rocks commonlyexhibit open-system behavior, the diffusion model ofJoesten (1977) assumes, for all components with appre-ciable chemical potential gradients across the reactionband, that no transport ofmaterials beyond the bound-aries of the reaction band takes place. Because of thisdiscrepancy, the attempts ofNishiyama (1983) and Grant(1988) to replicate natural olivine-plagioclase reaction

757

textures were not completely successful. Joesten (1986b,p.479) briefly considered the possibility ofopen-systembehavior by arbitrarily specifoing an external flux for asingle component in an olivine-plagioclase reaction co-rona. Johnson and Carlson (1990) extended Joesten'smodel to fully open systems by specifuing, on the basisof material-balance calculations, the magnitudes of allcomponent fluxes beyond the boundaries ofreaction bandsin olivine-plagioclase coronas in metagabbros of the Ad-irondack Mountains. With this approach, the two mostcommon types of olivine-plagioclase coronas found inthe Adirondacks were modeled successfully, and it wasdemonstrated that those coronas probably did not origi-nate by a single steady-state diffusion process, but insteadevolved with time through one or more transient states.

Although olivine-plagioclase reaction bands are by farthe most common and most carefully studied coronaltexture, equally well-developed coronas are often foundsurrounding garnets in mafic amphibolites (e.9., Mischand Onyeagocha, 1976). The coronas around garnet pre-sent an intriguing comparison with the olivine-plagio-clase coronas insofar as they originate at lower tempera-tures and pressures, and possibly at higher activities ofaqueous fluid; in addition, there is no question ofa pos-sible igneous contribution to their origin (cf. Joesten,1986a). Thus this inquiry into the origin ofcoronas sur-rounding garnets in amphibolites of the Llano Uplift al-lows a test, under different geologic conditions, of theassertion ofJohnson and Carlson (1990) that many nat-ural coronal textures can be quantitatively understoodonly as the consequence of open-system diffirsion-con-trolled reactions.

OccunnrNcE AND DEscRrPTroN oFCORONAL TEXTURES

With few exceptions, almandine garnets of the LlanoUplift show textural evidence, compositional evidence,or both, of partial resorption, whether they occur in mafic,pelitic, or semipelitic units. Although almandine garnetsin many localities across the uplift are surrounded byroughly concentric zones of plagioclase + cordierite +biotite (in pelitic rocks) or plagioclase + hornblende +magnetite (in mafic rocks), this article restricts its atten-tion to a single locality, the informally named "WhittRanch metagabbro," in which the resorption textures showsubstantial mineralogic variety and particularly strongspatial organization. The Whitt Ranch metagabbro isconsidered below in its regional context, and the petro-graphic and compositional features of the coronal reac-tion textures found within it are described in detail.

Geologic setting

Regional geology. The Llano Uplift exposes metaig-neous and metasedimentary rocks of the Llano Super-group, for which protolith ages approach 1300 Ma (Walk-er, 1988). Between -1167 and - 1080 Ma, the protolithsunderwent amphibolite-facies metamorphism at moder-ate to high pressures (Carlson and Nelis, 1986; Wilkerson

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

758

Fig. l. Photomicrograph of a coronal texture produced bygarnet-omphacite reaction. The approximate location of theoriginal surface of a euhedral garnet crystal is now marked by ahexagonal rim ofmagnetite crystals, separated from central relictgarnet by a radial symplectite of amphibole and plagioclase withscattered magnetite crystals. Close to the relict garnet (especiallyto its right), linear concentrations of magnetite mark the outerboundary ofan incomplete concentric secondary corona. Longdimension of image is 1.2 mm.

et al., 1988) accompanied by complex polyphase defor-mation (Nelis et al., 1989; Carter, 1989). Deformationceased and the metamorphic rocks rose to shallower lev-els of the crust before the intrusion of numerous graniticplutons, stocks, and dikes at - 1090-1050 Ma (Zartman,1964; Garrison et al., 1979). At the time of this intrusiveepisode, the uplift experienced a regionally extensive re-heating and hydration event sufficient to reset K-Ar sys-tems in metamorphic hornblende and biotite (Garrisonet al., 1979) and to induce both widespread static recrys-tallization (Nelis et al., 1989; Carter, 1989) and miner-alogic and isotopic reequilibration to fluids geneticallylinked to the granitic intrusions (Bebout and Carlson,1986). For a more detailed review ofthe regional geologyof the Llano Uplift and a discussion of related Precam-brian rocks in Texas, consult Mosher (in press).

In this regional context, the abundant petrologic evi-dence for the instability of almandine in the uplift is at-tributed to a polymetamorphic history in which garnetgrew in an early dynamothermal event of moderate tohigh pressure but was partially resorbed by reaction dur-ing a later static metamorphic event at low pressure. Phaseequilibria (e.g., Holdaway and Lee, 1977) and field rela-tions elsewhere (e.g., Chesworth, 1972) indicale that gar-net-resorption reactions should occur when garnetiferousassemblages in typical pelitic and mafic bulk composi-tions are reequilibrated at pressures in the range of 2-4kbar. Such pressures aro consistent with barometric es-timates for metamorphic assemblages in the uplift whosedecussate textures and association with local fluid infil-tration clearly indicate growth during the late static meta-morphic event (Bebout and Carlson, 1986).

The Whitt Ranch metagabbro. The Whitt Ranch me-

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

Fig.2. Photomicrograph of a coronal texture produced bygarnet-quartz reaction. Central garnet crystal is separated frompolycrystalline quartz by an inner layer of plagioclase speckledby tiny crystals of magnetite and an outer layer of intergrownclinopyroxene and orthopyroxene. Long dimension of image is4.5 mm.

tagabbro body is located approximately 2 km northwestof the community of Baby Head, just northeast of TexasHighway 16, at Universal Transverse Mercator coordi-nates I4RNK3 I 8 I 8 I . (This locality is entirely on privateproperty. Prior permission must be obtained from theowner before visiting the locality. The senior author willbe pleased to provide assistance in contacting the land-owner.) Mapping by Jordan (1970) and Moseley (1977)shows the metagabbro body to be an elliptical mass ofgarnet amphibolite 0.5 by 0.9 km in diameter, completelysurrounded by quartzofeldspathic Valley Spring Gneiss.A metasomatic border zone extends inward about 30 mfrorn the contact of the metagabbro with the gneiss; it ischaracterized by larger grain sizes, by the presence ofal-kali feldspars, and by the complete replacement of garnetby biotite * plagioclase + amphibole. Alkali feldspar andbiotite are very rare or absent elsewhere in the metagab-bro. The interior of the body is comprised of locally fo-liated and isoclinally folded metagabbro cut by numerousdikes of granite pegmatite up to 10 m wide.

Mineral assemblages are locally variable. The mostcommon rock type is dominated by numerous large (0.5-3 mm) porphyroblasts of manganiferous almandine,strongly zoned from spessartine-rich cores to spessartine-poor rims. Included in the garnets are small crystals ofepidote, edenitic hornblende, plagioclase, and rare om-phacite. The garnets are embedded in a very fine-grained(0. l- l0 pm) symplectitic matrix of intimately intergrownhornblende, plagioclase, and clinopyroxene. They areseparated from this matrix by narrow (-200-300 pm)coronal reaction bands comprised of varying amounts ofplagioclase, amphibole, in most cases magnetite, and insome cases orthopyroxene (Fig. l). For reasons detailedbelow, these textures are believed to arise from reactionsbetween garnet and omphacitic clinopyroxene. Samplescollected in the vicinity of large dikes of postkinematic

z lslFig. 3. Textures produced by garnet-fluid reaction along frac-

ture in sample SS2. (a) Photomicrograph. Central zone of Mag(black) is separated from Alm (at left and right) by a vermicularintergrowth of Fe-Prg (darker gray) and Lab (lighter gray). Longdimension of image is 300 pm. (b) Backscattered-electron image.Numerals at bottom identify layers as follows: I : Alm; 2 :I-ab (black) + Fe-Prg (gray); 3 : Mag. Scale bar at top right is100 pm long.

pegmatite often show a matrix assemblage enriched inhornblende at the expense ofclinopyroxene and plago-clase, and many of these samples contain garnets in whichmore than one resorption event is made evident by aconcentric repetition of the sequence of zones of coronalproducts. In some specimens, one finds pods of poly-crystalline quartz that are free of plagioclase and amphi-bole but that contain slightly more manganiferous garnetssurrounded by coronas of plagioclase, magnetite, ortho-pyroxene, and clinopyroxene (Fig. 2). These quartz-richvolumes may be remnants of quartz veins introduced intothe gabbro prior to its metamorphism. Further petro-graphic and chemical description of the Whitt Ranch rocksmay be found in Moseley (1977) and in Harris (1986).

On the basis of the regional geologic setting, the pet-rologic features noted above, and the thermobarometricdata of Schwarze (1990), the geologic evolution of the

759

Fig. 4. Textwes produced by garnet-quartz reaction in sam-ple SS2. (a) Photomicrograph. Qtz (right) is adjacent to layer ofOpx + Aug (darker gray); Alm (left) is adjacent to layer of Olg(lighter gray) speckled with scattered crystals of Mag (black).Long dimension of image is 160 pm. (b) Backscattered-electronimage. The contrast with a in the abundance and grain size ofmagnetite reflects the extent of variation within the sample. Nu-merafs at bottom identiff layers as follows: I : Alm; 2 : Olg(black) + Mag (white); 3 : Opx (light gray) + Aug (dark gray);4 : Qtz. Scale bar at top right is l0 pm long.

Whitt Ranch metagabbro is believed to include this se-quence of events: (l) gabbro crystallization and veiningby quartz; (2) metamorphism to -650 'C at 6-8 kbaraccompanied by polyphase deformation; (3) static re-heating and hydration of the entire body to -600 "C at2-3 kbar; and (4) additional local episodes of static re-heating and hydration, restricted to the vicinity of latepegmatite dikes.

Petrography and mineral chemistry of coronas

Garnet reaction textures in the Whitt Ranch metagab-bro are ofthree types: (l) reaction between garnet and ametamorphic fluid penetrating transcurrent fractures (Fig.3); (2) reaction between garnet and quartz (Fig. 4); and.

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

422

760

3 l t lFig. 5. Textures produced by garnet-omphacite reaction in

sample SS2. (a) Photomicrograph. Alm (at left) is surrounded bya layer of Lab (white) with sparse elongate crystals of Fe-Prg(light $ay); this is ringed by Mag (black); material to the rightof the Mag is a layer of intergrown Opx + Olg + Mg-Hbl (witha narrow band of the amphibole immediately adjacent to themagnetite layer), and a layer of very fine-grained Olg + Na-Aug+ Mg-Hbl (slightly darker material at right). Boundary betweenthe two outerrnost layers is indistinct in this view because theyare so fine grained, but each occupies about half of the darkregion to the right of the Mag. Long dimension of image is 750pm. (b) Backscattered-electron image. The contrast with a in thethickness of the reaction band and the layer modes reflects theextent of variation within the sample. Numerals at bottom iden-tify layers as follows: I : Alm; 2: Lab (black) + Fe-Prg (gray)+ minor Mag (white); 3 : Opx (whire) + Mg-Hbl (gray) + Olg(black); 4 : Na-Aug (gray) + Mg-Hbl (gray) + Olg (black). Scalebar at top right is 100 pm long.

(3) reaction between garnet and omphacite (Fig. 5). Allthree reaction textures may be present in a single thinsection, and in fact, individual garnet crystals lying astridean interface between a quartz-rich volume and a clino-pyroxene-rich volume commonly exhibit all three reac-tion bands at different places along the garnet's periphery.

Modal proportions of phases in the product layers were

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

2

obtained by an image-analysis technique that digitizedbackscattered-electron images of the product zone, seg-regated phases according to their intensities in the image,and computed relative areas of all phases in the image.Multiple determinations were made, proportions in threeto eight separate areas were evaluated, and the mean andstandard deviation of the measurements are reported.

Compositions were obtained by electron microprobeanalysis using wavelength-dispersive techniques. Datawere collected with an accelerating potential of 15 kV, asample current of l5 nA on brass (or l0 nA for feldspar),and counting times of 40 s for each element. Data reduc-tion employed the empirical correction scheme of Albeeand Ray (1970). Natural and synthetic silicates and ox-ides were used as standards. In most cases, at least threeanalyses were made of each phase in each layer. Repre-sentative individual analyses are reported in the tablesthat follow, but idealized compositions based on the av-erage ofseveral analyses (also tabulated below) were usedin all calculations. The garnet analyses selected for inclu-sion in the tables and used for the transport calculationsare from points -100 pm away from the edges ofgarnetcrystals. Because the crystals are zoned and partially re-sorbed, analyses at these locations best represent thecomposition of the garnet involved in the corona-formingreaction, insofar as they are as near to the crystal rim asis possible without encountering interference from the ef-fects of intracrystalline diffirsion engendered by the re-sorption reaction. In the following petrographic descrip-tions, nomenclature for amphibole follows Hawthorne(1983), with Fe'z*/Fe3+ ratios fixed by renormalizing, onthe basis of 22 O and 2 OH, the sum of all cations exceptK, Na, and Ca to a total of 13. Pyroxene nomenclatureconforms to Morimoto et al. (1988). These abbreviationsare used in tables and figures: Alm : almandine; Aug :augite; Fe-Prg : ferroan pargasite; I^ab : labradorite; Mg-Hbl : magnesio-hornblende; Mag : magnetite; Na-Aug: sodian augite; Olg : oligoclase; Omp : omphacite;Opx : orthopyroxene; Qtz : quartz.

Garnet-fluid reaction textures. Garnets that were frac-tured prior to the resorption event display symmetricbands of reaction between the garnet and the metamor-phic fluid penetrating the fracture (Fie. 3). These bandsconsist of a central layer of magnetite separated from thegarnet by a fine-grained symplectitic intergrowth of pla-gioclase and amphibole crystals, both elongated in thedirection perpendicular to the fracture walls. Although insome cases the amphibole crystals extend across the en-tire reaction zone, in many instances one end of an am-phibole crystal will be in contact with the magnetite layer,and the crystal will taper to a point or terminate morebluntly before reaching the garnet at the wall of the frac-ture. Plagioclase in these reaction bands is labradorite; itscomposition varies little, in contrast to plagioclase in gar-net-quartz and garnet-omphacite reaction bands. Am-phibole crystals are ferroan pargasite; they are too smallto permit the detection of zoning along their lengths. Fromone hand sample to another, mineral compositions are

Trele 1, Compositions and proportions of minerals in garnet-fluid reaction texture

761

TABLE 2. Compositions and proportions of minerals in garnet-quartz reaction texture

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

Mag Fe-Prg olg Mag opx Aug

SiO, 38.3 62j n.d. 51.0 52.7Al,o3 21.1 23.6 0.30 O.23 0.66FeO- 24.8 0.36 90.4 30.3 12.2MgO 4.58 n.d. n.d. 15.8 12.0MnO 3.29 n.d. n.d. 2.08 0.83TiO, 0.14 n.d. 0.09 0.03 0.04CaO 7.82 4.89 n.d. 0.57 21.4Na,O n.d. 8.80 n.d. n.d. 0.66GO n.d. n.d. n.d. n.d. n.d.

Total 100.0 99.8 90.8 1 00.0 100.5ldealized tormulas used in modeling

si 3.00 2.75 0.00 1.99 2.O0 1.00Al 1.96 1.25 0.00 0.01 0.03 0.00Fe 1.63 0.00 3.00 0.99 0.36 0.00Mg 0.49 0.00 0.00 0.92 0.70 0.00Ca 0.67 0.25 0.00 0.03 0.85 0.00Na 0.00 0.75 0.00 0.00 0.04 0.00o 1 2 8 4 6 6 2Modal % 63.3 5.0 17 .8 13.9+1 esd 1 .5 0 .8 1 .0 1 .0

/Vofe: Values in upper part of table are in wtTo ol oxides; n.d. : notpresent at levels above detection limit.

. All Fe reported as FeO.

nearly uniform, but proportions of phases in the productzone may change markedly. Table I presents electron mi-croprobe analyses and mineral modes of the products ofa representative garnet-fluid reaction in sample SS2.

Garnet-quartz reaction textures. Where garnet adjoinedquartz prior to the resorption event, reaction has pro-duced coronal structures consisting of an inner layer ofsodic plagioclase * minor magnetite surrounded by anouter layer oforthopyroxene + augite (Fig. 4). Feldsparcomposition changes systematically and gradually acrossthe layer: plagioclase adjacent to garnet (typically ande-sine, -An35) is more calcic than plagioclase adjacent topyroxene (typically oligoclase, -An,r). In some coronas,atrgite crystals may be slightly more abundant near thecontact ofthe pyroxene and plagioclase layers than theyare at the opposite edge ofthe pyroxene layer. Althoughmineral compositions are nearly uniform from one handsample to another in these reaction bands, the propor-tions of product phases are somewhat variable from rockto rock. In some samples, layers of plagioclase and py-roxene are nearly equal in width; in others, the plagio-clase may be as much as twice as thick as the pyroxenelayer. The volume percentage of orthopyroxene in thepyroxene layer ranges from -500/o in soms samples tol00o/o in others. Table 2 presents electron microprobeanalyses and mineral modes of the products of the rep-resentative garnet-quartz reaction from sample SS2 thatis imaged in Figure 4b.

Garnet-omphacite reaction textures. Reaction bands thatnow separate garnet from an extremely fine-grained (0. l-l0 pm) symplectitic intergrowth of sodian augite + oli-goclase * magnesio-hornblende are believed to have aris-en by reaction of garnet with an omphacitic clinopyrox-ene. This belief is founded upon three observations. First,plagioclase crystals in these intergrowths are in opticalcontinuity over polygonal areas l-4 mm in diameter, in-dicating that the intergrowths are pseudomorphs after acoarser-grained phase. When viewed petrographically un-der crossed nicols, the rock's texture takes on the ap-pearance ofan equigranular mosaic typical ofhigh-grade

sio,Al2o3FeO-MgoMnOTio,CaONa2OGo

TotalModal %+1 esd

38.7 53.721.5 29.625.6 0.356.23 n.d.0.31 n.d.0.09 n.d.7 .65 11 .5n.d. 5.03n.d. n.d.

1 00.1 100.069.70.5

n.d. 42.10.28 15.9

91.6 12.4n.d. 12.7n.d. 0.080 . 1 7 1 . 1 4n.d. 11.4n.d. 2.93n.d. 0.05

92.1 98.710.2 20.00.1 0.6

Note.'Values in upper part of table are in wty" of the oxide; in lowerpart, elements not used in the model are excluded and values are atomsper formula unit; n.d. : not present at levels above detection limit.

. All Fe reoorted as FeO.

mafic metaigneous rocks. Second, the field relations, bulkcomposition, and textures of the Whitt Ranch metagab-bro are closely analogous to those of eclogitic metamaficrocks in the northwestern Llano Uplift described byWilkerson et al. (1988) in which omphacitic clinopyrox-enes have been only partially reconstituted as very sim-ilar (although coarser) vermicular intergrowths of sodianaugite and plagioclase (cf. Fig. 2 of Wilkerson et al., 1988).Third, some of the garnets in the Whitt Ranch occurrencepreserve inclusions of omphacite close in composition tothat found as coarse primary crystals in the eclogite rem-nants.

In contrast to the garnet-fluid and garnet-quartz reac-tion bands described above, the garnet-omphacite reac-tion bands display substantial mineralogic variation fromrock to rock. The principal differences are in the anange-ment and relative proportions of orthopyroxene andhornblende. In one specimen in particular (sample SS2),the garnet-omphacite reaction bands exhibit a strikingvariety of product assemblages and modes. Some reac-tion bands contain an outer layer ofabundant orthopy-roxene intergrown with oligoclase and magnesio-horn-blende, separated from the garnet by an inner layer richin labradorite with minor ferroan pargasite and minuteamounts of magnetite (Fig. 5b). In other reaction bands,a nearly monomineralic layer of magnesio-hornblende(often incorporating a sharp internal break in composi-tion) is interposed between the inner and outer layers(Figs. 6a-6c). When this central amphibole layer is pres-ent, the amounts of orthopyroxene and oligoclase in theouter layer are reduced, and the amount of rnagnesio-hornblende in the outer layer is augmented; also, smallamounts of magnetite are sometimes concentrated nearthe interface between the inner labradorite-rich laver and

762

Ir lFig. 6. Variation of garnet-omphacite reaction textures in

sample SS2. Backscattered-electron images. Scale bars at top rightare 100 pm long. Numerals at bottom identify layers as follows:I : Alm; 2 : l-ab (black) + Fe-Prg (gray) + Mag (white); 3 :Mg-Hbl (gray) + Olg (black); 4 : Opx (white) + Mg-Hbl (gray)+ Olg (black); 5 : Na-Aug (gray) + Mg-Hbl ($ay) + Olg (black).The images from a through f are interpreted as the result ofprogressive replacement of Opx and Lab by Mg-Hbl in reactions

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

---r*

occurring at the interface between layers 2 and 3 and at tneinterface between layers 3 and 4. This interpretation suggeststhat an initial increase in the proportion of Mg-Hbl in layer 3(a) leads to a nearly monomineralic layer of Mg-Hbl (b) and (c)that engulfs Opx (d) and (e) and eventually eliminates it (0. Aconcurrent increase in the proportion ofFe-Prg within layer 2 isalso noticeable.

CARISON AND JOHNSON: CORONAL REACTION TEXTURES

Trar-e 3. Compositions and proportions of minerals in garnet-omphacite reaction texture

'163

Lab F+Prg opx Mg-Hblolg omp

sio,Alro3FeO-MgoMnOTio,CaONaroK.O

Total

39.521 .925.57.200 .140.o26.57n.d.n.o.

100.8

3.001.981.660.780.580.001 2

53.830.00.31n.d.n.o.n.o.

1 t . 54.90n.d.

42.813.613.612.70.040.80

11 .52.800.03

6.082.501.662.641.790.83

233.50.6

52.40.29

24.921.',|o.14n.o.0.440.04n.d.

1.99o.o20.781 . 1 90.020.006

10.01 . 7

61.624.50.26n.d.n.d.n.d.5.408.570.02

100.4

2.771.230.000.000.23o.778

34.72.7

45.59 .15

12.013.8n.d.2. ' t1

1 1 . 52.210 .13

96.4

6.691 .591.483.021 .810.63

238.50.7

55.08 .105.789.830.050 . 1 1

15.35.54n.o.

99.7

2.000.350.170.510.580.38o

100.5 97.9ldealized tormulas used in modeling

SiAIFeMgCaNaoModal o/"

+1 esd

2.401.600.000.000.600.40I

43.31 . 9

Nofe. Values in upper part of table are in wt% ol the oxide; in lower part, elements not used in the model are excluded and values are atoms performula unit: n.d. : not present at levels above detection limit.

- All Fe reoorted as FeO.

the central amphibole layer. In many bands, orthopyrox-ene is nearly or completely absent, oligoclase is sparse,and magnesio-hornblende dominates the outermost co-ronal layer (Figs. 6d-6f). A continuum oftextures inter-mediate between those of Figures 5b and 6f is observedin a single thin section of sample SS2.

Most garnet-omphacite reaction bands exhibit a sharpdiscontinuity in plagioclase cornposition near the centerofthe band: feldspar in the inner plagioclase-rich layer isapproximately Arruo, whereas feldspar in and near the out-er layers rich in hornblende + orthopyroxene averagesAnrr. This change, barely perceptible as lighter and darkerregions in the backscattered-electron image of the plagio-clase layer in Figure 5b, coincides with an abrupt changein the compositions of the associated amphibole. Am-phibole in the inner labradorite-rich layer has higher Al-Si and Fe-Mg ratios than amphibole in the orthopyroxene* magnesio-hornblende * oligoclase layer, and inter-mediate compositions characterize hornblende where itis found in central monomineralic layers. In Figure 5b,the sharp boundary between the different compositionsof amphibole and plagioclase, which lies within layer 2,does not coincide with the edge of the orthopyroxene-bearing layer. Textural details in the inner coronal zoneof labradorite + ferroan pargasite resemble those in thegarnet-fluid reaction bands: both feldspar and amphiboleare elongated in the direction perpendicular to the garnetsurface, and amphibole crystals are usually rooted in themagnetite layer (if it is present), tapering and often ter-minating before reaching the garnet surface (Fig. l).

For the metagabbro body as a whole, orthopyroxene-free coronas are much more common than their ortho-plroxene-bearing counterparts. Even in samples in whichsome coronas contain orthopyroxene, other coronas maynot. Garnet-omphacile reaction bands in proximity toquartz-rich regions of these rocks are more likely to in-

clude abundant orthopyroxene and to lack monomrner-alic amphibole layers than are similar reaction bands moredistant from quartz-rich regions.

Table 3 presents electron microprobe analyses andproduct mineral modes for the representative garnet-om-phacite reaction band from sample SS2 that is imaged inFigure 5b, along with the average analysis of an inclusionof omphacite in garnet from sample 13.

Ancillary reaction textures. Three additional reactionsin which garnet is not directly involved are also evidentin rocks of the Whitt Ranch metagabbro. Most significantis the breakdown reaction of omphacite to a fine-gminedsymplectite of oligoclase * sodian augite + magnesio-hornblende. In contrast to all other reactions in the rock,this reaction has generated textures vrithout strong spatialorganization. As in the eclogite remnants described byWilkerson et al. (1988), it probably reflects in situ re-placement of the omphacite by an assernblage of minerals

TABLE 4. Compositions and proportions of minerals in symplec-tite replacing omphacite

Na-Aug Mg-HblobSO,Alro"FeO-MgoMnOTio,CaONa2OKrO

TotalModal %+1 esd

66.02'1.7o.42

n.d.n.d.n.d.

2.688.95

n.d.

52.82.007.33

14.30.090.06

21.90.77

n.d.

47.19.86

12.214.50 .100.64'12.11.99n.o.

98.58.15.6

99.345.24.3

99.846.74 .1

Note: Values in upper part of table are in wt% of the oxide; n.d. : notoresent at levels above detection limit.

'All Fe reoorted as FeO.

764 CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

of lower density. It is probable that this reaction pro-ceeded simultaneously with the corona-forming reac-tions, because both represent static hydration and re-equilibration at lower pressures. Table 4 presents electronmicroprobe analyses of the phases in symplectites in sam-ple SS2 that are believed to be pseudomorphs after anoriginal omphacite matrix in which the garnets of thatrock were formerly embedded.

Two additional subsidiary reactions have also takenplace: where quartz and magnetite were once in contactwith omphacite, monomineralic reaction rims of clino-pyroxene (around quartz) and of hornblende (aroundmagnetite) are observed. Both of these monomineralicreaction bands are volumetrically inconsequential.

OnrcrNI oF THE coRoNAL TEXTuRESThe three dissimilar garnet resorption reactions repre-

sent three distinct responses to the same reheating andhydration event. Differences among the reactions are ev-idently the result of variations in the local chemical en-vironment at the scale of a few tens of micrometers, in-dicating that the extent of chemical exchange, at least forsome components, may be highly restricted. As noted inthe review above, many previous attempts to accountquantitatively for coronal reaction textures have reliedupon the dubious assumption that chemical exchange isso severely restricted that the reaction zones may be treat-ed as isochemical subsystems. The following discussionof reaction mechanisms will test the validity of that as-sumption in the Whitt Ranch coronas and will establishthat none of the reactions can be treated as isochemical.We then evaluate the extent to which these reaction zonesmust be in chemical communication with one another orwith other reactions taking place in the rock. That infor-mation will then be used to relax the restriction of iso-chemistry present in Joesten's (1977) steady-state diffirsionmodel, taking the approach of Johnson and Carlson(1990). This will lead to fully quantitative descriptions ofthe origin of the coronal reaction textures in the WhittRanch metagabbro as the products of open-system dif-fusion-controlled reaction.

Reaction mechanisrns

Evidence for diffusional kinetic controls. Fisher (1977,p. 383) set forth the following criteria for the recognitionof textures produced by diffusion-controlled reactions:". . . diftrsion-controlled structures characteristically havea strong spatial organization, with well-defined mineralzones showing sharp changes in compositions at zoneboundaries, all arranged in an orderly sequence of in-creasing or decreasing chemical potential." As illustratedin Figures 3, 4, and 5, all of the garnet resorption reac-tions in the Whitt Ranch metagabbro possess strong spa-tial organization and sharply defined zone boundaries.The data in Tables 1,2, and 3 confirm sharp changes atthe zone boundaries in mineral compositions and modes,and therefore in bulk compositions. The systematic ar-

rangement of phases in terms of the chemical potentialsof the diffusing components will become apparent below,when diffusion models presuming such an arrangementare shown to replicate the arrangement and proportionsof phases in the coronal reaction textures. Therefore, inthe analysis that follows, intergranular diftrsion will bepresumed to exert the predominant kinetic control on theformation of these textures.

The kinetic role of the metamorphic fluid. The influ-ence of the metamorphic fluid on the reaction kineticsdeserves careful consideration. Metasomatic reactionsdriven by massive infiltration of fluids are evidently re-stricted to a shell extending about 30 m inward from themargins of the metagabbro body. Substantial amounts ofalkalis have been imported into this shell from the sur-rounding quartzofeldspathic gneiss, producing assem-blages including alkali feldspars and biotite that areanomalous by comparison to the assemblages in the in-terior ofthe body.

Although material transport dominated by bulk mo-tion of a fluid brine was probably restricted to the meta-somatic shell, there is direct evidence that garnet resolp-tion reactions in the interior of the metagabbro body weretriggered by the introduction ofaqueous fluids along frac-tures and grain boundaries. Some thin sections are cut byanastamosing curvilinear veinlets, l-3 mm wide, com-posed predominantly ofrelatively coarse crystals of horn-blende, plagioclase, or both. Garnet crystals within a fewhundred micrometers of these veinlets, unlike othersslightly further away, have undergone two separate re-sorption events, as indicated by concentric double reac-tion bands. In some instances, the secondary (smaller)corona does not extend completely around the peripheryof the garnet, but is instead restricted to the side of thegarnet closest to the veinlet. A secondary corona of thistype is faintly visible around the garnet remnant in Figurel, outlined by a narrow zone of magnetite within thehornblende-plagioclase symplectite. Microtextures suchas these, combined with the observation that rocks inwhich all garnets show multiple resorption events arsfound only near large pegmatite dikes, establish that fluidaccess initiated at least the secondary garnet reso,rptionepisodes. The petrologic ewidence cited above for a re-gionally extensive hydration event at low pressures leadsto the conclusion that fluid played a similar role in acti-vating the primary resorption event that has affected allof the garnets in the metagabbro body.

Nevertheless, there is no evidence that the kinetics offluid motion or the diffusion of fluid components alongfractures or grain boundaries are rate-controlling factorsin the generation ofthese coronal textures. For example,in garnet-omphacite reaction bands like that of Figure 5b,assemblages in the reaction zones do become progres-sively less hydrous inward toward the garnet, but it wouldbe erroneous to infer from this that the coronal structurereflects the kinetics offluid transport, because an equallyimpressive coronal structure is evident in the completelyanhydrous assemblages of the garnet-quartz reaction

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES 765

bands. Also, because reaction bands along fracturesmaintain uniform width instead of tapering inward, andbecause they show no longitudinal differences in mineralassemblage, it is apparent that fluid access along the lengthof fractures is substantially more rapid than ionic diffu-sion along grain boundaries perpendicular to the fracturewalls. Finally, it will be shown below that diffusion mod-els treating the chemical potentials of HrO and O, asexternally controlled parameters satisfactorily replicate theobserved reaction textures, indicating that the kinetics oftheir transport does not play an essential role in the de-velopment of the coronas.

Estimates of material transport

Any analysis of the material transport involved inmetamorphic reactions relies upon an assessment of thematerial balance between reactants and products.Thompson (1959, p. 428-430) points out that statementsregarding the introduction or removal of material aremeaningless without a clear specification of the frame ofreference, but that all choices ofreference frame can pro-vide equally correct descriptions of the material trans-port. The most appropriate frame of reference for theanalysis of diffusion-controlled reactions is difficult tochoose, however, because reaction textures do not, ingeneral, preserve indisputable evidence for or against (1)any particular position for the original interface betweenreactants, (2) volume changes, and (3) gains or losses ofindividual components. Two of these three sources ofindeterminacy must be removed in order to produce anexplicit description ofthe reaction, but definitive petro-logic means of eliminating any of them are rare.

The resolution of this ambiguity is one of the mostformidable obstacles to rigorous analysis of materialtransport in rocks. Previous attempts to calculate mate-rial balance in coronal textures or reaction zones havenecessarily, but arbitrarily, assumed the conservation ofvolume, or the conservation of one or more elements, orboth (e.g., Al and Si: Whitney and Mclelland, 1973;Al:Thompson, 1975; Ca:. Misch and Onyeagocha, 1976; Nand Si: Mongkoltip and Ashworth, 1983; Al: Johnsonand Carlson, 1990) or have assumed the location of theoriginal interface on the basis ofsharp mineralogical dis-continuities (e.g., Thompson, 1975, p. 329-330). In thisarticle, gains and losses of individual components are cal-culated over the complete range of possible locations forthe original interface between reactants by assuming con-servation of volume in an inert-marker reference frame.For the modeling of diffusion, we have chosen estimatesfor positions of the original interfaces that correspondwith boundaries between Al-rich and Al-poor layers inthe reaction bands and with discontinuities in the com-positions of amphibole and plagioclase.

Conservation of volume. Although large changes in thevolume of the solid are likely in reactions dominated bydevolatilization (see, e.g., Thompson, 1975), there maybe a sound theoretical basis for a close approximation toconstant-volume replacement during diftision metaso-

matism involving other types of reactions (Carmichael,1987). Careful examination of the resorption textures inthe Whitt Ranch body revealed no evidence of a volumeincrease, such as the disturbance of structures in the sur-rounding fabric or outward displacements of garnet frag-ments separated by reaction bands developed along frac-tures. Likewise, nothing was seen in the textures thatwould preclude nearly constant-volume replacement. Thusit appears, in the light of Carmichael's (1987) theory andour petro$aphic observations, that the coronal reactionsin the Whitt Ranch metagabbro are reasonably approxi-mated by constant-volume replacement in an inert-mark-er reference frame.

We have examined in reconnaissance the effects of pos-

sible volume changes on the calculations to be presentedbelow. If one presumes small changes in volume, lessthan a few percent, one calculates shifts of similar mag-nitude in the transport of elements, which lead in turn tominor modifications in the calculated relative diffusivi-ties of elements required to form the coronal textures.Such changes do not, however, affect our fundamentalconclusion that the coronas constitute open diffi.rsionalsystems that may evolve through time. If one presumeslarger changes in volume, on the order of +10-200/0, itbecomes impossible to prescribe any set of relative dif-fusivities for the elements that will account quantitativelyfor the formation of the coronas; either the calculatedproportions of phases in the coronas disagree with theobserved proportions, or the observed arrangement ofproduct phases is calculated to be diffirsionally unstable,or both. Given the evidence cited above for a diffusionalorigin for these textures, this finding appears consistentwith the notion that volume changes during corona for-mation were small or negligible.

Locations of original interfaces between reactants. Evenunder the presumption of constant-volume replacement,ambiguities in calculations of material balance arise fromthe fact that the original interfaces between reactants can-not be located except under favorable circumstances. Forthe garnet-quartz and garnet-omphacite reactions, thereare no unambiguous indicators of the locations of theoriginal interfaces between reactants. Consequently, wehave calculated the material balance and modeled theintergranular diftrsion over the fi.rll set of possible loca-tions for the original interfaces. The modeling allows usto place limits on the range of possible choices for thepositions ofthe original interfaces that are consistent withthe arrangement of minerals observed in the reactionbands.

Garnet-fluid reaction. For the garnet-fluid reaction tak-ing place along fractures present in the garnets prior toresorption, the material balance for constant-volume re-placement is uniquely determined if the volume of thefracture is regarded as negligible: essentially all of thevolume now occupied by products was formerly occupiedby garnet. The material transport required under thesecircumstances is illustrated in Figure 7a. The ordinate inFigure 7 has been chosen to emphasize the degree to which

766

each element is conserved within the reaction band dur-ing coronal formation; it is the molar percentage of eachelement that is lost or gained by the reaction band, in-dicated by positive and negative quantities, respectively.For example, a transport percentage of +500/o signifiesthat half the number of moles originally present in thereactants has been exported beyond the boundaries ofthereaction band; a transport percentage of -500/o signifies

100

50

0

-50

-100o 20 40 60 80 100

POSITION OF ORIGINAL INTERFACE

100

50

0

-50

-1000 20 40 60 80 100

POSITION OF ORIGINAL INTERFACE

100

50

0

-50

-1000 20 40 60 80 100

POSITION OF ORIGINAL INTERFACE

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

Fcco(Laz(EFFzTIJocc]u(L

F(EoTLaz(EFFzl.Jl()TElrlTL

that half the number of moles now present in the prod-ucts had to be imported from regions outside the bound-aries ofthe reaction band; an element that is conservedduring reaction plots at 0olo on the ordinate. The abscissain the figure, expressed as a percentage of the distanceacross the reaction band, is used to show (in Figs. 7b and7c) how the calculated material balance would difler de-pending upon one's choice for the location ofthe originalinterface between reactants. In the case ofthe garnet-fluidreaction (Fig. 7a), in which almandine is present on bothsides of the original interface, the calculated materialtransport is unrelated to the presumed position of theinterface, so the lines in Figure 7a arc hoizontal. Thenonzero transport values express the fact that this reac-tion is not isochemical; for this reaction band, a substan-tial gain of Na and substantial losses of Mg, Fe, Al, andSi are required.

Garnet-quartz reaction. In the case ofthe garnet-qvartzreaction, there is no petrographic evidence from whichone can derive the position of the original interface be-tween reactants; that is, there is no way to determine whatfraction of the volume now occupied by plagioclase andpyroxene was formerly occupied by garnet, and whatfraction by quartz. It is nevertheless clear from Figure 7bthat no choice for the position of the original interfacewill permit this reaction to be treated as an isochemicalprocess. In Figure 7b, an isochemical reaction would berepresented by the simultaneous convergence of the linesfor all elements to a value of zero for some choice of theposition ofthe original interface. That all choices for theposition ofthe original interface yield nonzero transportfor most elements demonstrates that a closed-system dif-fusion model would approximate this reaction very in-accurately. The position of the original interface affectsthe calculated material transport in the diffusional mod-eling the dashed vertical line in the figure indicates theposition that is used in the examples below to estimatethe amounts of material that have been transported acrossthe outer boundary ofthe reaction band. That positioncorresponds to the contact between the plagioclase andthe pyroxene layers in the product zone, thal is, to the

(-

Fig. 7 . Material transport as a function of choice of locationfor original interface between reactants, assuming conservationof volume in an inert-marker reference frame. Ordinate in eachdiagram indicates the extent to which each element is conservedwithin the reaction band; it is the percentage of each elementthat is lost (positive values) or gained (negative values) by thereaction band (see text). Abscissa is percentage ofdistance acrossthe reaction band, with 00,6 representing the edge ofthe reactionband in contact with the garnet. All three reactions require sub-stantial import and export ofmaterial, regardless ofone's choicefor the position ofthe initial interface between reactants. Dashedvertical lines indicate the positions ofthe original interfaces be-tween reactants that were used in the calculations of materialbalance for the examples of difrrsional modeling that appear inTable 7.

Fcco(Laz(fF

FzLrJotrlrJ(L

-si- - . tZ;:"

IJ'a -. "

=-s i

--o,><

contact between the Al-rich and Al-poor product layers.(The successful diftrsion models below are consistent withplacing the original interface near this layer contact, justinside the pyroxene layer.)

Garnet-omphacite reaction. Figure 7c demonstrates theimpossibility of accurately representing the garnet-om-phacite reaction as an isochemical replacement. As inFigure 7b, there is no point along the abscissa for whichall transport values convergetozeto, so the reaction can-not be isochemical. Regardless of the position one choos-es for the original interface between reactants, substantialamounts of material must be transported across theboundaries of the reaction band as the corona forms. Onceagain, there is no petrographic evidence for the locationofthe original interfacp between reactants, and the dashedvertical line in the figure indicates the position that isused in the examples below to estimate the amounts ofmaterial that have been transported across the outerboundary ofthe reaction band. The position is again cho-sen to fall at the contact between the Al-rich 0abradorite+ ferroan pargasite) and Al-poor (orthopyroxene + mag-nesio-hornblende * oligoclase) product layers, at the dis-continuity in compositions of the amphibole and plagio-clase. (The successful diffusion models below areconsistent with placing the original interface near this lay-er contact, just inside the orthopyroxene-bearing layer.)

Material balances. Under the assumption of constant-volume replacement, any choice for the location of theoriginal interface in each reaction band uniquely specifiesthe amount of material that has been imported into orexported from the reaction band. Using the locations in-dicated by the dashed lines in Figure 7, Table 5 caststhese estimates of the material balance as the number ofmoles of each element that is imported into or exportedfrom the reaction bands, per mole of garnet resorbed.These time-integrated boundary fluxes will be incorpo-rated below into a quantitative model for the intergran-ular diffusion within the garnet-quafiz and garnet-om-phacite reaction bands.

It is unlikely that rocks of the Whitt Ranch metagabbrorepresent closed systems at any scale. Table 5 shows thatthe three garnet-resorption reactions cannot compensatefor one another: all three are net sources of Si, Fe, andCa; all three are net sinks for Na. The other reactionstaking place in the rock cannot account for all of thematerial lost and gained by the garnet reactions. Althoughthe omphacite-breakdown reaction is a source rather thana sink for Na, it (like all three garnet resorption reactions)is a source for Si, Fe, and Ca. The subsidiary reactionsproducing clinopyroxene around quartz and hornblendearound magnetite could serve as minor sinks for Si, Fe,and Ca, but these reactions are volumetrically insignifi-cant. These considerations suggest that Si, Fe, and Caarenot conserved within the rock at the scale of hand sam-ples and possibly at much larger scales. This conclusionis not surprising considering the ample petrologic evi-dence for substantial chemical interaction of the rockswith fluids at all scales.

t o l

TABLE 5. Boundary fluxes

Garnet-Garnet-fluid Garnet-quartz omphacite

(moles per mole of garnet resorbed)

0.530.480.750.530.00

-0.42

0.820.580.43

-0.210.07

-0.83

0 5 7-0.29

1 .531 .020.69

-0.40

Open-system diffusion models

The hypothesis of a diffusional origin for the garnet-quartz and garnet-omphacite reaction bands can be testedby constructing a quantitative open-system model basedupon that ofJoesten (1977), as modified by Johnson andCarlson (1990). The modeling below seeks an answer tothis question: Is there any range ofpossible values for therelative rates of intergranular diftrsion of Na, Ca, Fe, Mg,Si, and Al for which the observed arrangement of productlayers is stable with respect to continual diffirsion? If thereis none, then either the diftrsion model is oversimplifiedand unable to represent the natural reaction accurately,or these textures originated by some other process. Ifsucha range of relative diffusivities does exist, then it is ofgeologic interest to try to determine the relative magni-tudes ofthose diffusivities and their dependence upon theintensive parameters of metamorphism.

Steady-state diffusion models. The model of isochem-ical, steady-state diftrsion of Joesten (1971) evaluates,from the observed arrangement and measured composi-tions of minerals in a reaction band, the range of ratiosof intergranular diftrsion coefficients for which that re-action band could stably form. This is accomplished bysolving a large system of simultaneous linear equationsthat describes the fluxes and reactions within the growingreaction band; those equations impose upon the reactionband the constraints of local equilibrium, mass balance,and nonequilibrium thermodynamics. Joesten's (1977)article provides a lucid exposition ofthe approach and ofthe system of equations required, using a hypotheticalreaction as an example. Because Joesten's model has alsobeen abstracted in each of the later papers that sought toapply it to natural coronal textures (Nishiyama, 1983;Grant, 1988; Johnson and Carlson, 1990), details ofthemodel will not be reiterated here.

The only undetermined quantities in the system ofequations are the phenomenological coefficients (Zr) thatrelate the flux (1,) of a component to the chemical-poten-tial gradients (dp,,/dx) for each of the r" difusing com-ponents in the fundamental diffusion equation:

r , : ; - L . . ! t i . ( l )?, "" dx'

Like all earlier approaches except that of Grant (1988),this treatment neglects, when evaluating the flux of one

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

SiAIFeMgCaNa

768

element, the contributions of gradients in the chemicalpotentials of other elementst thus all cross coefficients(those l, for which i + j) arc taken as zero, and I,, iswritten more simply as I,. In addition, this treatment,like all earlier ones, regards the Z, as constants, despitethe fact that each is proportional to the concentration ofthe element in the intergranular medium and the fact thatthose concentrations must vary across the reaction bandin the presence of chemical-potential gradients for thediffusing components. There is some suggestion in theresults of the modeling below that while treatment ofthe l-ratios as constant within a given reaction texturemay be a valid approximation, variations in concentra-tions of elements in the intergranular medium among dif-ferent types of coronal reaction bands may be largeenough, even in a single sample, to affect values of the-L-ratios appreciably. (For discussion of the implicationsof considering Z-ratios to vary across a reaction band,consult Joesten, 1977, p. 661-665.)

In order to allow for the transport of materials beyondthe boundaries of the reaction band as required by thematerial-balance calculations, Joesten's (1977) modelmust be modified. The method of Johnson and Carlson(1990) is used again here. This method integrates intoJoesten's system of equations a set of boundary-fluxequations that specify the amount of each element lost orgained by the reaction band. Those amounts are obtainedfrom estimates of material balance, as illustrated abovefor the case of the Whitt Ranch coronas.

Application to the Whitt Ranch coronas. The garnet-quartz reaction imaged in Figure 4b, in which both re-actants are essentially Na-free, has been modeled in thediffusional system CaO-FeO-MgO-AlOr,,-SiO, by treat-ing the chemical potentials of NaO,rr, HO,rr, and O asexternally controlled (i.e., determined by equilibria beyondthe boundaries of the reaction band). The garnet-om-phacite reaction imaged in Figure 5b has been modeledin the diflusional system NaO,,,-CaO-FeO-MgO-AlO.,r-SiO, by treating the chemical potentials of HO,,, and Oas externally controlled. In the garnet-omphacite corona,labradorite was modeled as a phase distinct from oligo-clase, and ferroan pargasite was modeled as a phase dis-tinct from magnesio-hornblende; gradations in the com-position ofplagioclase across all zoned layers were ignored,and average compositions were used. In both cases, fluxesat the outer boundary of the reaction band were com-puted on the basis ofthe positions for the original inter-faces shown in Figure 7, and, fluxes across the innerboundary (the contact with the central garnet crystal) weretaken as zero. The equations used to describe these re-action bands appear in Table 6.'

The observed arrangement of phases in both reactionbands was calculated to be difrrsionally stable over a sub-

' A "opy "f

fable 6 may be ordered as Document AM -gl-457from the Business Office, Mineralogical Society of America, I130Seventeenth Street Nw, Suite 330, Washington, DC 20036,U.S.A. Please remit $5.00 in advance for the microfiche.

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

stantial range of possible values for the ratios of phenom-enological coefrcients in this open-system model. Table7 presents an example of the calculated interface reac-tions and diffusional transport for each of the reactionbands. The examples chosen are situations in which theI-ratios are near unity; these sets ofl-ratios are thereforerepresentative of the minimum differences in the phe-nomenological coefficients that are required in order togenerate diffirsionally stable solutions. Numerous diffu-sionally stable solutions also exist that involve greaterdiflerences between pairs of the phenomenological coef-ficients.

We emphasize, however, that attempts to treat thesecoronal textures as if they were isochemical are inade-quate. Such a treatment of the garnet-omphacite texturesfails to identifu any region in Z-ratio space for which thegiven arrangement of product assemblages is diftrsionallystable. Likewise, although an isochemical approach to thegarnet-quartz textures leads to the calculation of stableassemblage zonations over a restricted range of l-ratios,those solutions yield calculated modes that depart signif-icantly from the actual modes. It is evident that properspecification of the boundary fluxes is an essential pre-requisite to accurate estimation of the ratios of phenom-enological coeffi cients.

To explore further the dependence of estimates for theratios of phenomenological coefficients on the boundaryfluxes, the Whitt Ranch coronas were modeled over theentire range of possible choices for the positions of theoriginal interfaces between reactants. Not surprisingly, forsome values ofthese choices, for which garnet represent-ed small fractions of the reactant volume, no set of pos-sible values for the ratios of phenomenological coeffi-cients was found that produces diffusionally stableassemblage zonations matching the observed reactionbands. Less extreme values produced small regions inL-ratio space that would be consistent with the observedbands, but these regions occupy different portions ofthespace for different choices of the material balance.

Values of Z-ratios. Much of the interest in coronal tex-tures centers on extracting from them the actual ratios ofphenomenological coefficients, knowledge of which wouldmake it possible to evaluate the relative rates of inter-granular diftrsional transport during metamorphism. Inthe Whitt Ranch coronas, however, there are large irreg-ularly shaped regions of l-ratio space for which the ob-served reaction bands are computed to be diftrsionallystable; these regions shift somewhat when different valuesfor the boundary fluxes are employed. Consequently, fewgeneralities can be drawn from the modeling. For theboundary fluxes presented in Table 5, the garnet-quartzcoronas suggest Lr" * L", > L.^ > Ir, (with Z^ poorlyconstrained), whereas the garnet-omphacite coronas sug-gest Z*, = Lr. t L*" - L", - L* - Lor.

Joesten (1977) points out that in some reaction bands,the modal proportions of phases in some layers or zonesdepend upon the ratios of phenomenological coefficients,so that measurement of the modes sometimes permits

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

TABLE 7, Examples of diffusion models for corona formation

Garnet-quartz reaction band

769

Garnet-omohacite reaction band

Coetftcients of interlace reactions (moles per mole of garnet resorbed)Reaction interface*

1-2

Reaction interface-

1-2 34

AlmolgMagopxAugQtz

- 1 .0001.1830.543000

-0.2530.4810.0000.490o.374

0-0.092-0.346

0.0670.0950

-0.0700 . 1 1 10.939

-0.128-0.060

Layer-

0000.414o.273

-2.514

1 .143-0 .012-0.509-0.573-o.245

AlmLabFe-PrgopxolgMg-HblompSiAIFeMgCaNA

- 1 .0001 . 1 2 60.0590000

-0.0640.0301.5610.623

-0.202-0.500

0-0.017-0.027

0.0060.0200.0270

-0.0380.0290.002

-0.0150.007

-0.003

Layer'

0000.3980.7920.038

- 1 .9590.672

-0.349-0.033

0.4120.8860.103

SiAIFeMgCa

Fluxea across layers (moles per mole ot garnet resorbed)

SiAIFeMgCa

-0.2530.4810.0000.4900.374

Observed

-0.3230.5920.9390.3630.315

Model

-0.0640.0301.5610.623

-0.202-0.500

Observed

-0 .1020.0591.5630.608

- 0.1 96-0.503

Model

0.570-0.290

1.5301.0200.690

-0.400

0.820 si0.580 Al0.430 Fe

-0,210 Mg0.070 Ca

Na

Ploduct modes (vol. o/")

LabFe-PrgopxolgMg-Hbl

/Vote.'Garnet-quartz example gives results calculated lor |AtlLN: 1, t"/l*": 1, Ls/L6": 0.316 (: 1005), and loll"s: 1. Garnet-omphacite examplegives resufts calculated for L",/Lo, : 10, I*/I.. : 1, L"lL,.: 0.1, I€/LM. : 0.1, and IolL"" : 1.

t Numbers for interfaces and layers match those in Figures 4b and 5b.

olgMagopxAug

63.35.0

17.813.9

63.2c.u17.714.0

43.3J . C

10.034.78.5

45.43.610.533.37.2

one to deduce particular values of the l-ratios that wouldproduce the measured proportions ofphases. But, as theexample in Joesten (1977, p. 658) illustrates, the modalproportions in a reaction band will be invariant over thefield of Z-ratios unless one or more phases is present ina pair ofadjacent layers. Thus in the case ofthe garnet-quartz and garnet-omphacite coronas of the Whitt Ranchmetagabbro, in which no two layers share a commonphase, the modal proportions and the relative layer widthscalculated by the model are independent of the postulatedratios of phenomenological coefrcients.

In the Whitt Ranch occurrence, however, one mightconsider taking another approach to restricting the rangeof possible ratios for the phenomenological coefrcients.Because both the garnet-quartz and the garnet-omphacitereactions take place simultaneously in the same smallvolume of rock, the actual values of the Z-ratios mightbe expected to li€ in the intersection of the ranges foreach ofthe individual reactions, insofar as they representdiftrsion at the same temperature and pressure. That ex-pectation would be based upon the assertion that theprincipal determinants of the phenomenological coefr-

cients are the intrinsic mobility of each diffusing species,and intensive parameters such as the temperature, pres-sure, and chlorinity of the intergranular fluid; it wouldregard as negligible the effects ofdiffering concentrationsof elements in the intergranular fluid that might resultfrom local buffering by different assemblages.

In the Whitt Ranch coronas, there is indeed overlap inthe ranges of ratios for the phenomenological coefficientsfor which the individual reaction bands are calculated tobe diftrsionally stable, but the Z-ratios within the rangeof overlap require the ratio Lsi/LNu in the garnet-om-phacite corona to be l0 or greater. Such ratios may beinconsistent with qualitative notions of the relative mo-bility of these elements (e.g., Misch and Onyeagocha,1976; Whitney and Mclelland, 1973; Mongkoltip andAshworth, 1983). If instead one considers l-ratios thatlie nearer to the centers of each of the two individualregions of diffiisionally stable solutions in Z-ratio space,those inconsistencies disappear.

As an alternative, then, one might consider the possi-bility that the relative rates of diffusion for Mg, Fe, Ca,Al, and Si may have been slightly different in the garnet-

770 CARLSON AND JOHNSON: CORONAL REACTION TEXTURES

quartz coronas than in the garnet-omphacite coronas,rather than lying in the range ofoverlap. It is conceivablethat the mineral assemblages in the two coronas mighthave locally buffered the concentrations ofdiffusing spe-cies to different values. For example, the intergranularfluid at least at one end ofthe garnet-quartz reaction bandwas saturated with silica, but the entire garnet-omphacitereaction band was probably undersaturated in silica, asthe analysis presented in the following section indicates.Because the phenomenological coefficients are propor-tional to local concentrations in the intergranular fluid,and because those local concentrations are probably dif-ferent in the two types of coronas, it is plausible that theZ-ratios may vary appreciably from one coronal type tothe other. The most impressive difference between themost likely relative Z-values for the two reactions is thatthe Z-value assigned to Si is greater in the garnet-quafizreaction, which is consistent with the idea that a higherconcentration ofsilica in the garnet-qlrarlz reaction fluidaugments the value of the phenomenological coefficientthere. To the extent that concentrations must also varyacross each individual reaction band, this inference in-validates the assumption that the l-ratios may be treatedas constant across the band. However, the fact that dif-fusionally stable solutions (using constant l-ratios) forindividual coronas are found over wide ranges ofZ-ratiospace suggests that the effects of variable concentrationacross a given reaction band introduce acceptably smallerrors into the calculations.

Evor,urroN oF coRoNAL ASSEMBLAGES

The variety ofgarnet-omphacite reaction textures shownin Figures 5 and 6 suggests that these reaction bands mayhave undergone a history more complex than just a sin-gle-stage steady-state difiirsion episode. All ofthe texturesshown in Figures 5 and 6 are reasonably interpreted asrepresenting reaction bands in which orthopyroxene wasoriginally present over nearly half the width of the band,but in which hornblende has subsequently replaced or-thopyroxene along only one edge of the orthopyroxene-bearing layer, the edge closer to the central garnet crystal.This observation requires explanation, because it posesthe problems of identifuing the cause of the replacementreaction and accounting for its variable degree and itsselective and directional nature.

One conceivable explanation for the elimination of or-thopyroxene is simply that during the waning stages ofthe static metamorphic event responsible for the forma-tion of the coronas, temperatures dropped sufficiently tocause orthopyroxene to become unstable with respect tohornblende in the presence of an aqueous fluid. Thereare, however, several reasons to regard this explanationas unlikely, or at least incomplete. First is the preserva-tion in most garnet-quartz coronas of orthopyroxene inintimate association with plagioclase, the same phase withwhich it appears to react in the garnet-omphacite coro-nas. A second argument against declining temperature asan exclusive explanation for the elimination of orthopy-

roxene is the evidence for unidirectional replacement. Inall cases, only one edge ofthe orthopyroxene-bearing zone(the one nearer the garnet crystal) is subject to the re-placement reaction; by contrast, orthopyroxene-richregions in contact with the polymineralic matrix of so-dian augite + oligoclase * magnesio-hornblende appearunaffected (Figs. 6b-6e). Third, because the entire rangefrom incipient to complete replacement is found u/ithina single thin section (and, in some cases, on opposite sidesof a single garnet crystal), one cannot call upon local vari-ations in temperature to account for the local variationin degree of replacement; some sort of additional kineticfactors must be invokec..

An alternative explanation is suggested by the associ-ation ofthe garnet-omphacite coronas containing ortho-pyroxene with quartz-rich regions ofthe rock, and by thelack of reaction between orthopyroxene and plagioclasein many garnet-quartz coronas. These observations areconsistent with the notion that the stability of orthopy-roxene depends sensitively upon the local activity ofsil-ica and other components in the intergranular fluid. Theforegoing material-balance calculations indicate that boththe garnet-omphacite reaction and the simultaneous re-action of omphacite to sodian augite + oligoclase + mag-nesio-hornblende are sources of silica and other compo-nents. If, as the omphacite-breakdown reaction in thematrix surrounding the garnets progresses toward com-pletion, it ceases to function as a local source or sink forsome components, then their activities at the outerboundary of the reaction band will change. This will per-turb the boundary fluxes at the outer limit of the reactionband, which may in turn alter the internal diffusive fluxesin such a way that the orthopyroxene-bearing assemblageis rendered diffusionally unstable relative to hornblende.We hypothesize, therefore, that the variability and theunidirectionality of the orthopyroxene replacement tex-tures arise from local changes with time in the boundaryfluxes, changes that arise from the progressive exhaustionof the omphacite-breakdown reaction as that reactionnears completion.

This hypothesis has been tested by calculating the ef-fects ofchanges in the boundary fluxes ofSi and Al, thedominant components that are respectively produced andconsumed by the omphacite-breakdown reaction. Theboundary fluxes of these two elements were altered in thedirections and proportions that would simulate the grad-ual elimination of the omphacite-breakdown reaction inthe surrounding matrix, and the efects on the modal pro-portions of minerals in the reaction band were calculatedfor several ditrerent values ofthe boundary fluxes. Table8 presents the results of those calculations. As the hy-pothesis predicts, these changes in the boundary fluxeslead to gtreater amounts of hornblende in both productlayers, at the expense of orthopyroxene and plagioclase.For sufficiently large changes in the boundary fluxes(rightmost column in Table 8), the calculations predictthe eventual elimination of orthopyroxene as a diffusion-ally stable member of the product assemblage, mirroring

CARLSON AND JOHNSON: CORONAL REACTION TEXTURES 7 7 1

the progressive changes displayed in Figures 5b and 6a-6f.

These changes with time in the product modes andassemblages are directly analogous to those revealed, withequally explicit visual evidence, in the olivine-plagioclasecoronas ofmetagabbros ofthe Adirondacks (Johnson andCarlson, 1990). In the Adirondack coronas, there is pet-rographic confirmation that the changes in product as-semblages are caused by the gradual exhaustion ofone ofthe reactants, calcic plagioclase, as a source for Ca andSi. In the Whitt Ranch rocks, a similar role is apparentlyplayed by the gradual elimination of the breakdown re-action of omphacite as a source for Si and a sink for Al.These reaction textures that show evolution ofproductsover time have an important feature in common: one ofthe original reactants is undergoing chemical modifica-tion simultaneously with the corona-forming reaction.This modification affects the boundary fluxes and theoverall material balance, shifting the internal fluxes andaltering the internal reactions. In such circumstances, asteady-state difusion model that presumes an unchang-ing assemblage of product phases may lead to erroneousresults.

CoNcr-usroNs

This investigation confirms, in wholly diferent petro-logic circumstances, the inference drawn by Johnson andCarlson (1990) that closed-system approaches to diffu-sion-controlled reaction textures may yield incorrectanalyses ofthe nature, extent, and rates ofelement trans-port by intergranular diffusion. It demonstrates that non-equilibrium transport models for the origin of the WhittRanch coronas can be constructed by specifying bound-ary fluxes from material-balance calculations, and thatsuch models are quantitatively consistent with the ar-rangement and modes of mineral assemblages in the re-action bands. It suggests that the range ofpossible ratiosfor the phenomenological coefficients determined by adifusion model may be very poorly constrained, becausethe petro$aphically observable features of the reactionband are often not highly sensitive to those ratios. De-spite the overlap in the ranges ofpossible values for theratios of phenomenological coefficients in the two coronaltypes in the Whitt Ranch metagabbro, the actual valuesin the two coronal types may have been slightly different,reflecting differences between the two coronal environ-ments in the concentrations of dissolved species in theintergranular fluid.

A more general conclusion, again familiar from ourearlier work, stands out clearly: there is a misleading sim-plicity in many coronal textures. Although they preserveboth reactants and products and the spatial relationshipsamong them, many harbor subtle but unequivocal indi-cations of extraordinarily complex origins. In particular,changes in the product assemblages with time, as illus-trated in Figures 5b and 6a-6f, severely limit the appli-cability of steady-state models, which presume that a giv-en reaction band originates with all product layers present

TABLE 8. Effects on garnet-omphacite products of eliminationof omphacite-breakdown reaction

Boundary fluxes(moles per mole of garnet resorbed)

0.57 0.66 0.75-0.29 -0.37 -0.45

SiAIFeMg

Na

LabFe-PrgopxolgMg-Hbl

1.02 1 .020.69 0.69

1.53 1 .53 1.531.020.69

-0.40 -0 40 -0.40Vol. oA of products

0.80-0.49

1 .531 0 20.69

-0.40

35.75.2e o

34.121.1

at infinitesimal thickness and then simply enlarges overtime. Whenever one or more of the reactants undergoeschemical changes as the corona-forming reaction pro-gresses, the coronal assemblages may evolve with time.This potential for modification over time of product min-eral assemblages emphasizes the risk of assuming that theassemblages one observes in a diffusion-controlled tex-ture were present throughout the rock's diffirsional his-tory.

Because this study focuses attention on the complexchemical interactions between local corona-forming re-actions and their surroundings, it amplifies the admoni-tion of one of the earliest workers on these textur€s,namely Sederholm (1916, p. 142),who remarked: "Often. . . interpretations [of coronal and similar textures] seemto lie so close at hand, that it appears only necessary tograsp a pen and write them down on paper. But, as wehave found, . . . the reactions between mineral substancesderived from the adjacent minerals have been compli-cated by transfer of material from greater distances, thusincreasing the difficulty of an interpretation." This studyand its predecessor (Johnson and Carlson, 1990) bothargue that the starting point for such interpretations mustbe an attempt to solve quantitatively the perplexing prob-lems of the nature and magnitudes of the boundary fluxesand of the degree to which they remain constant or changeas reactron progresses.

AcxNowr,rocMENTS

We wish to extend special thanks to Susan Harris, whose Master's thesison the Whitt Ranch rocks under the supervision of the senior authoruncovered a number ofcrucial obs€rvations and demonstrated, in an eailyattempt to develop diffusion models for the Whitt Ranch coronas, thenecessity oftreating such textures as the products ofopen-system reaction.We are also grateful to Elizabeth Schwarze, who generously contributedher thermobarometric estimates for the Whitt Ranch and other localitiesprior to their publication and who provided the microprob€ analysis oftho omphacite inclusion that appears in Table 3. Ife Whitt graciouslyallowed access to his property for field work. The constructive criticismsoffered in reviews by Ray Joesten, Tom Foster, and John Brady materiallyimproved both the content and presentation of the article, and their con-tributions are gratefully acknowledged. Portions of this work were sup-ported by NSF $ant EAR-8804717 to W.D.C., Nicholas Walker, andSharon Mosher. The Geology Foundation ofthe University ofTexas helpedto defray costs ofpublication.

45.43.6

10.533.37.2

32.8 30.15.8 6.41 .9 0 .0

34.4 34.625j 28.8

772

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M.lNuscnrpr R.EcErl'ED JeNuanv 31, 1990MrNuscnrpr AccEprED FesRuAnv 14. 1991