Textural and Thermal History of Partial Melting in Tonalitic Wallrock

Textural and Thermal History of PartialMelting in Tonalitic Wallrock at the Marginof a Basalt Dike, Wallowa Mountains, Oregon

H. L. PETCOVIC* AND A. L. GRUNDER

DEPARTMENT OF GEOSCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331, USA

RECEIVED SEPTEMBER 15, 2002; ACCEPTED JUNE 18, 2003

Columbia River Basalt Group dikes invade biotite---hornblendetonalite to granodiorite rocks of the Wallowa Mountains. Mostdikes are strongly quenched against wallrock, but rare dikesegments have preserved zones of partial melt in adjacent wall-rock and provide an opportunity to examine shallow crustalmelting. At Maxwell Lake, the 4 m thick wallrock partialmelt zone contains as much as 47 vol. % melt (glass plus quenchcrystals) around mineral reaction sites and along quartz---feldspar boundaries. Bulk compositional data indicate thatmelting took place under closed conditions (excepting volatiles).With progressive melting, hornblende, biotite, and orthoclasewere consumed but plagioclase, quartz, and magnetite persistedin the restite. Clinopyroxene, orthopyroxene, plagioclase, andFe---Ti oxides were produced during dehydration-melting reac-tions involving hornblende and biotite. Reacting phases becamemore heterogeneous with progressive melting; crystallizingphases were relatively homogeneous. Progressive melting pro-duced an early clear glass, followed by light (high-K) and dark(high-Ca) brown glass domains overprinted by devitrification.Melts were metaluminous and granitic to granodioritic. Thermalmodeling of the partial melt zone suggests that melting tookplace over a period of about 4 years. Thus, rare dikes withmelted margins represent long-lived portions of the ColumbiaRiver Basalt dike system and may have sustained large flows.

KEY WORDS: Columbia River Basalt dike; crustal melting;

dehydration-melting; tonalite---granodiorite; thermal model

INTRODUCTION

Although there is broad consensus that basalt injectioncan be fundamental in crustal melting, there are fewplaces where stages of the interaction can be directly

sampled. The Wallowa Mountains of northeasternOregon, however, provide a natural laboratory inwhich to examine shallow crustal melting. In thisarea, hundreds of Columbia River Basalt Group(CRBG) feeder dikes cut granitoid rocks of theWallowaBatholith. The batholith is a biotite- and hornblende-bearing tonalite to granodiorite. Although most dikeswere strongly quenched against their wallrock, a fewdikes have developed partially melted contact zones,commonly with up to 50 vol. % quenched melt inthe wallrock at their margins. This melt is representedby devitrified silicic glass plus plagioclase, pyroxene,and magnetite quench crystals. We have examined thepartially melted zone in tonalite at the margin of aCRBG (Grande Ronde) dike where quenched melt ispreserved over a distance of 4m from the dike marginand reaches 47 vol. % near the dike---wallrock contact.Recent work has shown that dehydration-melting

plays a crucial role in the generation of silicic melts inthe crust. Dehydration-melting, also called fluid- orvapor-absent melting, is the incongruent reaction of ahydrous mineral assemblage to form melt plus residualminerals. Previous studies of crustal dehydration-melt-ing considered protoliths with either amphibole ormica. Melting of protoliths with both hydrous phaseswas examined by Rutter & Wyllie (1988) and Skjerlie& Johnston (1996) at high pressure (10 kbar). Thisstudy differs from previous work in that both horn-blende and biotite are present in the Wallowa parentrock and melting was shallow.

Related studies of partial melting

Dehydration-melting of mafic to intermediate compo-sition amphibolites at pressures510 kbar (1000MPa)

JOURNAL OF PETROLOGY VOLUME 44 NUMBER 12 PAGES 2287±2312 2003 DOI: 10.1093/petrology/egg078

*Corresponding author. Telephone: 541-737-1201. Fax: 541-737-1200. E-mail: [email protected]

Journal of Petrology 44(12)# Oxford University Press 2003; all rightsreserved

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have produced trondhjemitic, to tonalitic, to grano-dioritic melt coexisting with a restite of clinopyroxene�orthopyroxene � plagioclase � quartz � Fe---Ti oxides(Beard & Lofgren, 1991; Rapp et al., 1991; Rushmer,1991; Wolf & Wyllie, 1994; Pati~no Douce & Beard,1995). Biotite dehydration-melting reactions at510 kbar have produced granitic to granodioritic meltand a restite of orthopyroxene � plagioclase � Fe---Tioxides (generally ilmenite and/or magnetite) � alkalifeldspar � quartz from protoliths as diverse as biotitetonalites (Pati~no Douce & Beard, 1995; Singh &Johannes, 1996a, 1996b), a biotite-bearing metagrey-wacke (Vielzeuf &Montel, 1994), pelites (Le Breton &Thompson, 1988; Vielzeuf & Holloway, 1988), and ahigh-F tonalitic gneiss (also containing �2 wt % horn-blende; Skjerlie & Johnston, 1992, 1993). At pressuresaround 10 kbar and greater, garnet was a crucial phasein the restite for nearly all experimental protoliths.Partial melting experiments have been performed

on protoliths containing both biotite and amphiboleat 10 kbar. A garnet---biotite---hornblende tonaliteyielded up to 40 vol. % melt (melt compositionnot given) with a restite of orthopyroxene �clinopyroxene � rutile � garnet (Rutter & Wyllie,1988). A biotite---hornblende---epidote gneiss producedabout 35 vol.%peraluminous, granodioritic to graniticmelt and a restite of orthopyroxene � clinopyroxene �plagioclase � garnet � ferro-pargasitic amphibole(Skjerlie & Johnston, 1996).Whereas work on piston-cylinder experiments has

rarely reported coexisting, compositionally distinctmelts, the occurrence of multiple melts is common innatural examples and rock core experiments. Shallow(51 kbar) partial melting of Sierra Nevada biotitegranite at the contact with a trachyandesite plug pro-duced both brown and clear glass coexisting with relictplagioclase � sanidine � quartz, with magnetite,rutile, and Mg-cordierite replacing biotite (Al-Rawi& Carmichael, 1967; Kaczor et al., 1988; Tommasini& Davies, 1997). Brown and clear glass coexisting witha restite of plagioclase � quartz �orthoclase � Fe---Tioxides � minor orthopyroxene were also noted byGreen (1994) in partially melted biotite granodioritexenoliths hosted in andesitic to dacitic dikes. Philpotts& Asher (1993) noted abundant disequilibrium melt-ing textures, such as sieve-textured feldspar, in par-tially melted biotite gneiss at the contact of a basaltdike at paleodepth of about 10 km. Knesel & Davidson(1996) melted 2 cm cubes of biotite alkali granite underatmospheric conditions, yielding coexisting clear andbrown melts plus a restite of quartz � plagioclase �Fe---Ti oxides � alkali feldspar. Multiple compositionmelts attributed to muscovite and biotite (� quartz �plagioclase) dehydration-melting reactions in pelite at7 kbar were also noted by Rushmer (2001).

THE NATURAL LABORATORY

The Wallowa Batholith and CRBG dikes

The Wallowa Mountains are largely composed of theWallowa Batholith, a series of Late Jurassic plutons(140---160Ma; Armstrong et al., 1977) intruded intoisland arc terranes accreted to the western margin ofNorth America (Fig. 1). Dikes exposed in the batholithare part of the Chief Joseph dike swarm, which fed theColumbia River flood basalts (Taubeneck, 1970).Imnaha Basalt (17.3---17.0Ma; Baksi, 1989) is pre-served as erosional remnants on some peaks of theWallowa Mountains and as dikes. Most dikes in thebatholith are of Grande Ronde Basalt (16.9---15.6Ma;Baksi, 1989). Paleodepth at the time of Grande Rondedike emplacement was as great as 2.5 km, as estimatedfrom a thickness of about 1 km of Imnaha flows uncon-formably overlying about 1.5 km of relief in theWallowa Mountains.Individual basalt dikes within the Wallowa Moun-

tains can extend several kilometers along strike, are afew centimeters to 50m thick (average 7---10m), andare steeply dipping (average 70�). Overall, dikes strikeN10�W, although strikes may vary from N55�W toN30�E within sub-swarms where dikes occur in zonesof concentration of 7---12 dikes per km2 (Taubeneck &Duncan, 1997; Petcovic et al., 2001). Dikes have oneor more of the following morphologies: dikes with

Fig. 1. Simplified geological map showing the location of theMaxwell Lake dike, modified after Taubeneck (1995).

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quenched margins and no interaction with the wall-rock, dikes with partially melted wallrock at theirmargins, dikes that have eroded their wallrock, anddikes containing whole to disaggregated crustalxenoliths that constitute locally as much as 30% ofthe dike (Grunder & Taubeneck, 1997). The majorityof Wallowa dikes have an aphanitic quench zone(a few centimeters to 20 cm thick) at their margins.Rarely, these dikes may have localized zones ofpartial melting that extend into wallrock for up toabout 50 cm from the dike---wallrock contact. Typi-cally, localized melting zones occur where dikes haveeroded their own quenched margins so that coarse-grained basalt is directly in contact with the wallrock.Partial melting is most common in narrow wallrockscreens or in wallrock trapped between two cross-cutting dikes.Only a handful of dikes in the Wallowas exhibit

extensive partial melting (i.e. zones of partial meltingthat are meters thick and continuous for tens to hun-dreds of meters). These dikes consistently lack an apha-nitic quench zone at the dike margin. In these dikes,melted margins are typically one-quarter to one-thirdof the width of the dike, and in cases where dikes arenot vertical, the hanging wall has a thicker meltedmargin (Grunder & Taubeneck, 1997).

The Maxwell Lake Dike

This study focuses on a single, well-exposed GrandeRonde basalt dike with well-developed partiallymelted wallrock margins (Fig. 2). The dike strikesN20�E, dips steeply to the west (averaging about75�), and is from 2.6 to 7.8m thick. It extends as enechelon segments for at least 1 km along strike. Paleo-depth at the time of dike emplacement was at most2 km, as reconstructed from regional geology.The partial melt margin along the hanging wall

(western margin) of the dike is generally 2---2.5mthick but reaches nearly 5m thick at the southern endof the outcrop (Fig. 2). The footwall partial melt mar-gin is about 1.5m thick. The thickness of the melt zonein the hanging wall is typically 1.7---1.5 times as thickas that of the footwall and is 0.3---0.5 times as thick asthe dike.The dike margins are divided into four mappable

zones based on outcrop-scale textural characteristics(Fig. 2). These are: the unmelted country rock, themafics-out zone, the mottled zone, and the mushzone. These textural zones are 10 cm to 2m wide withgradational transitions from one zone to the next.Zones parallel the dike and, like the partial melt zoneoverall, are proportionally wider in the hanging wall.The unmelted wallrock is a hypidiomorphic

granular hornblende biotite granodiorite by IUGS

classification. Based on the compositional classificationof Barker (1979) it is a tonalite; we refer to it as atonalite owing to the paucity of orthoclase relative toplagioclase feldspar (Table 1; Fig. 3). The mafics-outzone is characterized by the absence of biotite andhornblende. Instead, fine-grained pyroxene and Fe---Ti oxides occupy former biotite and hornblendemineral sites. Glass may be present as thin seams sur-rounding quartz and feldspar grains. The mottled zoneis characterized by a blue---gray mottled texture madeup of residual quartz and feldspar grains that lackdistinct margins, mafic mineral reaction domains,and brown glass seams that surround grains. Themush zone is a discontinuous, 10---50 cm wide zoneparalleling the dike margin. This zone contains sparseamorphous grains of quartz and feldspar in a fine-grained, blue---gray groundmass. The presence of themush zone appears to correlate with thicker parts of thedike. A dense network of blue---gray veins and a cata-clastic texture occur in the partially melted tonalite atthe southern end of the outcrop.

PROGRESSIVE STAGES OF MELTING

Quenched partial melt zones in the margins of theMaxwell Lake dike have captured a continuum oftextural reactions, most of which can only be viewedin thin section. We have grouped samples collectedfrom the hanging wall partial melt zones into fiveprogressive stages based on interpretations of the meltreaction textures. Samples representing Stage 1 werecollected from the unmelted wallrock zone (Fig. 2) at adistance of 44m from the dike---wallrock contact.Samples for Stages 2 and 3 were collected from themafics-out zone (�2---4m from the contact), and sam-ples representing Stages 4 and 5 were collected fromthe mottled zone (0.5---2m from the contact) (Fig. 2).These stages of reaction range from unmelted wallrock(Stage 1) to nearly 47 vol. % quenched melt (Stage 5).

Stage 1

The unmelted tonalite is medium to coarse grainedwith an inhomogeneous distribution of biotite andhornblende (Fig. 4). Biotite rims are commonly alteredto chlorite whereas hornblende grains may containsmall quartz inclusions and/or clinopyroxene cores.Magnetite and trace phases, including (in decreasingorder of abundance) apatite, zircon, and titanite, areassociated with biotite and hornblende and also maybe enclosed within grains.

Stage 2

Stage 2 contains trace amounts (51 vol. %) of glass,thus representing the onset of melting (Table 1, Fig. 3).

PETCOVIC AND GRUNDER MELTING AT BASALT DIKE MARGIN


Hornblende contains sub-microscopic dusty reactionproducts whereas biotite contains dusty opaque reac-tion minerals along cleavage planes and up to 0.5mminward from crystal edges. No glass is present onquartz---feldspar boundaries, but thin (510 mm) seamsof clear glass are present along fractures within quartzcrystals. These seams locally grade into rare small poolsof yellow---brown glass at the margins of biotite andhornblende crystals. The sample of Stage 2 has a cata-clastic overprint as evidenced by abundant veins,minutely fractured quartz and feldspar crystals, andoffset twinning in plagioclase.

Stage 3

Stage 3 is most conspicuously characterized by theappearance of continuous glass seams and the absenceof both hornblende and biotite (Table 1, Fig. 3). Dustybrown reaction products rimmed by optically alignedpyroxene grains occur in hornblende reaction sites(Fig. 5a). Sites of biotite consumption are occupiedby a fine-grained intergrowth of glass, dusty magnetiteand lesser ilmenite, orthopyroxene, and plagioclasefeldspar with opaque oxides aligned in parallel bands(Fig. 6a). When in contact with glass, plagioclasegenerally has a spongy texture and poorly developed

Fig. 2. Outcrop map of the Maxwell Lake dike and its partially melted margins, showing sample labels and locations.



fritted margins. Individual cells in spongy plagioclaseare rounded and filled with brown glass. We estimate�4 vol. % glass is trapped within spongy plagioclase.About 12 vol. % devitrified brown glass is localized

around sites where biotite and hornblende have beenconsumed and along quartz---plagioclase grain bound-aries (Fig. 7a). Seams are as thin as a few tens ofmicrons and as thick as 1mm. The brown glass occursas two textural domains that grade into one another(Fig. 7b). The dominant light brown glass domain ischaracterized by radiating sheaves of microfibrouscrystals that we interpret as fine spherulitic devitrifica-tion. Opaque oxides occur as sparse needles. The darkbrown domain remains largely glassy and is character-ized by dusty appearance under the microscope. Lightbrown domains surround dark brown domains, andboth domains are irregularly distributed around bio-tite and hornblende reaction sites and along seamsbetween quartz and feldspar (Fig. 7a). Microlites ofacicular to hopper plagioclase, acicular pyroxene, andequant magnetite are associated with the brown glassdomains (Fig. 7b). These quench crystals make uptypically 2 vol. % of the bulk mode, but up to20 vol. % of the quenched groundmass. Minor clear,

granular domains have sharp boundaries with thebrown glass domains (Fig. 7b). These clear granulardomains are somewhat crystalline (i.e. not completelyisotropic) and almost always associated with embayedquartz crystals.

Stage 4

Stage 4 is characterized by the absence of orthoclase;the modal proportion of quartz is 53 vol. %, and thesample contains about 18 vol. % glass and about1 vol. % quench crystals (Table 1, Fig. 3). Small(5100 mm long), optically aligned clinopyroxene andorthopyroxene crystals occupy sites where hornblendehas been consumed. Magnetite and lesser ilmenite areconcentrated towards the center of the hornblendereaction sites. A fine-grained intergrowth of glass,aligned magnetite and lesser ilmenite, orthopyroxene,and plagioclase occupies sites of biotite consumption.The three devitrified glass domains described forStage 3 are also present in Stage 4. The sample ofStage 4 has a cataclastic overprint as evidenced bybrecciated crystal fragments, microscopic fractures incrystals, and offset twinning in plagioclase.

Table 1: Modal percentages of phases in each stage of melting

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5

Total counts: 1390 418 1092 954 1142

Primary/relict Plagioclase� 42.9 49.0 52.3 59.4 43.3

Quartz 19.3 19.6 8.2 2.5 3.0

Orthoclase 8.0 5.7 1.7 0 0

Hornblende 14.7 15.3 0 0 0

Biotite 14.0 9.3 0 0 0

Fe---Ti oxide 1.0 1.0 1.0 0.6 0.1

Hornblende site Pyroxene 0 0 12.5 7.4 8.8

Biotite site Pyroxene 0 0 6.5 7.1 2.9

Plagioclase 0 0 1.3 1.5 1.1

Fe---Ti oxide 0 0 2.1 2.4 1.5

Glass Browny 0 51 12.3 14.3 30.1

Clearz 0 51 0.1 3.9 1.1

Quench crystals Plagioclase 0 0 0.5 0.3 5.3

Pyroxene 0 0 1.0 0.3 1.9

Fe---Ti oxide 0 0 0.3 0.2 0.9

Numbers are given as vol. % of whole rock, and modal data were collected by point counts on thin sections. Stage 1 isrepresented by two samples. All other stages are represented by a single sample (for sample names and locations, see Fig. 2).Point counts were performed on multiple thin sections (one thin section for Stage 2). Error is �1% for most counts and maybe higher in Stage 2 because of the low count number. Error is probably higher in Stages 2 and 4 as a result of the cataclastictexture of these samples. All stages contain trace (50.5 vol. %) zircon, fluorapatite, and titanite.�Plagioclase data include plagioclase plus spongy rims containing trapped melt. Visual estimates are 4 vol. % trapped melt inStage 3, 5 vol. % trapped melt in Stage 4, and 7 vol. % trapped melt in Stage 5.yBrown glass data include both dark and light brown domains.zClear data are for granular domains in Stages 3---5. Description of clear glass from Stage 2 is given in text.



Stage 5

Stage 5 contains about 31 vol. % glass and 9 vol. %quench crystals (Table 1, Fig. 3), thus representing themaximum degree of partial melting at the MaxwellLake dike. Optically aligned orthopyroxene withminor magnetite occurs in hornblende consumptionsites (Fig. 5b). Aligned magnetite and lesser ilmeniteintergrown with orthopyroxene and plagioclase occurs

in biotite reaction sites (Fig. 6b). Although still veryfine, crystals are coarser than in Stage 3. Relict quartzcrystals are rounded and embayed, and may be sur-rounded by haloes of acicular clinopyroxene. Plagio-clase crystals in contact with glass have well-developedspongy texture, fritted margins, and a thin (525 mmwide) optically distinct rim. Spongy plagioclase maycontain up to 7 vol. % glass; �30% of the plagioclasehas a spongy texture (Fig. 3).As in Stages 3 and 4, glass is localized around reacted

mafic sites and as seams up to 2mm thick on quartz---plagioclase grain boundaries. Stage 5 glass occurs asdominant, microfibrous light brown domains, glassydark brown domains, and minor clear granulardomains. Acicular to hopper quench crystals of plagio-clase, pyroxene, and magnetite are localized in thebrown glass domains, typically making up 25 vol. %,but up to 50 vol. %, of the groundmass.

Fig. 4. Photomicrograph of unmelted tonalitic wallrock (Stage 1) incrossed nicols. Plag, plagioclase; Qtz, quartz; Or, orthoclase; Hbl,hornblende; Bio, biotite; Mag, magnetite; Ap, apatite; Zr, zircon; Ti,titanite.

Fig. 5. Photomicrographs of hornblende reaction sites in Stages 3and 5. (a) In Stage 3, a hornblende reaction site, in plane-polarizedlight, has a dusty core with a coarser fringe of aligned pyroxene. Seamof finely devitrified brown glass (BG, lower right) and spongy reac-tion in plagioclase should be noted. (b) In Stage 5, hornblende site incrossed nicols is a network of optically aligned orthopyroxene (Opx)and minor magnetite. Dark interstitial material is brown glass.

Fig. 3. Modal data as a function of melt fraction. Dashed linesindicate estimates of when phases react out. The modal abundanceof melt is shown by the stippled fields. Melt is represented by glassand devitrified glass (both grouped as glass here), quench crystals(shown separately), and reaction melt trapped in spongy plagioclase(shown separately). An estimate of the actual amount of plagioclasewas made by subtracting the estimated proportion of glass in spongyplagioclase from the modal plagioclase data. Ox, iron---titaniumoxide (magnetite and ilmenite); Plag or Pl, plagioclase feldspar;Qtz, quartz; Or, orthoclase feldspar; Hbl, hornblende; Bio, biotite;Pyx, pyroxene; xtls, crystals.



COMPOSITIONAL

CHARACTERISTICS OF

PROGRESSIVE MELTING

Analytical methods

Whole-rock bulk analyses were performed using theRigaku 3370 X-ray fluorescence (XRF) spectrometerat Washington State University's GeoAnalyticalLaboratory. Analyses of crystals and glass were per-formed using the CAMECA SX-50 Electron Micro-probe at Oregon State University. Analyses ofhornblende, pyroxene, feldspar, and biotite used abeam current of 30 nA, an accelerating voltage of15 kV, and a 3---5 mm diameter beam. Glass was ana-lyzed using the same conditions and a broad (20 mm)beam. Sodium was counted first in glass and crystals

because of its susceptibility to migration. Additionaldetails have been provided by Petcovic (2000).

Bulk composition

Most analyses of major and trace elements for the fivestages are identical within analytical error (Table 2).The samples from Stage 2 and, to a lesser degreeStage 4, are slightly altered, which is reflected in lowNa2O values. Nevertheless, minor loss of Na2O duringmelting is possibly indicated. Some variations greaterthan analytical error, such as higher La, Ce, and Th inStages 3, 4, and 5 compared with Stage 1, probablyreflect differences in the proportion of trace phases

Fig. 6. Photomicrographs of biotite reaction sites in Stages 3 and 5.(a) In Stage 3 (plane-polarized light), the biotite reaction site isoccupied by aligned, dusty opaque oxides (Mag � Ilm). Lightdomains are an intergrowth of plagioclase, orthopyroxene, andglass (Plag � Opx). (b) Stage 5 biotite reaction site in plane-polarized light. Magnetite and lesser ilmenite are slightly morecoarse-grained than in Stage 3.

Fig. 7. Photomicrographs of glass domains in Stage 3. (a) Domainsof light brown glass (LBG) and dark brown glass (DBG) distributedalong quartz---plagioclase grain boundaries. (b) Light brown glass,dark brown glass, and clear granular (CG) domains adjacent to aplagioclase grain and hornblende reaction site. Dark brown glassremains largely isotropic whereas light brown glass exhibits featherydevitrification. Sparse quench crystals of plagioclase (Q plag) andpyroxene (Q pyx) occur in brown glass domains.



between samples. Overall, however, the process ofpartial melting of the Wallowa tonalite was a closedsystem, excepting volatile components, based on thenear identity of bulk-rock analyses of unmelted andpartially melted rock.

Phase compositions

Hornblende and biotiteHornblende (Table 3, Fig. 8a) is present only inStages 1 and 2 (Fig. 3). Relative to unmelted rock,

Stage 2 hornblende is compositionally more hetero-geneous, and has lost virtually all Cl, half of the F,and much of the K (Fig. 8). Whereas Stage 1 horn-blende has a normal trend of increasing FeO withdecreasing MgO, Stage 2 hornblende has a scatteredincrease in FeO with increasing MgO (Fig. 8b).Like hornblende, biotite is present only in Stages 1

and 2 (Fig. 3, Table 3). Stage 1 biotite is tightly clus-tered in composition, relative to Stage 2 biotite, whichhas generally lower FeO and higher TiO2 and highlyvariable MgO concentrations (Fig. 9). Stage 2 biotitehas as great as 10-fold enrichment in Na2O with rela-tively modest loss in K2O (Fig. 9b). Fluorine concen-trations are also enriched in most Stage 2 biotite andcorrespond to high TiO2 compositions (Fig. 9c).Chlorine concentration is broadly the same in Stage 1and in Stage 2 (Cl of 0.5---0.15 wt %) but Stage 2

Table 2: Bulk-rock major and trace element

data

Stage 1 Stage 2 Stage 3 Stage 41 Stage 5

wt %

SiO2 59.32 62.61 60.02 61.05 61.15

TiO2 0.70 0.50 0.59 0.64 0.73

Al2O3 17.97 17.09 17.32 18.03 17.00

FeO� 5.35 4.23 5.24 4.77 5.17

MnO 0.10 0.07 0.12 0.08 0.10

MgO 3.84 2.59 3.93 3.25 3.89

CaO 6.45 7.28 7.52 7.06 6.07

Na2O 4.12 1.84 3.21 2.49 3.48

K2O 1.35 1.18 1.23 1.58 1.58

P2O5 0.18 0.13 0.18 0.16 0.19

Total 99.37 97.53 99.35 99.09 99.36

ppm

Ni 39 23 48 26 33

Cr 65 28 61 37 45

Sc 17 15 13 14 18

V 124 99 117 126 147

Ba 504 371 332 495 492

Rb 25 21 22 30 32

Sr 626 586 453 524 541

Zr 125 84 123 171 117

Y 20 11 22 14 17

Nb 5 5 4 4 5

Ga 18 17 19 17 17

Cu 34 22 2 26 15

Zn 44 44 67 48 66

Pb 4 1 4 0 4

La 8 0 21 16 22

Ce 21 28 42 43 41

Th 1 3 8 3 3

1Data for Stage 4 are averages of two analyses. All others area single analysis.2Unexplained total of 1133 ppm.�All Fe as FeO.Error is �10% for all except Ba, Zr, Y, Nb (�15%) and Al, Si(�0.5wt %).

Table 3: Selected biotite and hornblende

analyses

Hornbende Biotite

Stage 1 Stage 2 Stage 1 Stage 2

No. of analyses: 69 23 43 26

wt %

SiO2 (0.13/0.12) 49.28 49.52 36.54 43.16

TiO2 (0.02/0.03) 0.80 0.62 3.21 3.97

Al2O3 (0.05/0.07) 6.37 5.77 14.40 14.18

FeO� (0.09) 13.52 13.45 16.45 13.19

Fe2O3 (calc) 7.25 7.03

FeO (calc) 6.88 7.13

MnO (0.03) 0.45 0.39 0.14 b.d.

MgO (0.08/0.07) 14.55 14.48 13.36 11.82

CaO (0.06/0.01) 11.67 11.54 b.d. 0.50

Na2O (0.03/0.02) 0.69 0.74 0.12 0.73

K2O (0.01/0.05) 0.34 0.13 8.98 7.32

H2O (calc) 2.00 2.04 3.74 3.80

F (0.02) 0.18 b.d. 0.28 0.67

Cl (0.01) 0.04 b.d. 0.11 0.08

O � F, Cl 0.08 0.03 0.15 0.30

Total 100.42 99.36 97.18 99.12

Mg no. 66 66 59 62

Range Mg no. 63---74 61---67 58---60 24---74

�All Fe as FeO.Fe2O3, FeO, and H2O calculated by charge balance using theCameca SX Formula-1 program. b.d., concentration belowdetection limit. Numbers in parentheses indicate analyticalprecision determined from microprobe counting statistics;first number refers to precision for hornblende, second num-ber for biotite.



biotite with low F (50.3 wt %) also has low Cl(50.05 wt %; Fig. 9c).

FeldsparPlagioclase and orthoclase together make up nearly50 vol. % of the bulk unmelted wallrock (Fig. 3).Orthoclase is absent from the restite assemblage byStage 4 (Table 4, Fig. 3), indicating that it is consumedduring the early stages of melting. Stage 2 orthoclase isless potassic than Stage 1 orthoclase (Fig. 10a) andcontains up to three times more Ba (0.54---1.69 wt %in Stage 2 vs 0.08---0.55 wt % in Stage 1).Plagioclase makes up the bulk of the mode in all

stages (Fig. 3). In the partially melted wallrock, itoccurs as a residual phase (Stages 2---5), as interstitialmaterial in biotite reaction sites (Stages 3---5), and asquench crystals associated with glass seams (Stages3---5) (Table 4). Changes in residual plagioclase com-position are not systematic with increased melting(Fig. 10). Stage 5 plagioclase is less different fromStage 1 than are the intervening stages. On thewhole, residual plagioclase becomes richer in K2O,

CaO, FeO, and MgO, and poorer in Na2O with con-tinued melting (Fig. 10). Optically distinct plagioclaserims in Stage 5 are also compositionally distinct,having higher concentrations of CaO, FeO, andMgO than Stage 5 relict plagioclase (Fig. 10b).Plagioclase from reacted biotite sites in Stages 3---5

(Fig. 6) is predominantly labradorite (Table 4). Plagio-clase in biotite reaction sites is generally more calcicthan residual plagioclase and contains 51 wt % K2O,but is compositionally variable. MgO and FeO con-centrations are slightly higher than in residual plagio-clase (up to 0.34 wt % MgO and 1.54 wt % FeO inStage 5), and concentrations of these oxides increasewith continued melting.Andesine to labradorite quench crystals were ana-

lyzed in Stages 3---5 (Table 4). The compositionalrange of quench plagioclase is similar to that of relictplagioclase, but quench crystals may be slightly lesspotassic. FeO and MgO concentrations in quench pla-gioclase are slightly higher in more advanced stagesof melting. Stage 5 quench crystals contain up to0.13 wt % MgO and 1.31 wt % FeO.

Fig. 8. Hornblende compositions in Stages 1 and 2. (a) Amphibole classification after Deer et al. (1992). (b) FeO and MgO. (c) K2O andNa2O. (d) Cl and F.



PyroxeneExcepting rare cores in hornblende, pyroxene is notpresent in the wallrock assemblage until Stage 3,indicating that it is produced during partial meltingreactions (Fig. 3). Texturally, pyroxene occurs inStages 3---5 as microlites associated with hornblende

reaction sites, as interstitial material in biotite reactionsites, and as acicular quench crystals associated withbrown glass seams. Both orthopyroxene and clino-pyroxene occur in hornblende reaction sites as opti-cally aligned microlites (Fig. 5). In Stage 3, augite,pigeonite, and lesser enstatitic orthopyroxene occupydecomposed hornblende sites (Table 5). However, byStage 5, nearly all of this pyroxene is enstatitic ortho-pyroxene (Table 5). Early clinopyroxene and ortho-pyroxene crystals have heterogeneous compositions,but pyroxene compositions become more homogeneouswith continued reaction (Fig. 11). Concentrations ofNa2O in augite decrease from 1.3 wt % in Stage 3 to50.4 wt % in Stage 4. Stage 3 augite contains up to8wt%Al2O3whereas Stage 4augite contains55wt%.Orthopyroxene intergrown with plagioclase and

opaque oxides occurs in biotite reaction sites in Stages3---5 (Fig. 6). Enstatitic orthopyroxene in biotite reac-tion sites is slightly more Mg- and Al-rich and Ca- andNa-poor than enstatitic orthopyroxene in hornblendereaction sites (Table 5).Enstatitic orthopyroxene quench crystals are asso-

ciated with brown glass seams, and sparse augitequench crystals also occur in Stage 5 (Table 5).Quench enstatitic orthopyroxene is similar in com-position to enstatitic orthopyroxene in decomposedhornblende sites, but is Al-poor in comparison withenstatitic orthopyroxene in decomposed biotite sites(typically 0.8---1.8 wt%Al2O3 vs43 wt% for biotite).Quench augite is also Al-poor in comparison withaugite in reacted hornblende sites from Stages 3 and 4.

Magnetite and ilmeniteMagnetite is ubiquitous in all stages, occurring as aprimary phase in the unmelted wallrock, as a residualphase in Stages 2---5, texturally associated with horn-blende and biotite reaction sites in Stages 2---5 (Figs 5and 6), and as quench crystals associated with brownseams of glass. Residual magnetite, as well as magne-tite associated with decomposed hornblende andbiotite sites, may contain up to 15 wt % TiO2. Sparseilmenite is present as scattered grains in Stages 3 and 5,in Stage 4 decomposed hornblende sites, and in reactedbiotite sites from Stages 3---5. The small size of Fe---Tioxides made microprobe analysis difficult and oftenyielded low analytical totals (Table 6, and see Petcovic,2000). Only a few magnetite---ilmenite pairs in Stages 3and 4 yielded analyses suitable for geothermometry(see thermal modeling section, below).

GlassGlass analyzed from Stages 3---5 yields a different com-position for each textural domain (Table 7, Fig. 12).

Fig. 9. Biotite compositions in Stages 1 and 2. (a) FeO and MgO.(b) Na2O and K2O. (c) TiO2 and F. Cl concentration in all sampleswith50.3wt % F is50.05wt %. Sample marked with � has high Cl(0.28wt %). Others have Cl between 0.05 and 0.15wt %.



Table 4: Representative feldspar analyses

Orthoclase Plagioclase Plagioclase in biotite sites Quench plagioclase

Stage 1 Stage 2 Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 31 Stage 41 Stage 51 Stage 3 Stage 41 Stage 5

No. of analyses: 14 5 57 19 31 14 38 1 16

wt %

SiO2 (0.14) 64.03 63.01 59.34 58.18 57.42 55.32 56.59 53.00 51.56 53.80 56.18 57.32 54.81

Al2O3 (0.09/0.07) 18.07 18.89 26.10 25.52 26.33 26.92 26.64 28.35 29.24 27.75 26.23 25.23 27.12

FeO� (0.03/0.02) 0.12 b.d. b.d. 0.21 0.23 0.98 0.23 0.73 1.29 1.25 1.00 1.04 1.31

MgO (0.01) b.d. b.d. b.d. b.d. b.d. 0.11 b.d. 0.05 0.16 0.22 0.08 0.09 0.12

CaO (0.05/0.01) b.d. 0.37 7.74 8.00 8.55 10.61 9.51 10.74 12.93 10.43 9.52 10.22 10.47

BaO (0.01/0.02) 0.45 1.58 b.d. b.d. b.d. b.d. b.d. 0.59 b.d. 0.15 b.d. 0.13 b.d.

Na2O (0.06/0.03) 0.43 1.92 7.01 5.54 5.77 4.24 5.42 4.40 3.22 4.74 5.40 3.53 4.88

K2O (0.02/0.07) 15.56 12.20 0.10 1.38 0.61 0.63 0.40 0.55 0.42 0.62 0.55 1.16 0.33

Total 98.69 98.05 100.39 98.91 98.95 98.86 98.90 98.39 98.87 98.97 98.99 98.73 99.07

An 0 2 38 41 43 56 48 55 67 53 48 56 53

Ab 4 19 61 51 53 40 50 41 30 44 49 36 45

Or 96 79 1 8 4 4 2 3 3 4 3 8 2

Range An, Or2 94---97 74---80 32---50 35---49 34---56 40---64 38---58 55 65---69 48---57 49---63 48---59

1Average of two analyses.2Range in An for plagioclase, Or for orthoclase.�All Fe as FeO.b.d., concentration below detection limit. Numbers in parentheses indicate analytical precision determined from microprobe counting statistics; first number refers toprecision for plagioclase feldspar, second number for orthoclase feldspar.

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The light brown, microfibrous glass domains are meta-luminous to mildly peraluminous (generally 51%normative corundum) and granitic with up to 8 wt %K2O in Stage 3 and 6.5 wt % K2O in Stage 5(Table 7). The dark brown glass domains are meta-luminous and tonalitic in composition. The twodomains are best distinguished by the amount of K2Orelative to CaO (Fig. 12a). With respect to Na2O,TiO2, MgO, FeO, and Al2O3, the brown glass domainslargely overlap at each stage. With increased degree ofmelting (from Stage 3 to Stage 5), K2O and SiO2

concentrations decrease and FeO, MgO, and TiO2

concentrations increase slightly in both brown glassdomains (Fig. 12).The clear granular domains of Stages 3---5 are essen-

tially composed of SiO2 (Table 7, Fig. 12b). The clearto yellow---brown glass domains in Stage 2 were not

successfully analyzed. These seams proved to generallybe too thin and/or altered for successful analysis.

DISCUSSION

Partial melting at the margin of the Maxwell LakeCRBG dike in the Wallowa Mountains provides amacrocosm of textural and compositional informationabout progressive melting of continental crust at a scalebetween experimental charges and granitoid intru-sions. We first discuss the nature of the melting reac-tions, including the nature of melts produced. We thencompare this example of natural melting with othernatural examples and experimental work on similarprotoliths. Finally, the conditions of melting are eval-uated, followed by development of a thermal modelthat predicts the timescale over which meltingtook place.

Nature of the melting reactions

Closed-system meltingWithin the range of samples we have examined, partialmelting appears to have taken place under closed con-ditions. Nearly all major elements (as well as most traceelements) were conserved from Stage 1 to Stage 3 toStage 5 (Table 2), indicating that these samples repre-sent a chemically closed system. Stages 2 and 4 areweighted less in this discussion because of the cataclas-tic overprint and slight bulk compositional differencesattributed to alteration. We do not consider water andother volatile components here. Compositional near-identity during progressive melting also suggests thatmelts did not separate from their restite. Indeed, weobserved no textural evidence in thin section or inoutcrop that suggests large-scale melt flow.

The melting reactionsDuring the progress of partial melting observed over4m of wallrock adjacent to the Maxwell Lake dike,hornblende, biotite, and orthoclase were entirely con-sumed. The composition of these phases became highlyvariable as they broke down. Plagioclase, quartz, andmagnetite were partially consumed yet persisted in therestite with as much as 31 vol. % glass. With progres-sive melting, relict plagioclase developed a spongyresiduum by the reaction of andesine to producelabradorite plus an albitic melt partially trappedwithin the plagioclase.During progressive melting, orthopyroxene, clino-

pyroxene, secondary plagioclase, magnetite, sparseilmenite, and melt (now represented by devitrifiedglass � quench crystals) were produced. The composi-tions of new minerals were initially variable, yet

Fig. 10. Feldspar compositions in Stages 1---5. (a) Ternary feldsparcompositions of primary and residual feldspar. (b) CaO and FeO� inplagioclase. Stage 5 plagioclase rims have 0.5---1.5wt % MgO, com-pared with relict plagioclase with typically 50.5wt %.



Table 5: Representative pyroxene analyses

Clinopyroxene in

hornblende sites

Pigeonite in

hornblende sites

Orthopyroxene in

hornblende sites

Orthopyroxene in

biotite sites

Quench

orthopyroxene

Quench

clinopyroxene

Stage 3 Stage 41 Stage 3 Stage 4 Stage 5 Stage 3 Stage 4 Stage 5 Stage 4 Stage 51 Stage 31 Stage 41 Stage 5 Stage 5

No. of analyses: 34 5 14 1 17 32 51 1 6 3

wt %

SiO2 (0.10/0.09) 52.53 52.33 53.86 51.94 52.78 54.57 53.44 53.71 50.77 52.96 53.17 51.91 53.27 52.37

TiO2 (0.02) 0.41 0.64 0.30 0.37 0.41 0.19 0.47 0.26 0.49 0.56 0.26 0.34 0.45 0.46

Al2O3 (0.02) 2.49 3.77 2.86 2.42 1.43 1.11 1.52 1.20 3.67 3.30 0.83 1.75 1.72 1.47

FeO� (0.10/0.08) 10.23 13.38 15.82 17.31 14.51 17.30 17.70 15.48 18.43 14.42 18.57 18.22 16.10 10.18

MgO (0.10/0.08) 15.64 19.58 23.23 22.67 22.22 25.15 25.03 26.20 22.49 28.03 23.92 24.29 25.92 15.67

MnO (0.03) 0.41 0.35 0.46 0.47 0.37 0.62 0.46 0.40 0.44 0.39 0.54 0.37 0.46 0.37

CaO (0.03/0.07) 18.03 10.53 2.54 2.74 7.53 2.00 1.97 1.74 1.43 0.47 1.34 1.13 1.67 18.68

Na2O (0.02) 0.46 0.15 0.40 0.19 0.12 b.d. b.d. b.d. 0.18 b.d. b.d. b.d. b.d. 0.26

Total 100.22 100.74 99.47 98.12 99.37 101.00 100.66 99.03 97.90 100.15 98.70 98.06 99.63 99.46

En 46 56 68 66 62 69 69 72 66 77 68 69 72 45

Fs 17 22 26 28 23 27 27 24 31 22 30 29 25 16

Wo 38 22 5 6 15 4 4 3 3 1 3 2 3 39

Range En 40---52 43---44 64---69 57---67 66---73 65---72 64---75 76---78 66---69 68---69 65---76 42---46

Range Wo 25---43 39---41 5---11 5---16 36---40

1Average of two analyses.�All Fe as FeO.b.d., concentration below detection limit. Numbers in parentheses indicate analytical precision determined from microprobe counting statistics; first number refers toprecision for orthopyroxene, second number for clinopyroxene.

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became more homogeneous with continued melting.Melt developed within and around decomposed biotiteand hornblende sites, and also along seams up to 2mmthick between relict quartz and plagioclase crystals.In Stage 3, aligned clinopyroxene and orthopyroxenemicrolites, minor magnetite and rare ilmenite, andglass occupy decomposed hornblende sites, suggestingthat dehydration-melting reactions involving horn-blende produced these phases. By Stage 5, all of theclinopyroxene in hornblende decomposition siteshas reacted to produce enstatitic orthopyroxene. InStages 3---5, biotite dehydration-melting reactions pro-duced aligned magnetite and lesser ilmenite in anintergrown matrix of orthopyroxene, plagioclase, andglass. Because partial melting took place in a closedsystem, components released from phases that breakdown must have been accommodated either in therestite or in the melt.Consideration of the difference between modal pro-

portion of phases between Stage 1 and Stage 3 (Fig. 3),along with consideration of the composition of these

phases, allows us to determine a general, initial melt-producing reaction:

hornblende� biotite� quartz� orthoclase

� Ab from plagioclase � clinopyroxene

� orthopyroxene� plagioclase�magnetite

�melt �glass� quench crystals�: �1�This reaction was terminal for both amphibole andbiotite and yielded �18 vol. % melt (12 vol. % glass�2 vol. % quench crystals � 4 vol. % glass trapped inspongy plagioclase; Table 1). Between Stage 3 andStage 5 (Fig. 3), wallrock closer to the dike marginexperienced higher temperature conditions, suggestingthat additional melt was produced from the followingreaction:

plagioclase� quartz� orthoclase� clinopyroxene

�magnetite � melt �glass� quench crystals�: �2�This second reaction was terminal for orthoclase andclinopyroxene, and yielded an additional 29 vol. %melt for a total of nearly 47 vol. % melt (31 vol. %glass � 9 vol. % quench crystals � 7 vol. % glasstrapped in spongy plagioclase; Table 1).Although we have established general melt-

producing reactions, other work on partial melting incrystalline rocks (e.g. Wolf &Wyllie, 1991; Philpotts &Asher, 1993; Hammouda et al., 1996; Knesel &Davidson, 1996) has suggested that dehydration-melt-ing reactions are locally controlled by stoichiometry ofmelt reactions and kinetics rather than by the overallbulk assemblage. For example, melt in reaction (1)could have been produced by local reactions such ashornblende � quartz, biotite � plagioclase, biotite �hornblende� orthoclase, or quartz� plagioclase. Thisprocess results in a disequilibrium assemblage ofheterogeneous local melts, residual minerals and sec-ondary minerals in different stages of reaction. Abun-dant disequilibrium textures (e.g. spongy plagioclase)and initially variable phase compositions suggest thatpartial melting reactions in the Wallowa tonalite werealso controlled by local assemblages.

Composition of the melts producedWe analyzed three distinct glass domains in the par-tially melted wallrock: a dominant, light brown,high-K glass exhibiting feathery devitrification; a lessabundant, dark brown, high-Ca glass; and a sparse,clear, high-Si glass. The distribution of both brownglasses is highly irregular, occurring in seams betweenquartz and plagioclase, adjacent to hornblende andbiotite reaction sites, and within these reaction sites.The compositions of each glass domain largely overlapfor each stage, except for high K2O in the light brown

Fig. 11. Pyroxene compositions in Stages 3---5. (a) Ternary composi-tion of pyroxenes from hornblende reaction sites. Data for Ca-poorpyroxenes of Stages 3---5 are virtually overlapping. Augite is commononly in Stage 3. Stage 4 augites fall into the center of the distribution.(b) Al2O3 and Na2O in pyroxene from hornblende reaction sites.



Table 6: Selected magnetite and ilmenite analyses used in oxide geothermometry

Stage 3 Stage 4

Magnetite Ilmenite Magnetite Ilmenite

Analysis no.: A.C.3 F.4 A.K.3 A.K.4 A.A.4 B.C.1 .16 .6 .3 .13 G.1

wt %

SiO2 (0.01) 0.18 0.12 b.d. 0.11 b.d. 0.11 b.d. b.d. b.d. b.d. 0.49

TiO2 (0.07/0.21) 10.49 10.24 7.56 47.53 46.68 41.35 14.93 10.68 15.48 13.90 53.22

Al2O3 (0.02) 5.62 7.14 1.53 0.26 0.27 0.15 2.19 2.43 2.09 2.25 2.35

Cr2O3 (0.02/0.01) b.d. b.d. 0.14 b.d. b.d. 0.16 0.43 0.15 0.13 0.22 b.d.

V2O3 (0.01) 0.22 0.20 0.71 b.d. b.d. 0.34 1.10 0.80 0.67 0.97 1.42

FeO� (0.22/0.15) 71.27 69.18 77.43 41.30 43.45 48.94 73.18 77.15 73.60 73.87 31.99

Fe2O3 (calc) 40.87 39.74 47.87 9.28 10.55 18.22 35.96 43.97 35.30 37.61 0.00

FeO (calc) 34.49 33.41 34.36 32.95 33.95 32.55 40.83 37.59 41.83 40.03 31.99

MnO (0.02/0.03) 0.47 0.43 0.32 0.58 0.57 0.29 0.22 0.19 0.20 0.21 b.d.

MgO (0.02/0.03) 3.76 4.29 1.13 5.22 4.17 2.50 2.32 1.95 1.97 2.04 3.68

Total 96.16 95.64 93.66 95.93 96.24 95.67 98.03 97.85 97.72 97.29 93.31

Xmag 0.66 0.65 0.76 0.54 0.67 0.53 0.57

Xilm 0.90 0.89 0.81 1.00

Equilib. match B.C.1 A.K.4 A.A.4 A.C.3 A.C.3 A.K.3 G.1 G.1 G.1 G.1

T (�C) 969 808 745 804 830 861 1088 988 1098 1066

�All Fe as FeO.b.d., concentration below detection limit. Low totals are due to the small size of the oxides. Numbers in parentheses indicate analytical precision determined frommicroprobe counting statistics; first number refers to magnetite, second number to ilmenite. Magnetite and ilmenite recalculations by the method of Stormer (1983).Potential equilibrium pairs for geothermometry identified by Mg---Mn partitioning after Bacon & Hirschmann (1988). Geothermometry temperatures calculated afterGhiorso & Sack (1991).

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glass (4---8 wt %) and high CaO in the dark brownglass (3---5 wt %). It is possible that the high-K glasswas produced by dehydration-melting reactionsdominated by biotite, whereas the high-Ca glass wasproduced by reactions dominated by hornblende.Compositionally, high-K glass data in Fig. 12 largelyoverlap with the biotite dehydration-melting field inFig. 13, and high-Ca glass data overlap with thecompositional field for amphibole-derived melts.Additionally, the high-K glass is more abundant thanthe high-Ca glass.Texturally, we find that an overprint of devitrifica-

tion has obscured whether the light and dark brownglasses were initially separate melts originating from

biotite and hornblende breakdown reactions, respec-tively. Light brown glass, which exhibits spheruliticdevitrification, surrounds cores of dark brown, largelyisotropic glass. This textural relationship occurs in meltseams and adjacent to biotite and hornblende break-down sites. We are currently unable to determinewhether the dark and light brown glass domains arerelicts of coexisting melts, or whether they are theproduct of Ca---K migration during devitrification.However, it is likely that locally controlled meltingreactions initially produced distinct melts of variablecomposition, that these melts homogenized to somedegree during continued melting, and that they experi-enced devitrification after quenching.

Table 7: Representative glass analyses

Dark brown Light brown Clear granular

Stage 3 Stage 4 Stage 5 Stage 3 Stage 4 Stage 5 Stage 3

No. of analyses: 15 7 8 38 36 50 4

wt %

SiO2 (0.11) 74.18 76.19 78.90 75.99 72.73 76.26 90.44

TiO2 (0.02) 0.21 0.78 0.58 0.49 0.74 0.87 b.d.

Al2O3 (0.04) 11.49 11.40 11.46 11.62 10.46 11.02 1.43

FeO� (0.04) 0.49 0.89 0.56 0.44 0.97 0.95 b.d.

MgO (0.01) b.d. 0.14 b.d. b.d. 0.14 0.05 b.d.

CaO (0.02) 3.51 3.08 3.36 0.53 0.68 0.73 0.68

Na2O (0.04) 3.15 2.88 3.51 2.17 2.64 2.81 0.20

K2O (0.03) 0.31 1.50 0.33 6.21 4.40 5.35 0.06

P2O5 (0.03) 0.07 0.16 0.19 0.08 0.35 0.27 b.d.

Total 93.57 97.15 99.03 97.65 93.25 98.45 93.05

F (0.02) 0.14 0.16 0.15 0.14 0.18 0.14 0.15

Cl (0.01) 0.02 b.d. 0.02 b.d. 0.04 b.d. 0.02

CIPW norm (wt %)

Q 47.1 47.3 50.7 38.7 39.6 38.4 87.7

C 0 0 0 0.6 1.0 0 0

Or 1.9 8.8 1.9 36.7 26.0 31.6 0.4

Ab 26.7 24.4 29.7 18.4 22.4 23.7 1.7

An 16.3 13.8 14.6 2.1 1.1 1.7 2.8

Di 0.6 0.4 0.3 0 0 0.2 0

Wo 0 0 0.2 0 0 0 0.2

Hy 0.3 0.6 0 0.1 0.9 0.4 0

Il 0.4 1.5 1.1 0.9 1.4 1.7 0.1

Ap 0.2 0.4 0.4 0.2 0.8 0.6 0

�All Fe as FeO.b.d., concentration below detection limit. Number in parentheses indicates analytical precision determined from microprobecounting statistics. Concentrations of MnO and S were below detection limits for all samples. CIPW norm calculated with allFe as FeO. CIPW norm abbreviations: Q, quartz; C, corundum, Or, orthoclase; Ab, albite; An, anorthite; Di, diopside; Wo,wollastonite; Hy, hypersthene; Il, ilmenite; Ap, apatite.



The clear granular domains observed in Stages 3---5are nearly always associated with embayed to resorbedquartz grains. Texturally, clear domains are somewhatcrystalline with undulose to patchy extinction. Theyare composed essentially of SiO2, suggesting that theyrepresent fully (re)crystallized polymorphs of quartz.It is unclear whether these domains were ever melted.Clear glass localized on quartz fractures in Stage 2 isprobably a product of initial melting reactions invol-ving quartz and feldspar.To reconstitute the bulk melt composition from the

glass domains and quench crystals, we employed threemethods (Table 8). (1) A composite melt analysis wasachieved by averaging electron microprobe point ana-lyses from a grid of evenly spaced points in a regionencompassing glass domains and quench crystals.(2) Using a melt modal reconstitution, the melt com-position was calculated by summing the compositionsof the devitrified glass domains and plagioclase, pyr-oxene, and magnetite quench crystals, each weightedby their modal abundance. (3) Using a bulk modal

reconstitution, the difference between modally weigh-ted composition of restite phases and whole-rock bulkcomposition yielded the composition of the melt.On the whole, we think the grid analysis average is

the best approximation of the real bulk composition,because it is not subject to errors in modal estimates.There is poor agreement among the three compositionsreconstituted for Stage 3 and good agreement forStage 5 (Table 8). The bulk modal reconstitution wasdifficult to complete because of the variability inmineral phase compositions, and due to the difficultyin evaluating the mode of extremely fine mineral reac-tion products in decomposed biotite and hornblendesites. For example, the differences in Stage 3 are prob-ably due to difficulties in modal estimates, in particularoverestimation of the abundance of Fe---Ti oxides lead-ing to high values of FeO in the reconstructed melt.The reconstructed melt in Stage 3 is metaluminous to

mildly peraluminous (0.3 wt % normative corundum)and granitic (Fig. 13a). Stage 5 reconstructed melt ismetaluminous and granodioritic to granitic (Fig. 13a).

Fig. 12. Glass compositions in Stages 3---5. (a) Normative An (anorthite, CaAl2Si2O8)---Ab (albite, NaAlSi3O8)---Or (orthoclase, KAlSi3O8).Fields after Barker (1979). (b) Normative Q (quartz, SiO2)---Ab---Or. Cotectic line for haplogranite system at 2 kbar and Xmelt(H2O) � 0.7(water undersaturated) after Holtz et al. (1992). Filled star marks position of the granite minimum. (c) TiO2 and FeO�. The behavior of MgOlargely mimics that of FeO�.



With increased melting, concentrations of SiO2 andK2O decrease whereas concentrations of most othercomponents (notably Al2O3, CaO and FeO) increase(Fig. 13).

The natural vs the experimental laboratory

The Maxwell dike locality bears out the conclusionthat progressive partial melting in crystalline rocksproduces a heterogeneous assemblage of composition-ally variable melts, based on other examples of melting(e.g. B�usch et al., 1974; Kaczor et al., 1988; Wolf &Wyllie, 1991; Philpotts & Asher, 1993; Hammoudaet al., 1996; Knesel & Davidson, 1996; Rushmer,2001). In contrast, dehydration-melting experimentsof powdered starting materials do not produce suchmelt heterogeneity. Despite the inherent disequili-brium nature of melting in the natural examples, weobserve that the overall modal and compositional

record of progressive melting is similar to that of equi-librium experiments. In the following comparisons, webear in mind that the Wallowa tonalite differs frommost other melting studies of crustal rocks in that itcontains subequal proportions of amphibole andmica, and thatmelting took place at51 kbar. However,melting of granitoid protoliths appears to be insensitiveto pressures as great as �8 kbar (Singh & Johannes,1996a), which suggests that the results of melting inthe Wallowa example are applicable to much of theupper crust.

Comparison of mineral compositionsPlagioclase formed part of the parent rock in all gran-itoid natural and experimental protoliths, and residualplagioclase was a component of restite assemblagesunder nearly all pressure---temperature conditions upto 10 kbar. In these studies, as in the Wallowa rocks,

Fig. 13. Comparison between reconstructed melt compositions in Stages 3 and 5 and other natural and experimental melts. Field foramphibole-bearing protoliths includes data from Beard & Lofgren (1991), Rapp et al. (1991), Rushmer (1991), and Pati~no Douce & Beard(1995). Field for biotite-bearing protoliths includes data from Kaczor et al. (1988), Vielzeuf & Holloway (1988), Kitchen (1989), Skjerlie &Johnston (1992, 1993), Philpotts & Asher (1993), Pati~no Douce & Beard (1995), and Singh & Johannes (1996a, 1996b). Individual datapoints are shown for Skjerlie & Johnston (1996). (a) Normative An---Ab---Or of melts. (b) Normative Q---Ab---Or in melts. (c) TiO2 and FeO�in melts.



plagioclase was consumed only at large extents of melt-ing. Dissolution features, such as fritted margins andspongy textures observed in reacted Wallowa plagio-clase, were also observed by B�usch et al. (1974), Kaczoret al. (1988), Green (1994), and Philpotts and Asher(1993), who attributed fritted margins to melting alongcleavage planes. By Stage 5, Or and Ab components ofresidual Wallowa plagioclase had increased, and relictrims were high in An, FeO, andMgO. High-An rims inrelict plagioclase were also observed by B�usch et al.

(1974). Vielzeuf &Montel (1994) reported an increasein the Or component of residual plagioclase, and manystudies (B�usch et al., 1974; Kaczor et al., 1988; Beard &Lofgren, 1991; Philpotts & Asher, 1993; Singh &Johannes, 1996b) observed an increase in An in resi-dual plagioclase relative to starting plagioclase.Amphibole dehydration-melting reactions produced

clinopyroxene � lesser orthopyroxene in amphibolereaction sites under nearly all pressure---temperatureconditions up to 10 kbar (e.g. Beard & Lofgren, 1991;

Table 8: Reconstructed bulk melt compositions

Stage 3 Stage 5

matrix melt bulk bulk matrix melt bulk

average1 mode2 mode3 mode4 average1 mode2 mode3

wt %

SiO2 71.78 68.50 81.08 65.59 66.93 68.01 70.73

TiO2 0.28 0.50 ÿ1.80 0.54 0.60 0.97 0.40

Al2O3 11.89 10.65 25.58 18.63 13.47 12.42 14.45

FeO� 1.95 5.99 ÿ14.26 0.54 2.73 5.53 2.44

MnO 0.00 0.09 0.02 0.13 0.05 0.07 0.09

MgO 1.08 2.45 11.39 0.16 1.40 1.64 0.09

CaO 1.39 1.68 ÿ4.56 9.62 3.22 2.84 4.54

Na2O 2.17 2.20 1.69 1.02 3.07 2.82 3.57

K2O 5.31 3.82 5.08 3.74 3.81 3.40 3.21

P2O5 0.09 0.07 0.61 0.45 0.34 0.17 0.29

Total 96.18 96.08 104.81 100.41 95.86 98.01 99.76

F 0.18 0.11 0.20 0.10

Cl 0.02 0.01 0.02 0.03

CIPW norm (wt %)

Q 33.1 29.3 27.7 25.0 27.0 28.3

C 0.3 0 0 0 0 0

Or 31.4 22.6 22.1 22.5 20.1 19.0

Ab 18.3 18.6 8.6 26.0 23.9 30.2

An 6.3 7.8 35.2 11.7 11.2 13.4

Di 0 0 1.5 1.7 1.5 5.9

Hy 5.9 16.4 0 6.8 12.0 1.1

Il 0.5 0.9 1.0 1.2 1.8 0.8

Ap 0.2 0.2 1.0 0.8 0.4 0.7

�All Fe as FeO. CIPW norm abbreviations same as for Table 7.1Average of 79 analyses in a 212mm � 387mm grid for Stage 3, and average of 256 analyses in four 800mm � 800 mm(approximately) grids for Stage 5.2Modal melt reconstitution based on proportions of melt domains and quench crystals as follows: dark brown glass : lightbrown glass : clear granular glass : quench plagioclase : quench orthopyroxene : quench clinopyroxene : quench magnetite,3.5:8.8:0.2:0.5:1.0:0:0.3 for Stage 3 and 4.2:25.9:1.1:5.3:1.3:0.6:0.9 for Stage 5. Numbers are given in vol. % of the bulkrock.3Melt calculated by difference of bulk and restite given mode in Table 1 and average mineral compositions. Details have beengiven by Petcovic (2000). F and Cl data not available for bulk analyses.4Melt calculated by difference of bulk and restite with mode given in Table 1 except 0.5% primary magnetite, 10% pyroxenein hornblende reaction sites, 1.1% Fe---Ti oxides in biotite reaction sites, and 19% melt. Numbers are given in vol. % of bulkrock.



Rapp et al., 1991; Rushmer, 1991; Pati~no Douce &Beard, 1995). In the Wallowa rocks, dehydration-melting reactions involving hornblende initially pro-duced aligned augite � pigeonite � lesser enstatiticorthopyroxene � sparse magnetite. Beard & Lofgren(1991) observed enstatitic orthopyroxene, augite, andsparse pigeonite as amphibole reaction products, buttheir augite contained 55 wt % Al2O3. In contrast,Wallowa augite contained up to 8 wt % Al2O3.Studies involving biotite dehydration-melting reac-

tions document orthopyroxene � Fe---Ti oxides �alkali feldspar (rarely � amphibole) in biotite reactionsites (e.g. B�usch et al., 1974; Kaczor et al., 1988;Vielzeuf & Montel, 1994; Pati~no-Douce & Beard,1995; Singh & Johannes, 1996a; Rushmer, 2001).Reactions involving biotite breakdown in the Wallowawallrock produced orthopyroxene � plagioclase �high-Ti magnetite � sparse ilmenite. Similar to whatwe observed from Stage 3 to Stage 5, Pati~no Douce &Beard (1995) and Singh & Johannes (1996b) observedan increase in En component in orthopyroxene withrising temperature. In contrast to other studies involv-ing biotite breakdown, alkali feldspar was notobserved. It is possible that the high water content ofinitial dehydration melts and low total pressure desta-bilized alkali feldspar as a reaction product [compareexperimental work by Naney (1983) and Johnson &Rutherford (1989)]. On the other hand, hornblendedehydration-melting reactions may have contributedCa to form plagioclase in biotite reaction sites.

Comparison of meltsIn crystalline rocks such as the Wallowa tonalite, in situmelt of multiple compositions (preserved as glass orgranophyre) has been reported as seams around crys-tals, along fractures within crystals, and localizedaround decomposed mafic sites (e.g. B�usch et al.,1974; Kaczor et al., 1988; Philpotts & Asher, 1993;Green, 1994; Knesel & Davidson, 1996; Tommasini& Davies, 1997; Rushmer, 2001). In the partiallymelted granite studied by Kaczor et al. (1988), clearglass (high SiO2, alkalis, and Rb) localized aroundquartz was a product of biotite breakdown and earlyquartz---feldspar melting. Brown glass (high CaO,Al2O3, MgO, FeO, and TiO2), localized around oxidesand spongy feldspar, became more abundant at higherextents of melting. Philpotts & Asher (1993) reportedtwo distinct melt compositions: a high-K glass locatedbetween quartz and orthoclase, and a low-K glassbetween quartz and andesine.In their granite cube experiment, Knesel &

Davidson (1996) observed clear (trachytic) glass locallygrading into brown (mugearitic) glass that surroundedFe---Ti oxides replacing biotite. Rushmer (2001)

observed voluminous, granitic glass derived from mus-covite breakdown along grain boundaries and cracksin quartz grains, with minor, biotite-derived glass(higher FeO and TiO2) localized around spinel inbiotite reaction sites. Locally, melt compositionsmixed along grain boundaries, producing an inter-mediate-composition glass. Previous workers havepointed out that progressive disequilibrium melting ofthis type produces melts that were initially enriched inmany incompatible elements, particularly Rb as wellas 87Sr/86Sr derived from the breakdown of biotite---hornblende, whereas successive melts were enriched inthe restite component (particularly Sr) released withplagioclase (e.g. Kaczor et al., 1988; Hammouda et al.,1996; Knesel & Davidson, 1996; Tommasini & Davies,1997; Davies & Tommasini, 2000).Similar to other studies, melt in the Wallowa tonalite

occurs on quartz---plagioclase boundaries, trappedwithin spongy plagioclase, and around decomposedhornblende and biotite sites. The heterogeneity inquenched melt from the Wallowa samples may, inpart, be attributable to devitrification textures; how-ever, comparison with other studies suggests that localreactions also played a role. Dehydration-melting reac-tions dominated by either biotite or hornblende couldaccount for the high-K and high-Ca brown glasses,respectively. Sparse clear (Stage 2) glass, which wehave not analyzed, probably represents an early pro-duct of melting on quartz---feldspar boundaries, similarto the clear glass observed by Kaczor et al. (1988).The Stage 3 bulk (reconstructed) melt was produced

by dehydration-melting reactions involving biotite,hornblende, plagioclase, quartz, and orthoclase.Although biotite and hornblende are modally sub-equal in the unmelted Wallowa tonalite, the Stage 3bulk melt lies within the field for melts produced frombiotite-bearingprotoliths (Fig.13).Biotitedehydration-melting reactions, however, may produce 2---3 timesmore melt than amphibole dehydration-meltingreactions (Pati~no Douce & Beard, 1995). The compo-sition of Stage 5 bulk melt (Fig. 13) reflects continuedmelting and the consumption of clinopyroxene andplagioclase.Overall, the bulk Wallowa melts are metaluminous

to barely peraluminous, in contrast to peraluminous(1---5% normative corundum) melts produced frombiotite-bearing protoliths. Melts from amphibole-bearing protoliths are trondhjemitic to granodioritic,metaluminous to peraluminous (0---7% normative cor-undum), silicic, and low in mafic oxides; Wallowamelts differ in that they are more K-rich and Al-poor(Fig. 13). Our bulk melt composition reflects the invol-vement of both biotite and hornblende in dehydration-melting reactions. With increased degree of melting(Stage 3 to Stage 5), Wallowa melts become slightly



more mafic, more aluminous, less potassic, and lesssilicic, as is consistent with other partial melting stu-dies. Overall, Stage 3 Wallowa bulk melt is similar toaverage A-type granite (as compiled by Skjerlie &Johnston, 1992) and some high-silica rhyolites (seeStreck, 2002). Stage 5 Wallowa bulk melt is similar tometaluminous rhyodacites thought to be mainly ofa crustal-melt origin (see Feeley & Grunder, 1991;Johnson & Grunder, 2002).

Thermal history and implications of diking

Thermal conditions of meltingTemperatures recorded by minerals in the melt zonerepresent an integrated thermal effect from heatingduring diking followed by cooling when basalt flowceased. The maximum thermal gradient at the edge

of the Maxwell Lake dike was about 1090�C, assuminga geothermal gradient of 25�C/km, a depth of 2 km tothe dike, and a basalt magma temperature of 1140�C[liquidus temperature from Murase & McBirney(1973)]. The wallrock could have been hotter thanestimated if it had been subjected to heating by pre-vious dike intrusion. Mineral thermometry and com-parison with experimental phase equilibria have beenused to estimate the temperature conditions repre-sented by each progressive melting stage (Fig. 14a).The small grain size and heterogeneity of Fe---Ti

oxides compromise their use in geothermometry.Nevertheless, potential equilibrium pairs of magnetiteand ilmenite were identified in Stages 3 and 4 on thebasis of the Mg---Mn partitioning criteria of Bacon &Hirschmann (1988) (Table 6). Stage 3 magnetite---ilmenite pairs give a range of values with an averagetemperature of about 830�C (Fig. 14a). Stage 4 pairs

Fig. 14. Results of thermal model predicting wallrock heating due to dike flow followed by dike and wallrock cooling. Parameters include:initial wallrock temperature 55�C, dike temperature 1140�C, dike thickness 8m, a � 4.11 � 10ÿ7. Additional details are given in theAppendix. (a) Part A of the thermal model. Temperature plotted against distance from the dike center as a function of time simulating theperiod when the dike was active. Square symbols and vertical error bars indicate average and range (respectively) of results for Fe---Ti oxidethermometry (Table 6). Range of temperatures for hornblende and biotite breakdown are from comparison with partial melting experiments(see text). Horizontal error bars reflect typical thickness for each stage of melting (�1m). (b) Part B of the thermal model. Temperatureplotted against distance from the dike center as a function of time once basalt flow has ceased. Dashed line is initial isotherm, equivalent to 3.6years of heating in Part A. (c) Enlargement of framed area shown in (b).



suggest an average temperature of 1060�C (Fig. 14a).Stage 5 contained no equilibrium pairs. Pyroxenepairs did not produce equilibrium tie lines (afterLindsley, 1983), and were therefore not successful forthermometry.Comparison with experimental phase equilibria

from intermediate composition protoliths can help con-strain the temperature at which melt-forming reactionstook place. From a series of low-pressure experimentsdesigned to locate the solidus of a synthetic biotitetonalite, Singh & Johannes (1996a, 1996b) concludedthat the onset of melting in biotite-bearing rocks maybe as low as 700�C. However, 1---3 kbar melting experi-ments on a meta-greywacke indicated a biotite break-down range of �800---860�C (Vielzeuf & Montel,1994), and 3 kbar experiments on a synthetic biotitetonalite indicated a range of 850---930�C (Pati~no Douce& Beard, 1995). We have chosen an initial biotite-breakdown temperature of 800�C, which is therefore aminimum estimate for Stage 2 where the dehydration-melting of biotite produced thin glass seams (Fig. 14a).Under pressure conditions of �5 kbar, hornblende inmost intermediate composition amphibolites reactedout between about 850 and 925�C (Beard & Lofgren,1991; Pati~no Douce & Beard, 1995). Because Stage 3lacks both biotite and hornblende, temperature at thislocation probably reached at least 925�C (Fig. 14a).

Thermal model of wallrock meltingWe have constructed a one-dimensional thermal modelto estimate how long basalt flowed in the dike to pro-vide sufficient thermal flux to the wallrock to propa-gate partial melt zones and related temperatures as faras 4m from the dike---wallrock contact (additionaldetails are given in the Appendix). In Part A of themodel (Fig. 14a), the dike was held at a constanttemperature of 1140�C to simulate magma flow whilethe dike was active. The final conditions of this simula-tion provided the initial conditions for Part B (Fig. 14band c), in which magma flow has ceased and the dikeand wallrock were allowed to cool. Both parts of themodel assume all heat flow was via conduction. Addi-tionally, we assume that the latent heat of crystalliza-tion of the basalt was equal to the latent heat of meltingof the wallrock, and that both were released linearlyover the entire temperature interval. Latent heat ofcrystallization of the wallrock was neglected.Results for Part A of the model (for an intermediate

thermal diffusivity value of 4.11� 10ÿ7 m2/s; Fig. 14a)suggest that tonalite located within the first few centi-meters of the dike---wallrock contact began to undergopartial melting within a few hours of dike intrusion.This timescale is similar to that of experimental studies,such as the water-saturated melting experiments of

B�usch et al. (1974) and the granite cube experimentsby Knesel & Davidson (1996), where small volumes ofrock underwent melting over timescales of hours todays. Within about 1 year of dike injection, all Stage 5wallrock (0---1m from the dike---wallrock contact) hadreached temperatures in excess of 925�C. Stage 4 wall-rock (1---2m from the contact) had begun to melt afterabout 1 year, whereas Stage 3 wallrock (2---3m fromthe contact) underwent melting after nearly 3 years.Results suggest that about 4 years were required forrocks at 4m from the dike---wallrock contact to reach800�C. This, therefore, is the maximum time that thedike was active; wallrock beyond this distance showsno evidence of biotite breakdown or onset of partialmelting.Results of modeling the cooling history of both the

dike and wallrock are shown in Fig. 14b. Because theheat pulse continued to propagate during cooling,wallrock further than about 3m from the contactexperienced additional heating. However, at 4mfrom the contact (the critical transition betweenunmelted and partially melted wallrock), there wasonly about 10�C of additional heating (Fig. 14c)between 0.05 and 1 year following cessation of dikeflow. Isotherms were steep enough so that wallrock atdistances 44m never experienced temperatures inexcess of 800�C and therefore experienced no partialmelting.We found that both parts of the model were sensitive

to thermal properties of the dike and wallrock[expressed as the thermal diffusivity value (a)]. Forexample, a minimum thermal diffusivity value (1.22 �10ÿ6 m2/s) yielded model results indicating that wall-rock at 4m from the contact began to break down after1.2 years. Amaximum thermal diffusivity value (1.38�10ÿ7 m2/s) suggested that breakdown began afterabout 10.6 years. The model was rather insensitive todike thickness. Part A was independent of dike thick-ness, whereas both dike and wallrock cooled moreslowly with a thicker dike in Part B, yet the shape of theisotherms remained largely unaffected. For example, adike of 16m thickness was 200�C warmer after 20 yearsof cooling than the results shown in Fig. 14b.The 4 year heating interval implied by this model is a

maximum time if the wallrock were preheated by priordike activity. Preheating the wallrock to 200�C at thetime of dike injection allowed temperatures to reach800�C at 4m from the contact in 2.6 years, one yearfaster than with wallrock at 55�C. On the other hand,it is more likely that the 4 year timescale is a minimumif intermittent flow in the dike or cooling by meansother than conduction occurred. Long pauses in flowof the dike could potentially cause complex overprintsof melting and crystallization reactions. Although wecannot preclude some fluctuations, the regular textural



progression observed in the melt zone is consistent withcontinuous or pulsating flow. Cooling of dike and wall-rock by means other than conduction, such as coolingby circulating groundwater [as suggested by Delaney(1987)], may account for the rapid cooling historyimplied by the presence of glass and quench crystalsin the partial melt zones. If such cooling took place, thecalculated heating times based on conductive coolingare minima.

Implications of the thermal model for CRBG volcanismDikes with substantial wallrock melting are rare in theWallowas; in mapping of four sub-swarms we havefound only two dikes with well-developed wallrockpartial melt zones. In both cases where there is sub-stantial partial melt in wallrock adjacent to a dike, thedike is not prominently quenched. Instead, coarse-grained basalt extends to the dike---wallrock contact.The thermal model of the Maxwell Lake dike suggeststhat magma flowed through this dike for at least sev-eral years. We believe that, in general, the higherthermal flux experienced by dikes with partial melt attheir margins indicates that they had a prolonged his-tory of activity and therefore were more likely to havefed major CRBG flows. Dikes with quenched marginsand no interaction with the wallrock, on the otherhand, probably represent conduits used for short peri-ods of time before solidifying. We take this to be analo-gous to Hawaiian-style eruptions where early dike-fedfissure eruptions become localized to yield long-livedcentral vent eruptions that feed flows.

CONCLUSIONS

In rare instances, wallrock adjacent to Columbia RiverBasalt Group dikes in the Wallowa Mountains hasundergone partial melting, providing a unique oppor-tunity to examine crustal melting in a natural setting.The unmelted wallrock is a hornblende---biotite grano-diorite to tonalite, a lithology that is rarely examined inthe experimental partial melting literature but is prob-ably common in natural settings. Samples collectedfrom the margin of a Grande Ronde dike at MaxwellLake represent progressive stages of closed-system par-tial melting over a distance of about 4m from unmeltedtonalite (Stage 1) to about 47 vol. % quenched melt(Stage 5). Partial melt reactions took place at a paleo-depth of about 2---2.5 km.With the onset of melting, a trace amount of a clear

melt was produced, now preserved along fractures inquartz. Dehydration-melting reactions involving bothbiotite and hornblende, plus plagioclase, orthoclaseand quartz, produced melt (preserved as variably devi-trified glass and quench crystals) localized around

decomposed mafic sites, on quartz---feldspar bound-aries and in spongy plagioclase. Comparison withother natural examples and experimental work indi-cates that a dominant high-K (light brown) glassresulted from biotite dehydration-melting, leavingaligned magnetite and ilmenite intergrown withplagioclase and orthopyroxene in the former biotitesites. A less abundant high-Ca (dark brown) glass wasproduced during dehydration-melting of hornblendeleaving a dusty intergrowth of clinopyroxene, lesserorthopyroxene, and sparse magnetite in former horn-blende sites. Approximately 18 vol. % melt were pro-duced in these early stages of melting. Up to 29 vol. %additional melt were produced by the reaction oforthoclase, clinopyroxene, quartz, plagioclase andmagnetite. This reaction was terminal for both ortho-clase and clinopyroxene, leaving optically alignedorthopyroxenes in former hornblende sites. With pro-gressive melting, phases being consumed became strik-ingly more heterogeneous in composition, whereasreaction products were relatively homogeneous. Theprogress of disequilibrium melting reactions as well asthe composition of reaction products are broadly simi-lar to those observed in other natural and experimentalcase studies.The bulk composition of the reconstructed early

melts was granitic and metaluminous to barely pera-luminous and closely approximates the composition ofmany A-type granites. With progressive reaction, themelt became more granodioritic and metaluminousand closely mimics the composition of rhyodacitic vol-canic rocks thought to be the products of crustal melt-ing. In general, the Wallowa bulk melt, producedby simultaneous dehydration-melting of biotite andhornblende, was intermediate in composition betweengranitic peraluminous liquids produced from biotitedehydration melting and the tonalitic liquids producedfrom amphibole dehydration melting.Thermal modeling suggests that for a dike of 8m

thickness carrying magma at 1140�C, about 4 yearswere required to initiate breakdown reactions in wall-rock at a distance of 4m from the dike---wallrock con-tact. Depending on the choices of thermal properties ofthe dike and host rock, 1---10 years were required toinitiate the melting reactions at 4m. If flow of magmain the dike was intermittent, or cooling was enhancedby groundwater circulation, then the dike could havebeen active longer. We think that dikes with substan-tial partially melted margins represent long-lived por-tions of the Columbia River Basalt feeder system andmay have sustained large flows, possibly analogous toHawaiian-style central vent eruptions. In contrast, themajority of dikes, which are quenched against the wall-rock, represent dike propagation and fissure eruptionevents that were short-lived.



ACKNOWLEDGEMENTS

We would especially like to thank Bill Taubeneck forhis assistance with the petrographic analysis and hisguidance on the field component of this research. Wewould also like to thank Roger Nielsen of the OregonState University Electron Microprobe Laboratory forhis advice on and assistance with data collection.George Bergantz and Roy Haggerty provided valuableideas and input for the thermal modeling. Discussionswith Peter Reiners, John Dilles, and Joe Dufek helpedto clarify this work. Reviews by Tracy Rushmer andMike Williams substantially improved the manuscript.Thanks go to Mike Winkler, Brandon Browne, JesseDickinson, Lang Farmer, and George Bergantz forparticipation in field work. This research was sup-ported in part by the Geological Society of Americagrant number 6514-99 awarded to H.L.P.

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Symbols used in model

Symbol Units Value(s) used in model�

a distance from dike center (x � 0) to dike---wallrock contact (x � a) m 4

a modified thermal diffusivity m2/s 1.38 � 10ÿ7, 4.11 �10ÿ7, 1.22 � 10ÿ6

Cp specific heat capacity J/kg K 1000, 2000, 3000

r densityy kg/m3 2400

erfc complementary error function

fs solid fraction in magma none

k conductivity J/m s K 1, 2, 3

L latent heat of crystallization/melting J/kg 30 000

t time s

y dimensionless temperature y � �TÿTw�=�TdÿTw� none

Td initial dike temperature �C 1140

Tw initial wallrock temperature �C 55

wr wallrock value

x distance normal to dike-wallrock contact from dike center m

�Values used to calculate a for model results shown in Fig. 14 are in bold.yIntermediate value used for both dike and wallrock.

APPENDIX: THERMAL MODEL



Part A

To simulate heating of wallrock while magma is flow-ing through the dike, we model the system as heat flowvia conduction in a semi-infinite solid (e.g. Carslaw &Jaeger, 1959). The general heat equation, given aninitial condition of T(x 4 a, 0) � Tw and boundaryconditions of dike margin constant atTd [T(x� a, t)�Td], andTmust beTw at infinity [T(x!1, t)� 0] is

rwrCpwr@T

@t� kwr

@2T

@x2� rwrLwr

dfs

dt: �A1�

Because the solid fraction is a function of temperature,the heat equation is rewritten as

rwrCpwr@T

@t� kwr

@2T

@x2� rwrLwr

dfs

dT

@T

@t: �A2�

Introducing dimensionless temperature yields

@y@t

rwr Cpwr� Lwr

�Td ÿTw�dfs

dy

�� kwr

@2y@x2

: �A3�

Assuming that fs is linear in y (i.e. that jdfs=dyj � 1),the solution is

y�x4a, t� � erfcxÿa2��atp

� ��A4�

where

a � kwrrwr�Cpwr � Lwr=�Td ÿ Tw�� : �A5�

This equation was solved and plotted using theprogram Mathematica 4.2.

Part B

To simulate cooling of the dike and wallrock, we modelthe system as an infinite solid. At t� tp, dike injection isstopped, and dike and wallrock are allowed to cool.The general solution to equation (A4) at t � tp is (afterCarslaw & Jaeger, 1959)

y�x, t�� 1

2��pa�tÿ tp�p Z 1

ÿ1yp�x0�exp ÿ�xÿ x0�2

4a�tÿ tp�

" #dx0,

t4tp: �A6�This equation was solved by numerical integrationand plotted using Mathematica 4.2.



Documents

Textural and Thermal History of Partial Melting in Tonalitic Wallrock