Metamorphic evolution of sapphirine- and orthoamphibole-cordierite-bearing gneiss, Okanogan dome, Washington, USA

S. C. KRUCKENBERG* AND D. L. WHITNEYDepartment of Geology and Geophysics, University of Minnesota – Twin Cities, Minneapolis, MN 55455, USA(seth@geology.wisc.edu)

ABSTRACT Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about thepressure–temperature–fluid history of the domes. Three northern Cordilleran migmatite domes (Thor-Odin and Valhalla ⁄Passmore, British Columbia, Canada; Okanogan, Washington, USA), however,contain Mg–Al-rich orthoamphibole-cordierite gneiss as layers and lenses that record metamorphicconditions and pressure–temperature (P–T) path information not preserved in the host migmatite. TheseMg–Al-rich rocks are therefore a valuable archive of metamorphic conditions during dome evolution,although refractory rocks such as these commonly contain reaction textures that may complicate thecalculation of metamorphic conditions. In the Okanogan dome, Mg–Al-rich layers are part of the TunkCreek unit, which occurs at the periphery of an underlying migmatite domain. Bulk compositional layers(mm- to m-scale) consist of gedrite-dominated, hornblende-dominated and biotite-bearing layers thatcontain variable amounts of gedrite, hornblende, anorthite, cordierite, spinel, sapphirine, corundum,kyanite, biotite and ⁄ or staurolite. The presence of different compositional layers (some with reactiontextures, some without) allows systematic analysis of metamorphic history by a combined petrographicand phase equilibrium analysis. Gedrite-dominated layers containing relict kyanite preserve evidence ofthe highest-P conditions; symplectitic and coronal reaction textures around kyanite indicate decompres-sion at high temperature. Gedrite-dominated layers lacking these reaction textures contain layers ofsapphirine and spinel in apparent textural equilibrium and record a later high-T–low-P part of the path.Phase equilibria (pseudosection) analysis for layers that lack reaction textures indicates metamorphicconditions of 720–750 �C at a range of pressures (>8 to <4 kbar) following decompression. Elevatedcrustal temperatures and concordant structural fabrics in the Tunk Creek unit and underlying migmatitedomain suggest that the calculated P–T conditions recorded in Tunk Creek rocks were coeval withanatexis, extension, and dome formation in Palaeocene–Eocene time. In contrast to orthoamphibole-cordierite gneiss in the other Cordilleran domes, the Tunk Creek unit occurs as a discontinuous km-scalelayer rather than as smaller (m-scale) pods, is more calcic, and lacks garnet. In addition, kyanite did nottransform to sillimanite, and spinel commonly occurs as a blocky matrix phase in addition to vermiculesin symplectite. These differences, along with the compositional layering, allow an analysis of bulkcomposition v. tectonic (P–T path) controls on mineral assemblages and textures. Pseudosectionmodelling of different layers in the Tunk Creek unit provides a basis for understanding the metamorphichistory of these texturally complex, refractory rocks and their host gneiss domes, and other such rocks insimilar tectonic settings.

Key words: high-temperature metamorphism; Mg-Al gneiss; North American Cordillera; Okanogandome; Omineca belt; orthoamphibole-cordierite; pseudosection; reaction textures; sapphirine-spinel;symplectite; Washington.

INTRODUCTION

Mg–Al-rich rocks, including orthoamphibole-cordie-rite gneisses, occur in many metamorphic terranesworldwide. Metamorphic assemblages in these rockstypically involve gedrite and ⁄ or anthophyllite as theorthoamphibole phase. Other common minerals, inaddition to cordierite and plagioclase, are spinel,corundum, garnet, orthopyroxene, Al2SiO5 polymor-

phs and sapphirine (Kelsey et al., 2005). More calcicrocks may also contain hornblende and anorthite.

Orthoamphibole-cordierite gneisses are volumetri-cally minor in orogens and have unusual bulk com-positions that do not correspond to �typical� igneous orsedimentary protoliths (e.g. Eskola, 1914; Chinner &Fox, 1974; Robinson et al., 1982). Rocks with theseusual bulk compositions may have been modified bysevere infiltration-metasomatism, partial melting orinteraction with melt, or polyphase histories (e.g.Tilley, 1937; Vallance, 1967; Schumacher, 1988; Pan &Fleet, 1995; Peck & Smith, 2005; Raith et al., 2008 and

*Present address:Department ofGeoscience,Universityof Wisconsin – Madison, Madison, WI 53706, USA

J. metamorphic Geol., 2011, 29, 425–449 doi:10.1111/j.1525-1314.2010.00926.x

� 2011 Blackwell Publishing Ltd 42 5

references therein). Nevertheless, there have been manystudies of these rocks owing to their importance fordetermining pressure (P)–temperature (T) conditionsand paths, and because these rocks commonly dis-play textures that provide clues about reactionhistory (Robinson & Jaffe, 1969; James et al., 1978;Robinson et al., 1982; Schumacher & Robinson 1987;Schneiderman & Tracy, 1991; Diener et al., 2008 andreferences therein). Although reaction textures such ascoronas and symplectites are disequilibrium features,they nonetheless can provide important informationabout metamorphic history, and therefore tectonicprocesses.

Sapphirine-bearing gedrite-cordierite gneiss in theOmineca belt occurs in the Thor-Odin dome (Duncan,1984; Norlander et al., 2002), the Passmore dome ofthe Valhalla Complex (Marshall & Simandl, 2006),and the Okanogan dome (Harvey, 1994; this study)(Fig. 1). In these domes, orthoamphibole-cordieritegneiss occurs in close association with quartzofelds-pathic migmatite as lenses and pods within, or directlyadjacent to, high-melt fraction migmatite. Thesedomes contain the highest-grade rocks of the interiorCordilleran crystalline belt (Fig. 1) and record thehighest pressures of the core complexes (e.g. Norlanderet al., 2002).

In the Okanogan dome, part of the Okanogan–Kettle metamorphic core complex of Washington,USA, orthoamphibole-cordierite gneiss is one of thefew lithologies with assemblages suitable for deter-mining metamorphic conditions. However, the deter-mination of metamorphic conditions in Mg–Al-richorthoamphibole-cordierite gneiss is often complicatedby uncertainties in assessing equilibrium conditionsowing to factors such as anomalous bulk composition,the presence of disequilibrium reaction textures andthe wide stability fields of high-variance assemblagesfound in these rocks.

In the Tunk Creek unit, however, the orthoamphi-bole-cordierite gneiss has a prominent layering thatpreserves distinct mineral assemblage and reactiontexture domains that contain either evidence of localtextural equilibrium among phases or abundant reac-tion textures (e.g. symplectites, coronas). Using phasediagrams calculated for specific bulk compositionallayers where there is evidence for textural equilibrium,and combining these data with the information on thereaction history obtained from adjacent layerscontaining reaction textures, we develop amethodologyfor quantifying the metamorphic conditions andpetrological evolution of these texturally complex,refractory rocks. Phase diagrams calculated for repre-sentative bulk compositions in the Tunk Creek unit arefurther used to discuss the P–T evolution of theOkanogan dome, and the implications of these data forgneiss dome tectonics. In the following sections, wedocument how careful analysis of the variations in bulkcomposition, reaction textures and pseudosectionanalysis of select compositional layers may aid future

tectonic studies of Mg–Al-rich orthoamphibole-cordierite gneiss by integrating a quantitative assess-ment of metamorphic conditions with the rich reactiontexture history that commonly characterize these rocks.

GEOLOGICAL SETTING AND FIELD RELATIONS

The Omineca metamorphic core complex belt of thenorthern North American Cordillera occurs betweenthe Mesozoic continental arc (Coast Mountains–NorthCascades) to the west and the foreland thrust belt(Rocky Mountains) to the east (Fig. 1a). Core com-plexes in this belt contain migmatite-cored domalstructures (Fig. 1b). Migmatite crystallization in thesedomes occurred during Palaeocene–Eocene time andwas coeval with basin development and major motionon detachment faults (Vanderhaeghe et al., 1999, 2003;Glombick et al., 2006; Hinchey et al., 2006; Gordonet al., 2008; Kruckenberg et al., 2008). Metamorphismand deformation occurred during Late Cretaceous toearly Tertiary time (Brown & Read, 1983; Tempelman-Kluit & Parkinson, 1986; Parrish et al., 1988; Carlson& Moye, 1990).In the Okanogan dome, orthoamphibole-cordierite-

bearing gneiss occurs in a series of NW-SE trending,km-scale layers exposed east of the Okanogan Riverwithin the Tunk Creek valley (Fig. 1c). The Mg–Al-rich gneiss is part of the Tunk Creek amphiboliteunit (Harvey, 1994), which is predominantly composedof alternating layers of amphibolite and chlorite schist.The Tunk Creek unit is exposed within quartzofelds-pathic gneiss of the footwall rocks below the Okano-gan Valley detachment fault, and structurally overlies alarge domain of high-melt fraction (diatexite) migma-tite (Kruckenberg et al., 2008).Structural fabrics (foliation and lineation) in the

western Okanogan dome are consistent in orientationfrom the structurally lowest migmatite domain upwardthrough the detachment (Kruckenberg et al., 2008),including the Tunk Creek unit. A shallowly dippingfoliation is folded around NW–SE oriented open toisoclinal fold hinges that parallel the regional stretch-ing lineation in the Okanogan detachment. The TunkCreek unit is compositionally heterogeneous andcharacterized by mm- to m-scale compositionaldomains aligned subparallel to foliation in both theTunk Creek unit and the surrounding gneissic layers(Figs 1c & 2). At the outcrop scale, the contact of theTunk Creek unit is concordant with layering in thehost gneiss and commonly forms pinch and swellstructures (Fig. 2a,b), suggesting that the structuralfabrics in the Tunk Creek unit and host gneiss formedduring the same deformation event.The similarities in the structural fabrics preserved in

the Tunk Creek unit and the surrounding gneiss sug-gest that the prominent layering in the Tunk Creekunit is either early-formed, or formed pre-peak meta-morphism. Significant changes in the bulk compositionof the Tunk Creek unit therefore likely developed early

4 26 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

in its history, presumably by similar processes to thosein other Mg–Al-rich lithologies (e.g. infiltration-metasomatism, melting). In this study, we focus ontextural evidence from representative layers in theTunk Creek unit, thereby providing information aboutthe P–T conditions that these rocks experienced duringthe evolution of the Okanogan dome.

PETROLOGY

Snook (1965) recognized the unusual mineral assem-blages recorded in amphibolite layers within the TunkCreek Valley and proposed that metamorphism in thewestern Okanogan dome reached granulite faciesconditions. Harvey (1994) proposed the name TunkCreek amphibolite, documented the occurrence ofsapphirine-bearing orthoamphibole-cordierite gneissand provided the first estimate of metamorphicconditions of �825 �C based on Mg–Fe exchangethermometry of sapphirine + spinel pairs (Owen &Greenough, 1991). Despite the efforts of previousresearch, a quantitative understanding of the meta-morphic evolution of this compositionally distinct unitis still lacking, particularly in light of recent resultsfrom the underlying migmatite domain indicating thatpartial melting and deformation occurred in Palaeo-cene–Eocene time and was therefore associated withthe Cordilleran orogeny (Kruckenberg et al., 2008).The Tunk Creek unit is highly variable in compositionbut largely comprises the quartz-absent assemblageof anorthite with either orthoamphibole (i.e. gedrite)or clinoamphibole (i.e. hornblende).

Calcic amphibole (hornblende-bearing) layers aremost abundant, but m-scale gedrite-rich domainsoccur as layers or pods surrounded by hornblende-dominated assemblages (Fig. 2c). At the outcrop scale,mm- to dm-scale layering is also observed (Fig. 2d),and more Al-rich bulk compositions containing gedrite(Table 1) commonly also contain sapphirine, spinel,corundum, cordierite, staurolite (relict and neoblastic),kyanite (relict) and biotite. Layers dominated byhornblende have a more uniform assemblage ofhornblende + plagioclase + spinel. The texturallyyoungest phase of the Tunk Creek unit is composed ofthulite (Mn-zoisite) which forms blebs or elongatepods in the plane of the foliation that disrupt theprimary layering in hornblende- and gedrite-dominated layers. Thulite is variably deformed and iscut by epidote veins oriented oblique to foliation andapproximately perpendicular to the direction ofextension in the structurally overlying Okanogandetachment (Fig. 2e).

In the section that follows, observations from thethree most representative layer types of the TunkCreek unit are described: gedrite-dominated, biotite-bearing and hornblende-dominated layers. Bulkcompositions for each of the three layer types weredetermined by ICP–MS analysis from representativesamples (Table 1). Compositional layering in many

cases is planar, facilitating the separation of distinctbulk compositional domains and thereby avoidingmixing.

As is typical for quartz-absent, Mg–Al-rich rocks,SiO2 content is low (�38–46 wt%), Al2O3 content ishigh (�25–37%) and bulk Mg# = 0.50–0.67 for allthree layer types (Table 1). These major elementabundances are similar to those observed in othergedrite–cordierite localities in the Omineca belt (e.g.Thor-Odin dome; Hinchey & Carr, 2007).

Gedrite-dominated layers

Gedrite-dominated layers are characterized by gedrite,plagioclase, cordierite, sapphirine, spinel, corundum,±kyanite, ±staurolite parageneses, with minor latechlorite observed in some thin sections. Compositionallayering is defined by alternating zones composedprimarily of gedrite and recrystallized polygonal pla-gioclase with layers that contain either: 1) elongatecoronal and symplectite reaction clusters aligned par-allel to foliation and the main compositional layering(Fig. 3a,c; hereafter referred to as gedrite domainscontaining reaction clusters); or 2) layers of intergrownsapphirine and spinel that lack coronal and symplect-itic reaction clusters (Fig. 3a,b; hereafter referred to asgedrite domains with sapphirine + spinel layers).

Gedrite domains containing reaction clusters consistof symplectitic intergrowths of sapphirine, corundum,plagioclase, ±spinel that are rimmed by a corona ofpolygonal plagioclase and are everywhere separatedfrom gedrite by a moat of cordierite (Figs 3c & 4). Inmost of the thin sections studied, these coronal reactiontextures contain sapphirine + corundum + anorthite(Fig. 4b,c). In spectacular examples where the originalphase (kyanite or staurolite) is still present, vermicularspinel occurs with sapphirine +corundum+ anorthite ±staurolite (neoblastic in the case of kyanite replace-ment) (Fig. 4b,d,e). Given textural evidence for dis-equilibrium, these layers were not used in thesubsequent pseudosection analysis for calculation ofmetamorphic conditions.

In contrast, gedrite domains with sapphirine +spinel layers are characterized by coarse grain inter-growths of sapphirine and spinel within polygonal,unzoned cordierite and plagioclase (Fig. 3b). A char-acteristic feature of these layers is the lack of reactionclusters and evidence of textural equilibrium micro-structures between phases. In rare cases, large euhedralgrains of corundum are also found in sapphirine +spinel layers (Fig. 4f). Layers of sapphirine + spinelin gedrite domains are utilized in the subsequentpseudosection analysis.

Biotite-bearing layers

Biotite-bearing layers of the Tunk Creek unit are dis-tinguished from gedrite-dominated layers by theoccurrence of biotite, the minor abundance of gedrite

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 42 7

� 2011 Blackwell Publishing Ltd

(a)

(c)

(b)

4 28 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

and abundant relict staurolite (Fig. 3d). Mineralswithin these layers include plagioclase, biotite, stau-rolite (relict), corundum, spinel and sapphirine ±kyanite ± gedrite. Plagioclase in biotite-bearing layersis zoned and tends to display more irregular grainboundaries than those observed in the gedrite-domi-nated layers. Abundant randomly oriented or partiallydismembered porphyroblasts of staurolite, and lesscommonly kyanite or corundum, characterize thereaction textures within the biotite and plagioclasematrix (Fig. 5a–d). Partial replacement of staurolite inbiotite-bearing layers involves either intergrowths ofplagioclase + corundum (Fig. 5a) or symplectiticintergrowths of sapphirine, spinel, and plagioclase(Fig. 5b,c). In contrast to the gedrite-dominateddomains, sapphirine and spinel are minor phases andoccur only in association with staurolite reactiontextures. Sapphirine grains within these reactionclusters are euhedral and typically larger than sapphi-rine in symplectites associated with gedrite-dominatedlayers. Kyanite in biotite-bearing layers is fine-grained(<0.25 mm) and lacks reaction coronas (Fig. 5d).Biotite-bearing layers were not used in the subsequentpseudosection analysis.

Hornblende-dominated layers

These layers are characterized by calcic amphibole +plagioclase ± spinel ± corundum ± chlorite (Figs 3e& 5e–h). A well-developed foliation is defined by thepreferred alignment of amphibole. Spinel porphyro-blasts are commonly elongated parallel to foliation andform aggregates (Fig. 5e). Spinel also occurs as rims oncorundum porphyroblasts (Fig. 5f). Corundum por-phyroblasts may contain synkinematic inclusion trailsthat are aligned parallel to the foliation (Fig. 5g).Coronal reaction textures are common and are char-acterized by successive coronas of corundum, spineland plagioclase (Fig. 5e–g). Chlorite is texturally late,typically forming in localized zones oriented slightlyoblique to the primary foliation defined by amphibole.Only hornblende-dominated layers lacking coronalreaction textures were used in the subsequent pseudo-section analysis.

MINERAL CHEMISTRY

Chemical compositions of coexisting minerals wereobtained with a JEOL 8900 electron microprobe in theDepartment of Geology and Geophysics at the Uni-versity of Minnesota. Quantitative analyses (WDS)were obtained with operating conditions of 20 nA

beam current, 15 kV accelerating voltage, a focusedbeam (�1 lm) for minerals such as sapphirine andspinel, and a beam defocused to 5 lm for mineralssuch as cordierite and gedrite.

Sapphirine

Sapphirine occurs in a variety of textural relationshipswithin the Tunk Creek unit. In gedrite domains con-taining reaction clusters, it occurs as slender bluegrains <100 lm long and in contact with corundum,spinel and anorthite as symplectitic intergrowthsresulting from the breakdown of kyanite (Figs 4 & 6).In gedrite domains with sapphirine + spinel layers,sapphirine also occurs as subhedral grains intergrownwith spinel aggregates in thin (mm-scale) layers thatlack symplectitic or coronal reaction clusters, andmore rarely as relatively large (>100 lm) isolatedsubhedral crystals in plagioclase-rich regions. Sapphi-rine composition varies as a function of texturaloccurrence in both gedrite domains. Sapphirine inreaction clusters is, in general, slightly more peralu-minous than those in layers with spinel (Fig. 7). Inbiotite-bearing layers, symplectitic sapphirine is asso-ciated with staurolite reaction textures (Fig. 5b,c). Inall textural occurrences, sapphirine is dominantlyunzoned and Mg-rich [Mg ⁄ (Mg + Fetot) = 0.79–0.84;Mg ⁄ (Mg + Fe2+) = 0.80–0.87] and shows minorvariation in composition depending on textural setting(Figs 6 & 7; Table 2).

Spinel

Spinel occurs in all three of the main types of com-positional layers of the Tunk Creek unit and innumerous textural settings. In gedrite-dominated lay-ers, spinel occurs as vermicular intergrowths withsapphirine and corundum in kyanite symplectites, asaggregates intergrown with sapphirine in spinel +sapphirine-rich layers, and as large (>1 mm) clustersin the matrix foliation (Figs 3, 4 & 6). In biotite-bearing layers, spinel is less abundant and is restrictedto symplectitic intergrowths with sapphirine and pla-gioclase after staurolite (Fig. 5b,c). In hornblende-dominated layers, large clusters of spinel occurthroughout the amphibolite matrix and are similarto the rare clusters of spinel observed in gedrite-dominated layers. Unique to the hornblende-dominated layers, spinel occurs as coronas aroundlarge (�1 mm) corundum porphyroblasts. Spinel is theleast magnesian of the Mg-bearing reaction phasesand shows minor compositional variation between

Fig. 1. (a) Regional geological map showing the position of the Omineca crystalline belt and Shuswap metamorphic complex. (b)Simplified geological map of the Shuswap metamorphic complex (modified from Kruckenberg et al., 2008) showing the location ofmigmatite-cored gneiss domes and associated localities of orthoamphibole-cordierite gneisses. In the Okanogan dome, orthoamphi-bole-cordierite gneiss is closely associated with the migmatite domain (dark region to N-NE) on the western margin of the dome (inset,shown at lower left). (c) Geological map of a portion of the western Okanogan dome in the Tunk Creek Valley (modified after Harvey,1994). Locations of samples are indicated by stars.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 42 9

� 2011 Blackwell Publishing Ltd

symplectitic intergrowths after kyanite and staurolite(XMg = 0.39–0.46) and layers with sapphirine or largematrix spinel aggregates (XMg = 0.48–0.57) (Figs 6 &8; Table 3).

Corundum

Corundum occurs in all three of the compositionaldomains. In gedrite-dominated layers, corundum is

Host Gneissic

Foliation

Contact

TCU

0.5 m

0.5 m

Hbl-dominated Ged-dominated

Hbl-dominated

Thulite

Gedrite-dominated layersHornblende-dominated layers

(may include chlorite schist)Biotite-bearing

layersThulite lenses

Gne

iss

Tunk

cre

ek u

nit

(a)

(b)

(d) (e)

(c)

Fig. 2. (a) Illustration synthesizing the representative field relationships of the Tunk Creek unit (TCU). (b) Outcrop photograph(view to the NW) from a quarry on the north side of Tunk Creek Valley showing the upper contact of the TCU with quartzofeldspathicbiotite gneiss and concordant structural fabrics between the TCU and adjacent gneiss. (c) Field relationships between hornblende- andgedrite-dominated domains. Gedrite-dominated domains occur as continuous layers (light portion bottom third of photo) and asboudinaged segments wrapped by hornblende-dominated domains. (d) Outcrop photograph showing the compositional heterogeneityof the TCU on the mm- to cm-scale with interlayered hornblende-dominated (dark green layers), gedrite-dominated (brown-tanlayers), and biotite-bearing layers. (e) Outcrop photograph showing m-scale thick thulite veins and pods. Mineral abbreviations usedthroughout are from Whitney & Evans (2010).

4 30 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

typically restricted to kyanite reaction textures and isfine-grained (<100 lm) and clear in plane-polarizedlight (Fig. 4). A less common textural occurrence ofcorundum as large, blue-purple euhedral crystalsoccurs in gedrite domains with sapphirine + spinellayers (Fig. 4f). In biotite-bearing layers, corundumoccurs both as large (>2.5 mm) porphyroblasts asso-ciated with plagioclase-rich domains and as fine-grained vermicular intergrowths in reaction texturesaround staurolite (Figs 5a & 8d,e). In hornblende-dominated layers, corundum occurs only as large(�1 mm) crystals, commonly containing slightlywarped inclusion trails parallel to the primarycompositional foliation (Fig. 5g). In most texturaloccurrences within hornblende-dominated layers,corundum is surrounded by a thick corona of spinelthat is itself surrounded by anorthite (Fig. 5f,h).

Staurolite

Staurolite is present in gedrite-dominated and biotite-bearing layers. In both domains, staurolite is texturallyearly and partially replaced by either symplectiticintergrowths of sapphirine and spinel (gedrite-richlayers: Fig. 4d,e; biotite-bearing layers: Fig. 5b,c) orcorundum + plagioclase intergrowths (biotite-bearinglayers: Figs 5a & 8c). Neoblastic staurolite in gedrite-dominated layers is also observed in association withthe breakdown of kyanite and has a composition ofXMg � 0.38–0.43 (Fig. 6a; Table 4), similar to stauro-lite in other Mg-rich silica-deficient rocks (e.g. Visser &Senior, 1990; Shimpo et al., 2006). In biotite-bearinglayers, staurolite has a more variable composition,with XMg values ranging from 0.31 to 0.38 (Table 4)for staurolite partially replaced by sapphirine andspinel symplectites (Fig. 8a,b) and staurolite por-phyroblasts replaced by corundum and plagioclaserespectively (Fig. 8c). Matrix porphyroblasts of stau-

rolite lacking reaction clusters are the least magnesian(XMg � 0.32; Table 4). The concentration of zinc instaurolite is low (<0.2 wt% ZnO), as is chromium,with the exception of those associated with sapphirineand spinel symplectite in the biotite-bearing layers;these have up to 0.5 weight per cent Cr2O3.

Plagioclase

Plagioclase is abundant in the Tunk Creek rocks andcommonly displays a polygonal mosaic texture,particularly in plagioclase-rich regions of gedrite-dominated layers. Plagioclase shows systematicvariations in chemistry depending on its occurrence. Ingedrite-dominated layers, plagioclase has nearly uni-form composition (�An97) in all textural occurrences,from vermicular intergrowths with sapphirine–corun-dum–spinel, in coronas surrounding reaction clusters,or as a matrix phase (Table 5). In biotite-bearing layers,plagioclase in reaction clusters after staurolite has ananorthite-rich composition that is similar to thatobserved in gedrite-dominated domains (An94–87;Table 5). X-ray composition maps of plagioclase fromreaction clusters and matrix grains in biotite-bearinglayers also show zoning. The cores of reaction clustersare anorthite-rich and grade outward toward moresodic compositions (Fig. 8). Matrix plagioclase, incontrast, is reversely zoned and ranges from An47 in thecore to An72 at the rim of large matrix grains (Fig. 8;Table 5). As in the gedrite-dominated layers, plagio-clase in hornblende-dominated lithologies is unzonedand nearly pure anorthite (An99; Table 5).

Cordierite

Cordierite occurs in gedrite-dominated and biotite-bearing layers of the Tunk Creek unit and has adifferent composition in each domain (Table 6). In

Table 1. ICP–MS analyses of representative bulk compositional layers.

Textural

characteristics

Gedrite-dominated layers Biotite-bearing

layers with

staurolite reaction

clusters

Hornblende-dominated

layers with spinel

reaction clustersWell-distributed

reaction clusters

in gedrite

Spr–Spl

dominated

layers

Reaction

clusters in

gedrite adjacent

to Spr–Spl layers

Spr–Spl

dominated

layers

Sample OK60A OK60DA-1 OK60DA-2 OK60E OK60C OK65

SiO2 45.53 37.90 44.52 44.00 46.64 38.90

TiO2 0.04 0.04 0.14 0.03 0.42 0.01

A12O3 24.59 36.98 27.06 31.35 35.44 22.97

Fe2O3 6.54 5.92 5.34 5.67 2.80 6.33

MnO 0.10 0.08 0.08 0.08 0.02 0.08

MgO 11.24 9.57 8.47 9.58 2.79 12.78

CaO 7.88 8.82 10.68 7.69 6.91 11.24

Na2O 0.41 0.41 0.55 0.43 5.43 0.24

K2O 0.17 0.13 0.09 0.02 0.52 0.20

P2O5 0.00 0.02 0.04 0.02 0.33 0.02

LOI 1.26 1.74 0.97 1.27 0.80 3.94

Total 97.76 101.61 97.94 100.14 102.1 96.71

Sr (ppm) 558 899 1371 781 1060 479

Ba (ppm) 145 147 83 55 409 5

Bulk XMg 0.63 0.62 0.61 0.63 0.50 0.67

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 43 1

� 2011 Blackwell Publishing Ltd

gedrite-dominated domains, cordierite occurs as moatsseparating gedrite from reaction clusters after kyaniteand staurolite (Figs 4 & 6a–c), or is found within andadjacent to layers of sapphirine and spinel inter-growths (Fig. 6e). Cordierite is also found as a matrixphase partially replacing and intergrown with gedrite(Fig. 4a). Cordierite in gedrite-dominated domains isvery Mg-rich (XMg � 0.90; Table 6) and unzoned. Inbiotite-bearing layers, cordierite occurs adjacent tocorundum porphyroblasts or reaction clusters formed

from the breakdown of staurolite (Fig. 8c), and as amatrix phase. Cordierite in biotite-bearing layers is lessMg-rich than in gedrite-dominated domains (XMg �0.83; Table 6).

Gedrite

Gedrite has a distinct composition in each of the threebulk compositional domains (Fig. 9; Table 7). Ingedrite-dominated layers, gedrite is Mg- and Al-rich

SprSpl

(c)

Spr+Crn+Spl

Ged

Crd

CrdPl1 mm

(b)

(d) (e)

1 mm

1 mm1 mm

Pl

Bt

CrnStCrn

HblPl

Pl

Ged

(a)

Ged

rite-

dom

inat

edBi

otite

-be

arin

gH

ornb

lend

e-do

min

ated

Spr +

Spl

laye

rsRe

actio

n cl

uste

r dom

ains

Spr+Crn+Spl+Pl

Ged

Hbl

Spl

BtCrn+Pl

Crn

Spr

Spl

StSt

Pl+Crd

Pl

Ky

Crn+Pl

Pseu

dose

ctio

ns

OK6

0DA

-1 &

OK6

0EPs

eudo

sect

ion

OK6

5Re

actio

n cl

uste

r dom

ains

and

bio

tite-

bear

ing

laye

rs n

ot a

naly

zed

in p

seud

osec

tion

anal

ysis

0.5 mm

Gedrite-dominated: sapphirine-spinel layers

Gedrite-dominated: reaction cluster domains

Biotite-bearing Hornblende-dominated

Fig. 3. (a) Synoptic figure illustrating bulk compositional domains and textural relationships in the Tunk Creek unit. Cm-scalebulk compositional variations are shown for correlation with the thin section and textural associations analysed. Note that bulkcompositional layering is also observed at the m-scale in the Tunk Creek unit. Column at right of figure denotes samples and layersselected for pseudosection analysis. Photomicrographs illustrating representative compositional domains of the Tunk Creek unit: (b)Gedrite-dominated domains with sapphirine + spinel layers [OK60E1]. (c) Gedrite-dominated domains containing reaction clusters[OK60A1]. (d) Representative textures in biotite-bearing domains showing partial reaction of staurolite to symplectite containingcorundum + plagioclase [OK60C2]. (e) Characteristic assemblages in hornblende-dominated layers containing corundum (or spinel)porphyroblasts, commonly rimmed by successive coronas of spinel and plagioclase [OK60A].

4 32 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

(e)

Ky

Spr+Crn Pl

St

0.25 mm

(f)

Spl

Spr

Pl

Crn 0.25 mm

(c)

Pl

Pl

Crd

Crn

Ged

Spr

0.25 mm

(d)

Ky

StSpr

Crd

Spl

Crn

Spr

PlPl

Pl0.25 mm

(b)

Ged

Crd

Spr+Crn

Spr+Crn

Crd

Pl

Crd

Pl

0.25 mm

(a)

St

KySpr+Crn+Spl

Ged

Crd

CrdPl

1 mm

Crd Ky

Fig. 4. Plane-polarized light photomicrographs of reaction textures in gedrite-dominated layers. (a,b) Representative photomicro-graphs showing typical pseudomorph reaction textures. All samples in this domain involve sapphirine + corundum intergrowths withcoronas of anorthite separated from gedrite by a moat of cordierite. In Fig. 4b, relict kyanite is surrounded by gedrite, partiallypseudomorphed by sapphirine + corundum, and rimmed by anorthite and cordierite. The upper left half of (b) shows a similarreaction texture to those in (a,c), in which sapphirine + corundum + anorthite intergrowths have completely pseudomorphed kyaniteor staurolite [OK60A1, 2]. (c) Sapphirine + corundum + anorthite intergrowths after kyanite or staurolite [OK60DA]. (d,e) Rarerelict kyanite with spectacular symplectite development involving sapphirine + spinel + corundum + staurolite (neoblastic)[OK60A1]. (f) Photomicrograph showing large euhedral corundum adjacent to thin layer of sapphirine + spinel intergrowths withinthe gedrite domain [OK60E1].

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 43 3

� 2011 Blackwell Publishing Ltd

(b)

0.5 mm

(a)

(c)

0.5 mm

St

Pl

Spr

Crn+Pl

Pl

GedBt

Spl

Spl

Spr

St

Pl

StSpr

0.5 mm

1 mm

Ky

St

Pl

1 mm

1 mm 1 mm

(e) (f)

(g) (h)

1 mm

(d)

Spl

HblPl

Spl

Crn

Spl

Crn

Hbl

Pl

Crn

Hbl

Pl

Crd

Pl

St

Fig. 5. Photomicrographs of reaction textures in biotite-bearing (a–d) and hornblende-dominated (e–h) layers. (a) Partial replacementof staurolite by vermicular intergrowths of plagioclase and corundum ± cordierite. Plagioclase in reaction texture is more anorthite-rich than surrounding matrix plagioclase [OK60C2]. (b) Symplectite of sapphirine + spinel + plagioclase formed by the partialreplacement of staurolite. Large euhedral sapphirine developed in these textures, in contrast with sapphirine in other pseudomorphs[OK60C2]. (c) Reaction clusters involving sapphirine and spinel after staurolite in biotite-bearing layers [OK60C2]. (d) Dismemberedstaurolite and rare kyanite in biotite-bearing layers [OK60C3]. (e) Large aggregates of spinel rimmed by anorthite in hornblende-dominated layers. (f–h) Corundum porphyroblasts in hornblende-dominated layers, rimmed by successive shells of spinel andplagioclase (f, h) and well-developed syn-deformational inclusion trails (g) [OK60A, OK63B2, OK65].

4 34 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

(XMg = 0.75; Al2O3 � 18.5 wt%) for large matrixgrains and those adjacent to reaction clusters (Fig. 6-a,b; Table 7). Na varies slightly as a function of Alcontent, from �0.47 Na PFU near sapphirine–spinellayers to 0.36–0.50 Na PFU (23 oxygen basis) for largematrix grains (Fig. 9; Table 7). Gedrite in biotite-bearing layers and hornblende-dominated layers areslightly more aluminous (Al2O3 � 22 wt%) and havecompositions ranging from �0.6–0.7 Na PFU (23oxygen basis; Fig. 9; Table 7). Gedrite in biotite-bearing domains is the least magnesian compositionobserved (XMg � 0.68) (Fig. 9; Table 7).

Biotite

Biotite is a volumetrically minor phase (<10–20 mode%) in the Tunk Creek unit and occurs primarily assmall matrix grains along the margins of plagioclase inbiotite-bearing layers (Figs 3d, 5c & 8e), or rarely asthin alteration rims on some gedrite crystals associatedwith late chlorite. Biotite is Mg-rich and shows onlyminor variations in composition between matrix grains(XMg � 0.70) and grains adjacent to reaction clusters(XMg � 0.80; Table 8).

Hornblende

Hornblende does not typically coexist with gedrite inthe Tunk Creek rocks but is abundant in gedrite-freelayers, although there are exceptions (e.g. sample 60b,which contains intergrown gedrite and hornblende).Similar to gedrite, the orientation of hornblende lathsdefines the major foliation and compositional layeringin the Tunk Creek unit with alternating layers ofanorthite-rich plagioclase. Hornblende varies in com-position from pargasite to magnesio-hornblende(XMg = 0.78–0.83; Table 9).

Chlorite

Chlorite primarily occurs in late zones of alteration inthe calcic amphibole layers, but also occurs rarely asthin rims on some gedrite and biotite crystals in theother compositional domains. Chlorite is Mg-rich(XMg = 0.86) and aluminous (5.40 total Al PFU, 28oxygen basis (Table 9).

INTERPRETATION OF REACTION TEXTURES

The heterogeneity of the Tunk Creek unit, as definedby compositional layering on the mm- to m-scale,demonstrates the effect of bulk composition on thepreservation of reaction textures in different domainsthat experienced the same P–T history. The heteroge-neous mineral assemblages and textural developmentin the distinct bulk compositional domains of the TunkCreek unit, if interpreted with care, can be useful forinterpreting the reaction history. In particular, mineralassemblages and reaction histories in Al-rich, Si-poor

(sapphirine-bearing) rocks are very sensitive to bulkcomposition and may develop by a sequence of reac-tions (e.g. Kelsey et al., 2005), evidence for which maybe preserved in reaction textures. Interpretation ofthese textures in the context of metamorphic processesand conditions must also consider chemical potentialgradients and the possibility of open system behaviour(e.g. Tajcmanova et al., 2007; White et al., 2008).

Textural associations among Al-rich mineralstherefore provide information about the reaction his-tory and petrological evolution of the Tunk Creekrocks. In gedrite-dominated layers, symplectitic inter-growths of sapphirine + corundum + anorthite ±spinel are mantled by a thick corona of anorthite andseparated from gedrite by a moat of cordierite. Thesereaction textures formed on kyanite and in a fewinstances staurolite (Figs 4 & 5), which were likely inequilibrium with gedrite prior to corona formation.Relict kyanite and staurolite are rare in gedrite-domi-nated layers, indicating that the symplectite- and cor-ona-forming reactions have gone to completion inmost cases. In biotite-bearing layers, staurolite por-phyroblasts surrounded by sapphirine + spinel +plagioclase symplectitic intergrowths (Figs 5b,c &8a,b) or corundum + plagioclase intergrowths (Figs5a & 8c) are also abundant, along with kyanite lackingreaction coronas. The presence of partially reactedlaths of gedrite preserved adjacent to symplectitereaction clusters (Fig. 5c) provides further texturalsupport for the interpretation that kyanite andstaurolite coexisted with gedrite during an earlier (pre-reaction texture) stage of the high-grade metamor-phism of the Tunk Creek rocks.

Symplectitic intergrowths of sapphirine + corun-dum + plagioclase ± spinel that are successivelyrimmed by coronas of anorthite and cordierite repre-sent disequilibrium reaction textures developedbetween kyanite or staurolite and matrix phases (i.e.gedrite). Similarly, the partial replacement of kyaniteby a texturally younger generation of neoblastic stau-rolite (Fig. 6a) in reaction clusters may reflect incom-plete reaction during decompression and ⁄ or heating.In biotite-bearing domains, the decrease in the anor-thite content of plagioclase away from corundum-bearing reaction clusters (An85 – An50; Fig. 8d,f) mayprovide further evidence of the role of chemicalpotential gradients in controlling the development ofvermicular intergrowths within disequilibrium reactionclusters.

In gedrite-dominated layers, the preservation ofdistinct mm-scale layers of intergrown sapphirine +spinel may also provide more information about theP–T evolution of the Tunk Creek rocks. These layersare characterized by unaltered grain boundariesbetween euhedral to subhedral sapphirine and spinel;these boundaries are interpreted to reflect local texturalequilibrium (Fig. 6d,e). The formation of sapphirine+ spinel layers in gedrite-dominated domains suggestthat these layers represent bulk compositions that are

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 43 5

� 2011 Blackwell Publishing Ltd

distinct from those containing reaction clusters.Alternatively, as suggested by Raith et al. (1997),sapphirine + spinel layers may form through melt-enhanced recrystallization and grain-coarsening oforiginally fine-grained symplectites, although in thecase of the Tunk Creek rocks, there is no obviousevidence for such a process.

The interpretation of reaction textures in the horn-blende-dominated layers is limited by the high varianceassemblage hornblende + plagioclase + spinel ±corundum. Corundum (commonly with synkinematicinclusion trails; Fig. 5g) is characteristically rimmed byspinel, suggesting that corundum was an early-formedphase. In thin sections lacking corundum, aggregates

100 µm

250 µm

100 µm

200 µm

200 µm100 µm

(a) (b)

(c)

(d)

(e)Spl

XMg = 0.55

XMg = 0.53

XMg = 0.54

XMg = 0.54

0.52

0.54

XMg = 0.530.

53

XMg = 0.83

0.82

0.83

0.82

0.82

XMg = 0.81 XMg = 0.83

Spl

Spr

Spr

SplSpl

Spl

Spr

St (XMg = 0.43)

Ky

Ky

St

St (XMg = 0.38)

Spr (XMg = 0.79)

Spl (XMg = 0.43)

Crn

Crn

Crd (XMg = 0.89)

Crd (X Mg = 0.90)

Spr (XMg = 0.82)

Pl (XAn = 0.97)

Pl (X An = 0.97)

Pl

Ged (XMg = 0.73)

Ky

Crd (XMg = 0.92)

Pl (XAn = 0.95)

Crn

Spl (XMg = 0.44)

Ged (XMg = 0.75)

Spl (XMg = 0.48)

Spr (XMg = 0.83)

Crn

Pl (XAn= 0.99)

Crd

CrdXMg = 0.88

Fig. 6. Backscattered-electron (BSE) images annotated with mineral composition data in gedrite-dominated layers. (a) Sapphirine +spinel + corundum + staurolite symplectite after kyanite. Staurolite has partially replaced kyanite and also occurs as thin neoblasticcrystals within the corona texture. Polygonal plagioclase and moats of cordierite separate reaction clusters from gedrite [OK60A1]. (b)Relict kyanite in gedrite, rimmed by plagioclase and cordierite and partially replaced by corundum + spinel [OK60A1]. (c) Repre-sentative BSE image of symplectitic intergrowths of sapphirine + corundum + plagioclase in reaction clusters lacking relict phases(e.g. kyanite) [OK60DA]. (d,e) Sapphirine + spinel intergrowths within mm-scale sapphirine + spinel layers in gedrite-dominateddomains. Unaltered grain boundaries along intergrowths of euhedral to subhedral sapphirine + spinel suggest local textural equi-librium. Analytical traverses show homogeneity of composition. These sapphirine + spinel intergrowths are characteristics of thedomains used during pseudosection analysis [OK60E1].

4 36 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

of spinel are elongated parallel to the main foliationdefined by hornblende and are therefore interpreted tohave formed at least in part synkinematically. In rare

instances, hornblende occurs as asymmetric or spi-ralled porphyroblasts that indicate the synkinematicgrowth.

PHASE EQUILIBRIA AND PRESSURE–TEMPERATURE CONDITIONS

The compositional layering of the Tunk Creek unitand its reaction history as preserved in distinct bulkcompositional domains provides an opportunity togain additional information about the P–T evolutionusing phase diagrams calculated for specific bulkcompositions in representative layers. Given theproximity of the layers, the P–T results should besimilar for each one, but it is worth calculating a phasediagram for each layer in order to assess assumptionsabout bulk composition, model systems, and phaseequilibrium analysis, and to evaluate the variability ofthe calculated P–T conditions.

Pseudosections

Pseudosections were determined for three representa-tive gedrite- and hornblende-dominated layers ofthe Tunk Creek unit that are free of symplectite orreaction coronas (i.e. obvious disequilibrium tex-tures). Two samples are from gedrite-dominateddomains containing sapphirine–spinel layers (samplesOK60DA-1, OK60E; Table 1) and one sample of ahornblende-dominated layer with the high varianceassemblage hornblende + plagioclase + spinel(OK65; Table 1). Textural evidence in each of these

Sapphirine in layers associated

with spinel

Sapphirine within pseudomorphs

after kyanite and/or staurolite

5.0

4.5

3:5:1

7:9:3

2.0

2:2:1

Si per 20(O)

(Al+

Cr)/

2 pe

r 20(

O)

4.01.0 1.5

M:A:S

OK60DA-2

Fig. 7. Plot of Si v. (Al + Cr) ⁄ 2 per 20 oxygen for sapphirine insample OK60DA-2. Figure shows the compositional variation ofsapphirine in distinct textural occurrences. Sapphirine inpseudomorphs after kyanite and ⁄ or staurolite (diamonds) aremore aluminous than sapphirine in layers associated with spinel(squares). The latter are close to the 7:9:3 end-member ofsapphirine.

Table 2. Representative microprobe analyses of sapphirine.

Domain Gedrite-dominated layers Biotite-bearing layers

Texture Symp. on kyanite Reaction clusters after kyanite Spr + Spl rich layers Symp. on staurolite

Euhedral blue

Spr in An

Locality 60A1 60A1 60A2 60DA 60E1 60DA 60E1 60C2 60C2

Inner symp. Outer symp. Large euh. Btwn. Sp Inner symp. Outer symp. Rim Core

SiO2 10.61 10.19 10.30 9.65 10.83 11.61 12.22 12.14 11.05 10.11 10.63 11.31 10.95

TiO2 <d.l. <d.l. <d.l. <d.l. <d.l. 0.03 0.00 <d.l. 0.06 <d.l. 0.02 0.02 <d.l.

A12O3 67.84 67.73 67.60 68.29 67.62 65.17 65.42 65.07 65.57 67.08 65.92 66.41 66.51

Cr2O3 0.02 <d.l. 0.14 0.03 0.02 <d.l. 0.02 <d.l. <d.l. <d.l. 0.02 0.03 0.01

FeOa 6.63 6.08 5.64 6.61 5.50 6.27 6.29 5.94 7.37 7.58 7.31 6.94 6.93

MnO 0.06 0.03 0.04 0.04 0.04 0.01 0.04 0.05 0.06 0.04 0.07 0.08 0.04

MgO 14.42 15.22 15.73 14.74 16.19 16.00 16.33 16.68 15.16 14.96 15.28 15.20 15.45

ZnO <d.l. 0.10 <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. 0.06

CaO 0.45 0.12 0.12 0.13 0.07 0.07 0.01 0.01 0.07 0.07 0.11 0.02 <d.l.

Total 100.02 99.46 99.57 99.49 100.25 99.15 100.31 99.88 99.34 99.84 99.37 100.00 99.94

Structural formulae based on 14 cations

Si 1.26 1.21 1.22 1.15 1.27 1.38 1.44 1.43 1.32 1.20 1.27 1.34 1.30

AlIV 4.74 4.79 4.78 4.85 4.73 4.62 4.56 4.57 4.68 4.80 4.73 4.66 4.70

AlVI 4.74 4.68 4.64 4.71 4.62 4.53 4.51 4.48 4.55 4.59 4.53 4.62 4.58

Fe3+ 0.11 0.12 0.14 0.11 0.09 0.05 0.10 0.12 0.22 0.20 0.04 0.12

Fe2+ 0.65 0.49 0.43 0.52 0.43 0.54 0.57 0.49 0.61 0.54 0.53 0.65 0.56

Mg 2.55 2.69 2.77 2.61 2.83 2.84 2.86 2.93 2.70 2.65 2.72 2.69 2.73

Ca 0.06 0.01 0.01 0.02 0.01 0.01 0.00 0.01 0.01 0.01

Mg ⁄ (Mg + Fe2+) 0.80 0.85 0.86 0.84 0.87 0.84 0.83 0.86 0.82 0.83 0.84 0.81 0.83

Mg ⁄ (Mg + Fetot) 0.79 0.82 0.83 0.80 0.84 0.82 0.82 0.83 0.79 0.78 0.79 0.80 0.80

Btwn., between; d.l., detection limits; euh., euhedral; symp., symplectite.aTotal iron reported as FeO.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 43 7

� 2011 Blackwell Publishing Ltd

samples suggests that the minerals of interest likelygrew together (e.g. sapphirine + spinel) and the min-erals are unzoned, suggesting that the bulk composi-tions measured by ICP–MS are representative. Animportant assumption in the calculation of thepseudosections is that the bulk compositions obtainedby ICP–MS are representative of the equilibrium vol-ume of interest for the representative domains.Therefore, distinct mm- to cm-scale bulk composi-tional layers were sawed out of thin section billets tominimize the mixing of chemical subdomains and torestrict the bulk compositional analysis (by ICP–MS)to regions characterized by the assemblages of interest.

Pseudosections were calculated using the PERPLE_XPERPLE_X

software package (Connolly, 1990, 2005, April 2010version) using the updated version of the Holland &

Powell�s (1998) data set as described by Kelsey et al.(2004) for the calculation of high temperatureequilibria with sapphirine. The chemical systemNa2O–CaO–FeO–MgO–Al2O3–SiO2–H2O (NCFMASH)represents all major phase relations observed in theTunk Creek unit, with the exception of biotite. Toinvestigate the effect of K2O on the calculated topo-logies observed in the NCFMASH system, additionalcalculations were carried out in the NCKFMASHsystem for each of the three samples.The activity-solution models considered in the

pseudosection calculations for all three domains ofinterest are clino- and orthoamphibole (Diener et al.,2007), sapphirine (Kelsey et al., 2004), orthopyroxene(Powell & Holland, 1999), clinopyroxene (Green et al.,2007), garnet (White et al., 2007), spinel, staurolite,

Table 3. Representative microprobe analyses of spinel.

Domain Gedrite-dominated layers

Texture Symp. on kyanite Reaction clusters after

kyanite

Spr + Spl-rich layers Matrix aggregates

Locality 60A1 60A2 60DA 60DA 60E1 60E1 60DA 60DA

Vermicular Rare, vermicular

SiO2 <d.l. 0.37 0.04 0.01 0.01 <d.l. <d.l. <d.l. <d.l.

TiO2 0.04 <d.l. 0.02 0.03 0.00 <d.l. <d.l. <d.l. 0.04

A12O3 62.96 64.11 62.35 63.60 63.76 63.50 62.83 64.46 63.25

Cr2O3 0.02 0.03 <d.l. 0.06 0.00 <d.l. 0.01 <d.l. 0.07

Fe2O3

FeOa 25.79 23.90 24.06 22.23 21.92 22.73 23.92 20.09 22.30

MnO 0.08 0.11 0.08 0.16 0.12 0.10 0.12 0.09 0.11

MgO 11.04 11.44 11.75 12.96 13.24 13.74 13.24 14.73 13.02

ZnO 0.34 0.57 0.94 0.21 0.22 0.26 0.20 0.07 0.12

Total 100.27 100.52 99.23 99.26 99.26 100.32 100.32 99.44 98.91

Structural formulae based on 3 cations

Al 1.98 2.00 1.97 1.99 1.99 1.96 1.95 1.98 1.98

Fe3+ 0.02 0.00 0.03 0.01 0.01 0.04 0.05 0.02 0.02

Fe2+ 0.55 0.53 0.51 0.48 0.47 0.46 0.47 0.42 0.48

Mg 0.44 0.45 0.47 0.51 0.52 0.54 0.52 0.57 0.52

Mg ⁄ (Mg + Fetot) 0.43 0.46 0.47 0.51 0.52 0.52 0.50 0.57 0.51

Domain Biotite-bearing layers Hornblende-dominated layers

Texture Symplectite on staurolite Matrix Coronas around corundum

Locality 60C2 63B1 63B2 65 60B 63B2

Adjacent to euhedral Spr Vermicular in symp.

SiO2 <d.l. 0.09 <d.l. <d.l. <d.l. 0.01 <d.l.

TiO2 <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l.

A12O3 61.37 62.38 61.88 62.55 64.51 63.82 64.73

Cr2O3 <d.l. 0.01 0.13 0.08 <d.l. 0.05 0.00

Fe2O3

FeOa 28.07 25.24 25.20 22.98 22.38 22.04 18.07

MnO 0.16 0.12 0.44 0.44 0.25 0.12 0.41

MgO 10.27 11.74 12.90 14.17 13.38 13.76 16.54

ZnO 0.26 0.31 0.06 0.10 0.25 0.47 <d.l.

Total 100.12 99.90 100.60 100.31 100.77 100.27 99.75

Structural formulae based on 3 cations

Al 1.95 1.96 1.92 1.93 1.98 1.97 1.97

Fe3+ 0.05 0.04 0.07 0.07 0.02 0.03 0.03

Fe2+ 0.58 0.53 0.48 0.43 0.47 0.45 0.36

Mg 0.41 0.47 0.51 0.55 0.52 0.54 0.64

Mg ⁄ (Mg + Fetot) 0.39 0.45 0.48 0.52 0.52 0.53 0.62

d.l., detection limits; symp., symplectite.aTotal iron reported as FeO.

4 38 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

cordierite, epidote (Holland & Powell, 1998), chlorite(Holland et al., 1998), biotite (Tinkham et al., 2001;Tajcmanova et al., 2009), white mica (Coggon &Holland, 2002; Auzanneau et al., 2010) and feldspar(Fuhrman & Lindsley, 1988). No phases were excludedfrom consideration within the NCFMASH or NCKF-MASH systems and calculations were carried out withH2O in excess with aH2O = 1. Mn and Ti were notmodelled because of their low abundance (Table 1) andthe absence of phases in the Tunk Creek unit that mayconcentrate these components (e.g. garnet). In thepseudosection analysis, all Fe is calculated as FeO, andas such the pseudosections are not able to model theeffects of ferric iron (Fe3+) on the calculated topologies.Ferric iron may have a significant effect on the calcu-lation of phase equilibria as has been demonstrated bymany researchers (e.g.White et al., 2000, 2002; Johnson& Oliver, 2002; Diener et al., 2007, 2008; Green et al.,2007). Our results therefore need to be carefully con-sidered in this context as a possible source of error,along with the fact that activity-composition modelsthat incorporate the effect of Fe3+ are largely lacking inFe-bearing phases such as sapphirine, spinel and stau-rolite. However, indications frommineral compositions(e.g. Tables 2 & 3) suggest that Fe3+ is not present insignificant amounts in the samples analysed.

The NCFMASH system is sufficient to account forthe reactions observed in the Tunk Creek unit because:

(i) K2O is extremely low in the samples analysed (<0.2weight per cent for samples OK60DA-1, 60E, 65;Table 1); (ii) the addition of K2O in pseudosectionmodelling did not modify the position of the majorphase equilibria and the primary effect was to stabilizebiotite, or muscovite at lower-T, everywhere, but withextremely low mode (<1 vol.%); and (iii) biotite is theonly K-bearing phase in the Tunk Creek unit, and itsoccurrence is restricted to biotite-bearing layers thatwere not analysed in the pseudosection calculations.Therefore, discussion is restricted to pseudosectionmodelling results using the NCFMASH system.

Gedrite-dominated domain with sapphirine–spinel layers

Pseudosections were calculated for two samples,OK60DA-1 (Fig. 10a) and OK60E (Fig. 10b), whichwere selected because they lack disequilibrium reactiontextures and are characterized by euhedral to subhe-dral intergrowths of sapphirine and spinel, interpretedto have grown together and to represent local texturalequilibrium (Figs 3a,b & 6d,e). Furthermore, com-parison of these samples allows for a check of theconsistency of results on the outcrop scale for sampleswith different bulk composition (Table 1). Calculatedpseudosections for both samples yield similar topolo-gies, and the positions of major equilibria are similarin both samples (Fig. 10). The locations of the

Table 4. Representative microprobe analyses of staurolite.

Domain Gedrite-dominated

layers

Biotite-bearing layers

Texture Symplectite on kyanite With Spr + Spl

symplectite

With Crn + An

symplectite

Matrix porphyroblasts

Locality 60A1 60C2 60C2 60C3 60C1 60C2 60C3

Small Large Rim Core

SiO2 27.77 25.77 27.01 27.24 24.86 25.78 27.30 26.70 27.26

TiO2 0.02 0.22 0.27 0.21 0.73 0.65 0.88 0.75 0.70

A12O3 56.14 57.81 56.95 55.13 56.30 56.45 53.96 55.14 55.66

Cr2O3 <d.l. 0.01 0.02 0.05 0.01 <d.l. 0.02 <d.l. <d.l.

FeOa 10.18 10.88 12.16 12.17 12.74 12.28 12.80 12.94 11.53

MnO 0.07 0.09 0.11 0.10 0.09 0.08 0.12 0.09 0.12

MgO 4.26 3.80 3.68 4.10 3.16 3.30 3.48 3.28 3.21

ZnO 0.18 0.01 0.12 0.05 0.04 <d.l. 0.12 0.08 0.11

CaO <d.l. 0.04 0.05 0.02 0.01 0.03 0.03 <d.l. <d.l.

Na2O <d.l. 0.02 <d.l. <d.l. <d.l. 0.01 <d.l. 0.01 <d.l.

K2O 0.01 0.01 0.01 <d.l. <d.l. <d.l. <d.l. 0.01 <d.l.

Total 98.61 98.66 100.36 99.07 97.95 98.57 98.72 99.00 98.60

Structural formulae based on 48 oxygen

Si 7.85 7.32 7.58 7.75 7.20 7.38 7.82 7.64 7.76

Ti 0.05 0.06 0.04 0.16 0.14 0.19 0.16 0.15

Al 18.70 19.35 18.84 18.48 19.20 19.06 18.23 18.59 18.68

Cr 0.00 0.01 0.01

Fe2+ 2.41 2.58 2.85 2.90 3.08 2.94 3.07 3.10 2.75

Mn 0.02 0.02 0.03 0.02 0.02 0.02 0.03 0.02 0.03

Mg 1.79 1.61 1.54 1.74 1.36 1.41 1.49 1.40 1.36

Zn 0.04 0.02 0.01 0.01 0.02 0.02 0.02

Ca 0.01 0.01 0.01 0.01 0.01

Na 0.01 0.01 0.01

Mg ⁄ (Mg + Fetot) 0.43 0.38 0.35 0.38 0.31 0.32 0.33 0.31 0.33

d.l., detection limits.aTotal iron reported as FeO.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 43 9

� 2011 Blackwell Publishing Ltd

orthopyroxene-in and garnet-in equilibria provideimportant limits on the P–T conditions experienced bythe Tunk Creek unit, which lacks both orthopyroxeneand garnet: less than �750 �C, <7–8 kbar (Fig. 10).

Quartz and sillimanite are not stable at most P–Tconditions modelled in both pseudosections (Fig. 10).Both pseudosections also record similar conditions forsapphirine-in (>650–700 �C) and spinel-in (<5–6

Fig. 8. Backscattered-electron images annotated with mineral composition data in biotite-bearing layers. (a,b) Sapphirine + spinel +plagioclase symplectite after staurolite [OK60C2]. (c) Partial replacement of staurolite by vermicular intergrowths of plagioclaseand corundum and adjacent cordierite corona [OK60C3]. (d) Corona development on staurolite in biotite-bearing layers, showingvermicular intergrowths of plagioclase and corundum, and polygonal plagioclase. Plagioclase varies in composition from An-rich inthe vermicular intergrowths and polygonal clusters, to more sodic compositions in the cores of matrix plagioclase. (e) Biotite and zonedmatrix plagioclase [OK60C1]. (f) Plagioclase compositional traverses for reaction cluster and matrix grains for locality OK60C1.

4 40 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

kbar) assemblages. Minor variations in the topologies,such as increased stability of staurolite in sampleOK60DA-1 (Fig. 10a), illustrate the effect of thevariations of bulk compositions (e.g. decreasing SiO2

or increasing Al2O3) on mineral assemblages.The occurrence of unzoned intergrown sapphirine +

spinel suggests that the growth and equilibration ofthese minerals within a matrix of gedrite + cordierite +plagioclase took place within the stability field ofsapphirine and spinel (Fig. 10). Isopleths calculated forsapphirine and spinel have XMg values that correspondto measured sapphirine and spinel compositions inthese rocks at �720 �C and �4 kbar (e.g. Spr:XMg = 0.79–0.83, Table 2; Spl: XMg = 0.50–0.52,Table 3; Fig. 10). These values predict equilibration ofthe sapphirine–spinel layers within the calculatedstability field of coexisting gedrite + sapphirine +cordierite + plagioclase + spinel for the bulk com-

positions of samples 60DA-1 and 60E alike (starredregions in Fig. 10), and are interpreted to represent thehigh-T conditions of metamorphism. These metamor-phic conditions overlap within error of temperatureestimates calculated using the sapphirine–spinelexchange thermometer of Sato et al. (2006), whichindicate �730–790 �C for sapphirine–spinel mineralpairs in these layers.

The upper limit for pressure conditions that theTunk Creek unit may have experienced, as revealed bypseudosection analysis of the sapphirine–spinel bulkcompositional layers, is limited by the garnet-in equi-libria at �7–8 kbar (Fig. 10). However, the preserva-tion of relict kyanite in adjacent gedrite-dominatedlayers that contain reaction clusters (i.e. cm-scale lay-ers surrounding sapphirine–spinel layers; Figs 2 & 3)indicates that conditions of metamorphism must haveoccurred within the stability field of kyanite prior to

Table 5. Representative microprobe analyses of plagioclase.

Domain Gedrite-dominated layers Biotite-bearing layers

Texture Symp. reaction

clusters

Reaction coronas Matrix With Crn + An symp.

Locality 60 A1 60DA 60A2 60DA 60E1 60DA 60E1 60C1 60C3

Within vermicular

phases

Rim Core Rim Core Inner symp. Outer symp. Vermicular

SiO2 44.50 43.44 44.72 45.48 43.06 43.38 43.38 43.38 44.76 46.03 56.36 57.68

A12O3 36.47 36.92 35.99 35.28 36.92 36.42 36.75 36.62 36.02 34.44 27.72 27.16

FeOa 0.29 0.10 0.04 0.13 0.14 0.12 0.06 0.15 0.06 0.02 0.04 0.07

CaO 18.51 19.50 19.05 17.56 19.63 19.69 19.50 19.54 18.69 17.31 9.18 8.23

Na2O 0.27 0.29 0.38 0.92 0.28 0.22 0.36 0.25 0.61 1.46 6.23 6.95

K2O 0.01 <d.l. <d.l. 0.01 <d.l. <d.l. <d.l. 0.01 0.01 0.02 0.07 0.04

Total 100.06 100.26 100.18 99.37 100.03 99.84 100.05 99.95 100.15 99.28 99.59 100.13

Structural formulae based on 8 oxygen

Si 2.05 2.00 2.06 2.10 1.99 2.01 2.01 2.01 2.06 2.13 2.54 2.58

Al 1.98 2.01 1.95 1.92 2.01 1.99 2.00 2.00 1.95 1.88 1.47 1.43

Fe2+ 0.01 0.00 0.00 0.01 0.01 0.01

Ca 0.91 0.96 0.94 0.87 0.97 0.98 0.97 0.97 0.92 0.86 0.44 0.39

Na 0.02 0.03 0.03 0.08 0.03 0.02 0.03 0.02 0.05 0.13 0.54 0.60

XAn 0.97 0.97 0.96 0.91 0.97 0.98 0.97 0.98 0.94 0.87 0.45 0.39

Domain Biotite-bearing layers Hornblende-dominated layers

Texture Spr + Spl symp. Matrix Matrix aggregates Coronas around corundum or spinel

Locality 60C2 60C1 60C3 60B 63B1 65 60B 63B1 63B2 65

Vermicular Rim Core

SiO2 43.68 49.72 56.08 57.23 44.16 43.02 42.19 43.84 42.77 42.77 42.83

A12O3 35.86 32.05 27.98 27.60 36.18 36.77 36.24 36.47 36.88 36.94 36.96

FeOa 0.43 0.07 0.07 0.20 0.18 0.13 0.13 0.10 0.23 0.26 0.22

CaO 19.12 14.48 9.77 8.91 18.99 20.27 20.05 19.30 19.97 19.88 19.83

Na2O 0.65 3.17 6.02 6.34 0.42 <d.l. 0.03 0.26 0.01 0.03 0.06

K2O <d.l. <d.l. 0.01 0.06 0.01 <d.l. 0.01 <d.l. <d.l. <d.l. 0.01

Total 99.74 99.49 99.94 100.34 99.92 100.19 98.65 99.98 99.86 99.88 99.92

Structural formulae based on 8 oxygen

Si 2.03 2.28 2.52 2.56 2.04 1.99 1.98 2.03 1.99 1.98 1.99

Al 1.96 1.73 1.48 1.45 1.97 2.01 2.01 1.99 2.02 2.02 2.02

Fe2+ 0.02 0.01 0.01 0.01 0.01 0.01

Ca 0.95 0.71 0.47 0.43 0.94 1.01 1.01 0.96 0.99 0.99 0.99

Na 0.06 0.28 0.52 0.55 0.04 0.02 0.01

XAn 0.94 0.72 0.47 0.44 0.96 1.00 1.00 0.98 1.00 1.00 0.99

d.l., detection limits; symp. symplectite.aTotal iron reported as FeO.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 44 1

� 2011 Blackwell Publishing Ltd

the formation of symplectitic reaction textures andsapphirine + spinel layers in gedrite dominateddomains. Therefore, the peak pressures experienced bythe Tunk Creek unit were likely >8 kbar and, due tobulk compositional variations, the sapphirine–spinellayers record metamorphic conditions subsequent to atleast 3–4 kbar of decompression and re-equilibrationat high-T and lower-pressures. The specific P–T pathcannot be quantitatively determined using this analy-sis; however, a likely range of paths involving decom-pression, with or without heating, extend from thekyanite stability field to the calculated equilibrationconditions of sapphirine + spinel at �720 �C and�4 kbar. It is likely that decompression occurred alonga near isothermal path (possibly with minor heating) asa cooling path during decompression would predictassemblages dominated by chlorite or, in the case ofsignificant heating during decompression, formation oforthopyroxene for these bulk compositions (Fig. 10),neither of which are observed in the samples analysed.

Hornblende-dominated domain

Sample OK65 (Fig. 11, Table 1) was selected forpseudosection analysis because it contains polygonalspinel aggregates in a hornblende–plagioclase matrix(Fig. 5e) that lacks the coronal textures observed inother samples. Corundum-bearing samples containingsuccessive coronas of spinel and plagioclase (e.g.Fig. 5f) in hornblende-dominated domains were avoi-ded as the preservation of these corona textures mayreflect changes in the effective bulk compositionexperienced by the layers (e.g. Raith et al., 2008). The

Table 6. Representative microprobe analyses of cordierite.

Domain Gedrite-dominated layers Biotite-bearing layers

Texture Moats around reaction coronas Matrix Within Crn + Pl

symplectite

Matrix

Locality 60A1 60A2 60DA 60A1 60E1 60C2 60C3 60C1 60C2 60C3

SiO2 48.34 48.78 49.17 47.79 48.56 47.92 47.92 48.67 47.97 47.63

TiO2 0.00 0.02 0.00 <d.l. 0.01 0.04 0.01 0.04 0.01 <d.l.

A12O3 33.02 33.33 33.58 33.15 33.39 33.00 33.29 33.38 33.15 33.29

Cr2O3 <d.l. 0.03 0.03 0.01 <d.l. 0.05 <d.l. <d.l. 0.01 0.01

FeOa 2.67 2.11 2.03 2.17 2.26 4.09 4.11 3.93 4.14 3.88

MnO 0.01 0.01 0.01 0.01 0.03 0.03 0.09 0.03 0.07 0.07

MgO 11.90 11.95 11.53 12.50 11.64 11.19 10.80 10.86 10.88 11.22

CaO 0.03 0.05 0.05 0.03 0.12 0.05 0.05 0.04 0.07 0.06

Na2O 0.22 0.30 0.30 0.22 0.30 0.35 0.53 0.44 0.34 0.50

K2O <d.l. <d.l. 0.00 0.02 <d.l. <d.l. 0.01 <d.l. 0.01 0.02

Total 96.15 96.56 96.71 95.88 96.28 96.71 96.81 97.38 96.62 96.63

Structural formulae based on 18 oxygen

Si 4.97 4.98 5.00 4.92 4.97 4.93 4.93 4.97 4.94 4.91

Al 4.00 4.01 4.03 4.02 4.03 4.00 4.04 4.01 4.03 4.04

Fe2+ 0.23 0.18 0.17 0.19 0.19 0.35 0.35 0.34 0.36 0.33

Mn 0.01 0.01 0.01

Mg 1.82 1.82 1.75 1.92 1.78 1.72 1.66 1.65 1.67 1.72

Na 0.04 0.06 0.06 0.04 0.06 0.07 0.11 0.09 0.07 0.10

Mg ⁄ (Mg + Fetot) 0.89 0.91 0.91 0.91 0.90 0.83 0.82 0.83 0.82 0.84

d.l., detection limits.aTotal iron reported as FeO.

Fig. 9. (a) NaA–AlVI plot and (b) XMg–AlVI plot of gedritecomposition. Gedrite composition is distinct in each of the threerepresentative domains of the Tunk Creek unit.

4 42 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

topology of the calculated pseudosection for sampleOK65 (Fig. 11) is relatively simple and suggests atemperature of equilibration for the assemblage pla-gioclase + hornblende + spinel at �750 �C, within-

error to temperatures calculated for gedrite-dominatedlayers. Moreover, the composition of spinel in thisstability field is in agreement with the results of rep-resentative microprobe analyses (XMg = 0.48–0.51;Table 3, Fig. 11). The stability field of coexistingplagioclase + hornblende + spinel spans a range ofpressures from �4 to 8 kbar, similar to the pressurelimits obtained for the gedrite-dominated domains. Asin the sapphirine + spinel layers, maximum tempera-tures in the hornblende-dominated layers are limitedby the orthopyroxene-in reaction.

DISCUSSION

Reaction textures (coronas, symplectite) in the TunkCreek unit of the Okanogan dome are similar to thoseobserved in gedrite–cordierite rocks in the Thor-Odindome (Fig. 1b), but with some important differences.In the Tunk Creek unit:1 Kyanite did not transform to sillimanite during

high-T decompression, despite indications that therocks passed through the sillimanite stability fieldduring decompression, as evidenced by the calcu-lated P–T conditions in adjacent sapphirine–spinellayers and the presence of sillimanite in metapeliticschist elsewhere in the Okanogan dome.

2 Garnet was not observed in the Tunk Creek unit,but it is abundant in the gedrite–cordierite gneiss of

Table 7. Representative microprobe analyses of gedrite.

Domain Gedrite-dominated layers Biotite-bearing

layers

Hornblende-

dominated 1ayers

Locality 60A1 60A2 60DA 60E1 60C2 60B

Matrix

rim

Matrix

core

Thin,

in Crd

Large,

matrix

Adjacent to Spr + Spl lyr Adjacent to Spr + Spl lyr Matrix Matrix

rim

Matrix

core

Matrix

rim

Matrix

core

SiO2 43.26 45.72 44.18 45.04 44.95 43.69 44.70 42.03 41.63 42.55 41.91

TiO2 0.10 0.14 0.04 0.05 0.14 0.10 0.05 0.09 0.04 <d.l. 0.04

A12O3 19.65 16.10 18.91 18.50 19.11 19.75 19.03 21.57 22.49 22.33 23.29

FeOa 13.05 13.08 11.78 11.86 11.68 11.99 11.44 14.91 13.84 10.06 10.43

MnO 0.22 0.27 0.21 0.17 0.20 0.23 0.27 0.25 0.26 0.17 0.13

MgO 18.53 20.32 19.24 20.45 20.56 19.82 19.85 16.73 17.54 20.47 19.75

CaO 0.65 0.60 0.61 0.55 0.64 0.62 0.52 0.48 0.41 0.55 0.63

Na2O 1.69 1.33 1.84 1.79 1.84 1.65 1.72 2.56 2.67 2.29 2.37

K2O <d.l. <d.l. <d.l. <d.l. <d.l. 0.02 0.01 0.02 <d.l. <d.l. 0.01

F <d.l. <d.l. 0.02 0.01 <d.l. 0.02 0.01 0.01 <d.l. <d.l. <d.l.

Cl 0.01 <d.l. <d.l. <d.l. <d.l. 0.01 0.01 <d.l. <d.l. 0.01 0.01

Cr2O3 0.03 0.01 <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l. <d.l.

Total 97.19 97.57 96.78 98.43 99.11 97.87 97.53 98.58 98.82 98.38 98.55

Structural formulae based on 23 oxygen

Si 6.12 6.41 6.24 6.25 6.19 6.10 6.24 5.93 5.84 5.88 5.80

A1IV 1.88 1.59 1.76 1.75 1.81 1.90 1.76 2.07 2.16 2.12 2.20

A1VI 1.40 1.08 1.38 1.28 1.30 1.35 1.38 1.52 1.56 1.52 1.59

Ti 0.01 0.01 0.01 0.01 0.01 0.01 0.01

Fe3+ 0.12 0.08

Fe2+ 1.54 1.42 1.39 1.38 1.35 1.32 1.34 1.76 1.62 1.16 1.21

Mn 0.03 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.02 0.02

Mg 3.91 4.25 4.05 4.23 4.22 4.12 4.13 3.52 3.67 4.22 4.07

Ca 0.10 0.09 0.09 0.08 0.09 0.09 0.08 0.07 0.06 0.08 0.09

Na 0.46 0.36 0.50 0.48 0.49 0.45 0.46 0.70 0.73 0.61 0.64

Mg ⁄ (Mg + Fetot) 0.72 0.73 0.74 0.75 0.76 0.75 0.76 0.67 0.69 0.78 0.77

d.l., detection limits.aTotal iron reported as FeO.

Table 8. Representative microprobe analyses of biotite.

Locality 60C1

(matrix)

60C2 (adjacent

to reaction clusters)

60C2

(matrix)

60C3

(matrix)

SiO2 36.50 36.43 38.00 36.96

TiO2 2.23 0.33 1.28 2.47

A12O3 20.41 21.11 19.00 19.24

FeOa 12.22 9.41 11.46 12.01

MnO 0.02 0.03 <d.l. 0.05

MgO 16.41 19.73 16.46 15.71

CaO 0.02 0.13 0.02 0.02

Na2O 1.08 1.47 0.73 0.36

K2O 7.73 6.66 8.85 9.03

Total 96.61 95.29 95.79 95.83

Structural formulae based on 22 oxygen

Si 5.24 5.21 5.50 5.38

A1IV 2.76 2.79 2.50 2.62

A1VI 0.70 0.77 0.74 0.68

Ti 0.24 0.04 0.14 0.27

Fe2+ 1.47 1.13 1.39 1.46

Mn 0.01

Mg 3.51 4.21 3.55 3.41

Na 0.30 0.41 0.20 0.10

K 1.42 1.22 1.63 1.67

Mg ⁄ (Mg + Fetot) 0.71 0.79 0.72 0.70

d.l., detection limits.

F and Cl were analysed but not detected.aTotal iron reported as FeO.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 44 3

� 2011 Blackwell Publishing Ltd

the Thor-Odin dome (Duncan, 1984; Norlanderet al., 2002; Hinchey & Carr, 2007).

3 Sapphirine and spinel occur in symplectite, as inThor-Odin, but in the Tunk Creek unit they alsooccur as coarse matrix grains that are typically inmm- to cm-scale layers dominated by these twominerals.

4 The Tunk Creek unit contains calcic amphibole,which is not observed in gedrite–cordierite gneiss ofthe Thor-Odin dome. Furthermore, althoughanorthite occurs in corona textures in gedrite–cor-dierite gneiss in both domes, it is more abundant inthe Tunk Creek unit.A number of the differences in assemblages and

textures in the Tunk Creek unit when compared tosimilar rocks in the Thor-Odin dome are directlyattributable to the more calcic bulk composition of theTunk Creek rocks: representative Tunk Creek gneisscontains �7–11 wt% CaO (Table 1), when comparedto <1 wt% in Thor-Odin rocks (Norlander et al.,2002). Other factors, however, such as P–T conditions,paths and rates, must also be considered.

Phase diagrams calculated for spatially associatedbut compositionally and mineralogically distinct layersin the Tunk Creek unit are in good agreement witheach other in terms of predicted P–T conditions. Fieldand textural observations indicate that compositionallayering in the Tunk Creek rocks formed prior todeformation and associated decompression, and

therefore the mineral assemblages and textures areassociated with high-grade metamorphism thataccompanied dome formation. The mm- to m-scalelayering with distinct bulk compositions resulted indifferences in textural evolution during decompression,e.g. coronal reaction textures on kyanite. In contrast,sapphirine + spinel layers apparently record texturalequilibrium and equilibration at higher-T and lower-Pthan the kyanite-bearing layers (and hence were usedin the pseudosection analysis).The low-P equilibration of spinel + sapphirine

indicates elevated crustal temperatures (�720–750 �C)within the Okanogan dome at a relatively shallow levelof the orogenic crust (�4 kbar). Despite the high-Tconditions, sillimanite is not present in the Tunk Creekrocks, as is shown by pseudosection (Fig. 10) andtextural analysis. The lack of sillimanite in the TunkCreek unit may be controlled by factors such as bulkcomposition, the presence of other, more stable alu-minous phases at the P–T–X conditions of metamor-phism, or the sluggish reaction kinetics of the Al2SiO5

polymorphs.Alternatively, differences in the P–T evolution of the

Mg–Al rocks and host migmatites during dome for-mation could account for some of the differences inreaction textures and assemblages among the northernCordilleran gneiss domes. However, Mg–Al-rich rocksin the Thor-Odin dome record decompression from thekyanite zone (>8–10 kbar) to conditions in the

Table 9. Representative microprobe analyses of hornblende and chlorite.

Locality Hornblende Chlorite

60B 63B1 (core) 63B1 (rim) 63B2 65 (rim) 65 (core) 63B1 (within amphibole) 63B1 (late shear zone)

SiO2 43.89 46.94 46.03 47.62 47.36 45.88 28.25 28.19

TiO2 0.03 0.06 0.04 0.06 <d.l. 0.07 0.08 <d.l.

A12O3 18.87 15.42 15.39 15.22 14.29 15.94 24.19 24.06

FeOa 7.16 6.89 7.06 5.83 6.91 7.22 7.82 7.66

MnO 0.04 0.19 0.10 0.14 0.12 0.12 0.10 0.10

MgO 14.76 15.39 15.20 15.74 15.63 14.67 27.57 27.23

CaO 11.23 13.16 13.47 13.16 12.70 12.65 0.04 0.01

Na2O 2.39 0.45 0.79 0.55 1.02 1.11 <d.l. 0.01

K2O 0.21 0.09 0.10 0.08 0.10 0.14 0.01 <d.l.

F 0.01 0.02 0.01 0.01 0.00 0.00 0.01 <d.l.

Cl 0.01 0.01 0.01 <d.l. <d.l. <d.l. 0.01 0.01

Cr2O3 <d.l. 0.06 0.02 <d.l. <d.l. <d.l. 0.04 0.01

NiO

Total 98.59 98.66 98.20 98.37 98.10 97.64 88.11 87.25

Structural formulae based on 23 oxygen for hornblende and 28 oxygen for chlorite

Si 6.06 6.49 6.45 6.58 6.59 6.44 5.36 5.39

A1IV 1.94 1.51 1.55 1.42 1.41 1.56 2.64 2.61

A1VI 1.13 1.00 0.98 1.06 0.94 1.08 2.79 2.83

Ti 0.01 0.01 0.01 0.01

Cr 0.01 0.01

Fe3+ 0.81 0.46 0.29 0.28 0.39 0.35 0.10 0.12

Fe2+ 0.02 0.33 0.54 0.39 0.42 0.49 1.14 1.10

Mn 0.02 0.01 0.02 0.01 0.01 0.02 0.02

Mg 3.04 3.17 3.17 3.24 3.25 3.07 7.80 7.77

Ca 1.66 1.95 2.02 1.95 1.89 1.90 0.01

Na 0.64 0.12 0.22 0.15 0.27 0.30 0.01

K 0.04 0.01 0.02 0.01 0.02 0.02

Mg ⁄ (Mg + Fetot) 0.79 0.80 0.79 0.83 0.80 0.78 0.86 0.86

d.l., detection limits.aTotal iron reported as FeO.

4 44 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

sillimanite–cordierite zone (<5kbar) at �750 �C(Norlander et al., 2002), suggesting similar conditionsto those recorded in the Tunk Creek unit. Mg–Al-richgneiss in both domes record evidence of an early,high-P (>8 kbar) history with subsequent decom-pression to �720–750 �C at shallower crustal levels(�4–5 kbar).

Differences in the scale and structural position of theorthoamphibole-cordierite gneiss may have also influ-enced the extent of their interaction with the partiallymolten rocks (including fluids related to crystallizingmelt) and their deformation behaviour. In theOkanogan dome, the Tunk Creek unit occurs adjacentto a high-melt fraction migmatite domain and ischaracterized by km-scale layers of Mg–Al-rich gneiss(Fig. 1b,c). In contrast, similar rocks in the Thor-Odinand Valhalla ⁄Passmore domes occur in relatively small

(metre to tens of metres scale) pods surroundedby high-melt fraction migmatite (Duncan, 1984;Norlander et al., 2002), although layers up to 50 ·500 m have been reported (Hinchey & Carr, 2007).Based on similar calculated metamorphic conditions inboth domes, the differences in the recorded assem-blages and reaction textures were likely controlledprimarily by variations in bulk composition duringtheir tectonic evolution.

Metamorphism and gneiss dome evolution

In the Omineca belt, the mineral assemblages andtextures of orthoamphibole-cordierite gneiss werelikely related, at least in part, to gneiss dome evolution.Mg–Al-rich gneiss in the Thor-Odin, Valhalla ⁄Passmore and Okanogan domes record similar

0.72

0.74

0.76

0.78

0.78

0.80

0.52

0.48

0.44

0.400.36

0.28

0.52

0.48

0.28

Oam SprCrd Pl Grt

710

Oam ChlCrd Pl

Oam

Chl

Crd

Pl S

pl

Oam SprCrd Pl Spl

Oam

Crd

Pl

Spl O

px

Oam SprCrd Pl

Crd Pl Spl Opx

Oam SprCrd PlSpl Opx

Oam

Spr

Crd

Pl O

px

Spr Crd PlSpl Opx

Spr CrdPl Opx

Spr Crd

Pl Grt

2

4

6

8

3

5

7

700 750 800T (°C)

Pre

ssur

e (k

bar)

4

26

2729

30

31

34

32 33

35

650

2 3

56

1214

Oam Chl Crd Pl

Crn

242311

28

8 9

17

19

2018

2122

13

1 - Chl Pl Ky Qz2 - Chl Pl Grt Ky Qz3 - Oam Chl Pl Grt Ky Qz4 - Chl Pl Ky Sil Qz5 - Oam Chl Pl Ky Sil Qz6 - Oam Chl Crd Pl Ky Qz7 - Oam Chl Pl Ky Qz8 - Oam Hbl Chl Pl Ky Qz9 - Oam Hbl Chl Pl Ky

10 - Oam Chl Pl Ky11 - Oam Chl Crd Pl Ky12 - Oam Chl Crd Pl Ky Sil13 - Oam Chl Pl Ky Sil14 - Oam Chl Crd Pl Grt Sil

1

15 16

25

Oam CrdPl Spl

15 - Oam Chl Pl Sil16 - Oam Chl Crd Pl Sil Crn17 - Oam Chl Crd Pl Grt Crn18 - Oam Chl Crd Pl Crn19 - Oam Spr Crd Pl Crn20 - Oam Spr Crd Pl Grt Crn21 - Oam Crd Pl Crn22 - Oam Crd Pl Grt Crn23 - Oam Hbl Spr Crd Pl Grt24 - Oam Spr Crd Pl Grt Opx25 - Chl Pl Sil Qz26 - Oam Chl Crd Pl Sil Qz27 - Oam Chl Pl Sil Qz28 - Oam Chl Crd Pl Sil

29 - Oam Chl St Crd Pl Sil30 - Oam Chl St Crd Pl31 - Oam Chl St Crd Pl Crn32 - Oam Chl Spr Crd Pl Crn33 - Oam Chl Spr Crd Pl34 - Oam Chl Crd Pl Spl Crn35 - Oam Chl Spr Crd Pl Spl

Opx

Chl StPl Ky Oam SprCrd Pl Grt

Chl

Crd

Pl S

pl

Oam

Chl

Crd

Pl S

pl

Oam C

hl Crd

Pl C

rn

Oam

Spr

Crd

Pl S

pl O

px

Spr Crd Pl Spl Opx

Oam

Spr

Crd

Pl O

px

1

27

28

25

2930

31 Oam

Chl S

pr Crd P

l Spl

3233

34

35

36 3739

4038

2

4

6

8

3

5

7

700 750 800T (°C)

Pre

ssur

e (k

bar)

650

OK60DA-1, NCFMASH (+H2O)

3

2

5 6 8

410

11

Oam ChlCrd Pl

Grt Crn

1213

1415

16

1718

19

20 22

23

Oam Spr

Crd Pl Spl

Chl StPl KyCrn

1 - Chl St Pl Ky Sil Crn2 - Chl St Pl Ky Sil3 - Chl St Pl Sil4 - Oam Chl St Pl Sil5 - Hbl Chl St Pl Ky Sil6 - Hbl Chl St Pl Sil7 - Hbl Chl St Pl Ky8 - Oam Hbl Chl St Pl Ky9 - Oam Hbl Chl St Pl Sil

10 - Oam Chl St Pl Sil Crn11 - Oam Chl Pl Grt Sil Crn12 - Oam Hbl Chl Pl Grt Crn13 - Oam Chl Pl Grt Crn14 - Oam Chl Spr Pl Grt Crn

7

Chl StPl SilCrn

Chl CrdPl SplCrn

21

9

24

26

Spr Crd Pl

Grt Opx

15 - Oam Chl Spr Pl Crn16 - Oam Spr Crd Pl Crn17 - Oam Spr Pl Crn18 - Oam Spr Pl Grt Crn19 - Oam Spr Crd Pl Grt Crn20 - Oam Hbl Spr Pl Grt Crn21- Hbl Spr Crd Pl Grt22 - Oam Hbl Spr Crd Pl Grt23 - Oam Spr Crd Pl Grt Opx24 - Chl Pl Sil Crn25 - Chl Crd Pl Sil Crn26 - Chl Crd Pl Crn27 - Chl St Crd Pl Sil Crn28 - Oam Chl St Crd Pl Crn

29 - Oam Chl Spr Crd Pl Crn30 - Oam Chl Spr Crd Pl31 - Oam Chl Crd Pl Spl Crn32 - Chl Spr Crd Pl Spl33 - Chl Spr Crd Pl Spl Crn34 - Oam Hbl Chl Crd Pl Spl35 - Oam Hbl Spr Crd Pl Spl36 - Oam Hbl Crd Pl Spl37 - Oam Hbl Crd Pl Spl Opx38 - Hbl Crd Pl Spl Opx39 - Hbl Spr Crd Pl Spl Opx40 - Crd Pl Spl Opx

OK60E, NCFMASH (+H2O)

0.78

0.78

0.80

0.82

0.80

0.76

0.74

0.72

0.580.54

0.50

0.46

0.42

0.38

0.34

0.26

Oam SprCrd Pl

Spr CrdPl Opx

(a) (b)

Fig. 10. Calculated pseudosections for sapphirine + spinel intergrowth layers in gedrite-dominated domains of samples OK60DA-1(a) and OK60E (b) (Table 1). Calculated XMg isopleths are plotted as dot-dash contours for sapphirine (0.72–0.84) and dashes forspinel (0.26–0.58). The intersections of calculated XMg isopleths with measured values for Tunk Creek sapphirine and spinel (Tables 2& 3) are starred and occur at �720 �C and �4 kbar. Numbered phase fields in pseudosection correspond to assemblages listed belowfigure.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 44 5

� 2011 Blackwell Publishing Ltd

evidence for elevated crustal temperatures anddecompression at high-T (Duncan, 1984; Norlanderet al., 2002; this study). Orthoamphibole-cordieritegneiss is associated with migmatite in each of thesedomes, and consequently the petrological evolution ofthese rocks was likely linked to elevated thermal con-ditions during Palaeocene–Eocene partial melting(Vanderhaeghe et al., 1999; Hinchey et al., 2006;Gordon et al., 2008; Kruckenberg et al., 2008) and thedecompression of dome cores.

The Tunk Creek unit records high-T metamorphism(720–750 �C) over a range of pressures duringdecompression (Fig. 12a), including at shallow crustallevels (�9–12 km). Abundant evidence for melt-present deformation in the underlying migmatitedomain, early Cenozoic crystallization ages of domemigmatites, and the consistency of structural fabricsfrom the lowest structural levels in the migmatitedomain upward to the Okanogan Valley detachmentfault, suggest the temporal and kinematic linkages ofmigmatite crystallization, exhumation and detachmenttectonics in the Okanogan dome (Kruckenberg et al.,2008). The concordance of structural fabrics withinand surrounding the Tunk Creek unit (Fig. 2a,b), and

abundant evidence for elevated crustal temperatures inthe Tunk Creek unit and in the structurally lowermigmatite domain, indicates that the high-T–low-Pconditions were likely related to evolution of themigmatite dome, including the high-melt fractionmigmatites that structurally underlie the Tunk Creekunit (Fig. 12b). Thulite and epidote veins and podspreserve a later part of the history in the Tunk Creekunit and may record the infiltration and circulation ofwater-rich metamorphic fluids associated with crys-tallization of the dome migmatite at shallow crustallevels, or metamorphic fluids associated with detach-ment faulting during rapid exhumation and rapidcooling of the dome core (Fig. 12c).Linkages between the development of the Okanogan

dome, crustal anatexis in structurally underlyingmigmatites, and the high-T–low-P conditions recordedin the Tunk Creek unit are consistent with ideas aboutheat transfer during the ascent of migmatite domes(e.g. Teyssier & Whitney, 2002) and recent geodynamicmodels that document decompression P–T paths dur-ing the ascent of partially molten crust to shallowcrustal levels along a high geothermal gradient (e.g.Rey et al., 2009). Shallow emplacement of migmatitedomes is a mechanism by which high-T, low-P meta-morphic conditions can be attained without an extremeheating mechanism.

Significance for other tectonic studies

Mg–Al-rich orthoamphibole-cordierite rocks are tex-turally complex and commonly preserve high-varianceassemblages that have wide stability fields, hinderingquantitative assessment of the P–T conditions experi-enced by these rocks (e.g. Raith et al., 2008). However,in this study of the Tunk Creek unit, the presence ofdifferent mm- to m-scale bulk compositional layers,some of which contain reaction textures and some ofwhich show apparent textural equilibrium amongphases, allows both a systematic analysis of the P–Tconditions of high-grade metamorphism and petro-logical evolution. In particular, the application ofpseudosection analysis to three different layers thatlack disequilibrium reaction textures (symplectites,coronas) gave systematic results, despite differences inbulk composition. These results demonstrate the utilityof pseudosection analysis in deciphering the complexpetrological evolution of Mg–Al-rich rocks whencarefully applied to specific bulk compositional andtextural domains.The application of such techiniques, however,

requires the consideration of a number of possiblesources of error, including chemical subdomains,choice of solution models and the effects of Fe3+ onthe stability fields of calculated topologies. Layerscontaining symplectite or coronal reaction textures arethus not used in phase equilibrium analysis, but insteadprovide additional information about reaction historyand path. The integrated approach in this study allows

2

4

6

8

3

5

7

700 750 800T (°C)

Pre

ssur

e (k

bar)

650

Pl Cpx Hbl Chl

Pl HblChl Zo

Pl HblChl Grt

Zo

Pl CpxHbl Chl Grt

Pl H

bl S

pl O

px

Pl Hbl Chl Grt

Pl Hbl Spl

Pl C

px H

blS

pl O

px

Pl Cpx Hbl

Spl Opx Fo

1

23

45

6

7

Pl H

bl C

hl S

pl

8

0.60

0.58

0.56

0.54

0.52

0.50

0.48

0.46

0.44

0.42

0.34

OK65, NCFMASH (+H2O)

1 - Pl Cpx Hbl Chl Zo2 - Pl Cpx Hbl Chl Grt Zo3 - Pl Cpx Hbl Chl Spl Grt4 - Pl Hbl Chl Spl Grt5 - Pl Hbl Spl Grt6 - Pl Hbl Spl Grt Opx7 - Pl Cpx Hbl Chl Spl8 - Pl Hbl Chl Spl Opx

Fig. 11. Calculated phase diagram for hornblende-dominateddomains for sample OK65 (Table 1). Calculated XMg isoplethsfor spinel are plotted as dashed lines and are representative ofTunk Creek spinel compositions (Table 3) within the stabilityfield of spinel + hornblende + plagioclase (starred region).Similar to pseudosections for sapphirine + spinel layers, thestability field of spinel + hornblende + plagioclase occurs at�720–750 �C; maximum pressure is limited by garnet-inequilibria �7–8 kbar and maximum temperature is limited bythe orthopyroxene-in reaction.

4 46 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

an analysis of the bulk composition v. tectonic (P–Tpath) controls on mineral assemblage and textureformation during the metamorphic evolution of com-positionally and texturally heterogeneous rocks such asMg–Al-rich gneiss, and thus provide additional infor-mation about the P–T history of associated rocks; inthis case, migmatite of the Okanogan dome, withapplication to the metamorphic evolution of otherhigh-T gneiss domes.

ACKNOWLEDGEMENTS

This research was supported in part by NSF GrantEAR-0409776 to C. Teyssier and D.L. Whitney, aGeological Society of America grant to S. Kruckenberg,and summer research funds from the Department ofGeology and Geophysics, University of Minnesota. R.

Tracy, J.V. Owen and editor D. Robinson are gratefullyacknowledged for their constructive comments anddetailed reviews that significantly enhanced the qualityof this manuscript.We thank J. Connolly for addressingearly questions about pseudosection analysis in PER-PER-

PLE_XPLE_X. We also thank P. Davis, E. Goergen, C. Regalla,R-E. Farrell, and the support and analytical staff at theUniversity of Minnesota. This research also could nothave been conducted without the access to the TunkCreek rocks that was graciously provided by theWashington state landowners.

REFERENCES

Auzanneau, E., Schmidt, M.W., Vielzeuf, D. & Connolly,J.A.D., 2010. Titanium in phengite: a geobarometer for high

(a) (b)

(c)

Fig. 12. (a) Comparison of metamorphic conditions obtained from pseudosection analysis of gedrite- and hornblende-dominateddomains of the Tunk Creek unit. Equilibria for index phases (e.g. garnet, opx) are approximate in position. (b) Following Jurassic toCretaceous crustal thickening and metamorphism, crystallization of migmatites was coeval with extensional deformation duringPalaeocene–Eocene time (Kruckenberg et al., 2008). Structural fabrics within the Tunk Creek unit (TCU) are concordant to those insurrounding units. Replacement reactions of early-formed kyanite ⁄ staurolite + gedrite to form corona ⁄ symplectite textures werecoeval with extension, partial melting, exhumation (decompression) and dome formation. Crustal temperatures were high (�720 �C) atshallow crustal levels, consistent with evidence from underlying migmatites. (c) Schematic Eocene relationships along the westernOkanogan dome. Cooling of footwall rocks and the crystallization of dome migmatites below �325 �C was largely completed by c.47 Ma (Kruckenberg et al., 2008). The occurrence of thulite associated with epidote in pods and layers that cut the three representativedomains of the TCU is interpreted to suggest metamorphic fluid infiltration, perhaps derived from crystallizing migmatites and plutonsin the dome or the circulation of detachment fluids.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 44 7

� 2011 Blackwell Publishing Ltd

temperature eclogites. Contributions to Mineralogy andPetrology, 159, 1–24.

Brown, R.L. & Read, P.B., 1983. Shuswap terrane of BritishColumbia: a Mesozoic ‘‘core complex’’. Geology, 11, 164–168.

Carlson, D.H. &Moye, F.J., 1990. The Colville igneous complex;Paleogene volcanism, plutonism, and extension in northeasternWashington. In: The Nature and Origin of Cordilleran Mag-matism: Geological Society of America Memoir, Vol. 174 (ed.Anderson, J.L.), pp. 375–394. Geological Society of America,Boulder, CO.

Chinner, G.A. & Fox, J.S., 1974. The origin of cordierite-anthophyllite rocks in the Land�s End aureole. GeologicalMagazine, 111, 397–408.

Coggon, R. & Holland, T.J.B., 2002. Mixing properties ofphengitic micas and revised garnet-phengite thermobarome-ters. Journal of Metamorphic Geology, 20, 683–696.

Connolly, J.A.D., 1990. Multivariable phase diagrams – analgorithm based on generalized thermodynamics. AmericanJournal of Science, 290, 666–718.

Connolly, J.A.D., 2005. Computation of phase equilibria bylinear programming: a tool for geodynamic modeling and anapplication to subduction zone decarbonation. Earth andPlanetary Science Letters, 236, 524–541.

Diener, J.F.A., Powell, R., White, R.W. & Holland, T.J.B., 2007.A new thermodynamic model for clino- and orthoamphibolesin the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-O.Journal of Metamorphic Geology, 25, 631–656.

Diener, J.F.A., Powell, R. & White, R.W., 2008. Quantitativephase petrology of cordierite-orthoamphibole gneisses andrelated rocks. Journal of Metamorphic Geology, 26, 795–814.

Duncan, I.J., 1984. Structural evolution of the Thor-Odin gneissdome. Tectonophysics, 101, 87–130.

Eskola, P., 1914. On the petrology of the Orijarvi region insouthwestern Finland. Bulletin of the Geological Commissionof Finland, 40, 277.

Fuhrman, M.L. & Lindsley, D.H., 1988. Ternary-feldsparmodeling and thermometry. American Mineralogist, 73, 201–215.

Glombick, P., Thompson, R.I., Erdmer, P. et al., 2006. U-Pbconstraints on the thermotectonic evolution of the Vernonantiform and the age of the Aberdeen gneiss complex,southeastern Canadian Cordillera. Canadian Journal of EarthScience, 43, 213–244.

Gordon, S.M., Whitney, D.L., Teyssier, C., Grove, M. &Dunlap, W.J., 2008. Timescales of migmatization, meltcrystallization, and cooling in a Cordilleran gneiss dome, theValhalla Complex, southeastern British Columbia. Tectonics,27, TC4010.

Green, E.C.R., Holland, T.J.B. & Powell, R., 2007. An order-disorder model for omphacitic pyroxenes in the system jadeite-diopside-hedenbergite-acmite, with applications to eclogiticrocks. American Mineralogist, 92, 1181–1189.

Harvey, J.L., 1994. Sapphirine-bearing amphibolites in theOkanogan complex, Washington: thermobarometry and tectonicimplications MS thesis, Northern Arizona University, 237 pp.

Hinchey, A.M. & Carr, S.D., 2007. Protolith composition ofcordierite-gedrite basement rocks and garnet amphibolite ofthe Bearpaw Lake area of the Thor-Odin Dome, MonasheeComplex, British Columbia, Canada. The Canadian Mineral-ogist, 45, 607–629.

Hinchey, A.M., Carr, S.D., McNeill, P.D. & Rayner, N., 2006.Paleocene-Eocene high-grade metamorphism, anatexis anddeformation in the Thor-Odin dome, Monashee complex,southeastern British Columbia. Canadian Journal of EarthSciences, 43, 1341–1365.

Holland, T.J.B. & Powell, R., 1998. An internally-consistentthermodynamic data set for phases of petrological interest.Journal of Metamorphic Geology, 16, 309–343.

Holland, T.J.B., Baker, J.M. & Powell, R., 1998. Mixing prop-erties and activity-composition relationships of chlorites in thesystem MgO- FeO-Al2O3-SiO2-H2O. European Journal ofMineralogy, 10, 395–406.

James, R.S., Grieve, R.A.F. & Pauk, L., 1978. The petrology ofcordierite-anthophyllite gneisses and associated mafic andpelitic gneisses at Manitouwadge, Ontario. American Journalof Science, 278, 41–63.

Johnson, S.P. & Oliver, G.J.H., 2002. High ƒO2 metasomatismduring whiteschist metamorphism, Zambezi Belt, northernZimbabwe. Journal of Petrology, 43, 271–290.

Kelsey, D.E., White, R.W., Holland, T.J.B. & Powell, R., 2004.Calculated equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O forsapphirine-quartz-bearing mineral assemblages. Journal ofMetamorphic Geology, 22, 559–578.

Kelsey, D.E., White, R.W. & Powell, R., 2005. Calculated phaseequilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for silica-undersaturated sapphirine-bearing mineral assemblages.Journal of Metamorphic Geology, 23, 217–239.

Kruckenberg, S.C., Whitney, D.L., Teyssier, C., Fanning, C.M.& Dunlap, W.J., 2008. Paleocene-Eocene migmatite crystalli-zation, extension, and exhumation in the hinterland of thenorthern Cordillera: Okanogan dome, Washington USA.Geological Society of America Bulletin, 120, 912–929.

Marshall, D. & Simandl, G., 2006. Phase relations and meta-morphism in the sapphirine bearing granulites of the Valhallacomplex, Slocan Valley, BC. GAC ⁄MAC Annual MeetingAbstracts Program, 31, 96.

Norlander, B.H., Whitney, D.L., Teyssier, C. & Vanderhaeghe,O., 2002. Partial melting and decompression of the Thor-Odindome, Shuswap metamorphic core complex, Canadian Cor-dillera. Lithos, 61, 103–125.

Owen, J.V. & Greenough, J.D., 1991. An empirical sapphirine-spinel Mg-Fe exchange thermometer and its application tohigh grade xenoliths in the Popes Harbour dyke, Nova Scotia,Canada. Lithos, 26, 317–332.

Pan, Y. & Fleet, M.E., 1995. Geochemistry and origin ofcordierite-orthoamphibole gneiss and associated rocks at anArchean volcanogenic massive sulphide camp: Manito-uwadge, Ontario, Canada. Precambrian Research, 74,73–79.

Parrish, R.R., Carr, S.D. & Parkinson, D.L., 1988. Eoceneextensional tectonics and geochronology of the southernOmineca Belt, British Columbia and Washington. Tectonics,7, 181–212.

Peck, W.H. & Smith, M.S., 2005. Cordierite-gedrite rocks fromthe Central Metasedimentary Belt boundary thrust zone(Grenville Province, Ontario): Mesoproterozoic metavolcanicrocks with affinities to the Composite Arc Belt. CanadianJournal of Earth Science, 42, 1815–1828.

Powell, R. & Holland, T., 1999. Relating formulations of thethermodynamics of mineral solid solutions: activity modelingof pyroxenes, amphiboles and micas. American Mineralogist,84, 1–14.

Raith, M., Karmakar, S. & Brown, M., 1997. Ultra-high-temperature metamorphism and multistage decompressionalevolution of sapphirine granulites from the Palni HillRanges, southern India. Journal of Metamorphic Geology, 15,379–399.

Raith, M.M., Rakotondrazafy, R. & Sengupta, P., 2008.Petrology of corundum-spinel-sapphirine-anorthite rocks(sakenites) from the type locality in southern Madagascar.Journal of Metamorphic Geology, 26, 647–667.

Rey, P., Teyssier, C. & Whitney, D.L., 2009. Extension rates,crustal melting and core complex dynamics. Geology, 37, 391–394.

Robinson, P. & Jaffe, H.W., 1969. Aluminous enclaves ingedrite–cordierite gneiss from southwestern New Hampshire.American Journal of Science, 267, 389–421.

Robinson, P., Spear, F.S. & Schumacher, J.C., 1982. Phaserelations of metamorphic amphiboles: natural occurrence andtheory. In: Reviews in Mineralogy, Vol. 9B (eds Veblen, D. &Ribbe, P.), pp. 1–211. Mineralogical Society of America,Washington, D.C.

Sato, K., Miyamoto, T. & Kawasaki, T., 2006. Experimen-tal calibration of sapphirine-spinel Fe2+-Mg exchange

4 48 S . C . K R U C K E N B E R G & D. L . W H I T N E Y

� 2011 Blackwell Publishing Ltd

thermometer: implications for constraints on P-T condition ofHoward Hills, Napier Complex, East Antarctica. GondwanaResearch, 9, 398–408.

Schneiderman, J. & Tracy, R.J., 1991. Petrology of orthoam-phibole-cordierite gneisses from the Orijarvi area, southwestFinland. American Mineralogist, 76, 942–955.

Schumacher, J.C., 1988. Stratigraphy and geochemistry of theAmmonoosuc Volcanics, central Massachusetts and south-western New Hampshire. American Journal of Science, 288,619–663.

Schumacher, J.C. & Robinson, P., 1987. Mineral chemistry andmetasomatic growth of aluminous enclaves in gedrite–cordierite gneiss from southwestern New Hampshire, USA.Journal of Petrology, 28, 1033–1073.

Shimpo, M., Tsunogae, T. & Santosh, M., 2006. Firstreport of garnet-corundum rocks from southern India:implications for prograde high-pressure (eclogite-facies?)metamorphism. Earth and Planetary Science Letters, 242,111–129.

Snook, J.R., 1965. Metamorphic and structural history of‘‘Colville batholith’’ gneisses, north central Washington.Geological Society of America Bulletin, 76, 759–776.

Tajcmanova, L., Konopasek, J. & Connolly, J.A.D., 2007.Diffusion-controlled development of silica-undersaturateddomains in felsic granuliates of the Bohemian Massif (Vari-scan belt of Central Europe). Contributions to Mineralogy andPetrology, 153, 237–250.

Tajcmanova, L., Connolly, J.A.D. & Cesare, B., 2009. A ther-modynamic model for titanium and ferric iron solution inbiotite. Journal of Metamorphic Geology, 27, 153–165.

Tempelman-Kluit, D.J. & Parkinson, D., 1986. Extension acrossthe Eocene Okanagan crustal shear in southern BritishColumbia. Geology, 14, 318–321.

Teyssier, C. & Whitney, D.L., 2002. Gneiss domes and orogeny.Geology, 30, 1139–1142.

Tilley, C.E., 1937. Anthophyllite-cordierite granulites of theLizard. Geological Magazine, 74, 300–309.

Tinkham, D.K., Zuluaga, C.A. & Stowell, H.H., 2001. Metap-elite phase equilibria modeling in MnNCKFMASH: the effectof variable Al2O3 and MgO ⁄ (MgO + FeO) on mineral sta-bility. Geological Materials Research, 3, 1–42.

Vallance, T.G., 1967. Mafic rock alteration and isochemicaldevelopment of some cordierite–anthophyllite rocks. Journalof Petrology, 8, 84–96.

Vanderhaeghe, O., Teyssier, C. & Wysoczanski, R., 1999.Structural and geochronological constraints on the role ofpartial melting during the formation of the Shuswap meta-morphic core complex at the latitude of the Thor-Odin Dome,British Columbia. Canadian Journal of Earth Sciences, 36,917–943.

Vanderhaeghe, O., Teyssier, C., McDougall, I. & Dunlap, W.J.,2003. Cooling and exhumation of the Shuswap metamorphiccore complex constrained by 40Ar-39Ar thermochronology.Geological Society of America Bulletin, 115, 200–216.

Visser, D. & Senior, A., 1990. Aluminous reaction textures inorthoamphibole-bearing rocks: the pressure-temperature evo-lution of the high-grade Proterozoic of the Bamble Sector,south Norway. Journal of Metamorphic Geology, 8, 231–246.

White, R.W., Powell, R., Holland, T.J.B. & Worley, B.A., 2000.The effect of TiO2 and Fe2O3 on metapelitic assemblages atgreenschist and amphibolite facies conditions: mineralequilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology, 18,497–511.

White, R.W., Powell, R. & Clarke, G.L., 2002. The interpreta-tion of reaction textures in Fe-rich metapelitic granulites of theMusgrave Block, central Australia: constraints from mineralequilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology, 20,41–55.

White, R.W., Powell, R. & Holland, T.J.B., 2007. Progressrelating to calculation of partial melting equilibria formetapelites. Journal of Metamorphic Geology, 25, 511–527.

White, R.W., Powell, R. & Baldwin, J.A., 2008. Calculatedphase equilibria involving chemical potentials to investigatethe textural evolution of metamorphic rocks. Journal ofMetamorphic Geology, 26, 181–198.

Whitney, D.L. & Evans, B.W., 2010. Abbreviations for names ofrock-forming minerals. American Mineralogist, 95, 185–187.

Received 28 April 2010; revision accepted 3 December 2010.

MET A MO RP HI C E V O L UT I O N O F MG - A L -R I C H G N E I S S I N TH E O K A N O G AN D O M E 44 9

� 2011 Blackwell Publishing Ltd