ELSEVIER Tectonophysics 312 (1999) 35–55www.elsevier.com/locate/tecto

Pervasive melt migration from migmatites to leucogranite in theShuswap metamorphic core complex, Canada:

control of regional deformation

Olivier Vanderhaeghe *

Earth Sciences and Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada

Received 15 October 1998; accepted 17 March 1999

Abstract

The Shuswap metamorphic core complex, exhumed in the hinterland of the Canadian Cordillera, displays a ¾15 kmstructural section from migmatites to leucogranite. It comprises a lower unit dominated by anatectic migmatites exposed inthe core of domes. At higher structural level the transition from a middle amphibolite-facies unit to an upper crustal unit ismarked by a low-angle detachment associated with the emplacement of laccoliths of leucogranite. The amphibolite-faciesmiddle unit is permeated by a network of granitic veins feeding the laccoliths and structurally connected to the migmatitesof the lower unit. This paper describes the geometric, textural and structural characteristics, and the distribution of thegranitic fraction at the outcrop scale in the amphibolite-facies unit separating the migmatites from the leucogranite. Thisanalysis constrains the mechanisms of melt migration within a plurikilometer-scale section across high-grade rocks of themiddle crust. In fertile lithologies (metapelites, felsic amphibolites), the granitic fraction, in part generated in situ, formsa diffuse network of concordant veins feeding discordant veins, and structural sites such as the foliation, shear zones,and boudin necks. Discordant granitic veins are oriented perpendicular, and to a lesser extent parallel, to the mineral andstretching lineation. In refractory lithologies, the granitic fraction is dominantly intrusive and displays sharp contactswith the host rock. However, distribution of the granitic fraction shows the same characteristics as for fertile lithologies.The melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as thefoliation plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations. Thesegeometric and textural characteristics suggest that granite migration in these rocks is achieved dominantly by viscousflow of the melt (C=� solid) through the solid matrix driven by its buoyancy and controlled by mechanical anisotropyof the rock and local dilation created by heterogeneous deformation. The relationship between the regional fabric andthe distribution of the granitic fraction indicates that regional deformation (incremental and finite) played a major rolein providing pathways for melt migration. The formation of laccoliths of leucogranite at higher structural levels suggeststhat upward melt migration led to accumulation of granitic magmas. These observations are consistent with a model ofpervasive melt migration through a network of interconnected concordant and discordant granitic veins feeding laccolithicplutons extracted from the dominantly solid matrix during vertical thinning and compaction at a regional scale. Thepresence of both transposed and intact granitic networks indicate an overlap between deformation and granite migrationas a continuum or in pulses?). The methodology and terminology proposed in this paper provide some insights on the

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36 O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55

behavior of the granitic fraction in the middle crust. This approach should be applicable to other high-grade terrainsexhumed in the cores of orogenic belts. 1999 Elsevier Science B.V. All rights reserved.

Keywords: migmatites; granites; partial melting; structural analyses; Canadian Cordillera; Shuswap metamorphic corecomplex

1. Introduction

Two centuries ago, James Hutton, visiting theCaledonides in Scotland, described granite as “hav-ing been forced to flow, in a state of fusion, amongstrata broken by a subterraneous force, and distortedin every manner and degree” (Hutton, 1794). Sincethis statement, the study of migmatites and gran-ites has evolved into two distinct areas of research.Granites are within the scope of igneous petrologistswhereas migmatites are reserved to metamorphicpetrologists. This distinction partly reflects the iso-lation of most granitic plutons from their sourcerock, but is also related to the difficulties that struc-tural geologist have had in developing tools whichwould improve our understanding of the behavior ofgranitic magmas within the crust.

Understanding mechanisms of granitic magmamigration or ascent from the source, where themelt fraction is generated and segregated, to theemplacement level, where the melt is collected, re-quires an integrated approach at the crustal scale(Brown, 1994). Unfortunately, only a few exam-ples of crustal-scale sections display at the sametime the potential source and the emplacement levelof granitic magmas (Read, 1957; Wickham, 1987;Vanderhaeghe and Teyssier, 1997; Sawyer, 1999).Analysis of migmatites shows that migration of thelow-viscosity=low-density melt fraction is driven bypressure gradients resulting from an interplay be-tween regional gravity field and local deformation(Wickham, 1987; Burg and Vanderhaeghe, 1993;Sawyer, 1994; Brown and Rushmer, 1997). Sev-eral mechanisms of melt migration through the crusthave been proposed: (1) percolation which invokesmigration of the melt fraction through a networkof connected pores during deformation or com-paction of the solid matrix (McKenzie, 1984); (2)diapirism which corresponds to the development ofa gravitational instability with upward motion ofthe low-density magma accommodated by continu-

ous deformation of its surrounding (Biot and Ode,1965; Ramberg, 1981; Weinberg and Podladchikov,1994); (3) diking which describes the formation ofgranitic veins by fracturing of the host rock (Listerand Kerr, 1991; Clemens and Mawer, 1992; Rubin,1993; Petford, 1995).

These mechanisms are not mutually exclusive andeach of them is efficient at different levels of the crustand at various scales: porous flow and percolation ofthe melt fraction during compaction of the solid ma-trix is most efficient at the grain scale in the source ofgranitic magmas (Wickham, 1987); diapirism is pos-sible in a relatively high-temperature environmentand when granitic magma reaches a critical volume(Weinberg, 1996); and diking is favored in rela-tively low-temperature environments when the fluidpressure of the confined melt phase exceeds the ten-sile strength of the host rock (Clemens and Mawer,1992; Burg and Vanderhaeghe, 1993; Rubin, 1993;Petford, 1995; Weinberg, 1996). The orientation ofdikes is thus controlled by the orientation of the leastcompressive stress. Geologic studies in various tec-tonic settings indicate that whatever the mechanismof melt migration (percolation, diapirism, or fractur-ing), segregation, ascent and emplacement of graniticmagmas is primarily controlled by regional deforma-tion (Burg et al., 1984; Hutton, 1988; D’Lemos et al.,1992; Tikoff and Teyssier, 1992; Burg and Vander-haeghe, 1993; Vigneresse, 1995; Collins and Sawyer,1996; Brown and Solar, 1998).

The aim of this paper is to present new data fromthe Shuswap Metamorphic core complex (ShuswapMCC), exhumed in the hinterland of the CanadianCordillera, illustrating the ‘missing link’ betweenmigmatites and leucogranites. The Shuswap MCCdisplays a cross-section from anatectic migmatites,appearing in the core of domes, to laccolithicleucogranites emplaced below a low-angle detach-ment. The focus of this contribution is to documentthe relationship between regional strain field and for-mation of a network of granitic veins in the unit

O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55 37

in between the migmatites and the leucogranite. Inlight of the data presented in this study, I propose amodel of pervasive melt migration from the potentialmigmatitic source to laccolithic plutons through aninterconnected network of granitic veins permeatingthe solid-dominated host rock during regional defor-mation. By describing the distribution of the graniticfraction in the Shuswap MCC, this paper also definesa methodology for the description of the geometriccharacteristics of granitic veins in high-grade ter-rains.

2. Geologic setting: the Shuswap MCC in theCanadian Cordillera

2.1. Tectonic evolution of the Canadian Cordillera

The formation of the Canadian Cordillera is theresult of Mesozoic accretion and collision of mag-matic arcs to the western edge of the North Amer-ican craton (Monger et al., 1982). Eastward thrust-ing of allochthonous terranes over the sedimentarysequences accumulated on the North American pa-leomargin (Brown et al., 1986; Price, 1986) resultedin the formation of a 50–60-km-thick crustal welt(Coney and Harms, 1984; Parrish et al., 1988; John-son and Brown, 1996; Vanderhaeghe et al., 1999a)and the development of a flexural foreland basinat the front of the Rocky Mountain fold-and-thrustbelt (Price and Mountjoy, 1970). Crustal thicken-ing and burial of the paleomargin sedimentary se-quences was associated with widespread high-tem-perature metamorphism and crustal anatexis withpressures ranging from 4 to 7 kbar and tempera-tures from 620 to 820ºC (Reesor and Moore, 1971;Ghent et al., 1977; Duncan, 1984; Journeay, 1986;Lane et al., 1989; Sevigny et al., 1989; Nyman etal., 1995). Crustal thickening was followed by ex-humation of high-grade rocks in the hinterland ofthe Cordillera associated with the formation of theShuswap MCC (Fig. 1) (Reesor, 1970; Crittendenet al., 1980; Brown and Read, 1983). Recent tec-tonic models attribute exhumation to a period ofearly Tertiary regional extension (Tempelman-Kluitand Parkinson, 1986; Parrish et al., 1988; Bardouxand Mareschal, 1994; Johnson and Brown, 1996;Vanderhaeghe and Teyssier, 1997; Vanderhaeghe et

al., 1999a) with the activation of an array of normalfaults linked by strike-slip faults at the scale of thesouthern Canadian Cordillera (Ewing, 1981; Struik,1993) (Fig. 1).

2.2. The Shuswap metamorphic core complex

At the latitude of the TransCanada Highway 1(TCH1) the Shuswap MCC displays a¾15-km struc-tural section from migmatites to leucogranites in thecollapsed hinterland of the Canadian Cordillera. Itconsists of three superposed units (Figs. 2 and 3),identified on the basis of their contrasted structuralevolution during the formation of the metamorphiccore complex (Vanderhaeghe and Teyssier, 1997).A low-angle detachment zone (Figs. 2 and 3) sep-arates remnants of a dismembered upper unit fromthe exhumed metamorphic core (Read and Brown,1981; Lane, 1984; Tempelman-Kluit and Parkinson,1986; Johnson and Brown, 1996; Vanderhaeghe andTeyssier, 1997). Above the detachment, tilted blocksare associated with pull-apart and extensional basinsfilled by Eocene sedimentary and volcanic sequences(Church, 1981; Mathews, 1981). The low-angle de-tachment zone is localized at the top of a graniticlaccolith (Carr, 1992; Vanderhaeghe and Teyssier,1997). Below the detachment zone, exhumed high-grade rocks are characterized by a composite foli-ation reflecting transposition of original bedding ormagmatic layering of the various lithologies dur-ing a complex history from crustal accretion to ex-humation. The middle unit is composed of a domi-nantly metasedimentary amphibolite-facies sequenceintruded by leucogranites and pegmatites (Brownand Journeay, 1987; Carr, 1992; Johnson and Brown,1996; Johnston, 1998). Fertile lithologies of the mid-dle unit show evidences of incipient partial meltingsuch as leucosomes rimmed by melanosomes. Thelower unit consists of anatectic migmatites whichappear in the cores of dome-shaped culminations.It comprises, following the definitions of Mehn-ert (1968), a core of diatexites, or heterogeneousmigmatites, surrounded by layered metatexites, orstromatic migmatites. As reported by Vanderhaegheand Teyssier (1997) and further developed in thispaper, the laccoliths of leucogranite emplaced be-low the detachment zone are structurally connectedto the migmatites of the lower unit through a net-

38 O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55

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Fig. 1. Geology of the southern Canadian Cordillera. (a) Schematic geologic map modified after Wheeler and McFeely (1991). (b)Schematic cross-section indicated on the map by line 1b.

O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55 39

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Fig. 3. Schematic geologic cross-section along the Trans Canada Highway 1. Location indicated in Fig. 2. The small letters indicate the approximate location of the picturesof Fig. 5. Note that the trace of the major foliation plane is extrapolated from the information gained at the outcrop scale. The major foliation is axial planar to large-scalefolds and is warped by gentle upright undulations. The network of granitic veins is represented schematically.

O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55 41

work of granitic veins permeating the middle unit.Accordingly, the anatectic migmatites of the lowerunit represent a potential source for the leucograniteemplaced at a higher structural level, in addition tothe melt generated in situ within the middle unit.

2.3. Geochronologic constraints

In the area investigated in this study, leucogranitesyield U–Pb zircon and monazite ages ranging from¾60 Ma to ¾55 Ma (Parrish et al., 1988; Carr, 1991;Carr, 1992; Parkinson, 1992; Vanderhaeghe, 1997)and pegmatitic dikes yield ages as young as 49 Ma(Parkinson, 1992). These ages are similar to thoseobtained to the south in the Bitteroot Complex inIdaho (Foster and Fanning, 1997), but to the northof the Shuswap MCC, north of Frenchman’s Cap,similar leucogranites yield a wider range of agesfrom ¾135 Ma to 57 Ma (Sevigny et al., 1989;Scammell, 1993).

U–Pb dating of zircon from migmatites of theMonashee Complex yields discordant Proterozoicages with lower intercepts ranging from late Meso-zoic to early Tertiary (Wanless and Reesor, 1975;Armstrong et al., 1991; Parkinson, 1991; Crowley,1997). These results were interpreted to representProterozoic migmatization followed by Mesozoic toTertiary partial resetting due to lead loss or growth ofmetamorphic rims (Wanless and Reesor, 1975; Arm-strong et al., 1991; Parkinson, 1991; Crowley, 1997).In order to decipher the history of these complexlyzoned zircons, U–Pb dating of similar migmatites inthe Thor–Odin dome has been conducted using theSHRIMP facility. The analysis yields the same rangeof Proterozoic ages in the core of zircons. However,much younger ages (¾56 Ma) are obtained on thinU-rich euhedral magmatic rims grown around theolder cores (Vanderhaeghe, 1997; Vanderhaeghe etal., 1999b). These results are interpreted to reflectearly Tertiary partial melting of a protolith contain-ing Proterozoic zircon. Early Tertiary ages are con-sistent with the lower intercepts of discordant agesobtained by previous authors on zircons suggestingthat the latter might have gone through a similar his-tory. Furthermore, these ages are consistent with the60 Ma to 55 Ma U–Pb ages obtained on leucogran-ites emplaced within the middle unit (Carr, 1991,1992; Parkinson, 1992; Vanderhaeghe et al., 1999b).

Argon thermochronology performed in the stud-ied area on rocks from the middle and lower unitsshows that cooling through the hornblende closuretemperature (¾500ºC) occurred sometime between60 and 55 Ma, although scattering and irregularitiesin the 40Ar=39Ar age spectra obtained on hornblendeindicate a component of excess argon, and coolingthrough the white mica and biotite closure temper-atures (respectively ¾350ºC and ¾300ºC) occurredfrom 50 to 48 Ma (Parrish et al., 1988; Johnson,1994; Vanderhaeghe, 1997). As a consequence, peg-matites intruding the middle unit and yielding agesyounger than 55 Ma (Parkinson, 1992) had to begenerated at a crustal level below the current levelof exposure. In turn, the thermal history decipheredfrom argon thermochronology is consistent with gen-eration of the leucogranitic bodies yielding ages inthe range 60–55 Ma within the middle or the lowerunit of the Shuswap MCC.

3. Characteristics of the granitic vein network inthe central Shuswap MCC

Several papers describe the structural characteris-tics of the leucogranite laccoliths and the migmatitesin the Shuswap MCC (Reesor and Moore, 1971;Carr, 1992; Vanderhaeghe and Teyssier, 1997; Van-derhaeghe et al., 1999b). The goal of this contribu-tion is to build on this early work and focus on thedistribution of the granitic fraction with respect to thefabric within the middle unit of the Shuswap MCC,comprised in between the migmatitic domes of thelower unit and the leucogranite laccoliths emplacedbelow the detachment zone. This study is limited toa section across the Shuswap MCC along the TCH1between Revelstoke and Sicamous (Figs. 2 and 3).

3.1. Distribution of granites in the middle unit of theShuswap MCC at the latitude of the TCH1

The presence of granitic veins is ubiquitousthroughout the middle unit of the Shuswap MCCbelow the detachment zone as exposed along theTCH1 section (Fig. 3). The distribution, proportionand geometric characteristic of the granitic fractionare strongly controlled by the fertility and physicalproperties of the host rock, in other words, by the

42 O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55

lithologic composition and the structural level. Thegranitic fraction occurs in various sizes and shapesbut mostly as veins or sheet-like bodies (Fig. 4). Ittypically represents about 10–20% of the outcropand may reach about 30 to 40% without total loss ofthe host rock’s coherency.

Most outcrops display a network of veins con-cordant and discordant with respect to the foliationof the host rock, with the exception of two struc-tural levels where veins are dominantly transposed inthe mylonitic foliation and show a fabric reflectingsolid-state deformation. The first of these structurallevels corresponds to the top of the middle unit,marked by ductile to brittle mylonitic zones char-acterizing detachments at the western and easternborder of the Shuswap MCC (respectively the EagleRiver–Okanagan and Columbia River detachments;Figs. 2 and 3). The detachment zone is localized atthe top of a granitic laccolith which displays a strongmylonitic fabric developed from subsolidus to green-schist facies conditions (Carr, 1992; Vanderhaegheand Teyssier, 1999). U–Pb dating of the leucogran-ite, argon thermochronology and geologic argumentsconstrain the timing of deformation within the de-tachment zone between ¾60 and ¾50 Ma (Parkin-son, 1992; Johnson and Brown, 1996; Vanderhaeghe,1997). The second structural level which displaysdominantly transposed granitic veins is a myloniticzone referred to as the Monashee decollement byprevious authors (Brown and Journeay, 1987), whichis located within the middle unit and connects to theColumbia River detachment to the east (Fig. 3). TheMonashee decollement is localized at the top of aquartzite unit in the fertile metapelite investigated byNyman et al. (1995) from which a large amount ofgranitic magma was produced. U–Pb dating of peg-matites cross-cutting, or boudinaged in, the foliationindicates that the Monashee decollement was activeuntil ¾57 Ma (Carr, 1992).

Fig. 4. Field pictures illustrating the structural and geometric relationships between granitic veins and the fabric of the host rock. Mostpictures are from a layer of hornblende–granodiorite except (g), (i) and (j), which are from paragneisses. Approximate locations ofpictures are indicated by the corresponding letters on the cross-section of Fig. 3. (a) View perpendicular to the foliation plane showingdiffuse patches and more focused granitic veins in a granodiorite. Pegmatitic layers contained in the foliation are boudinaged. (b) Viewperpendicular to the foliation displaying diffuse patches and a connected network of diffuse concordant and discordant granitic veins ina granodiorite. (a), (b) and (c) indicate that the diffuse granitic veins form a 3-dimensional interconnected network. (c) View parallel tothe foliation plane showing a connected network of granitic veins in a granodiorite. (d) View perpendicular to the foliation plane. Theconcordant veins are in continuity with the discordant ones and both preserve a magmatic texture.

A recurring problem in the Shuswap MCC andin high-grade terrains in general, is to draw the linebetween a ‘host rock intruded by granitic veins’and a ‘granite containing enclaves’ (Brown et al.,1981). In the middle unit of the Shuswap MCC,increasing proportion of granitic fraction is associ-ated with a larger number of intrusive contacts andwider discordant veins. The transition from contin-uous host rock containing a network of veins todisrupted blocks or enclaves floating in a granite oc-curs for approximately 40–50% of granite. Enclavesof refractory lithologies such as calc-silicate, maficamphibolite, and quartzite are angular and preservean internal fabric typically discordant to the one ofthe surrounding granite. Deflection of the magmaticfabric around the enclaves and localization of un-deformed pegmatite into pressure shadows suggestthat the enclaves behaved as rigid blocks rotatingin the melt fraction during flow of the magma. Incontrast, enclaves of migmatitic gneisses have dif-fuse boundaries and display complex internal fabricsuch as disharmonic folds, and show superimposedstructures not observed in the granitic host. The mag-matic fabric of the granitic matrix is relatively wellorganized except in the vicinity of these enclaveswhere it displays complex patterns. With increasingstrain, as usually observed at higher structural leveltoward the detachment zone, the internal structure ofthe enclave tends to be less discordant to the onein the surrounding granite. These features suggestthat enclaves of migmatitic gneisses deform and arenot rigid, but nevertheless cause local perturbationduring flow of the magma. It is also important todetermine whether all the granite was present as apartially molten phase at once or if the granite in-truded as successive pulses. Rocks that contain acontinuous granitic phase in excess of 40–50% andin which the residual or refractory gneiss is disruptedare mapped as granite.

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3.2. Distribution and geometric characteristics ofthe granitic veins

In the following descriptions, various types ofveins are distinguished based on their geometric andstructural relationships with respect to the fabric ofthe host rock (Figs. 4 and 5). The studied area isdominated by fertile protoliths, such as metapelitesand intermediate amphibolites, in which a signif-icant part of the granitic fraction has been gen-erated in situ. The local origin of granitic veins,or leucosomes, is suggested by the presence of arim of coarse refractory mafic minerals (biotite inmetapelites, hornblende in intermediate amphibo-lites), and, in favorable cases, by the identificationof a mineral assemblage consistent with a partialmelting reaction. For metapelites, inferred meltingreactions are (Nyman et al., 1995):

MsC Qtz D SilC KfsCMelt

BiC SilC Qtz D GrtCMelt

Structurally above a thick package dominated bymetapelites, the middle unit of the Shuswap MCCat the latitude of the TCH1 contains a layer, severalhundred meters thick, of hornblende–granodiorite(Hbl, Pl, š Bi, š Gt, š Qtz) (Figs. 2 and 3). Thepresence of leucosomes with a tonalitic composition(Pl, Hbl, š Qtz) in the granodiorite suggests meltingpossibly involving the breakdown of hornblende.

In these fertile protoliths, the granite=host-rockcontacts are relatively diffuse. In homogeneous pro-toliths, leucosomes locally form patches, but morecommonly, the granitic fraction appears as a net-work of veins dominantly concordant and locallydiscordant with respect to the foliation of the rock(Fig. 4a,b). In the foliation plane, the granitic frac-tion also forms an interconnected network of conju-gate veins (Fig. 4c). A striking feature at the outcrop

Fig. 4 (continued). (e) Detail of connected concordant and discordant diffuse veins with hornblende phenocrysts embedded in tonaliticgranitoid. Veins are in textural continuity. (f) Detail of a diffuse granitic vein in a migmatitic paragneiss. The concordant veins arein textural continuity with the discordant ones. At the intersection of the concordant and discordant veins, the alignment of biotitessuggests a drag related to flow of the magma. (g) Localization of granite in boudin necks and shear zones. (h) Sills and dikes forming acontinuous network. The contacts of the granitic veins are sharp and cross-cut the fabric of the host rock suggesting that sills and dikesformed by fracturing of the host rock. (i) View perpendicular to the foliation plane with concordant veins in continuity with deformeddiscordant veins interpreted to be progressively transposed into the foliation plane. (j) View perpendicular to the foliation plane showinga discordant vein stretched and boudinaged.

scale is the textural continuity of the granitic materialfrom diffuse patches and=or concordant veins to dis-cordant veins (Fig. 4d,e). At the intersection betweenleucosomes and discordant veins the alignment ofmafic minerals, from an orientation parallel to thefoliation to an orientation parallel to the vein contact,suggests that these minerals were floating in a meltand were reoriented according to the flow of themagma (Fig. 4f). The cross-cutting veins thus appearto be fed by in-situ leucosomes or concordant veins(Fig. 4d–f). Assuming that the melt migrates prefer-entially upward, this latter structure corresponds toa new synmigmatitic way-up criterion to be addedto the structures described by Burg (1991) and Burgand Vanderhaeghe (1993).

The preferential alignment of granitic materialalong the foliation plane suggests a control either ofstrain or mechanical anisotropy on the distributionof the melt fraction. Such veins are defined as con-cordant diffuse veins (Fig. 5). For protoliths with aninherited compositional layering, concordant diffuseveins, also referred to as leucosomes, are superim-posed on the compositional layering. The relativefertility of the layers appears to control the abun-dance of leucosomes at the scale of the outcrop. Asingle compositionally distinct layer contains typi-cally several concordant diffuse veins. Their thick-ness ranges from a few centimeters to a meter, andthicker veins tend to be pegmatitic. Adjacent concor-dant veins are separated by a distance on the orderof 10 cm to a meter. Concordant diffuse veins, andpreferentially pegmatites, are often boudinaged.

Most discordant granitic veins, in relatively in-competent fertile protoliths, have rather diffuseboundaries at the grain scale, although they areclearly localized in discontinuities. These are re-ferred to as discordant diffuse veins (Fig. 5). Inparticular, granitic veins are typically localized inshear zones and boudin necks (Fig. 4g). This is in-

46 O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55

Sills and dikes

Heterogeneous ductile matrix

Concordant and discordant diffuse veins

Boudin necks and shear zones

Viscous flow of melt (+/- crystals) in fractures

Viscous flow of melt (+/- crystals)in ductilely deformed matrix

Homogeneous ductile matrix

Types of granitic vein networks

Diffuse concordant and discordantgranitic veins

Sharp, cross-cuttinggranitic veins

Brittle matrix

Fig. 5. Granitic vein networks: geometric, structural and textural characteristics. Granitic veins are either concordant or discordant withrespect to the fabric of the host rock, and sharp or diffuse. Sharp cross-cutting veins, i.e sills and dikes, suggest that they formed as aresult of fracturing of the host rock at the grain scale. Diffuse veins suggest that melt migration operated by porous flow during ductiledeformation of the host rock. Heterogeneous deformation of the host rock and dilation created in structural sites as shear zones or necksbetween boudins create preferential pathways for focused melt migration. Whether it is in sharp cross-cutting or diffuse veins, migrationof melt (š crystals) operates by viscous flow.

terpreted to represent mainly migration of the meltfraction into dilatant sites created during heteroge-neous deformation, although, in some instances, it ispossible that melting occurred preferentially in thesesites because of local decompression (see Brownand Rushmer, 1997). Melt migration primarily con-trolled by heterogeneous deformation is defined asheterogeneous viscous flow (Fig. 5).

Localization of the granitic veins in discordantdiffuse veins is not only restricted to rocks affectedby heterogeneous deformation, but also occurs inhomogeneous protoliths lacking dilatant sites relatedto shear localization or boudinage of competent lay-ers (Fig. 4b,d,e). In this case, the formation of apervasive network of concordant and discordant dif-fuse veins is referred to as homogeneous viscousflow (Fig. 5). On a given outcrop, discordant dif-fuse veins are usually less abundant than concordantones. Discordant veins range in width from a fewcentimeters to a few meters. Most discordant veinsare continuous at the scale of the outcrop and areat least meters to tens of meters long. Spacing ofdiscordant diffuse veins is not as well organized asalternations of concordant leucosomes, especially inmechanically layered protoliths affected by hetero-

geneous deformation. In homogeneous intermediateamphibolites or hornblende–granodiorite the typicalspacing between discordant veins is on the order of10 cm to a meter.

Refractory lithologies (massive mafic amphibo-lite, quartzite, marble, and calc-silicate) contain fewor no in-situ leucosomes and display a granitic frac-tion which is dominantly intrusive. The intrusivenature of granitic veins is indicated by sharp con-tacts and a mineralogical composition inconsistentfor being a melt generated from the nearby hostrock. Granitic veins are either localized along the fo-liation plane or form dikes cross-cutting the foliation(Fig. 4h). Large dikes and sills contain angular frag-ments of the host rock. Dikes and sills also dominateat high structural level.

3.3. Structural characteristics, textures, and internalfabric of granitic veins

Granitic veins display a variety of textures reflect-ing the entire range from a purely magmatic fabricto solid-state deformation, recrystallization and evencataclasis at the level of the detachment zone. Non-boudinaged concordant veins and non-transposed

O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55 47

discordant veins display magmatic textures, whichare also preferentially preserved in boudin necks.The magmatic fabric is delineated by coarse in-terfingered crystals and by euhedral mafic minerals(mainly biotite and hornblende). The magmatic foli-ation, delineated by phenocrysts and mafic minerals,is typically parallel to the vein contacts (concordantor discordant to the host rock foliation dependingon the type of veins). Most of these discordant dif-fuse veins are oriented at a high angle with respectto the foliation which probably reflects their initialorientation at the time of formation (Fig. 4b,d,e). Inparticular, shear zones containing granitic materialwith a preserved magmatic fabric show an anglebetween the foliation and the shear zone between50º and 70º. Estimated offsets of markers indicatethat the relative displacement associated with theseshear zones is limited. Activation of shear zones isthus apparently efficient at driving melt migration,although localization of the melt fraction in shearzones does not seem to play a significant role inenhancing shearing and displacement. In fact, shearzones affecting the middle unit of the Shuswap MCCappear as conjugate sets which probably reflects thedominant coaxial regime not favorable to localizedlarge-scale simple shear.

Many discordant veins are folded with an axialplane parallel to the host rock foliation or boudi-naged in a direction parallel to the main stretchingdirection (Fig. 4i,j). These veins show an internalfabric with geometric characteristics similar to theone in the host rock. In this case the texture ofthe vein shows signs of overprinting of the magmatictexture during solid-state deformation (stretching andrecrystallization of quartz and feldspar, boudinage orbending of mafic minerals). These features suggestthat discordant veins are transient features progres-sively transposed during deformation implying thatdeformation and migration of the granitic melt over-lapped in time.

3.4. Orientation of granitic veins at the latitude ofthe TCH1

Measured orientations of the various types of dis-cordant granitic veins described above (shear zones,boudin necks, diffuse veins and dikes) are reportedfor seven locations across the Shuswap MCC along

the TCH1 (Fig. 2). The results are presented in stere-ograms for each location (each location encompassseveral outcrops). A synthesis of all measurementsis presented in Fig. 6, where all veins have beenrotated using the nearby lineation and foliation asa reference (lineation is rotated on an east–west di-rection and the foliation in a horizontal position).The accuracy of the measurements is affected byseveral factors. First, veins are not regular structuresand their measurement requires a good three-dimen-sional observation which usually reveals that theirorientation varies at the outcrop scale. Second, thefoliation and lineation, used as a reference for therotation, also vary at the outcrop scale. Third, thesynthetic plot contains a number of veins deformedunder solid state which might have been reorientedduring transposition.

Results indicate that discordant veins are not ran-domly oriented. Since the studied area is dominatedby fertile lithologies, discordant veins are domi-nantly of the diffuse type. Discordant diffuse veinsare mostly oriented at right angles with respect tothe foliation plane (Figs. 2 and 6). Discordant diffuseveins that are neither oriented at a right angle tothe foliation plane nor localized in shear zones, arefolded or boudinaged. When all types of veins areconsidered, more than 25% of the discordant veinsare striking within 20º from the direction perpendic-ular to the mineral and stretching lineation (Figs. 2and 6). In addition to the predominant orientationperpendicular to the lineation, discordant veins showa relative concentration in a direction parallel to thelineation, perpendicular to the foliation and about10% of the veins strike within 20º from the lineationdirection (Figs. 2 and 6).

In particular, dikes are oriented either perpendicu-lar or parallel to the stretching direction of the finitestrain ellipsoid estimated in the host rock, hence theorientation of dikes is not random with respect tothe regional ductile fabric. This suggests that theincremental strain ellipsoid or stress tensor appliedto the host rock at the time of diking is relatedto the finite strain ellipsoid. In other words, at thetime of fracturing, principal axes of the stress el-lipsoid were contained in the foliation plane. Thetwo maxima for the orientation of dikes indicatethat the least compressive and the intermediate stressaxes switched orientation within the foliation plane

48 O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55

N

Equal Area(Schmidt)

N = 78

Discordant veinDikeShear zoneBoudin neck

2 %4 %6 %8 %

a b

c

10%

10%

20%

20%

Poles of granitic veins:

lineation

pole to foliation

lineation

Strike of granitic veins:

d

Granitic veins and strain ellipsoid

Structural characteristics of granitic veins

Fig. 6. Summary of geometric and structural relationships between granitic veins and regional fabric. (a) Stereonet representing allmeasured granitic veins after rotation of the foliation plane into the horizontal plane and the lineation into the east–west direction. (b)Rose plot reporting the strike of the same veins represented in the stereonet. The radius of the circle corresponds to 30% of the data andthe measurements are grouped for intervals of 20º. (c) Block-diagram illustrating the relationship between the preferential orientation ofgranitic veins and the finite strain ellipsoid. (d) Schematic representation of the various types of veins observed in this area.

O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55 49

with respect to the finite stretching direction duringdeformation. Similarly, the orientation of discordantveins is related to the finite strain ellipsoid and thusprobably controlled by deformation. However, theweak maximum perpendicular to the lineation andperpendicular to the foliation suggests that the areainvestigated recorded strain in the flattening field(Flinn, 1965).

4. Model of pervasive melt migration duringregional deformation

4.1. Mechanisms and factors controlling meltmigration

Before discussing the controlling factors andmechanisms of melt migration inferred from theseobservations, it is important to distinguish melt seg-regation at the grain scale to melt migration beyondthe grains. Melt generation probably occurs at thegrain scale (Mehnert et al., 1973). However, natu-ral rocks rarely preserve any textural evidence ofmelt generation and the overwhelming occurrence ofgranite as a distinct phase at the scale of the outcropreflects the efficiency of melt segregation at the grainscale in the formation of migmatites.

The granitic fraction permeating the middle unitof the Shuswap MCC, at the latitude of the TCH1,occurs as a network of (1) dikes and sills, or (2)diffuse concordant and discordant veins. The char-acteristics of the granitic network depend on thestructural level but are also a function of the lithol-ogy of the host rock which controls its ability to meltand its physical properties. The variability of the ge-ometric and textural characteristics of this networkreflects the nature of the key parameters controllingmelt migration. The two types of networks point outtwo distinct types of behavior of the host rock duringmelt migration.

The textural characteristics described above forthe network of diffuse concordant and discordantveins suggest that their formation did not requirefracturing of the host rock at the grain scale andrather indicate viscous flow of the melt fraction (šsolid) through the solid deforming matrix. The localformation of diffuse patches of granite in homo-geneous protolith probably reflects initial formation

of melt pools in a relatively isotropic environment.Concentration of granitic material along the folia-tion of the host rock indicates a strong control ofphysical anisotropy and=or stress on melt migration.In mechanically layered rocks, concentration of themelt fraction in veins concordant to the foliation islikely due to the combination of several processesincluding (1) movement of melt guided by gradientsin the effective stress in a rheologically layered rock(Robin, 1979; Van der Molen, 1985a,b), and (2) per-colation of the melt fraction during compaction ofa rock presenting permeability gradients (McKenzie,1984). The theoretical analysis of Stevenson (1989)predicts that melt migration occurs preferentially inthe direction of the least compressive stress. Thiscould explain the tendency of segregation of the meltfraction in veins concordant to the foliation, evenin lithologically homogeneous rocks. These mech-anisms are most efficient close to the melt sourcewhere the host rock is near solidus temperature andare proposed to account for segregation and migra-tion of the melt fraction at a centimeter to hectome-ter scale (McKenzie, 1984; Wickham, 1987; Brown,1994). In addition, it is likely that segregation ofgranitic veins in the foliation reflects migration ofthe melt along mechanically weak planes. Thesemechanisms account for the formation of concordantveins.

Focusing of the granitic fraction into discordantveins cannot be explained by the aforementionedmechanisms which require that the melt migratesacross mechanical barriers such as the foliation planeor competent impermeable layers. In a rock affectedby heterogeneous deformation, boudinage of a me-chanical layering or shear localization, the creationof dilatant sites provides pathways allowing melt mi-gration across these mechanical barriers. However,discordant veins are not restricted to mechanicallylayered protoliths and occur also in homogeneousprotoliths. Textural continuity of the melt fractionfrom diffuse patches and=or veins concordant to thefoliation to discordant veins suggests a transitionfrom grain-scale pervasive porous flow to focusedviscous flow of the melt (š solid) through conduits.The geometric characteristics of the conduits (pla-nar veins) and their orientation (perpendicular to thestretching direction) appear to be controlled by theregional strain pattern. However, in this case, the

50 O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55

mechanical homogeneity of the host rock does notfavor a control of heterogeneous deformation on theformation of these discordant diffuse veins. Alter-natively, focused viscous flow and the formation ofdiscordant veins fed by concordant veins are likelydue to the development of gravitational instabilitiesrelated to the buoyancy of the melt fraction. Theformation of discordant veins by this mechanismimplies that the melt fraction accumulates until itreaches a critical mass to respond to the buoyancyforce and migrate down the pressure gradient.

In contrast, the formation of dikes most likelyinvolves fracturing as indicated by the sharp cross-cutting contacts with the host rock and by the pres-ence of angular fragments disrupted in the veins.Associated sills propagate along the foliation plane.This suggests that fracture propagation is primarilycontrolled by the stress ellipsoid (equivalent to theinstantaneous strain) and by the tensile strength ofthe rock, the foliation probably representing a weakplane.

These observations indicate that the driving forcefor melt migration at the outcrop scale is relatedto the interplay between local pressure gradientsand the vertical gravity field. The apparent texturalcontinuity of sills and dikes on one hand, and ofconcordant and discordant diffuse veins on the otherhand, suggests that an interconnected melt phasewas present at the outcrop scale. Migration of themelt fraction through the solid matrix in such a sys-tem could thus be treated as percolation of the meltthrough a network of veins during deformation ofpolycrystalline aggregates. A batch of melt ascend-ing through the crust would preferentially migrate byviscous flow through a network of deformation-con-trolled dilatant sites at high temperature and closeto the melt source, whereas at a higher structurallevel and when intruding more competent refractorylayers, it would develop a network of fractures ac-cording to the orientation of the instantaneous strainellipsoid.

4.2. Formation of laccoliths of leucogranite in theShuswap MCC: continuous flux or pulses of magma?

The structural connection and the consistency inages obtained on leucogranites and leucosomes inmigmatites, suggest that the granitic vein network

permeating the middle unit acted as a plumbing sys-tem feeding granitic laccoliths from their source totheir emplacement level between ¾60 Ma and ¾55Ma. This model is consistent with the ages obtainedalong the TCH1, but to the north U–Pb geochronol-ogy yields a wider range of ages suggesting thatthis simple interpretation might not be valid through-out the entire Shuswap MCC. In addition, withinthe middle unit, the only ages available have beenobtained from pegmatites and intrusive leucogran-ites and not from leucosomes generated in situ. Theobservations and the model presented above predictthat the crystallization ages of in-situ leucosomeswithin the middle unit should not significantly differfrom the ages obtained on leucogranites (60–55 Ma).

Connection of the granitic fraction in a networkof concordant and discordant veins indicates thatmelt migration was pervasive at the outcrop scale.The granitic fraction present at the outcrop scalerepresents only a residual melt fraction which is amixture of melt generated in situ and melt trappedduring migration and=or accumulated against me-chanical barriers such as competent refractory litho-logic layers. Determining the origin of the graniticfraction (in situ or intrusive) requires further test-ing using geochemistry and=or petrology in orderto compare the granite and the host rock affinities.Nevertheless, accumulation of granite in laccolithsat a high structural level along the detachment, indi-cates a significant amount of upward migration of themelt fraction. Deeper than a few kilometers belowthe surface, lithostatic pressure precludes perennialmaintenance of a continuous network of melt fromthe source to the emplacement level. It is more likelythat upward melt migration, owing to its buoyancy,proceeded progressively taking opportunity of anypathway created by deformation. If we take 1 kmto represent the upper limit on the thickness of thelaccolithic leucogranite ponded in the detachmentand consider that it accumulated between 60 and 55Ma, the resulting accumulation rate, assuming thatthe flux of melt was constant, is 200 m=Ma. Forthe average geometric characteristics of the granitenetwork described above (one 5 cm wide discor-dant vein every 50 cm), the magma ascent velocitythrough each discordant vein, averaged over 5 Ma,is 2 km=Ma. These velocities are slow compared tothe velocity inferred for efficient melt ascent through

O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55 51

dikes without freezing (Clemens and Mawer, 1992).This implies either that the temperature of the hostrock was high enough to prevent freezing or thatmelt migration resulting in the formation of laccol-ithic leucogranites was not continuous and occurredin pulses. The presence of both transposed graniticveins and granitic networks with preserved magmatictextures indicates that melt migration and deforma-tion events overlapped, but without systematic datingof granitic veins it is not possible to determine ifthese events occurred in a continuum or as pulses.

4.3. Thermomechanical implications of pervasivemelt migration

Field observations in the Shuswap MCC suggestthat a significant portion of the Cordilleran orogeniccrust was partially molten at the onset of late-oro-genic collapse. A striking feature of the ShuswapMCC is the thickness of the structural section char-acterized by sillimanite-grade metamorphic rocks.Metamorphic conditions recorded by the high-graderocks are consistent with partial melting occurringat depths ranging from 15 to 30 km implying thatat some point the geothermal gradient was relativelysteep (¾60ºC=km for the first 15 km of crust). In ad-dition, the geothermal gradient was such that smallquantities of melt were allowed to migrate upwardefficiently enough to lead to the formation of lac-coliths without crystallizing on their way. Numericalmodels show that it is possible to obtain such highgeothermal gradient for a crust rich in radioactiveelements (Huerta et al., 1996; Henry et al., 1997).Heat advection related to melt migration to higherstructural levels is another potential mechanism toincrease the geothermal gradient (DeYoreo and Lux,1989; Weinberg, 1996). In the middle unit of theShuswap MCC at the latitude of the TCH1, amphi-bolite-facies metamorphism is pervasive and graniticveins are not associated with any apparent meta-morphic aureoles. However, argon thermochronol-ogy conducted on hornblende yields scattered agesand irregularities in the 40Ar=39Ar age spectra indi-cate a component of excess argon possibly relatedto imperfect degassing of the minerals. With thecurrent data on the middle unit of the ShuswapMCC, it is thus difficult to determine if the appar-ent high geothermal gradient was related to (1) heat

conduction in a crust with a high radioactive heatproduction, or (2) heat advection due to pervasivegranite migration.

Whether the melt fraction was generated in situor permeated within ductilely deformed rocks, thepresence of a melt fraction affects the rheologicalproperties of rocks (Arzi, 1978; Van der Molen andPaterson, 1979; Rushmer, 1996; Vigneresse et al.,1997). Experimental deformation indicates that theeffective viscosity of a partially molten rock de-creases by several orders of magnitude from ¾20%to ¾40% of melt fraction and the associated loss ofstrength of the rock is related to the formation ofmelt-enhanced fractures and cataclastic zones (Vander Molen and Paterson, 1979; Paquet et al., 1981;Dell’Angelo and Tullis, 1988; Rutter and Neumann,1995; Rushmer, 1996). In natural conditions, thepresence of a melt fraction induces (1) strain lo-calization in the weak melt fraction reflected bythe common association of granite emplacement andshear zones in various geologic settings (Burg et al.,1984; Gapais, 1989; Davidson et al., 1992; Tommasiet al., 1994; Brown and Solar, 1998), and also (2) avariation of the bulk physical behavior of the rockwhen the melt fraction exceeds ¾40–50% at whichpoint the rock behaves as a magma (Brown, 1973;Burg and Vanderhaeghe, 1993).

At the scale of the cross-section investigated inthis study, the large number of granitic veins foldedand transposed in the major foliation indicates thatpart of the fabric development affected the graniticveins. However, the ubiquitous presence of discor-dant granitic veins with preserved magmatic tex-tures, indicates that in most places (except for twostructural levels mentioned above) migration of meltwas synkinematic and not outlasted by ductile de-formation. On the other hand, the detachment zone,separating the middle from the upper unit, is spa-cially and temporally related to the emplacement ofgranitic laccoliths characterized by pervasive C=Sfabrics (Carr, 1992; Vanderhaeghe et al., 1999b).Thus, melt migration and deformation appear co-eval in the middle unit of the Shuswap MCC, andstrain localization related to the presence of graniteis only recorded at the top of the middle unit, in thedetachment zone.

Disruption of the host rock’s solid framework atthe outcrop scale, resulted in the formation of rafts

52 O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55

or enclaves embedded into the granitic fraction. In-creasing intrusive granitic fraction into refractory orsubsolidus host rocks is associated with the develop-ment of magmatic breccias or magmatic structurescharacterized by the formation of angular blocksof host rock with no signs of internal deforma-tion related to magmatic flow. However, in fertilelithologies, the combination of intrusive granite andin-situ generated leucosomes leads to the forma-tion of shollen or raft structures (Mehnert, 1968).Whether enclaves are rigid or passive is probablynot affecting the effective physical properties of themagma as long as the enclaves are not interacting.Hence, in the middle unit of the Shuswap MCC,the transition from a gneiss-dominated migmatitewith a load-bearing framework to an enclave-richgranite, seems related predominantly to the intru-sion of a melt. This contrasts with the metatexite–diatexite transition, characteristic of the lower unitof the Shuswap MCC, for which most of the graniteis generated in situ and leads to the disruption ofthe solid framework with increasing melt fraction(Vanderhaeghe et al., 1999b).

These observations suggest that partial meltingand redistribution of the melt from the outcrop to thecrustal scale generates a new crustal-scale rheologi-cal layering. The associated weakening of the crusthas been proposed to (1) trigger the transition fromcrustal thickening to late-orogenic collapse, and to(2) control the geometry of metamorphic core com-plex formation (Vanderhaeghe and Teyssier, 1999).

5. Conclusion

The middle unit of the Shuswap MCC ex-posed across a ¾5–10-km structural section betweenmigmatites of the lower unit and a low angle de-tachment zone, is permeated by a large volumeof granitic veins. Structural, geometric and textu-ral characteristics suggest that distribution of thegranitic fraction at the outcrop-scale is controlled bythe regional strain pattern.

In fertile lithologies (metapelites, felsic amphibo-lites), the granitic fraction, in part generated in situ,forms a diffuse network of concordant leucosomesfeeding discordant veins, and dilatant sites such asshear zones, boudin necks, and axial planes of folds.

Discordant granitic veins are oriented perpendicu-lar, and to a lesser extent parallel, to the mineraland stretching lineation. The geometric and texturalcharacteristics of discordant veins are consistent withformation by buoyancy-driven viscous flow of meltthrough the solid matrix into dilatant sites createdduring deformation.

In refractory lithologies, the granitic fraction isdominantly intrusive and displays sharp contactswith the host rock. However, distribution of thegranitic fraction shows the same characteristics asfor fertile lithologies. The granitic fraction is prefer-entially oriented within the foliation plane, probablytaking advantage of mechanical weaknesses, anddikes are oriented dominantly perpendicular and par-allel to the regional stretching and mineral lineation.

Melt migration is thus primarily driven by itsbuoyancy and melt migrates in an opportunisticmanner along mechanical weaknesses such as thefoliation plane or in dilatant sites created by hetero-geneous deformation. The relationship between theregional fabric and the distribution of the graniticfraction indicates that regional deformation played amajor role in providing pathways for melt migration.The formation of laccoliths of leucogranite associ-ated with detachment faults suggests that upwardmigration of the melt is efficient and leads to accu-mulation of granitic magmas in specific sites. Theseobservations are consistent with a model of pervasivemelt migration through an interconnected network ofconcordant and discordant granitic veins feeding lac-colithic plutons extracted from the dominantly solidmatrix during vertical thinning and compaction at aregional scale.

Acknowledgements

This publication is Lithoprobe contribution num-ber 987. The first phase of this work has been fundedby NSF while O.V. was a graduate student at theUniversity of Minnesota. Additional field investiga-tions, with the cheerful assistance of Iain Pirie, andredaction of the manuscript have been done whileO.V. was holding a postdoctoral fellowship at Dal-housie University funded by Lithoprobe. The ideaspresented in this paper have been developed dur-ing lively discussions with the inspiring Christian

O. Vanderhaeghe / Tectonophysics 312 (1999) 35–55 53

Teyssier from the field to the office (via the pub).The first draft of the manuscript benefitted from thecomments of Rebecca Jamieson, Christian Teyssierand Jean-Louis Vigneresse. The present version hasbeen improved thanks to the comments of the re-viewers, Tracy Rushmer and Michael Brown. Atlast, the author would like to thank Carol Simpsonfor a smooth and efficient editorial work.

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