answer the article

Geology

doi: 10.1130/G31017.1 2010;38;1067-1070Geology

G.C. Koteas, M.L. Williams, S.J. Seaman and G. Dumond Granite genesis and mafic-felsic magma interaction in the lower crust

Email alerting services articles cite this article

to receive free e-mail alerts when newwww.gsapubs.org/cgi/alertsclick

Subscribe to subscribe to Geologywww.gsapubs.org/subscriptions/click

Permission request to contact GSAhttp://www.geosociety.org/pubs/copyrt.htm#gsaclick

official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may post works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent their employment. Individual scientists are hereby granted permission, without fees or further Copyright not claimed on content prepared wholly by U.S. government employees within scope of

Notes

© 2010 Geological Society of America

on January 20, 2011geology.gsapubs.orgDownloaded from

GEOLOGY, December 2010 1067

INTRODUCTION The presence and character of continental

crust on Earth is fundamentally linked to the ori- gin and evolution of felsic igneous rocks. Many models for the origin of felsic magmas involve the input of heat from mafi c magmas emplaced within or beneath the crust (e.g., Huppert and Sparks, 1988; Annen et al., 2006). Mafi c mag- mas also play a role at shallower levels, where mafi c-felsic magmatic interaction can affect the longevity and composition of felsic magma sys- tems (e.g., Wiebe, 1996; Harper et al., 2004). The fact that mafi c and felsic igneous rocks typically have a signature of contamination even before emplacement in the upper crust (DePaolo et al., 1992; Annen et al., 2006) suggests that the shal- low crustal interaction may be the fi nal stage in a protracted set of petrogenetic processes. The Athabasca granulite terrane (AGT), located in northwestern Saskatchewan, Canada (Figs. 1A and 1B), is one of Earth’s largest exposures of intact lower continental crust and provides an unprecedented example of felsic magma gen- eration and felsic-mafi c magma hybridization in the deep crust. Relationships preserved in this region fi ll the gap between processes that yield heat and magma from the mantle and processes that yield bimodal suites in the shallower crust.

The purpose of this paper is to present detailed fi eld evidence for granitic magma genesis asso- ciated with mafi c magmatism within the lower continental crust and to document evidence for deep crustal mafi c-felsic magma mingling and mixing. Exposures in the AGT offer a view of a fundamental mechanism for contamination of mantle-derived materials (i.e., assimilation of crustal components via magma mixing) as

well as felsic magma production from melt- ing of orthogneiss in the deep crust. The region provides a picture of a heterogeneous, dynamic, and locally fertile cratonic deep crust. This set- ting may serve as a basis for new models of the behavior of the deep continental crust and provide insight into the petrogenesis of magmas observed at shallower crustal levels.

BACKGROUND The AGT is a >20,000 km2 domain of

Archean to Paleoproterozoic mafi c and felsic granulites and orthogneisses (Fig. 1B) that were deformed and metamorphosed at ~1.0–1.2 GPa (~40 km paleodepths) (Mahan and Williams,

Geology, December 2010; v. 38; no. 12; p. 1067–1070; doi: 10.1130/G31017.1; 2 fi gures; Data Repository item 2010294. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].

Granite genesis and mafi c-felsic magma interaction in the lower crust G.C. Koteas1, M.L. Williams1, S.J. Seaman1, and G. Dumond2 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA 2 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA

ABSTRACT Field observations from an exposed section of deep continental crust, the Athabasca granu-

lite terrane (AGT), Saskatchewan, Canada, provide a view of granite genesis and a mechanism for deep-seated contamination of felsic and mafi c magmas. The 1.9 Ga Chipman mafi c dike swarm was emplaced into the Chipman Tonalite (ca. 3.3 Ga) and the megacrystic Fehr granite (ca. 2.6 Ga) at a crustal depth of ~40 km. The Fehr granite shows evidence for extensive par- tial melting and generation of granitic leucosome. Mafi c dikes and granitic leucosome display magma mingling and mixing textures similar to those widely described from shallow crustal exposures. The AGT provides a view of a dynamic, heterogeneous, and locally fertile deep crust. Mantle-derived mafi c magma promotes extensive partial melting of fertile granitoids, which in turn fi lter and entrap later mafi c dikes and sills. The result is almost inevitable min- gling and hybridization (i.e., contamination) of mafi c and felsic end members. This interaction of magmas in the deep crustal environment may account for the isotopic and compositional signatures of igneous rocks at shallower crustal levels that typically record contamination of crustal melts by mantle material and vice versa.

1000 km

Pacific Ocean

Atlantic Ocean

N North

America

CHURCHILL Province - Rae

CHURCHILL Province -

Hearne

Tr an

s- Hu

ds on

Or og

en

S N O W

B IR

D

S N O W

B IR

D

T E C T O N IC

Z O N E

T E C T O N IC

Z O N E

Western limit

of exposed

Canadian Shield

GR sz

L L sz

110 W

60 N

Tr an

s- Hu

ds on

Or og

en

Ta ltso

n-T he

lon

Or og

en

Archean cratonic provinces

Athabasca granulite terrane

Archean greenstone belts

Hearne Domain (undivided)

Ductile and brittle faults Grease River shear zone (GRsz) Legs Lake shear zone (LLsz) Proterozoic basins

Focus area for this study

Aeromagnetic highs in covered basement

200 km Steinhauer Lake

Cross Lake

No Name Lake

Fehr Lake

Fe hr

g ra

ni te

C hi

pm an

to na

lit e

Fehr granite (ca. 2.6 Ga)

5 km

Legs Lake

Le gs

L ak

e S he

ar Z

on e

Lake Athabasca

Ch ip

m an

di ke

sw ar

m Chipman tonalite (ca. 3.0 Ga)

S2

N

Equal Area Projection n = 40

S1

B C

A

D N

Figure 1. A: Index map showing location of study. B: Generalized geologic map of Athabasca Granulite Terrane (AGT, after Gilboy, 1980; Hanmer, 1997; Mahan et al., 2005). C: Detailed map focused on easternmost portion of the AGT including the Fehr granite, Chipman to- nalite, and NE-SW−striking Chipman. Chipman dikes are present throughout area shown in C (modifi ed from Flowers et al., 2008). Color gradient within Fehr granite is a schematic depiction of interpreted intensity of partial melting, white (most-migmatitic) to gray (least- migmatitic). D: Stereogram of S

1 (crossed dots) and S

2 (black dots) tectonic fabrics within

migmatized Fehr granite. Hybrid fabrics of S 1 and S

2 commonly observed, especially in

areas of abundant migmatite.

on January 20, 2011geology.gsapubs.orgDownloaded from

1068 GEOLOGY, December 2010

2005; Williams and Hanmer, 2006). This region is interpreted to represent continental lower crust during the period 2.6−1.85 Ga (Mahan et al., 2006b; Flowers et al., 2006a) (Fig. 1B). It was uplifted and exhumed along the Legs Lake thrust-sense shear zone (Fig. 1C) (Mahan and Williams, 2005, Mahan et al., 2006a, 2006b). The eastern portion of the AGT is dominated by the Mesoarchean (3.3 Ga) Chipman tonal- ite batholith (Fig. 1C), a large body of banded hornblende tonalite with inclusions of anortho- site, pyroxenite, and a variety of mafi c and fel- sic granulites. The Fehr granite (ca. 2.6 Ga; W. Davis, 1997, personal commun.) occurs along the eastern fl ank of the Chipman tonalite, just west of the Legs Lake shear zone (Hanmer et al., 1994; Hanmer, 1997) (Fig. 1C). The Chip- man tonalite and the Fehr granite were simulta- neously intruded by the 1.9 Ga Chipman mafi c dike swarm (Flowers et al., 2006b).

The most pristine exposures of the Fehr gran- ite are characterized by euhedral to subhedral K-feldspar megacrysts (up to 8 cm diameter) in a matrix of quartz + plagioclase (~1 cm diam- eter) with fi ne-grained biotite and hornblende (Fig. 2A). Locally, the long axes of megacrysts are aligned in a relatively isotropic matrix sug- gesting preservation of a magmatic fl ow fabric. More commonly, the Fehr granite has a gneissic texture, and in many areas, a relatively strong early foliation (S

1 ) is warped into open folds

and cut by a moderately- to steeply-dipping, northeast-striking axial planar foliation (S

2 )

(Figs. 1D, 2C, and 2D). Aplitic granite pods, dikes, and sills are common in this area.

The Chipman mafi c dike swarm forms a lin- ear belt several tens of kilometers in width and extends for hundreds of kilometers to the north and south (Williams et al., 1995; Flowers et al., 2006a). Individual Chipman dikes range from centimeters to tens of meters in width. These dikes are composed of hornblende + plagio- clase + clinopyroxene + garnet and locally, tonalitic leucosome, which is interpreted as partial melt of Chipman dikes (Williams et al., 1995). Internal textures vary markedly based on the amount of tonalitic leucosome present. Relatively late-stage dikes are straight-sided and cut all fabrics in the Chipman tonalite and Fehr granite. Earlier dikes are commonly folded and locally contain metamorphic garnet and clinopyroxene in addition to tonalitic leu- cosome. The Chipman dikes are interpreted to have been syntectonically emplaced and meta- morphosed (Williams et al., 1995; Hanmer, 1997). Metamorphic assemblages within these dikes indicate high pressure (1.0−1.2 GPa) granulite facies conditions, with calculated temperatures on the order of 750−850 °C (Wil- liams et al. 1995; Flowers et al., 2006a). Geo- chemical and isotopic signatures are most con- sistent with derivation from a predominantly

depleted lithospheric or asthenospheric mantle source (Flowers et al., 2006a).

PARTIAL MELTING OF THE FEHR GRANITE

The Fehr granite is a pink, K-feldspar bearing megacrystic granite or gneiss. Most exposures also contain pink, aplitic granite in dikes, veins, pods, and fi ne stringers (interpreted as granitic leucosome) (Fig. 2D). With increasing abun- dance of granitic leucosome, K-feldspar mega- crysts are smaller, more anhedral, and in some cases are partially replaced by granitic leuco- some (Fig. 2B). At its most extreme, the replace- ment of megacrysts results in outcrop exposures with what appear to be deformed megacrysts but are, in fact, granitic leucosome pods within a fi ne grained granitic matrix. Tonalitic leucosome

veins are also locally present, and are probably derived from neighboring migmatitic Chipman mafi c dikes. The much more abundant granitic leucosome is interpreted to represent in situ par- tial melting of the Fehr granite itself. This inter- pretation is supported by major and trace ele- ment geochemical trends that show a cogenetic relationship between Fehr granitic leucosome and the Fehr granite protolith (see Fig. DR1 and Table DR1 in the GSA Data Repository1).

A

C

B

D

FE

1GSA Data Repository item 2010294, Figure DR1 (four bivariant element plots of the ten major divi- sions of rock units sampled from the study area) and Table DR1(complementary whole-rock data set used to generate the four plots in Fig. DR1), is available online at www.geosociety.org/pubs/ft2010.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

Figure 2. Field photos of Fehr granite and Fehr granite–Chipman mafi c dike inter action along eastern edge of Athabasca granulite terrane (AGT). A: Isotropic megacrystic Fehr granite preserving only weak deformation and limited in-situ granitic leucosome. B: Dif- fuse contact between Fehr granite–related leucosome pod with S

1 fabric. C: “Megacren-

ulation” cleavage in migmatitic Fehr granite. Note the crenulated S 1 fabric and spaced

granitic leucosome-rich S 2 crenulation cleavage. D: Aplitic leucosome (felsic dike) with

trails projecting in from S 2 fabric domains. E: Leucosome-rich Fehr granite chemically

and mechanically interacting with discontinuous mafi c Chipman dikes. F: Coarse- grained, leucosome-rich Fehr migmatite and Chipman dike termination.

on January 20, 2011geology.gsapubs.orgDownloaded from

GEOLOGY, December 2010 1069

Compositional and textural gradients from relatively pristine to highly migmatitic Fehr granite occur on several scales. Regionally, gra- nitic leucosome segregations tend to be more abundant in the central (Steinhauer Lake) areas of the Fehr granite exposure (Fig. 1C). In the northernmost and southernmost exposures, local transitions from pristine granite to mig- matitic granite occur on scales from meters to tens of meters. Single outcrops preserve varia- tion in rock texture from granite dominated by euhedral single-crystal megacrysts to exposures with few megacrysts and abundant granitic leu- cosome pods (Fig. 2B).

Garnet is rare or absent in the northern and southern exposures of the Fehr granite, but is locally abundant within the more penetratively deformed and more migmatitic central part of the exposure area (Fig. 1C). Outcrops in this area can have 10%–15% garnet, with crystals up to several centimeters in diameter. Commonly, pink granitic leucosome segregations occur adjacent to garnet crystals in triangular “tails” extending along the dominant foliation. The garnet-bearing migmatite is interpreted to be the result of a peritectic melt reaction in which garnet is produced during biotite dehydration melting. Textural evidence for in situ melting in exposures with no garnet is interpreted to refl ect a different melt reaction, eutectic melting of hydrous orthogneiss.

Deformational fabrics in the Fehr granite vary with the abundance of granitic leucosome. This leucosome fi rst appears along the north- west-striking, shallowly dipping S

1 foliation as

tails or as lozenge-shaped segregations. With increasing abundance, segregations are more commonly aligned along the upright S

2 foliation,

and typically defi ne steeply-dipping, northeast- striking axial planar foliations to meter-scale folds (Fig. 1D). In the highly migmatitic central region, a distinctive megacrenulation fabric is common (Figs. 2C and 2D) in which the granitic leucosome veins defi ne both the sigmoidal S

1

traces and a spaced S 2 cleavage. Whereas crenu-

lation cleavage in schists is typically spaced on a scale of millimeters, the S

2 spacing in the

migmatized Fehr granite is ~10−20 cm. With increasing granitic leucosome abundance, the S

2

domains typically host progressively larger and more continuous aplitic dikes (Fig. 2D). The dif- ferent fabrics and geometries in the Fehr granite migmatite are interpreted to refl ect the evolving rheology of the granite with increasing degrees of partial melting.

FEHR GRANITE - CHIPMAN DIKE INTERACTION: MAGMA MINGLING AND MIXING

The Chipman dike swarm intrudes both the Chipman tonalite and the Fehr granite, but the physical character of dikes is signifi cantly

different in the two host rocks. Most tonalite- hosted dikes have straight, sharp, parallel con- tacts. All observations support the interpretation that Chipman mafi c magmas intruded an essen- tially brittle tonalite host. In contrast, although contacts between Chipman dikes and the Fehr granite can be straight and sharp, they are more commonly curving, irregular, and distinctly non- parallel on opposite sides of individual dikes. Larger dikes commonly bifurcate into smaller, anastomosing dikes that end abruptly as thin fi n- gers or rounded terminations (Fig. 2E). Where dikes intrude granitic leucosome segregations and larger aplitic dikes and veins within the Fehr granite, dike margins typically have pillow-like shapes (Fig. 2F). These textures suggest that the Chipman dikes were emplaced into, and locally interacted with, partially melted Fehr granite.

Chipman mafi c magma and aplitic Fehr gran- ite-related leucosome were mutually contami- nated by mixing and mingling processes. Mega- crysts from the partially melted Fehr granite are typically present within Chipman dike margins, and locally, trains of megacrysts are present well within the mafi c dikes. Aplitic leucosome commonly projects into mafi c dikes (Fig. 2E), becoming progressively more diffuse and dis- persed along strike, ultimately producing thin, white, millimeter-scale schlieren in the Chip- man dikes. Chipman dikes that intrude migma- titic Fehr granite or aplitic partial melt segrega- tions locally disaggregate into concentrations of centimeter- to meter-scale, rounded mafi c accu- mulations (Fig. 2F). These pillow-like mafi c pods are preferentially oriented with long-axes parallel to the major northeast-southwest trend of the dike swarm and have been recognized in granitic leucosome segregations some distance from obvious mafi c dikes, suggesting that con- taminated granitic leucosome can be transported well away from sites of mafi c dike interaction.

DISCUSSION The Athabasca granulite terrane preserves

evidence of high high-pressure–high-temper- ature (P-T) metamorphism, deformation, and pluton emplacement at ca. 2.6 Ga (Williams and Hanmer, 2006; Dumond et al., 2010). The AGT may have remained at deep crustal levels or may have experienced minor exhu- mation, but a very large portion of the terrane underwent a second period of high P-T (1.0− 1.2 GPa) metamorphism and deformation at 1.9 Ga (Mahan and Williams, 2005; Williams and Hanmer, 2006; Flowers et al., 2006a). The Chipman mafi c dike swarm was emplaced dur- ing the second event into the relatively fertile 2.6 Ga Fehr granite and the adjacent Chip- man tonalite. Locally high temperatures, due to the proximity of mafi c dikes and/or addi- tional mafi c magma at depth, led to extensive anatexis of the Fehr granite. The abundance of

aplitic veins and dikes suggests that the gra- nitic magma was mobilized to some degree (Fig. 2D). As the fraction of partial melt in the Fehr granite increased, it may have become increasingly diffi cult for subsequent Chipman dikes to cross-cut the granite, as indicated by the abundance of irregular, pillow-like dike terminations in the Fehr granite migmatite (Figs. 2E and 2F). Instead, dikes apparently pooled beneath and within the Fehr granite, providing additional heat for further melting.

One critical question concerns the source of water for large degrees of melting in these deep crustal rocks. Melting can be explained by two mechanisms. Biotite dehydration melting is indicated by the abundant garnet + granitic leucosome textures in the central exposures of migmatized Fehr granite, east of Steinhauer Lake (Fig. 1C). However, large exposures of migmatized Fehr granite do not contain gar- net or other minerals indicative of dehydration (peritectic) melting, yet textural evidence sup- ports in situ melting. Partial melting in these areas is interpreted to represent eutectic melting resulting from the infl ux of hydrous fl uids. The Chipman mafi c dikes contain abundant horn- blende and are interpreted to have been hydrous at the time of emplacement (Williams et al., 1995, Flowers et al., 2006b). Crystallization and subsequent metamorphism of early Chip- man dikes, and possibly of a genetically related mafi c underplate, probably provided additional fl uids for partial melting reactions. The large degree of partial melt production can be attrib- uted to a combination of the fertility of the Fehr granite, the presence of hydrous phases, intro- duction of water from migmatized Chipman dikes, and especially, the very high temperatures (>800 °C) in the vicinity of the dense Chipman mafi c dike swarm .

Mid- and shallow-level igneous rocks com- monly have a signature of contamination (e.g., DePaolo et al., 1992; Barnes et al., 2002). Workers typically call upon assimilation of deep crust to explain this contamination and many envision the digestion of blocks of con- tinental crustal materials. Studies of the Fehr granite migmatite provide a different model, one in which felsic magma genesis and con- tamination are fundamentally linked. High temperatures (>800 °C) and hydration pro- duced during emplacement and migmatization of mantle-derived mafi c dikes can lead to exten- sive melting of granitoids in the deep crust. The presence of felsic partial melt during continued dike intrusion allows mixing and mingling of felsic and mafi c magmas. We suggest that this type of mafi c-felsic magma interaction can provide an effi cient means of contamination of both felsic and mafi c end members at lower crustal depths prior to migration of magma to middle or shallow crustal levels.

on January 20, 2011geology.gsapubs.orgDownloaded from

1070 GEOLOGY, December 2010

Many workers have suggested that mantle partial melts pond at the dry, mafi c base of the crust (Huppert and Sparks, 1988; Annen et al., 2006, their fi gure 1). Feeder dikes (Petford, 1996; Rushmer and Klepeis, 2003; Brown, 2005) or shear zones (Hollister and Crawford, 1986) have been invoked to convey magmas to shallower crustal levels where mixing and differentiation processes have been widely documented. However, the heterogeneity of lower crustal exposures in the AGT indicates that the deep crustal “hot zone” of Annen et al. (2006) is more complex than a site of intrusion of partial mantle melts into mafi c crust. Rather, underplating at the base of the crust provides a thermal engine such that true granites can form in close proximity to the mantle, creating a setting where mingling and contamination are inevitable. Exposures in the AGT provide a view of a linked and positively reinforcing sys- tem involving mafi c injection, partial melting of granitoids, fi ltering and entrapment of mafi c magma, and ultimately hybridization of felsic and mafi c end members.

ACKNOWLEDGMENTS We gratefully acknowledge Rebecca Flowers’ con-

tribution of fi rst recognizing mingling textures in the Fehr granite. The work was supported by National Science Foundation Grant EAR-0911421. We ap- preciate discussions with Kevin Mahan, Michael Jer- cinovic, Simon Hanmer, and Julien Allaz. We thank James McLelland, Sandra Wyld, and two anonymous reviewers for thoughtful reviews of the manuscript.

REFERENCES CITED Annen, C., Blundy, J.D., and Sparks, R.S.J., 2006,

The genesis of intermediate and silicic magmas in deep crustal hot zones: Journal of Petrol- ogy, v. 47, p. 505–539, doi:10.1093/petrology/ egi084.

Barnes, C.G., Yoshinobu, A.S., Prestvik, T., Nordgu- len, Ø., Karlsson, H.R., and Sundvoll, B., 2002, Mafi c magma intraplating: Anatexis and hy- bridization in arc crust, Bindal Batholith, Nor- way: Journal of Petrology, v. 43, p. 2171–2190, doi:10.1093/petrology/43.12.2171.

Brown, M.2005, Melt extraction from the lower con- tinental crust of orogens: The fi eld evidence, in Brown M., and Rushmer T., eds., Evolution and Differentiation of the Continental Crust: Cam- bridge, Cambridge University Press, p. 331–376.

DePaolo, D.J., Perry, F.V., and Baldridge, W.S., 1992, Crustal versus mantle sources of granitic

magmas; A two-parameter model based on Nd isotopic studies: Geological Society of Amer- ica Special Paper 272, p. 439–446.

Dumond, G., Goncalves, P., Williams, M.L., and Jer- cinovic, M.J., 2010, Subhorizontal fabric in ex- humed continental lower crust and implications for lower crustal fl ow: Athabasca granulite terrane, western Canadian Shield: Tectonics, v. 29, p. TC2006, doi:10.1029/2009TC002514.

Flowers, R.M., Bowring, S.A., and Williams, M.L., 2006a, Timescales and signifi cance of high- pressure, high-temperature metamorphism and mafi c dike anatexis, Snowbird tectonic zone, Canada: Contributions to Mineralogy and Pe- trology, v. 151, p. 558–581, doi:10.1007/s00410 -006-0066-7.

Flowers, R.M., Mahan, K.H., Bowring, S.A., Wil- liams, M.L., Pringle, M.S., and Hodges, K.V., 2006b, Multistage exhumation and juxtaposi- tion of lower continental crust in the west- ern Canadian Shield; linking high-resolution U/Pb and 40Ar/ 39Ar thermochronometry with pressure-temperature-deformation paths: Tec- tonics, v. 25, 20 p.

Flowers, R.M., Bowring, S.A., Mahan, K.H., Wil- liams, M.L., and Williams, I.S., 2008, Stabili- zation and reactivation of cratonic lithosphere from the lower crustal record in the western Canadian Shield: Contributions to Mineralogy and Petrology, v. 156, p. 529–549, doi:10.1007/ s00410-008-0301-5.

Gilboy, C.F., 1980, Sub-Athabasca Basement Geol- ogy Project, in Christopher, J.E., ed., Summary of Investigations by the Saskatchewan Geolog- ical Survey, I. 80–4, pp. 17.

Hanmer, S., 1997, Geology of the Striding-Athabasca mylonite zone, northern Saskatchewan and south eastern District of Mackenzie, Northwest Territories: Bulletin - Geological Survey of Canada, report 501, 92 p.

Hanmer, S., Parrish, R., Williams, M., and Kopf, C., 1994, Striding-Athabasca mylonite zone; complex Archean deep-crustal deformation in the East Athabasca mylonite triangle, northern Saskatchewan: Canadian Journal of Earth Sci- ences, v. 31, p. 1287–1300.

Harper, B.E., Miller, C.F., Koteas, G.C., Cates, N.L., Wiebe, R.A., Lazzareschi, D.S., and Cribb, J.W., 2004, Granites, dynamic magma cham- ber processes and pluton construction; the Az- tec Wash Pluton, Eldorado Mountains, Nevada, USA, in Ishihara, S., Stephens, W.E., Harley, S.L., Arima, M., Nakajima, T., eds., Fifth Hut- ton symposium on the origin of granites and related rocks, v. 95, p. 277–295.

Hollister, L.S., and Crawford, M.L., 1986, Melt- enhanced deformation: A major tectonic pro- cess: Geology, v. 14, p. 558–561, doi:10.1130/ 0091-7613(1986)14<558:MDAMTP>2.0.CO;2.

Huppert, H.E., and Sparks, R.S.J., 1988, The genera- tion of granitic magmas by intrusion of basalt into continental crust: Journal of Petrology, v. 29, p. 599–624.

Mahan, K.H., and Williams, M.L., 2005, Recon- struction of a large deep-crustal terrane; im- plications for the Snowbird tectonic zone and early growth of Laurentia: Geology, v. 33, p. 385–388, doi:10.1130/G21273.1.

Mahan, K.H., Goncalves, P., Williams, M.L., and Jercinovic, M.J., 2006a, Dating metamorphic reactions and fl uid fl ow; application to exhu- mation of high-P granulites in a crustal-scale shear zone, western Canadian Shield: Journal of Metamorphic Geology, v. 24, p. 193–217, doi:10.1111/j.1525-1314.2006.00633.x.

Mahan, K.H., Williams, M.L., Flowers, R.M., Jerci- novic, M.J., Baldwin, J.A., and Bowring, S.A., 2006b, Geochronological constraints on the Legs Lake shear zone with implications for regional exhumation of lower continental crust, western Churchill Province, Canadian Shield: Contri- butions to Mineralogy and Petrology, v. 152, p. 223–242, doi:10.1007/s00410-006-0106-3.

Petford, N., 1996, Dykes or diapirs?: Geological Society of America Special Paper 315, p. 105– 114.

Rushmer, T., and Klepeis, K.A., 2003, Generation and evolution of lowermost crust of an arc; examples from Fiordland, New Zealand: Eos, Transactions, American Geophysical Union, v. 84, p. f1584–f1585.

Wiebe, R.A., 1996, Mafi c-silicic layered intrusions; the role of basaltic injections on magmatic processes and the evolution of silicic magma chambers: Geological Society of America Spe- cial Paper 315, p. 233–242.

Williams, M.L., and Hanmer, S., 2006, Structural and metamorphic processes in the lower crust: evidence from the East Athabasca mylonite triangle, Canada, a deep-crustal isobarically cooled terrane, in Brown, M. and Rushmer, T., eds., Evolution and Differentiation of the Con- tinental Crust: New York, Cambridge Univer- sity Press, p. 231–267.

Williams, M.L., Hanmer, S., Kopf, C., and Darrach, M., 1995, Syntectonic generation and segrega- tion of tonalitic melts from amphibolite dikes in the lower crust, Striding-Athabasca mylonite zone, northern Saskatchewan: Journal of Geo- physical Research, v. 100, p. 15717–15734, doi:10.1029/95JB00760.

Manuscript received 11 January 2010 Revised manuscript received 15 June 2010 Manuscript accepted 1 July 2010

Printed in USA

on January 20, 2011geology.gsapubs.orgDownloaded from