Paleocene-Eocene Migmatite Crystallization, Extension, and Exhumation in the Hinterland of the Northern Cordillera: Okanogan Dome, Washington, USA

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Paleocene-Eocene migmatite crystallization, extension, and exhumation in the hinterland of the northern Cordillera: Okanogan dome, Washington, USA

Seth C. Kruckenberg† Donna L. Whitney Department of Geology and Geophysics, University of Minnesota-Twin Cities, Minneapolis, Minnesota 55455, USA Christian Teyssier Department of Geology and Geophysics, University of Minnesota-Twin Cities, Minneapolis, Minnesota 55455, USA Institut de Géologie et de Paléontologie, Anthropole, Université de Lausanne, CH-1015, Lausanne, Switzerland C. Mark Fanning W. James Dunlap* Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

ABSTRACT crystallization. Zircon from folded and dis- Keywords: Omineca belt, geochronology, par- cordant granitic leucosome in the diatexite tial melting, gneiss dome, metamorphic core The Okanogan gneiss dome, Washing- domain yields a calculated 206Pb/238U age of complex, migmatite, Okanogan dome, conti- ton, is located in the hinterland of the North 53.5 ± 0.5 Ma for migmatite crystallization. nental tectonics. American Cordillera and is part of a chain Zircon from discordant leucosome of the of metamorphic core complexes containing metatexite domain has a mean 206Pb/238U age of INTRODUCTION gneiss and migmatite domes exhumed dur- 59.8 ± 0.5 Ma, with ages as young as ca. 53 Ma ing Eocene extension of thickened crust. attributed to fi nal crystallization of the leuco- In recent years there has been increased rec- U-Pb sensitive high-resolution ion micro- some. Core domains of zircon samples have ognition that partial melting is a major orogenic probe (SHRIMP) analyses of zircon, mona- 206Pb/238U ages that range from ca. 85 to 70 Ma process that affects the construction and col- zite, and titanite, and 40Ar-39Ar analyses of and are interpreted to be related to an earlier lapse of mountain systems. Geophysical surveys biotite from migmatites exposed in the foot- phase of the orogeny. Monazite from two in young and active orogens (Tibetan Plateau- wall of the Okanogan detachment, coupled samples gives 206Pb/238U crystallization ages of Himalaya: Nelson et al., 1996; Andean Alti- with a detailed structural analysis, docu- 52.9 ± 0.6 Ma for the granodiorite diatexite plano: Schilling and Partzsch, 2001; Pyrenees: ment the timing and duration of migmatite and 52.0 ± 0.6 Ma for nearby boudinaged and Pous et al., 1995) have revealed that much of the crystallization and indicate coeval crystalli- foliated layers of biotite granodiorite. One orogenic crust contains a signifi cant fraction of zation, extensional deformation, and exhu- sample of folded granitic leucosome in meta- partial melt (~20 vol%) at mid to lower crustal mation of the dome. Okanogan migmatites texite contains titanite with a mean 206Pb/238U depths (>10–20 km). are folded and deformed, and preserve suc- age of 47.1 ± 0.5 Ma. The ca. 47 Ma age for The presence of a thick partial melt layer in cessive generations of leucosomes generated titanite is similar to biotite 40Ar-39Ar ages of actively deforming orogens has important impli- by synkinematic anatexis. 48.0 ± 0.1 Ma, 47.9 ± 0.2 Ma, and 47.1 ± 0.2 Ma cations for crustal rheology and the thermal- Analyses of migmatite samples from a high- for samples collected from the upper detach- mechanical evolution of orogens. The presence melt fraction subdome near Stowe Mountain ment surface downward over 1.5 km of struc- of large volumes of partially molten crust may suggest that the Okanogan dome records a tural thickness into the migmatite domain. contribute signifi cantly to crustal fl ow in orogens history of migmatite crystallization spanning Crystallization of the Okanogan migma- (Royden, 1996; McKenzie et al., 2000; Beau- at least 12 m.y., as indicated by 206Pb/238U ages tites was therefore coeval in part with upper mont et al., 2001; Soula et al., 2001; Babeyko et ranging from ca. 61 to 49 Ma for new zir- crustal extension and ductile fl ow of the al., 2002; Teyssier and Whitney, 2002), especially con growth and rim overgrowths attributed mid-crust. Leucosome crystallization largely when crustal fl ow at depth is coupled with upper to migmatite crystallization. Zircons from ceased by ca. 49 Ma, followed by rapid cool- crustal extension (Rey et al., 2001) or enhanced a granodiorite in a domain of diatexite near ing of footwall rocks through ~325 ºC by erosion (Beaumont et al., 2001; Jamieson et al., Stowe Mountain preserve rims that have a ca. 47 Ma. These data are similar to crystal- 2002). Flow of orogenic crust may: (1) contrib- mean 206Pb/238U age of 51.1 ± 1.0 Ma for the lization ages in migmatites from other domes ute to building orogenic plateaus through lateral youngest population attributed to migmatite in the northern Cordillera hinterland, sug- fl ow of the partial melt layer (Royden, 1996; gesting that crustal anatexis was widespread Royden et al., 1997; Clark and Royden, 2000; †E-mail: [email protected] *Present address: Department of Geology and over much of the mid-crust during Paleocene Beaumont et al., 2001); (2) facilitate or drive late- Geophysics, University of Minnesota–Twin Cities, to Eocene time, coeval with extension and orogenic extensional collapse and detachment Minneapolis, Minnesota 55455, USA exhumation of orogenic middle crust. formation (Bertotti et al., 2000; McKenzie et al.,

GSA Bulletin; July/August 2008; v. 120; no. 7/8; p. 912–929; doi: 10.1130/B26153.1; 13 ﬁ gures; 1 table; Data Repository item 2008079.

2000; Vanderhaeghe and Teyssier, 2001); and of cooling of dome lithologies relative to the age and minor calc-silicate rocks that make up the (3) affect the thermal budget of orogens through of melt crystallization. Tonasket gneiss (Snook, 1965; Fox et al., 1976); diapiric ascent of the partially molten layer to In this paper, we describe the temporal rela- (2) orthogneiss, commonly with megacrystic form gneiss and migmatite domes (Soula et al., tionships among partial melting, the cooling of augen, that intruded and structurally overlies the 2001; Teyssier and Whitney, 2002). dome migmatites, and the structural develop- Tonasket gneiss; (3) the amphibolite facies Tenas Field analysis of high-grade metamorphic ment of the Okanogan dome in Washington state Mary Creek Sequence, exposed primarily in the rocks in collapsed orogens is central to evalu- (USA). The Okanogan gneiss dome is part of a NE Okanogan dome and correlative with rocks of ation of these ideas about the role of partially belt of metamorphic core complexes and gneiss similar type in the Kettle dome, including pelitic molten crust in orogeny and the thermal and domes that extends from British Columbia schist, quartzite, marble, interlayered orthogneiss, mechanical consequences of crustal fl ow. Many (Canada) into Washington and Idaho (USA). and minor amphibolite rocks (Cheney, 1980); and exhumed orogens are characterized by a high- (4) early Tertiary granitic gneiss and granodioritic grade core of middle to lower crustal rocks GEOLOGICAL FRAMEWORK plutons of the Colville Batholith that are variably exposed in the footwall of large detachment mylonitized and that concordantly intruded the faults, resulting in the characteristic metamor- The Okanogan gneiss dome lies within the Tonasket gneiss and mantling orthogneiss (Fox et phic core complex geometry (Coney et al., interior of the Omineca crystalline belt near al., 1976, 1977; Holder and Holder, 1988; Carl- 1980; Crittenden et al., 1980; Armstrong, 1982). its southernmost limit, and is structurally cor- son and Moye, 1990). Therefore, metamorphic core complexes, partic- relative with rocks of the Shuswap metamorphic The metamorphic pressure-temperature-time ularly those that contain substantial regions of core complex of British Columbia (Figs. 1 and (P-T-t) paths of rocks from the Okanogan-Kettle migmatite, are excellent sites to study the fi eld 2). The tectonic history of the Okanogan gneiss gneiss domes are not well known. The meta- record of fl ow involving partially molten crust. dome involved Jurassic and Cretaceous crustal morphic rocks of the Okanogan dome are gen- It has long been known, based on pioneering thickening, shortening, and metamorphism asso- erally characterized as upper amphibolite facies work in metamorphic core complexes (Coney ciated with terrane accretion to the western mar- (Snook, 1965; Goodge and Hansen, 1983; Men- et al., 1980; Crittenden et al., 1980; Lister and gin of North America (Armstrong, 1982; Monger zer, 1983; Hansen and Goodge, 1988). Goodge Davis, 1989), that extension in the upper crust et al., 1982; Archibald et al., 1983; Brown and and Hansen (1983) and Hansen and Goodge is accommodated by ductile fl ow of the mid to Read, 1983; Okulitch, 1984), and Early Tertiary (1988) confi rmed that rocks of the Tonasket lower crust. This ductile fl ow may involve a tectonic denudation, regional extension, volumi- gneiss in the SW Okanogan dome experi- low-viscosity layer of partially molten rock. In nous plutonism and volcanism, and exhumation enced upper amphibolite facies metamorphism many metamorphic core complexes, a carapace of high-grade metamorphic rocks (including and reached the sillimanite + K-feldspar zone. of high-grade metamorphic rocks exposed in migmatites) by detachment tectonics (Hansen Snook (1965) proposed that some rocks of the the footwall of detachment systems mantles and Goodge, 1988; Parrish et al., 1988). Okanogan dome experienced granulite facies one or more domal structures composed of The nearly 100-km–wide Okanogan gneiss conditions based on relics of spinel, corun- high-melt fraction migmatites, interpreted to dome is part of a larger composite structural dum, and olivine in amphibolite. In addition, be anatectic in origin (Whitney et al., 2004). In and metamorphic culmination that includes the Harvey (1994) documented a suite of unusual addition, the application of high-spatial resolu- Kettle gneiss dome to the east and is bounded to minerals in a sapphirine-bearing amphibolite tion geochronology techniques has shown that the east and west by the Kettle and Okanogan in the western part of the dome south of Stowe dome migmatites formerly considered “old detachments, respectively. These 1- to 2-km– Mountain (Fig. 2), providing the fi rst quantita- basement” are in fact the exhumed expression thick mylonite zones defi ne the major structural tive estimates of metamorphic conditions of ~10 of the low-viscosity layer of partially molten fabric at the periphery of the domes and grade kbar and ~825 °C. The upper amphibolite facies crust (e.g., Gans, 1987; MacCready et al., upward into a diffuse zone of chloritic breccia assemblages are locally overprinted by green- 1997; Vanderhaeghe et al., 1999; Ledru near the contact with the upper plate (Goodge schist-facies minerals; e.g., epidote and chlorite et al., 2001). Furthermore, geochronology and and Hansen, 1983; Hansen and Goodge, 1988). near late faults and shear zones. thermochronology from numerous orogens These detachment zones display consistent kine- Structural features in footwall units of the increasingly document short time intervals matic criteria with top to the east shear sense on Okanogan dome include well-developed, shal- for the crystallization, exhumation, and cool- the Kettle detachment and top to the west-north- lowly dipping gneissic foliation, pervasive lin- ing of dome migmatites, detachment faulting, west shear sense on the Okanogan detachment eation, micro- to meso-scale folds, mylonites, and extensional basin development (e.g., Keay (Fig. 2), consistent with east-west regional exten- breccia, and a series of northwest–trending, et al., 2001; Whitney et al., 2003). This strongly sion. The composite Okanogan-Kettle complex doubly plunging anticlines or subdomes that suggests that the dynamics of the partially is cut at its core by three en echelon graben ori- warp the regional fabric. Two of these smaller molten layer and upper crustal deformation are ented northeast-southwest, the Toroda Creek, domal features are defi ned as regions of radially temporally and kinematically linked. Republic, and Keller graben (Fig. 2), which are dipping foliations in the Tonasket gneiss and are In order to more fully explore the role of bounded by high-angle normal faults. informally named the Burge and Stowe Moun- crustal fl ow in the exhumation of the partially tain subdomes (Fig. 2). These subdomes expose molten crust and its interactions with upper Overview of Petrology and Structure of the the lowest structural levels in the complex and crustal deformation, studies of these systems Okanogan Gneiss Dome contain migmatite in their cores. need to address: (1) the timing of melt crystal- In the Stowe Mountain region along the west- lization in migmatites relative to the structural Lithologies exposed in the core of the Oka- ern margin of the Okanogan complex (Fig. 2), development of domes and metamorphic core nogan gneiss dome can be broadly divided into a continuous structural section exposes the complexes; (2) the duration of crustal melting four groups: (1) upper amphibolite-granulite transition from a domain of migmatite within and whether anatexis was a continuous or dis- facies layered paragneiss (including migmatites), the Tonasket gneiss upward to the base of the continuous process; and (3) the timing and rate quartzofeldspathic banded gneiss, amphibolite, Okanogan detachment mylonite (Fig. 3). Fold

Geological Society of America Bulletin, July/August 2008 913 Kruckenberg et al.

O berta

A S RO T I C N K Y P T Al M British Columbi O L E U U R N M O TA T IN O M O FO N I N N LD IC T E A A C N N A D A E T N H R D B U S E T N L T B FC O E L

R T TH 120 W C C A R 118 S Y 51 N + C S + A T D A L E L S IN

CANADA E Shuswap

B Complex USA E B L E T L TO T

Idaho

Washingto PPnn 50 + OCOC +50

VCVC

050km

PsPs

49 CANADA + +49 USA

KCKC

OCOC Idaho

PRCPRC Montana

48 N + + + 48 N 120 W 118 116 W

Primary Units of the Shuswap Metamorphic Complex

Eocene volcanic and stratified rocks

Cretaceous to Eocene plutonic rocks

Washington Idaho Geologic contact Footwall orthogneiss and paragneiss High-angle normal fault High-grade gneiss domes (commonly containing migmatites) Low-angle normal fault

Figure 1. Simpliﬁ ed geologic map of the southern Omineca belt (after Parrish et al., 1988; Stoffel et al., 1991; Doughty and Price, 1999). Inset shows tectonic belts of the northern Cordillera. High-grade gneiss domes within the Shuswap complex from north to south are: FC—French- man Cap; TO—Thor-Odin; Pn—Pinnacles; VC—Valhalla Complex; Ps—Passmore Complex; PRC—Priest River Complex; KC—Kettle Complex; OC—Okanogan Complex (note: north of the 49th parallel, Okanogan is spelled Okanagan).

914 Geological Society of America Bulletin, July/August 2008 Migmatite crystallization and exhumation in the Okanogan dome, Washington

119°W 118°W 49°N

01km t A 97 n e y a m ghw Graben h

Hi c a

Burge Mountain t e

Subdome D 10 Okanogan Kettle n t a n Toroda Creek 04OK338 og e n hm a c k ta r O e e Gneiss iv D R nogan Location of inset ka O and photo below

20 Gneiss

Stowe Mountain raben Dome

Subdome ic G Kettle 04OK315 35

epubl

23 R

Dome r

er e v v

15 i i

R R

n 16

a Omak

04OK311 g a

o i n b

a Fault m k Lake lu

O o

ben

Keller 04OK307b Gra Lincoln 48°N

Dome A kilometers p p ro 0 10 20 30 40 x 25 ans im tr it of io at e na e as l zone b f 17 o Primary units of the Okanogan dome n tio ca lo Volcanic and sedimentary 04OK305 graben strata 04OK304 MOUNTAIN Allocthonous Heterogeneous Tonasket STOWE units metamorphics paragneiss 41 Colville batholith rocks Orthogneiss Migmatitic Tonasket Gneiss Hardy Spring 10 Symbology Normal fault 04OK300a,b Trace of foliation Large-scale fold 03OK80a,b,c Lineation orientation Detachment fault Thrust fault

B STOWE MOUNTAIN

ent Zone

Okanogan DetachmTransition Zone Top to W-NW Migmatite Domain

Figure 2. (A) Simpliﬁ ed geologic map of the Okanogan-Kettle dome region with inset showing location of study (lithologic con- tacts drawn after Holder and Holder, 1988, and references therein; Stoffel et al., 1991). Note similarity in fold axis orientation, lineation orientation, and orientation of en echelon graben in the Okanogan dome. Inset shows transect for U-Pb and 40Ar/39Ar sampling near the Stowe Mountain subdome. Red arrows denote the axis of the Stowe Mountain subdome, purple stars are U-Pb sample localities, and yellow circles are locations of 40Ar/39Ar samples. (B) Image of the western margin of the Okanogan dome as viewed to the north from south of Stowe Mountain along the Okanogan River. Image highlights the structural section along which samples were obtained from the migmatite domain, zone of transition, and the base of the Okanogan detachment zone.

Geological Society of America Bulletin, July/August 2008 915 Kruckenberg et al. NE-vergent, lineation parallel fold in Wispy granodiorite diatexite Stromatic metatexite and leucosome

MIGMATITE DOMAIN MIGMATITE F D in inter-boudin regions E metatexite domain (view NW)

chment. Note the leucosome in inter-boudin chment. Note the leucosome in inter-boudin

ain Migmatite Dom Migmatite e and document the changes in deformational nsition zone. Figure is not drawn to scale. nsition zone. Figure

Domain Diatexite Metatexite Domain

Transition Zone Fold axes in migmatite domain ~ 300-400 m thickness Subsolidus Deformation Brittle Upper Crust Detachment Zone ~ 1.5 km thickness

Dome Poles to foliation (all units) NW Lineation (all domains) Fold axes in transition zone NW NW-vergent cascading fold (view NE) NW-vergent

Folded lineation around NW-vergent Folded lineation around NW-vergent Shear bands in Okanogan detachment C fold (view N) B A Figure 3. Structural relationships of the western Okanogan dome in vicinity Stowe Mountain. Field images (A–F) synthesiz 3. Structural relationships Figure style (melt-present deformation to solid-state deformation) from the diatexite domain (lowest) upward through the Okanogan deta the diatexite domain (lowest) upward through deformation to solid-state deformation) from style (melt-present the migmatite domain to tra data show structural fabrics and changes in fold orientation from in image D. Stereonet regions DETACHMENT AND TRANSITION DETACHMENT ZONE

916 Geological Society of America Bulletin, July/August 2008 Migmatite crystallization and exhumation in the Okanogan dome, Washington axes within the migmatitic Tonasket gneiss are 2002, 2003); (2) dilatancy sites such as shear of the lower migmatite domain (e.g., folded mig- dominantly oriented NW-SE and parallel the bands are commonly ﬁ lled with leucosome, and matitic lineation and foliation, Figs. 3B and 3C). regional lineation preserved throughout much of amphibolite layers boudinaged parallel to the Hansen and Goodge (1988) also recognized the Okanogan dome and the extensional Okano- stretching lineation show abundant leucosome the change in fold axis orientation in the Tonas- gan Valley detachment fault (~300° orientation). in inter-boudin regions (Fig. 3D); and (3) over- ket gneiss in the SW Okanogan dome and Migmatitic lineation in the Tonasket gneiss is printing relations indicate that leuco some layers attributed it to progressive reorientation during strong and deﬁ ned by the preferred orientation are folded and cut by more leucosome that is mylonitization of previously formed folds. In of streaked biotite or hornblende selvages and variably deformed (Fig. 4). the vicinity of Stowe Mountain, our work docu- elongate leucocratic lenses. Migmatitic lineation Above the migmatite structural domain ments a transition zone (~300–400 m) between is also expressed within the migmatite domain over a few hundred meters of structural thick- the structural fabrics in the migmatitic core and as an L>S gneissic fabric that is elongate paral- ness (~300–400 m), structures that indicate the those of the detachment fault, expressed by the lel to both the extension direction inferred from involvement of melt-present deformation in transition to solid-state deformation and cascad- boudin geometries in the migmatite domain and the Stowe Mountain migmatite domain show a ing W-NW vergent folds. These folds decrease the regional stretching lineation (Fig. 3). dramatic change in deformation style, fold ori- in amplitude and tighten progressively upward The migmatite domain on Stowe Moun- entation, and vergence direction. Deformation toward the base of the Okanogan Valley detach- tain can broadly be divided into a structurally within and above the transition zone affects the ment mylonites (Fig. 3). lower domain of diatexite and an overlying structural fabrics acquired in the lower migmatite domain of metatexite (Fig. 3). Several lines domain but also records evidence for deforma- Previous Geochronology of evidence indicate that deformation in both tion under subsolidus conditions, as evidenced by the diatexite and metatexite domains took crystal-plastic deformation of quartz and feld- There are few published geochronologic place in the presence of melt: (1) outcrop-scale spar, asymmetric mica grains and porphyroclasts, studies for the Okanogan dome, and the age and petrographic relations suggest that leuco- and pene trative quartz stretching lineation. Fold of migmatite crystallization has not been pre- some layers are largely the product of in situ axes in the transition zone trend N-NE, uniformly viously determined. Previous estimates of the melt segregation (McLellan, 1988; Brown, 1994; verge to the W-NW (parallel to the regional linea- age of metamorphism are based on relative Brown and Solar, 1998; Marchildon and Brown, tion), and fold the melt-present structural fabrics age relationships or are derived from tentative

Foliated discordant A granitic leucosome within B 04OK300a diatexite, locally pegma- Zircon: Range of ages from ca. titic and folded 61 to 53 Ma; dominant age peak + + Folded, foliated granitic + + ca. 60 Ma. (Mixed population?) + + leucosome (white) in metatexite (white+gray) . .. . Wispy granodiorite 300b . . . diatexite Disrupted, boudinaged, 04OK300b and foliated biotite Titanite: Ages ranging from ca. granodiorite layer 48 to 46 Ma; mean age ca. 47.1 Ma

03OK80a Monazite: Range of ages from ca. 80a 57 to 50 Ma; mean age ca. 52 Ma C 80b 03OK80c Zircon: Ages ranging from ca. 55 to 52 Ma; peak age ca. 53.5 Ma

03OK80b 80c Zircon: Range of ages from ca. 85 to 49 Ma (2 main populations); youngest peak age ca. 51 Ma

Monazite: Range of ages from ca. 58 to 50 Ma; mean age ca. 53 Ma 0.5 m

Figure 4. (A) Schematic drawing synthesizing leucosome relationships of sampled migmatite units on Stowe Mountain and summary of obtained sensitive high-resolution ion microprobe (SHRIMP) ages. Figure combines fi eld relationships observed over ~150 m of structural thickness within the migmatite domain. (B) Folded and foliated stromatic metatexite with arrow pointing to granitic leucosome of sample 04OK300b. (C) Diatexite gneiss and outcrop sampling locality of samples 03OK80a, 03OK80b, and 03OK80c showing the crosscutting relationships of these various phases and sampled layering at this locality. Sample 04OK300a is not shown on fi eld photographs but shares fi eld relationships similar to sample 03OK80c but within the metatexite domain.

Geological Society of America Bulletin, July/August 2008 917 Kruckenberg et al. correlations with rocks in the Shuswap meta- Diatexite of the lowermost migmatite domain collected on a map spacing of ~1 km covering morphic core complex in British Columbia. is fi ne grained and granodioritic, and migmatitic an approximate structural thickness of 1.5 km. Based on dated post-metamorphic plutonic textures are dominated by wispy schlieren-like This transect extends structurally upward from rocks and syntectonic plutons, peak meta- structures defi ned by alternating quartzofeld- diatexite exposed in the deepest structural lev- morphism is interpreted to have occurred in spathic layers and biotite selvages. Boudinaged els through migmatitic paragneiss and biotite Jurassic to Early Cretaceous time (Armstrong, layers of stromatic migmatite, amphibolite, ± hornblende quartzofeldspathic gneiss to the 1982; Archibald et al., 1983). Fox et al. (1976) calc-silicate, and other gneiss are also common, Okanogan Valley detachment fault. reported U-Pb ages of 100 Ma and 87.3 Ma for forming rafts (schollen texture) within leuco- zircons from upper amphibolite facies rocks of some-rich bodies (Fig. 4C; e.g., 80a). Three U-Pb GEOCHRONOLOGY the northern Tonasket gneiss. These ages were samples of diatexite were collected from a large interpreted as either the maximum time since outcrop just SW of Hardy Spring on eastern Results of U-Pb analyses are reported in crystallization, or the age of metamorphism Stowe Mountain (Figs. 2A and 4C). Samples the GSA Data Repository (Tables DR1–3)1. resulting in radiogenic Pb loss. K-Ar ages were 03OK80a and 03OK80b were collected from Analytical methods for geochronology are obtained from the youngest intrusive suite, the the most abundant and characteristic phases of described in the supplementary material in the Colville batholith (Fig. 2); these cooling ages diatexite observed. Sample 03OK80a is from a Data Repository. range between 53 and 45 Ma. Nearly all the discontinuous layer of stromatic, biotite grano- plutonic rocks of the Okanogan complex were diorite migmatite that contains a well-developed Zircon emplaced prior to, and are affected by, mylo- migmatitic foliation (Figs. 4A and 4C). This nitic deformation. layer is boudinaged within a more leucocratic Granodioritic Diatexite (03OK80b) Bounding faults of the Republic graben cut granodiorite diatexite containing wispy biotite Zircon crystals in sample 03OK80b are char- the intrusive suite and some zones of mylonitic selvages (sample 03OK80b; Figs. 4A and 4C). acterized by euhedral to subhedral grains, typi- deformation (Fox et al., 1976, 1977; Pearson Sample 03OK80c is from a discordant granitic cally 100–500 μm in length. Zircons commonly and Obradovich, 1977; Parkinson, 1985; Tem- leucosome that cuts across structural fabrics in form elongate di-pyramidal crystals with length pelman-Kluit and Parkinson, 1986; Holder and diatexite samples 03OK80a and 03OK80b but to width ratios >3:1. When viewed in transmitted Holder, 1988; Carlson and Moye, 1990; Box and may also be foliated and folded with these units light, zircon grains are clear and rarely contain Wooden, 1994). Basin fi ll of the Toroda Creek when followed parallel to layering (Figs. 4A and visible inclusions or cracks. Under cathodolu- and Republic graben is dated at ca. 54 to 48 Ma 4C). Based on crosscutting fi eld relationships, minescence (CL), grains have varying internal (Pearson and Obradovich, 1977; Atwater, 1985; 03OK80c is the youngest leucosome phase structure but typically display sector and oscilla- Suydam and Gaylord, 1997), and therefore sampled at this locality (Figs. 4A and 4C). All tory-zoned euhedral to subhedral core domains intrusion of the Colville batholith, mylonitic samples were separated for zircon, monazite, surrounded by rim overgrowths (Fig. 5A). A deformation, cooling and exhumation, and re- and titanite; sample 03OK80a yielded monazite, small number of grains have less defi ned or burial beneath the basin fi ll took place in a rela- both zircon and monazite were present in sample complexly zoned cores that contain highly tively short time interval. 03OK80b, and 03OK80c yielded only zircon. luminescent anhedral cores also surrounded The age of partial melting in the Okanogan Two samples, 04OK300a and 04OK300b, by oscillatory-zoned domains (e.g., grain 17, dome is unknown, and therefore important were collected from an outcrop in the metatex- Fig. 5A). Overgrowths on cores are common relationships between partial melting (migma- ite domain structurally above the diatexite at and display great variation in their thickness and tization), dome development, and extension are locality 03OK80. Sample 04OK300b (Figs. 4A luminescence and range from fi nely zoned rims unresolved. We present new data that bracket and 4B) is from a parasitically folded leucosome to homogeneous rims or tips (Fig. 5A). the timing of crystallization and cooling of layer within a large fold with a lineation-parallel We analyzed 28 zircon grains with 37 individ- partially molten crust during different stages fold hinge (280º orientation) characteristic of the ual spot analyses in sample 03OK80b. Analyses of deformation in the tectonic evolution of the folding style in the migmatite domain at Stowe were selected in both core and rim domains of Okanogan dome. Mountain. Sample 04OK300a is from a weakly grains with variable internal structure. The zircon foliated discordant leucosome layer that, like population of sample 03OK80b yielded the widest SAMPLING METHODOLOGY AND sample 03OK80c, cuts the folded migmatite leu- range of 206Pb/238U ages of all the zircon samples SAMPLE DESCRIPTIONS cosome layers at a high angle but also becomes analyzed. The majority of analyses (N = 24) have folded and foliated itself when followed along Late Cretaceous 206Pb/238U ages for both core U-Pb Samples its outcrop trace (Fig. 4A). Sample 04OK300a and rim domains, defi ning a broad group of ages and sample 03OK80c, though from different from ca. 85 to 70 Ma (Figs. 5B and 5C, Table Sampling for U-Pb isotopic analysis focused parts of the migmatite domain, show the same DR1). A second distinct group of 206Pb/238U ages, on migmatite exposed on eastern Stowe Moun- structural relationships and are compositionally ranging from ca. 57 to 49 Ma (Figs. 5B and 5C, tain where structural evidence indicates that similar. Sample 04OK300a yielded zircon, and Table DR1), was obtained from 13 analytical deformation took place in the presence of melt sample 04OK300b contained no zircon but had spots. These Paleocene-Eocene ages occur both (McLellan, 1988; Brown, 1994; Brown and abundant titanite. in zircon cores and rims (e.g., grains 1 and 4; Solar, 1998; Marchildon and Brown, 2002, Fig. 5A). The Paleocene-Eocene 206Pb/238U ages 2003). Five U-Pb samples were selected from 40Ar/39Ar Samples defi ne two sub-peaks, one centered at ca. 55 Ma folded, boudinaged, and crosscutting leucosome layers within both diatexite and metatex- Samples for 40Ar/39Ar analysis were collected 1GSA Data Repository Item 2008079, Tables ite domains where their relationships could be on a transect ~6 km long, extending from eastern DR1–4 and analytical methods, is available at www. tied to specifi c structural characteristics of the Stowe Mountain NW toward the western margin geosociety.org/pubs/ft2008.htm. Requests may also Okanogan dome (Fig. 4). of the Okanogan dome (Fig. 2). Samples were be sent to [email protected].

918 Geological Society of America Bulletin, July/August 2008 Migmatite crystallization and exhumation in the Okanogan dome, Washington

03OK80b zircons, wispy granodiorite diatexite A

5 48.8 52.3 3 54.6

4 78.7 7 54.5 73.7 50.9 1 52.3 79.4 10 50.2 81.8 84.4 85.0 73.8 15 11 14 17

80.0 74.2 82.0 13 55.9 20 70.4 8 23 24 μ 80.9 200 m 81.6 21 78.1 84.8 69.6

03OK80b 03OK80b 0.08 7 B Mean 206Pb/238U = 51.1 ± 1.0 Ma C 6 N=6 (young population), MSWD=1.6 Relative probability 0.07 5

207Pb 4 0.06 206 Pb 3 Number

2 0.05 120 100 80 60 50 1

0.04 0 50 70 90 110 130 150 45 50 55 60 65 70 75 80 85 90 238U/206Pb Age 206Pb/238U Age Figure 5. (A) Cathodoluminescence images of representative zircon grains dated using the sensitive high-resolution ion microprobe (SHRIMP) for sample 03OK80b (wispy granodiorite diatexite). Small numbers (1–24) refer to grain number designations in Table DR1. Larger numbers are 206Pb/238U ages in Ma for individual spot analyses (see Table DR1 for corresponding error on spot analyses). (B) Tera-Wasserburg concordia diagram of the calibrated total 238U/206Pb ratios versus the total 207Pb/206Pb ratio for zircon sample 03OK80b. Analyses plotted as one sigma error ellipses. (C) Probability density plots with stacked histogram for zircon sample 03OK80b; the weighted mean 206Pb/238U age calculation and uncertainty is given at 95% conﬁ dence limits and includes the uncertainty in the U/Pb ratio calibration of the reference zircon. Plots and calculations based on Isoplot of Ludwig (2003).

(Fig. 5C), and a second well-defi ned sub-peak of lar degrees of luminescence under CL and have ity distribution (Fig. 6C) with a weighted mean six analyses yields the youngest mean 206Pb/238U well-defi ned oscillatory and sector-zoned mag- for all 19 analyses of 53.7 ± 0.6 Ma (MSWD age of 51.1 ± 1.0 Ma (mean square of weighted matic cores (Fig. 6A). Unzoned zircon grains are = 2.0). Eliminating analyses 6.1, 9.1, 10.1, and deviates [MSWD] = 1.6) for this sample. visible in CL and commonly display a thin (<20 16.1, which fall more than 3 sigma from the μm), highly luminescent rim at their margin. mean, results in a weighted mean age of 53.5 Discordant Granitic Leucosome in Diatexite A total of 19 zircon grains was analyzed from ± 0.5 Ma (MSWD = 1.1, Fig. 6C). Only one (03OK80c) 20 total areas in sample 03OK80c. Nineteen analysis (7.1, Table DR1) had an age outside of The zircon population in sample 03OK80c of the 20 analyzed spots give 206Pb/238U ages the main distribution, yielding a 206Pb/238U age consists largely of inclusion-free euhedral grains ranging from ca. 55 to 52 Ma for both core and of ca. 76 Ma. This age is similar to Late Creta- that form elongate to equant crystals with lengths rim analyses (e.g., grain 1, Fig. 6A; Table DR1). ceous ages for zircon in the older population of typically 200–300 μm. Zircon grains have simi- These analyses defi ne a simple relative probabil- sample 03OK80b.

Geological Society of America Bulletin, July/August 2008 919 Kruckenberg et al.

03OK80c zircons, discordant granitic leucosome in diatexite A

53.9 7 53.5 52.9

1 3 75.5 2 52.2 52.5 54.3

55.4 9 11 54.4 12

55.2 52.3 13 52.6 200 μm 17 16 18

03OK80c 03OK80c 9 Mean 206Pb/238U = 53.5 ± 0.5 Ma B 8 C 0.07 N=15, MSWD=1.1 7 Relative probability 6 0.06 5 207Pb 206 4

Pb Number 3 0.05 2 60 58 56 54 52 50 1 0.04 0 106 110 114 118 122 126 130 49 51 53 55 57 59 238U/206Pb Age 206Pb/238U Age Figure 6. (A) Cathodoluminescence images of representative zircon grains dated using the sensitive high-resolution ion microprobe (SHRIMP) for sample 03OK80c (discordant granitic leucosome in diatexite). Small numbers (1–18) refer to grain number designations in Table DR1. Larger numbers are 206Pb/238U ages in Ma for individual spot analyses (see Table DR1 for corresponding error on spot analyses). (B) Tera-Wasserburg concordia diagram of the calibrated total 238U/206Pb ratios versus the total 207Pb/206Pb ratio for zircon sample 03OK80c. Analyses plotted as one sigma error ellipses. (C) Probability density plots with stacked histogram for zircon sample 03OK80c; the weighted mean 206Pb/238U age calculation and uncertainty is given at 95% conﬁ dence limits and includes the uncertainty in the U/Pb ratio calibration of the reference zircon. Plots and calculations based on Isoplot of Ludwig (2003).

Discordant Granitic Leucosome in Metatexite In this sample, a total of 20 areas on 20 grains Fig. 7A). When these four analyses, which are (04OK300a) were analyzed. Despite the apparent internal more than 3 sigma from the mean, are excluded, Zircon grains in sample 04OK300a con- complexity revealed in CL, the majority of rim the weighted mean age of the remaining 16 anal- tain more abundant inclusion trails and a analyses and all 11 cores analyzed gave Paleo- yses is 59.8 ± 0.5 Ma (MSWD = 1.2, Fig. 7C). higher density of microcracks than zircon in cene 206Pb/238U ages from ca. 61 to 58 Ma (Table diatexite. Grains are clear, euhedral to sub- DR1). Four rim analyses (13.1 16.1, 17.1, and 18.1; Monazite hedral, and range in size from 50 to 400 μm. Table DR1) yielded 206Pb/238U ages extending into Zircons range from unzoned to irregu- Eocene time and are younger than the dominant Biotite Granodiorite Layer in Diatexite larly zoned (Fig. 7A). Ten- to 60-μm–thick age peak (Fig. 7C). Three of the four analyses (03OK80a) euhedral overgrowths on complexly zoned cores recording younger 206Pb/238U ages are located Monazite grains obtained from sample are common. on distinct tips and rims (grains 13, 16, and 18; 03OK80a are typically brown to honey-colored

920 Geological Society of America Bulletin, July/August 2008 Migmatite crystallization and exhumation in the Okanogan dome, Washington

04OK300a zircons, A discordant granitic leucosome in metatexite 16

5 3 59.6 6 57.9

59.3 2 60.7 54.5 8 53.2 56.8 59.2 18 58.5 7 19

13 200 μm 60.5

04OK300a 04OK300a 0.09 9 Mean 206Pb/238U = 59.8 ± 0.5 Ma B 8 N=16, MSWD=1.2 C

0.08 7 Relative probability 6 0.07 207Pb 5 206Pb 4

0.06 Number 3

0.05 2 70 66 62 58 54 50 1 0.04 0 90 100 110 120 130 51 53 55 57 59 61 63 238U/206Pb Age 206Pb/238U Age Figure 7. (A) Cathodoluminescence images of representative zircon grains dated using the sensitive high-resolution ion microprobe (SHRIMP) for sample 04OK300a (discordant granitic leucosome in metatexite). Small numbers (2–19) refer to grain number designations in Table DR1. Larger numbers are 206Pb/238U ages in Ma for individual spot analyses (see Table DR1 for corresponding error on spot analyses). (B) Tera-Wasserburg concordia diagram of the calibrated total 238U/206Pb ratios versus the total 207Pb/206Pb ratio for zircon sample 04OK300a. Analyses plotted as one sigma error ellipses. (C) Probability density plots with stacked histogram for zircon sample 04OK300a; the weighted mean 206Pb/238U age calculation and uncertainty is given at 95% conﬁ dence limits and includes the uncertainty in the U/Pb ratio calibration of the reference zircon. Plots and calculations based on Isoplot of Ludwig (2003).

grains when viewed in transmitted light, and are ages on the Tera-Wasserburg diagram (Fig. 8B). structure in sample 03OK80b. Sector and sym- relatively uniform in size (150–200 μm). Most The range of monazite 206Pb/238U ages spans metric growth zoning are typical with a subor- grains are equant and anhedral to subhedral in ~7 m.y., and the dominant group (N = 14) yields dinate number of grains displaying complex shape with rare, well-formed euhedral crystals. a weighted mean age of 52.0 ± 0.6 Ma (MSWD or embayed zonation in their cores (Fig. 9A). Back-scattered electron (BSE) images of mona- = 0.8, Fig. 8C). Overgrowths are common and typically occur zite show a dominantly unzoned population of as 30–100 μm homogeneous rims that appear grains with a subordinate number displaying Granodioritic Diatexite (03OK80b) brighter in BSE images (e.g., grains, 12, 15, and sector zoning (e.g., grains 5 and 18, Fig. 8A). Sample 03OK80b is the only sample that 16, Fig. 9A). Eighteen monazite grains were analyzed for yielded both zircon and monazite. Mona- Twenty locations on 18 monazite grains a total of 21 analyses. The majority of analyses zite grains are commonly subhedral and form were analyzed, yielding 206Pb/238U ages from (N = 19) yield Eocene 206Pb/238U ages ranging 200–400 μm equant grains of light honey color. ca. 58 to 50 Ma and are similar to 206Pb/238U from ca. 57 to 50 Ma (Figs. 8B and 8C; Table Monazite grains in sample 03OK80b contrast monazite ages obtained from sample 03OK80a DR2) and show no correlation to zoned domains those in 03OK80a and typically have less erratic (Table DR2). Four analyses gave older Paleo- (where present) or location in monazite grains. shapes with better deﬁ ned crystal faces. Addi- cene 206Pb/238U ages from ca. 58 to 56 Ma Two analyses give Paleocene 206Pb/238U ages and tionally, BSE imaging shows that monazite and are older than the main cluster of uncor- are older than the main cluster of uncorrected grains characteristically have a zoned internal rected analyses on the Tera-Wasserburg plot

Geological Society of America Bulletin, July/August 2008 921 Kruckenberg et al.

A 03OK80a monazite, disrupted, boudinaged, and foliated biotite 03OK80a granodiorite layer in diatexite 0.060 B

0.056 6 54.2 0.052 207Pb 206 51.3 Pb 0.048 5 60 58 56 54 52 50 48 46 0.044

0.040

105 115 125 135 7 238U/206Pb Age

54.7 03OK80a 52.3 55 C 8 54

52.5 52.8 U Age 52 238 51 Pb/

11 206 18 50

200 μm 49 Mean 206Pb/238U = 52.0 ± 0.6 Ma N=14, MSWD=0.8 48 Figure 8. (A) Back-scattered electron (BSE) images of representative monazite grains dated using the sensitive high-resolution ion microprobe (SHRIMP) for sample 03OK80a (disrupted, boudinaged, and foliated biotite granodiorite layer in diatexite). Small numbers (5–18) refer to grain number designations in Table DR2. Larger numbers are 206Pb/238U ages in Ma for individual spot analyses (see Table DR2 for corresponding error on spot analyses). (B) Tera-Wasserburg concordia diagram of the calibrated total 238U/206Pb ratios versus the total 207Pb/206Pb ratio for monazite sample 03OK80a. Analyses plotted as one sigma error ellipses. (C) Weighted average age plot from monazite sample 03OK80a; the weighted mean 206Pb/238U age calculation and uncertainty is given at 95% conﬁ dence limits and includes the uncertainty in the U/Pb ratio calibration of the reference monazite. Plots and calculations based on Isoplot of Ludwig (2003).

of Figure 9B. The bulk of analyses (N = 16) metatexite, are clear to transparent and anhe- analyses. These analyses deﬁ ne a simple distri- yield Eocene 206Pb/238U ages from ca. 54 to dral (Fig. 10A), with only minor inclusions bution (Fig. 10C) with a weighted mean for all 50 Ma (Fig. 9C) and have a weighted mean and cracks. In BSE images, titanite displays 18 analyses of 47.1 ± 0.5 Ma (MSWD = 0.6, age of 52.3 ± 0.8 Ma (MSWD = 2.2) for all dramatic zoning characterized in some grains Fig. 10C). 16 spots. Excluding the three youngest analy- by a bright rim of varying width around a ses (6.1, 9.1, and 15.1, Table DR2), which fall darker core. The core domain in some grains is BIOTITE 40Ar-39Ar THERMOCHRONOLOGY more than 3 sigma from the mean, results in homogeneous and in others displays euhedral a weighted mean age of 52.9 ± 0.6 Ma and a growth zoning (Fig. 10A). Some grains also Results of Ar-Ar analyses are reported in lower MSWD of 0.8. show embayed rim and core relationships (e.g., the GSA Data Repository (Table DR4; see grain 8, Fig. 10A). footnote 1) and a summary of Ar-Ar age Titanite We analyzed both rim and core domains of determinations is reported in Table 1. Ana- 15 grains for a total of 18 analyses (Table DR3). lytical methods for thermochronology are Titanite grains from sample 04OK300b, a All analyzed spots gave Eocene 206Pb/238U ages described in the supplementary material in the folded and foliated granitic leucosome from a ranging from ca. 48 to 46 Ma for core and rim Data Repository.

922 Geological Society of America Bulletin, July/August 2008 Migmatite crystallization and exhumation in the Okanogan dome, Washington

03OK80b A 03OK80b monazite, wispy granodiorite diatexite 0.060 B

0.056 1 52.6 0.052 207Pb 206 51.9 Pb 0.048 3 64 60 56 52 48 0.044

52.1 0.040 100 110 120 130 140 12 238U/206Pb Age 8 03OK80b 53.1 57 C 52.3 56.5 55 15

U Age 53 238

50.2 Pb/ 16 206 51 200 μm Mean 206Pb/238U = 52.9 ± 0.6 Ma N=13, MSWD=0.8 49 Figure 9. (A) Back-scattered electron (BSE) images of representative monazite grains dated using the sensitive high-resolution ion microprobe (SHRIMP) for sample 03OK80b (wispy granodiorite diatexite). Small numbers (1–16) refer to grain number designations in Table DR2. Larger numbers are 206Pb/238U ages in Ma for individual spot analyses (see Table DR2 for corresponding error on spot analyses). (B) Tera-Wasserburg concordia diagram of the calibrated total 238U/206Pb ratios versus the total 207Pb/206Pb ratio for monazite sample 03OK80b. Analyses plotted as one sigma error ellipses. (C) Weighted average age plot from monazite sample 03OK80b; the weighted mean 206Pb/238U age calculation and uncertainty is given at 95% conﬁ dence limits and includes the uncertainty in the U/Pb ratio calibration of the reference monazite. Plots and calculations based on Isoplot of Ludwig (2003)

Samples with Unaltered Biotite (04OK305, Ages for individual heating steps range from high MSWD from the isochron reduction, the 04OK307b, and 04OK338) ca. 48 to 26 Ma and give an integrated age of 47.9 Ma isochron age is our preferred age for 44.7 ± 0.5 Ma (Table 1, Table DR4). Results this sample (Table 1). Biotite grains in this group of samples for sample 04OK305 exhibit a plateau-like Fourteen step-heating analyses, from T = (04OK305, 04OK307b, and 04OK338) are pattern over steps 5–8, and ages range from 570–1060 ºC, resulted in total release of Ar dominantly unaltered, dark brown, and range in ca. 48 to 46 Ma (Fig. 11A). Using size of step from biotite sample 04OK307b. Individual step size from <50 μm to >750 μm. These samples weighting, the calculated age for this sec- intervals gave ages of ca. 51 to 43 Ma with an yield the least discordant ages of the samples tion of the age spectrum is 47.1 ± 0.2 Ma for integrated age of 47.7 ± 0.4 Ma (Table 1, Table analyzed, and plateau-like segments for a signif- 61.9% of the gas released (Fig. 11A, Table 1). DR4). The age spectrum defi nes a large plateau- icant number of steps characterize the stepwise Younger ages are evolved in the early gas like section defi ned by steps 4–8, yielding an age heating and release of argon. release, with monotonically rising ages until of 47.9 ± 0.2 Ma over 83.0% of the gas evolved Sample 04OK305 yielded the most discor- the plateau-like section is reached. An inverse (Fig. 11B, Table 1). Inverse isochron analysis dant ages within this group. Total release of isochron analysis of all 14 step-heating analy- of all 14 steps defi nes an intercept age of 47.9 Ar for sample 04OK305 was achieved over ses yields an intercept age of 47.9 ± 0.1 Ma ± 0.1 Ma (1σ, MSWD = 3.4; Fig. 11B), identi- 14 step-heating intervals from 570 to 1120 ºC. (1σ, MSWD = 8.4; Fig. 11A). Despite the cal to the age obtained from the plateau-like seg-

Geological Society of America Bulletin, July/August 2008 923 Kruckenberg et al.

A 04OK300b titanite, folded, foliated granitic 04OK300b leucosome in metatexite 0.30 B 3 0.26

1 0.22 48.1 207Pb 0.18 47.2 206Pb 0.14

0.10 46.1 0.06 5 78 74 70 66 62 58 54 50 46 0.02 47.7 80 100 120 140 238U/206Pb Age 48.5 04OK300b 7 10 Mean 206Pb/238U = 47.1 ± 0.5 Ma 9 C 46.7 N=18, MSWD=0.6 8 Relative probability 7 47.8 48.4 6 5 8

Number 4 9 3 2 47.2 1 200 μm 0 43 45 47 49 51 53 206Pb/238U Age Figure 10. (A) Back-scattered electron (BSE) images of representative titanite grains dated using the sensitive high- resolution ion microprobe (SHRIMP) for sample 04OK300b (folded, foliated granitic leucosome in metatexite). Small numbers (1–9) refer to grain number designations in Table DR3. Larger numbers are 206Pb/238U ages in Ma for individual spot analyses (see Table DR3 for corresponding error on spot analyses). (B) Tera-Wasserburg concordia diagram of the calibrated total 238U/206Pb ratios versus the total 207Pb/206Pb ratio for titanite sample 04OK300b. Analyses plotted as one sigma error ellipses. The 3-D linear regression line is shown from the measured 204Pb/206Pb ratio for each analysis along with total 207Pb/206Pb and 238U/206Pb ratios to deﬁ ne a lower concordia intercept of 47.0 ± 0.5 Ma (mean square weighted deviate [MSWD] = 1.0). (C) Probability density plots with stacked histogram for titanite sample 04OK300b; the weighted mean 206Pb/238U age calculation and uncertainty is given at 95% conﬁ dence limits and includes the uncertainty in the U/Pb ratio calibration of the reference titanite. Plots and calculations based on Isoplot of Ludwig (2003).

TABLE 1. SUMMARY OF AR-AR AGE DETERMINATIONS FOR BIOTITE SAMPLES Sample Location Preferred age Isochron method Integrated age Age Y-intercept Ar40/Ar36 MSWD 04OK305 N48°36.31′, W119°25.92′ 47.9 ± 0.1 Ma (steps 5–8) 47.9 ± 0.1 Ma 258.9 ± 1.6 8.37 44.7 ± 0.5 Ma 04OK307b N48°36.75′, W119°26.48′ 47.9 ± 0.2 Ma (steps 4–8) 47.9 ± 0.1 Ma 289.8 ± 2.6 3.38 47.7 ± 0.4 Ma 04OK338 N48°38.19′, W119°27.84′ 48.0 ± 0.1 Ma (steps 2–14) 48.0 ± 0.1 Ma 293.2 ± 3.4 2.11 47.9 ± 0.7 Ma 04OK304 N48°36.30′, W119°25.87′ - 45.2 ± 0.1 Ma 233.6 ± 1.8 15.72 28.0 ± 0.8 Ma 04OK311 N48°37.04′, W119°27.04′ - 44.8 ± 0.1 Ma 257.5 ± 1.3 38.73 38.7 ± 0.7 Ma 04OK315 N48°37.35′, W119°27.49′ - 44.0 ± 0.2 Ma 241.9 ± 1.7 62.13 34.9 ± 0.6 Ma Note: Uncertainties given at the 1σ level. MSWD—mean square of weighted deviates.

ment. Our preferred age for sample 04OK307b steps (Table DR4). The integrated age is 47.9 total 39Ar released (Fig. 11C, Table 1). Like is 47.9 ± 0.2 Ma (Table 1). ± 0.7 Ma (Table 1). Biotite from 04OK338 dis- sample 04OK307b, inverse isochron analysis Total release of argon from biotite sample plays a plateau-like pattern for a large section of sample 04OK338 gives an intercept age 04OK338 was achieved over 14 step-heating of gas release. Size of step weighting of steps identical to that calculated for the plateau-like intervals (570–1120 ºC), with ages ranging 2–14 of the analysis yields a preferred plateau- segment at 48.0 ± 0.1 Ma (1σ, MSWD = 2.1; from ca. 49 to 44 Ma for individual heating like age of 48.0 ± 0.1 Ma over 97.1% of the Fig. 11 C).

924 Geological Society of America Bulletin, July/August 2008 Migmatite crystallization and exhumation in the Okanogan dome, Washington

A 04OK305 54 54 0.004 Plateau-like segment Isochron Age = 47.9 ± 0.1 Ma (1σ) 52 47.1 ± 0.2 Ma 52 Y intercept = 3.863E-3 over 61.9% of 39Ar released X intercept = 8.103E-2 50 50 0.003 + MSWD = 8.371 48 48 + 36 Age 46 46 Ar 0.002 + (Ma) 40Ar 44 44 +++ +++ 42 42 0.001 +

40 40 ++ + 38 0 0.0 0.2 0.4 0.6 0.8 1.0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Fraction 39Ar released 39Ar/40Ar

B 04OK307b 54 54 0.004 Isochron Age = 47.9 ± 0.1 Ma (1σ) Plateau-like segment 52 47.9 ± 0.2 Ma 52 Y intercept = 3.451E-3 over 83.0% of 39Ar released X intercept = 8.121E-2 50 50 0.003 MSWD = 3.381 + 48 48 Age 36Ar 46 46 0.002 + (Ma) 40Ar + 44 44 + 42 42 0.001 +++ + 40 40 + ++ 38 0 + 0.0 0.2 0.4 0.6 0.8 1.0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Fraction 39Ar released 39Ar/40Ar

C 04OK338 54 54 0.004 Plateau-like segment Isochron Age = 48.0 ± 0.1 Ma (1σ) 52 48.0 ± 0.1 Ma 52 Y intercept = 3.411E-3 over 97.1% of 39Ar released X intercept = 8.124E-2 50 50 0.003 MSWD = 2.111 + 48 48 36Ar Age 46 46 0.002 + (Ma) 40Ar 44 44 + 42 42 0.001 + + +++ 40 40 +++ +++ 38 0 0.0 0.2 0.4 0.6 0.8 1.0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Fraction 39Ar released 39Ar/40Ar

Figure 11. Age spectrum for stepwise release of argon and inverse isochron diagrams for the step-heating results of biotite samples (A) 04OK305, (B) 04OK307b, and (C) 04OK338. Note the similarity between the age of the selected plateau-like segments and the associated inverse isochron intercept age. Horizontal gray dashed interval is time window from 49 to 47 Ma, shown for clarifying temporal relationships among samples.

Geological Society of America Bulletin, July/August 2008 925 Kruckenberg et al.

Samples with Partial Alteration of Biotite 54 54 (04OK304, 04OK311, and 04OK315) 46 46 40Ar/39Ar results for biotite from samples 04OK304, 04OK311, and 04OK315 lack pla- 38 Age 38 teau-like features in the age spectra (Fig. 12). (Ma) These biotite grains are partially altered to chlo- 30 30 rite in discrete domains or along cleavages. In contrast, the three samples yielding plateau-like 22 22 segments, described in the previous section, lack chlorite. 40Ar/39Ar ages for the altered bio- 14 14 tite are scattered from ca. 46 to 17 Ma (Table DR4), suggesting that the alteration has had a 0.0 0.2 0.4 0.6 0.8 1.0 signifi cant impact on the retention of radiogenic Fraction 39Ar released argon in the biotite host. Figure 12. Combination plot of disturbed age spectrum INTERPRETATION OF THE for stepwise release of argon from samples 04OK304 GEOTHERMOCHRONOLOGY DATA (lower), 04OK311 (top), and 04OK315 (middle). Hori- zontal gray dashed interval is time window from 49 to U-Pb SHRIMP ages for zircon and mona- 47 Ma shown for clarifying temporal relationships of zite from Okanogan migmatite range from 85 samples with spectra reported in Figure 11. to 49 Ma, although within this range only two samples (03OK80b and 03OK80c) contain phases dated at greater than 61 Ma. Zircon clearly younger than the main age population ca. 54 to 48 Ma ages of Okanogan-Kettle horn- 206Pb/238U ages in sample 03OK80b, the wispy at ca. 60 Ma. Field relationships clearly show blende, biotite, and muscovite (Fox et al., 1976; granodiorite diatexite in the core of the Stowe this leucosome, along with discordant leuco- Pearson and Obradovich, 1977; Holder and Mountain subdome, yield the largest range of some in the diatexite domain, to be the young- Holder, 1988; Berger and Snee, 1992). ages with two populations at ca. 85 to 70 Ma est in terms of crosscutting relations (Fig. 4A). Considering the time interval from the mean and ca. 57 to 49 Ma. Three possible inter- Discordant leucosome in the structurally lower 206Pb/238U age of 51.1 ± 1.0 Ma for the diatexite pretations could explain the Late Cretaceous diatexite domain is dated at 53.5 ± 0.5 Ma. This and the titanite ca. 47 Ma age, these data indi- ages. These ca. 85 to 70 Ma ages may repre- ca. 53 Ma age is similar to the youngest ages cate rapid cooling of ~100 °C/m.y. over the time sent zircon crystallization related to regional obtained in sample 04OK300a, and is likely interval 51 to 47 Ma (i.e., from >800 to 325 °C metamorphism or an earlier migmatization closer to the true geologic age of the discor- in 4 m.y.). Rapid cooling and the lack of differ- episode. Both of these explanations are reason- dant leucosome in the metatexite domain. We ence in cooling age between the diatexite core able because the high-grade metamorphism therefore interpret the bulk of ca. 60 Ma ages and higher structural levels is likely related to of other domes in the Omineca belt has been in sample 04OK300a as inherited ages, likely rapid exhumation of the migmatite dome. dated as Jurassic to Cretaceous (Armstrong, derived from the host migmatites, during crys- 1982; Archibald et al., 1983). Alternatively, the tallization of the discordant leucosome. CRYSTALLIZATION AND range of ages may also refl ect melting of het- Despite the complexities in resolving the age EXHUMATION OF MIGMATITES erogeneous source rocks (including plutons) discrepancy in sample 04OK300a, all samples and could therefore represent inherited ages. analyzed record Paleocene to Eocene ages for Our new data suggest that the Okanogan The Paleocene to Eocene ages determined new zircon growth and rim overgrowths that we dome records at least 12 m.y. of migmatite crys- in sample 03OK80b from rims on older cores attribute to periods of migmatite crystallization. tallization during Paleocene to Eocene time. and from newly formed zircon in leucosome The Okanogan dome therefore records a com- Based on our existing data it is not possible to are recorded in all fi ve samples (three diatex- plex deformational history and has a history distinguish whether partial melting and associ- ite and two metatexite). Based on structural of migmatite crystallization spanning ~12 m.y. ated migmatite crystallization was achieved by evidence that indicates melt-present defor- (ca. 61 to 49 Ma range of 206Pb/238U zircon and a continuous process or the result of discon- mation, the structural context of the samples monazite ages) during Paleocene-Eocene time. tinuous events. For events prior to ca. 61 Ma, (Figs. 3 and 4A) and grain morphologies, these Following crystallization of the youngest it is diffi cult to determine the duration of par- results are interpreted as the timing of mig- leucosome, the rocks cooled rapidly from high- tial melting or whether the crust contained a matite crystallization. Within the diatexite, all temperature through the closure temperature signifi cant volume of partial melt. In addition, samples have U-Pb zircon ages for new zircon of Ar in biotite (~325 °C). All rocks from the the crust at current exposure levels represents growth and rim overgrowths ranging from 61 diatexite core to the detachment zone record a small fraction of the likely layer of former to 49 Ma. All monazite grains analyzed are less 40Ar/39Ar biotite ages of ca. 48 Ma. U-Pb titan- partially molten crust; therefore we have an than ca. 55 Ma. The oldest mean zircon age ite ages are consistently ca. 47 Ma. These ages incomplete view of the orogenic crust. How- obtained (ca. 60 Ma; Figs. 4 and 7) is from a likely represent cooling (in zoned grains, core ever, we can consider at least two distinct cases discordant leucosome in the metatexite domain and rim ages are the same), and the similarity for the duration of partial melting, and discuss (sample 04OK300a). Despite having the old- of biotite Ar ages and titanite U-Pb ages indi- the likely consequences of each model. est calculated age of all the migmatite samples cate rapid cooling through ~325 °C by ca. 48 Crustal melting may have been a protracted analyzed, a spread of ages to ca. 53 Ma are to 47 Ma. Biotite and titanite ages overlap with event, starting during Late Cretaceous crustal

926 Geological Society of America Bulletin, July/August 2008 Migmatite crystallization and exhumation in the Okanogan dome, Washington

Deformation / cooling of Okanogan and Kettle detachments: ca. 54 to 47 Ma This study: ca. 48 to 47 Ma (Ar-Ar biotite, U-Pb titanite) Previous studies: (1, 3, 5, 6, 7, 9, 10)

Graben formation: ca. 53 to 48 Ma Figure 13. Schematic cross sec- (3, 4, 11) tion and summary of timing West East relationships in the Okanogan- Okanogan Kettle Dome Dome Kettle metamorphic core com- 0 0 plex. References are: 1—Fox et al. (1976); 2—Fox et al. (1977); 3—Pearson and Obradovich Migmatites (1977); 4—Atwater (1985); km km 5—Parkinson (1985); 6—Tem- pelman-Kluit and Parkinson (1986); 7—Holder and Holder 0 km 50 (1988); 8—Carlson and Moye 50 Crystallization of dome migmatites: 50 (1990); 9—Berger and Snee ca. 61 to 49 Ma (1992); 10—Box and Wooden (This Study; U-Pb zircon, monazite) Plutonism: ca. 53 to 45 Ma (1994); 11—Suydam and Gay- lord (1997). Metamorphism/age of Tonasket (1, 2, 3, 7, 8) gneiss: ca. 100 to 70 Ma This study: ca. 85 to 70 Ma (U-Pb zircon) Previous studies: (1)

thickening and metamorphism, and continuing Whether partial melting was a continuous Paleocene to Eocene time (Fig. 1). These data until cooling and exhumation in the early Ter- or episodic event spanning ~12 m.y. or longer, further suggest that melt-rich orogens, once tiary. According to this model, the U-Pb results it is clear that melt crystallization is kinemati- subjected to kinematic or thermal changes, of this study are minimum ages for the time cally linked to the structural development of rapidly exhume deep orogenic crust and cool interval in which the Omineca Belt contained the Okanogan dome, that the Okanogan dome rapidly—essentially ending orogeny. signiﬁ cant amounts of partially molten crust, has been thoroughly reworked by high-grade and, if so, the duration of migmatization may metamorphism and crustal melting, and is not ACKNOWLEDGMENTS have spanned ~35 m.y. Alternatively, Creta- simply exhumed “old basement.” Dynamic ceous crustal thickening may not have resulted and temporal relationships among doming, This research is supported by National Science Foundation grant EAR-0409776 to Teyssier and in generation of abundant migmatite, and partial detachment faulting, and brittle faulting (basin Whitney, a Geological Society of America research melting occurred primarily in the early Tertiary. formation) are observed throughout the south- grant to Kruckenberg, and the Department of Geol- In this scenario, the duration of partial melting ern Omineca Belt, suggesting that the Cordil- ogy and Geophysics Graduate Research Fund at the events is shorter—perhaps not much longer than lera hinterland underwent orogenic collapse University of Minnesota. Insightful reviews and edi- the time indicated by the crystallization ages of in the presence of melt. Tertiary partial melt- torial comments by James Crowley, William McClel- land, Elizabeth Miller, and Cees van Staal greatly zircon and monazite (~12 m.y.). An implication ing has been documented over a widespread improved this manuscript. We thank the Australian of this model is that partial melting and exten- area within the Shuswap complex (400 km Nuclear Science Technology Organization and the sion were linked; i.e., the presence of high-melt north-south), including the Frenchman Cap Australian Institute of Nuclear Science and Engi- fraction crust may have facilitated extension dome (Crowley et al., 2001), Thor-Odin dome neering for facilitating neutron irradiations of biotite. (e.g., Vanderhaeghe and Teyssier, 2001), and the (Vanderhaeghe et al., 1999, 2003; Hinchey et We also thank Dr. Annia Fayon, Christine Regalla, Rebecca-Ellen Farrell, the support and analytical staff feedback between decompression and further al., 2006), Valhalla (Spear and Parrish, 1996; at Australian National University and the University partial melting may have enhanced the produc- Spear, 2004; Gordon et al., 2008), and Okano- of Minnesota, and the Washington state landowners tion and exhumation of partially molten crust gan dome (this study). who provided access to conduct this research. (Teyssier and Whitney, 2002). Elucidating the The tectonothermal evolution of the domes relationships among these processes is difﬁ cult and core complexes to the north of the Oka- REFERENCES CITED in the case of the northern Cordillera because nogan complex has been interpreted to sug- Archibald, D.A., Glover, J.K., Price, R.A., Farrar, E., and regional extension could have been driven by gest that extension and exhumation were in Carmichael, D.M., 1983, Geochronology and tec- (1) slowed convergence and the onset of major part concentrated at the boundary between the tonic implications of magmatism and metamorphism, dextral transcurrent motion along the western cold foreland (Rocky Mountains) and the more southern Kootenay arc and neighbouring regions, southeastern British Columbia. Part I: Jurassic to mid- boundary of North America (Price, 1979; Price melt-rich hinterland (Omineca Belt) (Teyssier Cretaceous: Canadian Journal of Earth Sciences, v. 20, and Carmichael, 1986) or extension triggered et al., 2005). Results from the Okanogan region p. 1891–1913. by rollback of the subducted slab (Parrish et al., document that migmatite crystallization and Armstrong, R.L., 1982, Cordilleran metamorphic core complexes—From Arizona to southern Canada: Annual 1988); or (2) thermal weakening following exhumation, detachment tectonics, and basin Review of Earth and Planetary Sciences, v. 10, p. 129–154, Laramide orogenesis and extensive Early Ceno- formation (Fig. 13) were temporally and kine- doi: 10.1146/annurev.ea.10.050182.001021. Atwater, B.R., 1985, Contemporaneity of the Republic Gra- zoic magmatism (Armstrong, 1982; Coney and matically linked, in a region as far as 200 km ben and Okanogan gneiss dome—Evidence from the Harms, 1984). west of the hinterland-foreland boundary in Coyote Creek pluton, southern Okanogan County,

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