Late Cretaceous and Early Tertiary Plutonism and Deformation in the Skagit Gneiss Complex, North Cascade Range, Washington and British Columbia

Home , Mount Triumph

Late Cretaceous and early Tertiary plutonism and deformation in the Skagit Gneiss Complex, North Cascade Range, Washington and British Columbia

RALPH A. HAUGERUD U.S. Geological Survey at Department of Geological Sciences, AJ-20, University of Washington, Seattle, Washington 98195 PETER VAN DER HEYDEN* Department of Geological Sciences, University ofBritish Columbia, Vancouver, British Columbia V6T2B4 Canada

ROWLAND W. TABOR 1 ,, c „ . . , c „ . „ , „ ,., . „.„„ ..„.. „ T,„v \ U.S. Geological Survey, Menlo Park, California 94025 JOHN o. SIALti ) ROBERT E. ZARTMAN U.S. Geological Survey, Denver, Colorado 80225

ABSTRACT

The Skagit Gneiss Complex forms a more-or-less continuous ter- others, 1987a, 1987b, 1989). In the core of the range, in what appear to rane within the northern, more deeply eroded part of the North Cas- have been the most deep-seated rocks, direct evidence for the protolith age cade Range. The complex comprises abundant plutons intruded at and age(s) of metamorphism, migmatization, and deformation has been mid-crustal depths into a variety of metamorphosed supracrustal scarce. Relations of gneisses in the northern part of the Chelan block—the rocks of both oceanic and volcanic-arc origin. A plethora of syntec- region bounded by the Straight Creek, Ross Lake, and Entiat faults (Fig. tonic pegmatite, small plutons, and granitic dikes gives the complex a 1)—to surrounding less-metamorphosed rocks have been problematic (for migmatitic aspect. example, Davis and others, 1978; Hamilton, 1978; Whitney and U-Pb zircon ages from gneissic plutons within and near the McGroder, 1989). Skagit Gneiss Complex indicate magmatic crystallization between 75 These gneisses are a complex of metamorphosed granitoid plutons and 60 Ma. Deformation, recrystallization, and migmatization in part with intimately related metamorphosed nongranitoid rocks. Parts of the postdate intrusion of the 75-60 Ma plutons. This latest Cretaceous and complex have been known as the "Custer Granite-gneiss" (Daly, 1912), earliest Tertiary plutonism and migmatization may reflect thermal re- "Custer Gneiss" (McTaggart and Thompson, 1967), and "Skagit Gneiss" laxation following early Late Cretaceous orogeny documented else- (Misch, 1952,1966). We herein adopt the name "Skagit Gneiss," revise it where in the North Cascades. to "Skagit Gneiss Complex," and extend its geographic scope (see Nomen- The complex was ductilely extended northwest-southeast shortly clature, App. A, and Fig. 2).1 after intrusion of granite dikes at -45 Ma, but before emplacement of As part of a restudy of the Custer Ridge area mapped by Daly the earliest (~34 Ma) plutons of the Cascade arc. Outcrops of Late (Haugerud, 1985) and reconnaissance mapping of the North Cascades Cretaceous and earliest Tertiary plutons, migmatites of the Skagit south of the 49th parallel (Tabor and others, 1987a, 1988, and work in Gneiss Complex, and rocks with young ductile deformation are progress), we have obtained U-Pb ages from several plutons within and roughly coextensive, all apparently marking a region of greater middle adjacent to the Skagit Gneiss Complex. Our mapping and these ages lead Eocene unroofing. Unroofing was apparently contemporaneous with us to believe that the magmatic component of the Skagit is largely of latest east-west extension in the Okanogan region to the east and north- Cretaceous and earliest Tertiary age, and that differences between the south and northwest-southeast strike-slip faulting within the North Skagit and surrounding rocks reflect latest Cretaceous and early Tertiary Cascades. deformation and recrystallization and subsequent differential unroofing. In this paper, we (1) outline the geology of the Skagit Gneiss Complex and INTRODUCTION the plutons that we have dated, (2) present and discuss our new U-Pb analyses, and (3) discuss the age of Skagit metamorphism; the significance An early Late Cretaceous age for plutonism and regional metamor- of latest Cretaceous and earliest Tertiary magmas in the North Cascades; phism in much of the North Cascades of northwest Washington and and the age, extent, and tectonic setting of post-45 Ma ductile deformation southwest British Columbia has been well documented and extensively in the Skagit Gneiss Complex. discussed (Mattinson, 1972; McTaggart, 1970; Evans and Berti, 1986; Plummer, 1980; Monger and others, 1982; Haugerud, 1987; Tabor and

'Figure 2 is a folded insert in this issue. Appendices A-C are available free of 'Present address: Geological Survey of Canada, Vancouver, British Columbia charge by requesting Supplementary Data 9125 from the GSA Documents V6B 1R8 Canada. Secretary.

Additional material for this article (appendices) is available free of charge by requesting Supplementary Data 9125 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 103, p. 1297-1307, 9 figs., 1 table, October 1991.

1297

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• Vancouver

49° BRITISH COLUMBIA WASHINGTON

48°

EXPLANATION H Eocene strata

47° - Q Crystalline rocks

i i 123° 122° 120°

Figure 1. Sketch map of the North Cascade Range, showing distribution of crystalline rocks, major early Tertiary fault zones, and some localities discussed in the text. Oligocene and younger rocks omitted. The Skagit Gneiss Complex crops out in northwestern part of the Chelan block.

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GEOLOGY OF THE SKAGIT GNEISS COMPLEX deformation at lower temperature or higher strain rate. This later deformation is associated with a northwest-trending subhorizontal stretching line- The North Cascade Range lies at the southern end of the Coast ation that is defined by elongate mineral grains and aggregates and is Mountains and at the northern margin of the post-Eocene volcanic rocks commonly more pronounced than is foliation. of the Columbia Embayment. The range constitutes the first extensive Foliations strike northwest and are steep throughout much of the exposure of pre-Tertiary rocks north of the Blue Mountains province of Skagit Gneiss Complex. Large, gently plunging folds of foliation and litho- northeast Oregon. Geology of the North Cascades was summarized by logic layering with steep axial planes are subparallel to a northwest- Misch (1966, 1977,1988) and by Tabor and others (1989). trending stretching lineation. At a few locations along the east and west Eocene faults divide the North Cascades into three belts (Figs. 1 and margins of the Skagit, foliation and lineation strike and trend to the north 2): a western belt of Paleozoic and Mesozoic, mostly oceanic and island- and northeast. The origin of this discordance is unknown. arc-derived strata, commonly overprinted by high-pressure, very low- grade metamorphism; a central core of strongly metamorphosed schist and Maselpanik Area, Northeast Margin of Skagit Gneiss Complex gneiss; and an eastern belt composed largely of unmetamorphosed Jurassic and Cretaceous strata. Many workers (for example, Misch, 1966; Skagit Gneiss Complex in the Maselpanik area (Fig. 2) consists of McGroder, 1989) considered the metamorphic core to be infrastructure to compositionally diverse paragneiss; orthogneiss, including the migmatitic the Late Cretaceous orogen that is recorded in the belts to the east and biotite-hornblende tonalite orthogneiss of Custer Ridge (see App. A, No- west. menclature); and dikes, sills, and a small plug of crosscutting, yet deformed The range is intruded by a suite of ~ 85 to -100 Ma tonalite and granite (Haugerud, 1985). Enstatite in metamorphosed ultramafic rocks granodiorite (Tabor and others, 1989). In the metamorphic core, most of indicates that peak-metamorphic temperatures were >680 °C, assuming a these plutons are deep-seated gneissic bodies at grade with their wall rocks. water-rich fluid phase. Many of the ~85 to ~ 100 Ma plutons are deformed, and radiometric ages Crosscutting relations demonstrate three deformational phases (Fig. from them are among the evidence for Late Cretaceous orogeny. 3). Foliation within amphibolitic xenoliths in the orthogneiss of Custer The metamorphic core is divided into two blocks by the Entiat fault. Ridge represents the earliest event. Subsequent deformation formed the Southwest of the Entiat fault, in the Wenatchee block (Fig. 1), radiometric main foliation (S2) in the orthogneiss of Custer Ridge and (apparently) in cooling ages are largely Late Cretaceous (Engels and others, 1976; Hau- the remainder of the Skagit. Later deformation (D3) produced fabrics in gerud, 1987; Tabor and others, 1987a, 1988). Northeast of the Entiat fault, dikes and sills of lineated granite that cut S2 (Fig. 3). D3 is marked by a in the Chelan block, radiometric cooling ages—especially to the north— pervasive northwest-trending stretching lineation (L3) and local, steep, are early Tertiary (Engels and others, 1976; Tabor and others, 1987a). The northwest-striking foliation (S3, Fig. 3). D3 fabrics are commonly mylo- Skagit Gneiss Complex forms a northwest-trending belt within the north- nitic; quartz is now in ribbons, biotite is smeared into aggregates of smaller ern part of the Chelan block. grains, and plagioclase is locally fractured and extended. Subsolidus origin Pre-Late Cretaceous rocks within the northern Chelan block, as well of the D3 fabric in the dikes of lineated granite is demonstrated by de- as adjacent parts of the Wenatchee block, are assigned to the Chelan formed crystals of quartz, potassium feldspar, muscovite, and plagioclase Mountains terrane by Tabor and others (1989). Major pre-metamorphic and the lack of undeformed late-crystallizing igneous matrix. Rare tourma- units within this terrane are (1) the oceanic Napeequa unit of pre(?)-Late line in pegmatites associated with the lineated granite is also boudinaged Triassic age, in part; (2) the locally arkosic, locally conglomerate-rich and extended parallel to L3. Cascade River unit, which is, at least in part, of Late Triassic age (Tabor The structure in the Skagit in the Maselpanik area is complex on and others, 1988, 1989; Dragovich and others, 1989); and (3) the Late surfaces normal to L3; structure is simple on surfaces parallel to the linea- Triassic Marblemount plutons (Misch, 1966; Mattinson, 1972). Tabor and tion (Fig. 4). Poles to foliation in the Skagit Gneiss Complex define a others (1989) proposed that the Marblemount plutons intruded the proto- great-circle girdle that wraps around the horizontal, northwest-trending lith of the Napeequa unit and that the Cascade River unit unconformably average lineation direction. This simple pattern appears to reflect whole-

overlies both. sale transposition of earlier-formed fabrics into the D3 orientation With increase in metamorphic grade and increase in the proportion (Haugerud, 1985). of orthogneiss, schist of the Chelan Mountains terrane passes into the Skagit Gneiss Complex. Extensive lit-par-lit intrusion of granitic material Skagit Gorge into paragneiss and development of pegmatitic leucosomes in both orthogneiss and paragneiss contribute to an overall migmatitic character. The western part of the Skagit Gorge is carved from orthogneiss of Several varieties of dikes and sills cut main-phase gneiss and pegmatite of the Skagit Gneiss Complex. East and south of Mount Triumph (Fig. 2), the complex, yet are themselves deformed. As mapped, the Skagit is more orthogneiss intrudes strongly foliated schist of the Napeequa unit in lit-par- than 50% orthogneiss. Perhaps half of the remaining banded gneiss is also lit fashion. Fabric in the orthogneiss becomes weaker to the west. On the orthogneiss, for an aggregate of more than 75% orthogneiss in the unit. south slopes of Mount Triumph, the plutonic rock is locally directionless, Thermobarometry by Whitney and Evans (1988) indicated that peak although it has a strong heterogeneous cataclastic overprint perhaps related metamorphic conditions in the gneiss were about 720 °C and 9 kb. to the north-striking Triumph Pass fault (Fig. 2). Screens of schist south of Dragovich (1989; Dragovich and others, 1989) reported metamorphic Mount Triumph are well foliated. Apparently the schist was deformed conditions of 650 °C and 8-9 kb for lower-grade schist at the western prior to intrusion of the orthogneiss, and both units were deformed after margin of the northern Chelan block. intrusion. Textures in quartz-poor amphibolites of the Skagit Gneiss Complex The tonalitic to granodioritic orthogneiss of The Needle, exposed a are crystalloblastic, whereas quartz-rich rocks are commonly mylonitic, few kilometers south of the Skagit Gorge on The Needle, Pyramid Peak, with undulatory extinction and sub-grain development in quartz, local and Snowfield Peak (Fig. 2), intrudes amphibolite, muscovite-bearing mortar texture in plagioclase, and bent and broken plagioclase grains. The metachert, and minor ultramafic rock and marble, suggesting that the wall textures suggest that the earlier, low-strain-rate fabric seen in amphibolites rocks of this pluton are at least locally correlative with the Napeequa unit. has been overprinted, in less-competent quartz-rich rocks, by subsequent The orthogneiss of The Needle locally contains a second foliation that is

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Figure 3. Crosscutting relations in the Skagit Gneiss Complex, Maselpanik area. Orthogneiss of Custer Ridge, with S2 foliation, contains amphibolite xenoliths (outlined dark patches in lower right corner) and is cut by strongly lineated granite dike that contains S3.

axial-planar to decimeter-scale folds in the main foliation. Elsewhere, the of the Skagit Gneiss Complex. This stretching postdates most metamorphic body has a distinctive texture; grains are small (~1 mm), equant, and mineral growth; lineation in both dikes and sills of granitic orthogneiss and polygonal, and yet these grains form centimeter-sized patches richer in in wall-rock gneiss is associated with grain size reduction and little new either quartz, feldspar, hornblende, or biotite. These patches appear to be mineral growth. pseudomorphs after large igneous crystals. This texture and the presence of Hidden Lake Peak Area, Southwest of Skagit Gneiss Complex two foliations suggest that this body is older than other Skagit orthogneisses, a surmise sustained by the isotopic data described below. Several plutons intrude schists in the Hidden Lake Peak area, Dikes and sills of late, lineated granitic orthogneiss, similar to the northwest of Cascade Pass, and suggest further constraints on the timing of lineated granite on Mount Daly, are locally abundant in the Skagit Gorge deformation within the Skagit Gneiss Complex. type section (compare with Misch, 1968, p. 13 and plate If), especially Strongly layered flaser gneiss of the Marble Creek pluton crops out 5 near Diablo dam. Babcock and others (1985) reported a whole-rock Rb- km northwest of Hidden Lake Peak (Fig. 2). The flaser gneiss ranges from Sr isochron age of 45 ± 3 Ma for these bodies. Mylonitic lineation in the granodiorite to tonalite and contains common relict magmatic epidote and dikes and sills is parallel to northwest-trending subhorizontal lineation in magmatic muscovite. Steep, northwest-striking foliation in the gneiss is the wall rock; yet the dikes commonly cut wall-rock foliation (compare parallel to that in schist septa within the pluton and in adjacent schist of the with Misch, 1968, 1977). From these relations, we conclude that north- Napeequa.unit. Both schist and pluton were deformed after intrusion of the west-southeast stretching was the last phase of ductile deformation in much pluton (see also Misch, 1979).

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fmZfl ir- . . Figure 4. Biotite paragneiss in the Skagit Gneiss Complex, Maselpanik area. Surface JÊJKM 1 on left side of photograph is normal to my. ^t '"S \ i • northwest-trending mylonitic stretching lin- r eation; surface on right side is parallel to f*» m(' rr yf lineation. Lens cap is 5 cm across.

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To the south, the Hidden Lake pluton intrudes schist of the Napeequa The gneissic Cardinal Peak pluton intrudes schist of the Chelan and Cascade River units. The schist is well foliated, with strongly aligned Mountains terrane in the region between the Railroad Creek and Duncan micas wrapping around earlier-formed, locally helicitic, porphyroblasts. Hill bodies. Descriptions by Cater (1982) suggest that the schist was Intrusion of the Hidden Lake pluton postdates most deformation of the deformed prior to intrusion of the pluton and that both schist and pluton schist, because most of the pluton is undeformed or is only weakly foliated; were then further deformed. Cater (1982) described a high-strain zone in an exception is the northwestern margin, where Dragovich (1989) de- the northern part of the Cardinal Peak pluton, located between the exten- scribed shearing at the margin of the pluton and in the adjacent schists. sively deformed parts of the Railroad Creek and Duncan Hill plutons. North-northeast-striking foliation and down-dip lineations at this contact Presumably this high-strain zone is also middle Eocene or younger. are discordant to northwest-striking vertical foliation and subhorizontal lineation in the schist a few hundred meters to the northwest (Dragovich, Minimum Age of the Skagit Gneiss Complex 1989). Where visible elsewhere, the pluton contact is concordant, with sills of slightly deformed granodiorite intruding well-foliated metachert. Drag- Skagit Gneiss Complex and associated schists were intruded by 34 ovich (1989) indicated that intrusion of the pluton postdated both primary Ma and younger (Engels and others, 1976; Vance and others, 1986) synmetamorphic deformation of the adjacent schist and subsequent de- plutons of the Cascade arc; indeed, much of the Skagit forms large roof formation associated with local retrogression and pervasive dextral shear pendants in the Chilliwack batholith (Fig. 2). Volcanic rocks of the Cas- along northwest-striking foliation. cade arc lie on top of gneiss in some locales (Fig. 2; see also Staatz and North of Hidden Lake Peak, the Eldorado Orthogneiss is strongly others, 1972; Misch, 1977). Cairnes (1944) and McTaggart and Thomp- ductilely deformed along its contact with schist of the Chelan Mountains son (1967) reported depositional contacts between lower Tertiary sedi- terrane. Along strike to the southeast, schists of the Chelan Mountains mentary rocks and gneiss at two locations north of 49 °N. At both terrane and Eldorado flaser gneiss are interlayered on an outcrop scale locations, the sedimentary rocks are recrystallized by the Chilliwack batho- (Tabor, 1961). Time relations between intrusion of the Eldorado pluton lith and have not been directly dated. and deformation of the schist are not clear; both have been deformed since intrusion, but the schist may also have been deformed before intrusion. NEW U-Pb ANALYSES The northeast contact of the Eldorado Orthogneiss with banded gneiss of the Skagit Gneiss Complex is concordant. New U-Pb analyses are presented in Table 1 and illustrated in Figures 5, 6, 7, and 8. Sample locations are shown in Figure 2, and petrographic Southern Part of the Skagit Gneiss Complex, Lucerne Quadrangle descriptions of the samples are given in Appendix B.2 These analyses do not provide unequivocal evidence for the age of intrusion of most of these Although mapping of the southern part of the complex is not com- bodies, and we postpone discussion of intrusive ages until the end of this plete, several features have been established. In this region, the Skagit is section. mostly orthogneiss. Cater and Wright (1967) assigned this orthogneiss to the Swakane Biotite Gneiss, the type area of which is many kilometers Maselpanik Area across strike to the southwest; we concur, however, with more recent mapping by Ford and others (1988) that this assignment is in error, and Zircons from the orthogneiss of Custer Ridge are clear, light pink, here reassign these rocks to the Skagit Gneiss Complex. euhedral, typically magmatic crystals with aspect ratios as high as 10:1. In In the Lucerne quadrangle and farther north (Fig. 2), the Skagit is the fine fractions, 10%-20% of crystals contain small opaque and cloudy concordantly intruded by the Railroad Creek pluton, which appears to white inclusions; these grains were avoided during hand picking. Five mark the southwestern boundary of the complex. According to Libby fractions of zircon gave U-Pb ages that range from 60 to 64 Ma. The (1964), both granodiorite of the Railroad Creek pluton and adjacent youngest ages are from the finest grains, and the oldest ages are from Skagit Gneiss Complex are mylonitized along a planar contact, suggesting abraded (-50%) cores of coarse grains. Within la uncertainty, U-Pb ages a deep-seated fault. Farther south in the Lucerne quadrangle, schist of the of each fraction are concordant, but not all fractions are concordant with Chelan Mountains terrane is intruded by the Duncan Hill pluton. In plan each other (Fig. 5). Older ages reported for this sample by Haugerud view, both the Railroad Creek and Duncan Hill plutons are northwest- (1985) are analytically defective and should be ignored. We have been southeast elongate tadpoles, with crosscutting, high-level heads at the unable to reproduce them. southeast and concordant, gneissic tails to the northwest (Fig. 2) (Hopson Zircon from a sample of lineated granite from Mount Daly is euhed- and others, 1970; Cater, 1982; Libby, 1964; Dellinger and Hopson, 1986), ral and forms clear, colorless to very light pink, slender, typically magmatic a pattern that reflects up-to-the-northwest tilting since their emplacement crystals. Monazite grains are light orange to buff, perfectly euhedral, pris- and deformation. matic grains with aspect ratios of about 3:1. Xenotime grains are relatively Concordant biotite and hornblende K-Ar ages from high-level parts coarse (>210 ju), subhedral to anhedral, yellow to light green, and of the Railroad Creek and Duncan Hill plutons indicate that these bodies stubby. Irregularities in xenotime crystal faces suggest partial resorption by are middle Eocene (Engels and others, 1976). This conclusion is confirmed melt. U-Pb ages from all three minerals lie between 44 and 47 Ma (Fig. 5). by an -47 Ma U-Pb zircon age from the gneissic tail of the Duncan Hill U-Pb ages of zircon (44 Ma) are concordant, and those of xenotime (45 pluton (Tabor and others, 1987a). The evidence for middle Eocene or Ma) are slightly discordant, although within 2 a uncertainty, zircon and younger deformation provided by these plutons has not been widely ap- xenotime ages are not analytically distinct. Reverse discordance of the preciated. Cater (1982) interpreted fabrics in the plutons of the Lucerne monazite presumably reflects preferential partitioning of Th (relative to U) quadrangle to be largely protoclastic, the result of late-magmatic deforma- into monazite and consequent excess radiogenic 206Pb (Scharer, 1984). tion driven by the intrusion process; his illustrations and descriptions, This hypothesized Th excess is verified by high measured 208Pb (derived however, show the fabrics to be largely "crystal-plastic strain fabrics" (in from Th decay) (Table 1). The negative monazite 207Pb/206Pb age is the sense of Hutton, 1988) that postdate melt crystallization. Foliations meaningless for the same reason. and lineations are commonly slightly to strongly discordant with pluton boundaries and continuous with fabrics in the wall rocks, and the fabrics are of regional extent. 2See footnote 1.

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TABLE 1. U AND Pb ISOTOPE ANALYSES AND AGES, SKAGIT GNEISS COMPLEX AND RELATED ROCKS

Fraction* Sample U Total Pb Measured Pb isotopie composition t Ages (Ma)§ R" weight (ppm) (ppm) 2«,pb/204Pbtt 204 207 208 206 238 Pb/ U 207pb/235u 207Pb/206Pb

RH83-H76b orthogneiss of Custer Ridge (Skagit Gneiss Complex) z -210+149(1 NMA 3.9 mg 326 3.29 1689 0.0441 5.3493 10.9106 64.4 ± 0.5 64.0 ± 0.8 49 ± 25 0.88 z -210+149(1 M 5.5 mg 390 3.84 1641 0.0480 5.3284 9.7500 63.1 ± 0.7 62.5 ± 0.8 36 ± 19 0.91 z -149+7411 NM 2.9 mg 342 3.34 1697 0.0309 5.2201 10.0375 62.9 ± 0.4 63.4 ± 0.5 82 ± 7 0.95 z -74+44/i M 0.8 mg 260 2.51 1070 0.0144 5.1171 10.4772 62.1 ± 0.5 64.4 ± 1.5 150 ± 51 0.87 z -74+44NM 1.3 mg 345 3.25 996 0.0367 5.2868 10.1573 60.4 ± 0.4 60.7 ± 0.9 73 ± 28 0.88

RH83-H77 lineated granite (Skagit Gneiss Complex) z 0.1 mg 5125 41 260 0.3154 9.3071 18.1295 43.9 ± 0.3 43.6 ± 0.7 25 + 33 0.87 m 0.9 mg 3125 155 531 0.1754 7.1745 652.37 46.9 ± 0.3 45.9 ± 0.4 -11 ± 9 0.93 x 1.2 mg 16252 113 7783 0.0108 4.8638 10.0474 45.1 ± 0.4 45.3 ± 0.4 51.5 ± 1.4 0.99

RWT 179-86 orthogneiss at Newhalem (Skagit Gneiss Complex) z -102+63(i NM 18.6 mg 258 2.45 3586 0.0126 4.9341 4.0678 64.8 ± 0.1 65.0 ± 0.3 74 ± 11 0.58 z -63/j NM 15.6 mg 259 2.41 2458 0.0198 5.0612 4.1908 63.3 ± 0.2 63.9 ± 0.4 84 ± 13 0.56

RWT 211-86 orthogneiss of The Needle (Skagit Gneiss Complex) z -103+63/1 NM 7.9 mg 602 10.64 4329 0.01535 5.1205 6.6657 116.9 ± 0.2 122.0 ± 0.7 221 ± 13 0.46 z -«3/i NM 15.7 mg 610 10.45 5652 0.01355 4.2410 7.2421 112.9 ± 0.2 117.7 ± 0.3 242 ± 6 0.59

RWT 97-86 orthogneiss at Newhalem Creek (Skagit Gneiss Complex) z +102(1 NM 11.8 mg 177 2.03 1288 0.04563 5.4558 8.4293 74.1 ± 0.1 74.7 ± 0.3 92 ± 4 0.50 z -63/1 NM 12.2 mg 167 1.99 613 0.1351 6.7100 12.2650 72.5 ± 0.1 72.1 ± 0.4 61 ± II 0.43

RWT 434-85 Marble Creek pluton (intrudes Napeequa unit of the Chelan Mountains terrane) z -102+63/1 NM 15.0 mg 437 5.36 5267 0.00937 4.9051 8.7090 75.6 ± 0.1 75.9 ± 0.3 83 ± 7 0.48 i -102+63/1 M 17.7 mg 472 5.53 3306 0.02405 5.1423 9.6903 75.7 ± 0.1 76.3 ± 0.2 94 ± 7 0.61 z -63(1 NM 18.4 mg 459 5.27 10730 0.00185 4.8626 9.0833 75.1 ± 0.1 76.4 ± 0.4 116 ± 11 0.73 z -63(i 17.0 mg 476 5.50 3360 0.02220 4.9944 9.7178 74.7 ± 0.2 73.4 ± 2.0 33 ± 16 0.82

RH86-B127a Hidden Lake pluton (intrudes schists of the Chelan Mountains terrane) z 60/140 mesh 21.2 mg 348 4.24 2554 0.0305 5.2089 14.407 75.4 ± 0.1 75.5 ± 0.4 80 ± 12 0.55 z -140 mesh 22.4 mg 425 4.60 3864 0.0195 5.0408 5.0985 72.9 ± 0.1 73.1 ± 0.4 79 ± 12 0.64

RWT 204-86 Eldorado Orthogneiss z 60/140 mesh 17.6 mg 543 7.71 3664 0.01817 5.0721 15.0290 87.7 ± 0.1 88.2 ± 0.3 102 ± 6 0.55 z 140/200 mesh 20.6 mg 702 10.28 2288 0.03675 5.3380 17.1817 88.3 ± 0.1 88.7 ± 0.3 99 ± 4 0.54

RWT 224-80§§ Cardinal Peak pluton (intrudes schists of the Chelan Mountains terrane) z 60/150 mesh 4.0 mg 766 8.79 1453 0.0688 5.9192 7.7793 74.2 ± 0.3 76.5 ± 0.4 151 ± 7 0.83 z -150 mesh 7.8 mg 822 9.18 1914 0.0523 5.6709 6.6479 73.8 ± 0.3 75.7 ± 0.3 137 ± 3 0.95

82-S11 Cardinal Peak pluton (intrudes schists of the Chelan Mountains terrane) z 60/150 mesh 8.5 mg 751 8.73 1294 0.0773 6.1418 7.7336 75.0 ± 0.3 78.9 ± 0.4 198 ± 9 0.81 z -150 mesh 6.3 mg 772 9.13 690 0.1449 6.9461 10.3264 73.1 ± 0.4 74.0 ± 0.4 106 ± 12 0.73

Note\ letters A-J correspond to locations in Figure 2. RH83 samples analyzed at University of British Columbia; Cardinal Peak pluton samples analyzed at U.S. Geological Survey, Denver; remainder analyzed at U.S. Geological Survey, Menlo Park. Analytical procedures outlined in Appendix C. * m, monazite; x, xenotime; z, zircon; n, micrometers; M, magnetic; NM, nonmagnetic; NMA, nonmagnetic abraded. -10 204 206 207 208 t206pb = 100 Corrected for mass fractionation and analytical blank. Analytical blanks for RH83 samples ranged from 1 to 2 x lO g Pb, with a composition of 1:17.75:15.50:37.30 ( Pb: Pb: Pb: Pb). Analytical blanks for other samples were 3 * 10"10 g Pb, with composition of 1:18.7:15.6:38.2. §Ages calculated assuming decay constants for 238U = 1.55125 x 10"10 yr"1,235U = 9.8485 x 10"10 yr"1. Errors are lo for RH83 samples, 2a for Cardinal Peak samples, 95% confidence level for others. "Correlation coefficient for errors in 206Pb/238U and M7Pb/235U ages. ^Corrected for mass fractionation. §§Th concentrations and 208Pb/232Th ages for these samples: RWT 224-80 60/150 mesh, 105.9 ppm Th, 85.0 Ma; -150 mesh, 104.5 ppm, 82.2 Ma; 82-S11A 60/150 mesh, 89.7 ppm, 92.3 Ma; -150 mesh, 104.4 ppm, 80.3 Ma.

Skagit Gorge 210+149// NMA^ orthogneiss of 149+74// NM Two zircon fractions from a sample of orthogneiss at Newhalem Custer Ridge yielded U-Pb ages of 63 and 65 Ma, similar to those of the orthogneiss of -210+149fi M- Custer Ridge. The coarse fraction is concordant and the fine, younger, 74+44/j M fraction is mildly discordant (Fig. 6A). U-Pb ages of coarse zircon from *3> 74+44/j NM orthogneiss at Newhalem Creek, mapped as part of the same unit as orthogneiss at Newhalem, are mildly discordant at ~74 Ma. U-Pb ages of fine zircon are slightly disconcordant at ~72 Ma (Fig. 6B). Two zircon fractions from the orthogneiss of The Needle gave strongly discordant 113 to 122 Ma U-Pb ages, with the older ages from the coarser fraction (Fig. 6C). The ages lie on a chord that extends from an upper intercept of 360 ± 270 Ma to a lower intercept of 69 ± 93 Ma. lineated granite Hidden Lake Peak Area 0.04 0.05 0.06 0.07

207pb/235U Four zircon fractions from the strongly deformed Marble Creek Figure 5. U-Pb concordia diagram for samples of Skagit Gneiss pluton gave U-Pb ages of 73-76 Ma. The large error in the 207Pb/235U Complex from the Maselpanik area. Error ellipses are 2a. age of the bulk fine fraction reflects an unusually short-lived analytical

orthogneiss at Newhalem Marble Creek pluton ^ }y ¿V * i / — coarse M £ J/ * i & \ y^ coarse NM 8 c00o * 1 / CM fine fc y PCO •-< * y^ coarse co o fine NM (M o & eu * 1 CD £ O ÛH CM CD o & fine £ J/ CM y^ A A y i i 1 i 1 1 i i 0.062 0.064 0.066 0.068 0.074 0.078 207 235 207Pb/235u Pb/ U

orthogneiss at Hidden Lake pluton Newhalem Creek $ \c 00 ¡3 co ' coarse CO * CM CO L^zf CM coarse

CL, c^u * X CO & o CD * i CM O \yfine CM

- £ fine

B B O i 1 i y> i 1 1 i 0.074 0.078 0.072 0.074 0.076 207Pb/235U 207Pb/235U

—

Eldorado Orthogneiss orthogneiss of The Needle S O Dd co co C0 CO CM CM - coarse £ eu PH 360+-270Ma CD Y CD O ? coarse O CM - CM 00 O fine , - " o - ^y to c c ' 69 +- 93 Ma 1 1 / i i 1 0.120 0.125 0.130 0.115 0.088 0.090 0.092 207 235 Pb/ U 207 Pb/235U Figure 6. U-Pb concordia diagrams for samples of the Skagit Gneiss Complex in and near the Skagit Gorge. Error ellipses are 95% Figure 7. U-Pb concordia diagrams for samples of plutons in the confidence limits. Hidden Lake Peak area. Error ellipses are 95% confidence limits.

signal. Nonmagnetic fractions are discordant, whereas bulk fine and coarse gave analytically distinct, although individually concordant, U-Pb ages of magnetic fractions are concordant. The spread of analyses off concordia 75 and 73 Ma (Fig. 7B). requires three-component mixing and suggests that one component is in- Two fractions of zircon from a sample of the Eldorado Orthogneiss heritance from a significantly older source (Fig. 7A). gave mildly discordant ~88 Ma U-Pb ages (Fig. 7C). These ages are Two zircon fractions from the little-deformed Hidden Lake pluton similar to the 91 Ma age (recalculated with currently accepted decay

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early Mesozoic plutonic suites in the North Cascades, and an upper- intercept age of 360 ± 270 Ma lead us to suggest that the orthogneiss of The Needle is a metamorphosed Late Triassic pluton of the Marblemount suite. For the remainder of these plutons, the small spread in ages among individual fractions strongly suggests that we have analyzed variable mixtures of zircons of different ages. Such mixtures could result from protracted igneous crystallization, retention of older zircon from the rocks that were melted to make the magma or through which the magma passed (inheritance or contamination), metamorphic neo- or recrystallization of zircon (with effects identical to lead loss), and (or) lead loss by unspecified mechanisms. With the relatively minor differences among probable ages of sources for melts (late Paleozoic or Mesozoic), intrusion (late Mesozoic or early Cenozoic), and metamorphism (late Mesozoic or early Cenozoic) in the North Cascades, such mixtures could plausibly produce concordant 0.074 0.076 0.078 0.080 0.082 analyses for any given zircon fraction. Three lines of reasoning suggest that the U-Pb ages from these plutons approximate intrusive ages, and that the youngest concordant U-Pb ages Figure 8. U-Pb concordia diagram for samples of the Cardinal are the best estimates of the ages of intrusion and primary igneous crystalli- Peak pluton, Lucerne quadrangle. Error ellipses are 2a. zation. (1) Zircons from the orthogneiss of Custer Ridge have typical magmatic shapes. Fifty percent abrasion of coarse zircons from this sample constants) determined by Mattinson (1972) for a sample collected 9 km to failed to disclose significantly older cores, suggesting that any hypothesized the southeast. lead loss was minor, that the mass fraction of any inherited zircon is small, or that any inherited zircon is not much older than igneous crystallization. Lucerne Quadrangle (2) The Cardinal Peak pluton analyses indicate two-component mixing between a very old source and igneous zircon. Despite subsequent defor- Two samples—four fractions—of zircon from the Cardinal Peak mation, the analyses give no hint of a third episode of metamorphic zircon pluton gave mildly discordant U-Pb ages of 73-79 Ma. A line fit to all four growth or lead loss. Similar two-component mixing with a younger, points intersects concordia at 72.5 ± 0.4 and 1940 ± 260 Ma (uncertainties Mesozoic, source would produce concordant analyses, and the youngest are 95% confidence limits), with MSWD = 0.42 (Fig. 8). analysis would best approximate the crystallization age. (3) The orthogneiss of The Needle, which on geologic grounds, we have reason to Interpretation of Intrusive Ages believe is significantly older and more metamorphosed than other orthogneisses in the northern Chelan block, gives recognizably discordant ages. Only two of the plutons in this study give unambiguous indications of We suppose that this is because there is a significant amount of metamor- their intrusive ages. Lineated granite in the Maselpanik area has a simple phic zircon, or lead loss induced by metamorphism, in zircons from the history (only one deformation event), has not been extensively recrystal- orthogneiss of The Needle. Note that unlike the Cardinal Peak sample, lized, and is constrained geologically as being younger than the orthogneiss which retained 0.03%-0.12% of old (source rock) Pb, these analyses indi- of Custer Ridge and older than the Chilliwack batholith, here with an age cate retention of 11%-12% old (igneous?) Pb. of 30 Ma (Richards and McTaggart, 1976). For these reasons, we consider For these reasons, we suggest that the several-million-year spread in the -45 Ma average of zircon and xenotime ages (Fig. 5) to be the best near-concordant zircon ages from a single sample that is typical of these estimate of the intrusive age of this body. Disagreement between zircon high-grade orthogneisses most likely reflects some combination of inherit- (44 Ma) and xenotime (45 Ma) U-Pb ages and the monazite 207Pb/235U ance from a relatively young melt source and, perhaps, protracted (several age (46 Ma) may reflect minor inheritance and (or) minor Pb loss. million years) igneous crystallization following intrusion into wall rocks Discordant U-Pb ages from the Cardinal Peak pluton reflect igneous with temperatures above the solidus. The latter is not unlikely, given the crystallization at the well-defined age of 72.5 Ma and an inherited compo- high metamorphic temperatures inferred for the Skagit Gneiss Complex. nent with an age of —2.0 Ga. The degree of discordance indicates inheritance of 0.03%-0.12% Pb of Precambrian age. Such inheritance is DISCUSSION consistent with proximity of the Cardinal Peak pluton to the Swakane Biotite Gneiss, the only regionally extensive Precambrian terrane in the Age of Skagit Metamorphism North Cascade Range, for which Mattinson (1972) reported a 207Pb/ 206Pb Significant mid-Cretaceous crustal thickening in regions now adja- age of -1.5 Ga. Similar inheritance is evident in zircon ages from cent to the Skagit Gneiss Complex (on the west: Brandon and others, other plutons near or intruding the Swakane Biotite Gneiss (Entiat pluton, 1988; on the south: Miller, 1985; on the east: Tennyson and Cole, 1978; orthogneiss of the Mad River terrane, and tonalité of the Chelan Complex Trexler and Bourgeois, 1985; McGroder, 1988) suggests that relatively of Hopson and Mattinson, 1971; see Tabor and others, 1987a). Although high-pressure metamorphism of the Skagit may have been initiated by late the Cardinal Peak pluton was subsequently ductilely deformed, in part Early Cretaceous tectonic burial. Direct evidence of pre-Tertiary tectonism after intrusion of the neighboring ~47 Ma Duncan Hill and Railroad and metamorphism within the Skagit is, however, limited to the rare Creek plutons, the data show no sign of post-Late Cretaceous disturbance foliated amphibolite xenoliths in the orthogneiss of Custer Ridge. of the U-Pb systems. On the southwest side of the Skagit Gneiss Complex, schist of the Intrusive ages of the remainder of these plutons are equivocal. Chelan Mountains terrane was deformed and recrystallized prior to intru- Strongly discordant U-Pb ages from the orthogneiss of The Needle do not sion of the Cardinal Peak and Hidden Lake plutons at 73 Ma. Continuity define an intrusive age. Wall rocks similar to those of the Late Triassic of fabrics and metamorphism between the Skagit and adjacent schists (—220 Ma) Marblemount plutons, the absence of other late Paleozoic or

suggests that the Skagit also experienced the pre-73 Ma event. Ages from The presence of two foliations in the older orthogneiss of The Needle, the Marble Creek and Hidden Lake plutons would seem to bracket this contrasting with only one foliation in the Eldorado Orthogneiss and event, but our ages for these bodies are analytically indistinguishable. younger plutons, is suggestive of pre-88 Ma metamorphism, although the Perhaps the Marble Creek body is slightly older, and deformation is very difference in fabrics may simply reflect inhomogeneous deformation. closely dated. We believe it is more likely that at the present erosion level, The orthogneiss of Custer Ridge has the main Skagit metamorphic the deformation seen in the Marble Creek pluton was areally limited. The fabric and is cut by pegmatitic leucosomes typical of the Skagit. Ortho- distribution of ductile deformation may in part reflect subsequent up-to- gneiss at Newhalem is locally cut by leucosomes. Whether the zircon ages the-northwest tilting, as is evident in the Lucerne quadrangle to the from these rocks are interpreted as igneous ages, as ages that are too old southeast. because of minor inheritance, or as ages partially reset by subsequent high-grade metamorphism, they demonstrate that a significant part of this metamorphism and migmatization is younger than 64 Ma. We have not directly dated the ~9 kb, -720 °C metamorphism reported for Skagit paragneiss by Whitney and Evans (1988), beyond establishing that it predates 45 Ma. Schist of the Chelan Mountains terrane and the immediately adjacent part of the Skagit Gneiss Complex were, however, deeply buried at -73 Ma, because the Marble Creek pluton contains magmatic epidote, suggesting a pressure greater than -6 kb (Zen and Hammarstrom, 1984). Magmatic(?) epidote is also present in the 73 Ma Cardinal Peak pluton, and in orthogneiss on Teebone Ridge that is contiguous with -65 Ma orthogneiss at Newhalem and -74 Ma orthogneiss at Newhalem Creek. It is plausible that peak metamorphism in the .45 British Columbia Skagit also occurred in the Late Cretaceous and earliest Tertiary, and thus arguably was a result of thermal relaxation following mid-Cretaceous Washington crustal thickening. 25 Brown and Talbot (1989) proposed that northwest-trending stretching lineations in the metamorphic core of the North Cascades reflect dextral simple shear throughout Late Cretaceous and early Tertiary time. Our ages, along with those previously published by other workers (see Fig. 9), show that northwest-trending stretching lineations in the northern Che- • y y, ' y y y\ lan block developed before intrusion of the Hidden Lake stock at -73 Ma and after intrusion of lineated granites at 45 Ma. Data are not sufficient to 34 prove or disprove deformation in the intervening period. • y^ys.ï, i68. 49 Significance of Latest Cretaceous and Earliest Tertiary Magmas 88

The ages reported here, as well as ages reported by Miller and others 18, 90 (1989; see Fig. 9) and Mattinson (1972), demonstrate the presence of a suite of latest Cretaceous and earliest Tertiary plutons within the Skagit i220\ ?.y y. Gneiss Complex and intruding adjacent schist. Miller and others (1989) interpreted these plutons as evidence for the latest Cretaceous position of the subduction-related magmatic arc along this part of the Pacific Rim. y y y* 59. Latest Cretaceous and earliest Tertiary volcanic rocks are not present \y y y y . in the North Cascades or southern Coast Mountains. Although such vol- 22 .45 • y y y y. \y y y y y y. y y y y y< ,72. 48

EXPLANATION 47 20 km yyy ' y y y * Skagit Gneiss iC? x / / ^ Complex <~72 • y y, Figure 9. Summary map of Chelan block of the Ductilely deformed pluton North Cascade Range, showing Skagit Gneiss -72 Complex, plutons, and extent of middle Eocene ductile deformation as demonstrated by dated dikes and plutons. Oligocene and younger volcanic rocks fi Demonstrable ig omitted for clarity. Pluton ages (in Ma) from Engels post- -45-Ma and others (1976), Richards and McTaggart (1976), deformation ir Hoppe (1984), Miller and others (1989), and this o **) study.

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canic rocks could have been eroded away, an equally compelling hypothe- Skagit Gneiss Complex indicate that ductile deformation of the Skagit had sis is that this magmatic event was restricted to the middle and lower crust, ended by about 34 Ma. Biotite K-Ar ages of 44 and 45 Ma from garnet- with melting perhaps induced by thermal relaxation following mid- biotite schist and biotite gneiss (Misch, 1964; Engels and others, 1976) Cretaceous crustal thickening. If the resulting magmas were relatively suggest that the rocks exposed in the Skagit Gorge cooled below 300 °C water- and crystal-rich (and thus had little or no superheat), then they (approximate closure temperature for biotite) during middle Eocene time. might have congealed before they ascended into the upper crust. If else- Presumably most ductile deformation ceased by then. where nearby we could see into middle and lower crust of this age, then The region of middle Eocene ductile deformation within the Chelan perhaps we would see more latest Cretaceous to earliest Tertiary plutons. block is not well defined. It extends from the Maselpanik area, through The early Tertiary Ladybird plutons of Parrish and others (1988), 200 km much of the Skagit Gorge, and south to the deformed northern part of the to the northeast, may similarly be restricted to deeper parts of the crust and middle Eocene Duncan Hill pluton (Fig. 9). Throughout this region, rocks not be directly related to subduction. are characterized by northwest-trending, shallowly plunging mylonitic ex-

tensional lineation (L3 in the Maselpanik area). West of this region, Eo- Comparison of Skagit Gneiss Complex with Adjacent cene deformation must die out before the Hidden Lake pluton is reached. High-Grade Gneissic Rocks The undeformed -49 Ma Golden Horn batholith (age from Engels and others, 1976; Hoppe, 1984) places a northeast limit to the region of young High-grade gneissic rocks crop out in several regions near the Skagit ductile deformation. Isotropic fabrics in the southern parts of the -47 Ma Gneiss Complex, and although some of these units have been correlated Duncan Hill and Railroad Creek plutons limit young ductile deformation with the Skagit, they appear to have different metamorphic histories and to the southeast. (or) protoliths. Gneiss and schist of the Chelan Complex of Hopson and This southeastern limit to middle Eocene ductile deformation may Mattinson (1971) crop out several kilometers southeast of the Skagit. The reflect a lateral change in temperature of the now-exposed crust rather Chelan Complex has the same Chelan Mountains terrene protolith as the than a lateral change in strain regime. By middle Eocene time, the southern Skagit, although the absence of ultramafic bodies in the Chelan Complex Chelan block appears to have been cool, as biotite K-Ar ages are older suggests that the Napeequa unit may be absent. Both the Skagit Gneiss than 50 Ma and most are between 60 and 65 Ma (Tabor and others, Complex and the Chelan Complex appear to have been deeply buried 1987a). This is consistent with up-to-the-northwest tilting of the Duncan during mid-Cretaceous orogeny. Deformed 70 to 80 Ma plutons and Hill and Railroad Creek plutons. An extensive swarm of northeast-striking hornblende K-Ar ages that range from 84 to 64 Ma (Tabor and others, dikes associated with the southern part of the Duncan Hill pluton (Tabor 1987a) demonstrate that the Chelan Complex was hot and ductile in Late and others, 1987a) indicates significant northwest-southeast brittle Cretaceous time; however, K-Ar ages (Tabor and others, 1987a) and dilation. contact relations of the Duncan Hill pluton indicate that the Chelan Com- The region of documented middle Eocene ductile deformation is a plex was cold and brittle during the middle Eocene, whereas the Skagit narrow, north-trending swath (Fig. 9); however, structural trends and was hot and ductile. mylonitic fabrics in rocks of the Skagit Gneiss Complex within and exter- Banded gneiss of the Nason terrane, in the Wenatchee block south- nal to this swath are similar, suggesting that much of the Skagit was west of the Skagit Gneiss Complex, also appears to have formed during ductilely deformed in the middle Eocene. early Late Cretaceous orogeny, although the supracrustal component is Post-45 Ma northwest-southeast elongation within the Skagit is different from that of Skagit and Chelan gneisses (Tabor and others, coeval with dextral strike slip along the northern end of the steep, 1987b, 1989). K-Ar, ^Ar-^Ar, and fission-track ages from the Wenat- northwest-striking Ross Lake fault (Haugerud, 1985). The large change in chee block indicate that much of this banded gneiss was mostly unroofed metamorphic grade across the Ross Lake fault implies a component of in the Late Cretaceous (Engels and others, 1976; Tabor and others, 1987a; several kilometers of west-side-up displacement, much of which is proba- Haugerud, 1987). bly early Tertiary in age (Haugerud, 1985; McGroder and Miller, 1989). Plutonic rocks east of the Chewack-Pasayten fault (Fig. 1) have been Where crosscutting relations between post-45 Ma northwest-southeast called "Okanogan batholithic complex" (Barksdale, 1948, 1975), "Eagle elongation of the Skagit Gneiss Complex and north-south faulting on the Granodiorite" (Camsell, 1913), and "Eagle plutonic complex" (Greig, Straight Creek-Fraset fault are visible north of 49°N, Straight Creek de- 1988). K-Ar ages from these rocks are late Early Cretaceous (Todd, 1987; formation is younger (Haugerud, 1985). Given the possible 48 to 50 Ma Greig, 1988), suggesting that their unroofing predated the onset of meta- inception of the Straight Creek-Fraser fault (Vance, 1985; but see Tabor morphism in the region to the west. Limited K-Ar data (Raviola, 1988) and others, 1984), however, it is also possible that the crosscutting rela- from schist and gneiss at the southern end of the Methow belt, between the tions merely reflect the last increments of each process and that extension possible southern extensions of the Ross Lake and Chewack-Pasayten of the Skagit Gneiss Complex, displacement on the Ross Lake fault, and faults, suggest that these rocks have closer affinity to the Okanogan batho- displacement on the Straight Creek-Fraser fault are broadly contempo- lithic complex than to the Chelan Complex and Skagit Gneiss Complex to raneous. The lack of Tertiary regional metamorphism west of the Straight the southwest. Creek-Fraser fault and preservation of Eocene(?) fluvial sedimentary rocks on the west side of the Straight Creek fault a few kilometers north of the Skagit River (Misch, 1979; Brown and others, 1987) demand substan- Age, Extent, and Tectonic Setting of Post-45 Ma Deformation tial late Eocene to early Oligocene (pre-Chilliwack batholith) east-side-up of the Northern Chelan Block vertical displacement on this part of the Straight Creek-Fraser fault. Much of the northern Chelan block was ductilely deformed after Taken together, available geologic and geochronologic evidence sug- intrusion of the Duncan Hill pluton (47 Ma; Tabor and others, 1987a), gests that youngest ductile deformation—mostly northwest-southeast Railroad Creek pluton (-45 Ma; Engels and others, 1976), granitic dikes elongation—of the Skagit Gneiss Complex, cooling of the Skagit, and and sills along the Skagit Gorge (45 Ma; Babcock and otliers, 1985), and strike slip and dip slip on the adjacent Straight Creek-Fraser fault and lineated granite in the Maselpanik area (45 Ma; see above). Published northern Ross Lake fault all occurred in the latter part of middle Eocene radiometric ages on the undeformed, high-level Chilliwack composite time. batholith of Misch (1966) (see compendium by Engels and others, 1976; The Skagit Gneiss Complex of the North Cascades is thus similar to Richards and McTaggart, 1976; Vance and others, 1986) that intrudes the gneisses in the Tatla Lake metamorphic complex (Friedman and Arm-

strong, 1988), the Okanagan gneiss dome (Parkinson, 1985), the Valhalla Huntting, M. T., Bennett, W.A.G., Livingston, V. E., and Moen, W. S., compilers, 1961, Geologic map of Washington: Washington State Division of Mines and Geology, 2 sheets. complex (Carr and others, 1987), the Bitterroot lobe of the Idaho batholith Hutton, D.H.W., 1988, Granite emplacement mechanisms and tectonic controls: Inferences from deformation studies: Royal Society of Edinburgh Transactions, Earth Sciences, v. 79, p. 245-255. (Garmezy and Sutter, 1983; Toth, 1987; Criss and Fleck, 1987), and other Irving, E., Woodsworth, G. J., Wynne, P. J., and Morrison, A., 1985, Paleomagnetic evidence for displacement from the high-grade metamorphic terranes in the hinterland of the northern Cordil- south of the Coast Plutonic Complex, British Columbia: Canadian Journal of Earth Sciences, v. 22, p. 584 598. Libby, W. G., 1964, Petrography and structure of the crystalline rocks between Agnes Creek and the Methow Valley, lera that were ductilely deformed and rapidly unroofed during the Eocene. Washington [Ph.D. dissert.]: Seattle, Washington, University of Washington, 133 p. Mattinson, J. M., 1972, Ages of zircons from the Northern Cascade Mountains, Washington: Geological Society of Deformation and unroofing of many of these terranes were responses to America Bulletin, v. 83, p. 3769-3784. east-west extension (Parrish and others, 1988), but where these events McGroder, M. F., 1988, Structural evolution of the eastern Cascades foldbelt: Implications for late Mesozoic accretionary tectonics in the Pacific Northwest [Ph.D. dissert.]: Seattle, Washington, University of Washington, 140 p. reached into the North Cascade Range, ductile deformation and rapid 1989, Elements of the Cascades "collisional" orogen: Introduction to a transect from the Methow basin to the San Juan Islands, Washington, in Joseph, N. L., and others, eds.. Geologic guidebook for Washington and adjacent unroofing were associated with dextral strike-slip faulting along the con- areas: Washington Division of Geology and Earth Resources Information Circular 86, p. 91 95. tinental margin. McGroder, M. F., and Miller, R. B., 1989, Geology of the eastern North Cascades, in Joseph, N. L„ and others, eds.. Geologic guidebook for Washington and adjacent areas: Washington Division of Geology and Earth Resources Information Circular 86, p. 97 118. McTaggart, K. C., 1970, Tectonic history of the Northern Cascade Mountains, in Wheeler, J. O., ed.. Structure of the ACKNOWLEDGMENTS southern Canadian Cordillera: Geological Association of Canada Special Paper 6, p. 137-148. McTaggart, K. C., and Thompson, R. M., 1967, Geology of part of the Northern Cascades in southern British Columbia: Canadian Journal of Earth Sciences, v. 4, p. 1199-1228. Analytical work at the University of British Columbia was supported Miller, R. B., 1985, The ophiolitic Ingalls Complex, north-central Cascade Mountains, Washington: Geological Society of America Bulletin, v. 96, p. 27-42. by a contract from the Geological Survey of Canada and a Canadian 1987, Geology of the Twisp River Chelan Divide region, North Cascades, Washington: Washington Division of Geology and Earth Resources Open-File Report 87-17, 12 p. National Science and Engineering Research Council operating grant to Miller, R. B., Bowring, S. A., and Hoppe, W. J., 1989, Paleocene plutonism and its tectonic implications, North Cascades, R. L. Armstrong. A. B. Ford provided sample 82-S11 A. We thank E. H. Washington: Geology, v. 17, p. 846-849. Misch, P., 1952, Geology of the Northern Cascades of Washington: The Mountaineer, v. 45, no. 13, p. 5-22. Brown, J. R. LeCompte, D. M. Miller, J. K. Mortenson, and J. L. Wooden 1964, Age determinations on crystalline rocks of Northern Cascade Mountains, Washington, in Kulp, J. L., and others. Investigations in isotopic geochemistry: Palisades, New York, Columbia University, Lamont Geological for reviews of the manuscript. Observatory (U.S. Atomic Energy Commission Publication NYO-7243), Appendix D, p. 1-15. 1966, Tectonic evolution of the Northern Cascades of Washington State: Canadian Institute of Mining and Metallurgy Special Volume 8, p. 101-148. 1968, Plagioclase compositions and non-anatectic origin of migmatitic gneisses in Northern Cascade Mountains of REFERENCES CITED Washington State: Contributions to Mineralogy and Petrology, v. 17, p. 1 70. 1977, Bedrock geology of the North Cascades, in Brown, E. H., and Ellis, R. C., eds., Geological excursions in the Babcock, R. S„ Armstrong, R. L., and Misch, P., 198S, Isotopic constraints on the age and origin of tbe Skagit Pacific Northwest: Bellingham, Washington, Western Washington University, p. 1 62. Metamorphic Suite and related rocks: Geological Society of America Abstracts with Programs, v. 17, p. 339. 1979, Geologic map of the Marbiemount quadrangle, Washington: Washington Division of Geology and Earth Barksdale, J. 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