Paleocene-Eocene Migmatite Crystallization, Extension, and Exhumation in the Hinterland of the Northern Cordillera: Okanogan Dome, Washington, USA
Total Page：16
File Type：pdf, Size：1020Kb
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. 912 For permission to copy, contact [email protected] © 2008 Geological Society of America Migmatite crystallization and exhumation in the Okanogan dome, Washington 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.,