Symplectite breakdown of Ca-rich almandines in upper amphibolite-facies Skagit Gneiss, North Cascades, Washington

Misch, Peter; Onyeagocha, A.C.

doi:10.1007/bf00371006

Cited by 13 publications

(10 citation statements)

References 17 publications

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“…Textures in the diorite indicate that it has been metamorphosed and deformed, and that some deformation predated the high‐P (10 kbar) metamorphism [ Whitney , 1992; P. Pressly and D. L. Whitney, unpublished work]. Plagioclase coronas on garnets [ Misch and Onyeagocha , 1976], described as undeformed by Wernicke and Getty, are deformed in this rock and throughout much of the Cascades core, forming elongate, recrystallized tails in NW‐SE lineation‐parallel sections. We thus suggest that both the timing and driving mechanisms of exhumation suggested by Wernicke and Getty need modification.…”

Section: Discussionmentioning

confidence: 99%

Driving mechanisms for >40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington

et al. 2004

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[1] In the NW North American Cordillera, the Cascades core region of the Coast Plutonic Complex underwent Late Cretaceous (>96 Ma to locally 73 Ma) SW-NE contraction and crustal thickening followed by dextral transpression ($73 to 55 Ma), then transtension (<55 Ma). Exhumation occurred during all three phases. During contraction, slow exhumation ($0.6 mm/yr) occurred along the margins of the core, driven by isostatic rebound and erosion, and faster exhumation (>3 mm/yr) by local thrusting in regions undergoing crustal thickening. In the central part of the core (Chelan block), >40 km of exhumation occurred between 91 and 45 Ma, about half of which occurred during early contraction (driven by thrusting) and half during top-to-north, arc-oblique shear during reactivation of a midcrustal Cretaceous thrust, the Dinkelman decollement. The footwall of this thrust consists of the Swakane Biotite Gneiss, a Cretaceous, metaclastic assemblage with recorded pressures of 10-12 kbar, no arc-related magmatism, and structures dominated by pervasive top-to-north shearing. The hanging wall consists of the Napeequa Complex, an oceanic assemblage with recorded pressures of 6-12 kbar, voluminous arc-related magmatism, and complex structures indicating early top-to-WSW shearing, younger top-to-north shearing, and widespread folding. In the Napeequa, top-to-north shearing started by 73 Ma during melt-present conditions at pressures !6 kbar. Top-to-north shearing in both hanging wall and footwall continued during exhumation ($1.6 mm/yr) and cooling to greenschist facies conditions during which slip became increasingly localized, eventually resulting in formation of pseudotachylite on discrete slip surfaces. We suggest that arc-oblique extension was driven by along-arc heterogeneity in displacements/ erosion, initially during transpression and underplating of continental sediments, and later during transtension.

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Section: Discussionmentioning

confidence: 99%

Driving mechanisms for >40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington

et al. 2004

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“…Some garnet has a post‐kinematic symplectitic corona consisting of a vermicular intergrowth of plagioclase and hornblende (cf. Misch & Onyeagocha, 1976). The garnet is unzoned, as is common for the Skagit Gneiss.…”

Section: Petrography and Mineral Chemistrymentioning

confidence: 99%

Metamorphism and deformation at different structural levels in a strike‐slip fault zone, Ross Lake fault, North Cascades, USA

Gordon

Whitney

Miller

et al. 2010

Journal Metamorphic Geology

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Continental crust is displaced in strike-slip fault zones through lateral and vertical movement that together drive burial and exhumation. Pressure-temperature-deformation (P-T-d) histories of orogenic crust exhumed in transcurrent zones record the mechanisms and conditions of these processes. The Skagit Gneiss Complex, a migmatitic unit of the North Cascades, Washington (USA), was metamorphosed at depths of 25-30 km in a continental arc under contraction, and is bounded on its eastern side by the long-lived transcurrent Ross Lake fault zone (RLFZ). The P-T-d conditions recorded by rocks on either side of the RLFZ vary along the length of the fault zone, but most typically the fault separates high-grade gneiss and plutons from lower-grade rocks. The Ruby Mt-Elijah Ridge area at the eastern margin of the Skagit Gneiss exposes tectonic contacts between gneiss and overlying rocks; the latter rocks, including slivers of Methow basin deposits, are metamorphosed and record higher-grade metamorphism than in correlative rocks along strike along the RLFZ. In this area, the Skagit Gneiss and overlying units all yield maximum P-T conditions of 8-10 kbar at >650°C, indicating that slices of basin rocks were buried to similar mid-crustal depths as the gneiss. After exhumation of fault zone rocks to <15 km depth, intrusion of granitoid plutons drove contact metamorphism, resulting in texturally late andalusite-cordierite in garnet schist. In the Elijah Ridge area of the RLFZ, an overlapping step-over or series of step-overs that evolved through time may have facilitated burial and exhumation of a deep slice of the Methow basin, indicating that strike-slip faults can have major vertical displacement (tens of kilometres) that is significant during the crustal thickening and exhumation stages of orogeny.

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“…The pioneering studies of Misch (1966Misch ( , 1968Misch ( , 1977Misch ( , 1988 contain extensive descriptions and discussions of the petrology, geochemistry, and structure of the North Cascades, with emphasis on the Skagit Gneiss. Additional studies of the petrology and structure of the crystalline core of the North Cascades include Misch & Rice (1975), Misch & Onyeagocha (1976, Yardley (1978), Haugerud (1985), Babcock & Misch (1988), Brown & Talbot (1989) and Miller et al (1989. Misch (1968) divided the Skagit Gneiss into an epidote-bearing and an epidote-free subfacies (Fig.…”

Section: P R E V I O U S Workmentioning

confidence: 99%

“…The P-T path followed by these rocks has not previously been determined. Misch & Onyeagocha (1976) interpreted the presence of well-developed sillimanite in the matrix of rocks that contain kyanite inclusions in garnet to indicate growth of sillimanite in response to rising temperature. They similarly interpreted the presence of symplectitic halos of calcic plagioclase and hornblende around garnets in some amphibole-bearing rocks to be the result of the breakdown of Ca-rich almandine garnets in response to an increase in temperature.…”

Section: 'mentioning

confidence: 99%

High‐pressure metamorphism in the Western Cordillera of North America

Whitney

1992

Journal Metamorphic Geology

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The Skagit Gneiss, a major component of the crystalline core of the North Cascades, was metamorphosed during a mid-Cretaceous(?) to early Tertiary high-P event driven by the collision of the Insular and Intermontane superterranes. Maximum pressures recorded by metapelitic rocks are 8-10 kbar at 650-725" C. High pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the Skagit Gneiss and adjacent Cascade River Schist.Mineral reactions continued during nearly isothermal decompression from 8-10 kbar to c. 3-5 kbar. Early high-P minerals (e.g. kyanite) are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass. Skeletal relics of kyanite are also present in the groundmass of lower-grade, staurolite-bearing schists that contain texturally later cordierite. This matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature.These results represent a revision of the metamorphic history of the Skagit Gneiss, which was formerly thought to have experienced only relatively low-P Barrovian metamorphism. Qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression.Geobarometric results for the Skagit Gneiss are interpreted as evidence for major crustal thickening in the North Cascades. Recognition that pressures of c. 9 kbar were attained supports a contractional model for North Cascades orogenesis and requires that tectonic syntheses account for the burial of the Skagit Gneiss protoliths to a depth of c. 25-30 km.Gneiss presented in this paper relies on: (i) reinterpretation of textures and mineral assemblages in metapelitic rocks; (ii) discovery of cordierite in metapelitic rocks in the main part of the Skagit Gneiss; and (iii) geothermobarometric results combined with P-T estimates from petrogenetic grids. It should be noted that previous estimates of relatively low Barrovian metamorphic pressures were based on the apparent instability of almandine garnet in amphibolites and the occurrence of calcic plagioclase with zoisite in calc-silicate rocks (Misch, 1968), and not on the presence of cordierite. Cordierite was observed only recently in the main part of the Skagit Gneiss as part of this investigation.

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Symplectite breakdown of Ca-rich almandines in upper amphibolite-facies Skagit Gneiss, North Cascades, Washington

Cited by 13 publications

References 17 publications

Driving mechanisms for >40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington

Driving mechanisms for >40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington

Metamorphism and deformation at different structural levels in a strike‐slip fault zone, Ross Lake fault, North Cascades, USA

High‐pressure metamorphism in the Western Cordillera of North America

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