The transition from amphibolite-facies mylonite to chloritic breccia and role of the mylonite in formation of Eocene epizonal plutons, Bitterroot dome, Montana

Abstract

ZusammenfassungDer Bitterroot-Mylonit ist ein duktil deformierter amphibolitfazieller Mylonit (A-Mylonit), der yon einem duktil bis spr6d deformierten griinschieferfazieUen Mylonit (G-Mylonit) scharf getrennt wird. Das Bewegungsbild des A-Mylonites ist anhand seiner Lineationen und seiner S-C-Gefiige stark ausgerichtet; die Durchschnittsorientierung fiir die G-Mylonite ist ~ihnlich, streut aber breiter. Der Regionalmetamorphose, den Intrusionen yon Quarzdiorit, Orthogneis, Granodiorit und dem Granitbatholithen von Idaho-Bitterroot zwischen 105 und mindestens 60 Ma folgte eine regionale Dehnungsphase, dann die Bildung der A-Mylonite, danach ein rascher Temperaturabfall yon ungef~ar 700~ auf ca. 280 ~ wahrscheinlich im Zeitraum vor 57 his 53 Ma. Druckabh~n-giges Schmelzen wasserunters~ittigter Gesteine in tieferen Krustenniveaus krnnte als Reaktion auf den rapiden Druckabfall zur Bildung der nahezu trockenen Magmen gefiihrt haben, die dann vor ca. 52 oder 53 Ma als sehr flachliegende Plutone intrudierten oder wie der Challis-Rhyolith eruptiert wurden.Der griinschieferfazielle Mylonit und die Chloritbrekzien bildeten sich wiihrend der weiteren Abkiihlung bis anf ca. 100 ~ mit der Entwicklung yon listrisch geformten Abschiebungen anf der Ostflanke des Bitterroot-Domes vor ca. 40 Ma. AbstractThe Bitterroot mylonite is a ductile-deformed amphibolitefacies mylonite (A-mylonite), abruptly capped by ductile-to brittle-deformed greenschist-facies mylonite (G-mylonite). The movement picture of the A-mylonite from its lineation and S-C-surfaces is strongly focused; the average orientation for the G-mylonite is similar but much more diffuse.Regional metamorphism, and intrusion of quartz diorite orthogneiss, granodiorite, and granite of the Idaho-Bitterroot batholith between 105 and less than 60 m.y. ago was followed by regional extension, formation of the A-mylonite, and rapid drop in temperature from about 700 ~ to about 280 ~ at a time inferred to be about 57 to 53 m.y. ago. Pressurerelief melting of water-undersaturated rocks at deeper crustal

Extension of the Anaconda metamorphic core complex: 40Ar/39Ar thermochronology and implications for Eocene tectonics of the northern Rocky Mountains and the Boulder batholith

“…Extension in the Anaconda metamorphic core complex was coeval with early Eocene extension throughout the northern U.S. Rocky Mountains and Canadian Cordillera, and very similar in timing to the Bitterroot metamorphic core complex (Foster et al, 2007). The extension direction in the Bitterroot complex (100°-110°; Hyndman and Myers, 1988;Foster, 2000;House et al, 2002) is identical to the extension direction in the Anaconda core complex.…”

“…Eocene extension within the Bitterroot mylonite initially took place at upper-amphibolite-facies conditions (Foster et al, 2001) Extension of the Anaconda metamorphic core complex | RESEARCH considerably higher than the greenschist-facies conditions within the Anaconda mylonite. Overprinting deformation in the Bitterroot mylonite occurred at progressively lower temperatures and produced greenschistfacies mylonite, ultramylonite, and shear-banded chloritic breccia (Hyndman and Myers, 1988;Foster, 2000). The total amount of exhumation is greater for the Bitterroot complex than the Anaconda complex.…”

Neogene exhumation of the Variscan Szeghalom Dome, Pannonian Basin, E. Hungary

“…These two tectonic zones may have been connected in time and space and can be interpreted as the result of the same movement at different depths. The early ductile shear zone (Simpson and Schmid 1983) probably changed to a brittle zone closer to the surface (Hyndman and Myers 1988). The breccia zone of borehole A-180 separates rock units of signi®cantly different P±T±t characteristics, suggesting that a substantial section of lithosphere was excised tectonically between zones II and III.…”

Magnetic fabric and microstructure of a mylonite: example from the Bitterroot shear zone, western Montana

“…The core complex is composed 144 D. SIDMAN ET AL. primarily of the Idaho-Bitterroot Batholith, which began forming during Mesozoic-Palaeogene continental thickening, and metamorphosed Proterozoic Belt Supergroup sediments (Chase 1973;Hyndman 1980;Kerrich & Hyndman 1986;Hyndman & Meyers 1988;Foster et al 2001). Between 85 and 70Ma, intermediate-mafic plutons intruded (Armstrong et al 1977;Shuster & Bickford 1985;Foster et aL 2001).…”

“…Towards the top of the BSZ, fabric is characterized by a low-temperature, cataclastic overprint of the mylonitic fabric (Hyndman & Meyers 1988;Foster 2000). The BSZ ends abruptly at a brittle fault that separates the Bitterroot block from the Sapphire block, defining the Bitterroot Valley (Foster & Raza 2002).…”