Formation, subduction, and exhumation of Penninic oceanic crust in the Eastern Alps: time constraints from Ar/Ar geochronology |
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| Authors: | Lothar Ratschbacher Christian Dingeldey Christine Miller Bradley R Hacker Michael O McWilliams |
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| Institution: | aInstitut für Geowissenschaften, Technische Universität Bergakademie Freiberg, Bernhard-von-Cottastr. 2, D-09596 Freiberg/Sachsen, Germany;bInstitut für Mineralogie und Petrographie, Universität Innsbruck, A-6020 Innsbruck, Austria;cDepartment of Geological Sciences, University of California, Santa Barbara, CA 93106, USA;dDepartment of Geology and Environmental Sciences, Stanford University, Stanford, CA 94305-2215, USA |
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| Abstract: | New 40Ar/39Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186±2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine–Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57±3 Ma amphibole from the Penninic subduction–accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through ≤15 kbar and >500 °C at 42 Ma to 10 kbar and 400 °C at 39 Ma. The Tauern Eclogite Zone pressure–temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure–ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at ≤45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at 35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscan-style subduction–accretion processes that are recorded in the Rechnitz Complex. |
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| Keywords: | Alps Penninic unit of the Eastern Alps Ar/Ar geochronology Spreading Subduction Exhumation |
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