Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps) |
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Authors: | Gerhard Vavra Urs Schaltegger |
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Institution: | Institut für Mineralogie, Universit?t Tübingen, D-72074 Tübingen, Germany; e-mail: gerhard.vavra@uni-tuebingen.de, DE Departement Erdwissenschaften, Eidgen?ssische Technische Hochschule Zürich, CH-8092 Zürich, Switzerland, CH
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Abstract: | U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the
presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant
monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper
intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling
age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence
of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are
also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess 206Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism.
The age of growth of the new monazite crystals is approached by their 207Pb/235U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond
to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with
decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal
activity and alkaline magmatism.
Received: 7 July 1998 / Accepted: 4 November 1998 |
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