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Geochronological and Thermochronological Constraints on the Carboniferous Magmatism from the Armorican Massif: from the source to the exhumation

Abstract : During the Carboniferous, the Armorican Massif experienced an intense magmatism resulting in the emplacement of numerous granitic intrusions of various types (Capdevila, 2010). In southern Brittany, most of these intrusions are syntectonic peraluminous leucogranites and can be associated with significant U mineralization. To the north, most intrusions are U barren granites with more variable compositions such as the composite intrusion of Ploumanac’h. Vigneresse et al. (1989) proposed that some of these granites belong to the “so-called” High Heat Production Belt (HHPB), and represent melting of a Proterozoic to Paleozoic U-rich metasedimentary lower crust. However, the source, the U-enrichment processes as well as the intrusion ages and the exhumation history of most of those granites are still poorly known. To the south, petrological, geochemical and geochronological studies (Tartèse and Boulvais, 2010; Tartèse et al., 2011 a, b; Ballouard et al., 2015) as well as apatite fission track dating (AFT) (this work) were performed on the Lizio, Questembert and Guérande leucogranites to constrain their magmatic and exhumation history: (1) Nd and Sr isotope data suggest that they result from partial melting of metasedimentary formations. The southward increase of the εNd and ISr could be explained by sedimentary sources being gradually dominated by Paleozoic sediments relative to Brioverian sediments. (2) Along the South Armorican Shear Zone, the Lizio and Questembert granites emplaced at ca. 316 Ma (zircon U-Pb) while, to the south, the Guérande granite emplaced at ca. 310 Ma (zircon and monazite U-Th-Pb). (3) AFT dating revealed that the three leucogranites were exhumed to the near surface during the Middle Jurassic (AFT central ages range from 195 to 155 Ma). To the north, petro-geochemichal data from the Ploumanac’h composite intrusion revealed that mantle and metasedimentary sources were successively involved. The oldest unit (Bt-Hbl granite) was emplaced at 308.8 ± 2.5 Ma while the youngest unit (Bt–Ms leucogranite) was emplaced at 301.3 ± 1.7 Ma (zircon U-Pb). This intrusion was exhumed during the Triassic (AFT central age of 207 ± 9 Ma).
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Submitted on : Tuesday, August 18, 2015 - 9:34:37 AM
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Christophe Ballouard, Marc Poujol, Marc Jolivet, Philippe Boulvais, Romain Tartese, et al.. Geochronological and Thermochronological Constraints on the Carboniferous Magmatism from the Armorican Massif: from the source to the exhumation. Variscan 2015, Géosciences-Rennes, Jun 2015, Rennes, France. ⟨insu-01184856⟩

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