PÉTROLOGIE ET GÉOCHRONOLOGIE DES GRANULITES DE ULTRA-HAUTES TEMPÉRATURES DE L'UNITÉ BASIQUE D'ANDRIAMENA (CENTRE-NORD MADAGASCAR). Apport de la géochronologie in-situ U-Th-Pb à l'interprétation des trajets P-T.

Abstract : The thermomechanical evolution of the Andriamena unit is characterized by the superposition of 4 thermal events: 2.7 Ga, 2.5 Ga, 790 Ma et ~500 Ma. The significance of the Archaean event (2.7 Ga) is still unclear. The late Archaean (2.5 Ga) is associated with the UHT metamorphism (1050°C, 11.5 kbar). A mafic-ultramafic magmatism coeval with granulite metamorphism (850↓C, 7 kbar) occurred during the middle Neoproterozoic (790 Ma) and is interpreted as en evidence of a continental arc setting. The finite strain pattern results from the superposition of two deformation events D1 and D2 under amphibolite conditions (700°C, 5-6 kbar) during Cambrian (~500 Ma). The deformation is consistent with an horizontal East-West shortening related to the continental convergence during the Gondwana amalgamation. UHT Mg-granulites preserve numerous coronitic textures used to reconstruct an apparent continuous petrographical PT path. In-situ electron microprobe dating reveals that this petrographical path must be considered as a discontinuous PT path that results from the superposition of two events: 2.5 Ga et 790 Ma. Furthermore, a part of the petrographical PT path is interpreted as an apparent PT path with no geological meaning that results from the equilibration of the refractory UHT assemblages at lower pressure during the 790 Ma event. This distinction between petrographical and real PT path shows that it is critical to determine absolute ages of metamorphic assemblages. In-situ dating allows us dating minerals in their petrographical context and thus correlate ages and petrography. Using a new complementary approach of in-situ dating that combines chemical (EMP) and isotopic dating (ID-TIMS), allow us combining high spatial resolution and high analytical precision. These two techniques are applied on the same monazite grains extracted by micro-drilling directly from thin-section.
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Géologie appliquée. Université Blaise Pascal - Clermont-Ferrand II, 2002. Français
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  • HAL Id : tel-00003569, version 1

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Philippe Goncalves. PÉTROLOGIE ET GÉOCHRONOLOGIE DES GRANULITES DE ULTRA-HAUTES TEMPÉRATURES DE L'UNITÉ BASIQUE D'ANDRIAMENA (CENTRE-NORD MADAGASCAR). Apport de la géochronologie in-situ U-Th-Pb à l'interprétation des trajets P-T.. Géologie appliquée. Université Blaise Pascal - Clermont-Ferrand II, 2002. Français. 〈tel-00003569〉

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