Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

One of the most debated issues about the Carajás Mineral Province in Brazil is the tectonic setting that prevailed at the time of the major Neoarchean (ca. 2.7 Ga) volcanic event, with views ranging from subduction- to rift-related. During this event, thick lava flows of mainly basaltic and basaltic-andesitic composition extruded at the Earth's surface, cooled and underwent significant alteration by hydrothermal fluids rising through deep-seated crustal faults. Archean settings have been inferred for hydrothermally altered volcanic rocks using as a proxy their whole-rock compositions in discrimination diagrams for modern geotectonics. Despite its debatable application to Archean volcanic sites, the same working approach is used in reconsidering the geodynamic view about the Carajás volcanic event. We use an artificial neural networks-based method to perform a full mass balance calculation on a large compiled database of metavolcanic rocks from Carajás. Predicted precursors reproduce a linear and limited FeO*/MgO increase with SiO2 typical of modern calc-alkaline arc environment. Moreover, the identified least-altered rocks show that they may petrogenetically involve subducted water-bearing sediments. But contrary to what is expected for such a volcanic arc environment, amphibole and magnetite fractionations have not been observed in the precursors. Discrimination diagrams using incompatible trace elements were unable to rule out the subduction hypothesis so other geological and geochemical evidence from the granitoid record of Carajás was taken into consideration. In combination with the Archean granitoid evolution of CMP, we conclude that crustal contamination associated with continental rifting appears to be the most likely process for inducing the calc-alkaline affinity of the Carajás Neoarchean metavolcanic rocks. This calc-alkaline affinity in least-altered volcanic rocks appears to be best reproduced by petrogenetic modelling involving wall-rock assimilation and melting of the Carajás basement (TTG and other K-rich granitoids) by parental magmas derived from depleted mantle source, prior to their fractional crystallization. This volcanic event was coeval with the intrusion of A-type syntectonic granitoids at the transition from a stagnant lid to mobile plate tectonic regime associated with localized translithospheric transpressional rift.