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

Résumé

The Obatogamau Formation is a volcanic unit located in the Chibougamau area, NE corner of the Abitibi subprovince. It consists mainly of mafic lavas with up to 20 % feldspar macrocrysts. Based on this feature the formation has been recognized over a 100 of km in the E-W direction. In the studied area, the formation is sandwiched between the ~ 2.76-2.79 Ga Chrissie and des Vents Formations and the ~ 2.73 Ga Waconichi Formation. The Obatogamau Formation is usually viewed as a monotonous, ~ 4 km thick, pile of lava flows. The chemical and petrological heterogeneity of this formation, as well as the origin of the feldspar macrocrysts, is undocumented. Similar macrocrysts are contained by the mafic lavas of the Chrissie Formation and by two synvolcanic layered intrusions containing anorthosite units of the Chibougamau area, whose relationship with the Obatogamau Formation is undetermined. To address these issues, the chemistry and petrology of the Obatogamau Formation was investigated. The rocks range from basalt to andesitic basalt and have a tholeiitic affinity, with a high compositional homogeneity. Field mapping revealed several episodes of volcanic quiescence, marked by levels of claystones, tuffs, chert and rhyodacite flows. The felsic flows have a calc-alkaline affinity. As the age of the Obatogamau Formation is unknown, the rhyodacite flows have been sampled for dating. A detailed characterisation of the feldspar macrocrysts from the Obatogamau Formation, the Chrissie Formation, the DorÈ Lake and Opawica River complexes was undertaken in order to distinguish between the Obatogamau Formation and older volcanic units, and to discuss the relationship between the studied lava flows and exposed anorthosite-bearing intrusions.