Le Complexe d'Ascot des Appalaches du sud du Québec: pétrologie et géochimie

Abstract

This paper reviews new geochemical data obtained from Ordovician volcanics in the Ascot Complex of the Quebec Appalachians. The Ascot Complex is divided into three distinctive lithotectonic assemblages: the Sherbrooke, Eustis, and Stokes domains. The Sherbrooke Domain includes basalts, pyroclastic breccias, and felsic tuffs. The Eustis Domain is composed of volcanoclastic and pyroclastic rocks. The Stokes Domain is an assemblage of rhyolitic tuffs and basalts. The geochemical composition of the volcanics varies between basaltic and rhyolitic end members. Basaltic rocks are arc tholeiites (La/Yb = 0.5–2). Some basalts found in the Sherbrooke Domain show a composition very close to that of frontal-arc boninites (La/Yb = 1–2) and are depleted of rare-earth elements. Felsic rocks from the Sherbrooke Domain are poorer in light rare-earth elements (La/Yb = 0.5–0.7) than those from the Stokes Domain (La/Yb > 2). All of the felsic rocks are characterized by a negative Eu anomaly (Eu*/Eu = 1.3–3). The Stokes felsites are derived from an enriched magma contaminated by a continental crust component. The Sherbrooke Domain felsites are derived from a source impoverished in incompatible elements and enriched in high-partition-coefficient elements. Volcanic rocks from Sherbrooke and Eustis are thought to represent an incipient intra-oceanic arc. The rocks of the Stokes assemblage represent a more mature section of the same arc, explaining the contamination of generated magmas by continental crust components. The modern architecture of the Ascot Complex is essentially tectonic and may not correspond to the initial internal structure of this Ordovician arc. [Journal Translation]