Early Silurian orogenic andesites from the central Quebec Appalachians

Abstract

The Pointe aux Trembles Formation is a deep- to shallow-water volcaniclastic apron with massive lava flows deposited in Early Silurian time. The apron deposits extend laterally to a deep-water accumulation of finer grained siliciclastic and tuffaceous material intruded by gabbroic sills, defined as the Lac Raymond Formation. The suite has most of the characteristics of high-K orogenic andesites. The lavas are highly porphyritic, with plagioclase and augite as major phenocrysts, subordinate orthopyroxene (chlorite pseudomorph), and rare hastingsitic hornblende. They form a continuous series that is intermediate in composition, hypersthene normative, and high in Al2O3 (> 16%), low in TiO2 (< 1.5%), and without iron enrichment. The Lac Raymond gabbroic sills have geochemical features closely related to those of the lavas, suggesting their cogenetic origin. With magmatic differentiation, the suite becomes depleted in most major and trace elements, indicating crystal fractionation of plagioclase, pyroxene, amphibole, apatite, and oxides. Negative Nb and Ta anomalies and clinopyroxene geochemistry confirm that the magmatism occurred in a volcanic-arc environment. It can then be inferred that plate convergence and subduction were still active in Early Silurian time. The petrographic features of the suite also led to the conclusion that a thin rather than a mature continental crust was involved in the genesis of the volcanism. The earliest portion of the Taconian–Acadian interlude was thus an orogenic period in continuation with the early tectonic activity of the Taconian. Consequently, tectonic reconstruction of that part of the Appalachian orogen should not be viewed in terms of isolated events.