Hydrothermal circulation and metamorphism in crustal gabbroic rocks of the Bay of Islands ophiolite complex, Newfoundland, Canada: evidence from mineral and oxygen isotope geochemistry

Abstract

AbstractThe gabbroic crust of the Ordovician Bay of Islands ophiolite complex formed in an island-arc setting near the North American continental margin. Detailed structural studies on the North Arm Mountain massif provide us with a scheme of syn-oceanic deformation events recorded in the crust. During a first transtensional stage, which generated gabbroic rocks, sheeted dykes and lavas, the temperature of formation of amphiboles in the gabbroic unit fell with time in three steps from 880–745 °C, to 790–680 °C and to 550–500 °C. The Ti, Na and AlIV contents of amphiboles decreased, whereas the Si activity of the fluid increased with time. The first amphibole to form has typical mid-ocean ridge basalt δ18OVSMOW indicating equilibration with a magmatic fluid or evolved seawater at low fluid/rock ratio. Lower δ18O values for some amphiboles (0–2.5‰) indicate the circulation of large volumes of seawater. The lowest δ18O values are found in the inner part of the shear zones, which channelled deep infiltration of seawater into the gabbroic unit. During brittle deformation, infiltration of low-temperature seawater produced prehnite, carbonate and quartz veins, and plagioclase with high δ18O. This study documents that the hydration of ophiolitic crust in the Bay of Islands ophiolitic complex occurred mainly along pre-obduction oceanic structures in an intraoceanic setting.