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

The 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 ~ to 790-680 ~ and to 550-500 ~ The Ti, Na and A1 rv 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 dlSOvsMow indicating equilibration with a magmatic fluid or evolved seawater at low fluid/rock ratio. Lower 6180 values for some amphiboles (0-2.5%0) indicate the circulation of large volumes of seawater. The lowest 6180 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 lowtemperature seawater produced pretmite, carbonate and quartz veins, and plagioclase with high 6180. 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.Most oceanic and ophiolite gabbros experienced syn-oceanic hydrothermal alteration and metamorphism. According to spreading rates distinct processes occur and display their own metamorphic characteristics. Evidence from both oceanic and ophiolitic rocks provides insight regarding these particular problems. For modern oceanic slow-spreading ridges, M~vel & Can_nat (1991) have proposed a model for high-temperature seawater penetration distinct from the cracking front model (Lister 1974) in which thermal contraction enhances cracks and allows seawater penetration. Thermal contraction occurs at temperature below 500 ~ According to the model of Mfvel & Cannat (1991), early shearing starts at high temperatures, between 800 and 900 ~ at low water/ rock ratios, and produces granular textures with neoblast compositions similar to magmatic phases. With continued shearing, external fluid penetration increases as temperature falls and enhances am-

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“…(c) Ophiolite obduction along a regional-scale metamorphic sole thrust (after Searle et al, 2003, redrawn and modified). penetration during uprising (e.g. Giguère et al, 2003;Mével, 2003). In the second case, divergent plate motion at the oceanic ridge produces dilatation and cracking in the thin oceanic floor with consequent permeation of mantle rocks at surficial conditions.…”