Abstract
Abundant oceanic gabbros created in slow-spreading ridges have been collected by dredging, drilling or with submersibles (Atlantic ocean crust, Mid-Cayman Rise, South West Indian Ridge). A review of published studies as well as work in progress show that these gabbros may be extensively metamorphosed and more or less deformed. Structural and petrological investigations suggest that shearing starts in the lower crust at very high temperature, before the complete solidification of the magma chamber. Continuing shearing allows seawater penetration and formation of synkinematic amphibole as temperature decreases. In the absence of ductile deformation, metamorphic reactions result from interaction between gabbros and a seawater-derived fluid phase circulating through a crack network.
We propose a model to explain the metamorphic and deformational characteristics of oceanic gabbros. We suggest that early lithospheric stretching beneath the ridge allows seawater penetration in the lower crust when it is still very hot, through permeability created by shear zones and associated synkinematic cracks. Therefore, hydration of the lower crust starts at high temperature (750°C), in contrast with a simple cracking front model in which hydration starts at temperature below 500°C. The amount of stretching may be related to the spreading rate through the magma budget.
Gabbroic series from ophiolite complexes may show either this early stretching and high temperature metamorphism associated with ductile shear zones (Western Alps ophiolites) or a crack network related to the cracking front and moderate temperature metamorphism (Haylayn massif, Oman ophiolite).
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Mével, C., Cannat, M. (1991). Lithospheric Stretching and Hydrothermal Processes in Oceanic Gabbros from Slow-Spreading Ridges. In: Peters, T., Nicolas, A., Coleman, R.G. (eds) Ophiolite Genesis and Evolution of the Oceanic Lithosphere. Petrology and Structural Geology, vol 5. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-3358-6_16
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DOI: https://doi.org/10.1007/978-94-011-3358-6_16
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