Oxygen-isotope and geochemical characterization of hydrothermal alteration in ophiolite complexes and modern oceanic crust

Abstract

Stable isotopic, geochemical, and mineralogic variations in plutonic and hypabyssal rocks from oceanic crust (mainly from the Indian Ocean) and ophiolitic terranes (principally the Semail complex, Oman) are very similar. Several stages of seawater-oceanic crust interaction are recognized in these gabbros and diabases. Isotopic and chemical compositions of secondary mineral assemblages reflect changing temperatures and water-rock ratios, and record the effects of (i) pervasive seawater-hydrothermal circulation associated with the main stage of crustal formation at an oceanic spreading centre, (ii) subsequent alteration associated with off-axis volcanism (upper pillow-lava sequences), and (iii) progressively lower-temperature alteration associated with hydrothermal 'ageing' of the oceanic crust. Along the contact between the high-level, isotropic gabbro of the ophiolite, and the overlying sheeted dyke complex, repeated stoping of hydrothermally altered roof rocks into the magma chamber appears to be a ubiquitous process. Stoping is a maximum where the roof is intruded by large (off-axis) gabbro-diorite-plagiogranite bodies which may be 60% xenoliths. Abundant quartz-epidote-sulphide veins originate near these silicic intrusions and alter the overlying sheeted dyke complex. Diabase from the sheeted dyke complex in Oman, typically much more altered than dredged oceanic rocks of similar texture, exhibits the integrated effects of both the axis and off-axis hydrothermal systems.

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