A sequence of rocks characterized by ultramafic rocks at the base and (in ascending order) gabbro, sheeted dikes, pillow lavas, and deep-sea sediments. The typical sequence of rocks constituting the oceanic crust. We see ophiolites when they have been abducted onto the edge of continental crust.
A group of mafic and ultramafic igneous rocks derived by metamorphism, whose origin is associated with an early phase of the development of a geosyncline.
(o'-phi-o-lite) A stratigraphic section consisting of a suite of mafic and ultramafic igneous rocks and associated marine sediments and their metamorphic equivalents representing the oceanic crust and upper part of the mantle formed either at a oceanic spreading ridge or in a back arc basin behind a island arc. A complete section consists of a basal peridotite representing the upper layer of oceanic mantle, layered ultramafic and mafic igneous rocks ranging in composition from peridotite through gabbro, a dike complex consisting of steeply inclined basic feeder dikes, basaltic lavas and pillow lavas (formed by submarine eruption of basaltic lavas), capped by a layer of chalcedony and other marine sediments. This sequence of rocks is typically extensively altered to serpentine and other secondary minerals, and strongly tectonically deformed.