Since the late 1970s, the term ophiolite has been used to describe sections of oceanic crust and upper mantle, along with sedimentary rocks deposited on the sea floor, emplaced as thrust slices onto continental lithosphere . This process, called obduction, results from continent-continent collision following subduction of oceanic crust and the closure of oceans or back-arc basins. A typical ophiolite suite comprises (from base to top):
- Harzburgite, dunite, peridotite, and pyroxenite (ultramafic igneous rocks composed of varying amounts of olivine and pyroxene) representing upper, oceanic mantle. They are commonly altered to the slippery, shiny green-black rock serpentinite. Serpentinite is named after its resemblance to the skin of a snake. Indeed, the word ophiolite is derived from the Greek words ophis, meaning snake, and lithos, meaning stone, because of the presence of serpentinites.
- Iron-titanium and magnesium gabbros. Layered, cumulate-textured gabbros dominate the basal section of oceanic crust. Higher-level gabbros tend to be more massive and associated with plagiogranites.
- Sheeted mafic dykes, feeders to overlying volcanics.
- Pillow basalts, formed as lava flows on the sea floor. The characteristic pillow shapes result from rapid cooling when lava contacts seawater.
- Radiolarian chert and limestone , graywacke and mud-stone or their metamorphic equivalents marble , quartzite and mica schist. Oceanic crust in ophiolites is produced at spreading centers, above zones of subduction in extensional or transtensional arcs (supra-subduction zone ophiolites), and along some leaky transform faults . Ophiolites that lack ultramafic, upper mantle rocks may represent obducted slices of seamounts or oceanic plateaus.
Many ophiolites have undergone high pressure-low temperature , blueschist metamorphism in subduction zones prior to their emplacement. Blueschists are named after blue-colored glaucophane and other sodium-rich amphiboles formed in rocks of appropriate composition. Eclogite facies metamophism occurs when rocks are subducted to greater depth. In eclogites, pyroxene, olivine, and plagioclase recrystallize to sodium-rich pyroxene and garnet. During collision, blueschists plus or minus eclogites are thrust as a series of imbricate slices onto lower-grade, continental rocks. Ophiolites may be overprinted by greenschist facies metamorphic assemblages and exhumed during collapse of a thrust-thickened orogen.
Ophiolites in orogenic (mountain building) belts represent sutures between two continental plates. Their recognition is therefore important in tectonic reconstructions. Ophiolites host a range of mineral deposits. Ultramafic and gabbroic rocks may contain deposits of chromium or platinum-group elements. Chrysotile asbestos occurs in serpentinites. Copper, zinc, cobalt and nickel sulfides (marine exhalatives) may occur in economic amounts. Some ophiolites host shear controlled epithermal or mesothermal gold mineralization.
See also Plate tectonics; Subduction zone