Ophiolite

An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. Ophis is Greek for 'snake', and lite (from Greek lithos) means 'stone', after the green-color rocks that make up many ophiolites. An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. Ophis is Greek for 'snake', and lite (from Greek lithos) means 'stone', after the green-color rocks that make up many ophiolites. Their great significance relates to their occurrence within mountain belts such as the Alps and the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction. This insight was one of the founding pillars of plate tectonics, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts. The stratigraphic-like sequence observed in ophiolites corresponds to the lithosphere-forming processes at mid-oceanic ridges: The Penrose field conference on ophiolites in 1972 redefined the term ophiolite to include only the igneous rocks listed above, excluding the sediments formed independently of the crust they sit on. This definition has been challenged recently because new studies of oceanic crust by the Integrated Ocean Drilling Program and other research cruises have shown that in situ ocean crust can be quite variable, and that in places volcanic rocks sit directly on peridotite tectonite, with no intervening gabbros. Scientists have drilled only about 1.5 km into the 6- to 7-kilometer-thick oceanic crust, so their understanding of oceanic crust comes largely from comparing ophiolite structure to seismic soundings of in situ oceanic crust. Oceanic crust has a layered velocity structure that implies a layered rock series similar to that listed above. In detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating oceanic crust and ophiolites is that the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. Seismic sounding of mid-ocean ridges has revealed only a few magma chambers beneath ridges, and these are quite thin. A few deep drill holes into oceanic crust have intercepted gabbro, but it is not layered like ophiolite gabbro. The circulation of hydrothermal fluids through young oceanic crust causes serpentinization, alteration of the peridotites and alteration of minerals in the gabbros and basalts to lower temperature assemblages. For example, plagioclase, pyroxenes, and olivine in the sheeted dikes and lavas will alter to albite, chlorite, and serpentine, respectively. Often, ore bodies such as iron-rich sulfide deposits are found above highly altered epidosites (epidote-quartz rocks) that are evidence of (the now-relict) black smokers, which continue to operate within the seafloor spreading centers of ocean ridges today. Thus there is reason to believe that ophiolites are indeed oceanic mantle and crust; however, certain problems arise when looking closer. Compositional differences regarding silica (SiO2) and titania (TiO2) contents, for example, place ophiolite basalts in the domain of subduction zones (~55% silica, <1% TiO2), whereas mid-ocean ridge basalts typically have ~50% silica and 1.5–2.5% TiO2. These chemical differences extend to a range of trace elements as well (that is, chemical elements occurring in amounts of 1000 ppm or less). In particular, trace elements associated with subduction zone (island arc) volcanics tend to be high in ophiolites, whereas trace elements that are high in ocean ridge basalts but low in subduction zone volcanics are also low in ophiolites. The crystallization order of feldspar and pyroxene (clino- and orthopyroxene) in the gabbros is reversed, and ophiolites also appear to have a multi-phase magmatic complexity on par with subduction zones. Indeed, there is increasing evidence that most ophiolites are generated when subduction begins and thus represent fragments of fore-arc lithosphere. This led to introduction of the term 'supra-subduction zone' (SSZ) ophiolite in the 1980s to acknowledge that some ophiolites are more closely related to island arcs than ocean ridges. Consequently, some of the classic ophiolite occurrences thought of as being related to seafloor spreading (Troodos in Cyprus, Semail in Oman) were found to be 'SSZ' ophiolites, formed by rapid extension of fore-arc crust during subduction initiation. A fore-arc setting for most ophiolites also solves the otherwise-perplexing problem of how oceanic lithosphere can be emplaced on top of continental crust. It appears that continental accretion sediments, if carried by the downgoing plate into a subduction zone, will jam it up and cause subduction to cease, resulting in the rebound of the accretionary prism with fore-arc lithosphere (ophiolite) on top of it. Ophiolites with compositions comparable with hotspot-type eruptive settings or normal mid-oceanic ridge basalt are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes.