The stratigraphic sequence observed in ophiolites corresponds to the lithosphere-forming processes at mid-oceanic ridges:. Research Scientists have only drilled about 1.
Oceanic crust has a layered velocity structure that implies a layered rock series similar 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 only revealed 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 fluid 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.
These chemical differences extend to a range of trace elements as well that is, chemical elements occurring amounts of 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 in the gabbros is unexpectedly 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 s to acknowledge that some ophiolites are more closely related to island arcs than ocean ridges. Ironically, some of the classic ophiolite occurrences used to relate ophiolites 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 crust, 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 continental crust with forearc 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. Most ophiolites can be divided into one of two groups: Tethyan and Cordilleran.
Tethyan ophiolites are characteristic of those that occur in the eastern Mediterranean sea area, e. Cordilleran ophiolites are characteristic of those that occur in the mountain belts of western North America the "Cordillera" or backbone of the continent. These ophiolites sit on subduction zone accretionary complexes subduction complexes and have no association with a passive continental margin. Despite their differences in mode of emplacement, both types of ophiolite are exclusively SSZ in origin.
Ophiolite assemblages in the Alps and some other collisional mountain belts are not formed during subduction, but rather represent the thinned margin of the continent that forms during rifting and continental drift. This incipient ocean crust remains locked to the continental margin when the ocean basin closes, emplacing the incipient ocean crust into the collision zone.
Interestingly, the age of ophiolite formation is often surprisingly close to the age of their emplacement into the continental crust.
Ophiolites are found in all the major mountain belts of the world whether collisional e. Himalayas or not e. The subduction-related chemistry of ophiolites and their association with mountain belts suggests that their formation and emplacement are related to oceanic closure and continental collision final stages of the Wilson Cycle rather than oceanic opening and seafloor spreading as was first thought.
Furthermore, the occurrence of ophiolites throughout Earth history is not constant but rather they were formed and emplaced at specific intervals. These intervals correspond closely to times of super-continent break-up and dispersal—not because they form at the ridges that separate the drifting continents, but because the large ocean basin that must coexist with any super-continent must subduct along new subduction zones as rifting progresses.
Significance of ophiolites Their great significance relates to their occurrence within mountain belts such as the Alps or 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.
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