The sequences of rock types described above result from a specific set of processes that occurred along the oceanic-spreading centers where the ophiolites formed. As the mantle convects and the astheno-
sphere upwells beneath mid-ocean ridges, the mantle harzburgites undergo partial melting of 10-15 percent in response to the decreasing pressure. The melts derived from the harzburgites rise to form a magma chamber beneath the ridge, forming the crustal section of the oceanic crust. As the magma crystallizes, the densest crystals gravitationally settle to the bottom of the magma chamber, forming layers of ultramafic and higher mafic cumulate rocks. Above the cumulate a gabbroic fossil magma chamber forms, typically with layers defined by varying amounts of pyroxene and feldspar crystals. In many examples the layering in ophiolites is parallel to the fossil margins of the magma chamber. An interesting aspect of the magma chamber is that periodically, new magma is injected into the chamber, changing the chemical and physical dynamics. These new magmas are injected during extension of the crust so the magma chamber may effectively expand infinitely if the magma supply is continuous, as in fast-spreading ridges. In slow-spreading ridges the magma chamber may completely crystallize before new batches of melt are injected.
As extension occurs in the oceanic crust, dikes of magma shoot out of the gabbroic magma chamber, forming a diabasic (fine-grained, rapidly cooled magma with the same composition as gabbro) sheeted dike complex. The dikes tend to intrude along the weakest, least crystallized part of the previous dike, which is usually in the center of the last dike to intrude. In this way, each dike intrudes the center of the previous dike, forming a sheeted dike complex characterized by dikes that have only one chill margin, most of which face in the same direction.
Many of the dikes reach the surface of the sea-floor where they feed basaltic lava flows. Basaltic lava flows on the seafloor are typically in the form of bulbous pillows that stretch out of magma tubes, forming the distinctive pillow lava section of ophio-lites. Seafloor metamorphism typically alters the top of the pillow lava section, including having deposits of black smoker-type hydrothermal vents. sediments deposited on the seafloor overlie the pillow lavas. If the oceanic crust forms above the calcium carbonate compensation depth, the lowermost sediments may be calcareous. These would be succeeded by siliceous oozes, pelagic shales, and other sediments as the seafloor cools, subsides, and moves away from the mid-ocean ridge. A third sequence of sediments may be found on the ophiolites. These would include sediments shed during detachment of the ophiolite from the seafloor basement and its thrusting or emplacement onto the continental margin.
The type of sediments deposited on ophiol-ites may have been very different in some of the oldest ophiolites that formed in the Precambrian.
For instance, in the Proterozoic and especially the Archean, organisms that produce the carbonate and siliceous oozes would not be present, as the organisms that produced these sediments had not yet evolved.
There is considerable variation in the classical ophiolite sequence described above, as first formally defined by the participants of a Penrose conference on ophiolites in 1972. First, because most ophio-lite sequences are deformed and metamorphosed, recognizing many of the primary magmatic units, especially sheeted dikes, is difficult. Deformation associated with emplacement typically causes omission of some or several sections of the complete sequence, and repetition of others along thrust faults. Therefore the adjectives metamorphosed, partial, and dismembered often serve as prefixes to descriptions of individual ophiolites. The thickness of individual units also varies considerably—some may be totally absent, and different units may be present in specific examples. Similar variations are noted from the modern seafloor and island arc systems, likely settings for the formation of ophiolites. Most ophiol-ites are interpreted to be fragments of the ocean floor generated at mid-ocean ridges, but the thickness of the modern oceanic crustal section is about 4 miles (7 km), whereas the equivalent units in ophiolites average about 1.8-3.1 miles (3-5 km).
some of the variations relate to the variety of tectonic environments in which ophiolites form. Results from the Ocean Drilling Program, in which the oceanic crust has been drilled in a number of locations, have helped geologists to determine which units in what thickness are present in different sections of oceanic crust. Fast-spreading centers such as the East Pacific Rise typically show the complete ophiolite sequence, whereas slow-spreading centers such as the Mid-Atlantic Ridge may be incomplete, in some cases entirely lacking the magmatic section. other ophiol-ites may form at or near transform faults, in island arcs, back-arc basins, forearcs, or above plumes.
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