DEFINITION OF PROCESSES OF OPHIOLITE AND OCEANIC CRUST FORMATION

The sequence of rock types described above are a product of

a specific set of processes that occurred along the oceanic

spreading centers that the ophiolites formed along. As the

mantle convects and the asthenosphere upwells beneath midocean

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 has been shown to be

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 have a

tendency 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 dikes, forming a sheeted dike complex characterized

by dike that have only one chill margin, most of which

face in the same direction.

Many of the dikes reach the surface of the seafloor,

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 pillowlava

section of ophiolites. The top of the pillow-lava section

is typically quite altered by seafloor metamorphism including

having deposits of black smoker-type hydrothermal vents.

The pillow lavas are overlain by sediments deposited on the

seafloor. 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 (obduction)

onto the continental margin.

The type of sediments deposited on ophiolites 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, most ophiolite sequences are deformed and metamorphosed

so it is difficult to recognize many of the primary

magmatic units, especially sheeted dikes. Deformation associated

with emplacement typically causes some or several sections

of the complete sequence to be omitted, and others to

be repeated along thrust faults. Therefore the adjectives metamorphosed,

partial, and dismembered are often added as prefixes

to descriptions of individual ophiolites. There is also

considerable variation in the thickness of individual units,

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 ophiolites 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 may be related to the variety of

tectonic environments that ophiolites form in. The Ocean

Drilling Program, in which the oceanic crust has been drilled

in a number of locations, has resulted in the recognition that

differences in spreading rate and magma supply, among other

factors, may 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 ophiolites may

form at or near transform faults, in island arcs, back arc

basins, forearcs, or above plumes.

See also CONVERGENT PLATE MARGIN PROCESSES; DIVERGENT

OR EXTENSIONAL BOUNDARIES.

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