Page 41 - Global Tectonics

28 CHAPTER 2

ophiolite sequences on land. However, this simple of formation of the crust is very low (Section 6.10), in
analogy has been challenged, and it has been suggested the vicinity of transform faults at low accretion rates
that ophiolites do not represent typical oceanic litho- (Section 6.7), and in the initial stages of ocean crust
sphere, and were not emplaced exclusively during formation at nonvolcanic passive continental margins
continental collision (Mason, 1985). (Section 7.7.2). It seems probable that Hess (1962), in
Dating of events indicates that obduction of many suggesting that layer 3 of the oceanic crust is serpen-

ophiolites occurred very soon after their creation. Con- tinized mantle, was in part influenced by his experience
tinental collision, however, normally occurs a long time and knowledge of ophiolites of this type in the
after the formation of a mid-ocean ridge, so that the age Appalachian and Alpine mountain belts.

of the sea floor obducted should be considerably greater
than that of the collisional orogeny. Ophiolites conse-
quently represent lithosphere that was obducted while
young and hot. Geochemical evidence (Pearce, 1980; 2.6 METAMORPHISM
Elthon, 1991) has suggested that the original sites of
ophiolites were backarc basins (Section 9.10; Cawood & OF OCEANIC CRUST
Suhr, 1992), Red Sea-type ocean basins, or the forearc
region of subduction zones (Flower & Dilek, 2003). The
latter setting seems at first to be an unlikely one. Many of the rocks sampled from the ocean basins show

However, the petrology and geochemistry of the evidence of metamorphism, including abundant green-
igneous basement of forearcs, which is very distinctive, schist facies assemblages and alkali metasomatism: In
is very comparable to that of many ophiolites. Forma- close proximity to such rocks, however, are found com-
tion in a forearc setting could also explain the short time pletely unaltered species.
interval between formation and emplacement, and the It is probable that this metamorphism is accom-
evidence for the “hot” emplacement of many ophiol- plished by the hydrothermal circulation of seawater
ites. A backarc or forearc origin is also supported by the within the oceanic crust. There is much evidence for
detailed geochemistry of the lavas of most ophiolites, the existence of such circulation, such as the presence
which indicates that they are derived from melts that of metalliferous deposits which probably formed by the
formed above subduction zones. leaching and concentration of minerals by seawater,
There have been many different mechanisms pro- observations of active hydrothermal vents on ocean
posed for ophiolite obduction, none of which can satis- ridges (Section 6.5), and the observed metamorphism
factorily explain all cases. It must thus be recognized within ophiolite sequences.
that there may be several operative mechanisms and Hydrothermal circulation takes place by convective
that, although certainly formed by some type of accre- flow, probably through the whole of the oceanic crust

tionary process, ophiolite sequences may differ signifi - (Fyfe & Lonsdale, 1981), and is of great signifi cance. It

cantly, notably in terms of their detailed geochemistry, influences models of heat production, as it has been
from lithosphere created at mid-ocean ridge crests in estimated that approximately 25% of the heat escaping
the major ocean basins. from the Earth’s surface is vented at the mid-ocean
Although many ophiolites are highly altered and tec- ridges. The circulation must modify the chemistry of
tonized, because of the way in which they are uplifted the ocean crust, and consequently will affect the chem-
and emplaced in the upper crust, there are defi nite indi- ical relationship of lithosphere and asthenosphere over
cations that there is more than one type of ophiolite. geologic time because of the recycling of lithosphere
Some have the complete suite of units listed in Table that occurs at subduction zones. It is also responsible
2.3 and illustrated in Fig. 2.19, others consist solely of for the formation of certain economically important ore
deep-sea sediments, pillow lavas, and serpentinized deposits, particularly massive sulfi des.
peridotite, with or without minor amounts of gabbro. These hydrothermal processes are most conve-
If present these gabbros often occur as intrusions within niently studied in the metamorphic assemblages of
the serpentinized peridotite. These latter types are ophiolite complexes, and the model described below
remarkably similar to the inferred nature of the thin has been derived by Elthon (1981).
oceanic crust that forms where magma supply rates are Hydrothermal metamorphism of pillow lavas and
low. This type of crust is thought to form when the rate other extrusives gives rise to low-temperature (<230°C)