Page 42 - Global Tectonics

THE INTERIOR OF THE EARTH 29

Figure 2.20 Schematic model for hydrothermal metamorphism of the oceanic crust at a spreading center (redrawn
from Elthon, 1981).

and greenschist facies assemblages (Fig. 2.20). The dis- of the distribution of metamorphic facies is conse-
tribution of alteration is highly irregular, and is con- quently explained by a similarly heterogeneous distribu-

trolled by the localized fissuring of the extrusive rocks. tion of circulating fluids rather than extreme temperature
Higher temperature metamorphism is widespread variations. As indicated in Sections 2.4.7 and 2.5, parts
within the sheeted dike complex, producing assem- of the oceanic crust consist of serpentinite, that is,
blages typical of the actinolite facies, although pockets hydrated ultramafic rock. The ultramafic rock may be

of unaltered rock do occur. The highest metamorphic formed by magmatic differentiation within the gabbro
temperatures are achieved at the base of the sheeted layer, or derived directly from the mantle.
dike complex and the upper part of the gabbroic
section. Rarely, retrograde rocks of the greenschist
facies occur at this level. Alteration decreases to only
about 10% within the top kilometer of the gabbroic 2.7 DIFFERENCES
section and thereafter metamorphism is restricted to

the locality of fissures and dikes, although metamor- BETWEEN
phism does not completely terminate at depth. Accord-
ing to this model, seawater circulation occurs extensively CONTINENTAL AND
in the upper 3 km of the crust, producing the metamor-
phic assemblages and cooling the crust. High-tempera- OCEANIC CRUST
ture metamorphism only occurs near the spreading
center. At depth the circulation becomes diminished as
secondary minerals are deposited within the fl ow
channels. On the basis of information presented in this and
As the ridge spreads continuously, oceanic litho- following chapters, the major differences between
sphere is moved laterally from the heat source and continental and oceanic crust can be summarized as
undergoes retrograde metamorphism. This depends follows:
upon an adequate water supply, as water distribution is 1 Layering. The large-scale layering of the
the major control of metamorphic grade. The absence continental crust is ill defined and highly

of sufficient water allows the preservation of relict high variable, refl ecting a complex geologic history.

temperature assemblages. The heterogeneous nature In places there is a broad subdivision by the