Geoscience Reference
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Figure 7.23. (a) The burial model after 20 Ma of erosion of the country rock and
deposition on the sediment. Numbers along the surface are the maximum
temperature/depth ( C/km) attained by rocks exposed at the surface.
(b) Temperature-depth paths followed by rocks originally at 8, 14 and 20 km depth
and finally exposed at the surface. The solid line shows the initial equilibrium
geotherm in the country rock; dots indicate temperatures every 2 Ma from the start
of erosion. (From Fowler and Nisbet (1982).)
those formed when the rock reaches its highest temperature. As it cools, the rock
equilibrates to a lower temperature, and the mineral composition alters. However,
the reaction kinetics become markedly slower as the rock cools, and there is a
good chance of preserving some of the higher-grade minerals if erosion is fast
enough.
In this simple model of burial metamorphism, the trough of buried sediment
has no heating effect on the country rock. Therefore, the P - T curve (which can
be plotted from the highest-grade minerals in the exposed rocks) is simply that
of the initial, equilibrium thermal gradient in the rock. In this case, the metamor-
phic geotherm is identical to the equilibrium geotherm, no matter what the rate
of erosion (provided that erosion is fast enough to 'quench' the mineral composi-
tions at their highest temperatures). The metamorphic facies series (temperatures
and pressures recorded in the rocks) produced by the event is that of a normal
equilibrium geotherm in the country rock (facies series I in Fig. 7.24).
Intrusion models with erosion
If the country rock is eroded, little metamorphic effect is seen even from these
very large intrusions. With the exception of a localized contact zone (of the order
of 5 km across) in both cases, the country rock gives a metamorphic facies series
identical to the equilibrium facies series 1, and the net result is similar to that in
Fig. 7.23(a).Areal example of this could be the Great Dyke of Zimbabwe, which
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