Heat - The Solid Earth - page 317

The maximum temperatures attained in this underthrust block are controlled
by the time of initiation and the rate of erosion and also by any shear heating in
the upper slab. Eventually the thrust surface is exposed by the erosion, and the
following metamorphic events are seen.

1. An early (prior to thrusting) high-grade event in the overthrust block, shown as facies
series 1 in Fig. 7.24.
2. A post-thrusting retrograde event in the remaining part of the overthrust block which
overprints (1). In real rocks this event is controlled by the introduction of large amounts
of volatiles from the underthrust rocks. The overthrust block is rapidly cooled and
hydrated, and the degree of overprinting depends on reaction kinetics (exponentially
related to temperature) and the availability of volatiles.
3. A high-pressure, low-temperature event in the underthrust rocks, shown as facies series
4inFig.7.24.

An overthrust of the dimensions modelled here produces twin metamorphic belts:
one of high grade (facies series 1) and one of low grade (facies series 4). Much
smaller overthrusts (e.g., at shallow levels in mountain belts) would be qualita-
tively similar, but the metamorphic effects might be restricted by kinetic factors
or be small or difficult to distinguish.

7.8.5 Dating and metamorphism
The age of a radiometric system such as a mineral generally depends on the way
in which the daughter product became sealed into the system as it cooled. For a
typical system a blocking temperature exists. This is the temperature below which
the system can be thought to be closed (see Section 6.2). Different minerals, dating
techniques and rates of cooling all produce different blocking temperatures; thus,
we have a powerful tool for working out the thermal history of a mountain belt.
For moderate cooling rates, suggested closure temperatures are 80-110 ◦ C for
apatite fission tracks, about 175-225 ◦ C for zircon fission tracks, 280 40 ◦ C for ±

40 ◦ C for hornblende K-Ar (see Table 6.4 and Fig. 6.7).
In many cases, the Rb-Sr whole-rock ages probably reflect the original age of
the rock. Consider first the basaltic and granitic intrusions illustrated in Fig. 7.19
and 7.20. Underneath the intrusion, the radiometric clocks of minerals that close
at about 500 ◦ Cwould start recording about 1-2 Ma after the start of cooling.
Under the basalt intrusion, which has little internal heat generation, minerals that
close at about 300 ◦ Cwould not start recording until 20 Ma. In the case of the
granite, a mineral with a blocking temperature of 300 ◦ Cwould not close at all.
The depth-temperature paths in Fig. 7.25 show the great spread in radiometric
ages that would be obtained from various mineral 'clocks'. Furthermore, the
blocking temperatures are dependent on the rate of cooling, which is different in
each case.

biotite K-Ar and 530 ±