Geoscience Reference
In-Depth Information
Figure 7.24.
Pressure-temperature curves
(metamorphic facies series) obtained from
exposed rocks in the models after erosion.
Thick dotted lines show facies series: curve 1,
equilibrium series (Fig. 7.18), high grade;
curve 2, basalt intrusion after erosion
(Fig. 7.25); curve 3, granite intrusion after
erosion (Fig. 7.25); and curve 4, overthrust
model (Fig. 7.26), low grade. Facies fields: Z,
zeolite; P, prehnite-pumpelleyite; G-L,
glaucophane-lawsonite; Gs, greenschist; A,
amphibolite; E, eclogite; Gn, granulite; and
a-H, hb-H and px-H, albite, hornblende and
pyroxene hornfels. Dashed line MM,
minimum melting for some metamorphic
mineral assemblages. (From Fowler and
Nisbet (1982).)
o
Temperature ( C)
500
1000
0
0
px-
H
hb-H
MM
Z
10
2
Gs
P
3
A
20
Gn
G-L
30
4
40
E
1
50
has only a restricted contact zone. Figure 7.23(b) again shows depth-temperature
paths followed by individual points exposed on the surface after 20 Ma.
On the other hand, when the intrusion is eroded, and deposition takes place
on the country rock, marked effects are seen because deep-seated rocks close
to the intrusion are now being eroded. The resulting facies series is one of very
low d
P
d
T
(high temperatures at low pressures: facies series 2 for basalt and
facies series 3 for granite, as shown in Fig. 7.24). It can be seen from Fig. 7.25
that, although the original intrusions have been almost completely eroded, the
metamorphic imprint of intrusion and erosion is widespread and lasting. The
small body of granite present today (Fig. 7.25(c)) appears to have had a major
metamorphic effect. Figures 7.25(b) and (d) show some depth-temperature paths
for points exposed on the surface after 20 Ma.
/
The overthrust model with erosion
Erosion of the overthrust model provides clues regarding the origin of paired
metamorphic belts. For thinner thrust sheets, the results would be similar to those
illustrated here but with lower temperatures and pressures.
Initially, only the overthrust unit is exposed at the surface. Since the thrusting
event has a general cooling effect on the upper block, the highest temperature any
part of the upper block experiences is its initial temperature prior to thrusting.
Rocks from the upper block probably retain relict minerals from their previous
high-grade environment. The extent of re-equilibration of the mineral is probably
dependent on the availability of volatiles rising from the underthrust sheet into
