Laboratory Manual for Introductory Geology 4e

■ Metamorphic index minerals: Some metamorphic minerals
can survive only under a specific range of temperature-pressure
(T-P) conditions. They are called index minerals because their
presence in a rock indicates what the T-P range was during metamorphism.
Index minerals provide a relative intensity scale; that is,
we know which of these minerals represents the highest
metamorphic grade, but we can’t determine the precise intensity
of the metamorphism experienced in terms of degrees Celsius
or kilobars of pressure. Estimates of absolute (numerical)
T-P conditions are based on experimental work in which materials
with compositions similar to particular protoliths have
been heated and squeezed to replicate conditions at which
index minerals form. Estimates of metamorphic conditions
for regionally metamorphosed shales in northern Scotland are
shown in FIGURE 7.13.
Index minerals were discovered when field geologists noted
the change in mineralogy in wall rocks adjacent to plutons. Exercise
7.5 will guide you through the reasoning used by those metamorphic
pioneers.
FIGURE 7.13 Estimated temperature-pressure
conditions in regionally metamorphosed shales
from northern Scotland, superimposed on the
aluminosilicate phase diagram.
Pressure (kbar)
10
9
8
7
6
5
4
3
2
1
Chlorite
Andalusite
Biotite
Garnet
Sillimanite
0 0
300 400 500 600 700 800
Temperature (ºC)
Kyanite
Note that the garnet zone could be followed by either a
kyanite or a sillimanite zone, depending on the precise
T-P conditions.
30
20
10
Depth (km)
EXERCISE 7.5
Index Minerals and Metamorphic Zones
Name:
Course:
Section:
Date:
It wasn’t until the early 20th century that geologists recognized that contact metamorphic minerals were not randomly
distributed throughout the contact aureole (the area around a pluton affected by contact metamorphism). The figure
below shows the distribution of minerals in the contact aureole around the Hartland pluton, a small granite batholith in
central Maine. The field mapper has noted that the minerals chlorite, biotite, and andalusite appeared in metamorphosed
shales at different distances from the pluton.
Examine the figure on the right and answer the following
questions:
(a) Chlorite and biotite are platy minerals. Do you think they
are foliated in this contact aureole? Explain.
(b) Where in the contact aureole would the highest
temperatures have occurred? Explain your reasoning.
0 2 4
Miles
B
B
Hartland granite
A
A
Metamorphic zones
B
A
Andalusite Biotite Chlorite
B
(continued)