Laboratory Manual for Introductory Geology 4e

EXERCISE 7.5
Name:
Course:
Index Minerals and Metamorphic Zones (continued )
Section:
Date:
(c) What does the distribution of minerals in the contact aureole suggest about the temperatures at which these
mapped minerals are stable?
(d) Based on your answers above, list the three minerals mapped in this figure in order of increasing metamorphic
grade. 1. 2. 3.
Temperatures in the area containing biotite, but not chlorite or andalusite, must have been appropriate to form
biotite, but either too hot or too cool to form chlorite and andalusite. Biotite is therefore an index mineral for the
T-P conditions in that part of the aureole, and this area is called the biotite zone. Similarly, the chlorite and andalusite
zones represent areas in which those minerals were stable. Chlorite, biotite, and andalusite are therefore index
minerals for metamorphosed shales. If limestones and dolostones had also been present in the aureole, they would
have produced different index minerals because they don’t have the chemical composition needed to make these
three minerals.
Line B in the figure separates the biotite and chlorite zones, which means it separates the areas that were under
conditions at which chlorite was stable from areas that were under conditions at which biotite was stable. This line is
called the biotite isograd (iso 5 equal, grad 5 grade) because the metamorphic temperatures were exactly the same
everywhere along it: just hot enough to make biotite. Similarly, dashed line A in the figure is the andalusite isograd.
Metamorphic zones and isograds are also mapped in areas of regional metamorphism and may involve other metamorphic
index minerals.
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What Do You Think Geologists in Africa
have found economically valuable concentrations
of chromite (the principal ore of chromium) that formed
by magmatic differentiation in thick mafic sills. In the
same area, there are also mafic lava flows that look very
much like the sills, but which contain none of the valuable
minerals.
An investment firm is considering purchasing mineral
rights for a mountain near the chromite deposits that
has numerous tabular bodies of mafic igneous rock. You
have been hired as a consultant to determine whether
these bodies are intrusive sills, and therefore potential
chromite sources worth the investment, or extrusive
flows that would not be worth the money.
What metamorphic evidence would you look for that
would distinguish the two possibilities? On a separate
sheet of paper, write a brief report to the firm explaining
your reasoning and what you would expect to find in
either of the two cases.
192 CHAPTER 7 INTERPRETING METAMORPHIC ROCKS