Laboratory Manual for Introductory Geology 4e

EXERCISE 7.3
Identifying the Protolith
Name:
Course:
Section:
Date:
On the study sheets at the end of the chapter, suggest possible protoliths for your metamorphic rock specimens, using the
mineral assemblages you recorded from Exercise 7.2 and the information in Table 7.2.
7.5.1 Identifying the Protolith
A rock’s metamorphic mineral assemblage is the best evidence for the composition
of its protolith. During metamorphism, ions from protolith minerals recombine with
one another to form new minerals that are more stable under the new conditions. The
same ions are still present, so even if the protolith minerals have disappeared completely,
we can get a good idea of the protolith’s chemical composition (TABLE 7.2).
Knowing the protolith’s chemical composition is a good start, but chemistry can’t
tell us exactly what kind of rock it was. For example, a gneiss rich in quartz and
feldspar could have been a felsic igneous rock, but there’s no way to tell whether
it was intrusive (granite) or extrusive (rhyolite). It could also have been a clastic
sedimentary rock such as arkose or conglomerate. We would know that a protolith
composed mostly of minerals containing calcium was a limestone or dolostone, but
not what kind (e.g., fossiliferous, micritic, oolitic, or coquina).
Our ability to deduce more specific details about the protolith depends on how
much a rock has changed. For example, despite metamorphism, the original bedding
style of the metasandstone in Figure 7.9 has been preserved, and it indicates
deposition by a turbidity current. Had the protolith been metamorphosed more
intensely to form a gneiss, that information would have been lost.
TABLE 7.3 Types of metamorphism and their geologic settings
Type of
metamorphism
Agent(s) of
metamorphism*
How agents of metamorphism
are applied
Contact (thermal) Heat Cooling magma heats rocks cut by an
intrusive body or beneath a lava flow
Burial
Dynamic
Metasomatism
Pressure 1 a little
heat
Stress 1 a little
heat
Hydrothermal
fluids
Gravity causes increased pressure as
rocks are buried more deeply
Fault blocks generate stress as they grind
past one another; heat comes from depth
in the Earth and, to a small degree, friction
Supercritical fluids diffuse through rocks,
adding and/or removing ions
Impact Heat 1 stress Meteorite survives passage through the
atmosphere and collides with the Earth
Regional
Heat, pressure,
stress, hydrothermal
fluids
Heat and pressure come from burial in
thickened crust; plate collisions generate
compressional stress; partial melting
and metamorphic reactions produce
hydrothermal fluids
Geologic/tectonic setting
Contact between a pluton or lava flow
and wall rock
Lower parts of thick sedimentary basins
(e.g., Gulf of Mexico)
Fault zones, including transform faults
Mid-ocean ridges; pluton/wall rock
contacts; deep continental collision zones
Meteorite impact craters
Mountains formed at ancient convergent
boundaries (e.g., Alps, Himalayas,
Appalachians)
*Hydrothermal fluids may play a role in any type of metamorphism.
7.5 WHAT CAN WE LEARN FROM A METAMORPHIC ROCK?
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