Geoscience Reference
In-Depth Information
￿ protolith composition - what the rock type was prior to
metamorphism;
￿ metamorphic grade - key index minerals, or mineral
assemblages, may place constraints on pressure and/or
temperature conditions of metamorphism;
￿ metamorphic history - how metamorphic reactions have
progressed.
9
The appearance of quartzites and meta-limestones (marbles) can
be very similar, but if the interlocking grains can be scratched by
steel, then the rock is most likely quartzite. A clue to the rock
composition can aid the search for appropriate index minerals
in turn. Assigning a rock to a metamorphic facies or Barrow
Zone (Figures A9.1 and A9.2) is not simply cataloguing: it can
constrain the P and T conditions of metamorphism, perhaps
very tightly, and these data improve our understanding of
large-scale tectonic processes such as subduction and
continental collision. Some minerals are characteristic of certain
conditions (e.g. glaucophane suggests a relatively high P/T ratio;
cordierite occurs at low pressures; chlorite tends to occur at low
temperatures). However, composition and mineralogy are
intimately linked, and mineral stability fi elds change in different
rock types (e.g. garnet occurs at lower temperatures in Mn-rich
metasediments). Some bulk-rock compositions do not contain
enough of the elements required to grow useful index minerals.
For instance, a metamorphosed sandstone may only develop
quartz and perhaps some biotite, and these two minerals could
coexist over wide P and T ranges. Table 9.1 (p. 201) lists some of
the common minerals that are useful for constraining the
conditions of metamorphism in different bulk-rock
compositions at different conditions, and some that occur across
wide P and T ranges and are therefore less useful (bottom row).
Minerals lose water with
increasing temperature, so
hydrous minerals such as
chlorite and talc are
generally characteristic of
low-T rocks, whereas
anhydrous minerals
(pyroxene, garnet, feldspar,
spinel) dominate in high-T
assemblages.
Table 9.1 is a very simplifi ed summary; recognizing a single
metamorphic mineral may not constrain the conditions of
metamorphism very well. Try to identify assemblages of
minerals that have grown in equilibrium together, and as many
minerals as you can in the fi eld in order to refi ne your
interpretations of metamorphic grade. Plotting the fi rst
appearance of index minerals on a map to construct isograds is
a good way to summarize the metamorphic evolution of an area
(see Section 10.3.2).
9.3.3 Classifi cation of metamorphic rocks
The full complexity of a metamorphic rock can seldom be
revealed by fi eldwork, but classifying a metamorphosed rock
guides observation and encourages systematic recording of
observations. A good convention is to combine the key
minerals with a textural term, for instance: 'kyanite-chlorite
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