When rocks are metamorphosed their bulk chemistry remains
about the same, except for water and CO2, which are fairly
mobile. The mineral assemblages constantly change but the
chemistry remains the same. Thus, the temperature and pressure
of metamorphism control the mineral assemblages in
metamorphic rocks. In 1915 Pentti Eskola presented the concept
of metamorphic facies. Simply put, this concept states
that different assemblages of metamorphic minerals that reach
equilibrium during metamorphism within a specific range of
physical conditions belong to the same metamorphic facies.
Eskola studied rocks of basaltic composition, so he named
his facies according to the metamorphic names for basaltic
rocks. His classification, shown here, stands to this day.
Regional metamorphism is a response to tectonic activity and
different metamorphic facies are found in different tectonic
environments.
The figure on page 274 shows the distribution of metamorphic
facies in relationship to the structure of a subduction
zone. Burial metamorphism occurs in the lower portion of the
thick sedimentary piles that fill the trench, whereas deeper
down the trench blue schist facies metamorphism reflect the
high pressures and low temperatures where magmas come up
off the subduction slab and form an island arc; metamorphism
is of greenschist to amphibolite facies. Closer to the plutons of
the arc, the temperatures are high, but the pressures are low, so
contact metamorphic rocks are found in this region.
See also MINERALOGY; PETROLOGY; STRUCTURAL
GEOLOGY.
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