Metamorphic facies show how
metamorphic minerals form.
It is a series of
metamorphic mineral assemblages that form in a specified range of
pressures and temperatures.
In the early 1900s,
Scandinavian scientists realised that metamorphic minerals
don't
form randomly.
They realised that metamorphic rocks contain distinct
mineral assemblages formed under certain pressures and
temperatures.
Eclogite facies. By GOC53 via Flickr.com
Those assemblages depended not only on pressures
and temperatures,
but
also the chemical
composition
of the original rock.
In other
words, the same metamorphic
rock contains different minerals depending
on at which temperatures and pressures it was formed.
And two different
rocks don't contain the same minerals just because they formed under
the same pressures and temperatures, because the composition of the
original rock was different.
But - two separate masses of the
same rock, located at different locations, will have the same minerals
if they were formed under the same pressures and temperatures.
This observation
lead to Eskola's
facies:
Zeolite -
low pressures and temperatures
Hornfels -
low pressure, high to medium temperatures.
Greenschist - low to
medium
pressures and temperatures
Amphibolite - high
to medium
pressures and temperatures
Granulite -
highest temperatures, medium pressures
Blueschist -
high pressures, low to medium temperatures
Eclogite -
highest pressure, high to medium temperatures.
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