Geoscience Reference
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geometry of entire rock structures rather than on their
internal lithological nature.
Metamorphism takes two forms and is graded in
intensity. Contact metamorphism bakes or recrystallizes
rock in a localized metamorphic aureole around a
magma intrusion. Regional metamorphism deforms
and recrystallizes rock on a large scale by compression
and heating in crustal shortening/subduction zones.
Magmas reveal that temperature/pressure effects are
difficult to separate, but our familiarity with some forms
of metamorphism may help. Wrought iron is hardened
by hammering (compression). Simultaneous heating
greatly assists by making the minerals more ductile,
capable of adjusting to the required shape such as a sword
blade. When hot, it can also be bent without losing
strength. Similarly, fragile snowflakes may assume a dense,
crystalline form (ice) by compression, accompanied by
pressure melting at crystal edges. This does not break the
rule about not melting! Tiny films of water are essential
catalysts and permit realignment of constituent minerals
in all forms of metamorphism. The snowflakes have not
melted and refrozen; like the iron, their original
constituents merely recrystallize into a new, stronger
configuration.
Proceeding beyond diagenesis, in which a given rock
will have reached textural and chemical equilibrium,
regional metamorphism may first compress the material.
This causes shortening in the direction of compressive
stress by squeezing any remaining plate-shaped grains or
minerals into line, enhancing its foliated or planar
structure. Poorly cemented clastic (granular) sediments
deform most readily, and their grain size and relative
abundance of phyllo- or sheet silicates determine the
quality of foliation (see Figure 12.2 ). Thus mudstones with
weak bedding - horizontal diagenetic structures acquired
during deposition - are converted into shale and then
slate by progressive compression. Increasing foliation or
cleavage converts flagstone suitable for pavement into
strong slate suitable for roofs. Coarser silty clays and sands
are transformed into phyllites and schists respectively, the
latter developing coarser schistosity rather than closely
spaced cleavage.
The role of phyllosilicates in pressure metamorphism
is a vital clue to an important function of thermal
metamorphism. Parallel rise of temperature with pressure
in regional metamorphism converts mineral species stable
in a less compressed state to mineral species more
comfortable in a more compressed state by solid-state
recrystallization . Foliation is conditioned therefore not by
initial textures alone but by the extent to which new
crystalline textures can develop. This depends largely on
the available minerals and is exemplified by granite and
its metamorphic 'twin', gneiss. Random quartz-feldspar-
biotite crystal assemblages in granite contrast with marked
foliations in gneiss in which the phyllosilicate mineral
biotite forms bands. Absence of phyllosilicates leads to
massive rather than foliated structure. Marble, quartzite
and amphibolite represent metamorphic forms of
carbonate, quartz and basalt ( Plate 12.5 ).
As with magmas, high temperature allows recrys-
tallizing minerals to grow with time and the right mineral
assemblage can replicate fine or coarse textures over short
or long time scales. Small amounts of pore fluids enhance
recrystallization by diffusing dissolved minerals through
the rock, or depositing others as they are driven off at
higher temperatures, often as vein minerals in fractures.
Some minerals depend on particular temperature/
pressure environments exclusive to metamorphic rocks.
They form facies or packages diagnostic of those environ-
ments and reflect the grade or severity of metamorphism.
Grade is, in part, a trade-off between temperature and
pressure. Principal metamorphic facies and environments
are shown in Figure 12.8 .
Metamorphic zones cover large tracts of continental
crust through their formative association with tectonic
belts. The cumulative effects of six to eight global orogenic
episodes have created complex metamorphic belts around
the cores of contemporary and older orogens. Together
Plate 12.5 Metamorphic rocks (clockwise from top left).
Greenschist, with foliation and micro thrust nappes. Gneiss,
with augen (eye-shaped) phenocrysts. Amphibolite, with
foliation and micro-folds. Marble, with a sugary semi-
crystalline texture. Slate, shown normal to cleavage and with
a pale reduction spot showing oval deformation. Phyllite, with
a shiny semi-schistose structure. For scale, amphibolite core
is 20 cm long.
Photo: Ken Addison
 
 
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