Wilson J. Richard. Minerals and Rocks
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Minerals and Rocks
161
Metamorphic rocks
7.5.4 Thermal metamorphism
This is also known as contact metamorphism because rocks are heated in the proximity of an intrusion of
magma. A large granite pluton will initially have a temperauture of ca. 800°C. Depending on their depth
below the surface, the country rocks will have a temperature of, for example, 100°C. The rocks nearest to the
pluton will be heated most and the thermal effect will decrease with distance from the heat source. A contact
metamorphic aureole will develop around the intrusive body (Fig.7.10).
Fig.7.10. Contact metamorphism around a pluton.
The country rock envelope develops a contact metamorphic aureole. The rocks are not deformed during
heating and hornfels facies rocks develop. High temperature minerals will develop close to the heat source
and progressively lower temperature minerals with distance from the pluton. The minerals named here are
appropriate for the contact metamorphism of pelitic rocks. As in Fig.7.6, the lines separating the different
metamorphic zones are isograds.
The size of the aureole and the minerals that develop will depend on a variety of factors. These include the
temperature contrast between the magma and the country rock envelope. Basaltic magma (which will form
gabbro on slow cooling) may have a temperature of 1200°C, whereas rhyolitic magma (which will form a
granite) may be ca. 800°C. If the magma is emplaced at depth, the country rock envelope may be as hot as
400°C; near the surface the envelope will be <100°C. Another factor is the size of the intrusion; a narrow
dyke will not have the same thermal effect as a large pluton. The nature of the country rocks will also be
important. The sequence of minerals that can develop in shales will be very different from those that can
develop in, for example, calc-silicate rocks.
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Minerals and Rocks
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Metamorphic rocks
7.5.5 Dynamic metamorphism
The geological environment in which dynamic metamorphism occurs is in the vicinity of faults. Any heat
involved is either due to the depth below the surface or to heat supplied by friction. Rocks near the surface
react in a brittle way to faulting and can become crushed or brecciated (broken into angular fragments) to
form a fault breccia. At depth, where rocks behave plastically, recrystallization can take place to form fine-
grained, strongly-foliated rocks called mylonite (Fig.7.11).
Fig.7.11. Rocks on either side of a deep, major fault plane can recrystallize to a fine-grained
“mixed rock” called a mylonite.
The composition of a mylonite depends on the nature of the rocks on either side of the fault. Here the
mylonite (grey) will be a mixture of four rock types illustrated with different colours.
7.5.6 Metamorphism at mid-ocean ridges
The oceanic crust forms at mid-ocean ridges and consists dominantly of basaltic rocks. There is a thin
covering of deep sea sediments, increasing in thickness away from the ridge. Seawater sinks into the oceanic
crust through cracks and faults. This water seeps down into hot rocks and a convection system is set up
(Fig.7.12). The hot water passing through the basalts reacts with them (at 300-400°C) and the basaltic
mineralogy changes into that equivalent to a greenschist (chlorite, epidote, albite). There is no deformation
during the metamorphism, so no schistosity develops. This type of metamorphism is also known as “ocean
floor metamorphism”.
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Metamorphic rocks
Fig.7.12. Hot water reacts with basaltic rocks near mid-ocean ridges.
A convection system is set up as seawater sinks into hot oceanic crustal rocks. The hot water reacts with the
basaltic rocks and gives rise to metamorphism under conditions equivalent to the greenschist facies (at low
pressure). The basaltic mineralogy becomes transformed into chlorite + epidote + albite. The resulting rocks
are called “greenstones” since they are mineralogically similar to greenschists but are not foliated.
This means that most of the oceanic crust comprises metabasalts, and the oceanic crust that is subducted is
“wet”. As the wet oceanic crust is subducted it becomes metamorphosed in blueschist facies (section 7.5.2.).
But some of the water is released into the overlying wedge of mantle peridotite. The presence of water in the
peridotite means that it can begin to melt at a lower temperature than dry peridotite. This is a major reason
why subduction zones are regions of volcanic activity.
7.6 Where do metamorphic rocks occur?
Most metamorphic rocks at the surface of the earth occur in the Precambrian shield areas (ancient continents)
and mountain belts. The Precambrian shields are dominated by gneissic rocks. Mountain belts are formed as
a result of continental collision. The “young” mountain belt that stretches roughly east-west from the Alps to
the Himalayas was formed (and is still being formed) as a result of the collision of the African and Indian
continental plates with the European and Asian continental plates. Gneissic areas represent the “root zones”
of ancient mountain belts.