Download 3A8 Week 01 Lecture 03-Rocks and minerals 02

Geology for Engineers
Minerals and Rocks (II)
Gneiss
Metamorphic Rocks
Rocks may be altered through
• Temperature
• Pressure
• Deformation
• Fluid influx
Metamorphic Rocks
• The original rock is termed a protolith
• The composition of the protolith and the
conditions of metamorphism and
deformation will largely determine the end
structure and composition of the
metamorphic rock
Metamorphic Rocks
• Protoliths may be sedimentary, igneous or
even metamorphic
• Metamorphism may result from a general
increase (prograde), or decrease
(retrograde) of temperature and pressure
Pressure-Temperature-Time
Metamorphic Rocks
Metamorphism may be:
• Dynamic (i.e. some deformation involved),
e.g. gneiss
• Static (i.e. the end rock does not have a
preferred shape fabric) e.g. hornfels
Gneiss
Metamorphic Grades
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Anchi-metamorphic
Greenschist
Amphibolite
Granulite
Eclogite
• Blueschist
• Increasing
temperature and
pressure
P-T fields of metamorphic grades
Index of Metamorphic Minerals
csmres.jmu.edu/
Index of Metamorphic Minerals
csmres.jmu.edu/
Isograd Map
Different Styles of Metamorphism
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Barrovian (regional) metamorphism
Buchan (regional) metamorphism
Contact metamorphism
Shock metamorphism
Dynamic metamorphism (e.g. shearing)
Contact Metamorphism
• This shows a
basalt (igneous
rock) intruding
into a limestone.
You can see that
it has been
metamorphosed
into marble near
the contact with
the basalt.
Mylonite
• Intensely sheared
rock. Note the
recrystallization of
feldspar (lightest
crystals) and the
ribbons of quartz
(grey ) in a dark,
biotite-rich matrix.
• Difficult to
determine the
protolith!
Tectonic Settings of Metamorphism
Metamorphic Rocks
• Sandstone can
be
metamorphosed
to quartzite.
• Sutured quartz
boundaries
(changes
mechanical
properties of the
rock)
Metamorphic Rocks
• A limestone
protolith can be
metamorphosed
into a marble (this
example is quite
pure, i.e. white).
Metamorphic Rocks
• Mudstone
(pelite)
Metamorphic Rocks
• Phyllite - is the
metamorphic
step beyond
slate. New
layers of mica
minerals give
phyllite a shiny,
“wavy”
appearance.
Metamorphic Rocks
• Schist - heavily
foliated rock in
which all the
original clay
minerals are fully
transformed
Metamorphic Rocks
• Greenschist –
minerals such as
epidote, chlorite
and actinolite
Metamorphic Rocks
• This is a
blueschist. It was
formed in a
subduction zone in
high pressure and
relatively low
temperature
• Blue mineral =
glaucophane
Metamorphic Rocks
• Migmatite
Partial melting of
protolith
Dark parts =
restite
Light parts =
leucosome
Metamorphic Rocks
• Garnet Gneiss
Metamorphic Rocks
• This is an eclogite.
You can see the
presence of
garnet (pink) and
pyroxene (green)
Metamorphic Rocks
• This image
shows chert
(red), which is a
sedimentary
rock being
replaced by
silica (SiO2)
veins.
Metamorphic Rocks
• This shows a
basalt (igneous
rock) which has
been
metamorphosed
into an
amphibolite (i.e.
pyroxene
changed to
amphibole)
Metamorphic Rocks
• This shows an
olivine-rich
igneous rock
(dunite) which
has been
metamorphosed
into an
serpentinite