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
• Anchimetamorphic
• 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
•
•
•
•
•

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