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Regional Metamorphism
I. G. Kenyon
Regional Metamorphism
Associated with destructive plate margins
during subduction and continental collision
Occurs on a regional scale and involves
thousands of cubic kilometres of rock
Regional Metamorphism – Temperature/Pressure Ranges
Regional Metamorphism
Depth /km
Pressure /MPa
Temperature / °C
Regional Metamorphic Rocks show Foliation
Regional metamorphic rocks show foliation, a
banding/layering/alignment of crystal long axes
as they crystallised under directed (tectonic) stress
Pink and blue mica
crystals aligned
parallel to each
other produces
the foliation
Arrows indicate
directed stress
Regional Metamorphism of Argillaceous Rocks
Argillaceous rocks are rich in clay minerals
and include siltstone, mudstone, clay and shale
Different metamorphic rocks are formed from argillaceous
rocks under increasing temperature and pressure conditions
Low Grade Regional Metamorphism - Slate
Forms 5-15 km depth at high
pressure, low temperature
(<300°c) conditions
Clay minerals recrystallize into
chlorite and biotite mica and
are coarser grained than the
original clay minerals
Chlorite and biotite mica are
platy minerals have their long
axes aligned and at right angles
to the principal stress direction
to form slaty cleavage
Low Grade Regional Metamorphism - Slate
Slaty cleavage direction
Slate is coarser grained
Principal stress directions and crystalline than
the shale from which
it has been derived
but the crystals are too
small to be seen with
the naked eye
Thin section of slate under the
microscope, field of view 2.5mm
Platy crystals of chlorite
and biotite have their
long axes aligned
parallel and at right
angles to the principal
stress direction
Low Grade Regional Metamorphism - Slate
Relic bedding
and laminations
Slaty Cleavage Direction
cuts across relict bedding
At low grade, some relic sedimentary structures
may be preserved such as bedding or lamination.
Low Grade Regional Metamorphism - Slate
Delabole Slate Quarry
Delabole Butterfly (a brachiopod)
Fossils may be present in
slates but they will be
deformed - stretched,
elongated or compressed
for example ‘the delabole
butterfly’ a spirifer type
of brachiopod
Slate splits readily along its cleavage
Medium Grade Regional Metamorphism - Schist
Formed under higher
temperatures 400 to 500
degrees centigrade and
at depths of 15 to 25 km
Higher temperature results
in larger crystals 1 – 2mm
and the growth of new
minerals such as garnet
along with quartz and micas
Coarser foliation is known
as Schistose Texture
Garnet crystals occur as
porphyroblasts up to 5mm
in diameter and often
distort the foliation
Medium Grade Regional Metamorphism - Schist
Garnet porphyroblast
8mm in diameter
Principal Stress
Directions
Coarse foliation
formed by aligned
micas is known as
Schistose Texture
Growth of garnet has
distorted the foliation
High Grade Regional Metamorphism - Gneiss
Formed under higher
temperatures 500⁰c to 700⁰ c
and at depths of 25 to 40 km
Higher temperature results in
larger crystals over 2mm and
the growth of new minerals
such as feldspar along with
quartz and mica
1 cm
Principal Stress
Direction
Gneissose
banding
Grey/white quartz
Pink feldspar
Minerals are segregated into
discontinuous layers to
produce a coarse foliation
known as gneissose banding
Mineral composition
now similar to granite
Black biotite mica
Principal Stress
Direction
Very High Grade Regional Metamorphism - Migmatite
Banded Gneissose Component
Gradational junction
between the gneiss
and granite-no chilling
or baking evident
Granitic Component
Temperatures above 700⁰ c and
depths of 40 to 50 km results in
the gneiss starting to melt
The rock produced is a
migmatite or ‘mixed rock’
comprising a banded gneissose
component and a non-foliated
granitic component
Beyond 50 km depth and
temperatures >800⁰c the
migmatite melts completely
to form magma, when cooled
and solidified forms granite.
That’s All Folks!