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Transcript
Metamorphic rocks
Geology 115
Metamorphic rocks
• Unlike what you may have heard, it’s
not just “heat and pressure” applied to
existing rocks
• Also, not due to partial melting of rocks
• What it is: “the solid-state reaction of
minerals within the rock to produce new
minerals and thus new rocks”
Metamorphism
• Metamorphism is a
series of chemical
reactions that occur
to stabilize minerals
in relatively high
temperature and/or
pressure conditions
• Notice this is not
freezing minerals,
like in igneous rocks
Conditions for metamorphism
Besides heat and pressure, time is needed to
complete the chemical reactions and fluids
(either water, or more rarely, carbon
dioxide) are needed to transport ions
Recognizing metamorphic rx
• Metamorphic
rocks subjected to
directed pressure
typically result in
foliated rocks
• The rock fabric
gives a sense of
the pressure
direction
Recognizing metamorphic rx
• Metamorphic rocks subjected
to confining (or lithostatic)
pressure do not show foliation
• However, because of
metamorphic reactions, the
rock tends to be the same
hardness all the way through;
cracks in a rock go through
grains, rather than around
them – intragrain fracture
Three types of metamorphism
Contact metamorphism
• Contact
metamorphism
occurs when a hot
body (pluton or lava
flow) cools in
contact with preexisting cold
“country rock”.
• Relatively low
pressures (high T,
low P)
Blueschist metamorphism
• Also known as
“dynamothermal” or
“subduction zone”
metamorphism
• Relatively low
temperatures (high
P, low T)
Regional metamorphism
• “Standard”
metamorphic
conditions - both
temperature and
pressure rise due to
increasing depth of
burial
Regional metamorphism
• Characteristic facies –
an index mineral or
combinations of
minerals - are found
that indicate a more
precise maximum
pressure and
temperature
• For instance, garnets
are geobarometers
PT diagrams
Metamorphic grade diagram
Facies diagrams
Units are °C
and kb (kilobars)
where 1 bar is
roughly 1
atmospheric
pressure
So what rocks do you find?
Sed/met rx boundary
• If there is very hot
(>200°C) water
flowing through
rocks, minerals may
be hydrothermally
altered, or, in some
cases, deposited
• Metasomatism
creates ore deposits
North Cascades NP (1968)
The entire North American
west coast is a series of
accreted terranes, which are
volcanic island arcs, sea floor
sediment, parts of other
continents, or the sea floor
itself that has been brought
by plate tectonics to this
coast and, instead of
subducting, fused to the
existing North American
craton.
Wrangellia and Sonoma are
the names of two such
terranes.
Terrane accretion is due to a subduction-type convergent bounda
which leads
to the
formation of
volcanic
arcs
Within the North Cascades, two stories:
Terrane accretion
Volcanic arc orogenies
Combining the two stories (terrane accretion and volcanic
arc orogenies), you get the quite complicated story of the
North Cascades. In essence, there are two accreted terranes
surrounding a volcanic arc.
The bit of the North Cascades that we are interested in this
week is the Northwest Cascades Thrust System, which was
directly pushed against the side of North America during
subduction at a shallow depth, and thus led to blueschist (low
T, high P) metamorphism.
Rocky Mountain NP (1915)
These Rocky
Mountains were
uplifted by the
Laramide orogeny
(starting 70 million
years ago), but the part
of the story of interest
this week occurred in
the Archean Eon
The Archean rocks at the foothills of Rocky Mountain NP
show a distinctive set of minerals, typified by these index
minerals
microcline feldsparstaurolite
sillimanite
garnet
increasing temperature and pressure that the rocks were subjected
This progression is a characteristic of regional metamorp
Just east of the park, there are a series of metamorphic
rocks, each with a key index mineral, indicating the degree
of metamorphism
Yellowstone NP (1872)
but none of this is
relevant this week
About 53 to 43
million years ago,
just at the
northeastern edge
of Yellowstone, a
series of 13
volcanoes erupted
at different times;
these are the
Absaroka Volcanic
Province. The
products of the
eruption were
mostly basalt and
basaltic andesite.
Since then, the
volcanoes have all
mostly eroded
Breccia from the
Absaroka field,
which contains
some clasts of the
pre-existing
limestone “country
rock”, which was
then selectively
mineralized by
sulfide minerals
pyrite replacing skarn and gold.
Skarn is a metamorphosed limestone
that was subject to high temperatures
but not high pressures – something
that might happen near the surface of a
volcano. This is contact (low P, high T)
metamorphism.
In 1989, a Canadian firm
wanted to develop claims on
the New World Mine just
outside of the park
boundary. This causes
concern among
environmentalists and park
conservationists. Eventually,
the mineral rights were
purchased by the US Forest
Service from the original
owner, ending the prospect
of mining in this area.