Download Metamorphic rocks

Metamorphic rocks
Geology 101
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)
Dynamothermal metamorphism
• Also known as
“dynamic” 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