Download 7 Metamorphic

Geology 12
Presents
Metamorphic Rocks
Chp 7 Metamorphic Rocks
Change
Shape
• Process by which heat, pressure and
fluids change pre-existing rock in the solid
state
• Where?: mainly associated with the
continents. Sedimentary rocks (&
sediments) cover much of the continents,
but they are underlain by mainly igneous
and metamorphic rocks making up the
basement/craton. Not crouton!!
• Where the basement is exposed is the shield.
• Basement/shield: very stable over the last 600
ma and as 4 ba.
million years
billion years
Canadian Shield
Agents of Metamorphism: (3)
• 1. Heat: increases rate of chemical
reactions
– 2 sources of heat:
• a) Magma body: heats surrounding rock
– b) geothermal gradient: Earth warms up as
you descend
• Average of 25’C/km
• Question: what is temperature at 30 km?
T = 30 km x 25’C/km + 10’C
T = 750’C + 10’C
T = 760’C
• 2. Pressure:
– 2 types
• A) Lithostatic Pressure = confining
pressure = weight of overlying rock
–Applied equally in all directions
–Increases 0.25 Kbars/km of depth
»(1 Kbar = 14,700 psi)
• b) Differential Pressure = directed
pressure = pressure due to mountain
building
–Pressure is unequal so rock is distorted
conglomerate
Meta-conglomerate
•
3. Fluid Activity: water & CO2 enhance
metamorphism by increasing chemical
reactions by moving ions through
solution
Ex: 2Mg2SiO4 + 2H2O
Mg3Si2O5(OH)4 + MgO
Olivine
water
serpentine
away in sol’n
• 3 sources of water:
a) Sedimentary rock pore space (sh, sst)
b) Released from magma
c) Dehydration of water bearing minerals such
as gypsum (CaSO4 2H2O)
Types of Metamorphism (3)
• 1. Contact Metamorphism: when a hot
body of magma alters surrounding rock
with increased temperatures and the
release of hot fluids (Hi T/Low P + fluids)
Aureole = concentric zones of alteration
Country
rock
slight metamorphism (rare)
Igneous
intrusion
intermediate metamorphism
(medium)
extensive metamorphism
(well-done)
rare
medium
Well done
• The larger the intrusion (more heat + more
fluids) = more alteration
– Contact Met’ is generally localized (small
areas affected)
• i) dikes & sills: only rock in immediate
contact affected
• ii) laccoliths & batholiths: larger area
affected
• 2 Types of contact Met’
– i) baked (porcelain) clay: HEAT
– ii) as magma cools, water is released making
new minerals -> hydrothermal alteration –
valuable ore deposits
Contact Metamorphism
Country Rock
Contact Metamorphism
Intrusion
• 2. Dynamic Metamorphism: very
localized alteration around fault zones due
to differential pressure (Hi P/Low T)
Slikensides or fault breccia
Dynamic Metamorphism
Fault breccia
• 3. Regional Metamorphism: results in
most metamorphic rocks
– Occurs over large areas
– Caused by tremendous pressures,
temperatures, and deformation, deep in the
crust
– Usually along convergent plate margins, but…
Oceanic crust
Upper mantle
Low T/Hi P
(differential) zone
Continental crust
Upper mantle
Hi T/Hi P (lithostatic) zone
Regional Metamorphism
• …but, sometimes at divergent boundaries
(hi T + fluids)
ocean
Descending
sea water
Met’ rocks
Hot
magma
High heat
flow
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Hand out note helper WS 7.1a
Classification of Metamorphic
Rocks
• 2.5 types!
• 1. Foliated: minerals have arranged in a
parallel fashion…result of differential
pressure
Foliated Metamorphic Rocks
Rock
Parent Rock
Slate
sh, volcanic
ash
Appearance
fine grained,
splits into flat
pieces
Phyllite sh
fine grained,
silvery sheen
Schist sh, carbonates flakey micas
mafic ign’ rx
Gneiss sh, sst,
dark & light
felsic ign’ rx bands
“Zebra Rock”
Metamorphic
Grade
low
low-med
low-high
high
Slate
Phyllite
Notice “slight sheen”
Schist
Gneiss = “Zebra Rock”
“Zebras are gneiss”
• 2. Non-foliated: minerals have no
orientation, “equidimensional” (no layers,
no banding)
Rock
Parent Appearance
Rock
Lst
sugary, HCl
Metamorphic
Grade
low-high
Quartzite Qtz sst sugary, H=7
med-high
Marble
Non-foliated Metamorphic Rocks
Marble
Quartzite
Ford tough
• 2.5 If nature of original rock can be
recognized…
Rock
Parent
Appearance
rock
congloflattened
Metaconglomerate merate
pebbles
Metamorphic
Grade
low – med
Partially Foliated Rock
Meta-conglomerate
Metaconglomerate
• Generally, metamorphic grade is indicated
by:
– 1. increase in crystal size
– 2. increase in foliation, mineral
segregation, or granitization of minerals
– 3. index minerals: reversal of Bowen’s
Reaction Series (move up to dark/Ca
plag’ Fs minerals)
Pressure increasing
DYNAMIC MET’
diamonds
Igneous rx
REGIONAL
granite
graphite
gneiss
METAMORPHISM
schist
phyllite
coal
slate
Sh
Sedimentary
rocks
Lst
marble
CONTACT
METAMORPHISM
Temperature increasing
X
Y
Z
X
Y
Z
Low P/Hi T Contact Meta’
Hi P/Low T Regional Meta’
Hi P/Hi T Regional Meta’
High T
Low P
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Do Lab 7.1 Metamorphic Rocks ID
Do WS 7.1
Do Chp 2-7 Review WS