Download Met Pet Part 2

Met Pet Part 2
Metamorphism
• Changes in a igneous or sedimentary rock that
are caused by P, T, fluids or deformation
• How do geologists systematically study
metamorphic rocks?
•The Zone and Facies concepts
• Why doe these concepts work?
•Because metamorphic rocks are created by
reactions that occur under equilibrium
conditions, and we can predict what minerals
should occur under specific P-T conditions, and
vise versa
Metamorphic Textures
• See handout
The Zone Concept
• Barrow (ca. 1900) noted significant and systematic
mineralogical changes in pelitic rocks
• Metamorphic Zones: can subdivide an area into a
series of zones that represent changing (normally
increasing) metamorphic conditions from one zone
to another zone
• Each zone are subdivide from one another based
on the appearance of a new mineral as
metamorphic grade increase, termed the index
mineral
• can still have minerals from lower grade zones in
higher grade zones, change is controlled by first
appearance of the index mineral
Metamorphic Zones
• Isograd = line that separates the zones
delineated by the first appearance of the index
mineral
•Typical sequence:
Low grade
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Kyanite zone
High grade Sillimanite zone
Barrow’s Metamorphic Zones
Pros and Cons of Metamorphic Zones
• Concept has
been applied in
many locations
world wide, lots of
classical and typelocalities
• Convenient for
field geologists:
there are easy
mapable
boundaries from
zone to zone
• Has been applied to
different rock types,
but mostly used for
pelitic rocks
•Can be hard to
related P/T conditions
between different rock
types
• Doesn’t take into
account fluid or stress
changes
The Facies Concept
• Eskola (1915), more rigorous method of dealing with
metamorphic rocks
• In a metamorphic rock, bulk chemistry of rock is
same as protolith, and any change in mineralogy is
controlled by this original bulk chemistry and are a
result of P/T change under equilibrium conditions
• As a result, can define a set of mineral
assemblages for all bulk chemistries which occur
in same P/T range (regardless of protolith)
•Metamorphic facies: is a set of repeatedly associated
metamorphic mineral assemblages in a specific P/T
range
Metamorphic Facies: Quick Example
• Chemically identical mineral pairs occur under
different P-T conditions
MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5
Enstatite Anorthite
Diopside Andalusite
Metamorphic Facies Diagram
• Facies are defined by a mineral assemblage that
occur over a specific P-T range
Metamorphic Stability and Equilibrium
• If we can find a mineral or set of minerals are in
equilibrium and are formed under a narrow range
of P/T conditions, we can determine the
corresponding P/T conditions of metamorphism
•This is because there is a good understand of
what reactions take place during metamorphism
specific P-T conditions
P-T Diagram #1
• Lines represent equilibrium reactions between different
minerals, or phases
• Can be broken down into simpler diagrams
P-T Diagram #2
b
a
Metamorphic ACF Diagrams
Can build metamorphic
diagrams using oxide
composition of
rock/minerals that show
co-existing mineral
assemblages (Tie Lines)
under conditions that the
diagram is constructed
z If you can determine
mineral assemblages in
your rocks, you can see
what diagram they all plot
on,deduce P-T conditions
z
A=Al2O3+Fe2O3-Na2O-K2O
C=CaO-3.3P2O5
F= FeO+MgO+MnO
Metamorphic Summary
• Changes in a igneous or sedimentary rock that are
caused by P, T, fluids or deformation
• 5 different types of metamorphism
• Rocks can be broadly classified as foliated or
unfoliated
• Use concepts of metamorphic zones and
metamorphic facies to study metamorphic rocks
• We can systematically study metamorphic rocks
because the changes are based on reactions that
take place under equilibrium conditions at specific PT conditions