Lecture 1 – Basic Met Pet
Announcements
Syllabus is online
Project – get two rocks, make thin sections, ID minerals, take
photos, determine protolith, conditions of met. (generally). Turn
in full description, with photos. Due in three weeks – no time to
lose! Later: will do geothermobarometry, and make
pseudosections.
Objectives & schedule: under construction
Field trip: 10/12-10/13. More info to come.
Pre-course assessment (take 15 minutes).
Textbooks
Two needed to address phase equilibria and textures.
Metamorphism = change
What do metamorphic petrologists do?
Try to deduce protolith
Try to infer nature of metamorphic and deformation events
Numerous ways of classifying metamorphism (ASK
don’t’ TELL):
Style
Thermal
Dynamic
Dynamothermal
Geologic Setting
Contact
Regional Contact
Shock
High-strain = fault / shear zone
forms cataclasite (brittle)
or mylonite (ductile)
Regional
Burial
Ocean-floor / ocean-ridge
Orogenic
Tectonic Setting
Plate interior
contact, burial, regional (deep, and must be exhumed somehow)
Divergent margin
ocean-floor/ridge, contact
Transform margin
high-strain
Convergent
orogenic, dynamothermal, regional
contact
high-strain
Agents of metamorphism
Temperature (200 – melt migration)
How do rocks heat up?
Igneous
Isothermal relaxation after burial (sedimentary or tectonic)
Pressure
How do rocks get pressurized?
Burial
Units
SI unit for pressure is Pa = 1 kg m-1 s-2
Traditional: bars or kilobars (kb)
1 kbar = 0.1 GPa
1 bar ~ 1 atm
1 bar = 105 Pa = 100 kPa = 0.1 MPa
What is the pressure under 1 km of granite ( = 2700 kg/m3)? P =  g h
At the same depth, under the same type of rock, how much greater is
the pressure in the Himalayas (being squeezed) vs. in Kansas? (< 100
bars)
Stress
Leads to most textures
Fluids
typically H2O-CO2 solutions
typically supercritical fluid under at least middle-crust conditions.
can transport elements in or out of a rock = metasomatism
generally liberated by reactions as temperature rises
affects shape of reaction in P-T space
Bulk composition
determines the minerals that form (primary compositional effect)
determines the position of reactions in P-T space (detailed
compositional effect)
determined by Protolith ± metasomatism
Silicic (SiO2)
quartzite
Quartzofeldspathic (SiO2-Al2O3-CaO-K2O-Na2O-H2O)
Greywacke
Granitoid
Feldspathic sandstone
Pelitic (SiO2-Al2O3-FeO-MgO-K2O-Na2O-H2O)
shale
Calcareous pelite
limey shale (marl)
Carbonate (CaO-MgO-CO2)
Limestone / dolostone
Calc-silicate (SiO2-CaO-MgO-CO2-H2O)
Limey sandstone
reaction of granitic fluid & limestone (contact aureoles)
Mafic (Spear: “basic”) (SiO2-Al2O3-FeO-MgO-Na2O-H2O-CO2)
Mafic volcanics
Volcanic sediments
Ultramafic (SiO2-CaO-MgO-FeO-H2O)
Peridotite
Progressive metamorphism: P-T paths
Orogenic metamorphism
tectonic burial
Barrow, Scotland: chl – bio – grt – sta – ky – sill
Buchan: bio – crd – and – sill
Suggestive of increasing grade (Temperature) – linked to
each mineral
Facies
Eskola: found that the key was the set of minerals (assemblage)
that are linked to PT conditions
Facies = a set of metamorphic mineral assemblages, repeatedly
associated in time and space and showing a regular relationship
between mineral composition and bulk chemical compositions,
such that different metamorphic facies (sets of mineral
assemblages) appear to be related to different metamorphic
conditions, in particular temperature and pressure, although
other variables may also be important.
Name all facies
Describe parageneses (Qm assemblage of mineral
phases)
Correlate with Pelitic zones (Spear p. 16)
Facies Series (varying P/T ratio)
Very high P/T: Franciscan (z-pp-bs-ec)
High P/T: Sanbagawa (z-pp-bs-gs-am)
Medium P/T: Barrovian (ky – sil)
Low P/T: Buchan / Abukuma (and – sil)
Very Low P/T: Contact/Sanidinite
Grade = Temperature
Isograd = Reaction on the ground
Note that many zonal boundaries are not isograds in a
strict sense
Combinations of many isograds in slightly different bulk
compositions