Basic Metamorphism

Lecture 2 – Basic Met Pet
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
Objectives & schedule: under construction
Field trip: 10/13-10/14. More info to come.
Reading for Friday: Ch. 1, Ch 4.
Agents of metamorphism (ctd.)
Leads to most textures
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)
Quartzofeldspathic (SiO2-Al2O3-CaO-K2O-Na2O-H2O)
Feldspathic sandstone
Pelitic (SiO2-Al2O3-FeO-MgO-K2O-Na2O-H2O)
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)
Progressive metamorphism: P-T paths
Orogenic metamorphism
tectonic burial
Facies, etc.
Facies = region of P,T that produces the same set of minerals in
most metamorphosed basalts
Why does this work?
Compare to quartz arenite: no changes with P,T
Compare to pelite: too many changes, and too sensitive to vagaries
of bulk composition.
Metamorphic mineral zone
mostly used in pelites
maps increasing grade (~T), but not comparable between regions
Isograd = line of constant grade (on the ground)
typically mapped based on mineral appearance/disappearance
ideally, via the same discontinuous reaction
Reaction = mechanism by which minerals appear, disappear, or
change composition
Facies series = set of facies found in a region
distinguished by P/T ratio
Very high P/T:
High P/T: Sanbagawa (glc – jd)
Medium P/T: Barrovian (ky – sil)
Low P/T: Buchan / Abukuma (and – sil)
Very Low P/T: Contact
Metamorphic minerals: see handout