Lecture 2 – Basic Met Pet Announcements 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: 10/13-10/14. More info to come. Reading for Friday: Ch. 1, Ch 4. Agents of metamorphism (ctd.) 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 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