Metamorphism • Changes in rocks due to increasing P-T conditions and/or interaction with fluids. Importance 1. Mineral Resources 2. Mountain Building Events 3. History of Continental Crust Oldest Earth Uncut rocks Ruby on andthe Sapphire (4.0 billion year old gneiss from Northern Canada) Metamorphism usually involves changes in: • mineralogy formation of new metamorphic minerals • texture development of metamorphic “fabrics” Mineralogical Changes Textural Changes Metamorphic Conditions • All changes occur in the SOLID state between ~100C and 800 C “Solid State Recrystallization” = Metamorphism • Metamorphic “Grade” refers to general P-T conditions • High-temperature limit grades into partial melting migmatites (“mixed rocks”) Agents of Metamorphism • Temperature: depends on geothermal gradient (avg. 30°C/km) • Pressure: 1. lithostatic - uniform P, due to weight of overlying rock; 1 kb (0.1 GPa) = 3.3 km depth. 2. differential - unequal P in different directions; produces metamorphic rock fabrics • Fluids: H2O-dominated ± CO2. Derived from metamorphic reactions (internal) or magmatic fluids (external). Types of Metamorphism Two main types at tectonically active regions: (1) Contact Metamorphism (2) Regional Metamorphism Contact Metamorphism • thermal metamorphism due to heat of igneous intrusions • narrow zones (<1 km wide) Regional Metamorphism • Large, regional areas of crust affected (thousands of km2); one or more episodes of orogeny with combined elevated geothermal gradients and deformation • Associated with mountain building processes at convergent plate boundaries (subduction zones; collision zones) Examples: Andes, Himalayas, Appalachians • Full range of P-T metamorphic conditions; foliated rocks are a characteristic product Variable P-T Conditions in a Convergent Plate Setting Low P, high T (contact) high P, low T (“blueschist”) high P and T (regional) Non-foliated Foliated Common Metamorphic Fabrics Slaty Cleavage Schistocity Gneissic Banding Origin of Metamorphic Foliation Produced by differential stress Compressive Shearing Origin of Metamorphic Foliation Rotation and flattening of platy (clays, micas) or elongate minerals (hornblende, feldspars) Granite Granitic Gneiss Broad Compositional Categories based on mineralogy and textures ultimately inherited from the “protolith”. “Protolith” = parent rock type prior to metamorphism Quartz Sandstone (a) Limestone (fiossiliferous) Shale Schist IMPORTANT CONCEPT: Metamorphic assemblages are a function of P-T and protolith chemistry Different protoliths will yield different mineral assemblages at the same P-T conditions 3 Most Important Compositional Categories 1. Pelites: protolith = Al-rich, fine-grained clastic sediments (shales, siltstones). Classic slate-phylliteschist-gneiss sequence. 2. Calcareous: protolith = carbonate rocks (limestones, dolostones, shaly ls). Marbles, calc-silicate rocks. 3. Mafic and Ultramafic: protolith = ultramafic to mafic igneous rocks. Greenstones, amphibolites, granulites. P-T Classification metamorphic grade (low, intermediate, high) is the most basic way to classify based on P-T BUT, we can be more specific than that! P-T diagram showing “Metamorphic Facies” Metamorphic Facies are broad characterizations of the P-T conditions experienced by metamorphic rocks in an area. They are represented by “fields” or “polygons” on a P-T diagram. Adirondacks, NY NJ Highlands rocks If we find rocks in the field with a particular mineralogy, then a certain facies (P-T conditions) may be assigned to the area. Protolith = mafic igneous rocks • Facies are defined by distinctive mineral assemblages • Facies boundaries are defined by important mineral reactions and the disappearance/appearance of distinctive minerals.