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Metamorphism and Metamorphic Rocks

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Metamorphism and Metamorphic Rocks Big Island Field Trip • Space Still Available • Group Airline Tickets – May be available if enough people sign on • If interested send email to Greg Ravizza <[email protected]> • Planning Meeting Next Week – Will be announced by email 1 Metamorphic Rocks Metamorphism: to change form Metamorphic rock: any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in texture or mineralogical composition in the solid state No Single Mode of Origin • Metamorphic rocks result from the partial or complete recrystallization of minerals in rocks over long periods of time • Rocks remain essentially solid during metamorphism 2 Metamorphic Processes HEAT: stability region of mineral sensitive to T. With increased T, pore fluid decreases PRESSURE: greater pressure tends to decrease space available; metamorphic mineral tend to be dense Increased P can come from any directed stress (burial, differential stress) Differential stress will bring about a preferred orientation of minerals FLUIDS (H2O, CO2): acts as catalysts during metamorphism; aids the exchange of ions between growing crystals Metamorphic Grade Refers to the intensity of metamorphism High grade: high T, P Low grade: low T, P 3 Pressure and Temperature vs. Depth Temperature and Pressure Conditions of Metamorphism 4 Metamorphic Grade Evidence of Metamorphism Shale e.g., Sedimentary Rocks – Made of minerals derived from weathering of a parent rock – Stable at atmospheric temperatures and pressures (low T & P) – Originally horizontal, continuous and uniform layers • 1. Bent (deformed) layers Gneiss 5 Evidence of Metamorphism • 2. Flattened Pebbles Conglomerate Differential pressure “squashes” rock and included features Metaconglomerate 6 Evidence of Metamorphism Quartz Sandstone Quartzite • 3. Crystalline Texture Minerals tightly interlocking due to recrystallization under pressure Evidence of Metamorphism • 4. New mineral assemblages e.g., Shale: Clay minerals (some quartz) Æ Metamorphism (Mid-grade) • Forms Schist: Mica, Feldspar and other silicate minerals Æ 7 Metamorphism •Parent Rock –Even though minerals will change Shale – Most elements are provided by parent rock – Except water and some dissolved ions Schist Metamorphism Temp. (ºC) C 0 600 1200 A B 600 1200 A B C • Increased Temperature (geothermal gradients) – Minerals stable at lower temperatures converted to minerals stable at higher temperatures – Solid state chemical reactions are accelerated 8 Metamorphism • Increased Pressure (and stresses) – Increased Confining Pressure as rocks are buried – Compression at convergent plate boundary or – Sheared as plates slide past each other Types of Metamorphism Regional: Widespread changes in temperature and pressure bring about changes in rocks due to tectonic forces Contact: Intrusion of magma against colder rocks (affected area is proportional to the size and temperature of the intrusion but always only a local phenomenon) Deformational: Changes in rocks associated with faulting and folding (regional or local) 9 Regional Metamorphism Contact Metamorphism 10 Contact Metamorphism adjacent to a Granite Intrusion Contact Metamorphism of Limestone 11 Contact Metamorphism of Sandstones & Shales Deformational Metamorphism 12 Types of Metamorphism (cont.) Burial: Changes in a rock due to the gradual changes in T and P due to successive burial (regional) Impact: Changes due to rapid increase in pressure (only localized) Plate Tectonics and Metamorphism 13 Plate Tectonics and Metamorphism Metamorphic Reactions Mineralogical changes (e.g., clay to mica): Many complicated reactions — depend on pressure, temperature, composition Common metamorphic minerals include amphiboles, garnet, mica, staurolite, and kyanite Textural changes: recrystallization (grain boundaries more compact) and foliation (preferred orientation of minerals) 14 How Much Can a Rock Change? The amount of change during metamorphism depends on: • grade of metamorphism • duration of metamorphism • composition of the rock 15 Changes in Texture Grain size • Recrystallization • Mineral size can either decrease or increase Orientation of minerals • Recrystallization • Directed stress will orient minerals: – Lineation – Foliation GraniteGranite 16 GneissGneiss Granite Gneiss 17 Metamorphic Foliation 18 Metamorphic Foliation Direction of Compressive Forces 19 Slaty metamorphic cleavage Regional stresses Slate with Foliation and Relict Bedding 20 Slaty Cleavage Formation of Slaty Cleavage 21 Classification of Foliated Rocks Metamorphism of Shale (and the classification of metamorphic rocks) Shale: Fine grained Clay (and quartz) • Minerals stable under low T&P (atmospheric) •Compaction due to accumulation of sediment • Fissility along laminations • Water bound in crystalline structure of clay 22 Metamorphism of Shale 1: Low Grade MetamorphismÆSlate • Fine grained Mica, (Chlorite and Quartz) • Low grade metamorphic T&P (Water is expelled from crystalline structure of clay) • Minerals stable under low grade metamorphic conditions • Slaty Cleavage due to realignment of platy minerals Slate 23 Metamorphism of Shale 2: Medium-Grade Metamorphism Æ Phillite • Courser grained Mica and quartz • Medium-grade metamorphic T&P Causes minerals to grow • Slaty cleavage becomes rippled and rock has a sheen Phyllite 24 Metamorphism of Shale 3: Medium to High-Grade Metamorphism Æ Schist •Course grained Mica and Quartz • Medium-grade metamorphic T&P Causes minerals to grow • Minerals stable under medium grade metamorphic conditions appear: Garnet, amphibole and biotite • Schistosity due to alignment of platy and needle- like minerals Schist 25 Schist in Thin Section 26 Metamorphism of Shale 4: High-Grade Metamorphism Æ Gneiss • Course grained Feldspar, Quartz, Amphibole, Biotite •High-grade metamorphic T&P Causes minerals to separate into bands • Minerals stable under high grade metamorphic conditions appear: feldspar • Gneissic banding bands of dark and light minerals Gneiss 27 Metamorphism of Shale 5: Very High-Grade Metamorphism and partial meltingÆ Migmatite • Silica rich minerals melt first (quartz and feldspar) • Forming silicic magma •Injected into fractures resulting in silicic veins if intrusive igneous rock 28 Migmatite Quartzite 29 Quartzite in Thin Section Marble 30 Marble in Thin Section Garnet Porphyroblasts Schist Matrix 31 Stability of Minerals • Most minerals are stable over a relatively narrow range of P and T (e.g., ice unstable above 0°C) • The stability range of different minerals overlap and provide constraints on the metamorphic history of rocks Mineralogic Changes in Metamorphosed Shales 32 Mineralogic Changes in Metamorphosed Shales 33 Mineralogic Changes in Metamorphosed Mafic Rocks Changes in Composition Chemical composition little changed during metamorphism Addition and loss of volatile constituents (H2O, CO2) Non-volatile constituents (anions, cations) redistributed – bulk chemical composition constant Principal changes are in mineral assemblages Mineral assemblage for a given rock composition determined by temperature and pressure 34 • Addition or removal of fluids (and elements) – Water (and other fluids) within rocks and minerals – Moving during metamorphism – Accelerates solid-state chemical reactions and – May change rock composition Metasomatism • Metamorphism with a high water:rock ratio – Metasomatism water:rock ratio 10:1 – 100:1 – Metamorphism water:rock ratio ~ 1:10 • Water acts as a conduit for changes in chemical composition – Fluids can add and remove materials from rock • Metasomatism associated with – Contact metamorphism – Alteration of rocks at mid ocean ridge systems 35 Submarine hot springs Metamorphic Facies • A given set of metamorphic conditions • Each facies is characteristic of particular tectonic environments and will have certain minerals that are diagnostic of those conditions • Therefore, the minerals in a rock can be clues to the (P,T) history of the rock 36 Metamorphic Facies Metamorphic Reactions Prograde: Mineral changes that take place during an increase in temperature Retrograde: Mineral changes that take place during an decrease in temperature 37 38 Plate Setting and Metamorphism 39.
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