Metamorphic_Notes_2006

Metamorphic_Notes_2006 - Metamorphism: A Process of Change...

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1 Metamorphism: A Process of Change Metamorphic Rocks : Rocks formed from pre-existing rocks that are changed mineralogically, texturally, or chemically in the solid state by pressure(P), temperature (T), and/or solutions (X). Regional Metamorphism – large area due to temperature, pressure Contact Metamorphism – small area due to heat from adjacent intrusion Protolith - The parent rock to a metamorphic rock Metamorphic Conditions Metamorphic Conditions : High Pressure Deep Burial Moderate to High Temperature Geothermal Gradient Proximity to Intrusions Differential Forces Tectonic Forces Chemical Action with Water Dehydration Metasomatism Metamorphism occurs because the protolith is chemically and/or physically unstable under new pressure, temperature, and/or stress conditions it is subjected to. Examples : New minerals may form: chemical reactions Clays + Heat = Micas New textures may form: recrystallization due to reactions and/or pressure Sedimentary Bedding + Heat = Banding The metamorphic rock forms in order to maintain equilibrium with its surroundings – equilibrium implies that the minerals and/or structures are stable under the applied conditions
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2 Factors Controlling Metamorphic Rock Characteristics : The pre-metamorphic composition (mineral/chemical) of the protolith The temperature and pressure of metamorphism The effects of tectonic forces The effects of fluids associated with metamorphism (generally H 2 O, CO 2 ) 1) Pre-Metamorphic Composition Protolith composition controls the mineral composition of the resulting metamorphic rock: Sandstone Examples : 1.Quartz SS with Quartz Cement Quartz + Heat = Quartz Quartz Sandstone + Heat = Quartzite SiO 2 + Heat = SiO 2 2.Quartz SS with Calcite Cement SiO 2 + CaCO 3 = CaSiO 3 + CO 2 (g) Quartz + Calcite = Wollastonite + CO2(g) Two important things happened in the above reaction: Minerals reacted to form new minerals A gas (CO 2 ) was produced – this is the process of dehydration Dehydration reactions are common, and are a very important way that ions move through metamorphic rocks
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3 2) Temperature: Metamorphic temperatures ~ 200°C – 700°C (between lithification and melting) Metamorphic Heat Sources: Heat from a nearby magma Radioactive decay Regional pressure increases Geothermal gradient
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Metamorphic_Notes_2006 - Metamorphism: A Process of Change...

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