Metamorphic Energy Flow. Categories of Metamorphism. Inherited Protolith Character. Inherited Fabric. Chemical Composition

Transcription

1 Metamorphic Energy Flow Categories of Metamorphism Best, Chapter 10 Metamorphic processes are endothermic They absorb heat and mechanical energy Absorption of heat in orogenic belts Causes growth of mineral grains Drives endothermic reactions Inherited Protolith Character Preservation of parts of original fabric Chemical composition similar Inherited Fabric Best preserved in weak metamorphism Only distinct original features preserved Some mineral textures may remain Pseudomorphs e.g. Porphyritic textures Bedding may be preserved Chemical Composition Most metamorphic rocks have compositions similar to their protoliths Only volatiles move through the system Metasomatism involves a large transfer of major chemical elements

2 Metasomatism Massive replacement due to fluid flow Generally occurs adjacent to an intrusive Carbonate replacement is one type Tourmaline replacement of feldspars is another type Categories of Metamorphism Contact metamorphism Regional metamorphism Sea-floor metamorphism Burial metamorphism Shear metamorphism Contact Metamorphism Adjacent to a magmatic intrusion Caused by high thermal gradients Local metasomatism in carbonate hosts Minerals produced include: garnet, epidote, pyroxene, idocrase Regional Metamorphism Occurs over broad areas Originates deep in orogenic zones Changes occurs over 10 7 years Syntectonic schists and gneisses Continuous T & P changes Sea-floor Metamorphism Occurs near mid-ocean rifts Metasomatic and thermal processes Low temperature hydrothermal effects

3 Burial Metamorphism Occurs in thick sequences of sedimentary rocks Unrelated to orogeny and magmatism Equilibrium rarely attained Continuation of low-t diagenesis Shear Metamorphism Occurs in deep-seated shear zones Local cohesive solid-state flow Mylonite is a typical product Metamorphic Fabric Original fabrics displaced or destroyed Competing processes Deformation fabric imposed Reduces grain size Solid state crystal growth Increase grain size Deformation Fabric Near-surface deformation produces cataclasis Deeper deformation produces solution and redeposition of grains Ductile deformation causes bending and plastic slip within grains Solid State Crystal Growth Simple grain boundary adjustments Quartzite or marble Triple grain boundary angles are 120 Crystalloblastic fabric Polymineralic rocks Triple grain boundaries related to surface energies of the adjacent minerals Crystalloblastic series

4 Crystalloblastic Series 1. Sphene, rutile, pyrite 2. Garnet, sillimanite, staurolite, tourmaline 3. Epidote, magnetite,ilmenite 4. Andalucite, pyroxene, amphibole 5. Micas, chlorite, dolomite, kyanite 6. Calcite, idocrase, scapolite 7. Plagioclase, quartz, cordierite Timing of Mineral Growth Pre tectonic fabric Bent, kinked, and strained grains Syn tectonic fabric Simultaneous deformation and growth Preferred orientation of grains Post tectonic fabric Grains not bent Over-growth includes old fabrics Metamorphic Grade Controlled by the temperature of metamorphism Low-grade rocks contain hydrated and carbonated phases High-grade rocks are dehydrated and decarbonized Metamorphic Zones Mappable metamorphic units of similar grade in a rock of distinct composition Isograds are lines marking the first appearance of key minerals Zones of Barrow Biotite, garnet, staurolite, kyanite, sillimanite

5 Metamorphic Facies The set of mineral assemblages occurring in rocks of diverse composition Facies develop under restricted P,T conditions Mineral assemblages may be plotted on ACF diagrams Main Facies Zeolite Prehnite-pumpellyite Blueschist Greenschist Amphibolite Granulite Eclogite Pressure Temperature Relations Review of Facies Mineral assemblages in metamorphosed mafic rocks Correlation of Barrow s zones with facies from different protoliths P, T diagram for various facies Review of Phase Diagrams Solid-solid reactions Governed by Clausius Clapeyron equation dp/dt = 10 H/T V H is the heat of reaction The slope of the stability is given by dp/dt

6 Examples Open System Models - H 2 O Dehydration curves Al 2 O 3 phase stability Dehydration curves Example of the general case Specific minerals Breakdown of chlorite, muscovite, biotite, etc Open System Models - CO 2 Dehydration curves Example of the general case Specific minerals Breakdown of calcite, Stability of Iron Oxides P O2 vs. Temperature diagram Main phases Hematite Magnetite Native Iron/Wustite dolomite, etc Miyashiro s Facies Series Low geothermal gradient Zeolite, pumpellyite-prehnite, blueschist Intermediate geothermal gradient Barrow s zones High geothermal gradient Andalucite present in pelitic rocks P,T Relations Jadeite + Quartz = Albite Al 2 O 3 phases Relationship to Miyashiro s series

7 P-T T Space Subduction Model Multiple phase relation fields Facies in P-T space Miyashiro s series and facies Igneous solidus, wet/dry Granites Basalts High pressure series Medium P/T series High temperature series Polymetamorphism Sometimes there are repeated episodes of metamorphism The last event may be weak or of short duration Polymetamorphism is common in post tectonic environments and in contact aureoles