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Types of Metamorphism!
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The Types of Metamorphism
2 different approaches to classification
1. Based on principal process or agent Dynamic Metamorphism Thermal Metamorphism Dynamo-thermal Metamorphism
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The Types of Metamorphism
2. Based on setting 1) Contact Metamorphism 2) Regional Metamorphism
Orogenic Metamorphism, Burial Metamorphism, Ocean Floor Metamorphism
3) Fault-Zone Metamorphism 4) Impact or Shock Metamorphism
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1. Contact Metamorphism
Adjacent to igneous intrusions (forms a contact aureole)
Thermal (± metasomatic) effects of hot magma intruding cooler shallow rocks
Occurs over a wide range of pressures, including very low
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Contact aureole!
View over the Race Track valley to the quartz-monzonitic Ubehebe Peak intrusion and its contact aureole. Ubehebe Peak belongs to a series of alkaline intrusions which formed the Hunter Mountain Batholith. The field area is located in the Death Valley National Park, CA (USA).
Hornfels
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2. Regional Metamorphism
Metamorphism that affects a large body of rock, and thus covers a great lateral extent
Three principal types: Orogenic metamorphism Burial metamorphism Ocean-floor metamorphism
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2a. Orogenic Metamorphism
Type of metamorphism associated with convergent plate margins
Dynamo-thermal: one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress)
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Orogenic Metamorphism
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From Understanding Earth, Press and Siever. Freeman.
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2b. Burial metamorphism
Occurs in areas that have not experienced significant deformation or orogeny Restricted to large, relatively undisturbed
sedimentary piles away from active plate margins The Gulf of Mexico Bengal Fan: sedimentary pile > 22 km, Extrap. → 250-300oC at the base (P ~ 0.6 GPa)
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2c. Ocean-Floor Metamorphism
Affects the oceanic crust at ocean ridge spreading centers
Considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na
Highly altered chlorite-quartz rocks- distinctive high-Mg, low-Ca composition
Exchange between basalt and hot seawater
Example of hydrothermal metamorphism
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Ocean-Floor Metamorphism
Hydrothermal metamorphism Hot H2O-rich fluids
http://www.youtube.com/watch?v=EK9riJ-GpNU http://www.youtube.com/watch?v=bCWsQ9OAc24&feature=related
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Mid Ocean Ridges!
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3. Fault-Zone metamorphism
High rates of deformation and strain with only minor recrystallization
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(a) Shallow fault zone with fault breccia
(b) Slightly deeper fault zone (exposed by erosion) with some ductile flow and fault mylonite
Figure 21-7. Schematic cross section across fault zones. After Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.
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4. Impact Metamorphism
Impact metamorphism at meteorite (or other bolide) impact craters
Dynamic metamorphism
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The Progressive Nature of Metamorphism
A rock at a high metamorphic grade progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today
Age of metamorphism??
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Prograde and retrograde metamorphism
Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions
Retrograde: decreasing grade as rock cools and recovers from a metamorphic
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Retrograde metamorphism typically of minor significance
Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature
Why? 1. Prograde reactions are endothermic and easily driven
by increasing T 2. Devolatilization reactions are easier than reintroducing
the volatiles
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Types of Protolith
Lump the common types of sedimentary and igneous rocks into six chemically based-groups
1. Ultramafic - very high Mg, Fe, Ni, Cr 2. Mafic - high Fe, Mg, and Ca 3. Shales (pelitic) - high Al, K, Si 4. Carbonates- high Ca, Mg, CO2 5. Quartz - nearly pure SiO2. 6. Quartzo-feldspathic - high Si, Na, K, Al