6-Metamorphic Rocks

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University of Alberta *

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Geology

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Oct 30, 2023

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• By the end of this topic you should be able to: – Explain what metamorphism is with respect to rocks, and what variables are responsible for metamorphism – Describe the three main mechanisms for metamorphism and where they tend to occur – Explain the difference between recrystallization and neocrystallization with respect to metamorphism – Discuss the occurrence and utility of metamorphic index minerals – Describe how metamorphic texture informs us with regards to stresses applied to the rock during metamorphism Learning Goals
What is metamorphic rock?
Metamorphic (Greek): of changed form Metamorphosis: biological transformation
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Metamorphism begins with a parent rock ( protolith ): the type of rock that was metamorphosed. igneous sedimentary
Metamorphic rocks can also be further metamorphosed, but the parent rock will be either igneous or sedimentary metamorphic
What causes metamorphism?
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Metamorphism occurs when protoliths are exposed to: heating hot fluids squeezing protolith
Metamorphism causes rocks to change form (size, shape, texture, colour, and/or mineralogy)
Over which temperatures and pressures does metamorphism occur?
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Metamorphism occurs over a set range of P & T conditions. The higher the P/T, the higher the degree of metamorphism, or metamorphic grade .
Through burial, compaction and cementation, sediments turn to stone (lithify) at temperatures below ~200°C.
At low pressures in the shallow crust, metamorphism occurs between ~200°C to ~850°C
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At high pressures deeper in the crust, metamorphism can occur at much lower temperatures
Above ~850°C, rocks melt 200 800
What are the 3 types of metamorphism?
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Remember- metamorphism occurs when protoliths are exposed to: heating squeezing hot fluids
Range of temperatures and depths at which metamorphism will occur under "normal" conditions Heating The average (normal) geothermal gradient in the crust is ~25°C/km
Range of temperatures and depths at which metamorphism will occur under "normal" conditions Under normal conditions, rocks at >8 km depth are >200°C and will metamorphose. But, metamorphism can also happen at shallower depths. What could cause the temperature of the crust to rise above normal?
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Temperatures around magma intrusions are unusually hot because magma is above ~850°C. Very high temperatures can occur near the surface at mid-ocean ridges
The hot magma body will "bake" the surrounding country rock resulting in contact metamorphism The zone of contact metamorphism is narrow (mm to 10s of m) and completely surrounds the intrusion. This is called the contact aureole.
Hot fluids Water exists everywhere in the crust and in magma. Therefore, hot water is involved in nearly all metamorphic reactions. Fractures
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Water speeds up metamorphic reactions. End result may be the same. water water water water
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hydrothermal fluids often contain dissolved elements rejected from normal silicate minerals that form as magma solidifies.
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Fluids migrate into cracks in the surrounding rocks. Changes in P/T cause minerals to precipitate out of solution. water water
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Gold is not a component of silicate minerals, so it often gets flushed into cracks as magma cools and solidifies.
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Pegmatite formation is likely also linked to hydrothermal fluids. Pegmatites are igneous rocks, but are often found in veins and dikes. Pegmatite in veins
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Hot fluids Hot, mineral-rich water is involved in nearly all metamorphic reactions. When water is the main agent of metamorphism, it is called hydrothermal metamorphism .
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• Pressure also increases with depth due to the increasing mass of the overlying rock • This pressure is ~equal in all directions and is called lithostatic stress Squeezing
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Balloon gets smaller because pressure is equal on all sides. Deeply buried rocks experience lithostatic stress.
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What processes can produce directed stress in the crust? Directed stress : stress that is higher in one direction
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The crust adjacent to convergent plates experiences directed stress. Compression causes crust to thicken, producing mountains. Directed stress results in large areas of regional metamorphism.
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Directed stress also deforms rock, so regionally metamorphosed rock is often folded .
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heating squeezing hot fluids Contact metamorphism Regional metamorphism, Burial Metamorphism Hydrothermal metamorphism Summary
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The distinction between types of metamorphism are often blurred: suduction zones are prone to contact, hydrothermal and regional metamorphism
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How does metamorphism affect composition?
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In many cases, the bulk composition of metamorphic rocks is the same as the protolith from which it formed. Elements are not added or removed, just rearranged. heating Contact metamorphism squeezing Regional metamorphism, Burial metamorphism hot fluids* Hydrothermal metamorphism Protolith *hot fluids can (but not always) introduce/remove elements.
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Elements rearrange themselves because minerals that were stable at the surface of the Earth become unstable as P/T rise. The stability of a mineral is a function of P, T and fluids present
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Temperature Some minerals are stable at low T; some at high T Quartz can be stable up to 1800⁰C+ Clay is stable up to ~200⁰C Minerals are stable at higher T when rocks are under pressure
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Pressure Pressure can cause atoms in minerals to become more closely packed. Metamorphic minerals tend to be dense.
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All metamorphic reactions happen in the solid state (no melting!) What do we call rocks that form from the cooling and crystallization of molten rock?
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The minerals in metamorphic rocks may change form in many ways: Recrystallization: minerals change size and shape without changing identity Mineral recrystallize to form new, interlocking grains of the same mineral. Grains generally grow larger. Protolith Metamorphic rock
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Quartz Arenite Quartzite Protolith Metamorphic rock
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Limestone Marble Marble In limestone, calcite crystals grow larger Protolith Metamorphic rock
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The minerals in metamorphic rocks may change form in many ways: Neocrystallization: New minerals grow that were not present in the protolith Protolith Metamorphic rock Phyllite (chlorite + mica + quartz) Shale (clay + quartz)
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At low P/T, clay minerals become unstable, break down, and reform into chlorite As P/T continues to rise, the newly formed chlorite becomes unstable and breaks down to form micas, then garnet, then kyanite, then sillimanite.
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Index minerals only appear under specific P/T conditions. The presence of a given index mineral can tell us how much heat and pressure a rock experienced, or the metamorphic grade .
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How does metamorphism affect texture?
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As P/T increases, minerals become unstable, but don’t melt. • Some will break down and form new minerals ( neocrystallization ) • Some will grow larger and become interlocked ( recrystallization) Limestone Marble Marble As a result, most metamorphic rocks have a crystalline texture.
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Grains in a metamorphosed sandstone undergo recrystallization, but some remnant clastic texture may still be visible. Sedimentary structures can also be preserved. This is called remnant clastic texture. The rock may appear granular or sugary, but the grains won’t rub off. Quartzite
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Metaconglomerate Metaconglomerates also have a remnant clastic texture, but the clasts are often (but not always) deformed.
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Additional metamorphic textures: Foliated Layered Non-Foliated Not layered
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When pressure is equal in all directions, minerals can grow in any orientation, developing a non-foliated crystalline texture. (Often a result of deep burial or contact metamorphism.) Marble Limestone Recrystallization
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Non-foliated metamorphic rocks Quartzite (sandstone) Metaconglomerate * (conglomerate) Hornfels (shale) Anthracite (coal) Marble * (limestone) *can develop foliation under pressure
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Directed stress causes platy or elongate minerals to grow perpendicular to the main stress direction. Metamorphic Rock Directed Stress
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Slate Phyllite Schist Gneiss Shale is the parent rock of most foliated rocks In which tectonic environment do we expect to find foliated metamorphic rocks?
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Metamorphism at a continent-continent convergent boundary • Slate will form at relatively shallow depths; • gneiss will form deep in the cores of mountains
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