Modification of Rocks by Temperature and Pressure Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure © 2011 by W. H. Freeman and Company

Alteration of Rocks by Temperature and Pressure Chapter 6: Metamorphism: Alteration of Rocks by Temperature and Pressure

About Metamorphism Changes in heat, pressure, and the chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic. Metamorphic changes occur in the solid state, so there is no melting.

Lecture Outline Causes of metamorphism 2. Types of metamorphism 3. Metamorphic textures 4. Regional metamorphism and metamorphic grade 5. Plate tectonics and metamorphism

● internal heat of Earth ● internal pressure of Earth 1. Causes of Metamorphism ● internal heat of Earth ● internal pressure of Earth ● fluid composition inside Earth

● temperature increases with depth ● rate = 20º to 60ºC per km 1. Causes of Metamorphism ● temperature increases with depth ● rate = 20º to 60ºC per km ● at 15 km depth: 450ºC

1. Causes of Metamorphism ● pressure and temperature increase with depth in all regions

1. Causes of Metamorphism

● the role of temperature ● geothermal gradient ● shallow (20ºC / km) 1. Causes of Metamorphism ● the role of temperature ● geothermal gradient ● shallow (20ºC / km) ● steep (50ºC / km)

● the role of pressure (stress) ● confining pressure 1. Causes of Metamorphism ● the role of pressure (stress) ● confining pressure ● directed pressure

● the role of pressure (stress) ● rate of increase = 1. Causes of Metamorphism ● the role of pressure (stress) ● rate of increase = 0.3 to 0.4 kbar / km ● minerals are geobarometers

Thought questions for this chapter At what depths in the Earth do metamorphic rocks form? What happens if temperatures get too high? Why are there no metamorphic rocks formed under natural conditions of very low pressure and temperature, as shown in Figure 6.1?

● accelerated chemical reactions 2. Types of Metamorphism ● the role of fluids ● metasomatism ● accelerated chemical reactions

2. Types of Metamorphism Depth, km Oceanic crust 35 Continental crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

Shock metamorphism Depth, km Oceanic crust 35 Continental crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

Shock metamorphism Regional metamorphism Depth, km Oceanic crust 35 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

Shock metamorphism Regional metamorphism High-pressure metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

Shock metamorphism Regional metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

Shock metamorphism Regional metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Burial metamorphism

Shock metamorphism Regional metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Water Seafloor metamorphism Burial metamorphism

Thought questions for this chapter Draw a sketch showing how seafloor metamorphism might take place.

3. Metamorphic Textures

3. Metamorphic Textures Staurolite crystal Mica

Foliated rocks contain platy minerals that are aligned along a preferred orientation. Staurolite crystal Mica

3. Metamorphic Textures Feldspar Quartz Mica Pyrite Staurolite

Foliation is the result of compressive forces. Feldspar Quartz Mica Pyrite Staurolite

Foliation is the result of compressive forces. Mineral crystals become elongated perpendicular to the compressive force. Feldspar Quartz Mica Pyrite Staurolite

● preferred orientation of crystals ● slaty cleavage ● foliation 3. Metamorphic Textures ● preferred orientation of crystals ● slaty cleavage ● foliation

3. Metamorphic Textures shale sandstone layers 5 cm

The original bedding can be seen in the thin sandy layers. shale sandstone layers 5 cm

The original bedding can be seen in the thin sandy layers. foliation plane shale sandstone layers original bedding 5 cm

Regional metamorphism causes cleavage planes to develop. The original bedding can be seen in the thin sandy layers. Regional metamorphism causes cleavage planes to develop. foliation plane shale sandstone layers original bedding 5 cm

3. Metamorphic Textures Low grade Intermediate grade

3. Metamorphic Textures Diagenesis Low grade Intermediate grade High grade Slate Phyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Slaty cleavage Schistosity Banding Banding

Foliated rocks are classified by the degree of cleavage, schistosity, and banding. Diagenesis Low grade Intermediate grade High grade Slate Phyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Slaty cleavage Schistosity Banding Banding

● classification of foliated rocks ● metamorphic grade ● crystal size 3. Metamorphic Textures ● classification of foliated rocks ● metamorphic grade ● crystal size ● type of foliation ● banding

● nomenclature of foliated rocks ● slate ● phyllite ● schist ● gneiss 3. Metamorphic Textures ● nomenclature of foliated rocks ● slate ● phyllite ● schist ● gneiss ● migmatite

Foliated texture: schist with garnet porphroblasts

● nomenclature of granoblastic (non-foliated) metamorphic rocks 3. Metamorphic Textures ● nomenclature of granoblastic (non-foliated) metamorphic rocks ● hornfels ● quartzites ● marbles ● greenstones ● amphibolites ● granulites (granofels)

Granoblastic Texture

Thought questions for this chapter How is slaty cleavage related to tectonic forces? What forces cause minerals to align with one another? Would you choose to rely on chemical composition or type of foliation to determine metamorphic grade? Why?

4. Regional Metamorphism and Metamorphic Grade ● grades of metamorphism ● low ● intermediate ● high

4. Regional Metamorphism and Metamorphic Grade ● mineral isograds (zones of change) ● index minerals reflect pressure and temperature conditions ● groups of 2 to 3 index minerals form an isograd

4. Regional Metamorphism Canada ME NY Isograds VT NH MA Key: Not metamorphosed CT Chlorite zone Low grade RI Biotite zone Garnet zone Medium grade Staurolite zone High grade Sillimanite zone

Index minerals define metamorphic zones. Canada ME NY Isograds VT NH MA Key: Not metamorphosed CT Chlorite zone Low grade RI Biotite zone Garnet zone Medium grade Staurolite zone High grade Sillimanite zone

Index minerals define metamorphic zones. Isograds can be used to plot the level or degree of metamorphism. Canada ME NY Isograds VT NH MA Key: Not metamorphosed CT Chlorite zone Low grade RI Biotite zone Garnet zone Medium grade Staurolite zone High grade Sillimanite zone

Low Grade Increasing metamorphic grade Slate Intermediate Grade Phyllite High Grade Pressure (kilobars) Schist Depth (km) Gneiss Blueschist Migmatite Temperature (°C)

Metamorphic facies correspond to particular combinations of Hornfels Contact metamorphism Zeolite 5 Conditions beneath mountain belts 5 10 Blueschist 15 Partial melting begins Amphibolite Granulite Greenschist Pressure (kilobars) Depth (km) 10 20 25 13.5 15 Subduction zone 30 Eclogite 35 20 40 200 400 600 800 1000 Temperature (°C) Metamorphic facies correspond to particular combinations of pressure and temperature... … and can be used to indicate specific tectonic environments.

Thought questions for this chapter You have mapped an area of metamorphic rocks, such as the region in Figure 6.9a, and have observed a series of metamorphic zones, marked by north-south isograds, running from sillimanite in the east to chlorite in the west. Were metamorphic temperatures higher in the east or west? Which kind of pluton would produce the highest grade of metamorphism, a granite intrusion 20 km deep or a gabbro intrusion at a depth of 5 km?

● metamorphism occurs in or near ● plate interiors 5. Plate Tectonics and Metamorphism ● metamorphism occurs in or near ● plate interiors ● divergent plate margins ● convergent plate margins ● transform plate margins

Tectonic transport moves rocks through different pressure-temperature zones, … Low P, Low T High P, High T

Tectonic transport moves rocks through different pressure-temperature zones, … Low P, Low T High P, High T …and then transports them back to the shallow crust or the surface.

● metamorphic pressure-temperature paths 5. Plate Tectonics and Metamorphism ● metamorphic pressure-temperature paths ● history of burial and exhumation ● prograde and retrograde paths

5. Plate Tectonics and Metamorphism

The garnet crystal initially grows in a schist but ends up growing in a gneiss. Low Grade Slate RETROGRADE PATH Intermediate Grade Phyllite PROGRADE PATH Depth (km) Pressure (kilobars) Schist Gneiss High Grade Temperature (°C)

Pressure (kilobars) Depth (km) Temperature (°C) Low Grade RETROGRADE PATH RETROGRADE PATH Pressure (kilobars) Depth (km) PROGRADE PATH High Grade PROGRADE PATH Peak metamorphism Temperature (°C) Low temperature– high pressure metamorphism within a subduction zone Deep-ocean sediment Trench Continental crust Shelf sediment Oceanic crust Mélange ophiolites Continental crust Prograde path Peak metamorphism Retrograde path

Pressure (kilobars) Depth (km) Temperature (°C) Low Grade RETROGRADE PATH RETROGRADE PATH Pressure (kilobars) Depth (km) PROGRADE PATH High Grade PROGRADE PATH Peak metamorphism Temperature (°C) Low temperature– high pressure metamorphism within a subduction zone High temperature– high pressure metamorphism within a mountain belt Suture Deep-ocean sediment Trench Continental crust Deformed and metamorphosed shallow- and deep- ocean sediments Shelf sediment Continental crust Oceanic crust Multiple thrusts Mélange ophiolites Continental crust Continental crust Prograde path Prograde path Peak metamorphism Retrograde path Peak metamorphism Retrograde path

● rapid erosion (exhumation) rates of mountain ranges show a 5. Plate Tectonics and Metamorphism ● rapid erosion (exhumation) rates of mountain ranges show a relationship between ● tectonics (orogeny) ● climate ● controls the flow of metamorphic rocks to the surface

Thought questions for this chapter Draw a sketch showing how seafloor metamorphism might take place. Subduction zones are generally characterized by high-pressurelow temperature metamorphism. In contrast, continent-continent collision zones are marked by moderate pressurehigh temperature metamorphism. Which type of plate boundary has a higher geothermal gradient? Explain.

Key terms and concepts Blueschist Burial metamorphism Amphibolite Blueschist Burial metamorphism Contact metamorphism Eclogite Exhumation Foliated rock Foliation Gneiss Granoblastic rock Granulite Greenschist Greenstone High-pressure metamorphism Hornfels

Key terms and concepts Mélange Metamorphic facies Metasomatism Marble Mélange Metamorphic facies Metasomatism Migmatite Phyllite Porphroblast Quartzite Regional metamorphism Schist Seafloor metamorphism Shock metamorphism Slate Stress Suture

Key terms and concepts Ultra-high pressure metamorphism Zeolite