Metamorphosis
"When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed into a monstrous bug. Metamorphosis, by Franz Kafka
Metamorphism The transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Igneous, sedimentary and even other metamorphic rocks
One of the oldest rocks in the world. A gneiss produced by metamorphosis of an even older shale.
Origin of Metamorphic Rocks Metamorphism begins when Temperature exceeds 200 O C Pressure exceeds 300 M Pa (approx. 10,000 ft deep) Metamorphism ends when melting begins
Metamorphism Recrystallization of minerals in the solid state Caused by changes in T, P or pore fluids New environment = new minerals Growing minerals create a new texture Metamorphism progresses from low to high grades
Metamorphism Textural changes occur during metamorphism New minerals grow during metamorphism replacing old minerals The new minerals reflect the Temperature and Pressure of the metamorphic event Precursor rock textures are modified or destroyed
Heat What causes metamorphism? Most important agent Heat drives recrystallization - creates new, stable minerals Pressure (stress) Fluids Increases with depth Pressure can be applied equally in all directions or differentially, i.e. directed The flow of hot mineral-rich water through the rock can have a big impact on metamorphism Referred to as hydrothermal alteration and creates specific easily identified minerals
Main factor affecting metamorphism Parent rock Metamorphic rocks typically have the same chemical composition as the parent rock. They contain different minerals, but the same chemicals; just rearranged. Exception: at sometimes gases like carbon dioxide (CO 2 ) and water (H 2 O) are released Examples: Quartz Sandstone Quartzite Shale Slate Schist Gneiss Granite Granite, though minerals might align
Source of pressure Confining or hydrostatic pressure: equal in all directions Directed pressure: largely in one direction or along a particular axis
Source of Heat Proximity to igneous intrusions Contact metamorphism Zones if different metamorphic grade ring the intrusion Depth of burial 15-30 o C increase per km below the surface Regional scale burial, mountain building events
Source of Heat
Source of Fluids
Metamorphism Three types of metamorphic settings: Contact metamorphism from a rise in temperature within host rock Hydrothermal metamorphism chemical alterations from hot, ion-rich water Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock
Contact metamorphism Produced mostly by local heat source
Hydrothermal Metamorphism Circulation of hot fluids through cracks and porous rock Important source of ores
Regional Metamorphism: Subduction zones.. Continental Crust Oceanic Crust
Regional Metamorphism: Subduction zones.. High T Low P High P Low T High T High P
and/or deep burial
Why it is called regional Colors represent different levels of Temperature and Fig. 6.15. Regional Metamorphic Pressure as Gradients recorded in the minerals. This regional pattern was caused by the collision of two continents
Metamorphic Grade T & P combined determine degree of metamorphism & mineral assemblage Low-grade metamorphism- 200 to 350 O C and relatively low pressures Intermediate-grade metamorphism - 350 to 550 O C and moderate to high pressures High-grade metamorphism - very high temperatures, usually above 550 O C and/or very high pressures
Mineral stability depends largely on temperature and pressure. Example: the Al 2 SiO 5 system Low T High P High T High P Low T Low P These minerals can be formed by the metamorphosis of shales
Other minerals behave similarly Metamorphic Index Minerals
Regional Metamorphic Zones Index minerals A mineral that forms within a specific, often narrow range of conditions Identifies a specific grade of metamorphism
Index Minerals in metamorphic rocks Each of these minerals is an index of T and P
Different kinds of tectonic settings can produce distinct types of metamorphism
Regional metamorphism High pressure is dominant factor Occurs as large belts in intensely deformed mountain ranges Results in rocks with foliated textures May occur over wide temperature range Higher pressure and temperature will produce increased metamorphic grade Prograde metamorphism common
Regional Metamorphism and plate tectonics Most regional metamorphism occurs along convergent plate boundaries Compressional stresses deform along the plate boundaries Cores of subduction zones contain linear belts of metamorphic rocks Occurs in major mountain belts: Alps, Himalayas, and Appalachians High-P, low-t zones near trench High-T, low-p zones in region of igneous activity (arc)
Ocean-Continent convergence
Juan de Fuca plate
Increasing Depth Directed pressure Increasing Pressure Increasing Temperature
Change in metamorphic grade with depth
Progressive metamorphism of a shale Shale
Progressive metamorphism of a shale Slate
Progressive metamorphism of a shale Phyllite
Progressive metamorphism of a shale Schist
Progressive metamorphism of a shale Gneiss
Metamorphic Environments Metamorphic grade A group of minerals that form in specific conditions of Pressure and Temperature Zeolite (really low T,P; <200C) Greenschist (low T, P; 200-450C, 10-15 km) Blueschist (low T, high P - subduction zones) Amphibolite (high T, P; 450-650C, 15-20 km) Granulite (super high T, P; >700C, >25km)
Metamorphic facies
What are metamorphic textures? Texture refers to the size, shape, and arrangement of mineral grains within a rock Foliation planar arrangement of mineral grains within a rock
Metamorphic textures Foliation Foliation can form in various ways: Rotation of platy or elongated minerals Recrystallization of minerals in a preferred orientation Changing the shape of equidimensional grains into elongated and aligned shapes
Development of foliation due to directed pressure
Flattened Pebble Conglomerate = flattening
Granites
Granites
Foliated vs. Nonfoliated textures under the microscope
Common metamorphic rocks Foliated rocks Slate Very fine-grained Excellent rock cleavage Made by low-grade metamorphism of shale Think of a pool table or chalkboard
slate
slate
Slate roof
Common metamorphic rocks Foliated rocks Phyllite Grade of metamorphism between slate and schist Made of small platy minerals Glossy sheen with rock cleavage Composed mainly of muscovite and/or chlorite
Phyllite (left) and Slate (right) lack visible mineral grains Phyllite Slate Has a sheen No sheen
Common metamorphic rocks Foliated rocks Schist Medium- to coarse-grained Made of platy minerals (micas) The term schist describes the texture To indicate composition, mineral names are used (such as mica schist)
Mica Schist - note well developed foliation
Schist
A mica garnet schist
Common metamorphic rocks Foliated rocks Gneiss Medium- to coarse-grained Banded appearance High-grade metamorphism Composed of light-colored feldspar layers with bands of dark mafic minerals
Gneiss displays bands of light and dark minerals
Progressive metamorphism of shale
Metamorphic rocks exposed at Mt. Everest. Deformation occurs at various scales
Outcrop of gneiss Canadian Shield
Multiple Folds First, vertical pressure makes the gneiss Then horizontal pressure folds it Fig. 8-13d, p. 245
Common metamorphic rocks Nonfoliated rocks Quartzite Formed from a parent rock of quartz-rich sandstone Quartz grains are fused together Forms in intermediate T, P conditions
Sample of quartzite Thin section of quartzite
Common metamorphic rocks Nonfoliated rocks Marble Coarse, crystalline Parent rock usually limestone Composed of calcite crystals Fabric can be random or oriented
Marble (Random fabric = annealing; nonfoliated)
Marble
Question: Where do we see metamorphic rocks in outcrops?
North American Craton Shield Western North American Mobile Belt Eastern North American Mobile Be Platform
Answer: In continental shields and uplifted basement rocks What are these?
The three types reviewed
Fig. 8-5, p. 240