"When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed into a monstrous bug. Metamorphosis, by Franz Kafka

Transcription

1 Metamorphosis

2 "When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed into a monstrous bug. Metamorphosis, by Franz Kafka

3 One of the oldest rocks in the world. A gneiss produced by metamorphosis of an even older shale.

4 Metamorphism The transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Igneous, sedimentary and even other metamorphic rocks

5 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

6 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

7 Metamorphism Textural changes occur during metamorphism New minerals grow during metamorphism replacing old minerals Precursor rock textures are modified as old minerals are replaced by new Environment of formation influences crystal growth Diagnostic features of the original rock are masked or destroyed

8 What causes metamorphism? Heat Most important agent Heat drives recrystallization - creates new, stable minerals Pressure (stress) Increases with depth Pressure can be applied equally in all directions or differentially, i.e. directed

9 Main factor affecting metamorphism Parent rock Metamorphic rocks typically have the same chemical composition as the rock they were formed from Different minerals, but made of the same stuff. Exception: at higher T and P gases may be released, e.g., carbon dioxide (CO 2 ) and water (H 2 O)

10 Source of pressure Confining or hydrostatic pressure: equal in all directions Directed pressure: largely in one direction or along a particular axis

11 Source of Heat

12 Source of heat Proximity to igneous intrusions Contact metamorphism Zones if different metamorphic grade ring the intrusion Depth of burial o C increase per km Regional scale burial, mountain building events

13 Source of Fluids

14 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

15 Contact metamorphism Produced mostly by local heat source

16 Hydrothermal Metamorphism Circulation of hot fluids through cracks and porous rock Important source of ores

17 Regional Metamorphism: Subduction zones..

18 and/or deep burial

19 Why it is called regional Fig Regional Metamorphic Gradients

20 The three types reviewed

21 Origin of 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 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

22 Mineral stability depends largely on temperature and pressure. Example: the Al 2 SiO 5 system

23 Other minerals behave similarly Metamorphic Index Minerals

24 Regional Metamorphic Zones Index minerals A mineral that forms within a specific,often narrow range of conditions Identifies a specific grade of metamorphism Allows further subdivision of rock types

25 Index Minerals in metamorphic rocks

26 Different kinds of tectonic settings can produce distinct types of metamorphism

27 Regional metamorphism High pressure is dominant factor Results in rocks with foliated textures Prevalent in intensely deformed mountain ranges May occur over wide temperature range Higher pressure and temperature will produce increased metamorphic grade Metamorphism occurs in large belts Prograde metamorphism common

28 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)

29 Change in metamorphic grade with depth

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31 Metamorphic Environments Metamorphic grade A group of minerals that form in a particular P-T environment Zeolite (really low T,P; <200C) Greenschist (low T, P; C, km) Blueschist (low T, high P - subduction zones) Amphibolite (high T, P; C, km) Granulite (super high T, P; >700C, >25km)

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33 Metamorphic facies

34 Progressive metamorphism of a shale Shale

35 Progressive metamorphism of a shale Slate

36 Progressive metamorphism of a shale Phyllite

37 Progressive metamorphism of a shale Schist

38 Progressive metamorphism of a shale Gneiss

39 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

40 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

41 Development of foliation due to directed pressure

42 Flattened Pebble Conglomerate = flattening

43 Granites

44 Granites

45 Foliated vs. Nonfoliated textures under the microscope

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47 Common metamorphic rocks Foliated rocks Slate Very fine-grained Excellent rock cleavage Made by low-grade metamorphism of shale

48 Example of slate

49 Fig. 6.10a. Slate

50 Slate roof

51 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

52 Phyllite (left) and Slate (right) lack visible mineral grains

53 Common metamorphic rocks Foliated rocks Schist Medium- to coarse-grained Comprised of platy minerals (micas) The term schist describes the texture To indicate composition, mineral names are used (such as mica schist)

54 Mica Schist - note well developed foliation

55 Schist

56 A mica garnet schist

57 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

58 Gneiss displays bands of light and dark minerals

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61 Progressive metamorphism of shale

62 Metamorphic rocks exposed at Mt. Everest. Deformation occurs at various scales

63 Outcrop of foliated gneiss

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65 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

66 Sample of quartzite Thin section of quartzite

67 Common metamorphic rocks Nonfoliated rocks Marble Coarse, crystalline Parent rock usually limestone Composed of calcite crystals Fabric can be random or oriented

68 Marble (Random fabric = annealing; nonfoliated)

69 Marble

70 Question: Where do we see metamorphic rocks in outcrops?

71 North American Craton Shield Western North American Mobile Belt Eastern North American Mobile Be Platform

72 Answer: In continental shields and uplifted basement rocks