Metamorphism. Igneous rocks Sedimentary rocks Other metamorphic rocks

Transcription

1 Metamorphic Rocks

2 Metamorphism The transition of one rock into another by temperatures and/or pressures unlike those in which it formed Metamorphic rocks are produced from Igneous rocks Sedimentary rocks Other metamorphic rocks

3 Metamorphism Metamorphism progresses incrementally from low-grade to high-grade During metamorphism the rock must remain essentially solid Metamorphic settings Contact or thermal metamorphism driven by a rise in temperature within the host rock

4 Agents of Metamorphism Heat The most important agent Recrystallization results in new, stable minerals Sources of heat? Radioactive decay Thermal energy from formation An increase in temperature with depth due to the geothermal gradient Upper crust deg C/km

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6 Agents of metamorphism Pressure (stress) Increases with depth Confining pressure applies forces equally in all directions Rocks may also be subjected to differential stress, which is unequal in different directions Behavior of rocks - Shallow----- brittle Deep ductile

7 Figure 7.3

8 Agents of metamorphism Chemically active fluid Mainly water with other volatile components Enhances migration of ions removed in areas of high stress deposited in areas of low stress Aids in recrystallization of existing minerals tend to recrystallize and grow longer in a direction perpendicular to the compressional stress

9 Agents of metamorphism Chemically active fluids Sources of fluids Pore spaces of sedimentary rocks Fractures in igneous rocks Hydrated minerals such as clays and micas Metasomatism - an overall change in the composition of the surrounding rock due to the interaction of these fluids

10 Metamorphic textures Texture refers to the size, shape, and arrangement of grains within a rock Foliation any planar arrangement of mineral grains or structural features within a rock

11 Metamorphic textures Foliation can form in various ways including Rotation of platy and/or elongated minerals new crystals align perpendicular to max shortening Recrystallization of minerals in the direction of preferred orientation shale to slate clay to mica Changing the shape of equidimensional grains into elongated shapes that are aligned

12 stretch pebble conglomerate

13 Development of foliation due to directed pressure Figure 7.5

14 Foliated textures Rock or slaty cleavage Closely spaced planar surfaces along which rocks split Can develop in a number of ways depending on metamorphic conditions and parent rock

15 Figure 7.6

16 Figure 7.7 (top)

17 Schistosity Foliated textures Platy minerals are discernible with the unaided eye and exhibit a planar or layered structure Rocks having this texture are referred to as schist

18 Foliated textures Gneissic During higher grades of metamorphism, ion migration results in the segregation of minerals Gneissic rocks exhibit a distinctive banded appearance

19 Figure 7.8

20 Other metamorphic textures Those metamorphic rocks that lack foliation are referred to as nonfoliated Develop in environments where deformation is minimal Typically composed of minerals that exhibit equidimensional crystals

21 Other metamorphic textures Porphyroblastic textures Large grains, called porphyroblasts, are surrounded by a fine-grained matrix of other minerals Porphyroblasts are typically garnet, staurolite, and/or andalusite

22 Figure 7.9

23 Figure 7.10

24 Common metamorphic rocks Foliated rocks Slate Very fine grained less than 0.5 mm Excellent rock cleavage Most often generated from low-grade metamorphism of shale, mudstone, or siltstone

25 Slate Figure

26 Common metamorphic rocks Foliated rocks Phyllite Gradation in the degree of metamorphism between slate and schist Platy minerals not large enough to be identified with the unaided eye Glossy sheen and wavy surfaces Exhibits rock cleavage Composed mainly of fine crystals of muscovite and/or chlorite

27 Phyllite Figure

28 Common metamorphic rocks Foliated rocks Schist Medium to coarse grained Platy minerals predominate Commonly include the micas The term schist describes the texture To indicate composition, mineral names are used» mica schist» talc schist

29 Mica schist Figure

30 Common metamorphic rocks Foliated rocks Gneiss Medium to coarse grained Banded appearance High-grade metamorphism Often composed of white or light-colored feldsparrich layers with bands of dark ferromagnesian minerals

31 Gneiss typically displays a banded appearance Figure

32 Common metamorphic rocks Nonfoliated rocks Marble Coarse, crystalline Parent rock was limestone or dolostone Composed essentially of calcite or dolomite crystals Used as a decorative and monument stone Exhibits a variety of colors

33 Marble A nonfoliated metamorphic rock Figure

34 Common metamorphic rocks Nonfoliated rocks Quartzite Formed from a parent rock of quartz-rich sandstone Quartz grains are fused together

35 Quartzite Figure

36 Low grade>>>>>>>>>>>>high grade Metamorphism

37 Metamorphic environments Contact or thermal metamorphism Occurs due to a rise in temperature when magma invades a host rock A zone of alteration called an aureole forms in the rock surrounding the magma Most easily recognized when it occurs at the surface, or in a near-surface environment

38 Contact metamorphism Figure 7.13

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40 Figure 7.14

41 Metamorphic environments Hydrothermal metamorphism Chemical alteration caused when hot, ion-rich fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock Most widespread along the axis of the midocean ridge system

42 Figure 7.16

43 Figure 7.15 example of shallow depth hydrothermal metamorphism

44 Metamorphic environments Burial metamorphism Associated with very thick sedimentary strata Required depth varies from one location to another depending on the prevailing geothermal gradient Subduction zone metamorphism

45 Metamorphic environments Regional metamorphism Produces the greatest quantity of metamorphic rock Associated with mountain building Rocks usually display zones of contact and/or hydrothermal metamorphism

46 Regional metamorphism Figure 7.17

47 Metamorphic environments Other metamorphic environments Metamorphism along fault zones Occurs at depth and high temperatures Pre-existing minerals deform by ductile flow Impact metamorphism Occurs when high speed projectiles called meteorites strike Earth s surface Products are called impactites

48 Figure 7.18 Metamorphism along a fault zone brittle vs. ductile

49 Textural variations Metamorphic zones systematic variations in the mineralogy and often the textures of metamorphic rocks are related to the variations in the degree of metamorphism Index minerals and metamorphic grade Changes in mineralogy occur from regions of low-grade metamorphism to regions of highgrade metamorphism

50 Metamorphic rocks and associated environments Figure 7.19

51 Figure 7.20

52 Metamorphic zones in the Northeastern U.S. Figure 7.21

53 Migmatites Highest grades of metamorphism that is transitional to igneous rocks Contain light bands of igneous components along with areas of unmelted dark layers of metamorphic rock

54 Figure 7.22