metamorphism and metamorphic rocks

Transcription

1 metamorphism and metamorphic rocks

2 the rock cycle

3 metamorphism high enough temperature & pressure to change rocks "but not high enough to melt rocks " " changes to rocks occur in the solid-state hot, reactive fluids also contribute old minerals, unstable under new P, T "conditions, re-crystallize into " "new minerals new rocks are metamorphic rocks metamorphism occurs at depth; cannot "see metamorphic rocks unless " "they are uplifted

4 Metamorphism: Fundamental Questions What are the subsolidus changes in fabric and composition that occur during metamorphism? What transfers mass and energy to cause these changes? In what tectonic settings does metamorphism occur? How does the study of metamorphic rocks and processes help us understand plate tectonics and Earth s evolution?

5 Metamorphism: Simple Definitions Metamorphism is defined as physical and chemical changes that occur in pre-existing rocks (igneous, metamorphic, or sedimentary) in the solid state that yield in a lower free energy state as a result of changes in conditions (e.g. T and P). Prograde: Increasing T and P Retrograde: Decreasing T and P Diagenesis occurs at relatively low T and P and grades into metamorphism. Melting or migmatization occurs at the most intense, i.e. highest T an P limits of metamorphism and therefore grades into igneous processes.

6 General P/T Conditions of Metamorphism From Spear, 1993

7 metamorphic rocks: controlling factors parent rock composition (also called protolith) temperature and pressure during metamorphism tectonic forces fluids

8 parent rock composition no new material is added to rock during metamorphism metamorphic rock will have similar composition to parent rock if parent material contains only one mineral resultant metamorphic rock will only have one mineral "--mineral will be recrystallized (texture changes)-- marble (CaCO 3 ) limestone limestone (CaCO 3 )

9 limestone under microscope (stained) (note fragments of shells) texture changes marble under microscope (note interconnecting grains)

10 if parent material contains many minerals old minerals will recombine to form new minerals clay, quartz, mica, and volcanic fragments in a sandstone "will combine to form new metamorphic minerals example is garnet: which grows during metamorphism garnet growing garnet schist (metamorphic rock)

11 temperature during metamorphism heat from Earth s deep interior all minerals stable over finite temperature range higher temperatures than range cause melting " "(and therefore generates igneous rocks) heat is essential think about mixing flour, yeast, water, salt..nothing happens until they have a heat source " " and then they make bread

12 pressure during metamorphism pressure in the Earth acts the same in all directions pressure is proportional to depth in the Earth Increases about 1 kilobar per 3.3 km or 30 MPa/km (megapascals/km) pressure increases with depth look at example with deep water consequence on cube is squeezing into smaller cube --in rocks, grains pack together-- volume decreases with depth high pressure minerals: more compact and dense

13 tectonic forces - driven by plate motion! lead to forces that are not equal in all directions (differential stress) compressive stress (hands squeeze together) causes flattening at 90 to stress shearing (hands rubbing together) causes flattening parallel to stress

14 fluids hot water (water vapor) most important heat causes unstable minerals to release water water reacts with surrounding rocks "and transports dissolved material and ions time metamorphism may take millions of years longer times allow new minerals to grow larger "--coarser grained rocks

15 Controlling Factors in Metamorphism Note that metamorphic equilibration is also strongly affected by kinetic factors, which are not illustrated. Four Factors: 1) Temperature 2) Pressure 3) Fluid activity 4) Deviatoric stress

16 metamorphic rocks: basic classification based on rock texture foliated (layered) type of foliation -- e.g. slaty non-foliated (non-layered) composition -- e.g. marble

17 flattened pebbles in metamorphic rock

18 foliated (layered) metamorphic rocks results from differential stress (not equal in all directions) foliation

19 appearance under microscope non-foliated foliated

20 foliated metamorphic rock: slate

21 foliated metamorphic rock: slate

22 foliated metamorphic rock: phyllite (higher T, P than slate)

23 foliated metamorphic rock: schist (higher T, P than phyllite) new minerals grow -- garnet (large, roundish grains) garnet

24 schist under microscope garnet

25 foliated metamorphic rock: gneiss (higher T, P than schist) banding of quartz/feldspar and ferromagnesian minerals

26 non-foliated (non-layered) metamorphic rocks results from equal pressure (or stress) in all directions named on the basis of their composition limestone (CaCO 3 ) marble (CaCO 3 )

27 non-foliated metamorphic rocks: quartzite metamorphosed quartz sandstone

28 non-foliated metamorphic rocks: hornfels metamorphosed basalt Photo credit: R. Weller

29 types of metamorphism contact metamorphism occurs adjacent to magma bodies intruding "cooler country rock -- contact produces non-foliated metamorphic rocks happens in a narrow zone of contact "(~1 to 100 m wide) known as aureole forms fine-grained (e.g. hornfels) or "coarse-grained (e.g. marble) rocks

30 types of metamorphism regional metamorphism occurs over wide region and mostly in deformed "mountain ranges produces foliated metamorphic rocks happens at high pressures and over a range of temperature increases in pressures and temperatures forms rocks of "higher metamorphic grade

31 other types of metamorphism (less common) partial melting during metamorphism produces migmatites, which have both intrusive and "metamorphic textures shock metamorphism occurs during impact events yields very high pressures forms shocked rocks around impact craters

32 Shock Metamorphism Related to impacts of objects on planetary surfaces (asteroids and meteorites). Deformation (metamorphism) is extremely rapid and transient. Achieve much higher peak metamorphic temperatures and pressures than commonly observed in terrestrial environments. Products include impact breccias, melts (tektites), and planer deformation features in quartz and feldspar.

33 Canyon Diablo Iron Meteorite

34 Planetary Impact Structures Apollo 11 Image: The Moon Galileo Image: Ganymede From:

35 Impact Simulation - Gravity Dominated Model Comet Impact Body: 1.5 g/cc 6 x 3 km Simulation by: James Richardson and H. Jay Melosh, Univ. of Arizona

36 Shock Metamorphism P/T Conditions

37 PDS in Shocked Quartz Planar Deformation Structures From:

38 migmatite igneous and metamorphic textures

39 hydrothermal alteration along mid-ocean ridge cold sea water encounters hot basalt, forms steam, alters minerals

40 black smoker offshore Pacific Northwest hot steam/sea water cools as it emerges into ocean and precipitates metals

41 plate tectonics and metamorphism regional metamorphism associated with convergent boundaries pressure increases with depth temperature varies laterally different P, T conditions yield different degrees "of metamorphism temperatures cooler in down-going (subducting) plate (dashed purple line is isotherm -- line of equal T)