Fig Sources of metamorphic change

Transcription

1

2 Metamorphic Rocks Rocks that recrystallize without melting (solid state) at high temps and pressures Caused by changes in T, P or pore fluids New environment often = new minerals Growing minerals create a new texture*

3 Fig Sources of metamorphic change

4 Fig. 6.1b. Metamorphic rocks exposed at Mt. Everest. Deformation occurs at various scales

5 Origin of Metamorphic Rocks Metamorphism begins when Temperature exceeds 200 O C Pressure exceeds 3 kb (approx. 3 km or 10,000 ft deep) Metamorphism ends when melting begins (~700 C)

6 Two types of Metamorphic Environments Contact metamorphism Close proximity to igneous intrusions; high temps and chemical activity of fluids (hydrothermal alteration) Few meters to a few hundred meters from intrusion Small volumes of rock; localized Regional metamorphism Roots of mt. belts; plates collide; high T&P and directed stresses (foliated rocks) Tens to hundreds of kilometers Involves large volumes of rock

7 Fig Sources of heat for metamorphism Contact Metamorphism Regional Metamorphism

8 Origin of Metamorphic Rocks 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

9 Deformation Mineral growth responds to stress Differential stress causes metamorphic rocks to have distinct textures - Foliation Uniform stress does not create a mineral fabric Nonfoliated Generally, grain size in foliated rocks increase with intensity of metamorphism In foliated rocks, mineral alignment is perpendicular to regional horizontal stress

10 Metamorphic Rocks Classified by texture and composition Texture is divided into foliated & nonfoliated rocks Mineral composition qualifies textural name ex. Chlorite schist Index minerals provide information of specific T & P conditions

11 Foliated vs. Nonfoliated textures

12 Slate Foliated Rocks Fine grained rock showing slaty cleavage Slaty cleavage produced by recrytallization of platy minerals Growth is perpendicular to stress Slaty cleavage does not always coincide with bedding planes Originate from shales

13 Fig. 6.10a. Slate

14 Foliated Rocks Schist Strongly foliated rock Medium to coarse grained, dominated by platy minerals Further classified by dominant platy mineral & accessory minerals, ex. Chlorite schist Numerous parent rock types

15 Fig. 6.10b. Schist

16 Foliated Rocks Gneiss Coarse grained granular rock Foliation occurs as alternating bands of light & dark minerals Light: Quartz, feldspar Dark: Amphibole & biotite Numerous parent rock types

17 Fig. 6.10c. Gneiss

18 Nonfoliated Rocks Marble Interlocking, coarse grained calcite Recrystallization of limestone or dolostone Sedimentary features are destroyed

19 Fig. 6.10f. Marble

20 Nonfoliated Rocks Quartzite Metamorphism of quartz sandstone Pore space in sandstone is filled with silica cement Entire rock is recrystallized

21

22 Nonfoliated Rocks Metaconglomerate A conglomerate altered by heat & pressure Individual pebbles are stretched, deformed, and fused Very hard and dense (indurated); if fractured or broken, could break across pebbles as easily as around them

23

24 Regional Metamorphic Zones Large scale changes in thick masses of rock Systematic change in degree of metamorphism Changes in P & T Metamorphic zones are defined by index minerals

25 Regional Metamorphic Zones Precursor rock controls final mineral assemblage Different mineral assemblages form at a given P & T for various parent rocks Metamorphic facies describe a group of minerals formed under similar conditions

26

27 Metamorphic Rocks & Tectonics Most metamorphic rocks develop due to plate collisions at convergent boundaries high grade metamorphism (hi T &P) but can also get low grade (low T&P) in shallow crust Divergent boundaries usually just low grade metamorphism

28 Fig The origin of metamorphic rocks

29 End of Chapter 6

30 Extras

31

32 Textural changes: Metamorphism New platy minerals grow in direction of least stress producing planar texture called foliation. Three types of foliation: Slaty cleavage tendency of rock to split into thin layers Schistosity parallel arrangement of large platy minerals like mica, chlorite, talc Gneissic layering alternating light & dark layers

33 Regional Metamorphic Zones Index minerals A mineral that forms within a specific,often narrow range of conditions Identifies a specific grade of metamorphism, ex. Low grade: chlorite, High grade: sillimanite

34 Metamorphic Index Minerals