Chapter 8: Metamorphism & Metamorphic Rocks

1. Chapter 8: Metamorphism & Metamorphic Rocks

2. Introduction • “Meta” = change & “morph” = form • Metamorphic = to change form • Transition of one rock into another by application of pressure and/or temperature unlike those from which it formed. • Metamorphic rocks are produced from • Sedimentary rocks • Igneous rocks • Other metamorphic rocks • Parent rock = protolith • Rock from which the metamorphic rock was formed

3. Types of Metamorphism • Progresses incrementally • Low-grade metamorphism (< 200oC) • High-grade metamorphism (> 600oC) • ** Rock must remain solid!! • If melting occurs, then igneous rocks are formed • Agents of Metamorphism • Heat • Pressure & differential stress • Chemically active fluids

4. Settings of Metamorphism • Three settings: • Thermal metamorphism • AKA contact metamorphism • Intrusion of magma body • Change driven by rise in temperature • Hydrothermal metamorphism • Chemical alterations that occur as hot ion-rich water circulates through rock • Regional metamorphism • AKA large-scale deformation • Large quantities of rock are subject to P and high T

5. Heat as Metamorphic Agent • Most important agent • b/c provides energy for chemical reactions & recrystallization • Sources: • Geothermal gradient • Contact metamorphism

6. Pressure & Differential Stress • Confining pressure  compaction • Equal pressure in all directions • Differential stress = directed pressure • Causes folds & faults

7. Brittle vs. Ductile Behavior • At surface, rocks are brittle • Tend to fracture & break into smaller pieces • At high-T, rocks are ductile • Grains tend to flatten and elongate

8. Chemically Active Fluids • Mostly water (H2O) and carbon dioxide (CO2) • Sources of fluids: • Hydrated minerals • e.g. Clays & amphiboles • Movement: • Pore spaces of sedimentary rocks • Fractures in igneous rocks

9. Importance of Protolith • Remember: protolith = parent rock • Most metamorphic rock have same chemical composition of their parent rock • Except for gain or loss of volatiles • e.g., H2O & CO2

10. Metamorphic Textures • Texture = size, shape & orientation (arrangement) of grains in rock • Foliation = any planar arrangement of mineral grains or structural features within a rock • e.g., parallel alignment of platy and/or elongated minerals • e.g., parallel alignment of flattened mineral grains or elongated pebbles • e.g., compositional banding • e.g., slaty cleavage where rocks can be easily split into thin, tabular sheets

11. Foliation: Alignment of Platy Minerals

12. Foliation: Alignment of Elongated Pebbles

13. Foliation: Compositional Banding Granite protolith resultant Gneiss with compositional banding

14. Foliation: Slaty Cleavage

15. Foliated Textures • Rock cleavage (AKA slaty cleavage) • Closely spaced planar surfaces along which rocks split • e.g., slate (originally shale) • Schistosity • Platy minerals (e.g., micas) are discernible with unaided eye • Exhibit planar or layered structure • e.g., schist (originally slate) • Gneissic banding • During higher grades of metamorphism, ion migration results in separation of light and dark minerals • Exhibit distinctive light & dark compositional banding

16. Formation of Slate

17. Garnet – Mica Schist

18. Gneissic Banding

19. Other Metamorphic Textures • Nonfoliated • Form in environments where deformation is minimal • e.g., fine-grained limestone + heat = marble • Porphyroblastic texture • Large grains surrounded by smaller grains • Porphyroblast = large grains • Matrix = fine-grains around porphyroblast

20. Foliated Rocks: Slate Phyllite Schist Gneiss Nonfoliated Rocks: Marble Quartzite Common Metamorphic Rocks

21. Slate • Very fine-grained • Excellent rock cleavage • Slaty cleavage • Most often generated from low-grade metamorphism of shale, mudstone or siltstone • Different colors: • Black = carbonaceous • Red = Fe-oxide • Green = chlorite

22. Phyllite • Degree of metamorphism between slate & schist • Platy minerals not large enough to be identified with unaided eye • Glossy sheen & wavy surfaces • Has rock cleavage • Composed of platy minerals such as micas & chlorite Phyllite Slate

23. Schist • Medium- to coarse-grained • Medium-grade metamorpism • Platy minerals dominate • Mainly micas • Schistositic texture • To indicate composition, mineral names are used • e.g., mica schist 

24. Gneiss • Medium- to coarse-grained • Banded layered appearance • High-grade metamorphism • Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian-rich layers

25. Marble • Coarse-grained • Crystalline • Protolith = limestone • Composed of essentially calcite [CaCO3] and/or dolomite [CaMg (CO3)2] crystals • Exhibits a variety of colors • Used as decorative & monument stone

26. Quartzite • Medium- to coarse-grained • Crystalline • Protolith = sandstone • Composed dominately of quartz [SiO2]

27. Classification of Metamorphic Rocks

28. Metamorphic Environments • Contact Metamorphism • AKA Thermal Metamorphism • Hydrothermal Metamorphism • Regional Metamorphism • Other Metamorphic Environments • Burial Metamorphism • Metamorphism along fault zones • Impact metamorphism • AKA Shock Metamorphism

29. Contact Metamorphism

30. Hydrothermal Metamorphism

31. Regional Metamorphism

32. Burial Metamorphism

33. Fault Zone Metamorphism

34. Impact Metamorphism

35. Metamorphic Zones • Metamorphic Grade: • Systematic variation in mineralogy & textures are related to the variations in the degree of metamorphism • Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism • Index Minerals: • Certain minerals which are good indicators of the degree of the metamorphic conditions under which they form • e.g., chlorite ~200oC

36. Progressive Regional Metamorphism • With increasing P & T, higher degree of metamorphism

37. Index Minerals • Typical transition in mineralogy due to progressive metamorphism of shale

38. Metamorphic Zones in New England, USA • Grades concentric with high-grade in center of mountain range

39. Migmatites • Highest grade metamorphic rock • Transitional to Igneous rock • Partial melting of felsic minerals • Contain light bands of igneous components along with dark metamorphic rock

40. Fault Zone Metamorphism • Fault breccia at and near surface • Mylonite (AKA migmatite) forms at depth

41. Metamorphism & Plate Tectonics • Most metamorphism occurs along convergent boundaries • Compressional stresses deform edges of plates • Formation of Earth’s major mountain belts • e.g. Alps, Himalayas, Appalachians • Mountainous terrains along subduction zones contain linear belts of metamorphic rocks • High P, low T rocks nearest the trench • e.g. blueschist facies (glaucophane = index min) • High T, low P zones further inland • In region of igneous activity • Large-scale metamorphism also occurs along subduction zone at convergent boundaries • Several metamorphic environments exist here • Important site for magma generation

42. Metamorphism & Plate Tectonics High P deformation Mountain Building Migmatites Blueschist facies Decompression melting

43. Ancient Metamorphism Canadian shield AKA craton

44. Homework Assignment #1 • Investigate rocks in courtyard • Determine if igneous, sedimentary, metamorphic • Give five observations to support choice F-A F-C 2 3,4 5 F-B 6 1 7