1. Gregory G. Dimijian/Photo Researchers cd/EM - F

2. Metamorphic Rocks Metamorphism Metamorphism: to change form Metamorphic rock solid state. Metamorphic rock: any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in mineralogical or texture composition in the solid state.

3. No single mode of origin recrystallizationMetamorphic rocks result from the partial or complete recrystallization of minerals in rocks over long periods of time. solidRocks remain essentially solid during metamorphism.

4. Metamorphic processes HEAT: stability region of mineral sensitive to T. With increased T, pore fluid decreases. PRESSURE: greater pressure tends to decrease space available; metamorphic minerals tend to be dense. Increased P can come from any stress (burial). Directed stress will bring about a preferred orientation of minerals. FLUIDS (H 2 O): acts as a catalyst during metamorphism; aids the exchange of ions between growing crystals.

5. Metamorphic grade Refers to the intensity of metamorphism. High grade: high T (>550 0 C), P Intermediate grade: T (320- 550 0 C), P Low grade: low T (200-320 0 C), P

6. Metamorphic Grade

7. Pressure (depth) and Temperature vs. Rock Type

8. Types of Metamorphism Regional: Widespread changes in temperature and pressure bring about changes in rocks due to tectonic forces. Contact: Colder country rocks “cooked” by intrusion of magma (affected area is proportional to the size and temperature of the intrusion but always only a local phenomenon). Deformational: Changes in rocks associated with faulting and folding (regional or local).

9. Types of metamorphism (cont.) Burial: Changes in a rock due to the gradual changes in T and P due to successive burial (regional). Impact: Changes due to VERY rapid increase in pressure (only localized). Hydrothermal: heated fluids leach surrounding rocks (local)

10. Fig. 9.3 Plate Tectonics & Metamorphism

11. How much can a rock change? Depends on:  grade of metamorphism  duration of metamorphism  composition of the rock

12. Metamorphic reactions Mineralogical changes (e.g., clay to mica): Many complicated reactions in which new assemblages form — depend on pressure, temperature, composition. Textural changes: recrystallization (grain boundaries more compact) and foliation (preferred orientation of minerals).

13. Mineralogical Changes ( Mineral Stability) Most minerals are stable over a relatively narrow range of P and T (e.g., ice unstable above 0°C). The stability range of different minerals overlap and provide constraints on the metamorphic history of rocks -> METAMORPHIC ZONES.

14. Mineralogic Changes in Metamorphosed Shales (new assemblages develop -> zones)

15. Mineralogic Changes in Metamorphosed Shales

16. Contact Metamorphism adjacent to a Granite Intrusion

17. Changes in Texture Grain size Recrystallization - Granular Texture Grain size can either decrease or increase. Orientation of minerals Directed stress will orient minerals perpendicular to stress: –Lineation –Foliation

18. Fig. Story 9.4

19. Textural Changes: Non Foliated or Granular Granular texture –Coarse to fine interlocking grains –Little or no preferred orientation

20. Quartzite (from sandstone) Breck P. Kent

21. Quartzite in Thin Section

22. Marble (from limestone) Breck P. Kent

23. Marble in Thin Section

24. FOLIATED ROCKS Mineral grains show preferred orientation See different kinds of foliation: –Slaty cleavage –Schistosity –Gneissic Foliation

25. (sheen) (sparkles)

26. Slate with Foliation and Relict Bedding

27. Martin Miller Slaty Cleavage

28. Metamorphic Foliation

29. S. Dobos Direction of Compressive Forces

30. Classification of metamorphic shale Based on the texture and composition of the rock: Low gradeSlate Phyllite Intermediate gradeSchist Gneiss High gradeMigmatite

31. Slate Andrew J. Martinez/Photo Researchers

32. Schist Biophoto Associates/Photo Researchers

33. Schist in Thin Section

34. Gneiss Breck P. Kent

35. Metamorphism In Hawai’i Not important except for CONTACT METAMORPHISM (of course) at dike margins Only see Low Grade metamorphic rocks in rock quarries with lots of Zeolites (The following slides are for your edification only, so you can say that you have seen these figures!)

36. Metamorphic facies A given set of metamorphic conditions Each facies is characteristic of particular tectonic environments and will have certain rocks that are diagnostic of those conditions. Therefore, certain rocks can be clues to the (P,T) history of the region. Facies are: zeolite*, greenschist, amphibolite, granulite, blueschist, eclogite

37. Metamorphic Facies *

38. Plate Setting and Metamorphism The end!