Metamorphic Rocks

1. Metamorphic Rocks

2. Goals for mastering metamorphic rocks • Understand what it means to “metamorphose” a rock • Explain the differences between various types of • metamorphism processes – types of metamorphism • Describe the various common metamorphic textures • Associate the common textures with c0mmon • metamorphic rocks • Describe an index mineral – and how it relates to • metamorphism • Explain the association of metamorphic rocks with • various types of plate tectonic settings

3. What is “metamorphism” ? • meta(change)morphism(form) • transformation of pre-existing • rock from one to another • throughpressureand • temperaturerelationships • all changes take place through • solid state transformation

4. Types of Metamorphism • takes place around 150oC (temperature) • typically at depths of 5 km below the • earth’s surface (Pressure) At this temperature and pressure new minerals are formed – changed from one mineral to another

5. Low-grade Metamorphism • between 5-15 km below the earth’s • surface • temperatures up to 400oC • common low-grade minerals • quartz, chlorite, muscovite Chlorite Muscovite Quartz

6. High-grade metamorphism • 15 km below the surface to the point • melting (when melting occurs) • Temperatures greater than 400oC • Common high-grade minerals • garnet, kyanite, sillimanite kyanite sillimanite Garnet

7. How do fluids enhance metamorphism? • minerals dissolve – re-precipitated in the rock • fluids speed up chemical reactions – reduces • mineral melting points creating “migmatites” Igneous rock material Migmatite Metamorphic rock Melting occurs from lowered melting points creating small pockets of igneous rocks surrounded by metamorphic rocks

8. When does metamorphism take place? • solid rock coming in contact with hot • magma • solid rock contacts hot fluids (hot • water) • during episodes of intense mountain • building • along earthquake fault zones

9. What are the various types of metamorphism processes? • contact metamorphism • hydrothermal metamorphism • regional metamorphism • Metasomatism

10. What is contact metamorphism • Contact metamorphism: • magma “contacts” solid rock – • heat is transferred into overlying • rocks through conduction – changing • the pre-existing rock it intruded • high heat / low pressure • localized metamorphism • “fast acting” like touching a • hot frying pan

11. Country rock (surrounding) is baked by conduction

12. What is regional metamorphism? • Regional Metamorphism • pre-existing rock is subjected to intense • stresses and strains (deformation) usually • from the forces of mountain building • high heat / high pressure • very wide spread geographically • like experiencing a slow sun burn

13. Mountain building Wide spread belts

14. What is hydrothermal metamorphism • Hydrothermal metamorphism • the chemical alteration of pre-existing • rock from the action of hot water – • usually hot water from a magma source • that is ion rich • low pressures / high heat • hot water alteration

15. Black smokers on the ocean floor hydrothermal alteration – “spewing” hot water concentrated in sulfide metals

16. Metasomatism • fluids pass in / out of the rock changing the • overall chemical composition • fluids take away or add chemical compounds • to the rock Chemical alteration from fluids --- moving and out of the rock

17. What does each metamorphic process have in common? Heat: provides the energy to cause recrystallization of pre-existing minerals into “new” type of minerals Heat sources: contact with hot magma – heat penetrates overlying rocks (conduction) geothermal gradient – heat increases with depth – rocks are hotter when buried

18. Geothermal Gradient • Temperature increases • as depth increases • Large thermal gradient • at the surface • gradient decreases • after 1000 km depth 0 1000 2000 3000 4000 Steep gradient

19. Pressure – increases with depth various types of pressure rearrange minerals – compacts or elongates Differential pressure Confining pressure

20. Chemically active fluids • commonly hot water carries ions in • solution and ppt various solids (metals) • hot water dissolves minerals and deposits • minerals to other sites – concentrated • gold, silver, other earth metals Gold in Quartz vein Native Copper (Cu)

21. How does one know how much a rock has been metamorphosed? • the degree of metamorphism is • reflected in the rocks texture • and composition • Regional Metamorphism(HP/HT) – high grade • rock exhibits afoliationtexture • rocks show a “preferred” mineral • orientation • parallel mineral alignments

22. How does one know how much a rock has been metamorphosed? Contact Metamorphism(HT / LP) low-grade • rock textures decrease in • foliated textures – “sub” parallel type • textures • rocks composed of one type of • mineral (same chemistry) are called • non-foliated

23. How does one know the amount of temperature / pressure the rock has been subjected? • The Metamorphic facies • the set of metamorphic mineral assemblages • that form under various temperature/pressure • relationships • Associated with different plate boundaries • Blueschist / Eclogite facies occur in the • high pressure environment --- subduction • Greenschist, amphibolite, granulite facies • occur along convergent boundaries

24. Metamorphic facies – mineral assemblages that represent various P / T relationships

25. What plate tectonic settings do metamorphic rocks form? • Divergent boundaries • hydrothermal metamorphism • with black smokers • Convergent plate boundary • metamorphism along plate • boundary • regional and contact • metamorphism Contact Regional

26. Metamorphism and Metamorphic Rocks Define the following metamorphism terms: contact metamorphism nonfoliated foliation parent rock hydrothermal regional metamorphism metamorphism confining pressure differential pressure index mineral