METAMORPHISM

1. METAMORPHISM Introduction

2. METAMORPHISM What is metamorphism? • The IUGS-SCMR has proposed the following definition of metamorphism: • “Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.”

3. METAMORPHISM • Limits of Metamorphism: • Low-temperature: • limits grades intodiagenesis • the boundary is somewhat arbitrary • diagenetic/weathering processes are indistinguishable from metamorphic • metamorphism begins in the range of 100- 150oC for the more unstable types ofprotolith • some zeolites are considered diagenetic, others metamorphic – pretty arbitrary

4. METAMORPHISM Limits of Metamorphism: • High-temperature: • limits grade intomelting • over the melting range solids and liquids coexist • if we heat a metamorphic rock until it melts, at what point in the melting process does it become “igneous”? • xenoliths, restites, and other enclaves are considered part of the igneous realm because melt is dominant, but the distinction is certainly vague and disputable • migmatites(“mixed rocks”) are gradational

5. METAMORPHISM Limits of Metamorphism: High-temperature: migmatite

6. METAMORPHISM Fabric of Metamorphic Rocks Textures: Crystalloblastic -blastic = results from metamorphism idioblastic hypidioblastic xenoblastic vs. Noncrystalloblastic relict blasto- = relict

7. METAMORPHISM Fabric of Metamorphic Rocks Textures: Crystalloblastic Series Idioblastic Rutile, sphene, magnetite Tourmaline, kyanite, staurolite, garnet, andalusite Epidote, zoisite, forsterite Pyroxenes, amphiboles, wollastonite Micas, chlorite, talc, stilpnomelane Dolomite, calcite Scapolite, cordierite, feldspars quartz Xenoblastic

8. METAMORPHISM Fabric of Metamorphic Rocks Textures: Shape: granoblastic nematoblastic lepidoblastic

9. METAMORPHISM Fabric of Metamorphic Rocks: Textures: Other textures: porphyroblastic poikiloblastic augen blastoporphyritic blastoophitic

10. METAMORPHISM Fabric of Metamorphic Rocks: Structure: Lineation - any linear fabric elements Foliation - any planar fabric element Metamorphic rocks with predominantly nematoblastic minerals are more correctly termed lineated, but are usually lumped in with the foliated rocks.

11. METAMORPHISM Fabric of Metamorphic Rocks: Structures – Types of Foliation: Slaty cleavage: • Traditionally: the property of a rock to split along a regular set of sub-parallel, closely- spaced planes • A more general concept adopted by some geologists is to consider cleavage to be any type of foliation in which the aligned platy phyllosilicates are too fine grained to see individually with the unaided eye

13. METAMORPHISM Fabric of Metamorphic Rocks: Structures – Types of Foliation: Schistosity: • A preferred orientation of inequant mineral grains or grain aggregates produced by metamorphic processes • Aligned minerals are coarse grained enough to see with the unaided eye • The orientation is generally planar, but linear orientations are not excluded

15. METAMORPHISM Fabric of Metamorphic Rocks: Structure: Gneissic (gneissose): • Either a poorly-developed schistosity or segregated into layers by metamorphic processes • Gneissose rocks are generally coarse grained

17. METAMORPHISM Naming Metamorphic Rocks: • Metamorphic rocks may be named based on: • protolithology • structure (type of foliation) • structure and/or composition (nonfoliated + comp.) • chemistry

18. METAMORPHISM Naming Metamorphic Rocks: • Metamorphic rocks may be named based on: • protolithology • e.g.: meta-quartzite, meta- basalt, etc. • In the case of non-crystalloblastic rx, usually the most appropriate method, otherwise not as descriptive as other methods

19. METAMORPHISM Naming Metamorphic Rocks: • Metamorphic rocks may be named based on: • protolithology • structure (type of foliation) • structure and/or composition (nonfoliated + comp.) • chemistry

20. METAMORPHISM Naming Metamorphic Rocks: • Metamorphic rocks may be named based on: • Structure (type of foliation) • The classical (and our) way of naming the foliated • (or lineated) metamorphic rocks • slate • phyllite • schist • gneiss

21. METAMORPHISM Naming Metamorphic Rocks: Slate: compact, very fine-grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface. Phyllites with both a foliation and lineation are very common.

22. a b Slate Phyllite

23. METAMORPHISM Naming Metamorphic Rocks: Schist: a metamorphic rock exhibiting a schistosity. By this definition schist is a broad term, and slates and phyllites are also types of schists. In common usage, schists are restricted to those metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen.

24. METAMORPHISM Naming Metamorphic Rocks: Gneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers. Gneisses may also be lineated, but must also show segregations of felsic-mineral-rich and dark-mineral-rich concentrations

25. METAMORPHISM Naming Metamorphic Rocks: • Metamorphic rocks may be named based on: • protolithology • structure (type of foliation) • structure and/or composition (nonfoliated + comp.) • chemistry Composition may be used for some rocks which are structurally nonfoliated

26. METAMORPHISM Naming Metamorphic Rocks: Granofels - a comprehensive term for any isotropic rock (a rock with no preferred orientation; aka granulites) Hornfels - a type of granofels that is typically dark, very fine-grained and compact, and occurs in contact aureoles Marble - composed predominantly of calcite or dolomite; protolith is typically limestone or dolomite Quartzite - composed predominantly of quartz; protolith is typically sandstone Amphibolite – composed primarily of hornblende and plagioclase; may also be foliated, e.g. amphibolite gneiss

27. Marble

28. Quartzite

29. METAMORPHISM Naming Metamorphic Rocks: • Metamorphic rocks may be named based on: • protolithology • structure (type of foliation) • structure and/or composition (nonfoliated + comp.) • chemistry

30. METAMORPHISM Naming Metamorphic Rocks: • Common metamorphic rx fall into 6 chemical classes (approximated by mineralogy): • Pelitic – derivatives of aluminous sediments, e.g. shales, mudstones • Quartzo-feldspathic – derivative of quartz and/or feldspar-rich rocks such as arenites, felsic igneous rx • Calcareous – derivatives of calcareous sediments • Mafic – derivatives of mafic to intermediate igneous rocks • Magnesian – derivatives of ultramafic rx • Ferruginous – derivatives of Fe-rich protoliths, usually sedimentary

31. METAMORPHISM Naming Metamorphic Rocks: • Other names: • there are a number of names used for very specific rock types, e.g. serpentinite, skarn, blueschist, greenschist, mylonite, eclogite, cataclasite – will deal with later • Modifiers: • names usually modified based on: • mineralogy • e.g. mica schist, tourmaline schist • chemistry • e.g. magnesian schist, ferruginous slate • protolithology • e.g. granite gneiss, pelitic hornfels