Metamorphism and Metamorphic Rocks

What are metamorphic rocks? Increases in heat and pressure and changes in chemical environment can alter the mineral composition and crystalline textures of sedimentary and igneous rocks Metamorphism is the solid-state (without melting) transformation of pre-existing rock into texturally or mineralogically distinct new rock as the result of high temperature, high pressure, or both. Limestone Marble Sandstone Quartzite (meta-quartzite) Shale Slate Shale/sandstone Schist Shale Hornfels Shale/sandstone Gneiss

Metamorphic Rock Cycle Sedimentary Igneous Erosion + Deposition Melting Erosion + Deposition Burial + Heating Melting Burial + Heating Metamorphic

What are metamorphic rocks? Causes of metamorphism Heat & Pressure

Sedimentary Surface 15o C 5 km 200o C Metamorphic 30 km Igneous 600o C Occurs at depths greater than that of lithification (Diagenesis -> Sedimentary Process) Sedimentary Surface 15o C 5 km 200o C Metamorphic 30 km Igneous 600o C

The 3 “R”s of Metamorphism Regrowth - crystals grow larger and more uniform in shape. Recrystallization - new minerals form from existing minerals. Reorientation - platy or elongated mineral crystals become layered and aligned.

Factors Controlling Metamorphism Metamorphic changes bring a pre-existing rock into equilibrium with new surroundings Temperature Re-crystalisation – new mineral assemblages Segregation of new minerals – banding Pressure Regrowth - Confining pressure – acts in all directions – alters mineralogy by squeezing atoms into a denser state Reorientation - Directed pressure – acting in a particular direction – convergent plate boundaries – causes preferred orientation - foliation

Kinds of Metamorphism Regional Regional Contact Contact Burial Cataclastic Hydrothermal

Kinds of Metamorphism Regional Metamorphism Occurs where both high temperature and high pressure are imposed over large belts of the crust Destroys all original sedimentary or igneous textures through growth of new minerals Occurs in deeper levels of the crust along convergent plate boundaries particularly continental-continental boundaries where active mountain building is taking place.

Regional Metamorphism Most metamorphic rocks Occurs over a large area as a result of intense heat and pressure Most obvious at convergent plate boundaries where rocks are intensely deformed and compacted during subduction and compression Metamorphic grades seen through index minerals

Taconic Mountain Building Event (450 million years ago)

Taconic Mountain Building Event (450 million years ago)

Taconic Mountain Building Event Regional Metamorphism Taconic Mountain Building Event (450 million years ago)

Regional Metamorphism

Metamorphic Grade

Progression of metamorphism Start with a shale and then hit it with pressure and heat. Slate Phyllite Schist

You end up with something that is really Gneiss!

Sedimentary Rock 200° C 3000 ATM Low Grade Metamorphic Rock High Grade Metamorphic Rock 600° C Migmatite Magma

Migmatite gneiss that begins to partially melt into granite.

Kinds of Metamorphism Contact Metamorphism Heat and pressure generated by igneous intrusions Affects only a thin region of intruded rock. At shallow depths the mineral transformations generally associated with heat – pressure becomes important for intrusion at great depth A zone of alteration called an aureole forms in the rock surrounding the magma

Contact Metamorphism: The main metamorphic agent is heat. Types of Metamorphism Contact Metamorphism: The main metamorphic agent is heat.

Contact Metamorphism

Contact metamorphism

Kinds of Metamorphism Dynamic / Cataclastic Metamorphism Found along faults Fault movement causes rocks on either side to fragment The product is a rock with a broken and pulverised texture Generally found in strongly deformed mountain belts – often found with regionally metamorphosed rocks

Dynamic Metamorphism Associated with Fault Zones Low temp/high pressure Mylonites – hard, dense, fine-grained rocks, with thin laminations (less than 1 cm thick) San Andreas Fault, CA and Northern Scotland

Granite away from active fault

Granite next to active fault

Kinds of Metamorphism Hydrothermal Metamorphism Often associated with mid-ocean ridges Seawater heated by upwelling magma promotes chemical reactions with basalt causing alteration Alteration is also common in other igneous rocks such as granite. Alteration can result in the rock becoming weaker, more compressible and more permeable – the end product is very similar to that produced by chemical weathering

Kinds of Metamorphism Burial Metamorphism (precursor to Regional) Diagenisis grades into burial metamorphism Low grade metamorphisms resulting from heat and pressure exerted by overlying sediments and sedimentary rocks Bedding and other sedimentary structures are preserved Burial metamorphism can grade into regional metamorphism

Burial Metamorphism

Classification of Metamorphic Rocks Foliated Rocks A set of flat or wavy parallel planes produced by the preferred orientation of minerals, particularly platy minerals like mica

Foliated Rocks This slide is indicative of a schist. A foliation is any planar fabric in a metamorphic rock. In this case, the foliation is defined by aligned sheets of muscovite sandwiched between quartz grains. This slide is indicative of a phyllite. The foliation in this rock is a crenulation cleavage, and is developed after the horizontal foliation. This slide is indicative of a schist.

Classification of Metamorphic Rocks Criteria Nature of foliation Grain size Degree of banding Metamorphic grade Foliated Rocks

Common metamorphic rocks Foliated rocks Slate Very fine-grained Excellent rock cleavage Most often generated from low-grade metamorphism of shale, mudstone, or siltstone

Low Grade - Slate

Burial Mountain Building

Common metamorphic rocks Foliated rocks Phyllite Gradation in the degree of metamorphism between slate and schist Platy minerals not large enough to be identified with the unaided eye Glossy sheen and wavy surfaces Exhibits rock cleavage Composed mainly of fine crystals of muscovite and/or chlorite

Phyllite (left) and Slate (right) lack visible mineral grains

Intermediate Grade – Phyllite

Common metamorphic rocks Foliated rocks Schist Medium- to coarse-grained Platy minerals predominate Commonly include the micas The term schist describes the texture To indicate composition, mineral names are used (such as mica schist)

A mica garnet schist

High Grade - Schist

Common metamorphic rocks Foliated rocks Gneiss Medium- to coarse-grained Banded appearance High-grade metamorphism Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian minerals

Gneiss typically displays a banded appearance

High Grade - Gneiss

Classification of Metamorphic Rocks Foliated Rocks Gneiss Schist Slate

Regional Revisited Slate Phyllite Schist, Gneiss

Metamorphic Grade

Non-foliated Metamorphic Rocks granular texture interlocking grains composed primarily of one mineral uniform grain size

Classification of Metamorphic Rocks Non-foliated Rocks Hornfels Product of contact metamorphism Uniform grain size- little or no deformation – plate/elongate crystals are randomly orientated Often harder and more brittle than parent rock Marble Product of heat & pressure acting on limestone or dolomite Often show irregular banding or mottling due to impurities Granular texture Coarse, crystalline Parent rock was limestone or dolostone Composed essentially of calcite or dolomite crystals Used as a decorative and monument stone Exhibits a variety of colors Quartzite Derived from quartz-rich sandstones (contact or regional metamorphism) often very hard and extremely strong

Marble – a nonfoliated metamorphic rock

Nonfoliated - Marble metamorphosed limestone (CaCO3) bedding and fossils obliterated

Quartzite

Nonfoliated - Quartzite metamorphosed quartz-rich sandstone Pore spaces filled with SiO2

Metamorphic Rocks

Metamorphic environments associated with plate tectonics

Now go to Metamorphic Revision PPT