METAMORPHISM METAMORPHIC ROCKS Metamorphism Remember the rock cycle

METAMORPHISM & METAMORPHIC ROCKS

Metamorphism

Remember the rock cycle!

Metamorphism Basics Metamorphism: to change form Metamorphic Rock: formed by the alteration of pre -existing rock deep within Earth (but still in the solid state) by heat, pressure, and/or chemically active fluids. Most metamorphic changes occur at elevated temperatures and pressures. Conditions formation are found a few kilometers below the Earth’s surface and extend into the upper mantle.

Metamorphic Rocks & Tectonics Most metamorphic rocks develop due to plate collisions � Processes are not observable in the field � Conditions at various plate boundaries define possible P & T range � Parent rocks present control mineral assemblages

Contact vs Regional Metamorphism Contact: occurs when magma moves into rock � Occurs near a body of magma � Changes are driven by a rise in temperature � Ex. Marble Regional: results in large-scale deformation and high grade metamorphism � Produces greatest volume of metamorphic rock

Metamorphic Rocks Recrystallization in the solid state Caused by changes in T, P or fluids New environment = new minerals Growing minerals create a new texture

Agents of Metamorphism Heat • Provides the energy needed to drive chemical reactions • Heat comes from magma and the change in temperature with depth Pressure • Causes a more compact rock with greater density • Also increases with depth

Agents of Metamorphism Hydrothermal Solutions • Hot water-based solutions escaping from the mass of magma • Promote recrystallization by dissolving original minerals and then depositing new ones

Sources of metamorphic change

Metamorphism Recrystallization � T, P or change in pore fluids initiate change in the mineral assemblage � Reaction occur entirely in the solid state Shale Schist

Textural changes occur during metamorphism �New minerals grow during metamorphism �Precursor rock textures are modified

Protolith The parent rock subjected to Metamorphism -can be any rock type: igneous, sedimentary, or metamorphic Shale Schist

Protolith

Origin of Metamorphic Rocks Metamorphism � Temperature exceeds 200 OC �Pressure exceeds 300 M Pa (approx. 10, 000 ft deep) Metamorphism ends when melting begins Melting begins at ~700 o. C

Source of heat �Proximity to igneous intrusions Contact metamorphism Different metamorphic grade ring the intrusion �Depth of burial 15 -30 o. C increase per km Geothermal Gradient

Zone of Contact Metamorphism Magma Body -source of heat

Pressure and Stress Lithostatic Pressure- pressure increases with depth due to the weight of the overlying rocks. -equal in all directions: uniform stress -a cubic foot of granite weighs 167. 9 pounds.

Tectonic Pressure/Stress -directional/non-uniform -referred to as deviatoric stress

Folded Metamorphic Rocks -subjected to non-uniform deviatoric stress

T & P determine degree of metamorphism Low-grade metamorphism- 200 to 350 OC Intermediate-grade metamorphism-350 to 550 OC High-grade metamorphism - very high temperatures, above 550 OC

Progressive metamorphism of shale

Classification of Metamorphic Rocks

Classification Metamorphic Rocks are classified by �Texture Foliated vs Non-foliated �Composition Qualifies textural name

Texture Classification 1. Foliated � � Banded or layered appearance Ex. -Gneiss, Shist, Slate

Gneiss Typically Displays a Banded Appearance

Foliated Nonfoliated textures

Slate

Phyllite � Similar to slate � More Recrystallization Crystals � Common � shales are larger Protolith:

Shist �Strongly foliated rock �Medium to coarse grained -dominated by platy minerals �Protoliths: multiple? ?

Gneiss �Coarse grained granular rock �Foliation: alternating bands of light & dark minerals Quartz, feldspar, amphibole, biotite �Protoliths: ? ?

Texture Classification 2. Non-Foliated � No banded or layered appearance � Ex. -Marble, Quartzite, Anthracite

Marble—A Nonfoliated Metamorphic Rock

Nonfoliated Rocks Marble �Interlocking, coarse grained calcite �Recrystallization of limestone or dolostone � Sedimentary � destroyed features

Quartzite �Metamorphism of quartz sandstone �Entire rock is recrystallized

Classification of Metamorphic Rocks

Classification of Metamorphic Rocks