METAMORPHIC ROCKS

METAMORPHISM• Alteration of any previously

existing rocks by high pressures, high temperatures, and/or chemically active fluids.

Agents of Metamorphism– high temperatures - • most important agent because it provides energy to drive

chemical reactions

• geothermal gradient (25o C /km) & compressional heat

• proximity to magmatic intrusions

– high pressures -• compressive stresses during mountain building

• increases with depth

• can cause rock to flow or bend (deform plastically)

• sliding (shearing) can pulverize rock

– chemically active fluids -• water is in pore spaces of most rocks

• many minerals are hydrated

• hot water contains ions in solution that form new minerals

Textural, Structural & Mineralogical Changes

• Degree of metamorphism is reflected by– mineral composition– texture of rocks

• Rocks become more compact (denser)• Minerals recrystallize and grow larger in solid state• Some new minerals form• Preferred (structural) mineral orientations develop – perpendicular to stress• platy minerals

• elongated minerals (needle-like)

– causing foliation, a layered structure • chemical mineral segregation into light and dark bands

Foliation• Rock cleavage (slaty cleavage)-

– low-grade metamorphism (low pressure, low temp.)

– clay minerals recrystallize to minute mica flakes

– develops parallel fracturing

– forms slate

• Schistosity

– high-grade metamorphism

– mica crystals become visible (scaly appearance)

– forms schist

• "banding"

– highest-grade metamorphism

– silicate minerals segregate into light & dark bands

– forms gneiss

Development of Rock Cleavage (Slaty Cleavage)• High pressure, associated with folding, develops parallel

alignment of growing microscopic mica crystals (in a former clayey shale) that are perpendicular to the stress direction

Rock Cleavage in Outcrop• Rock cleavage enhances

erosion

Development of Banding• Light and dark minerals segregate out into parallel layers, such as

in a gneiss, that are perpendicular to the stress direction

Low-Grade Foliated Metamorphic Rocks• Shale metamorphoses to slate

• Clay minerals alter to microscopic mica crystals

• Rock cleavage develops as the mica crystals align parallel to each other and perpendicular to the main stress direction

• Breaks into parallel sheets

• Once used for school blackboards

• Slate metamorphoses to phyllite• Microscopic mica crystals

enlarge, giving the rock a glossy sheen

High-Grade Foliated Metamorphic Rocks• Phyllite metamorphoses to

schist– Mica crystals become

easily visible to the naked eye

Highest-Grade Foliated Metamorphic Rocks• Schists metamorphose to

gneiss– Minerals alter to form

abundant quartz & feldspar

– Light and dark minerals separate out into bands

Further Metamorphism Produces Migmatites• When the temperatures

during metamorphism are high enough, low-melting-point minerals melt and the hot fluid separates from the still-solid, high-melting-point minerals

• Forms bands of alternating igneous and metamorphic rock– light bands = igneous– dark bands = meta.

Non-Foliated Metamorphic Rocks• Foliation will not develop in rocks that are– Equant grained (equidimensional in all directions)– Monomineralic (e. g., all quartz, all calcite, etc.)

• Example #1: Quartz sandstone metamorphoses to a quartzite which may look the same whether it forms under low-grade to high-grade metamorphic conditions

• Example #2: Limestones metamorphose to marble– Equant grained– All calcite

Non-Foliated Metamorphic Rocks

Metamorphism Along Fault Zones• Movement along a fault zone may break up and grind

the adjacent rocks, due to shear, to form a breccia that is called a "fault breccia" near the surface where rocks are brittle.

• The fragments in a breccia are very angular• A high pressures deeper in Earth, rocks flow & don't

form breccias

Contact Metamorphism• Magma comes into contact with cooler country rock– base of thick lava flows– near-surface dikes and sills– around deep stocks & batholiths

• Heat and fluids from magma alter country rock• Forms concentric zones of varying degrees of mineral

alteration• Hydrothermal solutions from the magma can form

metallic ore deposits in the country rocks

Regional Metamorphism• Intense metamorphism and deformation during mountain

building associated with lithospheric plate collisions

• increases towards the core or major mountain ranges

• most common type of metamorphism

Mineral Changes with Increasing Metamorphism