METAMORPHISM

AND

METAMORPHIC ROCKS

WHAT IS METAMORPHISM?

•recrystallisation of pre-existing rocks, leaving larger crystals

•new minerals form, but composition usually stays the same

•metamorphic rocks form from igneous, sedimentary or other metamorphic rocks

•3 types of metamorphism are recognised

thermal/contact metamorphism

regional metamorphism

dynamic/dislocation metamorphism

WHAT ARE THE AGENTS OF METAMORPHISM?

•heat, usually from proximity to igneous intrusions, but also from depth and temperature increases with the geothermal gradient

•pressure, due to overlying rocks, but more especially in tectonically active zones

•hydothermal fluids, especially adjacent to igneous intrusions

HEAT

•breaks chemical bonds•mobilises hydothermal fluids of hot water and gases•softens rocks to allow deformation

Geothermal gradient

PRESSURE

•Can be lithostatic, equal in all directions, or compressive/ non uniform which gives a degree of foliation

Compressive pressure

HYDROTHERMAL FLUIDS

•Water comes from minerals, rocks, seawater or from magma

•fluids are water with dissolved gases and elements

•work most effectively at high temperatures and pressures when reaction rates are speeded up

•fluids move through the fissures and pore spaces in rocks

CONTACT/THERMAL METAMORPHISM

CONTACT/THERMAL METAMORPHISM

•forms around an igneous intrusion•the zone of alteration is called a metamorphic aureole•metamorphism is by heat and hydrothermal fluids•pressure does not operate so the rocks are unfoliated•metamorphism varies from HIGH GRADE to MEDIUM GRADE to LOW GRADE away from the intrusion•high grade metamorphic rocks have undergone the most heat and alteration by hydrothermal fluids•unaltered rock well away from the intrusion is called country rock•a good example is the altered slates around the Skiddaw granite in the Lake District

SIZE OF THE AUREOLE

Determined by:-

•Size of the intrusion•shape of the intrusion•type of magma - lower temperature acidic intrusions may have larger aureole due to producing more hydrothermal fluids

•Type of country rocktransfer of heatporositypermeability

•mudstone produces a larger aureole than sandstone

ROCK TYPES

Argillaceous country rock: shale/slate•Away from the intrusion large crystal called

porphyroblasts form in a generally unchanged rock•this medium grade metamorphism may give large andalucite crystals in the form of chaistolite•the loose patterns of porphyroblasts shows no orientation•cordierite crystals may contain other minerals to give a poikiloblastic texture

•Near the intrusion high grade metamorphism gives hornfels•this is completely recrystallised •with an equi-sized granoblastic texture

Spotted slate

andalusite

porphyroblasts

Poikiloblast of cordierite(poikilitic texture)

hornfels

CONTACT METAMORPHISM OF OTHER ROCKS

Sandstone•metaquartzite results•large interlocking crystals with 120 degree angles•a granoblastic texture

Limestone•pure calcite produces marble•with a large grained sugary or saccharoidal texture•impure limestones have other elements•eg dolomite, and magnesium oxide (periclase) remains

metaquartzite

marble

Contact MetamorphismContact Metamorphism

•• Adjacent to igneous intrusionsAdjacent to igneous intrusions

•• Thermal (Thermal (±± metasomatic) effects of hot magma metasomatic) effects of hot magmaintruding cooler shallow rocksintruding cooler shallow rocks

•• Occurs over a wide range of pressures, includingOccurs over a wide range of pressures, includingvery lowvery low

•• Contact Contact aureoleaureole

Table 2 Common Metamorphic Rocks

Rock Parent Rock Key Minerals Metamorphic Environment

Foliated Slate Shale Clay minerals, micas. chlorite. graphite Relatively low temperature and

pressure

Foliated Schist Shale,

basalt, graywacke sandstone, impure limestone Mica, chlorite, garnet, talc, epdiodte, hornblende, graphite, staurite, kyanite Intermediate - to - high temperature and pressure

Foliated Gneiss Shale, felsic igneous rocks, graywacke, sandstone, granite, impure limestoneGarnet, mica, augite, hornblende, staurolite, kyanite, sillimanite High Temperature and pressure

Nonfoliated Marble Pure limestone calcite, dolomite Contact with hot magma, or confining pressure from deep burial

Nonfoliated Quartzite Pure sandstone, chert Quartz Contact with hot magma, or confining pressure from deep burial