Introduction to metamorphism

Lecture 2

Metamorphism

mineralogical and textural changes to a rock that are a response to temperature, pressure, or in some fluid activity

Lower Limits of Metamorphism

Di agenesis grades into Meta: begins around 150C, and only happens to reactive protoliths

High Temperature Upper Limits

Melting in the earth where solids and liquids coexist- ranges in temperature and pressure

Migmatite

A rock mixed with both igneous and metamorphic properties from under going partial melt

Temperature

Agent of metamorphism, usually the biggest factor, increasing temperature causes recrystalization and drives reactions.

Recrystalization

changes in size and shape of the grain, increased grain size is very common

Pressure

lithostatic pressure increases with depth P=pgh, where g is the acceleration due to gravity. It influences the stability of minerals and can lead to denser rock formations.

Geothermal Gradients

Temperature and pressure increase with depth

Shallow Gradients

Older, stable continental cratons and is 10-20C/km

Volcanic geothermal gradient

may be more than 80-90C/km

Metamorphic Grade

The degree of change in mineralogy and texture of a rock due to increasing temperature and pressure during metamorphism. P and T and grade increase with depth usually

Prograde

increase in metamorphic grade with time

Retrograde

the decrease in metamorphic grade as temperature and pressure decrease during the cooling of a rock.

Hydrostatic

uniform stress and pressure

Differential Stress

unequal and in different directions

Foliation

the alignment of mineral grains within a rock, typically resulting from directed pressure during metamorphism. This happens during shear stress

Fluids

Evidence - fluid inclusions and violates

Fluid Pressure

Sum of partial pressures exerted by fluids in rock

Bulk Composition

chemical composition of the whole rock, protoliths have different bulk compositions that influence the metamorphic process.

Types of Protoliths

Ultramafic, mafic, Shales/Mudstones, Carbonates, Quartzo-feldspathic, quartz

Ultramafic

very high Mg, also Fe Ni, Cr and low Si

Mafic

high Fe, Mg, and Ca
Basalt, Gabbro & Certain Immature Sediments (Graywacke)

Shales/Mudstones (Pelites)

high Al, K, Si

Carbonates

high Ca, Mg, CO2
Limestone - Dolostone

Quartzo-feldspathic

igh Si, Na, K, Al
Granite, Granodiorite, Diorite(?), Extrusive Equivalents
Immature Sediments - Arkose

Quartz

very nearly pure SiO2

Classification of Metamorphic Rocks

Rocks with foliation are based on textures, slaty cleavage, schistosity, and gneissic layering

Meanwhile, non-foliated rocks, such as marble and quartzite, are classified based on mineral composition and texture.

classification for foliated metamorphic rocks

grain size, fol. minerals, rock name

Slate and Phyllite

slate- very fin grained salty cleavage, splits into thin slabs

Schist

Schistosity - modify rock name by listing minerals in order of increasing abundance ex. Chlorite schist ,

Gneiss Banding

Modify rocxk name by lkisting order of minerals of increasing abundence

Marble

contains calcite and dolomite, and includes other Ca-Mg minerals

Calc-silicate

carbonate-subordinate, wollastonaite, grossular, diopsiode, vesuvian, may be metamorphized marble

Amphibole

amphibole - plagioclaise

Eclogite-high pressure

metamorphic rock formed under high pressure and temperature, primarily composed of garnet and clinopyroxene.

hornfels

a fine grained non-foliated rock, caused by contact metamorphism

Granofels

medium to coarse-grained, granoblastic, metamorphic rocks without or with indisuctinct foliation,

Granoblastic

phaneritic equi-granular texture in metamorphic rocks typical of quartz and other non-foliated rocks

porphyroblast

a larger mineral which has grown within the finer grained groundmass

Contact Metamorphism

driven by heat, nearby igneous intrusion,

Aureole

area of metamorphic rocks surrending intrusion

contact metamorphism process

that occurs when rocks are heated by nearby molten rock, leading to changes in mineral composition and texture.

Thermal (meta somatic) effects of hot magma intruding cooler shallow rocks

result in localized metamorphism, altering mineralogy and texture.

contact metamorphism - aureole size and shape

nature of the pluton size, shape, temperature, nature of the country rocks, depth and metamorphic grade prior to intrusion.

Regional Metamorphism

metamorphism that affects a large body (region) of rock, typically associated with orogenic activity like orogenic metamorphism

Orogenic Metamorphism

typically associated with plate margins, convergent and divergent boundaries, resulting from tectonic forces and high pressures.

Dynamo-thermal

elevated geothermal gradients, deformation, foliated rocks, characteristic product

Burial metamorphism

occurs when rocks are buried under significant overburden, leading to increased pressure and temperature without tectonic forces. happens in passive margins that becomes active.

Hydrothermal Metamorphism

occurs when rocks are altered by hot, chemically active fluids, often associated with mid-ocean ridges and volcanic activity. difficult to constrain, and happens in most other types of metamorphism. It can lead to the formation of minerals such as talc and serpentine, and is significant in ore deposit formation.