Metamorphic petrology

Metamorphism

a sub-solidus process leading to a change in mineralogy, texture and sometimes chemical composition

texture refers to...

grain size and internal alignment of minerals

when does metamorphism start

after diagenesis; 100-150 degrees C

when does metamorphism end

anatexis (approx. 900 degrees C); when melting starts; partial melting

causes of metamorphism

• with depth temp. and pres. increases

metamorphic agents

temp., pressure, shear stress, protolith, fluids, time

increasing temperature...

• promotes crystallisation

• increases grain size

• drives chemical reactions that consume unstable minerals

physical conditions are described in terms of...

pressure-temperature (P-T) coupling e.g. High-T - low-P

P-T conditions at magmatic intrusions

High T/Low P

P-T conditions at subduction zones

Low T/ High P

typical increase in temp.

30 deg.C/km

lithostatic pressure

uniform stress (hydrostatic); pressure equal in every direction

deviatoric pressure

pressure varies in different direction

effect of deviatoric pressure

induces grain rotation and realignment (according to the stress (direction)) that causes characteristic texture in met. rocks; produces a fabric (cleavage, schistosity)

shear stress produces...

localised metamorphism

tensional stress

The stress produced by two tectonic plates moving apart

compressional stress

squeezes a rock and shortens a rock body

shear stress

stress that occurs when forces act in parallel but opposite directions, pushing parts of a solid in opposite directions

protolith

the original rock

what does the protolith determine?

type and overall structure

what do fluids do

facilitate the metamorphic process

what are fluids needed for

to from hydrous or carbonate phases -> rocks become more hydrated as they become metamorphosed

types of fluids

• meteoric

• juvenile magmatic

• subducted material

• sedimentary brines

• degassing of the mantle

classification of metamorphic rocks

by metamorphic grade; indicates the intensity of metamorphism; high/ intermediate/ low grade

what can met. rocks be classified on the basis of?

• texture

• fabric

• composition

fabric

internal structure of the rock e.g. foliation, lineation

foliation

mineral grains are arranged in planes or bands; formed by platy minerals; e.g. cleavage, schistosity, banding

lineation

any linear fabric formed by elongated minerals; minerals that are more long, prismatic

process of metamorphic foliation

pressure causes mineral rotation and realignment; results in lepidoblatsic texture

process of metamorphic lineation

in shear zones; minerals realign parallel to the shear direction; nematoblatsic texture

What is cleavage in geology?

The property of a rock to split along a regular set of sub-parallel, flat planes.

Which types of met. rocks typically exhibit cleavage?

Fine-grained, low grade rocks such as slate and phyllite.

What is schistosity?

Preferred orientation of mineral grains or grain aggregates produced by metamorphic processes.

What type of mineral grains are associated with schistosity?

Coarse grained minerals.

What is the fabric of schistosity like?

Planar and more wavy.

Give an example of a rock that exhibits schistosity.

Schist.

what are non-foliated met. rocks created by?

contact metamorphism i.e. granite intrusions

non-foliated met. rocks

• granofels

• hornofels

granoblastic texture

equidimensional grains with straight edged frequently forming at a triple junction at 120 degrees

porphyroblast texture

one or more met. minerals that grew much larger than other

gneissose texture

alternating dark and light mineral bands; coarse grained with layering or banding composed of alternating felsic and mafic minerals

augen texture

some gneisses, high grade met. rocks have large eye-shaped grains called 'auge'