Metamorphism

metamorphism

mineralogical + structural changes in solid rocks due to differing conditions from which they originated

controls on metamorphism

• P

  • lithostatic load from overlying rocks

• T

  • geothermal gradient up T w/ up depth

conduction

flow of heat in the mantle

convection

fluid circulation (hydrothermal)

advection

intrusion of igneous bodies

metamorphic environments

• regional

• contact

• hydrothermal

• impact

• fault-related

regional

• caused by crustal thickening + burial/heating

• accompanied by deformation

• slow reaction time + recrystallization

contact

• contact aureoles from around igneous intrusion

• T driven

• don’t always involve deformation

• shorter timescale

hydrothermal

• w/ regional + contact, near MORs

• T/P gradients drive fluid flow

• mobilizes metals

impact (shock)

• short timescales

• P from impact itself

• T from friction

fault-related

• brittle/ductile depending on depth

• intense deformation allows new minerals

• associated w/ hydrothermal

classification scheme

• mineral assemblage

• texture

• protolith

textures

• slate

• schist

• gneiss

• mylonite

• migmitite

• hornfel

slate

w/ cleavage b/c of ica orientation

schist

layers of mica/amphibole that cleaves along foliation

gneiss

• coarse-grained w/ cm-scale layering

• light/dark banding

mylonite

fine-grained formed b/c of deformation in shear zones

migmitite

• formed from partial melting

• characteristic light/dark layers

hornfel

lacks obvious deformation features

mineral assembleges

• minerals that stably coexist under 1 set of P + T conditions

• ultimately determined by protolith composition

• preserve peak metm. conditions

high temperature pelite composition

• cordierite

• garnet

• sillimanite

• K-feld

• plag

• quartz

high temperature mafic composition

• opx

• cpx

• amphibole

• plag

• quartz

types of phase diagrams

• petrogenetic grids

• compatibility diagrams

• pseudosections

petrogenetic grids

P-T projections → groups of rxn lines

compatibility diagrams

linear/triangular compositional diagrams

pseudosections

show mineral stability @ specific rock compositions (most useful)

varience

degrees of freedom in a chemical system → # of independent variables that can be changed w/out altering the # of phases in a system

• P/T diagrams only allow 1 variable to change @ a time

the phase rule

V = C - P + 2

variance = components - phases + 2

component

basic chem building block that defines mineral + rock composition

schreinemakers rules

1. no mineral can exist in a greater than 180º arc

2. a reaction will stop at an invariant point

   1. only one end of the reaction that passes through an invariant point is stable

   2. the metastable extension of a reaction that lacks a phase lies between rxns that produce the phase

compatability diagrams

for fixed P-T conditions but chnaging chemical comps

prograde rxns

cause dehydration (rocks lose water w/ heating) → peak assemblage cannot be undone w/ retrogression

isograd

mapped line between metm. zones representing first appearence of an index mineral in rocks of similar composition

Courtney

The hottest bitch alive

barrow zones

chlorite → biotite → garnet → staurolite → kyanite → sillimanite

high P - low T

• blueschist → eclogite

• assoc w/ subd + early mountain building

• from magmatic heat input/crustal thinning

  • often subduction

  • glaucophane → omphicite

subduction metm.

• cold zones

• cause partial melting from dehydration rxn in subducting slab

ultra high P metm.

• rocks w/ coesite

• w/ micro diamonds

• represent subducted + exhumed slabs

Low P - high T

similar minerals to contact metm but w/ deformation

buchan zones

biotite → cordierite → andalusite → sillimanite

causes of Low P-High T

• lithospheric thinning

• loss of mantle lithosphere

• magmatic heat b/c of depth

• high internal heat production from radiogenic elements

metamorphic fluids

• mobile phase exhibiting open system behavior

• produced by heating

• host to ionic species

• from breakdown of hydrous mins → those w/ OH in structure

• released during prograde phase

fluid production + loss

• below solidus hydrous mins break down → release water

• above solidus minerals melt + add water to melt

why does metm. dehydrate mins?

• w/ higher P little space for free water in pores

  • water moves up in crust to area of lower pressure

retrogression

• requires the readdition of water into the system

• occurs at low T

• usually finer grained

• usually in gradient across rock as fluids are distributed unevenly

mixed fluids

• CO2 and H2O

• intrudes a new degree of freedom into the system

mixed fluid factors

• comp of rock

• comp of fluid

• amount of fluid influx

• rocks w/ hydrous mins + carbonates

external buffering

rock w/ infinite external fluid

• mineral assemblage + compositions change to be in equilibrium w/ fluid

• any fluid released by rock to small to change overall fluid composition

internal buffering

mineral assembleges, modes, comps control composition of the fluid in pores

• fluid only exists w/in pore spaces

• any fluid released changes overall fluid comp

fluid-present melting reactions

fluid-absent melting reactions

A + B(hydrous) = liq + E

fluid-present melting reactions

A + B + h2o = liq + (D)

peritectic phases

mineral phases produced during a melt-producing rxn

rhestitic phases

mineral phases left after melt is produced

metatexites

migmatites w/ preserved protolith features

• form layered structure w/ alternating felsic + ferromagnetic layers

diatexites

no original features preserved

• contain rafts of metatexite

melt movement

• above 8% melting melt can migrate + be lost from rock

• controlled by fault systems

• causes crustal differentiation

porphyroblast-matrix relationships

• pre-tectonic

• syn-tectonic

• post-tectonic

retrograde textures

• incomplete retrogression of peak phase

• islands of original mineral (peak) preserved

• retrogressive pseudomorphs

granoblastic

very similar to an igneous texture

pseudomorph

new mineral mimicks shape of original mineral

symplecite

2 integrated minerals replacing a previous one together

spiral inclusion trail

reflect rotation of porphyroblast during deformation