metamorphism

protolith

parent rock before metamorphism - main inference or interpretation based on the mineral assemblage of the metamorphic specimen

what are the 6 types of protoliths?

1. carbonates

2. quartzose

3. quartzofelspathic

4. pelitic

5. mafic

6. ultramafic

**can people maybe understand quartzose and quartzofelspathics?

carbonates

• minerals: calcite, dolomite

• elements: Ca, Mg, C, O

• paleoenvironment: shallow, warm, marine, carbonate compensation depth (below this, calcite dissolves in colder water)

quartzose

• minerals: qtz

• elements: Si, Fe-oxides

• paleoenvironment: beach, passive margin or inland seaway

quartzofeldspathic

• minerals: qtz, k-spar, plag

• elements: Si, Fe, Al, Na, K

• paleoenvironment: continental arc, convergent boundaries (high Mg = high T, high Fe = low T)

pelitic

• minerals: shale-rich parent (clay minerals)

• elements: SiO2, Al, Mg, Fe, OH, Na, K

• paleoenvironment: deep, still water (off continental shelf, glacial lakes)

mafic

• minerals: Ol, Cpx, Opx, Pl

• elements: Mg, Fe, Ca, SiO2

ultramafic

• minerals: Ol, Pl, Cpx, Opx

• elements: Mg, Ca, Al, Fe (lower in Si than everything except carbonates!)

what helps us to diagnose the protolith?

assemblage of minerals! AND P/T conditions, timing, fluid flux

the protolith becomes a metamorphic rock that now consists of…

• a change in composition

• different mineral assemblage

• new or old minerals have new orientations

contact metamorphism

high T but no change in P, recrystallization

regional metamorphism

orogeny, directed P (strain and shear)

shock metamorphism

bolide impact (high P reached)

hydrothermal metm.

change in fluid present, development of clays, economic minerals

burial metm.

low grade T, directed P

what is the metm. facies diagram used for

linking rock type to formation environment… only used for metabasites

metabasites

start with a basalt (3000 kg/m³)

what are relict minerals?

minerals from protolith that have survived metm. event (can have a changed shape), while others from original rock have changed

what mineral do metm. rocks often have?

amphiboles!! which vary (appear in different facies) depending on PT conditions

what happens in retrograde metm.?

overprint lower grade minerals begin to replace or surround higher grade ones as P/T conditions lower

EX: rxn rims (observation) tell us there’s been >1 metm. event (interp.)

EX: if you see both high and low grade minerals, the high grade likely formed first and then transitioned to lower grade as metm. grade lowered… or maybe a relict mineral from a lower grade is still present if the metm. grade isn’t too high for it to have survived

when is the only time we can use the metm. facies diagram?

if we can identify the stable mineral assemblage for metabasites

petrogenetic grid for metabasites

shows rxns whose P-T conditions were experimentally determined

rxn boundaries match up with boundaries in metm. facies diagram/different minerals on either side of rxn line show changing PT conditions… “Chl out” moves you into amphibolite facies

how was the metm. facies diagram made?

in the field… general relationships! linking rock type to formation environment

dehydrating = ?

hydrating = ?

dehydrating = prograde

hydrating = retrograde

what is a type of low P/T metm?

what is a medium P/T metm. event?

orogenic events!

where does high P/T metm. occur?