Metamorphic facies

1
Metamorphic facies

• Chapter 14 and 18

2
Metamorphic terranes

• Geologic setting where metamorphism occurs
• Tracts of exposed, distinct rock from adjacent
  rock

3
Metamorphic terranes

• Ocean-ridge metamorphism
• Seen in ophiolites
• Lots of metasomatism
• Hydration of minerals, adding Na

4
Metamorphic terranes

• Regional metamorphism
• Most common in cont. crust
• Deformation with contractional stress, P and T up
  
• Alps, Appalachians, Caledonides

5
Metamorphic terranes

• Burial metamorphism
• Thick sed basins with T up leading to
  recrystallization

6
Metamorphic terranes

• Contact metamorphism
• Intrusions bake surrounding coutnry rock, fluids
  too
• Skarns form here

7
Metamorphic terranes

• Ductile shear zones
• Brittle at surface, ductile at depth
• San Andreas fault, Brevard fault

8
Metamorphic grade

• Equilibration T during metamorphism, P not
  restricted
• Mineralogy establishes grade
• Low grade or high grade with most hydrous
  minerals?
• For met. grade, consider rocks of similar bulk
  comp.
• Typically low, medium, high grade

9
Metamorphic zones

• Zone defined by distinctive fabric or mineral
  assemblage with similar protolith
• Sequence of zones shows gradient of metamorphism
• If correspond to a thermal gradient, basically
  same as met. Grade
• Zones based on fabric are different

10
Metamorphic zones

• In skarns
• Forsterite zone, monticellite zone, vesuvianite
  zone, garnet zone
• Each mineral is an index mineral marking a new
  zone

11
Metamorphic zones

• Pelites in Scotland
• Zones IDed by first appearance of index mineral
• Chlorite zone, biotite, garnet, staurolite,
  kyanite, sillimanite (ascending zones)
• Mapped by isograds a mappable line marking 1st
  appearancce of mineral

12
Metamorphic facies

• Comparison of suites of mineral assemblages
• Looking at changes of met. conditions in diverse
  rock types
• Compare met. conditions of pelites with mafic
  rocks
• Facies used to contrast P-T conditions of
  metamorphism

13
Metamorphic facies

• See Table 14.2
• Greenschist, amphibolite, granulite, blueschist
  facies
• Minerals expected in different rock compositions.

14
Metamorphic facies

• High P/T series subduction zones
• Medium P/T series regional met.
• Low P/T series contact met.

15
Key terms

• Metamorphic zone linked to minerals
• Metamorphic facies linked to P-T conditions and
  composition independent

16
P-T-t paths for common tectonic scenarios
17
P-T-t paths for common tectonic scenarios

• Crustal thickening
• Shallow magmatism
• Underplating of basalt, magma generation at base
  of crust

18
P-T-t paths for common tectonic scenarios

• Crustal thickening
• Shallow magmatism
• Underplating of basalt, magma generation at base
  of crust

19
P-T-t paths for common tectonic scenarios

• Crustal thickening
• Shallow magmatism
• Underplating of basalt, magma generation at base
  of crust

20
Metamorphic field gradient

• Series of P-T-t paths in one area
• Link maximum T from each loop, form metamorhpic
  field gradient

21
Metamorphic field gradients
22
P-T-t path exercises
23
Mafic metamorphic rocks

• Protolith basalts
• Start with few minerals, not much variety after
  met.
• Plag. amphiboles common
• Greenscist facies epidote, chlorite, actinolite,
  plag, quartz, calcite, titanite common

24
Mafic metamorphic rocks

• Amphibolite facies
• epidote and chlorite down
• Amphibole and garnet up

25
Mafic metamorphic rocks

• Granulite facies
• Hornblende reacts to form pyroxene
• At high P
• Blueschist facies with glaucophane
• Eclogite facies omphacite (px) and garnet, no
  plag

26
Expected minerals in mafic rocks for different
met. facies
27
Mineral changes in medium P/T series, mafic rocks