Isograds and metamorphic facies Flashcards
How did Barrow try to classify metamorphic rocks?
Looked at pelitic rocks, saw systematic changes.
- Saw increasing grade by increase in grain size!
- Made metamorphic zones based on the appereance of new higher grade minerals
What are index minerals?
New Minerals that characterise a specific metamorphic zone
What are Barrovians metamorphic zones (pelitic protolith)? (Increasing grade)
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Kyanite zone
SIlliminate zone
What is the correlation between the minerals in the metamorphic zones?
They are crystalized under the same T and P conditions (same grade)
What is isograds?
They are the boundary between metamorphic zones.
Represents:
- lineof constant metamorphic grade
- The first occurence of an index mineral (upgrade)
What is required for a new index mineral to occur?
A metamorphic reaction has taken place
What does index minerals depend on and what does this mean?
Depends on the bulk composition of the protolith (+ P,T)
Means that different index minerals can occur depending on the bulk composition, even for same type of protolith.
- difficult to compare grades acros different protoliths (!)
Is each index minerals made from one specific metamorphic reaction?
No, they can be made from different reactions
- hard to know which one
What is a metamorphic facies?
(an answer to the problems with isograds incomparability)
Its a range of P,T conditions under which a particular common mineral assemblage is stable.
Defined by mafic protoliths
What is the advantage with facies over isograds?
They defined so protoliths with a wide range of bulk compositions is in the same facies
What is the main difference between isograds and facies?
Isograds focus on the appearnce of one mineral, while Facies focus on the appearance of a mineral assemblage
What are Eskolas metamorphic facies, and what are the related critical minerals?
Greenschist
- epidote + albite
Amphibole
- hornblende + plagioclase
Granulite
- orthopyroxene+ clinopyroxene
Eclogite
- pyrope (garnet) + omphacite(clinopyroxene)
Blueschist
- glaucophane + lawsonite
What other metamorphic facies is there?
- Zeolite facies
- Prehnite-pumpellyite facies
- Albite-epidote Hornfels facies
- Hornblende hornfels facies
- Pyroxene hornfels facies
- Sanidinite facies
What is mineral reaction isograds?
The diffuse boundaries (isograds?) between facies.
They represent a mineral reaction.
Why are the boundaries (mineral reaction isograds) so wide/ diffuse?
They have 2 or more degrees of freedom
- bc. occur over range of P and T
- bc. there can be different compositions (?)
