Earth Materials Lecture 18: Metamorphic Textural relationships Flashcards
Define Foliation
Foliation in geology refers to repetitive layering in metamorphic rocks.
Describe the different kinds of Foliation
Primary foliation – related to the protolith, e.g. sedimentary bedding, magmatic layers
Diagenetic foliation – formed by compaction of minerals during diagenesis
Secondary foliation – formed as a result of deformation and metamorphism (includes cleavage, schistosity, mylonitic fabrics etc)
Important to recognize these and reconstruct the structural evolution from S0 onwards (S1, S2 etc)
Describe what s0 s1 and s2 refers to
s0 refers to initial bedding/layers before any tectonic processes
s1 Refers to the first foliation or cleavage created through metamorphosis
S2 refers to the next foliation or cleavage
Describe Continuous and Spaced Foliations
Continuous foliation refers to rocks in which all minerals are homogeneously distributed and aligned
Rocks with spaced foliations contain two domains: CLEAVAGE domains and MICROLITHONS – if the latter contain an earlier cleavage, the term CRENULATION cleavage is used, if not the structure is DISJUNCTIVE cleavage/foliation
Define Kinematic
We refer to the relationship of porphyroblast growth to the fabric as “kinematic”
If a porphyroblast grows before the fabric forms, then it is pre-kinematic
If it grows during fabric formation it is syn kinematic
If it grows after fabric formation it is post-kinematic
Describe Pre Kinematic crystals
Bent crystal with undulose extinction
Foliation wrapped around a porphyroblast
Pressure shadow or fringe
Kink bands or folds
Microboudinage (crystal is compressed and elongated, then splits into smaller crystals)
Deformation twins
Describe Post kinematic crystals
a. Helicitic folds
b. Randomly oriented crystals
c.Polygonal arcs
d.Chiastolite
e. Late, inclusion-free rim on a poikiloblast (?) f.Random aggregate pseudomorphDe
Describe Syn kinematic crystals
Spiral porphyroblasts are syn kinematic crystals.
That are affected by the foliaton direction and have their shape altered by it.
Paracrystalline microboudinage is when Compressed and elongated crystals split into many parts and those many parts become seperate crystals.
Describe the terms for analysing Metamorphic rocks
Deformational events: D1 D2 D3 …
Metamorphic events: M1 M2 M3 …
Foliations: So S1 S2 S3 …
Lineations: Lo L1 L2 L3 …
Define Paragenesis
The evolution of the mineral assemblage characterising a given rock (reaction history)
Define the equilibrium mineral assemblage
Minerals that occur together at chemical equilibrium are said to comprise an equilibrium mineral assemblage.
Chemical equilibrium a state in which the rate of the forward reaction equals the rate of the backward reaction
Describe equillibrium and disequilibrium textures
General recognition of equilibirum:
-Minerals are all in contact and idioblastic
-No compositional zoning
-No pseudomorphs
-No reaction textures
Disequilibrium textures:
-Reactions not gone to completion
-Minerals not in contact
-Compositional zoning and inclusions
-Reaction textures, coronas and pseudomorphs
