Metamorphic Rock Textures and Relationships

Flashcards about metamorphic rock textures and relationships.

What does the diagram from Best (1982) show about structural and fabric elements in metamorphic rocks?

Structural and fabric elements are generally consistent in style and orientation at all scales.

At what scales can textural relationships among different metamorphic minerals and porphyroblasts be observed?

Grain contact scale (inter-grain) or within grains (intra-grain).

What can fabrics and mineral assemblages be related to, respectively, in the metamorphic history of a rock sample?

Relating fabrics to deformation history and mineral assemblages to metamorphic history.

What are the three main types of foliations that can be found in metamorphic rocks?

Primary, diagenetic, and secondary.

What is a primary foliation related to?

Related to the protolith, e.g., sedimentary bedding, magmatic layers

How is diagenetic foliation formed?

Formed by compaction of minerals during diagenesis

How is secondary foliation formed?

Formed as a result of deformation and metamorphism (includes cleavage, schistosity, mylonitic fabrics etc.).

What are the two main classifications of foliations based on mineral distribution?

Continuous and Spaced.

What characterizes continuous foliation?

All minerals are homogeneously distributed and aligned

What two domains do rocks with spaced foliations contain?

Cleavage domains and microlithons.

What term do we use to describe the relationship of porphyroblast growth to the fabric?

Kinematic.

What are the three classifications of porphyroblasts based on their timing relative to fabric formation?

Pre-kinematic, syn-kinematic, and post-kinematic.

When does a pre-kinematic porphyroblast grow?

Grows before the fabric forms.

When does a syn-kinematic porphyroblast grow?

Grows during fabric formation.

When does a post-kinematic porphyroblast grow?

Grows after fabric formation.

What key textural features are observed in the analysis of deformed rocks (Figure 23.42)?

Asymmetric crenulation cleavage (S2) developed over S1 cleavage. S2 is folded.

What is paragenesis?

The evolution of the mineral assemblage characterising a given rock (reaction history).

How is mineral parageneses defined?

The succession of mineral assemblages that follow and replace one another during the metamorphic evolution of a given terrain.

What is an equilibrium mineral assemblage?

The group of minerals that make up a rock at equilibrium

What are some general recognitions of equilibrium in mineral assemblages?

Minerals are all in contact and idioblastic, no compositional zoning, no pseudomorphs, no reaction textures.

What characterizes disequilibrium textures in metamorphic rocks?

Reactions not gone to completion, minerals not in contact, compositional zoning and inclusions, reaction textures, coronas and pseudomorphs.