Lecture 18

Metamorphic facies

Metamorphic facies

A set of metamorphic assemblages that are indicative of a specific P-T condition of the rock formation process

why are facies named based on basalt subject to different P-T conditions?

Basalts are wide spread common and the mineral changes are limited

Metamorphism of basalt

If a basalt is metamorphosed to an eclogite, it will fall under the eclogite facie

Hornfels

A metamoprhic rock formed by the contact between a rock and a hot magmatic body and represents the altered equivalent of the original rock

• no foliation and interlocking texture

Blueschist

Rock with a blueish colour due to glaucophane

Eclogite

Plagioclase free metamorphic rock mainly pyroxene garnet (christmas tree rock)

Green schist

A rock whose greenish colour is due to chlorite actinolite and epidote

Amphibolite

Gneissose granofelsic metamorphic rock composed of green or black amphibole and plagioclase

Granulite

Composed mainly of feldspars quartz and lots of OPX CPX with lots of garnet

Skarns

A metamoprhic rock that forms by chemical metasomatism of rocks during metamorphism in the contact zone of magmatic intrusions like granulites with carbonate rich rocks like limestone or dolostone

What are Skarns closey associated with?

Granitic intrusions

-generate Late stage fluids rich in silica incompatible elements and halides

Metasomatism

The reaction of carbonates like limestone or dolostone producing alteration

• Calc silicate rock

What is the common mineralogy of skarns?

pyroxene, Garnet, wollastonite, actinolite, magnetute hematite epidote

How are skarns formed

Formed from incompatible element rich siliceous aqueous fluids, uncommon minerals are found here

Why are skarns target for mineral exploration?

The formation of incompatible minerals create rare and distinct minerals that are worth money

• related to economic minerals like gold iron tin zn pb

What is the pinchi fault lake area

Separates the early Mesozoic Takla Group to the east from the late Paleozoic Cache Creek Group to the west. Between these regions a complex fault system involves a series of elongate fault-bounded blocks of contrasting lithology and metamorphic grade

Dora Maira Italy

Coesite bearing rocks that are ultra high pressure that were cretaceous time

What is rock cleavage

Rocks cleave describe the tendency of a rock to break along parallel or sub parallel surfaces. It reflects the textural alignment of mineral grains

Slaty cleavage

Low metamoprhic grade growth of fine chlorite and clay minerals

What axial plane does cleavage align to?

normal to the major compressive stress responsible for folding

What is a porphyroblast?

Large mineral growth with a mass of smaller minerals are always euhedral

Pokiloblastic

Porphyroblastic intrusion that contains small abundant inclusions

pressure shadow

Halo of minerals surrounding a porphyroblast which differs from the matrix commonly find quartz in it

Rotated porphyroblasts

Are pretechtonic, the fabric greatly alters the poikiloblast

What is a migmatite?

At the frontier between igneous and metamrphic rocks under extreme temperature conditions during prograde metamorphism

Leucosome

Component of migmatic rocks which is a new mineral that crystallized from melting within the darker coloured amphibole

Ptygmatic folds

Tightly incoherent folds, where the lighter part of the rock is the leucosome and the darker is melanosome

-from due to highly ductile deformation and have no defined foliation

Cataclasis:

Deformation of the rock caused by fracture and rotation of mineral grains

• in the brittle shear zone of faulting

Mylonite

Ductile process where grain size reduction has occured during intense shearing and deformation

• occured in deep fault zones and crystal plastic deformation has occured

Protomylonite

A mylonite that less than 50% of rocks have undergone grain size reduction

porphyroclast

A large mineral crystal in finer grained matrix where the clast is older than the matrix

• occurs in rocks that have undergone shearing

What are the characteristic features of mylonite?

• Ductile shearing

• grain size reduction

• strong banding ribbon textures

• larger crystal grains called porphyroclasts

• forms eye shaped augen due to development of pressure shadows during rotation of crystals.

Fundemental principle of the influence of fluids

The mineral assemblage of metamoprhic rocks reflect the physical conditions (P-T) at the time when the rock formed

Equilibrium of P-T exposure to rocks

If we maintain these conditions for long enough the system will become the most stable configuration and will become equilibrium

What is the most stable mineral configuration?

The configuration with the lowest gibbs free energy

Syntechtonic porphyroblast

deformation occurs at the time of deposition and the pressure shadow is caught uo in the mineral being rotated