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