Metamorphic rocks Flashcards
What creates metamorphic rocks?
Heat and pressure/just heat causes a change in shape and form of a rock
What type of chemical reaction is metamorphism?
Isocheimal reaction
What are 5 contact metamorphic rocks?
- Metaquartzite (Parent rock = Orthoquartzite)
- Marble (Parent rock = Pure limestone)
- Low grade: Spotted rock (Parent rock = Clay/Mudstone/shale)
- Medium grade: Andelusite (Parent rock = Clay/Mudstone/shale)
- High grade: Hornfels (Parent rock = Clay/Mudstone/shale)
What is granoblastic texture?
The heat makes grains/clasts weld together. The crystals recrystallise creating sparkly/sugary texture.
What are porphyroblasts?
When new crystals grow within a metamorphic rock when it undergoes metamorphism
What are contact and regional metamorphic rocks?
Regional = Both heat + pressure
Contact = Just heat
At what temperature do rocks melt and become igneous rocks?
Around 900 degrees
What is the minimum temperature is required for a metamorphic rock to form?
200 degrees
What is a metamorphic aureole?
Altered zone around an intrusion. Around 10% of the width of the intrusion.
What causes a wider aureole?
- Gently dipping intrusions
-Volume of intrusion - Magma temperature
- Composition and structure of country rock
Why may a metamorphic aureole be larger at wet sandstone country rock and dry sandstone?
Magma from the intrusion will heat the wet sandstone creating steam, which then moves the heat creating more metamorphic rock.
In mudstones, what changes when it undergoes regional metamorphism?
They have a foliation (Orientation of long minerals) which line up when they undergo regional metamorphism.
Define grade
Grade = the size of the crystals in a metamorphic rock
What are 5 regional metamorphic rocks?
- Metaquartzite (Parent rock = Orthoquartzite)
- Marble (Parent rock = Pure limestone)
- Low grade: Slate (Parent rock = Clay/Mudstone/shale)
- Medium grade: Schist (Parent rock = Clay/Mudstone/shale)
- High grade: Gneiss (Parent rock = Clay/Mudstone/shale)
What is schistosity ?
At high temperatures and pressure (Medium Grade) it creates schistosity (porphyroblasts of shiny mica crystals) May also get garnet porphyroblasts.
What is gneissose banding?
High grade. Is the seperation of light and dark bands of minerals.
If the gneiss gets above a certain temperature where it starts to melt, it is called migmatite.
Describe the characteristics of marble/metaquartizite
Granoblastic texture (Sugary)
Recrystallised
Monominerallic
Sandstone = metaquartzite
Limestone –> marble
contact or regional
Bigger crystals = higher grade
Describe the characteristics of spotted rock
Low grade (small crystals)
Clumps or iron/carbon (are the spots)
Clay/mudstone = Spotted rock
Granoblastic texture
Contact
Complex aluminium silicate
Sedimentary structures preserved
> 200 degrees
Describe the characteristics of andelusite slate
Medium grade (> 400 degrees)
Porphyroblasts of andelusite
clay/mudstone = andeslusite slate
granoblastic texture
contact
same sedimentary structures preserved
Describe the characteristics of hornfels
Recrystallised
clay/mudstone = hornfels
High grade
granoblastic texture
contact
Hard
> 600 degrees
Describe the characteristics of slate
Low grade
Regional
Foliation due to pressure (salty cleavage)
small grain size
mudstone/shale - slate
Describe the characteristics of schist
Porphyroblasts of shiny mica/garnet (at higher temperatures)
Medium grade
regional
easily spotted
Schistosity foliation
Shale/mud/clay parent rock
medium grain size
Describe the characteristics of gneiss
High grade
Segregation of colours of dark and light bands (gneissose banding)
Wider bands = hotter temperatures
Regional
Made of felsic / mafic minerals
Large crystals
What are index minerals?
Minerals that indicate temperature and pressure
What are 5 index minerals?
Chlorite
Biotite
Garnet
Kyanite
Sillimanite
ONLY IN REGIONAL METAMORPHISM
What are polymorphs?
Minerals that have the same composition but occur in different crystal forms
What 3 polymorphs of aluminium silicate?
Depending on the pressure aluminium silicate can combine to form 3 minerals:
1. Kyanite
2. Andelsuite
3. Sillimanite
What graph do we use to see which minerals form at different temperatures for the polymorphs of AlSiO2
KAS pyramid with a wonky Ariel
Temperature increasing on the x axis
Pressure increasing on the y axis
What does phyllite show?
Crenulation cleavage
What are isograds?
A map that connects equal metamorphic grade, they join places of equal metamorphic grade
What is retrograde metamorphsm?
Where temperatures and pressure are reduced as the rock is returned to the surface and the reactions reverses.
Water and co2 are introduced. Creates a pattern of progressively lower grade metamorphism.
What causes metamorphic rocks to rise?
Mountains eroding, causing a reduce in pressure, making them rise
What conditions determine the nature of a metamorphic rock?
Strain rate - Slower strain rate allows rocks to respond in a ductile manner
Temperature - Higher temperature will allow rocks to behave in a ductile manner
Pressure - Higher confining pressure allow rocks to behave in a ductile manner
Presence of water - Enhances metamorphism, wet means they are more ductile
Means folding will occur if ductile, whereas brittle will cause faults
What is pressure solution?
Grain contact points are at high pressures, so a miner may dissolve from this place then get precipitated nearby where the pressure is lower (in a pore space). This turns a sedimentary granular texture into a metamorphic sugary/granoblastic texture.
What is boudinage stretching?
Made when a layer is stretched (at the crest of a fold) causing it to break into parallel cylinders that in cross-section look like strings of sausages.
What would the index mineral be of spotted rock?
Biotite
What would the index mineral be of Andelsuite “slate”?
Andalusite
What would the index mineral be of Hornfels?
Sillimanite
What are Barrovian zones?
Are metamorphic zones mapped using index minerals
What are relict structures?
Such as bedding, present in the parent rock, which is partially preserved in a metamorphic rock
What are metamorphic facies?
Is a set of metamorphic minerals that were formed under similar pressures and temperatures.
What is the Dalradian supergroup?
A regionally metamorphosed group of rocks that were deposited in the late Precambrian found in Scotland and Ireland.
What does inclusion mean?
The term used to mean an early formed mineral enclosed by one that grew later.
What are the three metamorphic fabrics that show metamorphic history?
- Pre-kinematic minerals - Formed BEFORE the deformation (eg mica crystals grew, then were folded or crenulated)
- Syn-Kinematic minerals - Grew at the SAME TIME as the deformation (eg garnet porphyroblast grew at the same time as the mica crystals, so has overgrown them, BUT MAY ALSO RECORD ROTATION DUE TO SHEAR FORCES
- Post-kinematic minerals - Grew AFTER the deformation (eg porphyroblast encloses earlier folds)
What are the 9 metamorphic facies?
- Zeolite
- Prehnite
- Blueschist
- Greenschist
- Eclogite
- Hornfels
- Amphibolite
- Sanidinite
- Granulite
Where does Eclogite form?
Under the crust (mantle)
what metamorphic facies are seen in contact metamorphism?
Hornfels and Sanidinite
What are paired belts?
Are parallel areas of regionally metamorphosed rocks that have experienced different pressure and temperature conditions.
For example, greenschist may form under volcanoes inside intrusions where temperature and pressure are both high, but next to it in the subduction zone where temperature is lower due to cold oceanic crust, but still high pressure, blueschist may form.
