Lecture Seventeen - Metamorphic processes and occurrences I (how to classify metamorphic rocks) Flashcards
What are the factors which need to be considered when classifying a metamorphic rock?
1) Colour.
2) Grain size.
3) Mineral content.
4) Texture.
5) Chemical composition of the protolith.
Describe foliation.
Foliated metamorphic rocks:
Foliation = repetition of planar surfaces or layers in a metamorphic rock.
Layers can be 1m thick.
Gives rocks a striped or streaked appearance and can cause them to split into thin sheets.
Occurs at right angles to the pressure exerted on the rock.
Occurs when preferential alignment of long thin minerals.
Foliation is not necessarily along the bedding plane.
What causes foliation?
1) Alignment of inequant minerals parallel to each other and/or
2) Rock has alternating dark and light layers.
Remember that the application of differential stress is required to cause foliation.
What is a progression of foliated metamorphic rocks at different T and P.
Slate:
Fine-grained, metamorphic rock with a slaty cleavage.
Low P and T metamorphism of shale (pelitic) protolith.
Foliation results from alignment of fine grained platy minerals (e.g. mica) such as clays or chlorite.
Slate splits of think slaty cleavage plants.
Slaty cleavage is caused by differential stress.
Cleavage planes form perpendicular to compression direction.
Cleavage can often be at a high angle to bedding.
Increase T and P …
Phyllite:
Formed by metamorphism of slate at temperature high enough to cause neocrystallisation 9forming new minerals) of chlorite and white mica.
The alignment (foliation) of the mica gives the rock a silky sheen - phyllitic lustre.
Increase T and P …
Schist:
Medium to course grained - courser grained than phyllite.
Has schistosity or a schistose foliation - alignment of large mica flakes (e.g. muscovite and biotite).
Occurs due to differential stress from shearing and shortening during metamorphism - mica flakes grow parallel to the foliation.
Start to see new minerals forming e.g. garnet, amphibole, kyanite, feldspar, quartz.
Some of these minerals can grow larger than surrounding minerals = porphyroblasts.
Smaller surrounding grains = matrix or groundmass.
Increase P and T …
Gneiss:
Layered metamorphic rock.
High T and P (high grade) metamorphism.
Alternating light and dark bands or lenses ranging from millimetres to meters.
Compositional layering - ‘gneissic banding’ - striped appearance.
Different coloured layers = different compositions.
Light coloured layers = felsic minerals (e.g. quartz and feldspar).
Dark coloured layers = Mafic minerals (e.g. amphibole, pyroxene and biotite).
Banding forms from:
- The original bedding of the protolith.
- From extreme shearing of the protolith (rock can flow) and the stretches, folds and smears out pre-existing compositional contrasts.
- From metamorphic differentiation, where in some layers, minerals dissolve, their constituents then move to another layers and new minerals grow. This creates segregation of mafic and felsic minerals in alternating layers.
Increase P and T …
Migmatite:
At very high T (or is hydrothermal fluids enter rock and allow it to melt at lower temperatures) -> gneiss can start to partially melt.
Melt produced is enriched in silica (high SiO2 %).
Sometimes, this melt doesn’t travel very far - forms a light coloured (felsic) igneous rock.
Rock is a mixture of igneous and relict (left behind, non-melted) metamorphic rock = migmatite.
Describe non-foliated metamorphic rocks.
Contain minerals that are recrystallised or new minerals that grew during metamorphism (neocrystallisation).
Lack of foliation is due to:
1) Metamorphism occurred without differential stress.
2) All the new crystals were equant (non flattened/elongated).
What is a progression of non-foliated metamorphic rocks at different T and P.
Hornfels:
Produced by heating of a rock without differential stress.
No foliation = crystals grow in random orientations.
Produces a dark sugary texture.
Protolith is commonly mudstone or sandstone.
Igneous intrusions often cause these rocks by heating clay rich country rock.
Increase P and T …
Amphibolite:
Metamorphism of mafic igneous rocks (basalt or gabbro).
A mafic protolith will transform into an amphibole - metamorphic rock with mail hornblende (a type of amphibole), plagioclase feldspar (and some biotite mica).
Poorly developed foliation as rock contains little mica.
Can start to see larger garnet porphyroblasts.
Increase P and T …
Quartzite:
Formed by metamorphism of quartz sandstone (mostly SiO2).
Pre-existing quartz grains recrystallise and make new, larger grains than are interlocking (no longer any empty pore spaces as in sandstone).
This metamorphic recrystallisation makes the grain weld together into a tight, hard matrix.
Quartz breaks/fractures through grains, sandstone breaks/fractures around grain boundaries.
Appears sugary.
Commonly white to grey.
Non-folliated - no aligned micas or compositional layering.
Increase P and T …
Marble:
Metamorphosis of limestone (mostly CaCO3).
Calcite (CaCO3) recrystallises, making original sedimentary features (e.g. fossil shells) hard to recognise.
Like quartzite, marble contains interlocking crystals (of calcite, not quartz).
Other minerals may be present if small amounts of quartz, clay and Fe-oxide were win the protolith.
No foliation - mostly equant grains.
Impurities like graphite or Fe-ocide can produce colour banding - original bedding in the limestone.
Graphite could be present due to fossils in the limestone - carbon from organisms converted to graphite.
How can metamorphic rocks be identified by the chemical compositions of the protoliths?
Chemical composition of protoliths:
Mudstone/shale:
Create pelitic metamorphic rocks.
Protolith = fine grained muds deposited in stable platforms or off shore platforms.
High proportion of Al, K and Si.
Contain many different salting minerals and their constituent parts.
Metamorphism produces Al-rich minerals like muscovite.
Shale -> Phyllite/Slate -> Schist -> Gneiss -> Migmatite.
Mafic (basic) igneous rocks -> Creates mafic metamorphic rocks:
Protolith = gabbro or basalt.
Little SiO2, high Fe and Mg.
Metamorphism produces a lot of biotite and amphibole.
Basalt -> greenschist -> amphibole -> granulite, (mountain belts).
OR
Basalt -> blueschist -> greenschist (subduction zones).
Name of the rock = The name of the metamorphic facies.
Ca-rich sedimentary rocks (carbonates) create calcareous metamorphic rocks:
Protolith = sedimentary limestones, dolostones, marls.
High in Ca, Mg and CO2.
Metamorphism of mainly calcite (CaCO3) - e.g. limestone.
Marble is most common metamorphic rock produced.
Starting impurities can produce striking colours.
Quartzo-feldspathic starting rocks:
Protolith high in silica (low SIO2), lots of quartz and feldspars.
Metamorphic rocks contain the same minerals - as quartz and feldspars are stable under high T and P.
Quartz crystals can flow plastically forming thin bands - foliation.
- Arkose protolith (feldspar-rich sandstones ~25% feldspar) -> paragneiss.
- Granitoid and rhyolitic protoliths (felsic igneous rocks) -> orthogneiss.
Quartz rich -> Create quartz rich metamorphic rocks:
Nearly pure SiO2.
Chert (oceanic) and sandstone (continental clastics) quartz roch protolith.
Forms quartzite - still nearly pure SiO2 (same as protolith).
Larger grain size with a sugary texture.
What minerals in a metamorphosed rock poport to which protoliths?
1) Micas, garnet, aluminosilicates (plagioclase, kyanite, andalusite, sillimanite, cordierite etc) = pelite (mudstone/shale).
2) Chlorite, amphiboles, pyroxenes = mafic rocks.
3) Calcite = limestone (carbonates).
4) Calcite, amphibole, clinopyroxene, garnet = ‘dirty/muddy’ limestone (marl).
5) Quartz, feldspar = felsic igneous rock or arkose (feldspar-rich) sandstone.
6) Quartz = quartz rich sandstone.
