Metamorphism Flashcards
Define metamorphism
Changes in the mineralogy and/or texture of any pre existing rock in response to changing conditions (usually burial/heating). It is isochemical
What changes may be seen during metamorphism using micro and macro observation?
Mineralogical, textural and development of new fabrics in response to deformation
What are the controls of metamorphism? (7)
Temperature, T
Pressure, P (measured in kbars)
Stress, S (directed pressure, usually from plate collisions)
Fluids, F
Time, t
Composition X
Tectonic processes
What is metamorphic grade?
The intensity of metamorphism in response to the degree of pressure and or heat applied
Draw the graph representing the rock type in relation to depth and heat as seen on page 2
What are the results of the metamorphic process? (5)
Destruction of fossils, beds and sedimentary structures
Hardening of the rock
Change in colour
Alignment of minerals
Growth of new metamorphic minerals
Is burial metamorphism attainable in the crust?
No
What is the approximate geothermal gradient with depth?
25°C km-1
When are hornfels formed?
During contact metamorphism at shallow depths
II - What happens to a rock during metamorphism? (7)
Porosity reduction
Dehydration - hydration
Compaction and volume reduction
Grain growth (diffusion, nucleation, growth)
Grain size increase
Chemical reactions
Destruction of primary fabrics
Explain the development of a metamorphic rock from fresh sediment on the ocean floor (P5)
Fresh sediment is deposited in the ocean floor. It undergoes COMPACTION and DIAGENESIS including PORE WATER EXPULSION, DEHYDRATION, POROSITY REDUCTION (including pressure solution between grains)
BEYOND 200°C the rock undergoes RECRYSTALLISATION. quartz recrystallises into new shapes, quartz growth and mica growth occurs and existing grains undergo DISSOLUTION.
The pressure and heat causes ENHANCED DIFFUSION RATES and therefore GRAIN GROWTH
a GRANOBLASTIC POLYGONAL MATRIX develops. There may be PORPHYROBLAST GROWTH including andalusite or garnet.
overall there is VOLUME REDUCTION.
Describe the fluid release curves in regards to mudstone and basalt
Describe the development of granoblastic fabrics
Under higher pressure and temperature conditions, there is any greater rate of ion diffusion and solution within rocks. Some of these ions begin to nucleate at nucleation sites between crystals, growing outwards until they come into contact with other crystals resulting in an interlocked, equidimensional texture. This texture characterised by 120° triple junctions is called granoblastic (grains) polygonal texture
Describe the formation of porphyroblasts/poikiloblasts
Mica and quartz can recrystallise into garnet and H2O under the right conditions. White mica and quartz breakdown and migration of ions occurs along the grain boundary - this is GRAIN BOUNDARY DIFFUSION. Inclusions of quartz (when in excess) remain in the centre of the garnet. Over time, slow growth and volume diffusion occurs. There is a depletion halo within the garnet
Which ions correspond to which colour in a garnet?
Ca - green
Mg, Fe - red
Ma - Orange
Give two examples of contact metamorphism in Britain
Rinsey Cove, Cornwall
Skiddaw, Lake District
Describe the qualities of spotted rock
Medium gray ground mass with dark grey spots
Fine grained <1mm
Mica, clays and cordierite composition
Spotted texture
In what grade of metamorphism are spotted rocks found?
Low grade
What is a baked margin?
A zone a few mm in width surrounding an igneous intrusions where a portion of the country rocks have been metamorphosed due to the intense temperature of the intrusion.
What is the term for the alteration of rocks by the action of hot fluids?
Metasomatism
Explain the role of granite in metasomatism.
As granite in an intrusion cools it releases fluids. These fluids permeate the surrounding rock, which causes mineralogical changes or growth of new minerals. Fluid from the granite may also contain metal ions and form metal rich veins and larger metallic deposits or “skarns”
How are hornfels formed during contact metamorphism?
When the country rock surrounding an intrusion is made up of shale, the rock subjected to the highest grade contact metamorphism (the part of the aureole closest to the intrusion), the shale is recrystallised into a hard, fine grained, splintery, granoblastic rock called hornfel.
How does andalusite form?
During medium grade metamorphism of shale. The shale is partially recrystallised and clusters of andalusite porphyroblasts formed, forming an andalusite slate
How are spotter rocks formed?
In low grade contact metamorphism of shale, in the outer part of the aureole. Temperatures are low and cause the growth of dark minerals in spots - which can be biotite mica, carbon or iron.
What is the cleavage of calcite?
120°
What type of grain boundaries may be found in quartzite?
Sutured grain boundaries
From what rock does quartzite form?
Orthoquartzite
From what rock does marble form?
Calcite limestone
Describe the formation of quartzite from a sandstone
Sediment is buried leading to increasing pressure and heat. The quartz grains in orthoquartzite sandstone recrystallise and the boundaries are deformed (sutured appearance) as they undergo pressure dissolution and are compacted against each other
Describe the formation of marble from a limestone
Under very high T and P calcitic limestone undergo grain coarsening of calcite crystals. These recrystallise to form an interlocking, equidimensional granoblastic marble which is white in colour. Any fossils in the limestone are destroyed
Describe the qualities of quartzite
White/grey
Granoblastic, 1-3 mm grains (medium£
Grain size increases w/metamorphic grade
Quartz
Regional or contact
From quartz sandstones
Describe the qualities of marble
White
Granoblastic
1-3mm grains (medium)
Grain size increases w/metamorphic grade
Calcite
Contact or regional
Calcitic limestone
Describe the qualities of spotted rock
Grey, purple, green, black with darker spots
Slaty cleavage if slate is the parent rock
Fine grain size <1mm
Clay minerals, mica, poorly formed minerals - mica, andalusite; graphite in spots, iron, magnesium, carbon
Contact
Slate/shale, composed of some clay minerals, mica and quartz
How does metamorphic grade relate to grain size and foliation?
High stress causes foliations in clays and micas
High temperatures lead to coarser rocks
What are rocks subjected to regional metamorphism affected by? (5)
Low - high temperatures
Burial
Stress as a result of plate collision
Fluids passing through rocks
Time
What are the main three regional metamorphic rocks - low, medium and high?
Low - slate
Medium - schist
High - gneiss
Define foliation
The texture in metamorphic rocks formed by the preferred alignment of flat, platy minerals
Define porphyroblast
A large crystal thats has grown during recrystallisation in a metamorphic rock and is surrounded by a finer grained ground mass of other crystals
Define matrix
The finer grained mass of material in which larger grains/crystals/clasts are embedded
What is slaty cleavage?
The texture in fine grained rocks formed by low grade regional metamorphism. Platy minerals recrystallise perpendicular to the direction of stress so the rock splits into thin sheets
Schistosity
Texture in medium- coarse grained metamorphic rocks formed by the preferred alignment of flat/tabular minerals. The alignment is perpendicular to the direction of stress. No traces of the original bedding remain.
Gneissose banding
The segregation of light and dark coloured minerals into layers/bands at the scale of mm to cm. Light band is normally granoblastic whilst the dark band shows foliation.
Quartzite
An extremely compact, hard granular rock consisting essentially of quartz. Often occurs as silicified sandstone. They are free of pores and have smooth fracture.
What is granoblastic texture?
An equigranular texture where crystals adopt a polygonal morphology w/grain triple junctions of 120°
What is relict texture?
A texture or structure such as bedding or fossils present in the parent rock that is partially preserved in a metamorphic rock
What is an inclusion?
An early formed mineral enclosed by one that grew later
What textures will be included in post-tectonic/post-kinematic porphyroblasts?
The same as in the groundmass - the porphyroblast overprints the foliation
What textures will be included in syn-tectonic/kinematic porphyroblasts?
A snowball, or spiral texture. The fabric within the porphyroblast is continually rotated and regrowing.
What textures will be included in pre-kinematic/tectonic porphyroblasts?
A fabric overprinted by the second event. The porphyroblast includes the early fabric, but the matrix around it (within the pressure shadow- the area protected by the mass of the porphyroblast) may be deformed. The porphyroblast fabric may also show kinks, folds or boudinage (stretching that cracks the mineral)
What is a metamorphic aureole?
A large area around a batholith where rocks have been metamorphosed
What is country rock?
The rock into which an igneous rock has intruded, unaffected by metamorphism.
Define spotted rock
A rock formed by low grade metamorphism in the outer part of the metamorphic aureole
Define hornfels
A fine grained, hard splintery granoblastic metamorphic rock formed when shale is completely recrystallised close to the contact with an igneous intrusion
Define unfoliated
The random orientation of minerals in a metamorphic rock
What is crenulation cleavage?
A fabric formed in metamorphic rocks ljke phyllite, schist and some gneiss by two or more stress directions causing the formation of super imposed foliations
What are the sources of heat within the earth?
Residual heat from formation of earth, cooling
Breakdown/decay of radioactive sources - exothermic - U, Th, K which tend to be in feldspars
Tectonic processes
Igneous intrusions
How is heat transferred in the lithosphere?
Conduction
How is heat transferred in the mantle?
(Solid state?) convection
What is lithostatic pressure?
The pressure from weight of overlying rock
What is deviatoric pressure?
Directed pressure or stress from tectonic load
What is pore pressure?
The pressure exerted by fluid
What is the unit for pressure?
Kbars
Where does contact metamorphism occur?
Adjacent to intrusions
Where does regional metamorphism occur?
Linked to orogeny
What is orogeny
Mountain building
Where does burial metamorphism occur?
Burial in sedimentary basins
Where does dynamic metamorphism occur?
Local in brittle faults/ductile shear zones/shock metamorphism (impacts)
What temp and depth for sedimentation?
20 - 100 °C
0 - 5km
Temp and depth for diagenesis
100 - 200 °C
5km
Temp and depth for metamorphism
200 - 650°C
10-30km
Temp and depth for partial melting
650°
35-45 km
Temp and depth for magmatism (igneous processes)
800°C+
50-100km
Describe the effect of temperature on metamorphism
- High temps near to igneous intrusions where magma hears surrounding rocks
- Temp increases w depth and the thermal gradient
- temp increases, rate of metamorphic reactions increases as activation energy is met and rate of ion diffusion is higher
Sped up by water which helps ion diffusion
What are the common minerals found in metamorphic rocks?
Biotite, muscovite, quartz, hornblende and some feldspars and occasionally garnet
What is migmatite?
A metamorphic rock that melts to form igneous rock
What occurs during dehydration?
Rocks dry out as water leaves through veins or permeates the rock, evidenced by veins cutting through the rock
Give six examples of minerals that contain water in their chemical structures
Chlorite
Gypsum
Talc
Muscovite
Hornblende
Biotite
What % of Muscovite is water?
6
What % of hornblende is water?
4
What % of biotite is water?
2.5
In regards to fluid release, what happens to igneous rocks during metamorphism?
They do not contain water at the start. Therefore they absorb water and the minerals are converted to hydrated minerals. These then break down and release the water.
What is the role of fluid in metamorphism?
They are an important transport medium for diffusion
They transport ions around the rock to growing minerals
Diffusion is at the greatest rate when fluid is available
Where does diffusion primarily occur at lower temperatures?
At grain boundaries
Where does diffusion occur at higher temperatures?
Through the whole crystal lattice of the rock
Where does nucleation begin in the Crystal lattice and what are the original grain shapes?
Weak points in the lattice
Spheres
Why are minerals that crystallise at greater depths denser?
Pressure increases causing volume reduction e.g. basalt recrystallises into eclogite
What is a poikiloblast?
A porphyroblast that is full of inclusions
What are the exposed structures of a batholith called?
Stocks
What feature indicates a metamorphic rock has not formed during regional metamorphism
Random orientation of minerals or porphyroblasts due to a lack of stress
What is a saccharoidal texture?
Sugary texture
What is pegmatite?
A type of coarse grain granite or other igneous rock
What minerals could be found as porphyroblasts in metamorphic rocks?
Garnet/Kyanite/Sillimanite
What is cataclasis?
The process where stress is rapidly applied to rocks - the mineral grains suffer brittle fracture - they suffer brittle fracture and are crushed and ground mechanically as the fault moves
What does cataclasis produce?
Variety of fabrics - coarse grained angular clasts in a fine grained matrix to almost entirely fine grained crushed particles
What process resulted in deformation rather than crushing
Gradual application of pressure or a long period of low strain rates
Pale layers in gneiss?
Granoblastic, quartz, plagioclase
Dark layers in gneiss?
Biotite, mafic minerals like amphiboles, pyroxenes, sillimanite, foliated
Where is regional metamorphism found?
Along orogenic belts
What rock types are found in Precambrian shield areas or cratons?
Gneisses, granites, schists
What type of rocks are found in young fold mountain belts?
Slates, schists, gneisses, granites
What are the three main stages in development of an orogenic belt?
Sinking - subsidence and deep burial of rocks
Compression and thickening - metamorphism, compression and folding
Uplift - uplift into mountain chain, erosion
What three factors control the mineralogy of a rock?
Pressure
Temperature
Composition
What are the orientations of foliations?
Parallel to axial planes of the folds
Perpendicular to the direction of pressure
What mineral forms the shape fabric in slate
Elongate quartz grains
What is phyllite formed from
Metamorphosed pelite mudstone
Qualities of phyllite
Medium grained
Rich in chlorite, micas
Doesn’t split as easily - grain coarsening
Wavy foliation often deformed by microfolds (looks like corrugated iron)
What is schist formed from
Metamorphosed pelite mudstone
What are the lenses in migmatite
Lenses and layers of granitic material - possibly formed by partial melting
What is the progression of rock formation with increasing metamorphic grade (5)
Slate
Phyllite
Schist
Gneiss
Migmatite
