Module 8 - Metamorphic Processes and Products Flashcards
What does Isochemical mean?
It means that no elements are added or removed, with the exception of volatiles such as water and carbon dioxide
What is metamorphism?
The isochemical process by which rocks are changed by either heat or pressure, or both heat and pressure.
What is the same for metamorphic rocks?
The chemical composition of the parent rock is the same as the metamorphic rock produced
How does metamorphism work?
The rock undergoes the very slow process of solid state recrystallisation without melting. Different temperatures and pressures cause new minerals to grow in rocks that have the same composition
What is the lower limit for metamorphism?
Between 200 and 150 degrees C.
What happens if the temperature is below 150 degrees C?
The changes in the rock are now part of diagenesis.
What is the upper limit of temperature for metamorphism?
Around 800 degrees C where melting occurs
What can the processes of metamorphism result in?
- destruction of fossils, beds and sedimentary structures
- hardening of the rock
- change in colour
- alignment of minerals
- growth of new metamorphic minerals
How does temperature affect metamorphism?
- High temperatures occur near to igneous intrusions, where the magma heats the surrounding rocks
- Temperature also increases with depth due to the geothermal gradient
What happens as temperature increases?
The rate of metamorphic reactions also increases. This is because many of the chemical reactions require heat to take place. It increases the rate at which ions diffuse between minerals, though it is still a slow process because the ions have to move through solid rock during metamorphism.
What speeds up the rate of metamorphism?
Water because it allows the ions to diffuse more rapidly
What are the three ways that pressure is applied to rocks?
- Pore pressure - pressure is exerted by fluids between grains in a porous rock. Water speeds up the reaction by acting as a catalyst
- Load pressure - the weight of overlying rocks and the physical bringing of minerals into contact with each other very long periods of time
- Tectonic stress - caused as the rocks undergo folding or faulting and very high pressures are exerted. Usually over short periods of time
How long does it take for metamorphism to occur?
Usually takes millions of years to occur
What are the different types of metamorphism?
- Contact
- Burial
- Regional
What is country rock?
The rock into which an igneous rock has been intruded
What is contact metamorphism?
It occurs adjacent to igneous intrusions, which increase the temperature in the surrounding country rock. On a local scale it produces a metamorphic aureole. Temperatures are generally high but pressure is low. As pressure isn’t a significant factor, the minerals are not aligned in contact metamorphic rocks
What is Burial metamorphism?
It occurs in conditions of medium to high pressure and relatively low temperature. It affects rocks deeply buried by the weight of overlying sediments. It also occurs in subduction zones where sea floor sediments and basalts are buried. Almost always contain the blue mineral glacucophane. They are blueshists
What is regional metamorphism?
Affects larger areas than contact metamorphism, extending over hundreds or thousands of square kilometers. Caused by low to high temperature and low to high pressure at convergent plate margins. Results from either subduction or continental collision. Minerals have preferred alignment. They occur in the core of fold mountain belts where mountain ranges have been eroded
What is foliation?
It is the texture in metamorphic rocks, formed by the preferred alignment of flat, platy minerals
What is slaty cleavage?
It’s a texture bin fine grained rocks formed by low grade regional metamorphism. Platy minerals recrystallise perpendicular to the direction of stress applied during metamorphism so that the rock splits into thin sheets
How are all metamorphic rocks formed?
From parent rocks, the original rocks that existed prior to metamorphism. The composition of these rocks affect the mineralogy of the meatamorphic rocks so they have a likeness
Which type of metamorphism produces foliated rocks?
Regional metamorphism.
They have been affected by pressure, to some degree. Any Platy minerals they contain take on preferred alignment known as foliation
What is the most common Platy mineral and what does this mean most foliated rocks have as a parent rock?
Clay and so they have shale as a parent rock
What are the foliated rocks produced by regional metamorphism?
Slate, schist and gneiss
Describe shale?
- composed of clay minerals and fine quartz particles
- rich in aluminum because clay minerals are rich in aluminum and slate is mainly composed of clay minerals and micas
- chlorite and quartz may also be present
- fine grained <1mm diameter
- shows slaty cleavage
- traces of original bedding may still be preserved as relict bedding
Describe Schist
- shale parent rock
- produced by higher temperatures and pressure than slate was
- medium grained - 1-5mm
- crystalline
- can occur in a variety of colours
- always has a shiny appearance where the flat surfaces of muscovite and biotite mica crystals are visible
- typically composed of mica and garnet
- garnet often forms large crystals called porphyroblasts
- mica crystals are all aligned at right angles to the maximum pressure
- has a schistosity texture
Describe Gneiss
- formed by the highest temperatures and pressures during regional metamorphism
- coarse grained >5mm
- crystalline rock with gneissose banding
- typically composed of quartz and feldspar in the light bands and biotite mica (and other mafic minerals) in the dark bands
What is a porphyroblast?
A large crystal that has grown during recrystallisation in a metamorphic rock and is surrounded by a finer grained groundmass of other crystals
What is schistosity?
The texture in medium and coarse grained metamorphic rocks formed by the preferred alignment of flat/tabular minerals. The alignment is perpendicular to the direction of stress applied during metamorphism. No traces of original bedding remains
What is gneissose banding?
The segregation of light and dark coloured minerals into layers or bands at the scale of mm to cm in thickness. The light band is normally granoblastic (granular) and the dark band normally shows schistosity
Slate fact file
- colour - grey/purple/green/black
- texture - salty cleavage, fine grain size (<1mm)
- mineral composition - clay/muscovite mica/chlorite/quartz
- type of metamorphism - LOW grade
Schist fact file
- colour - silvery sheen
- texture - schistosity/medium grain size (1-5mm)
- mineral composition - muscovite and biotite mica/quartz/garnet/kyanite
- type of metamorphism - MEDIUM grade
Gneiss factfile
- colour - dark and light bands
- texture - gneissose banding/coarse grain size (>5 mm)
- mineral composition - biotite mica/mafic minerals/quartz/k feldspar/sillimanite
- type of metamorphism - HIGH grade
Which rocks can be produced by regional or contact metamorphism?
Quartzite and Marble
What is the parent rock of Quartzite?
Orthoquartzite
What is the parent rock of Marble?
Oolitic limestone and Fossiliferous limestone
What is orthoquartzite composed of?
Sandstone composed of quartz grains held together by quartz cement
What happens to orthoquartzite to change it to quartzite?
Quartz grains in the sandstone recrystallise, forming interlocking quartz crystals. The quartz crystals are equidimensional so there can be no foliation. The texture is described as granoblastic. Any sedimentary structures or fossils in the parent sandstone are destroyed.
What is the colour of quartzite?
White or grey, unless there were other minerals in the original rock. For example, if any iron oxide was in the parent rock, there will often be a pink colour
What is granoblastic?
It describes the texture of metamorphic rocks that contain interlocking equi-dimensional crystals
How is Marble formed?
Limestone’s are essentially made of one mineral, calcite, which is stable over a wide range of temperatures and pressures. As a result, metamorphism of limestone only causes the original calcite crystals to grow larger. Calcite grains and fossil fragments in the limestone parent rock recrystallises to form an interlocking mosaic of calcite crystals. The crystals are equidimensional so there can be no foliation. Marble has a granoblastic texture, but the crystals of calcite make it look sugary in texture. Calcite will react with dilute HCl. Fossils are destroyed during metamorphism
What colour is Marble?
From pure limestone it’s white. From impurities, it can have a range of streaks
• clay can cause green or red minerals such as garnet to form
• sand can cause a reaction between calcite and quartz and therefore produce wollastonite which can be light green, pink, brown, red or yellow
Which rock is produced by contact metamorphism?
Spotted rock
How is spotted rock formed?
Because contact metamorphism involves only increased temperatures, it can not produce foliation. During contact metamorphism, spots may form in the rock where the heat has only partially recrystallised the rock. Spotted rock contains the same minerals as shale or slate. If slate is the parent rock of spotted slate, it will show foliation, but this was produced due to pressure during regional metamorphism. The random spots may contain - biotite, andalusite or graphite
What are the metamorphic textures?
Slate cleavage, schistosity, gneissose banding, porphyroblastic texture and granoblastic texture
What is slaty cleavage and how does this effect rocks?
Rocks with slaty cleavage will split into thin sheets along the cleavage planes. It occurs in fine grained rocks formed by low grade regional metamorphism.
•can only form in rocks consisting of Platy minerals - clay, chlorite and micas
•become aligned at 90 degrees to the direction of maximum pressure during metamorphism
•may be at an angle to the bedding put usually parallel to axial planes of the fold
•cannot occur in rocks with rounded grains such as quartz in sandstones
•bedding and fossils might not be completely destroyed leaving relict structures
What type of texture is schistosity?
Found in schists. Results from the alignment of flat Platy minerals, commonly muscovite mica, at 90 degrees to the direction of maximum pressure during metamorphism. Light coloured muscovite mica is concentrated into thin parallel bands, giving the rock a characteristic shiny appearance where flat surfaces of mica are visible.
What is a relict structure?
A structure such as bedding present in the parent rock, which is partially preserved in a metamorphic rock
What is a dalradian?
The name of a group of rocks formed in late Precambrian times (found in Scotland)
What type of texture is gneissose banding?
Gneissose banding is formed when light and dark coloured minerals are separated into bands. The mica - rich is foliated and the pale layer has granoblastic texture. The bands may be contorted or folded but are roughly at 90 degrees to the maximum pressure direction
What is Porphyroblastic texture?
It occurs in both regional and metamorphic rocks. Porphyroblasts are large crystals that grow during metamorphism and are surrounded by a finer grained groundmass. Metamorphic rocks that contain these large crystals are known as porphyroblastic.
What is an inclusion?
A fragment of an early formed mineral enclosed by one that grew later
What does unfoliated describe?
The random orientation of minerals in a metamorphic rock
What is granoblastic texture?
This is an unfoliated texture and is formed by thermal metamorphism. Pressure is not a factor in the formation of a granoblastic texture. The main characteristics are randomly orientated, equidimensional crystals usually in rocks with few, and sometimes only one mineral. Sometimes described as a sugary texture
What is a metamorphic aureole?
A region surrounding an igneous intrusion in which the country rocks have been recrystallised and changed by heat from the intrusion
What is a metamorphic grade a measure of?
The intensity of metamorphism. Although increases in temperature only result in increasing grade in contact metamorphism, grade is also used to describe regional metamorphism where both pressure and temperature vary
When does contact metamorphism occur?
When the country rock is affected by heat from a large igneous intrusion.
Why is contact metamorphism described as high temperature, low pressure metamorphism?
Because temperature differences between the surrounding rock and the intruded magma is greater at shallow levels in the earths crust
What does the high temperature of contact metamorphism do?
Leads to the formation of altered, recrystallised, unfoliated rocks in a zone surrounding the intrusion. (The metamorphic aureole)
How wide can the metamorphic aureole be around a large igneous intrusion such as a batholith?
Up to 10km wide
What is the contact metamorphism like of shale?
- close to the contact with the intrusion - high temp, so high grade metamorphism. Shale is completely recrystallised to form a fine grained, hard, splintery, granoblastic metamorphic rock called hornfels
- further away from contact - heat is less intense, medium grade M. Clusters of new metamorphic mineral andalusite, form porphyroblasts. This is andalusite slate or rock
- in the outer part of the metamorphic aureole - low temp, some recrystallisation occurs, causing s cluster of dark minerals to grow in different spots. (Iron/carbon/biotite). Low grade M creates spotted rock.
What are the factors controlling the width of metamorphic aureoles?
Volume of magma, temperature of the magma, composition of the Magma, composition of the country rock, dip of the contact.
How does the volume of magma affect the width of a metamorphic Aureole?
The size of intrusions ranges from batholith down to minor intrusions. Dykes and sills are not large enough and do not produce enough heat to develop a metamorphic aureole. Because the volume of magma is small it cools quickly and there is only sufficient heat to change the rock for a few centimeters on either side. This narrow zone of bleached and hardened rock is known as a baked margin.
Large intrusions cool slowly and heat surrounding rocks over long periods of time (10^4 - 10^6 years) allowing a wide metamorphic aureole to develop
How does the temperature of the magma control the width of metamorphic aureoles?
The volume of the magma in an intrusion affects the maximum temperature reached at any point and also the time it takes for temperatures to rise in the country rocks. Metamorphism will not occur unless the temperature rises above 200°C for an extended period of time. A small intrusion produces little metamorphic change because the rock has a little time to warm up and there is not enough time for metamorphic reactions to occur before the rock cools down.
How does the composition of magma control the width of metamorphic aureoles?
Mafic Mark Mum may be interested at a temperature of 1200°C, whilst silicic magma maybe and treated at 850°C. Silicic Magmas contain more volatiles. When they enter the country rock they speed up the metamorphic reactions. This compensates for the lower temperature of the magma, because metamorphic aureoles surrounding silicic intrusions are of similar size to those around mafic ones
How does the composition of country rock affect the width of metamorphic aureoles?
Rocks largely composed of one mineral, such as limestone and orthoquartzite, show much less variation then clay rich rocks such as shale. Quartzsite and marble have larger crystals the neighbor they are to the igneous intrusion and are uniform. Metamorphic aureoles formed in sandstone country rocks are typically narrower than those formed in Clay rich rocks. If the country rock is permeable and contains groundwater, sheets will be able to move by convection, allowing a wider aureole to develop
How does the dip of the contact control the wets of metamorphic a aureoles?
The dip of the sides of the intrusion have a major effect on the width of the metamorphic aureole. A shallow angle of dip gives a wide aureole and a steep angle of dip gives a narrow one. If the sides of the intrusion dip at different angles, then the metamorphic aureole will be asymmetric
What is the thermal gradient and index minerals in a metamorphic aureole like?
When a batholith is intruded into beds of shale, increases in metamorphic grade are marked by the appearance of an index mineral.
Contact metamorphism is only caused by temperature so an increase in grade represents a thermal gradient
What are index minerals?
Metamorphic minerals that are stable under specific temperatures and pressure conditions. They indicate the metamorphic grade.
What is the low grade mineral found in spotted rock (contact metamorphism)?
Biotite
Where is Al2SiO5 polymorph andalusite found?
Andalusite rocks (medium grade)
What does the sillimanite polymorph Al2SiO5 found?
The high grade rock hornfels.
Why is the kyanite Al2SiO5 polymorph not found in contact metamorphism?
The lack of pressure
What is a polymorph?
A mineral that has the same composition but occurs in different crystal forms?
How does quartzite form?
When orthoquartzite is afffected by contact metamorphism, all sedimentary structures including cross bedding and graded bedding are destroyed. The quartz grains in the sandstone recrystallise to form an interlocking mosaic of crystals giving it s granoblastic texture
How does Marble form?
Where limestones are affected by contact metamorphism. All sedimentary structures and fossils are destroyed. The grains and cement composed of calcite will recrystallise to form an interlocking mosaic of crystals giving it a granoblastic or sugary texture.
What is regional metamorphism?
Results from both heat and pressure generated at convergent plate margins during subduction and continental collision
Convergent plate margins with subduction zones?
- oceanic and continental plates collide - high pressure is produced as the oceanic plate is subducted - the result is high pressure, low temp Burial metamorphism and the formation of blueschists
- further away from the subduction zone, magma is rising from the melting oceanic plate and pressures are lower. High temp, low pressure metamorphism occurs
- high temperatures lead to the formation of igneous intrusions and metamorphic aureoles
Convergent plate margins continental - continental
Fold mountains form where the Earths crust is deformed thickened and there is extensive intrusive igneous activity. (The Himalayas)
What is migmatite?
A coarse grained mixed rock with some of the characteristics of gneiss and some of the characteristics of granite, formed by partial melting of the rock during the highest grade metamorphism, at the high temperature boundary between metamorphism and igneous activity
What are the grades of regional metamorphic rocks?
L - slate
M - schist
H - gneiss
What is an isograd?
A line on a map joining points of equal metamorphic grade. They join places where the first appearance of an index mineral appears
What is a metamorphic zone?
An area between two isograds. The zone is named after the lower grade isograd. All locations within a metamorphic zone experienced the same metamorphic grade.
What is a barrovian zone?
A metamorphic zone mapped using index minerals identified by George Barrow
What is the Dalradian supergroup?
Precambrian rocks in Scotland. Found about 400Ma
Index minerals and metamorphic zones?
Low Medium High
Slate Schist Gneiss
ChloriteBiotiteGarnetKyaniteSillimanite
