G1 : Rock Forming Processes KI2 (metamorphic Rocks) Flashcards
What is metamorphic grade
Describes the temperature and pressure conditions under which metamorphic rocks form
Metamorphic aureole
A region surrounding an igneous intrusion in which the country rocks have been recrystallised and changed by heat from the intrusion
Spotted rock
Formed by low grade metamorphisms in the outer part of a metamorphic aureole
Index minerals
Metamorphic minerals, which are so stable under specific temperature and pressure conditions. They indicate the metamorphic grade
What is contact metamorphism
When country rock is affected by heat from a large igneous intrusion. Because temperature differences between the country rock and the intrusion are greater at shallow depths in the earths crust, where pressure is low, contact metamorphism is described as high temperature, low pressure.
What does high temperature and low pressure lead to
The formation of altered, recrystallised, unfoliated ricks in a zone surrounding the intrusion.
How does distance from a metamorphic aureole impact metamorphism
Temperature decreases with distance from the contact with the intrusion for this reason the effects of contact metamorphism are greatest near to the contacts and decrease with distance. The outer part of the metamorphic aureole is only weakly metamorphosed and forms spotted rock
How does colour and hardness change in a metamorphic aureole
It gets darker and harder towards the centre
What do index minerals do in a metamorphic aureole
Index minerals are metamorphic minerals, which are stable under specific temperature and pressure conditions, they indicate the metamorphic grade.
Because contact metamorphism is caused by temperature only, an increase in grade represents a thermal gradient
What does the regional metamorphism of shale produce and with what minerals
Low grade : slate (chlorite)
Medium grade : schist (kyanite)
High grade : gneiss (sillimanite)
Regional metamorphism in orthoquartzite and limestone
Produces the same products and contact metamorphism - meta quartzite and marble. Each of these rocks is monomineralic as they are composed of one mineral only quartz and calcite respectively. The minerals are equidimensional, so they cannot align under pressure
Grade and metamorphic minerals
Low grade metamorphic rocks contain a high proportion of hydrous minerals (clay minerals and chlorite contain h2o in their crystal structure). High grade metamorphism takes place at higher temperatures. H2O is lost and anhydrous minerals (e.g. garnet) become more common. However, biotite, a hydrous mineral, is stable at a very high grade if metamorphism
Why is metamorphic grade not the same in contact and regional metamorphism
In regional metamorphism the metamorphic grade is made up of both temperature and pressure but only temperature is in contact
Why does the metamorphism of shake produce so much more information
Clay minerals can have many different chemical changes so can produce more rock types
Index minerals and contact metamorphism
-Biotite is the low grade and is found in spotted rock
-Andalusite is the low to medium temperature, low pressure alumina silicate polymorph found in andalusite slate. It indicates the medium grade
-with increasing grade, it follows a path from andalusite to sillimanite on the alumina silicates polymorph phase diagram, sillimanite indicated high grade grade and is found in hornfels
-kyanite the high pressure, low temperature alumina silicates polymorph, is not found in contact metamorphic rocks due to the lack of pressure
Where did George barrow map and when
In 1893 he mapped a sequence of highly deformed regionally metamorphosed rocks in the south eastern part of the Scottish highlands, this metamorphism and deformation occurred during the closure of the Iapetus ocean and during the Caledonian orogenic, about 400ma, these Precambrian rocks are known as the dalradian supergroup
What pattern did George barrow notice
There was a pattern to the occurrence of metamorphic minerals. He used the first appearance of these minerals, which he termed index minerals, to plot isograds and map metamorphic zones. Some of the minerals that crystallise at low grades are stable at higher grades so more than one index minerals may be found in one rock
What was George barrow able to map
Metamorphic zones using index minerals and isograds which define the boundaries of the zones. Although he did not do all the mapping personally, the system he devised was names after him and the zones are called barrovian zones
What does the chlorite zone represent
Low grade regional metamorphism. The rock is skate where most of the rock has recrystallised but some clay minerals may still exist
Where do schists develop
As a result of increasing temperatures and pressure and can be found in both bigots and garnet zones. The crystal sizes increase with metamorphic grade. Schists formed at lower temperatures and pressures are composed of quartz, Muscovite and biotite mica.
Medium grade metamorphism
Results from higher temperatures and pressure and many schists formed at this grade contain garnet, and less commonly, kyanite porphyroblasts
Where are the alumina silicates polymorphs kyanite and sillimanite found
In regional metamorphic rocks
Kyanite zone
Kyanite is typically found in gneisses and the kyanite zone represents high grade regional metamorphism. With increasing metamorphic grade, regional metamorphism follows a oath from kyanite to sillimanite, shown in the alumina silicates polymorph phase diagram
The silimanite zone
High grade regional metamorphism with very high temperatures and pressures. The rocks a gneisses. A maximum temperature if about 700c and pressure if 700mpa. This pressure exists at a depth of 25km below the surface of the continental crust. It gives us a geothermal gradient of 28Ckm-1
Why are quartz and plagioclase feldspar not useful as index minerals
They are stable throughout the whole range of grades
Why are there changes in mineral assemblages
Due to change in temperature and pressure conditions.
What’s a metamorphic facies
Metamorphic minerals that were formed under similar pressures and temperatures
What can rocks of a particular assemblage be linked to
Certain tectonic setting and times and can be used to reconstruct the conditions of metamorphism
What does the sequence of metamorphic facies seen in an area depend on
The geothermal gradient during metamorphism
Prograde metamorphism
The recrystallisation of a rock in response to an increase in the intensity of metamorphism
Why is andalusite not formed by contact metamorphism of pure limestone
Pure limestone os monomineralic so does not contain any impurities such as clay minerals which contain alumina silicates
Why are clay rich sediments the most useful in mapping metamorphic zones
Good indicator of environment of formation and facies and show a latter to the occurrence of these minerals
Why are metamorphic rocks classified mainly based on their textures
Grain size and orientation tell us a lot about the conditions of metamorphism
What happens if rocks are subjected to directed pressure
A preferred orientation for the minerals develops at 90 degrees to the pressure. if the minerals are Flat or Plath, foliation is produced. Foliation results from pressure and is a characteristic of rocks formed. H regional metamorphism
Dalradian
Regionally metamorphosed group of rocks originally deposited in the late Precambrian, they are found in Scotland and Ireland
Inclusion
An early formed mineral enclosed by one that grew later
Unfoliated
The random orientation of minerals in a metamorphic rock
Hornfels
Fine grained, hard, splintery, granoblastic metamorphic rock formed when shale is completely recrystallised close to the contact with an igneous intrusion
What is slaty cleavage
Rocks with slaty cleavage will split into thin sheets along the cleavage okayed. It occurs in fine grained rocks formed by low grade regional metamorphism
-form in rocks consisting of platy minerals such and clay, chlorite and micas
-these minerals align at 90 to the direction of maximum pressure during metamorphism
-usually parallel to axial planes of folds
-cannot occur in rocks with rounded grains such as quartz in sandstone
-relic structures may be found
What is schistosity
Texture found in phylites (fine grained) and schists (medium grained) and form by regional metamorphism. Results from the alignment of flat, platy minerals, commonly Muscovite mica and 90 to direction of maximum pressure during metamorphism. Light coloured Muscovite mica is concentrated into thin parallel bands giving the rock a characteristic shiny appearance where fk
At surfaces of mica are visible
Garnet porphyroblasts may be present
Both schists and phyllites are dalradian rocks
What is gneissose banding
Found in gneisses (high grade regional - coarse grained) formed when light (usually quartz and feldspar) and dark coloured minerals (usually biotite and mafic minerals) are separated into bands. The mica rich layer is foliage and the pale layer is granoblastic. The bands may be contorted or folded but are roughly 90 to the maximum oressure
What is porphyroblastic texture
Both regional and contact, porphyroblasts are large crystals that grow during metamorphism and are surrounded by finer grained ground mass. Metamorphic rocks that contain these are described as porphyroblastic.
Pyrite porphyroblasts in slate (often cubic crystals)
Garnet porphyroblasts in schist may contain inclusions (provides clues about the timing of events during formation)
Granoblastic texture
Unfoliated fabric by thermal metamorphism. Randomly orientated, equidimensional crystals usually in rocks with few or one minerals.
E.g. hornfels, marble and quartzite.
Crenulation cleavage
When several foliations are present but not all in the same direction. In rocks such as phyllite, schist and some gneisses where two or more stress directions cause the formation of one or more foliations superimposed in earlier ones, giving a wavy appearance to the texture. This allows us to determine the number if phases of metamorphism that have affected a rock
Fabrics stress and time during mountain building
High temperatures increase the rate of ions diffusing between minerals, process is sped up by water (diffuse more rapidly), these reactions take place of ma so temperature and pressure conditions must remain for a long time.
How do conditions change during mountain building
Several periods of deformation. Minerals that form early in metamorphism sometimes grow over existing sedimentary features and thus can help to identify the parent rock. This evidence can be defrocked by deformation and later metamorphic recrystallisation and pressure and temperature change and increase. Using a microscope allows more detailed information about the relationship between the growth of new minerals and the effects of deformation to be studied.
What happens if stress is applied to rocks rapidly
E.g. fault zones
Mineral grains suffer brittle fracture and are crushed and ground mechanics,lay as the fault moves thus process is cataclasis. It produces a variety of textures ranging from coarse grained angular class in a fine grained matrix, to rock consisting almost entirely of finely crushed particles. These fabrics are characteristics of dynamic metamorphism, but will occur in fault zones as part of mountain building
What happens if stress is applied to rocks slowly
Over a long period at low strain rates, the rock does not fracture but deforms gradually. Although temperature is important, foliation develops largely in response to stresses. Later phases of deformation may produce foliation with a different orientation and by recognising these differences in metamorphic fabric, a history of changing pressure and temperature during regional metamorphism can be determined
Spotted rock
In the outer part of the metamorphic aureole, temperatures are lower. Some recrystallisation occurs causing clusters of dark minerals to grow on separate spots. Iron, carbon, or biotite mica will form the spots. The rock in this outer part of the aureole is called spotted rock and is formed by low grade metamorphism
Andalusite rocks/slate
Further away from the contact, where heat is less intense, medium grade metamorphism occurs. Clusters of a new metamorphic mineral andalusite form porphyroblasts. Thus partly recrystallised rock is andalusite slate or rock
Hornfels
Close to the contact with the intrusion, temperatures are high and so high grade metamorphism occurs, shake is completely recrystallised to form a fine grained, hard, splintery, granoblastic, metamorphic rock called hornfels.
Contact metamorphism
-Associated with major igneous intrusions such as plutons and batholiths
-the volume of country rock altered by heat from the intrusion is know as the metamorphic aureole
-recrystallisation occurs with a random orientation of minerals (non foliated)
-volume of magma affects size of aureole (small have baked margins)
-not occur below 200c
-mafic 1200c and felsic 850c (contains more volatiles)
-aureoles narrower in sandstone that shake
-limestone contains carbonate which may react with hydrothermal fluids to create skarn
Regional metamorphism
Slate, schist, phyllite, gneiss, migmatites. Occurs when temperature and heat become too great and a mineral e.g. clay has to adapt to the new environment producing chlorite. Occurs in a large scale over regions in orogenic events and volcanic arcs. The process can affect igneous rocks and sedimentary rocks, but they produce different results.
Migmatite
A rock composed of metamorphic host material that is streaked or veined with granite rock; the name means ‘mixed rock’ such rocks are usually gneissic (banded) and felsic rather than mafic in composition; they may occur on a regional scale in areas of high grade metamorphism
Dynamic metamorphism
High pressure (cataclasis) and low temperature (along major fault zones pieces of crust are forced to slip past each other)
Often involves a change in texture of the rock with little to no change in minerals content. The resulting texture is that of finely pulverised rock powered called mylonite. There may be a thin layering of the rock (parallel to the fault zone)
Produces fault breccias and fault gouge
