Metamorphic Processes and products

Temperature

- High temperatures occur near igneous intrusions where magma heats the surrounding rocks. - Temperatures also increase with depth due to the geothermal gradient. - As temperature increases the rate of metamorphic reactions also increase.

Regional Metamorphism

- Affects larger areas than contact metamorphism extending over hundreds of thousands of kms - Caused by low to high temperatures and pressures at convergent plate boundaries. - Pressure is significant and therefore minerals have preferred alignment. - Occurs in fold mountain cores.

Spotted rock(contact)

- Because contact metamorphism only includes incased temperatures it cannot produce foliation. - During contact metamorphism spots may form in dark rock where the heat only partial recrystallised the rock. - Contains the same minerals as shale or slate. - If slate is the parent rock it will shows foliation(spotted slate) - Randomly orientated spots may contain biotite, andalusite and graphite.

Burial Metamorphism

- Burial metamorphism occurs in conditions of medium to high pressure and relatively low temperatures. - If affects rock deeply buried by the weight of overlying sediment. - Occurs as subduction zones where the seafloor sediments and basalts are buried.

Unfoliated Rocks

- By contact or regional metamorphism

Contact Metamorphism Of Shale

- Close in contact with the intrusion temperatures are high so high grade metamorphism will occur. The shale completely recrystallises to form fine grained hard rock thats granoblastic which is called hornfel. - Further away form contact the heat is less intense and medium grade metamorphism occurs forming cluster of a metamorphic mineral called andalusite porphyroblasts from andalusite slate or rock. - In the outer part of the metamorphic aureole temperatures are lower, some recrystallisation occurs causing clusters of dark minerals to grow in separate spots ,iron, carbon and biotite. Spotted rock forms form low grade metamorphism.

Foliated Metamorphic Rocks

- Foliated is the texture produced in metamorphic rocks by the preferred alignment of flat platy minerals. - Affected by pressure to some agrees during regional metamorphism.

Gneissose Banding

- Found in gneiss is formed when light coloured (quartz of feldspar) minerals and dark coloured (biotite) minerals are separated into bands. - The mica rich layer is foliated and the pale layer is granoblastic texture. - The bands may be contorted or folded but are roughly at 90 degrees to maximum pressure.

Schistosity

- Found in schist results from the alignments of flat platy minerals commonly Muscovite mica at 90 degrees to the direction of pressure. - Light coloured muscovite mica is concentrated into thin parallel bands giving the rock a shinny appearance.

Metamorphism

- Is the iso chemical process by which rocks are changed by either heat to pressure or both. The chemical composition of the parent rock will be the same as that of the metamorphic rock produced. - May result in the destruction of fossils, beds and sedimentary structures. - Hardening of rocks - Change in colour - Alignment of minerals - Growth of new metamorphic minerals

Marble (regional)

- Limestones are composed of one minerals mostly which is calcite which is stable over a wide range of pressures and temperatures. - So metamorphism only causes the original crystals crystals to grow larger. - Recrystallise to form interlocking mosaic of calcite crystals. - The crystals are equi-dimensional so ether is no foliation and theres a gran plastic texture. - Calcite will react with dilute HCI

Contact metamorphism

- Occurs adjacent to igneous intrusions which increases the temperature of the surrounding country rocks. - The metamorphism is important on a local scale as producing a metamorphic aureole. - Temperatures are higher but pressure low - As pressure is not a significant factor the minerals are not alined.

Contact Metamorphism

- Occurs where the country rock is affected by the heat from a large intrusion. - High temperature low pressure - Metamorphic Aureole

Slaty cleavage

- Rocks with slaty cleavage will split into thin sheets along the cleavage planes and occurs in fine grained rock formed by low grade regional metamorphism. - It can only form in rocks consisting of platy minerals such as clay minerals and chlorite and micas. - AT the microscopic scale these minerals become aligned at 90 degrees to maximum pressure during metamorphism. - Slaty cleavage may be at any angle to bedding but is usually parallel to axial planes. - Can not occur in rocks with rounded grains such as sandstones and quartz.

Factors Controlling The Size Of The Aureole

- SIZE, The size can range from batholiths down to minor intrusions so minor intrusion produce less heat and can not form an aureole as it cools quickly resulting in a small cm thick naked margin. - TEMPERATURE, The volume of magma in the intrusion affects the maximum temperature reached and also time it takes for country rock temperature to rise as metamorphism will not occur below 200c for extended period of time. Large intrusions means there is time for the country rock to heat up and recrystallisation to occur. - COMPOSITION OF MAGMA , Mafic magma will intruded at 1200c whilst silicic magma will at 850c. Silicic mage contains more volatiles which speed up metamorphic reactions. - COMPOSITION OF COUNTRY ROCK, If the rock is permeable heat will move through the rock by convection causing a wider metamorphic aureole to form. - DIP OF CONTACT, The dip of the side has a major effect on the width of the aureole. Shallow angle gives a wider aureole and vice versa, if the sides dip at different angles then the aureole will be asymmetrical.

Gneiss (regional)

- The parent rock of Gneiss is shale and is formed at the highest pressures and temperatures of regional metamorphism. - Its coarse grained >5mm crystalline with gneissose banding. - typically composed of quartz and feldspar in the light bands and and biotite mica and other mafic minerals in the dark and.

Quartzite (regional)

- The parent rock of quartzite is orthoquartzite an sandstone composed of quartz and quartz cement. - The quartz in the sandstone recrystallise to form interlocking crystals. - The crystals are equi-dimensional so there can be no foliation, the texture is described as granoblastic. - Any sedimentary structures or fossils in the rock are destroyed. - White or grey in colour unless other minerals present in the parent rock.

Schist (regional)

- The parent rock of schist is shale, schist is produced by higher temperatures and pressures than slate. - Its medium grained (1-5mm) and crystalline - The garnets often form large crystals called porphyroblasts. - The mica crystals alined at right angle to the maximum pressure forming the texture schistosity.

Slate (regional)

- The parent rock of slate is shale which is composed of clay minerals and fine quartz minerals. - Because clay minerals are rich in aluminium so are the metamorphic minerals in slate. - Mainly composed of clay minerals and mica. - Slaty cleavage by stress applied whilst metamorphism takes place.

Granoblastic Texture

- This is an un-foliated texture and is formed through thermal metamorphism. - Pressure is not the only factor in the formation of this texture. - Randomly orientated equi-dimensional crystals. - Example is hornfel fine grained granoblastic texture - Marble and quartzite are also granoblastic

Porphyroblastic Texture

- This texture occurs in both regional and contact metamorphism. - Porphyroblasts are large crystal that grown during metamorphism and are surrounded by finer grained groundmass. - Metamorphic rocks that contain these crystals are porphyroblastic. - Garnet porphyroblasts can be found in schist.

Thermal Gradient Index minerals

- When batholiths intrude into beds of shale, increase in metamorphic grade are marked by the appearance of minerals. - Index minerals are metamorphic minerals which are stable under specific temperatures and pressures and indicate metamorphic grade. - In contact metamorphism biotite is a low grade minerals found in spotted rock. - The Al2Sio5 polymorph andalusite indicates medium garde metamorphism and is found in andalusite rich rocks. - Sillimanite another polymorph is caused by temperature only and increase in grade represents a thermal gradient.

Unfoliated

Describes the random orientation of minerals in a metamorphic rock

Metamorphic Grade

Is a measure of the intensity of metamorphism.

IsoChemical

Means that no elements are added or removed with the exception of volatiles such as water and carbon dioxide.

Metamorphic Textures

Metamorphic rocks are identified by their textures as their textures can tell us a lot about the overall conditions of metamorphism.