2.4 1 Temperature, pressure and metamorphism

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1 2.4 1 Temperature, pressure and Key definition Isochemical means that no elements are added or removed, with the exception of volatiles such as water and carbon dioxide. Metamorphism is the isochemical process by which rocks are changed by either heat or pressure, or both heat and pressure. The chemical composition of the parent rock will be the same as the metamorphic rock produced. 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. The minerals produced are directly related to pressure and temperature conditions. The lower temperature limit for is between 200 and 150 o C. Below these temperatures, changes are part of diagenesis. There is no lower pressure limit. The upper temperature limit is where melting occurs. This happens at around 800 o C. The process of may result in: destruction of fossils, beds and sedimentary structures hardening of the rock change in colour alignment of minerals growth of new metamorphic minerals Sedimentary Temperature/ C Contact 200 Low grade 10 Case study Making a brick it s! Take a mass of soft, grey-coloured, sticky, crushed clay mixed with water and a little limestone. Stir well. Press into a brick shape. Put in the furnace at 1400 o C for 2 days. Cool slowly and you will have a hard, red brick. Pressure/MPa Burial Medium grade Regional High grade Figure 1 Relationship between, temperature and pressure Igneous rocks Temperature Temperature is a key variable in : High temperatures occur near to igneous intrusions, where the magma heats the surrounding rocks. Temperature also increases with depth, due to the geothermal gradient. As temperature increases, the rate of metamorphic reactions also increases. This is because many of the chemical reactions require heat to take place. Higher temperatures increase 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. The whole process is greatly speeded up by water, which allows the ions to diffuse more rapidly Depth/km 138

2 Pressure Pressure steadily increases with depth and is applied to rocks in three different ways: Pore pressure is the pressure exerted by fluids between the grains in a porous rock. The presence of water speeds up reactions by acting as a catalyst and increasing the rate and ease of ion exchange. In an experiment where two dry solids were heated together for 2 hours at 1300 o C, only 10% reacted. When the same solids were heated in water for the same time, the reaction was completed at only 600 o C. Load pressure is the weight of overlying rocks and physically brings minerals into contact with each other over very long periods of time. Tectonic stress or pressure is caused as the rocks undergo folding or faulting and very high pressures are exerted, but usually over relatively short periods of time. In all cases the higher the pressure, the greater the degree of. Reactions that depend on pressure only are less common than temperature dependent reactions. Module 4 Metamorphic Processes and Products Temperature, pressure and Time Time is very important because metamorphic reactions take place very slowly. These reactions usually take millions of years to occur. Pressure and temperature conditions that produce have to exist over long periods of time, in order for the reactions to occur. Types of Contact Contact occurs adjacent to igneous intrusions, which increase the temperature in the surrounding country rock. The is important on a local scale, producing a metamorphic aureole. Temperatures are generally high but pressure is low. As pressure is not a significant factor, the minerals are not aligned in contact metamorphic rocks. Key definition Country rock is the rock into which an igneous rock has been intruded. Burial Burial occurs in conditions of medium to high pressure and relatively low temperature. To some extent, burial overlaps with diagenesis, and grades into regional as temperature increases. It affects rocks deeply buried by the weight of overlying sediments. It also occurs at subduction zones where sea floor sediments and basalts are buried. Rocks buried at the deepest levels almost always contain the blue mineral glaucophane. They are called blueschists. Regional Regional affects larger areas than contact, extending over hundreds or thousands of square kilometres. It is caused by low to high temperature and low to high pressure at convergent plate margins. It can result from either subduction or continental collision. Pressure is significant and so minerals have a preferred alignment. Regionally metamorphosed rocks occur in the cores of fold mountain belts where mountain ranges have been eroded. Questions 1 Describe the temperature and pressure conditions associated with each of the three types of. 2 Describe the effects on the surrounding rocks of an igneous intrusion that cooled in 500 years. 3 Explain why it is more difficult to know the pressure at which metamorphic rocks form than the temperature at which they form. 139

3 2.4 2 Identifying regional metamorphic rocks All metamorphic rocks are formed from parent rocks, the original rocks that existed prior to. The composition of these rocks affects the mineralogy of the metamorphic rocks, so they have a kind of family likeness. Key definitions Foliation is the texture in metamorphic rocks, formed by the preferred alignment of fl at, platy minerals. Slaty cleavage is the texture in fi ne grained rocks formed by low grade regional. Platy minerals recrystallise perpendicular to the direction of stress applied during so that the rock splits into thin sheets. Figure 1 Shale and slate Foliated rocks produced by regional These rocks have all been affected by pressure, to some degree, during regional. Any platy minerals they contain take on a preferred alignment known as foliation. All the rocks described in this spread are foliated. (For an explanation of how foliated textures form, see spread ) The most common platy mineral is clay so the rocks described below all have shale as the parent rock. Slate The parent rock of slate is shale (for a photo of shale, see spread 2.3.6). Shale is composed of clay minerals and fi ne quartz particles. Because clay minerals are rich in aluminium, so are the metamorphic minerals in slate. This is mainly composed of clay minerals and mica (although chlorite and quartz may also be present). Shale is fi ne grained (grains <1 mm diameter) and shows slaty cleavage. Traces of original bedding may still be preserved as relict bedding. Alignment of minerals has changed due to pressure during Magnification x50 Shale, composed of clay minerals aligned by compaction during diagenesis Slate, composed of clay minerals and platy metamorphic minerals mica and chlorite Slate is a fine grained metamorphic rock that has slaty cleavage i.e. it splits into thin sheets Key definitions A porphyroblast is a large crystal that has grown during recrystallisation in a metamorphic rock and is surrounded by a fi ner grained groundmass of other crystals. Schistosity is the texture in medium and coarse grained metamorphic rocks formed by the preferred alignment of fl at/tabular minerals. The alignment is perpendicular to the direction of stress applied during. No traces of original bedding remain. Gneissose banding is 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. Schist The parent rock of schist is shale. Schist is produced by higher temperatures and pressures than those producing slate. It is medium grained (1 to 5 mm) and crystalline. Although it can occur in a variety of colours, it always has a shiny appearance where the fl at surfaces of muscovite and biotite mica crystals are visible. Schist is typically composed of mica and garnet. The garnets often form large crystals called porphyroblasts. The mica crystals are all aligned at right-angles to the maximum pressure, forming the texture schistosity. Gneiss The parent rock of gneiss is shale. Gneiss is formed by the highest temperatures and pressures during regional. It is a coarse grained (>5 mm), crystalline rock with gneissose banding. Gneiss is typically composed of quartz and feldspar in the light bands and biotite mica (and other mafi c minerals) in the dark bands. 140

4 Module 4 Metamorphic Processes and Products Thin section drawing of schist showing garnet porphyroblasts (black) and micas (brown) in preferred alignment Identifying regional metamorphic rocks Garnet porphyroblasts 0 2mm Mica crystals give the surface a sheen Figure 2 Schist Garnet mica schist Light coloured band Feldspar showing two cleavage directions at right angles Quartz Quartz rich band Dark coloured band Biotite mica is dark and shows preferred orientation Figure 3 Gneiss 0 2mm Thin section drawing of gneiss Gneiss showing gneissose banding Band rich in biotite mica Summary table of rocks produced by regional only Parent Rock Metamorphic rock Colour Texture Mineral composition Type of Shale, composed of clay minerals Slate Grey or purple or green or black Slaty cleavage, fine grain size (<1 mm) Clay minerals and muscovite mica, with some chlorite and quartz Low grade regional Shale Schist Silvery sheen Shale Gneiss Dark and light bands Schistosity, medium grain size (1 5 mm) Gneissose banding, coarse grain size (>5 mm) Muscovite and biotite mica Quartz Garnet Kyanite Biotite mica Mafic minerals Quartz K feldspar Sillimanite Medium grade regional High grade regional Questions 1 Which metamorphic rock would you associate with high grade regional 2 With the aid of labelled diagrams, describe the differences between schist and slate. 3 With the aid of labelled diagrams, describe one similarity and one difference between schist and gneiss. 141

5 2.4 3 Identifying metamorphic rocks The parent rock, for all the metamorphic rocks described in spread 2.4.2, was shale but this does not mean that shale is the only parent rock. The chemical composition of the minerals that make up shale is more varied than that of limestones, which are composed of calcite (CaCO 3 ), or of sandstones, some of which can be almost pure SiO 2 in the form of quartz. There is a much wider range of metamorphic minerals that can recrystallise from clay minerals. This means that shale can be the parent of different metamorphic rocks, but limestone and sandstone cannot. Unfoliated rocks produced by contact or regional Orthoquartzite parent Quartz grain Quartz cement Calcite Oolitic limestone parent Concentric layers of calcite Fossil fragments Fossiliferous limestone parent Calcite Quartzite Nucleus Marble 1mm 0.5mm 1mm Interlocking quartz crystals 1mm Figure 1 Thin section drawings of quartzite, marble and their parent rocks Because these rocks do not contain platy minerals such as mica they do not show foliation. They can be produced by either regional or contact. Key definition Granoblastic describes the texture of metamorphic rocks that contain interlocking equi-dimensional, crystals. Quartzite The parent rock of quartzite is orthoquartzite, sandstone composed of quartz grains held together by quartz cement. Quartz grains in the sandstone recrystallise, forming interlocking quartz crystals. The quartz crystals are equidimensional so there can be no foliation. This texture is described as granoblastic. Any sedimentary structures or fossils in the parent sandstone are destroyed. The colour of quartzite is 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. Marble Limestones are made essentially of one mineral, calcite, which is stable over a wide range of temperatures and pressures. As a result, of limestone only causes the original calcite crystals to grow larger. Calcite grains and fossil fragments in the limestone parent rock recrystallise to form an interlocking mosaic of calcite crystals. The crystals are equi-dimensional, so there can be no foliation. Marble has granoblastic texture, but the crystals of calcite make it look sugary in texture. Calcite will react with dilute HCl. Fossils are destroyed during. Marble from pure limestone is white. Impurities in the parent limestone give some marble a range of coloured streaks: If there are clay minerals in the limestones, then a number of green or red minerals such as garnet may form. If there are sand grains present, a chemical reaction between calcite and quartz will produce wollastonite, which can be light green, pinkish, brown, red or yellow. 142

6 Rocks produced by contact Spotted rock Because contact involves only increased temperatures, it cannot produce foliation. During contact, spots may form in the rock where the heat has only partially recrystallised the rock. A 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 (see spread 2.4.2). The randomly orientated spots may contain biotite, andalusite and graphite, but they are usually too indistinct to be identified in hand specimen. Relict bedding Module 4 Metamorphic Processes and Products Identifying metamorphic rocks Alignment of micas and clay minerals Dark spots Summary table of unfoliated rocks Parent rock Metamorphic rock Limestone Marble White Granoblastic composed of Medium grain calcite size (1 5 mm) (CaCO 3 ) grain size increases with metamorphic grade Sandstone composed of quartz (SiO 2 ) Slate or shale composed of some clay minerals, mica and quartz Colour Texture Mineral composition Quartzite White or grey Granoblastic Medium grain size (1 5 mm), grain size increases with metamorphic grade Spotted rock Grey or Slaty cleavage if purple or slate parent rock green or Fine grain size black with (<1 mm) darker spots Calcite (reacts with dilute HCl) Quartz Clay minerals and mica Poorly formed minerals (mica, andalusite, graphite) in spots Type of Contact or regional Contact or regional Contact 10 Figure 2 Thin section diagram of spotted slate showing clay minerals and mica aligned at 90º to maximum pressure. The remains of the sedimentary bedding can just be seen as relict bedding Yes Minerals have preferred alignment No Fine grained Medium to coarse grained Medium to coarse grained Fine grained Mica prominent. May contain garnet Contains dark and light bands Composed of quartz Composed of calcite Contains dark spots Slate Schist Key Produced by regional Figure 3 Identifying metamorphic rocks Gneiss Quartzite Produced by regional or contact Marble Produced by contact Spotted rock Questions 1 Explain why marble is not always white. 2 Explain why limestone and sandstone produce the same metamorphic rocks in both contact and thermal. 3 State two pieces of evidence indicating that quartzite is a metamorphic rock. 143

7 2.4 4 Metamorphic textures Metamorphic rocks are classifi ed mainly based on their texture. This is because grain size and orientation tell us a lot about the conditions of. If rocks are subjected to directed pressure, a preferred orientation of the minerals develops at 90 degrees to the pressure. If the minerals are fl at or platy, foliation is produced (see spread 2.4.2). Because it results from pressure, foliation is a characteristic of rocks formed by regional. Direction of maximum stress during Sandstone no cleavage Compression Shale beds Cleavage planes Sandstone no cleavage Foliation produced by the alignment of flat minerals e.g. mica Flat minerals like mica align so that their long axis is at 90 to the direction of pressure Slaty cleavage Relict bedding is at a different angle from the cleavage Figure 1 Foliation and slaty cleavage Direction of maximum stress during Slaty cleavage developed at 90 to maximum stress Slaty cleavage Rocks with slaty cleavage will split into thin sheets along the cleavage planes. It occurs in fi ne grained rocks formed by low grade regional : It can only form in rocks consisting of platy minerals such as clay minerals, chlorite and micas. At the microscopic scale, these minerals become aligned at 90 degrees to the direction of maximum pressure during. Slaty cleavage may be at any angle to bedding, but is usually parallel to axial planes of the folds. It cannot occur in rocks with rounded grains, such as quartz in sandstones. Bedding and fossils may not be completely destroyed by, leaving traces or relict structures. Fossils may be deformed due to the high levels of compressive stress. Slates are common in North Wales and the Lake District. Schistosity Found in schists (medium grained rocks formed by regional ), schistosity results from the alignment of fl at, platy minerals, commonly muscovite mica, at 90 degrees to the direction of maximum pressure during. Light coloured muscovite mica is concentrated into thin parallel bands, giving the rock a characteristic shiny appearance (micaceous sheen) where fl at surfaces of mica are visible. Garnet porphyroblasts are often present and they disrupt the alignment of mica minerals. Schists are found in the Highlands of Scotland in Dalradian rocks. Key definitions A relict structure is a structure such as bedding present in the parent rock, which is partially preserved in a metamorphic rock. Dalradian is the name of a group of rocks formed in late Precambrian times, found in Scotland. This rock shows porphyroblastic texture in a schist Figure 2 Porphyroblastic texture in a schist Schistosity caused by alignment of micas Garnet porphyroblasts 144

8 Gneissose banding Found in gneisses (coarse grained rocks formed by regional ), gneissose banding is formed when light (usually quartz and feldspar) and dark coloured minerals (usually biotite mica and mafic minerals) are separated into bands. The mica-rich layer 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 (for a thin section drawing and photo showing gneissose banding, see spread 2.4.2). Porphyroblastic texture This texture occurs in both regional and contact metamorphic rocks. Porphyroblasts are large crystals that grow during and are surrounded by a finer grained Gneiss dark and light bands Thin section drawing of a garnet porphyroblast groundmass. Metamorphic rocks that contain these large crystals are described as porphyroblastic. Garnet porphyroblasts found in schists may contain inclusions. Pyrite porphyroblasts can develop in slate, often forming clear cubic crystals. 2mm Garnet porphyroblasts commonly have curved cracks when seen in thin section Inclusions of early formed minerals enclosed by a garnet porphyroblast Module 4 Metamorphic Processes and Products Key definitions Metamorphic textures An inclusion is a fragment of an early formed mineral enclosed by one that grew later. Unfoliated describes the random orientation of minerals in a metamorphic rock. Figure 3 Gneissose banding Examiner tip Figure 4 Pyrite porphyroblast in slate Granoblastic texture This is an unfoliated texture and is formed by thermal. 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. Hornfels is an example of a fine grained rock with granoblastic texture. Marble and quartzite are also granoblastic. Because of their medium grain size and white colour, their texture is sometimes described as sugary. Granoblastic texture Equidimensional crystals with no preferred alignment Figure 5 Granoblastic texture How to tell the difference between the igneous texture porphyritic and the metamorphic texture porphyroblastic: porphyritic is where large crystals phenocrysts form first in the magma so grow larger than the groundmass, which cools later. porphyroblastic is where large crystals such as garnet grow after the groundmass has developed and they may distort the groundmass crystals. Questions 1 Name two metamorphic rocks that are unfoliated. 2 With the aid of labelled diagrams explain how a garnet porphyroblast affects the alignment of micas in a schist. 3 Explain why slaty cleavage commonly has a different orientation from relict bedding. 145

9 2.4 5 Contact 1 Key definitions A metamorphic aureole is a region surrounding an igneous intrusion in which the country rocks have been recrystallised and changed by heat from the intrusion. A metamorphic grade is a measure of the intensity of. Although increases in temperature only result in increasing grade in contact, grade is also used to describe regional where both temperature and pressure vary. Contact occurs when the country rock is affected by heat from a large igneous intrusion. Because temperature differences between the surrounding rock and the intruded magma are greater at shallow levels in the Earth s crust where pressure is low, contact is described as high temperature, low pressure. High temperature, not pressure leads to the formation of altered, recrystallised, unfoliated rocks in a zone surrounding the intrusion. This zone is the metamorphic aureole. Around a large igneous intrusion, such as a batholith, the metamorphic aureole may be up to 10 km wide. Temperature decreases with distance from the contact with the intrusion and for this reason the effects of contact are greatest near to the contact and decrease with distance. Metamorphic grade increases in all directions towards the intrusion. Contact of shale The chemical composition of minerals in shale is varied and so a range of different metamorphic rocks is formed, depending on the temperature and therefore the distance away from the intrusion: Close to the contact with the intrusion, temperatures are high and so high grade occurs. Shale is completely recrystallised to form a fi ne grained, hard, splintery, granoblastic metamorphic rock called hornfels. Further away from the contact, where the heat is less intense, medium grade occurs. Clusters of a new metamorphic mineral andalusite, form porphyroblasts. This partly recrystallised rock is 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 or biotite mica will form the spots. The rock in this outer part of the metamorphic aureole is called spotted rock and is formed by low grade. Andalusite porphyroblasts A metamorphic aureole showing contact of shale Shale with spots of partial recrystallisation Edge of metamorphic aureole Shale country rock Granite Hornfels 0 1 cm Cross-section of andalusite crystal Andalusite crystal Figure 1 Metamorphic aureole showing contact of shale and photo of a spotted rock and andalusite rock Andalusite slate Spotted rock Black spots of iron or carbon Factors controlling the width of metamorphic aureoles Volume of the magma The size of intrusions ranges from batholiths down to minor intrusions (see spread 2.2.9). 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 suffi cient heat to change the rock for a few centimetres on either side. This narrow zone of bleached and hardened rock is known as a baked margin. 1cm Larger intrusions cool slowly and heat the surrounding rocks over long periods of time ( years), allowing a wide metamorphic aureole to develop. 146

10 Temperature of the magma The volume of 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 little time to warm up and there is not enough time for metamorphic reactions to occur before the rock cools down. With larger intrusions there is time for metamorphic reactions to take place and for new minerals and recrystallisation to occur, because temperatures remain high for much longer periods of time. Composition of the magma Mafic magma may be intruded at a temperature of 1200 C, whilst silicic magma may be intruded at 850 C. Silicic magmas contain more volatiles. When they enter the country rock they speed up 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. Composition of the country rock Rocks largely composed of one mineral, such as limestone and orthoquartzite, show much less variation than clay-rich rocks such as shale. Quartzite and marble have larger crystals the nearer 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, heat will be able to move by convection, allowing a wider aureole to develop. Dip of the contact The dip of the sides of the intrusion has 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 aureole. If the sides of the intrusion dip at different angles, then the metamorphic aureole will be asymmetric. 5 X Gently dipping contact produces wider aureole N Key Granite Conglomerate Sandstone Quartzite Shale Spotted rock Limestone Marble Metamorphic aureole Temperature/ C Temperature/ C Module 4 Metamorphic Processes and Products Contact 1 Decrease in temperature with distance from intrusions of different size Distance from contact/km Granite 10km diameter Granite 5km diameter Granite 1km diameter Temperature change in intrusions of different compositions Distance from contact/km Gabbro Diorite Granite Temperature of intrusion Basic 1200 C; Intermediate 900 C; Acid 800 C; All intrusions are 5km wide Figure 2 Graph showing the effects of temperature and composition of magma 5 X Y Y Steeply dipping contact produces narrow aureole Figure 3 Map of an intrusion with dipping sides 1km Questions 1 What is the term for the zone surrounding a granite batholith 2 Hornfels forms at 460 C. Using Figure 3, state how far away from each intrusion hornfels will form. 3 Explain the relationship between metamorphic rocks and metamorphic grade. 147

11 2.4 6 Contact 2 Metamorphic aureole Increasing metamorphic grade Increasing temperature Granite High batholith grade Medium grade Low grade Unaltered country rock Shale The thermal gradient and index minerals in a metamorphic aureole When a batholith is intruded into beds of shale, increases in metamorphic grade are marked by the appearance of an index mineral: Index minerals are metamorphic minerals, which are stable under specifi c temperature and pressure conditions. They indicate the metamorphic grade. In contact, biotite is the low grade mineral found in spotted rocks. The Al 2 SiO 5 polymorph andalusite indicates medium grade and is found in andalusite rich rocks. Sillimanite, another Al 2 SiO 5 polymorph, indicates high grade and is found in hornfels. Because contact is caused by temperature only, an increase in grade represents a thermal gradient. Some of the minerals that crystallise at low grades are stable at higher grades, so more than one index mineral can be found in one rock. 7 Sillimanite appears Andalusite appears Biotite appears Figure 1 Sketch map showing index minerals and metamorphic grade Pressure/kbar KYANITE Triple point SILLIMANITE 2 1 ANDALUSITE Case study The Skiddaw granite is part of a major intrusion in the English Lake District. The intrusion is an oval dome shape measuring 10 km 6 km, with a wide metamorphic aureole. The zones around the granite are: Unmetamorphosed country rock Skiddaw slates are fi ne grained parent rocks showing slaty cleavage, formed by regional before the intrusion. Outer zone of spotted slate where the grain size is slightly coarser than in the country rocks and small round dark spots are visible. The spots contain biotite and organic material. Middle zone Andalusite slate is medium grained and generally crystalline, containing andalusite porphyroblasts. Close to the intrusion the parent slate has been completely recrystallised to hornfels that can Andalusite slate Dark grey fine grained rock Slaty cleavage Temperature/ C Temperature and pressure fields for the Al 2 SiO 5 polymorphs 0 1 cm Metamorphic path for contact Relict bedding Thin section drawing of andalusite crystals Crystals of andalusite contain sillimonite. Figure 2 Andalusite and sillimanite 148

12 The Al 2 SiO 5 polymorphs in contact The Al 2 SiO 5 polymorphs andalusite and sillimanite are found in contact aureoles andalusite is the low to medium temperature, low pressure polymorph found in andalusite slate, whereas sillimanite is the high temperature polymorph found in hornfels. With increasing metamorphic grade, contact follows a path from andalusite to sillimanite on the Al 2 SiO 5 polymorph phase diagram. Kyanite, the high pressure, low temperature polymorph, is not found in contact metamorphic rocks due to the lack of pressure. Formation of quartzite and marble When orthoquartzite, a sandstone composed entirely of quartz, is affected by contact, 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 a granoblastic texture. Near to the contact with the igneous intrusion, in the zone of high grade, the crystals are larger than they are further away from the contact where temperatures are not as high. The resulting rock is white or pale grey in colour and known as metaquartzite. Where limestones are affected by contact, 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. This metamorphic rock is called marble. Crystals are larger near to the contact with the igneous intrusion and smaller further away, due to the thermal gradient. If the parent limestone is composed purely of calcite, the resulting metamorphic rock is white in colour. Impurities in the limestone may give streaks of different colours in the marble. Module 4 Metamorphic Processes and Products Key definition Contact 2 A polymorph is a mineral that has the same composition but occurs in different crystal forms. Examiner tip Make sure that you know the products of contact. Do not write about slate (unless it is spotted slate or andalusite slate formed when the country rock was slate), schist and gneiss, if you are answering a question on contact. Edge of metamorphic aureole Coarse marble Marble Limestone Igneous intrusion Marble Coarse metaquartzite Quartzite Orthoquartzite 100m Quartzite Figure 3 Sketch map and photos showing contact of limestone and orthoquartzite Questions 1 Explain why andalusite is not formed by the contact of pure limestone. 2 Draw a cross-section through a metamorphic aureole and through shale country rock. Label the rock types that would be present on your cross-section. 3 Explain why there are no relict structures in quartzite. 149

13 2.4 7 Regional Most regional is accompanied by deformation, so these metamorphic rocks will have foliated textures. Regional and plate tectonics Regional results from both heat and pressure generated at convergent plate margins during subduction and continental collision. The geothermal gradient and plate tectonics Along subduction zones magmas are generated, rise and intrude into the crust. Temperatures are high near the surface result so the geothermal gradient may be in the range of 50 to 70 C/km, and contact results. Compression occurs at a subduction zone where the oceanic crust starts to subduct and the edge of the non-subducting plate is deformed. The geothermal gradient is normal at 25 C/km. Along a subduction zone, relatively cool oceanic lithosphere is pushed down to great depths. This produces a low geothermal gradient of 10 to 15 C/km. Case study Paired metamorphic belts include areas in New Zealand, Indonesia, Washington State in the United States, Chile, and the coast of South America. All these areas lie around the Pacifi c at convergent plate margins, where subduction has occurred. Case study The Dalradian sedimentary rocks were deposited in late Precambrian and Cambrian times in an ancient ocean called Iapetus, which existed between Scotland and England. Continental continental plate movements caused the ocean to close and the 13 km of sediments that had been deposited in the ocean were deformed and regionally metamorphosed to form the Caledonian orogenic belt. The area of extends both south and north of the Great Glen Fault into the Highlands of Scotland. The metamorphic zones are displaced by the fault. Convergent plate margins with subduction zones When oceanic and continental plates collide, high pressure is produced as the oceanic plate is subducted. The result is high pressure, low temperature burial and the formation of blueschists. Further away from the subduction zone, magma is rising from the melting oceanic plate and pressures are lower, so high temperature, low pressure occurs. High temperatures lead to the formation of igneous intrusions and metamorphic aureoles. Paired metamorphic belts will form at convergent margins with subduction zones. The zone closest to the trench will have high pressure due to compressive stress and low temperature as no magma is rising. The zone further away has high temperature due to rising magma and low pressure. Convergent plate margins continental continental Fold mountains form at these margins (see spread 1.3.7) where the Earth s crust is deformed, thickened and there is extensive intrusive igneous activity. The Himalayan mountain range began to form about 50 Ma when India collided with Asia. The Himalayas are still growing as the plates are still moving towards each other. High temperatures and pressures acting over such long periods create broad (>100 km 2 ) and often complex orogenic belts affected by all grades of regional. At the deepest part of the orogenic belt the pressures and temperature will be highest, giving high grade regional. Away from the collision zone and higher in the crust the grade of will be low. 150

14 Module 4 Metamorphic Processes and Products N Honshu Paired metamorphic belts in Japan High pressure low temperature Regional High temperature low pressure Kyushu Shikoku Crust High temperature low pressure belt Volcanoes Moho Low temperature high pressure belt Sediments Thrusts Trench Oceanic crust Moho Sea level Key definition Migmatite is 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, at the high temperature boundary between and igneous activity. Rising magma Partial melting of subducting plate Lithosphere plate Asthenosphere Figure 1 Paired metamorphic belts in Japan Grades of regional metamorphic rocks Regional of shale produces the following rocks (they are all described in spreads and 2.4.3): low grade: slate medium grade: schist high grade: gneiss Regional of orthoquartzite and limestones produces the same products as contact quartzite and marble. Each of these rocks is composed of only one mineral, quartz and calcite, respectively. The minerals are equi-dimensional, so they cannot align under pressure. Pressure/kbar Temperature/ C Slate Schist Gneiss Migmatite Increasing metamorphic grade Figure 2 Regional metamorphic rocks and their relationship to pressure and temperature Figure 3 Migmatite Questions 1 Describe the formation of a paired metamorphic belt. 2 Describe two general changes that would occur in a mudstone during regional. 3 Draw up a table with the following headings: metamorphic grade; parent rock; metamorphic rock; mineral composition; texture. Complete it using the information in this spread and spreads and

15 2.4 8 Regional metamorphic zones Key definitions An index mineral is a metamorphic mineral that is stable over a particular temperature and pressure range (e.g. mica, garnet, Al 2 SiO 5 polymorphs). They indicate the metamorphic grade (see spread for a defi nition of metamorphic grade). It is possible to map metamorphic grade using index minerals. The fi rst appearance of the index minerals is mapped as they may still remain stable at higher temperatures and pressures. An isograd is a line on a map joining points of equal metamorphic grade. They join places where the fi rst appearance of an index mineral occurs. A metamorphic zone is the area between two isograds. The zone is named after the lower grade isograd. All locations within a metamorphic zone experienced the same metamorphic grade. A Barrovian zone is a metamorphic zone mapped using index minerals identifi ed by George Barrow. Mapping the Dalradian Supergroup In 1893, George Barrow mapped a sequence of highly deformed regionally metamorphosed rocks in the south-eastern part of the Scottish Highlands. The and deformation occurred during closure of the Iapetus Ocean and the Caledonian orogeny about 400 Ma ago. These Precambrian rocks are known as the Dalradian Supergroup. As you already know, clay-rich sedimentary rocks such as shale produce a variety of metamorphic minerals, as temperature and pressure conditions change. When Barrow mapped rocks like these, he noticed that there was a pattern to the occurrence of metamorphic minerals. He used the fi rst appearance of some of these minerals, which he termed index minerals, to draw isograds. Some of the minerals that crystallise at low grades are stable at higher grades so more than one index mineral can be found in one rock. He was able to map metamorphic zones using index minerals and isograds, which defi ne the boundaries of the zones. Although he did not do all the mapping personally, the system he devised was named after him and the zones are called Barrovian zones. Index minerals S = Sillimanite K = Kyanite G = Garnet B = Biotite C = Chlorite 0 100km C C B G B K G Increasing grade Figure 1 Index minerals, isograds and metamorphic zones Index minerals and metamorphic zones G S K K North Isograds Metamorphic grade low medium high Rock type Slate Schist Gneiss Index minerals and metamorphic zones Chlorite Biotite Garnet Kyanite Sillimanite The chlorite zone represents low grade (low pressure and low temperature) regional. The rock is slate where most of the rock has recrystallised but some clay minerals may still exist. Schists develop as a result of increasing temperatures and pressures and can be found in both the biotite and garnet zones. The grain sizes increase with metamorphic grade. Schists formed at lower temperatures and pressures are composed of quartz, muscovite mica and biotite mica. Medium grade results from higher temperatures and pressures and many schists formed at this grade contain garnet, and less commonly, kyanite porphyroblasts. Kyanite is typically found in gneisses and the kyanite zone represents high grade regional. The sillimanite zone represents high grade regional with very high temperatures and pressures. The rocks are gneisses. Estimates based on the sillimanite zone indicate a maximum temperature of about 700 o C and maximum 152

16 Module 4 Metamorphic Processes and Products pressure of about 7 kb. This pressure exists at a depth of about 25 km below the surface of the continental crust. It gives a geothermal gradient of about 28 o C km 1. Regional metamorphic zones Quartz and plagioclase feldspar are stable throughout the whole range of grades. This makes them no use as index minerals. Moine Thrust Great Glen Fault 7 6 Highland Boundary Fault Pressure/kbar KYANITE Triple point SILLIMANITE 2 Dalradian metamorphic rocks Barrovian zones Chlorite Biotite 50km Garnet Kyanite Sillimanite Unknown Figure 2 Regional metamorphic zones 1 0 ANDALUSITE Temperature/ C Metamorphic path for regional Metamorphic path for contact Figure 3 Kyanite and sillimanite The Al 2 SiO 5 polymorphs in regional The Al 2 SiO 5 polymorphs kyanite and sillimanite are found in regional metamorphic rocks. A rock formed at high pressure and low temperature may contain kyanite. A rock formed at high temperature or at high temperature and high pressure may contain sillimanite, which can be found in contact and regional, both of which can involve high temperature. With increasing metamorphic grade, regional follows a path from kyanite to sillimanite on the Al 2 SiO 5 polymorph phase diagram. Questions 1 Describe the rocks found in each of the Barrovian zones. 2 Explain why clay-rich parent rocks are the most useful in mapping metamorphic zones. 3 Explain the difference between a polymorph and a pseudomorph. 153

17 Unit 2 Examination questions 1 Below is a diagram of the rock cycle. A MAGMA crystallisation crystallisation Rocks Igneous EXTRUSIVE B Partial melting Metamorphism Weathering Metamorphic rocks Erosion D Metamorphism C Deposition Products Processess Earth s surface From mantle Figure 1 (a) (i) Complete the table below using the diagram of the rock cycle. Location Process or product A B C D [4] (ii) Name two processes that occur after deposition to produce rock group D. [2] (b) Explain how the crystal grain size of igneous rocks is related to the depth at which they crystallised. [2] (c) Explain the difference between an era and a system. Give one example of each. [2] Total 8 (OCR 2832 May 06) 2 Descriptions of three igneous rocks are given in the table below. Rock A Rock B Rock C Description Flow banded Light grey or red or brown colour Very fine crystals <1 mm Conchoidal fracture Black colour No crystals Coarse crystal grain size Greenish black crystals of augite and homblende White crystals of plagioclase fedspar White crystals of potash fedspar (a) (i) Identify the three rocks A, B and C. [3] (ii) Describe with the aid of a sketch the term flow banding. [2] (iii) Explain why igneous rock B has no crystals. [1] (iv) Define the term conchoidal fracture. [1] (b) Plagioclase feldspar, augite and hornblende are all part of Bowen s Reaction Series. They have been entered on the reaction series diagram below. D Augite Hornblende Biotite E F Na rich plagioclase feldspar Ca rich plagioclase feldspar G Figure 2 (i) Name the minerals D, E, F and G from Bowen s Reaction Series. [4] (ii) Explain the relationship of Bowen s Reaction Series to temperature. [2] (iii) Name the minerals that form the discontinuous part of Bowen s Reaction Series. [1] (OCR 2835 June 06) (c) The table below shows the chemical composition by percentage of oxides of four igneous rocks H, J, K and L. (i) To which igneous rock groups do H, J, K and L belong [4] (ii) Describe the changes in the % of oxides of silicon and sodium compared to iron and magnesium across the four rock groups. [2] Oxide % A B C D SiO Al 2 O Fe oxides MgO CaO Na 2 O K 2 O others Total 20 (OCR 2835 June 06) 154

18 Unit 2 Rocks Practice questions 3 (a) The table below shows the results of a student s research into the world s top 12 most deadly volcanic eruptions. Rank Volcano Location Year of eruption Death Major cause of death 1 Tambora Indonesia Ash fall, starvation 2 Krakatau Indonesia Ash fall, tsunami 3 Mount Pelée Martinique Pyroclastic flows 4 Ruiz Colombia Lahars 5 Unzan Japan Volcano collapse, tsunami 6 Laki Iceland Starvation 7 Kelut Indonesia Lahars 8 Galunggung Italy Lahars 9 Vesuvius Italy Lava flows, lahars 10 Vesuvius Indonesia Ash falls, pyroclastic flows 11 Pandayan Indonesia Pyroclastic flows 12 Lamington Papua New Guinea Pyroclastic flows (i) Explain why Indonesia has so many volcanic eruptions. [2] (ii) Using the table calculate the percentage of eruptions that had starvation as a major cause of death. Show your working. [2] (iii) Suggest reasons why the global summer of 1816 was very cold. [3] Total 7 (OCR 2832 May 07) 4 The graphic log below shows a commonly found sequence of sedimentary rocks. E D C B A Clay Silt fmc Sand 50cm Granules Figure 3 (a) (i) Using the graphic log, name and explain the formation of the sedimentary structure shown by the change in grain sizes in bed A. [3] (ii) Flute casts are found at the base of bed A. Draw a labelled diagram of a flute cast. Explain how a flute cast is formed. [3] (OCR 2835 June 06) (b) The diagram below shows thin section drawings of two metamorphic rocks and their sedimentary parent rocks. Cement Quartz cement mm Quartz Rock L Rock M Rock N Rock O Figure 4 Calcite (i) Complete the sentences below by entering the correct rock letters. Rock is the parent of rock. Rock is the parent of rock. [2] (ii) Describe how rock L forms. [2] (iii) Rock N has symmetrical ripple marks on the bedding planes. Describe the environment in which rock N was deposited. [2] (c) i) Describe one mechanical weathering process, operating in a cold climate, that affects limestone. [2] (ii) State the shape of the scree fragments. [1] (iii) Describe one chemical weathering process that affects the limestone. [2] Total 17 (OCR 2832 May 07) 5 (a) Describe how the following factors control. (i) temperature [2] (ii) pressure [2] (b) Regional metamorphic rocks form as a result of changes in both temperature and pressure. (i) Name the rock type that is formed as a result of the regional of pure limestone and pure sandstone. [2] (ii) Explain why shales give rise to a wide variety of new metamorphic minerals when regionally metamorphosed. [2] (iii) Define the following terms: index mineral [1] isograd [1] (OCR 2835 June 06) (c) Explain why rocks formed by contact lack any foliation. [2] Total 13 (OCR 2832 May 06) 6 Using diagrams explain the differences between sills and lava flows. [8] (OCR 2832 May 06) 7 Describe with the aid of diagrams, the processes of compaction and cementation. [8] (OCR 2832 May 07) 155