2. ... the textural and mineralogical change rocks undergo when put under great heat and/or pressure. Metamorphism

3. Where heat and/or pressure changes a rock mineralogically and/or texturally but does not melt it, the rock is said to be metamorphosed. There are three types of metamorphism :  contact metamorphism - involving heat only  dynamic metamorphism - involving pressure only  regional metamorphism - involving both heat and pressure Both contact and dynamic are localised in extent ; regional metamorphism affects vast areas and is associated with mountain building episodes or orogenies.

4. dynamic metamorphism : fault breccia / mylonite Types of Metamorphism spotted slatehornfels fault breccia mylonite slate phyllite schist gneiss migmatite Depth (km) contact metamorphism : spotted slate / hornfels regional metamorphism : slate (low grade) phyllite (medium grade) schist (high grade) gneiss (very high grade) migmatite (partially melted gneiss)

5. Contact Metamorphism When a large intrusion is emplaced in the country rock, its heat bakes the country rock hard close to the contact to form hornfels. With increasing distance away from the contact the rock is heated to a lesser degree and spotted rocks result. The spots are often caused by the formation of the aluminium silicate - chiastolite. Chiastolite is a form of andalusite and is easily recognisable by its cruciform appearance under the microscope. unaltered country rocks igneous intrusion inner hornfels zone outer zone of spotted rock

6. Examples of Contact Metamorphism A baked shale or mudstone is called a hornfels. Metamorphosed sandstone is quartzite. Metamorphosed limestone is marble. In contact metamorphism heat only is involved. It occurs when hot magma is intruded into the country rock. The heat of the magma bakes the country rock it comes in contact with. Dolerite intruded into sandstone sandstone forced up and baked by hot basic magma Limestone metamorphosed to marble Granitic magma intruded into limestone the contact

7. Dynamic Metamorphism Dynamic metamorphism involves pressure alone. The rocks affected are crushed and ground down by immense shearing forces i.e. mylonitised. The rock produced is called mylonite. The presence of mylonite is a sure sign of thrust faulting. The mylonite seen below is a tiny part of the Moine Thrust Fault.

8. Dynamic Metamorphism Under lower pressures, i.e. in crustal rocks nearer the surface, the rock is shattered and forms a fault breccia. The spaces between the rock fragments may be filled in at later date by calcite or quartz. In some cases a clay gouge is created, which weathers away rapidly. This fault shows both. clay gouge fault breccia

9. Regional Metamorphism Regional metamorphism involves both heat and pressure. The rocks are much altered and deformed. Various types of metamorphic fabric are produced in the rock often depend on the grade of metamorphism involved. Slaty cleavage, schistosity and gneissose banding are the main fabrics. In the example opposite the rock displays gneissose banding.

10. Foliation A rock with a foliation that splits along mica-rich layers is said to have a schistosity. Mica schists and chlorite schists all display foliation. But, though most schists have foliation, some have lineation... When a rock rich in planar minerals such as mica is subjected to intense pressure, a foliation is produced in the rock. The pressure squeezes and flattens the micas into parallel layers at right angles to the forces exerted.

11. Lineation The mineral hornblende, one of the amphiboles, has long pencil shaped crystals. When compressed they line up in one direction at right angles to the forces exerted. The resultant fabric is known as a lineation. But lineations can be caused by other factors, e.g. by shearing or crenulation cleavage hornblende schist with lineation sheared feldspar crystals

12. Slaty Cleavage When mudstones, shales and siltstones are regionally metamorphosed at lower temperatures and pressures, a slaty cleavage is produced. It is a form of foliation in which the planar mineral chlorite is flattened at right angles to the compressive forces exerted. Slate is a low grade metamorphic rock.

13. Gneissose Banding Gneiss is a high grade metamorphic rock in which the minerals separate out into discrete bands. In many cases some partial melting appears to take place and thin granitic bands form. Whem this occurs, the rock is correctly known as a migmatite. gneissose banding

14. Foliation and bedding In sedimentary rocks that undergo metamorphism the foliation can cut across the original bedding of the rock. In this example the original bedding dips steeply to the right of the compass. The foliation, however, is seen dipping at a gentler angle just above the compass. In fact the foliation displayed here is a type called crenulation cleavage, where one foliation is later cut by a second at a different angle. line of bedding line of foliation

15. Augen Gneiss Inchbae augen gneiss is very distinctive and is only found in a small area just north of Garve. However the action of ice has distributed it widely across the inner Moray Firth and it is a useful erratic. When granite is regionally metamorphosed, only textural change occurs ; there is no mineralogical change. The resulting rock is augen gneiss with its distinctive ‘eye-shaped’ feldspar phenocrysts. direction of foliation augen gneiss erratic

16. Metamorphic rocks in thin section Metamorphic rocks are usually conspicuous by their layered or flow-like textures. In all the examples below, note the foliation in particular in the schist where the micas flow around the garnets. Schists under XPL with staurolite porphyroblasts

17. The Barrovian Sequence Aberdeen geologist, George Barrow, noticed that certain minerals appeared in the regionally metamorphosed Dalradian schists north of the Highland Boundary Fault and that these minerals formed a distinct pattern when mapped. Going north along the blue line the minerals change from :- chlorite to biotite to garnet to kyanite and to sillimanite. Today this is called the Barrovian sequence. The named minerals are index minerals, i.e. they indicate the grade of metamorphism.

18. Boudinage Because regionally metamorphosed rocks are often put under immense pressures, some layers get pulled and broken into fragments called boudins. Meta-dolerites are frequently affected as here in Lewisian gneiss boudin of amphibolite boudins of amphibolite