Metamorphic

• Metamorphism is the changes that occur to a rock’s mineralogy and texture in the solid state, brought about by the input of thermal energy (heat) and/or mechanical energy.
• Changes occurring during weathering and diagenesis are usually excluded from metamorphism.
• Temperature has a profound effect on metamorphism because it controls the stabilities of minerals and mineral assemblages.
• P-T diagram showing some of the reactions that can occur in metamorphosed mudstones (pelites).
• Metamorphic processes can be observed through a microscope as recrystallisation and neomineralisation.
• In response to heat, recrystallisation of existing minerals can change texture and form new minerals through neomineralisation due to chemical exchange between reacting minerals.
• Neomineralisation is the formation of new minerals at the expense of existing ones (new formulae and polymorphs).
• Neomineralisation involves chemical reactions between minerals (± fluid).
• A variety of elements in the clay minerals in the shale have recombined to form micas, garnets and stauralite.
• During metamorphism, existing minerals can adopt new textures (shapes and sizes) and some minerals can adopt a preferred orientation.
• Quartz has recrystalised in the solid state from a rounded, abraded sedimentary texture to an interlocking ‘granoblastic’ texture.
• Changes occurring during regional metamorphism can be seen in the metamorphism of mudstones, pelites, slate, and schist.
• Heating due to intrusion of magma can cause changes in minerals, leading to contact metamorphism, which is often seen in thermal aureoles around igneous intrusions.
• Regional metamorphism is almost always associated with contraction (deviatoric stresses), so development of cleavage to schistosity with increasing temperature and pressure is common.
• Regional metamorphism is associated with distinct mineral assemblages with increasing temperature and pressure, and grainsize increases with increasing temperature and pressure.
• Medium grade metamorphism, exemplified by muscovite crystals growing bigger and other new minerals forming, can be seen in schist.
• High grade metamorphism, exemplified by changes in texture, can be seen in gneiss.
• Contact metamorphism results in the formation of non-foliated rock, with distinct increase in crystal growth size, often with formation of recrystalised aggregate 'spots'.
• Low grade metamorphism, exemplified by changes in bedding and cleavage, can be seen in pelites.
• Dislocation, or dynamic metamorphism, occurs in fault zones, where minerals have moved due to deformation.
• Shock metamorphism occurs due to intense, short-lived shock pressures due to meteorite impacts.
• Rotation of flaky minerals into parallelism, and/or growth of new flakes in the new orientation, has resulted in the development of cleavage, also known as foliation.
• No new minerals have formed during the changes in quartz.
• Stress, in geology, is roughly to a pressure of 10kbar.
• Directed stress does not affect a rock’s volume but causes a rock to change in shape by being flattened or sheared.
• The nature of migmatite is described and its origin is explained in chapter 3.
• This temperature range is referred to as the anchizone.
• In sedimentary rocks, the process of these temperatures is called diagenesis (pronunciation: diagenesis), rather than metamorphism.
• At the upper end of the temperature range, metamorphism causes some rocks to partially melt, and a kind of gneiss is produced.
• Between roughly 100°C and 150°C, diagenesis is sometimes called very-low-grade metamorphism, also known as anchimetamorphism (pronunciation: anchimetamorphism).
• Stress can be resolved into two parts: Load Pressure, which acts uniformly in every direction, and Deviatoric Stress, which is different in different directions.
• Deviatoric Stress also results in plastic deformation in warm ductile middle & lower crust, as heat reduces the strength of rock.
• New minerals’ shape and orientation are also guided by deviatoric stress.
• Diagenesis follows lithification.
• Deviatoric Stress is caused by lateral transmission of tectonic forces through the solid Earth’s crust, for example at convergent plate margins.
• Directed stress (flattening) and directed stress pressure are terms used in the context of metamorphism.
• Deviatoric Stress results in fracturing (faulting) in cold, brittle upper crust.
• Deviatoric stresses have little effect on mineral stabilities, but can have a major effect on texture if they act over long periods of time because large cumulative deformation effects can build up.
• Existing minerals may be compressed, elongated or rotated due to deviatoric stress.
• Stress: Force per unit area, measured in Pascal (Pa), bar, or kbar.