Module 8: Metamorphism

• Metamorphism - Process of change undergone by an existing rock (protolith) in solid-state to another rock.
• Rocks undergo metamorphism because of heat, pressure and chemically active fluids. (agents of metamorphism)
• Diagenesis, a sedimentary process, is conserved as the lower limit of metamorphism.
• Melting, an igneous process, is observed as the upper limit of metamorphism.
• The three possible sources of heat for metamorphism to occur are geothermal gradients, intrusive bodies and radioactive decay.
• Heat mainly provides the energy to drive chemical reactions, such as the recrystallization and formation of new minerals
• There are two possible sources of pressure which may induce metamorphism which are confining pressure and differential pressure.
• Confining pressure succeeds diagenesis after sediments have been deposited. It is also referred to as lithostatic pressure and applies equal stress in all directions. Because of this, no distinct orientation of the minerals will be produced.
• Differential pressure is unequal pressure applied in different directions. Results in a specific orientation of minerals after metamorphism persists.
• Water trapped within sediments may be expelled due to increase in temperature or pressure. Similarly, these conditions may cause the release of hydroxide ions in water-bearing minerals such as amphiboles and mica. All of this is caused by the presence of chemical active fluids.
• Magmatic intrusions also contain fluids which react with the surrounding country rock to induce metamorphism. Chemically-active fluids leach ions from pre-existing rocks and accumulate them along planes of weakness such as fractures and faults. This process is referred to as metasomatism.
• The types of metamorphism based on the dominant agent of metamorphism are contact metamorphism (temperature dominant), dynamic metamorphism (deviatoric stress dominant) and static metamorphism (pressure dominant)
• In contact metamorphism, magma invades pre-existing rock that creates a zone of alteration (aureole) around the intrusion. Takes place at shallow depths and low pressure. Creates non-foliated rocks.
• Bake-and-Chill Margins show textural contrast between an intrusion and the country rock caused by contact metamorphism.Surfaces of the intrusion immediately in contact with the cold country rock become glassy due to a rapid drop in the temperature. Mineral grains of the country rock undergo recrystallization resulting to larger grains.
• In Dynamic Metamorphism, rocks are subjected to shear forces along a fault zone. Dynamic metamorphism may occur locally or regionally. When undergoing brittle deformation, fault breccias are produced; ductile deformation results to the formation of mylonites.
• In Burial/Static Metamorphism, massive amount of materials accumulating in a subsiding basin induces low-grade metamorphism in the deepest layers.
• Other types of metamorphism based on dominant agent of metamorphism are metasomatism (hydrothermal metamorphism by chemically active fluids), Dynamothermal/regional metamorphism(both high pressure and temperatures), and Impact metamorphism (short period of high temperature and pressure)
• Dynamothermal metamorphism typically occurs during orogenesis where large segments of crust are subjected to high pressure and high temperature.
• In Hydrothermal metamorphism (metasomatism), hot ion-rich fluids circulate through fissures and cracks in rocks associated with emplacement of magma(e.g. hydrothermal vents).
• In impact metamorphism, it occurs when high-speed projectiles called meteorites strike the earth's surface; high temperature and pressure over a short period; produces impactites.
• The types of metamorphism based on area or volume are local metamorphism and regional metamorphism.
• Types of metamorphism based on nature of chemicals affected are isochemical(textural change) and allochemical(chemical change).
• Protolith - literally "parent rock," represents the rock from which metamorphic rock was derived.
• Metamorphic rocks are typically classified by their texture, protolith and mineralogy.
• For foliated rocks, the trend of increasing metamorphism starts with shale, slate, phyllite, schist and finally gneiss.
• Metamorphic grade describes the relative temperature and pressure conditions under which metamorphic rocks form.
• Prograde metamorphism refers to the changes in a rock that accompany such increasing metamorphic grade. Retrograde refers to decreasing grade as a body of rock cools and recovers from a metamorphic or igneous event, and retrograde metamorphism describes any accompanying changes.
• Metamorphic facies are a set of rocks representing the full range of possible chemistries within a limited range of metamorphic conditions.
• Marble is formed from contact metamorphism of limestones, used primarily in construction
• Asbestos is formed from hydrothermal metamorphism of ultramafic rocks, used for insulation and firefroofing.
• Metals from Hydrothermal Deposits includes economically important minerals like copper, gold, zinc, etc.
• Some metamorphic rocks in the Philippines include Amphibolite(composed of foliated amphibole and plagioclases; protolith: gabbros and diorite) and Serpentinite(composed of serpentine as a product of hydrothermally alteration of peridotites -aka mantle rocks)
• In metamorphic facies, low P, high T: hornfels, Low P, contact: sandinite, Dynamothermal: greenschist, amphibolite and granulite, static: Zeolite, prehnite-pumpellyite and blueschist.
• Metamorphism at 3.5 kb and 300 C, Marble: calcite, dolomite and minor quartz; Metabasalt: chlorite, albite, actinolite, epidote and minor quartz; Pelitic Schist: Quartz, chlorite, muscovite and albite
• Metasomatism - fluids transport mineral matter from one mineral to the other
• Biology 11 1st Long Exam Reviewer