Metamorphism and Metamorphic Rocks French Alps, highly deformed and metamorphosed GLY 2010 - Summer 2013 - Lecture 11

Metamorphism The mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions These conditions have generally been imposed at depth below the surface zones of weathering and cementation They differ from the conditions under which the rocks in question originated

Metamorphic Rock Any rock derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state Derivation is in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at depth in the Earth's crust

Agents of Metamorphism Temperature Pressure Chemically Active Fluids Covered on following slides

Temperature Minerals in rocks which are heated may become unstable Crystals may grow together to form larger crystals New minerals may form The usual temperature of metamorphism is 200 to 800C Temperature increases with depth - geothermal gradient Two sources of heat Heat from the original accretion of the earth Heat from radioactive decay of long-lived isotopes 235U (Uranium) 238U (Uranium) 232Th (Thorium) 40K (Potassium)

Pressure Minerals in rocks which are under increased pressure may become unstable Pressure may be directed or hydrostatic

Directed Pressure Pressure may be directed (along a single axis) Causes shortening of the affected objects Tectonic collisions and impacts produce directed pressure Leads to foliation

Metamorphism from Compression

Effect of Compression Compression has deformed the strata and caused metamorphism This represents a continent-continent plate collision

Lithostatic Pressure Lithostatic pressure is equal in all directions Load pressure is a type of hydrostatic pressure and increases with depth Divers experience hydrostatic pressure Rock or sediment are much heavier than water, the corresponding pressures under these substances are 2-3x larger than under an equal thickness of water New, denser minerals may form Example: Calcite  Aragonite

Confining Pressure At depth, lithosphatic pressure is due to confinement

Chemically Active Fluids Fluids from different sources Fluids are capable of dissolving ions and ion-exchanging with existing minerals, to produce new minerals Metamorphism releases fluids as temperature and pressure increase Fluids may come from other sources, such as escaping from magma

Regional Metamorphism Burial Dynamothermal Fault Metasomatism Each topic is discussed on further slides

Burial When sediments are deposited, they begin to change Often associated with large sedimentary basins First changes are to sedimentary rock Increasing depth of burial raises the temperature and pressure, and begins to form new minerals

Dynamothermal Rocks caught between two colliding plates Seen as the roots of ancient mountain regions, such as in parts of Minnesota, Wisconsin, and Michigan Directed pressure may forces some rocks up, while others are thrust down into the earth Rocks deep within the earth are subject to additional load pressure, and heat

Fault Movement of rock along fault generates shearing stress in rock (increased pressure) Stress can lead to smearing of rocks along the fault, and the creation of some new minerals

Metasomatism The presence of interstitial, chemically active pore liquids or gases contained within the rock body or introduced from external sources are essential for the replacement process Often, though not necessarily, this occurs at constant volume with little disturbance of textural or structural features

Contact Metamorphism aka Thermal Proximity to a heat source may cause new minerals to form Zone of contact metamorphism is known as contact aureole Magma is usually the heat source Grain size is usually small Does not produce large volume of rock Image source: http://geollab.jmu.edu/Fichter/MetaRx/Contact.html

Contact Metamorphic Diagram Development of contact metamorphic aureole

Foliation A general term for a planar arrangement of textural or structural features in any type of rock Involves alignment of platy or elongated minerals grains Especially the planar structure that results from flattening of the constituent grains of a metamorphic rock

Foliation exposed in a mine Björnevatn Mine, Kirkenes, Norway

Foliated Vs. Nonfoliated Metamorphic rocks produced by directed pressure, such as tectonic or shearing forces, will be foliated Rocks produced by load pressure, or not involving pressure, will not be foliated

Development of Foliation

Metamorphic Grade Grade indicates the severity of conditions at the time of metamorphism Grade is judged on the basis of: Foliation Grain size Mineral assemblage - index minerals

Metamorphic Grade Indicators

Metamorphic Mapping The first appearance of grade minerals can be used to produce maps of metamorphism

Foliated Rocks Rocks shown following slides

Slate Red slate contains oxidized iron A compact, fine-grained metamorphic rock that possesses slaty cleavage and hence can be split into slabs and thin plates Most slate was formed from shale Cinema Expeditions CD-ROM 5 Rocks Upper Photo \metamorp\slate\449.051.jpg

Slaty Cleavage Pervasive, parallel foliation of fine-grained, platy minerals (mainly chlorite and sericite) in a direction perpendicular to the direction of maximum finite shortening Developed in slate or other homogeneous sedimentary rock by deformation and low-grade metamorphism Text photo 7-7 Gray slate showing foliation (slaty cleavage) in quarry near Alta, Norway

Uses of Slate Pool table at Campobello, F.D. Roosevelt's home Slate is used as the bed of high quality pool tables Slate may be used for roofing, and sometimes for walkways, although it is quite slippery when wet Cinema Expeditions CD-ROM 5 Rocks Left Photo \metamorp\slate\449.046.jpg Right photo: http://www.ansroofing.com/images/roof2.jpg Slate Roof

Phyllite A metamorphosed rock, intermediate in grade between slate and mica schist (Upper) Cleavage surfaces have a silky sheen (Lower) Phyllites commonly exhibit corrugated cleavage surfaces Cinema Expeditions CD-ROM 5 Rocks Upper Photo \metamorp\phllite\449.043.jpg Lower Photo \metamorp\phllite\449.044.jpg

Schist A strongly foliated crystalline rock, formed by dynamic metamorphism May be readily split into thin flakes or slabs due to the well developed parallelism of more than 50% of the minerals present (schistosity), particularly those of tabular or elongate prismatic habit Garnet is usually a metamorphic mineral Cinema Expeditions CD-ROM 5 Rocks Upper Photo \metamorp\schist\449.034.jpg Garnet schist showing well-developed schistosity

Mica Schist (Above) Mica imparts a shiny luster to rock (Left) Garnet mica schist Mica is very commonly found in schist - the island of Manhatten is composed of mica schist Cinema Expeditions CD-ROM 5 Rocks Upper Photo \metamorp\schist\449040.jpg Lower Photo: Text figure 7-9

Gneiss High grade regional metamorphic rock A foliated rock formed by regional metamorphism, in which bands of granular minerals alternate with bands in which minerals having flaky or elongate prismatic habits predominate Generally less than 50% of the minerals show preferred parallel orientation Cinema Expeditions CD-ROM 5 Rocks Photo \metamorp\gneiss\449.016.jpg High grade regional metamorphic rock

Gneiss Continued Cinema Expeditions CD-ROM 5 Rocks Upper Photo \metamorp\gneiss\449.039.jpg Gneissic foliation often involves felsic and mafic layers

Contortion in Gneiss Gneiss rocks form under great pressure and at high temperatures They may show contorted folding Folding is a response to directed pressure - the rock has shortened along the horizontal direction Cinema Expeditions CD-ROM 5 Rocks Upper Photo \metamorp\gneiss\449.013.jpg

Migmatite Figure 7.24 in text

Increasing Metamorphic Grade Figure 7-31 in text

Non-foliated Rocks Each rock type is shown on following slides

Marble Metamorphic equivalent of limestone Not foliated during metamorphism; may contain relict sedimentary bedding, as in the lower photo Dark band is organic matter that was deposited with the limestone Cinema Expeditions CD-ROM 5 Rocks Upper Photo \metamorp\marble\449.053.jpg Lower Photo \metamorp\marble\449.056.jpg

Quartzite A metamorphic rock consisting mainly of quartz Formed by recrystallization of sandstone or chert by either regional or contact metamorphism Cinema Expeditions CD-ROM 5 Rocks Photo \metamorp\quartzite\449.073.jpg A metamorphic rock consisting mainly of quartz Sioux Quartzite, South Dakota

Hornfels A fine-grained rock Composed of a mosaic of equidimensional grains No preferred orientation Typically formed by contact metamorphism Heat from the magma backs the rock it comes in contact with Sometimes a piece of rock falls into the magma - if it doesn’t melt, it will be backed on all sides http://www.uct.ac.za/depts/geolsci/cape.html Photo #d39.jpg

Table Mountain Capetown, South Africa Table Mountain is a famous landmark in South Africa The Malmesbury group is seen to the lower left of the photo, as shown in the diagram - It contains a hornfels http://www.uct.ac.za/depts/geolsci/cape.html Upper photo #d45.jpg Lower diagram tm1.gif

Malmesbury Hornfels Close up, the Peninsula granite is a coarse-grained rock consisting of large (2-5cm) white or pink feldspar crystals, glassy brown quartz and flakes of black mica containing inclusions (xenoliths) of dark Malmesbury hornfels The Malmesbury Group was made famous by Charles Darwin during his voyage of scientific discovery on H.M.S. Beagle in 1844

Anthracite Anthracite is metamorphosed coal It is almost pure carbon Represents the highest grade of coal May be recognized by its shiny appearance Cinema Expeditions CD-ROM 5 Rocks Photo \metamorp\anthrac\449.081.jpg Anthracite is metamorphosed coal

Fault Metamorphism Brittle fracture under low confining pressure Ductile flow under high confining pressure Figure 7-20 in text

Skarn Lime-bearing silicates, of any geologic age, derived from nearly pure limestone and dolomite with the introduction of large amounts of silicon, aluminum, iron and magnesium Skarns can form during regional or contact metamorphism Form from a variety of metasomatic processes involving fluids of magmatic, marine and/or metamorphic origin Mineralogy defines a rock as skarn

Skarns As Ores Gold-bearing vesuvianite and grossular skarn, near Benambra, northeast Victoria, Australia This deposit contains some exotic secondary phosphate and vanadate minerals Skarn formation often produces metal-containing minerals enriched enough to be ores http://www.mov.vic.gov.au/mineralogy/skarn.html Photo #vesu.jpg