Lecture 7 - Metamorphic Rocks PDF

METAMORPHIC ROCKS Review: What is metamorphism? • Metamorphism is the change/s experienced by an existing rock (e.g. igneous, sedimentary or metamorphic), in the solid state, which results to the formation of another group of rocks called metamorphic rocks. • The transition of one rock into ano...

METAMORPHIC ROCKS Review: What is metamorphism? • Metamorphism is the change/s experienced by an existing rock (e.g. igneous, sedimentary or metamorphic), in the solid state, which results to the formation of another group of rocks called metamorphic rocks. • The transition of one rock into another by temperatures and/or pressures unlike those in which it formed. • Metamorphic rocks are produced from • Igneous rocks • Sedimentary rocks • Other metamorphic rocks METAMORPHISM • Metamorphism progresses incrementally from low-grade to high-grade • During metamorphism the rock must remain essentially solid • Metamorphic settings • Contact or thermal metamorphism – driven by a rise in temperature within the host rock. • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism - occurs during mountain building. • Produces the greatest volume of metamorphic rock • Rocks usually display zones of contact and/or hydrothermal metamorphism. METAMORPHISM Prograde metamorphism results from increasing temperature and/or pressure conditions over time. Retrograde Metamorphism Retrograde metamorphism results from decreasing temperature and/or pressure so that lower temperature/pressure mineral assemblages develop that overprint earlier peak temperature/pressure mineral assemblages. Volatile components serve as catalysts in driving retrograde metamorphic reactions. Without the addition of volatiles from an external source, retrograde metamorphic conditions are difficult to attain because previously occurring prograde metamorphism has already depleted the rock in volatile components. AGENTS OF METAMORPHISM: HEAT • Heat • The most important agent. • Provides the energy to drive chemical reactions → recrystallization of minerals. • Recrystallization results in new, stable minerals. • Two sources of heat: • Contact metamorphism – heat from magma. •Large bodies of molten rock or intrusive bodies • An increase in temperature with depth due to the geothermal gradient. •Geothermal gradient - temperature increases with depth (20o – 30oC per km in the crust). AGENTS OF METAMORPHISM: PRESSURE • Pressure (Stress) • Increases with depth. • Two types of pressure: • Confining pressure applies forces equally in all directions. • Rocks may also be subjected to differential stress which is unequal in different directions. AGENTS OF METAMORPHISM: PRESSURE Confining pressure – equal stress in all directions; from overlying rock. Differential stress – unequal pressure in different directions AGENTS OF METAMORPHISM: PRESSURE AGENTS OF METAMORPHISM: CHEMICALLY ACTIVE FLUIDS • Chemically active fluids • Mainly water with other volatile components • Enhances migration of ions • Aids in recrystallization of existing minerals • Sources of fluids? • Water trapped in pore spaces of the original rock. • Water released during dehydration of minerals, such as amphibole or mica. • Water from magmatic bodies (hydrothermal fluids). THE IMPORTANCE OF PARENT ROCKS • “Protolith”= parent rock of metamorphic rocks. • A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed. • The importance of parent rock: • Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed; • Mineral makeup determines, to a large extent, the degree to which each metamorphic agent will cause change. THE IMPORTANCE OF PARENT ROCKS TYPES OF METAMORPHISM • Contact or Thermal Metamorphism • Hydrothermal Metamorphism • Regional Metamorphism • Other Types: • Burial Metamorphism • Metamorphism along Fault Zones • Impact Metamorphism TYPES OF METAMORPHISM: CONTACT OR THERMAL METAMORPHISM • Contact or thermal metamorphism • Occurs due to a rise in temperature when magma invades a host rock. • A zone of alteration called an aureole forms in the rock surrounding the magma. • Most easily recognized when it occurs at the surface, or in a near-surface environment. TYPES OF METAMORPHISM: CONTACT OR THERMAL METAMORPHISM Contact metamorphism occurs when magma invades pre-existing rock. A zone of alteration called an aureole (or halo) forms around the emplaced magma . Takes place at shallow depths (0-6km) and low pressure. Metamorphic rocks produced: non-foliated; fine-grained. Metamorphism associated with igneous intrusions (Contact metamorphism) It is increase in temperature which causes the most striking effects on the mineral assemblages in a rock. The heat for metamorphism was mainly transferred across the contact by conduction through the country rocks (Note: baked and chilled margins) Away from the immediate contact at a time shortly after intrusion, the temperature will fall off rapidly with increasing distance, but as time goes on, the temperature gradients will tend to flatten out. The maximum temperature is always at the contact itself (Note: width of the baked margins—can be several meters). TYPES OF METAMORPHISM: HYDROTHERMAL METAMORPHISM • Chemical alteration at high temperatures and moderate pressures by hot, ion-rich (hydrothermal) fluids that circulate through fissures and cracks. • This is common in basaltic rocks where hydrothermal metamorphism results in alteration to such Mg-Fe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. • Rich ore deposits are often formed as a result of hydrothermal metamorphism. • Most widespread along the axis of the mid-ocean ridge system TYPES OF METAMORPHISM: REGIONAL METAMORPHISM • Takes place at considerable depths over an extensive area (5-20 km, sometimes more than 30 km) under high pressure and is associated with the process of mountain buliding. • When continents collide (A) or oceanic crust subducts (B). • Produces the greatest quantity of metamorphic rock. TYPES OF METAMORPHISM: OTHERS • Other types of metamorphism: • Burial metamorphism • Associated with very thick sedimentary strata. • Required depth varies from one location to another depending on the prevailing geothermal gradient. • Metamorphism along fault zones • Occurs at depth and high temperatures. • Pre-existing minerals deform by ductile flow. • “Slickensides” TYPES OF METAMORPHISM: OTHERS • Shock or Impact Metamorphism • Impact metamorphism • Occurs when high speed projectiles called meteorites strike Earth’s surface. • Products are called “impactites”. When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock. These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite. METAMORPHIC TEXTURES • Texture refers to the size, shape, and arrangement of grains within a rock. • Foliation – any planar arrangement of mineral grains or structural features within a rock. • Examples of foliation • Parallel alignment of platy and/or elongated minerals; • Parallel alignment of flattened mineral grains and pebbles • Compositional banding • Slaty cleavage where rocks can be easily split into thin, tabular sheets. • Foliation can form in various ways including • Rotation of platy and/or elongated minerals • Recrystallization of minerals in the direction of preferred orientation • Changing the shape of equidimensional grains into elongated shapes that are aligned Foliation Round grains can become flattened Sheet silicate minerals can have a preferred orientation Development of foliation due to directed pressure 1- Foliation (Cont.) II- Gneissosity: defined by compositional layering of equant crystals (e.g. quartz, feldspars) alternate with platy or elongate mineral layers (e.g. micas). It is usually coarse-grained size. METAMORPHIC TEXTURES: FOLIATED • Gneissic • During higher grades of metamorphism, ion migration results in the segregation of minerals; • Gneissic rocks exhibit a distinctive banded appearance. 1- Foliation (Cont.) IIISchistosity: defined by alignment of platy (mica, chlorite) or inequent (amphiboles, quarz) minerals - Minerals defining schistosity are said to posses preferred orientation and usually are medium-grained. METAMORPHIC TEXTURES: FOLIATED • Schistosity • Platy minerals are discernible with the unaided eye and exhibit a planar or layered structure • Rocks having this texture are referred to as schist. 1- Foliation (Cont.) IVCleavage: Schistosity surface along which the rock may break (cleave). It include: a- Slaty cleavage in very fine-grained mica and/or chlorite in slate and phyllite, bCrenulation cleavage: alignments with cmto mm-scale periodic folding METAMORPHIC TEXTURES: FOLIATED • Foliated textures: • Rock or slaty cleavage • Closely spaced planar surfaces along which rocks split • Can develop in a number of ways depending on metamorphic conditions and parent rock 1- Foliation (Cont.) V- Mylonite layering: defined by layers of highly strained rock with elongated grains due to grain size reduction and dynamic recrystalization during shearing 2- Lineation Lineation: parallelism or alignment of linear elements in the rock Types of lineations: a. Preferred orientation of elongated mineral aggregates (e.g. quartz pebbles in metaconglomerates) b. Preferred orientation of elongate minerals (feldspars & Hb) c. Lineation defined by platy minerals d. Fold axes (especially of crenulations) e. Intersecting planar elements. Foliation and Lineation METAMORPHIC TEXTURES: NONFOLIATED • Other metamorphic textures • Those metamorphic rocks that lack foliation are referred to as nonfoliated • Develop in environments where deformation is minimal • Typically composed of minerals that exhibit equidimensional crystals C- Textures donating lack of preferred orientation or equigranular grains: - Hornfelsic textures: random orientation of fine-grained rocks, due to lack of stresses, granofelsic texture for the medium to coarse grained rock C- Textures donating lack of preferred orientation or equigranular grains (Cont.) - Granoblastic texture: A mosaic of fine to coarse grained anhedral grains, such as marble and granulites D- Textures donating Large grains within the rock: -Porphyroblastic texture: A relatively large crystal (e.g. garnet, staurolite) in smaller fine grained matrix. It could be -Idioblast (Euhedral), -subidioblast (subhedral) or, - xenoblast (anedral). D- Textures donating Large grains within the rock: -Porphyroclastic texture: A large strained grain in fine grained matrix -Blastoporphyritic texture: A relict of porphyritic volcanic texture in metamorphic rocks - Augen texture: Porphyroblast of feldspars with eye-shape cross section in fine grained gneissic matrix E- Textures donating inclusion within or rim on a porphyroblasts: - Poikiloblastic or sieve texture: porphyroblast containing numerous inclusions of one or more fine grains. E- Textures donating inclusion within or rim on a porphyroblast: Corona or reaction rim: A zone consisting of grains of a new minerals that have formed at rim around mineral. Common metamorphic rocks • Foliated rocks • Slate • Very fine-grained • Excellent rock cleavage • Most often generated from low-grade metamorphism of shale, mudstone, or siltstone Common metamorphic rocks • Foliated rocks • Phyllite • Gradation in the degree of metamorphism between slate and schist • Platy minerals not large enough to be identified with the unaided eye • Glossy sheen and wavy surfaces • Exhibits rock cleavage • Composed mainly of fine crystals of muscovite and/or chlorite Phyllite (left) and Slate (right) lack visible mineral grains Common metamorphic rocks • Foliated rocks • Schist • Medium- to coarse-grained • Platy minerals predominate • Commonly include the micas • The term schist describes the texture • To indicate composition, mineral names are used (such as mica schist) A mica garnet schist Common metamorphic rocks • Foliated rocks • Gneiss • Medium- to coarse-grained • Banded appearance • High-grade metamorphism • Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian minerals Gneiss typically displays a banded appearance Common metamorphic rocks • Nonfoliated rocks • Marble • Coarse, crystalline • Parent rock was limestone or dolostone • Composed essentially of calcite or dolomite crystals • Used as a decorative and monument stone • Exhibits a variety of colors Marble – a nonfoliated metamorphic rock Common metamorphic rocks • Nonfoliated rocks • Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together Quartzite Major Metamorphic Rock Types Temp C Temp F Coal Limestone Sandstone Basalt Shale Index Minerals Slate Chlorite Phyllite Biotite Schist Garnet Lignite Bituminous 300 500 Anthracite 600 Graphite Marble Greenstone 700 800 500 900 1000 600 1100 1200 700 Quartzite Amphibolite Staurolite Gneiss Kyanite Sillimanite Melting Begins METAMORPHIC ZONES • Systematic variations in the mineralogy and often the textures of metamorphic rocks are related to the variations in the degree of metamorphism. • Index minerals and metamorphic grade • Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism. • Index minerals and metamorphic grade • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form. METAMORPHIC GRADES • Metamorphic rocks classified according to grain size of matrix: • Coarse grained- crystals more than 1 mm in diameter • Medium-grained – from 1mm to 0.1 mm • Fine grained – less than 0.1 mm • **Coarse-grained metamorphic rocks frequently display compositional banding or layering. This can be a result of metamorphism, rather than the survival of sedimentary layering. • Generalization: • For metamorphic rocks from one metamorphic complex which have undergone metamorphism for approximately the same length of time, the grain size increases with temperature, so that fine-grained rocks have probably been metamorphosed at low temperatures, coarse grained ones at higher temperatures. This is the “rough” basis for the degree of metamorphism aptly referred to as “metamorphic grade”. • Metamorphic rocks often display different grain sizes in minerals formed during metamorphism, and may also inherit variable grain sizes from their igneous or metamorphic precursors. Rocks with large metamorphic crystals set in a finer grained matrix are said to be porphyroblastic, and the large crystals are called porphyroblasts. In such rocks, it is the matrix grain size which is indicative of metamorphic grade. METAMORPHIC FACIES • Facies concept: • The idea of metamorphic facies builds upon the recognition of mineral assemblages coexisting in equilibrium, by emphasizing that any mineral assemblage will coexist over a particular range of metamorphic conditions, temperature and pressure being the two most important. This idea was first propounded by Finnish petrologist, P. Eskola in 1915. • Features of metamorphic facies • In all facies classification schemes, individual facies are defined by specifying the diagnostic mineral assemblage for rocks of common bulk compositions, such as basic igneous rocks and pelitic rocks. • Individual metamorphic facies are given names after common metamorphic rock type stable under the appropriate conditions (e.g., greenschist, eclogite), and in some schemes subdivisions of metamorphic facies may be called after diagnostic mineral assemblages. • Another feature of metamorphic facies classification of rocks is that it treats metamorphism as a process which goes on under one set of conditions of temperature and pressure. These are often loosely described as the “pressure and temperature of metamorphism” or ‘P-T conditions’. In more careful discussions, they will be taken as the conditions at the highest temperature point on the P-T path, often called the metamorphic peak. https://openpress.usask.ca/physicalgeology/chapter/10-4-metamorphic-facies-and-index-minerals-2/ The descending slab heats up slowly compared with its rate of descent into the mantle, and therefore the rocks within it remain relatively cool to very considerable depths, greater than the depth of Moho in ordinary continental lithosphere. Subduction zones therefore provide suitable conditions for metamorphism at unusually high pressures and low temperature—forming blueschist facies. Blueschists are typically found as blocks within tectonic melange. The rock is called blueschist because in hand specimens, and under the microscope, it is coloured blue by the presence of sodium rich amphibole, glaucophane. The basic igneous rock contains this amphibole, rather than actinolite or hornblende which would form during lower pressure regional metamorphism, because high pressure favours the coupled replacement of Ca and Mg in amphibole by Na. In some melanges, large blocks show zoning on a scale of meters, with blueschist metamorphic assemblages in the cores of the blocks and assemblages such as albite + epidote + actinolite (greenschist facies) round their margins. Eclogite - Subduction zone metamorphism - Composed of pyrope garnet (red) and omphacite (green – pyroxene) https://www.sandatlas.org/eclogite/ https://commons.wikimedia.org/wiki/File:Eclogite_Almenning,_Norway.jpg Names of metamorphic facies and typical mineral assemblages of basic rocks and pelitic rocks