Presentation on theme: "Ch. 21 An Introduction to Metamorphism Ch. 21 An Introduction to Metamorphism Metamorphism is the transformation of rock by temperature and pressure Metamorphism."— Presentation transcript:
Ch. 21 An Introduction to Metamorphism Ch. 21 An Introduction to Metamorphism Metamorphism is the transformation of rock by temperature and pressure Metamorphism is the transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Metamorphic rocks are produced by transformation of: Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks Kyanite, Sillimanite, and Andalucite, greenschist, amphibolite, glaucophane
0 km Sedimentary rock Metamorphic rock Igneous rock 50 km 10 km ~200ºC ~800ºC Increasing depth and temperature Melting Metamorphism Sedimentary rock Sediment Metamor -phism occurs between about 10 and 50 km of depth The rocks don’t melt
North Cascades Canadian Shield Rocky Mountains Grand Canyon Appalachian Mountains Black Hills Llano Uplift Usually buried deep, metamorphic rocks are seen when erosion removes covering rocks, and in the cores of mountains Glaciers exposed the Best US exposures in New England and the South
What causes metamorphism? 1. Temperature Increase Greater Temps at depth Radioactive Isotopes Intruding Magma Friction Between Moving Bodies of Rock
Metamorphic Agents and Changes Temperature: typically the most important factor in metamorphism Temperature: typically the most important factor in metamorphism Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements Figure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311. LESSON: It gets hotter faster with depth in continental lithosphere
Metamorphic Agents and Changes Increasing temperature has several effects 1) Promotes recrystallization increased grain size 2) Drives reactions 3) Overcomes phase stability barriers
What causes metamorphism? 2. Pressure (stress) = Force/meters 2 Increases with depth Pressure can be applied equally in all directions or differentially All Directions = “Confining Pressure” also called “lithostatic pressure” Differential = “Directed Pressure”
Origin of pressure in metamorphism (due to burial) (Convergent Margin) Confining pressure aka “lithostatic”
Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation” Source: Kenneth Murray/Photo Researchers Inc. Original bedding is obliterated
Foliation Minerals Recrystallize Perpendicular to the Directed Pressure If the minerals are flat, such as sheet- like Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock.
Main factors causing metamorphism Main factors causing metamorphism 3. Parent rock Metamorphic rocks typically have the same chemical composition as the rock they were formed from. Different minerals, but made of the same atoms. Exception: when hot water is involved.
Types of Protolith Lump the common types of sedimentary and igneous rocks into six chemically based-groups 1. Ultramafic - very high Mg, Fe, Ni, Cr 2. Mafic - high Fe, Mg, and Ca 3. Shales (pelitic) - high Al, K, Si 4. Carbonates- high Ca, Mg, CO 2 5. Quartz - nearly pure SiO 2. 6. Quartzo-feldspathic - high Si, Na, K, Al Chemistry of the metamorphic rock is the most important clue toward deducing the parent rock Chemistry of the metamorphic rock is the most important clue toward deducing the parent rock
Metamorphic Settings Metamorphic Settings Four types of metamorphic settings: Four types of metamorphic settings: 1. Contact metamorphism – due heat from adjacent rocks 2. Hydrothermal metamorphism – chemical alterations from hot, ion-rich water 3. Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins). Makes great volumes of metamorphic rock. Includes: –a. Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated –b. Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes – foliated 4. Dynamic. Smearing of minerals as faults slip Most is Dynamothermal
Contact metamorphism Produced mostly by local heat source Baking due to nearby Magma Effect strongest in rocks in immediate contact
Hydrothermal Metamorphism Due circulation of water near Magma Important at mid-ocean ridge, or near continental volcanoes Hot water facilitates metamorphic reactions by allowing movement of atoms and ions Atoms in the rock may be added or removed by the water
Metasomatism Metamorphism by mobile fluids, e.g. H 2 O incl. salines, silica, CO 2 Evidence for the existence of a metamorphic fluids: Fluid inclusions Evidence for the existence of a metamorphic fluids: Fluid inclusions Fluids are required for hydrous H 2 O OH - or carbonate CO 3 -- phases Fluids are required for hydrous H 2 O OH - or carbonate CO 3 -- phases Ex: The fluids expelled from the subducting slab migrate upwards and form hot springs, mud- volcanoes or serpentinite Diapirs within the forearc system. Fluids or melts released at greater depth will hydrate and metasomatise the mantle rocks of the hanging plate (i.e. the mantle wedge). The source of melt for the volcanic arcs located above the subduction zones is this hydrated mantle wedge above the subducted slab. The composition changes
3. Regional Metamorhism Most Dynamothermal metamorphism occurs along convergent plate boundaries Most Dynamothermal metamorphism occurs along convergent plate boundaries Example 1: In Subduction Zones Example 2: Continent-Continent Collisions Compressional stresses deforms plate edge Continents Collide Major Folded Mountain Belts: Alps, Himalayas, and Appalachian Mts.
Dynamothermal Metamorphism, Before collision, in subduction zone Sediments are “unconsolidated”. They will fold if pushed.
Dynamothermal Metamorphism, After continental collision Felsic continental materials and sediments are buoyant, they have low density They float, cannot be subducted, so they get squashed.
Orogenic Regional Metamorphism of the Scottish Highlands George Barrow studied the metapelites in NE Scotland from 1884 to 1900. George Barrow studied the metapelites in NE Scotland from 1884 to 1900. He could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased He could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased
Figure 21-8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.
The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are: Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite Garnet zone. Schists with conspicuous red almandine garnet, usually with Biotite, chlorite, muscovite, quartz, and Albite or Oligoclase Staurolite zone. Schists with Staurolite, Biotite, Muscovite, Quartz, garnet, and plagioclase. Some Chlorite may persist Kyanite zone. Schists with Kyanite, Biotite, Muscovite, Quartz, plagioclase, and usually garnet and Staurolite Sillimanite zone. Schists and gneisses with Sillimanite, Biotite, Muscovite, Quartz, plagioclase, garnet, and perhaps Staurolite. Some Kyanite may also be present. Kyanite and Sillimanite are both polymorphs of Al 2 SiO 5 )
Metamorphic Grade and Index Minerals Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form
Index Minerals in metamorphic rocks Note Quartz and Feldspar are not index minerals: Why? Note Temperature gradient 220 o C 580 o C 460 o C690 o C Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form Stable temps vary based on pressure and other species present
Here is an internally heated pressure vessel at the AMNH With these you can study, for example: 1. the temperature and pressure conditions at which polymorphs change from one form to another. 2. the reactions of minerals with fluids (for example salty or alkaline water) at high temperatures and pressures. 3. the conditions necessary to change one assemblage of minerals to another http://research.amnh.org/earthplan/research/Equipment/Petrology
Thermometers and Pressure Gauges Polymorphs of Al 2 SiO 5 Kyanite Sillimanite Andalusite Note other minerals in system
7_21 CANADA NEW YORK CANADA U.S.A. Albany Boston Scranton Long Island Binghamton ATLANTIC OCEAN Unmetamorphosed Chlorite/muscovite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone High grade Medium grade Low grade Augusta PENNSYLVANIA NEW HAMPSHIRE MAINE MASSACHUSETTS Concord Montpelier VERMONT CONNECTICUT NEW JERSEY R.I. Providence Hartford Newark Increasing pressure and temperature DIAGENESIS LOW GRADE Chlorite and muscovite Biotite Garnet Staurolite Sillimanite INTERMEDIATE GRADE HIGH GRADEMELTING New England Dynamothermal Metamorphism r i f t v a l l e y Caused by C-C collision M-P Most of Appalachians
Metamorphic Facies in Subduction Zones We can look at minerals in Metamorphic Rocks and determine where they formed. Subducted Water, water from hydrated minerals, facilitates metamorphic reactions by allowing movement of atoms and ions Metamorphic grade or Facies: A group of minerals that form in a particular P-T environment. Can be used to deduce T-P conditions of formation
Greenschist Hand Sample Greenschist Thin Section Chl-Ep Chlorite is commonly found in igneous rocks as an alteration product of mafic minerals such as pyroxene, amphibole, and Biotite. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a Metasomatism product via addition of Fe, Mg, or other compounds into the rock mass.
Figure 10-4 or 10-5 (2 nd ed). Figure 10-4 or 10-5 (2 nd ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites. Eclogite: red to pink garnet (almandine-pyrope) in a green matrix of sodium-rich pyroxene (omphacite) Eclogite Ringwood, (1974) suggested that island arc lavas (IAT), which are basaltic, could be related to dehydration of the hydrated ocean crust (amphibolite) as it transforms to dense eclogite at depths of ~150km. The hydrous fluids rise up into the peridotite mantle wedge, promoting partial melting. Melting occurs at much lower temperatures in the presence of water. RINGWOOD, A.E. 1974. The petrological evolution of island arc systems. Journal of the Geological Society, London 130, 183-204.
basaltic greenschist amphibolite basaltic 2. Gibbs Free Energy calcs are an obvious check, and this drives research programs to provide the needed numbers experimentally in the presence of water, etc. from wet subducted ocean lithosphere eclogite Water out 1. There are many reactions suggested for the origin near subduction zones of glaucophane, which is a characteristic component of metabasite blueschists
Sample of quartzite Thin section of quartzite Field Geologists are grateful for quartzite. It doesn’t foliate, so you can see folds. Mudrocks foliate; much harder to map. Non-Foliated Rocks 1: Quartzite
7_18 Fracture Sandstone: grains and cement Quartzite: grains interlock
Common metamorphic rocks Nonfoliated rocks (cont.) Nonfoliated rocks (cont.) Marble –Coarse, crystalline –Parent rock usually limestone –Composed of calcite crystals –Fabric can be random or oriented
Marble (nonfoliated) Marble Coarse, crystalline Parent rock usually limestone Composed of calcite crystals Fabric can be random or oriented
Change in metamorphic grade with depth Increasing Directed Pressure and increasing Temps => Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions Mudstone is most common sedimentary rock. When metamorphosed, rocks reveal grade:
Common metamorphic rocks Foliated rocks Foliated rocks Slate –Very fine-grained –Excellent rock cleavage, often perp. to original –Made by low-grade metamorphism of shale
Example of slate Slate Very fine-grained Excellent rock cleavage, often perp. to original Made by low-grade metamorphism of shale
Phyllite also lacks visible mineral grains Phyllite Grade of metamorphism between slate and schist Made of small platy minerals Glossy sheen with rock cleavage Composed mainly of muscovite and/or chlorite
Common metamorphic rocks Foliated rocks Foliated rocks Schist –Medium- to coarse-grained –Comprised of platy minerals (micas) –The term schist describes the texture –To indicate composition, mineral names are used (such as mica schist)
Schist Foliated rocks Schist Medium- to coarse-grained Comprised of platy minerals (micas) The term schist describes the texture To indicate composition, mineral names are used, e.g. Garnet Schist
Common metamorphic rocks Foliated rocks Foliated rocks Gneiss –Medium- to coarse-grained –Banded appearance –High-grade metamorphism –Composed of light-colored feldspar layers with bands of dark mafic minerals
Gneiss displays bands of light and dark minerals Foliated rocks Medium- to coarse- grained Banded appearance High-grade metamorphism Composed of light- colored feldspar layers with bands of dark mafic minerals
What are metamorphic textures? Texture refers to the size, shape, and arrangement of mineral grains within a rock Texture refers to the size, shape, and arrangement of mineral grains within a rock Ex: Foliation – planar arrangement of mineral grains within a rock, perpendicular to the directed pressure, common near convergent margins Ex: Foliation – planar arrangement of mineral grains within a rock, perpendicular to the directed pressure, common near convergent margins
Development of lineation or foliation due to directed pressure
Migmatites- When Partial Melting Starts Heat the rock, when the minerals with the lowest melting points (Qtz, Feldspar) at that pressure melt then recrystallize, we get separate bands of Metamorphic and Igneous rock
► Metamorphic rocks are classified on the basis of texture and composition (either mineralogical or chemical) ► Unlike igneous rocks, which have been plagued by a proliferation of local and specific names, metamorphic rock names are surprisingly simple and flexible ► May choose some prefix-type modifiers to attach to names if care to stress some important or unusual textural or mineralogical aspects Chapter 22: A Classification of Metamorphic Rocks
► Foliation: any planar fabric element ► Lineation: any linear fabric elements
Cleavage The property of a rock to split along a regular set of sub-parallel, closely-spaced planes any type of foliation in which platy Phyllosilicates are aligned (parallel) but are too fine grained to see without a microscope
Schistosity ◦ A preferred orientation of mineral grains or grain aggregates produced by metamorphic processes ◦ Aligned minerals are coarse grained enough to see with the unaided eye ◦ The orientation is generally planar, but linear orientations are not excluded
Gneissose AKA gneissic structure ◦ Segregated into mafic and felsic layers by metamorphic processes ◦ Gneissose rocks usually have visible grains
Figure 22-1. Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. a b a. Slate: compact, very fine- grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull b. Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite and/or chlorite), although rarely coarse enough to see unaided, cause a reflective foliation surface.
Figure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Schist: metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen. Foliated Metamorphic Rocks
Figure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Gneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers. Gneisses may also be lineated, but must also show segregations of felsic-mineral-rich and dark- mineral-rich concentrations. Foliated Metamorphic Rocks
Non-Foliated Metamorphic Rocks Granofels: a comprehensive term for any rock with no preferred orientation An outdated alternative to granofels is granulite, but this term is now used to denote very high grade rocks (whether foliated or not), and is not endorsed here as a synonym for granofels. Hornfels is a subset, preferably for fine grained, contact metamorphosed rocks
Non-Foliated Metamorphic Rocks Marble: a metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone.
Non-Foliated Metamorphic Rocks Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone.
Metamorphic Rock Facies Greenschist/Greenstone: a low-grade metamorphic rock that typically contains chlorite, actinolite, epidote, and albite. Note that the first three minerals are green, which imparts the color to the rock. Such a rock is called greenschist if foliated, and greenstone if not. The protolith is either a mafic igneous rock or a graywacke.
Metamorphic Rock Facies Amphibolite: a metamorphic rock dominated by hornblende + plagioclase. Amphibolites may be foliated or non- foliated. The protolith is either a mafic igneous rock or graywacke.
Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.
Blueschist: a usually blue, usually glaucophane bearing metamorphosed mafic igneous rock or mafic graywacke. This term is even applied to non-schistose rocks.
Eclogite: a green and red metamorphic rock that contains the green clinopyroxene Omphacite and pink garnet Pyrope. The protolith is typically basaltic. It forms via the dehydration of Amphibolite at about 150km down a subduction zone.
Granulite: medium to coarse–grained metamorphic rocks that have experienced high-temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic (anhedral phaneritic equi-granular) texture and gneissose to massive structure. Metagabbro showing granoblastic texture Note how the interlocking plagioclase grains in this rock meet at ~120 degree triple junctions. This feature is characteristic of granoblastic texture.
Skarn: a contact metamorphosed and silica- metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, etc. Tactite is a synonym.
Migmatite: a composite silicate rock that is heterogeneous on the 1-10 cm scale, commonly having a dark gneissic matrix (melanosome) and lighter felsic portions (leucosome). Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins.
Additional Modifiers Porphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast Some porphyroblasts, particularly in low- grade contact metamorphism, occur as ovoid “spots” e.g. spotted hornfels, or spotted phyllite
Augen Some gneisses have large eye-shaped grains (commonly feldspar) that are derived from pre- existing large crystals by shear (as described in Section 23.1). Individual grains of this sort are called auge (German for eye), and the (German) plural is augen. An augen gneiss is a gneiss with augen structure (Fig. 23-18).
Additional Modifying Terms: Other modifying terms that we may want to add as a means of emphasizing some aspect of a rock may concern such features as grain- size, color, chemical aspects, (aluminous, calcareous, mafic, felsic, etc.). As a general rule we use these when the aspect is unusual. NOT a calcareous marble or mafic greenschist, as these are redundant, as is a fine grained slate.
Additional Modifying Terms: Ortho- a prefix indicating an igneous parent, and Para- a prefix indicating a sedimentary parent The terms are used only when they serve to dissipate doubt. For example, many quartzo- feldspathic gneisses could easily be derived from either an impure arkose or a granitoid rock. If some mineralogical, chemical, or field-derived clue permits the distinction, terms such as orthogneiss, paragneiss, or orthoamphibolite may be useful.