Presentation on theme: "Chp 7: Metamorphic Rocks"— Presentation transcript:
1 Chp 7: Metamorphic Rocks Metamorphism – From the Greek “meta” = to change, and “morpho” = shape.Metamorphism – “The altering of rock characteristics and mineral compositions due toheat and/or pressure, or other environmental factors. This changing is a Solid StateReaction, meaning that the rocks subjected to metamorphic processes do not melt(otherwise upon cooling, they would form igneous rocks). It is thought to be a relativelyslow geologic process. A great many areas of metamorphism yield abundant mineralreserves of gold, silver, copper, lead, zinc, and other valuable minerals.Metamorphic rocks are formed either by being exposed to heat, pressure, or chemicallyactive fluids, or a combination of these factors to create a rock that has a different textureand mineral contentThe “parent rock” is the term for the rock prior to metamorphism. It may be igneous,sedimentary, or another metamorphic rock. For example, here are some parent rocksand the rock that they may metamorphose into under certain conditions:Limestone – marbleClay stone – slateGranite – gneiss, etc.
2 Specimen of Chrysotile: fibrous form of serpentine asbestos Specimen of chrysotile from Thetford, Quebec, Canada. Chrysotile is the fibrous form of serpentine asbestos.p.186
3 Chp 7: Metamorphic Rocks The effect of metamorphism on rocks is analogous to baking a cake:the resulting cake is dependent upon the ingredients, the amount offluids, the temperature, and the length of time it was “baked”.A great portion of the continents is metamorphic formed during“continental accretion” during the formation of the Precambrian.Metamorphics form the stable basement rocks called “continentalshields” upon which surface sedimentary rocks have been deposited.Metamorphics also comprise a large portion of the crystalline coreof many mountain ranges.
4 Occurrence of metamorphic rocks: shields, core of mountain ranges Figure 7.1 Metamorphic rock occurrences. Shields are the exposed portions of the crystalline basement rocks underlying each continent; these areas have been very stable during the past 600 million years. Metamorphic rocks also constitute the crystalline core of major mountain belts.Fig. 7-1, p.185
5 Chp7: Metamorphic Rocks Factors Involved in MetamorphismHeat – The source of heat may be from a large intrusive body such as a pluton, orheat from activities associated with s plate tectonics.-At temperatures below 2000 C, only a small amount of fluid is present in most rocks.As the temperature increases many minerals release pore fluid that was trapped in therock or in crystal lattices of its minerals. This pore fluid may become very chemicallyreactive, altering the chemistry of the surrounding rocks.-The Geothermal Gradient – On average the temperature of the rocks in the earthincrease 25C per kilometer of depth. On the continental cratons, the average is20C/km. On the continental boundaries it is 40C/km. At subduction zones, it is10C/km because heat is dissipated into the sea.-At 700C, most rock components become “plastic” where many times the pre-existingcrystals rotate, or twist altering the texture of the rock.-Under conditions of high heat, pressure, and chemically active fluids, crystal latticesbegin to break down, recreate new types of crystal lattices, rearrange ions, and form newminerals in the process.-Some minerals only form at certain temperature and pressures. If these are found in ametamorphic rock, the temperature of formation can be deduced.
7 Chp 7- Metamorphic Rocks Pressure – When rocks are buried, they are subjected to lithostatic pressure -the pressures from all sides by the overburden weight of the country rock.-Differential pressures – may exist whereby the pressures exerted upon the rock are notequal in all directions. This results in a distortion or twisting effect on the rock.-Phenocryst rotation or distortion may occur. This can cause grains in the rock to stretch,rotate, bend, line up in rows, become platy, etc. (i.e. micas forming in mica schists)-Pressure distortion of metamorphic rocks is common around areas of high lithologicstress such as areas around tectonic boundaries.Chemically Active Fluids – Fluids released from igneous intrusions, or othermetamorphic processes can cause a constant interaction or exchange of ionsaltering the rocks.-Metasomatism – the introduction by fluids of ions from an external source not directlyassociated with the intrusion. Hydrothermal Metamorphism – changes due tomigrating superheated water and dissolved ions. Hydrothermal rocks many timesappear “bleached” because of the intense chemical reactions.
8 Sharp boundary (red line) between intrusion on left and country rock on the right.Figure 7.5 A sharp and clearly defined boundary (red line) occurs between the intruding light-colored igneous rock on the left and the dark metamorphosed country rock on the right. The intrusion is part of the Peninsular Ranges Batholith, east of San Diego, California.Fig. 7-5, p.190
9 Differential pressure- Pressure applied un- equally in all directions. Note garnet in schistFigure 7.3 Differential pressure is pressure that is unequally applied to an object. Rotated garnets are a good example of the effects of differential pressure applied to a rock during metamorphism. This rotated garnet (center) came from a schist in northeast Sardinia.Fig. 7-3, p.188
10 Chp 7- Metamorphic Rocks Sources of water –1. Juvenile Water – water given off by cooling magma.2. Metamorphic Water – water already present the country rock, which is given offduring metamorphic processes.3. Meteoric Water – “groundwater” contained in aquifers encountered in the countryrock during metamorphic processesHydrothermal activities – many times form economically rich mineral depositsof gold, copper, iron, lead, etc. This process is also responsible for the “veining”(“mother loads”) of gold and other valuable minerals.Volcanic events such as calderas usually have associated hydrothermal activitiesTypes of MetamorphismI. Contact – Effects of Heat and FluidsCharacteristics:a. “Heat” is the driving force in contact metamorphism.Common where hot magmatic plutons come into contact with the surrounding country rock.b. The degree of metamorphism is related to the temperature of the magma, the size of theintrusion, and the chemically active fluid content of the magma involved. Large intrusions suchas batholiths cool for long periods of time -more intense metamorphic change in the country rock.
11 Chp 7- Metamorphic Rocks c. Temperatures can reach 9000 degrees C next to the intrusion.d. As the heat and associated metamorphic changes alter the country rock, the countryrock closest to the intrusion is affected most, furthest from the intrusion is affected least.e.This sets up a “metamorphic halo” or “aureole” in the country rock around the intrusion.The aureole is a gradation of degrees of metamorphism surrounding the intrusion:1. Shale – unaltered country rock2. Slate – low grade metamorphism3. Phyllite –between low and med. Grade4. Schist – medium grade metamorphism5. Gneiss – High grade metamorphism6. Migmatite – Very high grade metamorphism7. Melting occurs at 900C,above this temperature = formation of igneous rock.f. Two types of contact metamorphic rocks are recognized:1. those resulting from the “baking” of the country rock2. those resulting from the actions of chemically active fluidsg. Many “baked” types have the texture of porcelain if they contain high amounts of claysuch as shale. This effect is seen in the firing of ceramics in a kiln.h. Hydrothermal activity is also common with contact metamorphism resulting in anenrichment of valuable ore deposits.
12 Metmorphic aureole commonly surrounds many intrusions: this Model has 3 zones of mineral assemblages around the intrusion, reflectdecreased pressure and temperature effects away from the intrusionFigure 7.4 A metamorphic aureole typically surrounds many igneous intrusions. The metamorphic aureole associated with this idealized granite batholith contains three zones of mineral assemblages reflecting the decreases in temperature with distance from the intrusion. An andalusite–cordierite hornfels forms adjacent to the batholith. This is followed by an intermediate zone of extensive recrystallization in which some biotite develops, and farthest from the intrusion is the outer zone, which is characterized by spotted slates.Fig. 7-4, p.189
14 Chp 7- Metamorphic Rocks II. Regional Burial – Effects of Lithostatic PressureCharacteristics:a. Occurs over a very broad areab. Rocks are altered due to tremendous pressures (and the resulting high temperatures),resulting in deformation within deeper portions of the crust.c. Very common along convergent and divergent plate boundaries.d. Index minerals are minerals that are known to form only under certain temperatures andpressures. The following is a sequence of known minerals that form from low grademetamorphism to high grade:chlorite – (forms around 200 C), muscovite, biotite, garnet, staurolite, kyanite(forms around 500C)e. Quartz and feldspars can be present in both igneous and metamorphic rocks, but someminerals such as andalusite, sillimanite, and kyanite (all 3 minerals are forms of Al2SiO5)form only from these metamorphic conditions.f. The presence or absence of these minerals is an indication of the degree of pressure(and resulting heat) in the formation of the rock in question.g. Examples of regional burial rocks are: marble from limestone, quartzite from quartzsandstone, and argillite from clay.
15 Mineralization due to metamorphism of shale: new minerals form at different Temperature levels. Progression of minerals indicatestemperature levelFigure 7.7 Change in mineral assemblage and rock type with increasing metamorphism of shale. When a clay-rich rock such as shale is subjected to increasing metamorphism, new minerals form, as shown by the various colored bars. The progressive appearance of particular minerals allows geologists to recognize low-, intermediate-, and high-grade metamorphic zones.Fig. 7-7, p.191
17 Chp7- Metamorphic Rocks III. Dynamic Metamorphism (“Dynamo-thermal”)-Characteristics:a. Usually associated with the pressures around fault zones.b. “Mylonites” is the term used to describe rocks formed in this way.c. Typically, the extent of metamorphism is restricted to narrow margins adjacent to faults.d. Myolinites are hard, dense, fine-grained rocks, many of which have laminations orlayerings.e. These also can be associated with tectonic settings.Textures of Metamorphic RocksI. Foliated Textures -a. Typically associated with contact metamorphism.b. Minerals are arranged in a platy, parallel fashion.c. The size and shape of the mineral grains determines if the foliation is fine or coarse.d. A coarse foliation usually indicates a higher degree of heat such as in gneiss.e. A fine foliation usually indicates a lower degree of heat such as in schist.Slate has a very fine foliation exhibiting the lowest grade of contact metamorphism.
18 Mylonite from Adirondack Mtns…note thin laminations Figure 7.6 Mylonite from the Adirondack Highlands, New York. Note the thin laminations.Fig. 7-6, p.190
19 When rocks are subjected to differential pressure, mineral grains Typically align in parallel fashion-producing a ‘foliated’ structure.b. Foliated metamorphic rock…Figure 7.8 (a) When rocks are subjected to differential pressure, the mineral grains are typically arranged in a parallel fashion, producing a foliated texture. (b) Photomicrograph of a metamorphic rock with a foliated texture showing the parallel arrangement of mineral grains.Fig. 7-8, p.192
20 Figure 7.8 (b) Photomicrograph of a metamorphic rock with a foliated texture showing the parallel arrangement of mineral grains.Fig. 7-8b, p.192
21 Chp 7- Metamorphic Rocks Examples of Foliated Textured Metamorphic Rocks:Slate –has a very fine foliation due to it having formed at the lowest grade of contactmetamorphism. It possesses a slaty cleavage, easily cleaving or parting along the axis oflayering. It is used for pool tables, chalkboards, and building tiles for this reason. Thedifferent colors of slates are due to the presence of minerals such as chlorite (green),graphite (black), or iron oxide (red).Phyllite – similar to slate but coarser grained. It is more lustrous or glossy due to tinymica minerals. Grains are too small to be identified with the unaided eye.Schist – is most commonly produced by regional burial metamorphism. It can also beproduced by medium grade contact metamorphism. Metamorphosed clay richsedimentary rocks typically produce schists (although other rocks may also produce them).All schists contain more than 50% platy and elongated minerals all of which largeenough to identify. The degree of schistosity reflects the temperature of formation: thegreater the temperature, the greater the degree of schistosity. Schists are common in lowto medium grade metamorphic environments. Schists are named as to the most abundantmineral: mica schist, talk schist, biotite schist, chlorite schist, etc.
22 Hand specimen of slate Fig. 7-9a, p.194 Figure 7.9 (a) Hand specimen of slate.Fig. 7-9a, p.194
23 Large sheet of slate-note the original bedding: up right to lower left Figure 7.9 (b) This panel of Arvonia Slate from Albemarne Slate Quarry, Virginia, shows bedding (upper right to lower left) at an angle to the slaty cleavage.Fig. 7-9b, p.194
24 Phyllite: note lustrous sheen and bedding at angle to cleavage Figure 7.10 Specimen of phyllite. Note the lustrous sheen as well as the bedding (upper left to lower right) at an angle to the cleavage of the specimen.Fig. 7-10, p.194
27 Chp 7 Metamorphic Rocks Texture continued…. Gneiss – is a streaked or has segregated bands of alternating light and dark minerals.Quartz and feldspar are the major light colored minerals and biotite and hornblende arethe principle dark colored minerals. Gneiss typically forms from regional metamorphismof clay-rich sedimentary rocks, from contact metamorphism of granites, or from meta-morphism of older metamorphic rocks.Amphibolite – a dark-colored, slightly foliated rock consisting primarily of hornblendeand plagioclase. The metamorphism of mafic rocks such as basalt produce amphibolites.Migmatites – “mixed metamorphics” – These have characteristics of both igneous andmetamorphic rocks indicating very high heat and pressure. Examples include the rockstouching an intrusion: the very highest grade contact metamorphism. Most containgranite components, or lenses (small pieces of other rocks), and appear to have beentwisted or wavy. This may be due to partial melting of the country rock.
28 Gneiss-recognized by segregated bands of light and dark minerals Gneiss-recognized by segregated bands of light and dark minerals. This gneiss has been folded…Figure 7.12 Gneiss is characterized by segregated bands of light and dark minerals. This folded gneiss is exposed at Wawa, Ontario, Canada.Fig. 7-12, p.195
29 Migmatites-high grade metamorphic rock, with streaks or lenses of granite. Picture from Lake Huron, Ontario, CanadaFigure 7.13 Migmatites consist of high-grade metamorphic rock intermixed with streaks or lenses of granite. This migmatite is exposed at Thirty Thousand Islands of Georgian Bay, Lake Huron, Ontario, Canada.Fig. 7-13, p.198
30 Chp 7 Metamorphic Rocks II. Nonfoliated Textures - Characteristics: These textures result from the metamorphosing of rocks whose minerals do not show apreferred orientation, and therefore are not foliated.Most non-foliated rocks result from contact or regional burial of rocks that are devoidof platy or elongated crystals.Two Types of Nonfoliated rocks:those composed of mainly one mineral (marble or quartzite)those composed of mineral grains that are too small to be seen- hornfels or greenstones.Examples of Non-foliated Textured Metamorphic Rocks:Marble – the parent rock is a limestone (mostly calcite) or dolostone (mostly dolomite)that was subjected to contact or regional burial. It may be fine-grained to coarse-grained.Color variation is due to impurities in the parent rock. Because of its texture and softness,marble has been used extensively for sculpturing.Quartzite – the parent rock is a quartz sandstone subjected to medium to high gradecontact or regional burial resulting in a hard, coarse-grained compact rock. Pure quartziteis white but impurities may alter the color. Since it is so hard from the re-crystallizationof the quartz, it is commonly used for the bases of roads and buildings.
31 Chp 7- Metamorphic Rocks Greenstone – this is the name given to any compact, dark green, altered, mafic igneousrock that formed under low to high grade metamorphic conditions. The green color is dueto the minerals chlorite, epidote, and hornblende. These are commonly the rocks foundin “greenstone belts” along the transitional zones of sialic continental plates to maficoceanic plates.Hornfels – fine-grained, nonfoliated rock formed from contact metamorphism. Thegrains are equidimensional with its composition dependent upon the composition of theparent rock. Most are formed from contact metamorphism of clay-rich sedimentary rocksor impure dolomites.Anthracite – is a black, lustrous, hard coal that is high in carbon and low in volatiles.Its parent rock is bituminous coal that was subjected to regional burial.
32 Marble results from the metamorphism of sedimentary rocks known Limestone or dolostoneFigure 7.15 Marble results from the metamorphism of the sedimentary rock limestone or dolostone.Fig. 7-15, p.199
33 Photomicrograph of marble-note the mosaic of roughly equidimensional minerals-indicates non-foliated textureFigure 7.14 Nonfoliated textures are characterized by a mosaic of roughly equidimensional minerals, as in this photomicrograph of marble.Fig. 7-14, p.198
34 Quartzite results from the metamorphism of quartz sandstone Figure 7.16 Quartzite results from the metamorphism of quartz sandstone.Fig. 7-16, p.199
35 Chp 7- Metamorphic Rocks Metamorphic Zones or Facies –a. A “metamorphic facies” is a group of metamorphic rocks characterized by particularmineral assemblages (more than one mineral is present) under the same broadtemperature/pressure conditions.b. Each facies is named after its most characteristic rock or mineral.c. Metamorphic facies are usually are applied to areas whose parent rocks were originallyclay-rich. Metamorphic facies cannot be applied to areas where the parent was purelimestone or pure quartz sandstones because they would produce only marbles andquartzites respectively.Examples of Metamorphic Facies:a. Greenschist Facies – forms whenever the rock is rich in the mineral chlorite and issubjected to relatively low temperatures and pressures.b. Granulite Facies and Amphibolite Facies – form under similar chemistries but thepressures are significantly greater.c. Blueschist Facies – form at subduction zones where, due to the presence of seawater,the temperature is low, but because of the tectonic activity, the pressure is high.This results in an abundance of a blue-colored amphibole mineral named glaucophane.The presence of a blueschist facies indicates to the geologist the presence of ancientsubduction zones.
36 Metamorphic facies produced along oceanic-continental boundary Figure 7.19 Metamorphic facies resulting from various temperature–pressure conditions produced along an oceanic–continental convergent plate boundary.Metamorphic facies produced along oceanic-continental boundaryFig. 7-19, p.201
37 Chp 7- Metamorphic Rocks-Summary Metamorphic Rocks form as a result of ‘metamorphism’…an alteration of rock charac-teristics and chemical composition due to application of heat and/or pressure, or chem-ically active fluids.“Parent rock” is term applied to the rock being metamorphosed-it may be igneous, sedi-mentary or even another metacmorphic rock.Metamorphic rocks commonly occur in--core of mountain ranges-continental shields (sedimentary rocks commonly deposited on top of them…)-original continental accretion in PreCambrianFactors applied during metamorphism:-Heat-Pressure-Chemically active fluids
39 Chp 7- Metamorphic Rocks-Summary Types of Metamorphism:A. Contact metamorphism: results from heat and fluids-metamorphic ‘halo’ known as aureole is generated (shale)-baked zones common-hydrothermal effects occur..B. Regional burial: occurs over large area-gradation of minerals common as a result of high pressure-specific minerals indicate different levels of pressure/temperatureC. Dynamic metamorphism: usually associated with fault zones- mylonitesEconomic uses- mining slate, hydrothermal minerals suggest proximityto gold or silver??
40 Slate mine in Wales, England Slate mine in Wales, England. Formed by mountain building process dated at 400 to 440 million years ago (MYA)Figure 7.21 Slate quarry in Wales. Slate, which has a variety of uses, is the result of low-grade metamorphism of shale. These high-quality slates were formed by a mountain-building episode that occurred approximately 400 to 440 million years ago in the present-day countries of Iceland, Scotland, Wales, and Norway.Fig. 7-21, p.202
41 Chp 7- Metamorphic Rocks-Summary Metamorphic textures:A. Foliated-results from contact metamorphism-varies from coarse to fineslate, phyllite, schist, gneiss, amphibolite, migmatiteB. Non-Foliated- no preferred orientation to minerals-2 types: single mineral, grains too small to be seen with naked eyemarble, quartzite, greenstone, hornfels, anthraciteMetamorphic Zones/Facies: metamorphic rocks characterized byspecific mineral assemblages that reflect pressure-temperature regimerock experienced:1. Greenschist: contain chlorite, low temperature, lo pressure2.Granulite/Amphibolite: similar but higher pressure3. Blueschist: fairly low temperature, high pressure. Indicative ofsubduction zones. Glaucophane mineral….
42 Pressure-temperature diagram showing where metamorphic facies occur. Each facies is named after its most characteristic mineral.Figure 7.18 A pressure–temperature diagram showing where various metamorphic facies occur. A facies is characterized by a particular mineral assemblage that formed under the same broad temperature–pressure conditions. Each facies is named after its most characteristic rock or mineral. Source: From AGI Data Sheet 35.4, AGI Data Sheets, 3rd edition (1989) with the kind permission of the American Geological Institute.Fig. 7-18, p.200
44 This Greek kouros, which stands 206 cm tall, has been the object of an intensive authentication study by the Getty Museum. Using a variety of geologic tests, scientists have determined that the kouros was carved from dolomitic marble that probably came from the Cape Vathy quarries on the island of Thasos. Garry Hobart/GeoImageryFig. 7-CO, p.182
45 Figure 7.8 (a) When rocks are subjected to differential pressure, the mineral grains are typically arranged in a parallel fashion, producing a foliated texture.Fig. 7-8a, p.192
46 Different colored slates used for roof in Michigan Different colored slates make up the roof of this elementary school in Mount Pleasant, Michigan.p.193
47 Slate roof in Switzerland Figure 7.9 (c) Slate roof of Chalet Enzian, Switzerland.Fig. 7-9c, p.194
48 Franciscan Complex in CA: low temperature, high pressure subduction Figure 7.20 Index map of California showing the location of the Franciscan Complex and a diagrammatic reconstruction of the environment in which it was regionally metamorphosed under low-temperature, high-pressure subduction conditions approximately 150 million years ago. The red line on the index map shows the orientation of the reconstruction to the current geography. Source: From “Effects of Late Jurassic-Early Tertiary Subduction in California,” San Joaquin Geological Society Short Course, 1977, 66, Figure 5-9.Fig. 7-20, p.201
49 Figure 7.20 Index map of California showing the location of the Franciscan Complex and a diagrammatic reconstruction of the environment in which it was regionally metamorphosed under low-temperature, high-pressure subduction conditions approximately 150 million years ago. The red line on the index map shows the orientation of the reconstruction to the current geography. Source: From “Effects of Late Jurassic-Early Tertiary Subduction in California,” San Joaquin Geological Society Short Course, 1977, 66, Figure 5-9.Fig. 7-20a, p.201
50 Figure 7.20 Index map of California showing the location of the Franciscan Complex and a diagrammatic reconstruction of the environment in which it was regionally metamorphosed under low-temperature, high-pressure subduction conditions approximately 150 million years ago. The red line on the index map shows the orientation of the reconstruction to the current geography. Source: From “Effects of Late Jurassic-Early Tertiary Subduction in California,” San Joaquin Geological Society Short Course, 1977, 66, Figure 5-9.Fig. 7-20b, p.201