Metamorphism and Metamorphic Rocks

1  Metamorphism is the change in form that happens in Earth’s crustal rocks in response to changes in temperature and pressure... Temperature And Pressure 2  Low-grade metamorphism: at

Metamorphism and Metamorphic

Rocks

What is Metamorphism? (1)

 Metamorphism is the change in form that happens in Earth’s crustal rocks in response to changes in temperature and pressure.

 Fluids: present or absent

 Time: how long a rock is subjected to high pressure or high temperature.

 Deformation type: whether the rock is simply compressed

or is twisted and broken during metamorphism.

Temperature And Pressure (1)

 Rock can be heated by burial or by nearby igneous intrusion.

 Burial is accompanied by an increase in pressure due to the weight of the overlying rocks.

 An intrusion may be shallow, resulting in low pressure, or deep, resulting in high pressure.

Fig: heat sources vs local & regional thermal metamorphism

Temperature And Pressure (2)

 Low-grade metamorphism: at low temperatures c.a 100oC to 500oC, and at relatively low

pressures.

 High-grade metamorphism: at high temperatures (above 500oC), and at high pressure.

Figure 8.1

Stress

 Stress is applied pressure that results in deformation in a solid, and the development of new textures

 Uniform stress occurs if pressure is equal in all directions.

 Differential stress occurs if pressure is different

in different directions.

 Texture is controlled by differential versus uniform stress.

Figure 8.2

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Figure 8.2A: Result of uniform stress

Figure 8.2B: Result of differential stress

Figure 8.3

Result of differential stress: extension resulting in boudinage

(boudins are lenticular parts of sandstone/quartzite beds

dismembered by “sub-horizontal” extension/stretching) & max stress/pressure: “sub-vertical”: schistosity of mudstone

Fluids and Metamorphism (1)

 Sedimentary rocks have open spaces between their grains filled by a watery intergranular fluid.

 This fluid:

 Is never pure water.

 Always contains small amounts of dissolved gases and salts.

 Contains traces of all the mineral constituents present in the enclosing rocks.

 Is an important transporting medium for ions

Fluids and Metamorphism (4)

 When intergranular fluids are absent, metamorphic reactions are very slow.

 When pressure increases due to burial of a rock, and as metamorphism proceeds, the amount of pore space decreases and the intergranular fluid

is slowly squeezed from the rock.

Fluids and Metamorphism (5)

 Any fluid that escapes during metamorphism will carry with it small amounts of dissolved mineral matter.

 Minerals precipitated in a facture are called a vein.

 Metamorphic changes that occur while temperatures and pressures are rising (usu In presence of intergranular fluid) are termed

prograde metamorphic effects (prograde metamorphism)

Figure B8.2

Prograde & retrograde metam

Fluids and Metamorphism (6)

 Metamorphic changes that occur as temperature and pressure are declining (and usually after

much of the intergranular fluid has been expelled) are called retrograde metamorphic effects (retrograde metamorphism)

Role of Time in Metamorphism

 Coarse-grained rocks are the products of long sustained metamorphic conditions (possibly over millions of years) at high temperatures and

pressures.

 Fine-grained rocks are products of lower temperatures, lower pressures or, in some cases, short reaction times.

thousand meters of overlying rock.

 At the upper end, metamorphism ceases to occur

at temperatures that melt rock.

How Rocks Respond To Temperature and Pressure Change In Metamorphism

 Lower-grade Metamorphism: Slaty Cleavage.

 the newly forming sheet-structure minerals create foliation that tends to be parallel to the bedding planes of the sedimentary rock being metamorphosed.

 Higher-grade Metamorphism: Schistosity.

 At intermediate and high grades of metamorphism, grain size increases.

 Foliation in coarse-grained metamorphic rocks is called schistosity (the parallel arrangement of coarse grains of the sheet-structure minerals)

Figure 8.5

Figure 8.6

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Mineral Assemblage Change

 As temperature and pressure rise, one mineral assemblage evolves into another

 Each assemblage is characteristic of a given rock composition.

Metamorphism of Shale and Mudstone

 Slate (low grade):

 The low grade metamorphic product of shale.

 Phyllite (intermediate grade):

 Pronounced foliation, larger mica grains.

 Schist and gneiss (high grade):

 Schist is a coarse-grained rock with pronounced schistosity.

 Gneiss is a high grade, coarse grained rock with layers of micaceous minerals segregated from layers

of minerals such as quartz and feldspar.

Figure 8.8: KNOW THIS WELL

Figure 8.9 KNOW THIS WELL: Fig A.

Metamorphism of Basalt

 Greenschist (= metamorphosed mafic volcanic rock) has pronounced foliation like phyllite, but also a very

distinctive green color because of its chlorite content.

 Amphibolite and granulite.

 When greenschist is subjected to intermediate-grade metamorphism, amphibole replaces the chlorite Foliation is present in amphibolites, but is not pronounced because micas and chlorites are usually absent

 At the highest grade of metamorphism, amphibole is replaced by pyroxene and an indistinctly foliated rock called a granulite

develops.

Greenschist

Folded quartz vein

in a greenschist (or greenstone) (green colour caused by chlorite)

 Pure marble is snow white.

 Pure grains of calcite.

 Many marbles contain impurities that result in various colors.

Figure 8.14

 Aureoles reach more than 100 m in thickness.

 Metamorphism that involves a lot of fluid is called metasomatism.

Burial Metamorphism

 When buried deeply in a sedimentary basin, sediments may attain temperatures of a few hundred degrees Celsius, causing burial

metamorphism.

 As temperatures and pressures increase, burial metamorphism grades into regional

metamorphism.

Regional Metamorphism—A Consequence of Plate Tectonics

 Regional metamorphism results from tectonic forces that build mountains.

 It results from pronounced differential stresses and extensive mechanical deformation in

addition to chemical recrystallization.

 Regional metamorphism produces greenschists and amphibolites.

Fig scanned large regional metam in orogen from another book

Metamorphic Facies

 Mineral assemblages caused by specific sets of temperature/pressure conditions:

 Granulite facies, - hornfels facies

 Amphibolite facies, - zeolite facies

 Epidote-amphibolite facies,

 Greenschist facies,

 Blueschist facies,

 Eclogite facies

Figure 8.16: BE ABLE TO DRAW THIS

Metasomatism

 Metasomatism is the process in which rock compositions are distinctively altered through exchange with ions in solution.

 Metasomatic fluids may carry valuable metals and form mineral deposits.

Figure 8.17 ADD CAPTION

Plate Tectonics And Metamorphism (1)

 There are five geologic settings where plate tectonics encourages metamorphism:

Figure 8.18 BE ABLE TO SKETCH THIS, AND EXPLAIN

Plate Tectonics And Metamorphism (2)

accumulated in ocean-floor trenches, such as those off the coasts of Peru and Chile.

 When oceanic crust with a covering of sedimentary rocks is dragged down by a rapidly subducting plate , pressure increases faster than temperature, subjecting the rock to high pressure but relatively low temperature

 This is observed today along the subduction margin of the Pacific Plate where it plunges under the coast of Alaska and the Aleutian Islands.

Figure 8.18 1-burial metamorphism; 2-high p, low T metamorphism

Plate Tectonics And Metamorphism (3)

thickened by plate convergence and heated by rising magma, greenschist and amphibolite facies

metamorphic condition occur.

 Examples include the Appalachians, Alps, Himalayas, and Andes.

 If the crust is sufficiently thick, when 10 percent or more of the crust has melted the magma (wet magma)

so formed will rise forming stock or batholith.

 As the granitic magma formed by wet partial melting rises, it heats and metamorphoses the rocks with

which it comes in contact.

Figure 8.18 4- wet granitic magma, 5-rises & contact metamorphism (see Fig 8.19 )

Figure 8.19