Presentation on theme: "Metamorphism – changes in mineralogy and texture brought about by perturbations in heat and pressure Compositional changes are generally restricted to."— Presentation transcript:
Metamorphism – changes in mineralogy and texture brought about by perturbations in heat and pressure Compositional changes are generally restricted to those related to devolatilization Metamorphism involves the recrystallization of a rock Metasomatism involves mass transfer, not just devolatilization ; commonly, introduced fluids carry elemental constituents that react with minerals to form new minerals. These fluid may be of various origins, but often they are derived from magmas (which crystallize as igneous plutonic rocks) which are also supplying heat.
Types of metamorphism Contact – high T, low P Hydrothermal – involving a fluid; often referred to as hydrothermal alteration Regional – high T, can be high P Burial – a continuation of diagenesis; related to burial in a sedimentary basin without active tectonism or plutonism T – P ranges are 100-800 O C and 1 bar(10 5 Pa) to 10 kb (10 9 Pa)
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 Slate: compact, very fine- grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface. Phyllites with both a foliation and lineation are very common. Chapter 22: Foliated Metamorphic Rocks
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: a metamorphic rock exhibiting a schistosity. By this definition schist is a broad term, and slates and phyllites are also types of schists. In common usage, schists are restricted to those metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen. Chapter 22: Foliated Metamorphic Rocks
Additional Modifying Terms: 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” If such spots occur in a hornfels or a phyllite (typically as a contact metamorphic overprint over a regionally developed phyllite), the terms spotted hornfels, or spotted phyllite would be appropriate. Chapter 22: A Classification of Metamorphic Rocks
Figure 23-14b. Spotted Phyllite. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Chapter 22: A Classification of 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. Chapter 22: Foliated Metamorphic Rocks
Marble: a metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone. Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone. Some confusion may result from the use of this term in sedimentary petrology for a pure quartz sandstone. Chapter 22: Specific Metamorphic Rock Types
Simpler than for foliated rocks Again, this discussion and classification applies only to rocks that are not produced by high-strain metamorphism Granofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation) Hornfels is a type of granofels that is typically very fine-grained and compact, and occurs in contact aureoles. Hornfelses are tough, and tend to splinter when broken. Chapter 22: Non-Foliated Metamorphic Rocks
Skarn: a contact metamorphosed and silica metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, etc. Tactite is a synonym. Granulite: a high grade rock of pelitic, mafic, or quartzo-feldspathic parentage that is predominantly composed of OH-free minerals. Muscovite is absent and plagioclase and orthopyroxene are common. Chapter 22: Specific Metamorphic Rock Types
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 graywacke. 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. Chapter 22: Specific Metamorphic Rock Types
Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine. Blueschist: a blue amphibole-bearing metamorphosed mafic igneous rock or mafic graywacke. This term is so commonly applied to such rocks that it is even applied to non-schistose rocks. Eclogite: a green and red metamorphic rock that contains clinopyroxene and garnet (omphacite + pyrope). The protolith is typically basaltic. Chapter 22: Specific Metamorphic Rock Types
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. Chapter 22: Specific Metamorphic Rock Types
The aluminium silicates : kyanite, andalusite and sillimanite Al 2 SiO 5 Al 2 SiO 5 ???? Why this formula for a silicate with isolated tetrahedra? Could be written Al.AlO(SiO 4 ) because there are two different Al structural sites as well as isolated [SiO 4 ] tetrahedra. They also have been considered as belonging to a separate silicate subclass – subsaturates, because there are conceptually too few oxygens, a naïve viewpoint. Kyanite, andalusite and sillimanite are polymorphs, because they have the same chemical composition but can exist with different crystal structures.
The crystal structures of the Al 2 SiO 5 polymorphs All three structures have straight chains of edge-sharing AlO 6 octahedra along the c axis. These chains contain half of the Al in the structural formula. In kyanite the remaining Al atoms are in 6-fold coordination (octahedra) In andalusite the remaining Al atoms are in 5-fold coordination In sillimanite the remaining Al atoms are in 4-fold coordination (tetrahedra) Density: kyanite > sillimanite > andalusite Therefore kyanite is stable at the highest pressures and lowest temperatures, while sillimanite is stable at high temperatures and lower pressures
The crystal structures of the Al 2 SiO 5 polymorphs All three structures have straight chains of edge-sharing AlO 6 octahedra along the c axis
The structure of kyanite 1. The AlO 6 octahedral chains
The structure of kyanite 1. The AlO 6 octahedral chains 2. Add the other Al polyhedra
The structure of kyanite 1. The AlO 6 octahedral chains2. Add the other Al polyhedra 3. Add the [SiO 4 ] tetrahedra
The structure of andalusite 1. The AlO 6 octahedral chains
The structure of andalusite 1. The AlO 6 octahedral chains 2. Add the other Al polyhedra
The structure of sillimanite 1. The AlO 6 octahedral chains
The structure of sillimanite 1. The AlO 6 octahedral chains 2. Add the other Al polyhedra
The structure of sillimanite 1. The AlO 6 octahedral chains 2. Add the other Al polyhedra 3. Add the [SiO 4 ] tetrahedra
Staurolite, Fe 2 2+ Al 9 O 6 [SiO 4 ] 4 (O,OH) 2 Monoclinic, 2/m (pseudo-orthorhombic)
The stability of the Al 2 SiO 5 polymorphs Pressure (kbar) The Al 2 SiO 5 polymorphs form in metamorphic rocks 3.55-3.66 3.16-3.20 3.23
The Metamorphic Facies Concept If P-T conditions were the same, different mineral assemblages must represent different starting compositions (i.e. different protoliths) If the starting compositions are identical, then different mineral assemblages must represent metamorphism under different physical conditions
Garnet compositions Solid solutions within each group are typical. At high T (above 700 o C) there is also solid solution between the 2 groups. Note: Solid solutions are always more extensive at higher Temperatures Grandite group Grossular Ca 3 Al 2 (SiO 4 ) 3 AndraditeCa 3 Fe 3+ 2 (SiO 4 ) 3 UvaroviteCa 3 Cr 2 (SiO 4 ) 3 Pyralspite group PyropeMg 3 Al 2 (SiO 4 ) 3 AlmandineFe 2+ 3 Al 2 (SiO 4 ) 3 SpessartineMn 3 Al 2 (SiO 4 ) 3
The garnet minerals Garnets: A large group of cubic minerals with general formula A 3 2+ B 2 3+ (SiO 4 ) 3 A – Ca 2+, Mg 2+, Fe 2+ or Mn 2+ B – Al 3+,Fe 3+ or Cr 3+ There are also many other synthetic compositions possible
Metamorphic facies High P/T series: subduction zones Medium P/T series: regional met. Low P/T series: contact met.
Metamorphic facies See Table 14.2 Greenschist, amphibolite, granulite, blueschist facies Minerals expected in different rock compositions.