Introduction to Metamorphism 2 IN THIS LECTURE Importance of Understanding Metamorphic Mineral Assemblages Progressive Nature of Metamorphism Stable Mineral Assemblages The Phase Rule in Metamorphic Systems The MgO-H2O system

What’s so Important About Metamorphic Mineral Assemblages Equilibrium mineral assemblages can tell us about P-T conditions P-T conditions tell us about tectonic environments P-T conditions combined with geochronology tells us about how tectonic environments evolve through time Optical Mineralogy is the starting point!!

The Progressive Nature of Metamorphism A rock at a high metamorphic grade probably progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today All rocks that we now find must also have cooled to surface conditions. Therefore, at what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate? The preserved zonal distribution of metamorphic rocks suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature) Metamorphic rocks usually maintain equilibrium as grade increases A rock at a high metamorphic grade thus probably progressed through a sequence of mineral assemblages as it passed through all of the mineral changes necessary to maintain equilibrium with increasing temperature and pressure, rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today

The Progressive Nature of Metamorphism Prograde reactions are endothermic and easily driven by increasing T Devolatilization reactions are easier than reintroducing the volatiles Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature Retrograde metamorphism is of only minor significance, and is usually detectable by observing textures, such as the incipient replacement of high-grade minerals by low-grade ones at their rims

Stable Mineral Assemblages in Metamorphic Rocks Equilibrium Mineral Assemblages At equilibrium, the mineralogy (and the composition of each mineral) is determined by T, P, and X “Mineral paragenesis” refers to such an equilibrium mineral assemblage Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated “Mineral assemblage” is used by some as a synonym for paragenesis, conventionally assuming equilibrium for the term Impossible to prove that a mineral assemblage now at the Earth’s surface represents thermodynamic (chemical) equilibrium at prior elevated metamorphic conditions Indirect textural and chemical support for such a conclusion is discussed in the text In short, it is typically easy to recognize non-equilibrium minerals (retrograde rims, reaction textures, etc.) We shall assume equilibrium mineral assemblages in the following discussion (will ignore retrograde…)

The Phase Rule in Metamorphic Systems Phase rule, as applied to systems at equilibrium: F = C - P + 2 the phase rule P is the number of phases in the system C is the number of components: the minimum number of chemical constituents required to specify every phase in the system F is the number of degrees of freedom: the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.) Remember we’ve seen this already with igneous systems

The Phase Rule in Metamorphic Systems Pick a random point anywhere on a phase diagram Likely point will be within a divariant field and not on a univariant curve or invariant point The most common situation is divariant (F = 2), meaning that P and T are independently variable without affecting the mineral assemblage In complex natural systems there may be one or more compositional variables as well, so that F may be greater than two The common occurrence of certain metamorphic mineral assemblages worldwide supports this contention that F  2, since such assemblages are much more likely to represent variable P-T-X conditions than more restricted situations

Remember this diagram?

The Phase Rule in Metamorphic Systems If F  2 is the most common situation, then the phase rule may be adjusted accordingly such that F = C - P + 2  2 and therefore P  C This is Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule The above simplified phase rule states that in the most common situation for a rock at equilibrium the number of phases is equal to or greater than the number of components It has been called Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule It is useful in evaluating whether or not a rock is at equilibrium

The Phase Rule in Metamorphic Systems If C has been determined for a particular rock then there are three potential situations according to the phase rule P=C This is the standard divariant situation in metamorphic rocks The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone P<C A situation that commonly arises in systems that display solid solution. We’ve seen this already with the binary phase diagrams for the albite-anorthite system To some authors the mineralogical phase rule is phi = C

Albite-Anorthite Phase Diagram

The Phase Rule in Metamorphic Systems If C has been determined for a particular rock then there are three potential situations according to the phase rule 1. P = C This is the standard divariant situation in metamorphic rocks The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone 2. P < C A situation that commonly arises in systems that display solid solution. We’ve seen this already with the binary phase diagrams for the albite-anorthite system 3. P > C A more interesting situation, and at least one of three situations must be responsible To some authors the mineralogical phase rule is phi = C

The Phase Rule in Metamorphic Systems For P > C then the following three situations could apply F < 2 Equilibrium not attained Choice of C not correct If (1) applies then the sample was collected from a location right on a univariant reaction curve or invariant point To some authors the mineralogical phase rule is phi = C The P-T phase diagram for the system Al2SiO5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The Phase Rule in Metamorphic Systems 2. Equilibrium is not attained The situation with (2) is more important in terms of optical mineralogy. The phase rule only applies to systems that are in equilibrium. If equilibrium is not attained or maintained then there could be any number of minerals co-existing Unfortunately, this is often the case, especially in rocks that have been partially retrogressed or rocks in the blueschist and eclogite facies. Optical Mineralogy is the main tool for decided which minerals represent the equilibrium mineral assemblage Graywackes typically contain fragments of a number of different rocks in a matrix of grains derived from a rapidly eroding source Dozens of minerals may be present, but they are not in equilibrium with one another A number of igneous and metamorphic rocks also have retrograde reactions that begin, but do not run to completion

The Phase Rule in Metamorphic Systems 3. The number of components was not correct Some guidelines for an appropriate choice of C Begin with a 1-component system, such as CaAl2Si2O8 (anorthite), there are 3 common types of major/minor components that we can add a)  Components that generate a new phase Adding a component such as CaMgSi2O6 (diopside), results in an additional phase: in the binary Di-An system diopside coexists with anorthite below the solidus b) Components that substitute for other components Adding a component such as NaAlSi3O8 (albite) to the 1-C anorthite system would dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below the solidus Fe and Mn commonly substitute for Mg Al may substitute for Si Na may substitute for K Graywackes typically contain fragments of a number of different rocks in a matrix of grains derived from a rapidly eroding source Dozens of minerals may be present, but they are not in equilibrium with one another A number of igneous and metamorphic rocks also have retrograde reactions that begin, but do not run to completion

The Phase Rule in Metamorphic Systems 3. The number of components was not correct (cont.) c) “Perfectly mobile” components Either a freely mobile fluid component or a component that dissolves in a fluid phase and can be transported easily The chemical activity of such components is commonly controlled by factors external to the local rock system They are commonly ignored in deriving C for metamorphic systems Graywackes typically contain fragments of a number of different rocks in a matrix of grains derived from a rapidly eroding source Dozens of minerals may be present, but they are not in equilibrium with one another A number of igneous and metamorphic rocks also have retrograde reactions that begin, but do not run to completion

The Phase Rule in Metamorphic Systems Consider the very simple metamorphic system, MgO- H2O Possible natural phases in this system are periclase (MgO), aqueous fluid (H2O), and brucite (Mg(OH)2) How we deal with H2O depends upon whether water is perfectly mobile or not A reaction can occur between the potential phases in this system: MgO + H2O  Mg(OH)2 Per + Fluid = Bru As written this is a retrograde reaction (occurs as the rock cools and hydrates)

The System MgO-H2O Cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O  Mg(OH)2 1) Suppose H2O is mobile and we ignore it as a component If we don’t treat a constituent as a component, we cannot treat it as a phase either, so we ignore the aqueous pore fluid in this case as well (the fluid phase is usually gone by the time we look at the rock anyway) Begin with periclase above the reaction temperature phi = 1 (water doesn’t count) and C = 1 (MgO), so the mineralogical phase rule holds As we cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O  Mg(OH)2

The System MgO-H2O Reaction: periclase coexists with brucite: P = C + 1 F = 1 (2nd reason to violate the mineralogical phase rule) To leave the curve, all the periclase must be consumed by the reaction, and brucite is the solitary remaining phase F = 1 and C = 1 again Reaction: univariant conditions periclase coexists with brucite (phi = C + 1) so F = 1 (the second reason to violate the mineralogical phase rule stated above) End: “Perfectly mobile” means that H2O behaves such that it can be added as it is needed, and any excess will leave as it is produced Water is simply an external reservoir, available in sufficient quantity that it enables the immobile magnesium to exist as either periclase or brucite, depending on which is stable under the given hydrous P-T conditions Imagine the diagram to be a field area, and the reaction an isograd For any common sample phi = 1 (since we ignore H2O and don’t really see the fluid in the rock), and phi = C in accordance with the mineralogical phase rule Only an uncommon sample, collected exactly on the isograd, will have phi > C

The Phase Rule in Metamorphic Systems Once the water is gone, the excess periclase remains stable as conditions change into the brucite stability field Thus periclase can be stable anywhere on the whole diagram, if water is present in insufficient quantities to permit the reaction to brucite to go to completion As a general rule, reactions such as this do indeed represent the absolute stability boundary of a phase such as brucite, because it is the only reactant in the prograde reaction (brucite  periclase + water) The reaction is not the absolute stability boundary of periclase, or of water, because either can be stable across the boundary if the other reactant is absent Of course this is true for any reaction involving multiple phases

The Phase Rule in Metamorphic Systems At any point (other than on the univariant curve itself) we would expect to find two phases, not one P = brucite + periclase below the reaction curve (if water is limited), or periclase + water above the curve To the right of the curve we appear to have only periclase, but if we count water as a component, we must also include it as a phase: the fluid phase

The Phase Rule in Metamorphic Systems How do you know which way is correct? The rocks should tell you The phase rule is an interpretive tool, not a predictive tool, and does not tell the rocks how to behave If you only see low-P assemblages (e.g. Per or Bru in the MgO- H2O system), then some components may be mobile If you often observe assemblages that have many phases in an area (e.g. periclase + brucite), it is unlikely that so much of the area is right on a univariant curve, and may require the number of components to include otherwise mobile phases, such as H2O or CO2, in order to apply the phase rule correctly

Metamorphism of Pelites IN THIS LECTURE Types of Protoliths Examples of Metamorphism Orogenic Metamorphism of the Scottish Highlands Barrovian vs Buchan Style Metamorphism Regional Metamorphism Otago New Zealand Contact Metamorphism of Pelitic Rocks

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, CO2 5. Quartz - nearly pure SiO2. 6. Quartzo-feldspathic - high Si, Na, K, Al Chemistry of the protolith is the most important clue toward deducing the parent rock 1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates 2. Mafic rocks. Basalts or gabbros, some graywackes 3. Shales (or pelitic rocks). Fine grained clastic clays and silts deposited in stable platforms or offshore wedges. 4. Carbonates. Mostly sedimentary limestones and dolostones. Impure carbonates (marls) may contain sand or shale components 5. Quartz rocks. Cherts are oceanic, and sands are moderately high energy continental clastics. Nearly pure SiO2. 6. Quartzo-feldspathic rocks. Arkose or granitoid and rhyolitic rocks. High Si, Na, K, Al Categories are often gradational, and cannot include the full range of possible parental rocks One common gradational rock type is a sand-shale mixture:psammite Other rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodies

Some Examples of Metamorphism Interpretation of the conditions and evolution of metamorphic bodies, mountain belts, and ultimately the evolution of the Earth's crust Metamorphic rocks may retain enough inherited information from their protolith to allow us to interpret much of the pre-metamorphic history as well When combined with geochemical and structural information can be used to reconstruct the tectonic environment

Orogenic Regional Metamorphism of the Scottish Highlands George Barrow (1893, 1912) SE Highlands of Scotland In Europe Caledonian orogeny ~ 500 Ma In Africa and other parts of Gondwana Pan-African Orogeny Nappes Granites George Barrow (1893, 1912): one of the first systematic studies of the variation in rock types and mineral assemblages with progressive metamorphism Metamorphism and deformation in the SE Highlands of Scotland occurred during the Caledonian orogeny, which reached its maximum intensity about 500 Ma ago Deformation in the Highlands was intense, and the rocks were folded into a series of nappes Numerous large granites were also intruded toward the end of the orogeny, after the main episode of regional metamorphism

Orogenic Regional Metamorphism of the Scottish Highlands 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. Barrow’s Area

Orogenic Regional Metamorphism of the Scottish Highlands Barrow studied the pelitic rocks Could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased Barrow noted significant and systematic mineralogical changes in the pelitic rocks He found that he could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased (which he could correlate to increased grain size) The new mineral that characterizes a zone is termed an index mineral

Orogenic Regional Metamorphism of the Scottish Highlands 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 (although kyanite and sillimanite are both polymorphs of Al2SiO5)

Barrovian Metamorphism of Pelites Sequence = Barrovian zones The P-T conditions referred to as Barrovian-type metamorphism (fairly typical of many belts) Now extended to a much larger area of the Highlands Isograd = line that separates the zones (a line in the field of constant metamorphic grade) This sequence of zones now recognized in other orogenic belts, and is now so well established in the literature that the zones are often referred to as the Barrovian zones Tilley, Kennedy, etc. confirmed Barrow’s zones, and extended them over a much larger area of the Highlands Tilley coined the term isograd for the line that separates the zones An isograd, then, is meant to indicate a line in the field of constant metamorphic grade Really = the intersection of the isogradic surface with the Earth’s surface

Barrovian Zones in the Scottish Highlands 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.

Barrovian Zones in the Scottish Highlands To Summarise An isograd (in this classical sense) represents the first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade When one crosses an isograd, such as the biotite isograd, one enters the biotite zone Zones thus have the same name as the isograd that forms the low-grade boundary of that zone Since classic isograds are based on the first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones Later we shall see broader categories: metamorphic facies Barrovian zones have become the norm to which we compare all other areas of regional metamorphism OK practice, but we shouldn’t let these zones constrain our thinking or our observations Other zones may be important and useful locally A chloritoid zone is prevalent in the Appalachians (X)

Variations on the Barrovian Zones in the Scottish Highlands A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district Here the pelitic compositions are similar, but the sequence of isograds is: chlorite biotite cordierite andalusite sillimanite

Barrovian vs Buchan Metamorphism The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite  sillimanite at the sillimanite isograd only above this pressure The molar volume of cordierite is also quite high, indicating that it too is a low-pressure mineral The geothermal gradient in this northern district was higher than in Barrow’s area, and rocks at any equivalent temperature must have been at a lower pressure This lower P/T variation has been called Buchan-type metamorphism. It too is relatively common Miyashiro (1961), from his work in the Abukuma Plateau of Japan, called such a low P/T variant Abukuma-type Both terms are common in the literature, and mean essentially the same thing The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodies Intrusions are shallow, and contact effects overprinted on an earlier low-grade regional orogenic metamorphism

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK The aureole around the Skiddaw granite was sub-divided into three zones, principally on the basis of textures: Unaltered slates Outer zone of spotted slates Middle zone of andalusite slates Inner zone of hornfels Skiddaw granite Increasing Metamorphic Grade

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. After Eastwood et al (1968). Geology of the Country around Cockermouth and Caldbeck. Explanation accompanying the 1-inch Geological Sheet 23, New Series. Institute of Geological Sciences. London. First effects (1-2 km from contact) = 0.2 - 2.0 mm sized black ovoid “spots” in the slates At the same time, recrystallization -> slight coarsening of the grains and degradation of the slaty cleavage Spots were probably cordierite or andalusite, since re-hydrated and retrograded back to fine aggregates of mostly muscovite Both cordierite and andalusite occur at higher grades, where they are often partly retrograded, but not farther out Spots that we now see in most of the spotted slates are probably pseudomorphs

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Middle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartz Cordierite forms ovoid xls with irregular outlines and numerous inclusions, in this case of biotite, muscovite, and opaques The biotite and muscovite inclusions often retain the orientation of the slaty cleavage outside the cordierites This indicates that the growing cordierite crystals enveloped aligned micas that grew during the regional event Excellent textural evidence for the overprint of contact metamorphism on an earlier regional one Micas outside the cordierites are larger and more randomly oriented, suggesting that they formed or recrystallized during the later thermal event Andalusites have fewer inclusions than cordierite, and many show the cruciform pattern of fine opaque inclusions known as chiastolite Cordierite-andalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London. 1 mm

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Inner zone: Thoroughly recrystallized Lose foliation Both andalusite and cordierite are minerals characteristic of low-pressure metamorphism, which is certainly the case in the Skiddaw aureole, where heat is carried up into the shallow crust by the granites The rocks of the inner zone at Skiddaw are characterized by coarser and more thoroughly recrystallized textures Same mineral assemblage as the middle zone Some rocks are schistose, but in the innermost portions the rock fabric loses the foliation, and the rocks are typical hornfelses 1 mm Andalusite-cordierite schist from the inner zone of the Skiddaw aureole. Note the chiastolite cross in andalusite (see also Figure 22-49). From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK The zones determined on a textural basis Better to use the sequential appearance of minerals and isograds to define the zones But low-P isograds converge in P-T Skiddaw sequence of mineral development with grade is difficult to determine accurately The zones at Skiddaw were determined by Rastall (1910) on a textural basis A more modern and appropriate approach would be to conform to the practice used in the regional example above, and use the sequential appearance of minerals and isograds to define the zones This is now the common approach for all types of regional and contact metamorphism The first new mineral in most slates is biotite, followed by the approximately simultaneous development of cordierite and andalusite Perhaps the textural zonation is more useful in some cases

Pelites in Southern Africa Barberton Granite-Greenstone Belt, Mpumalanga Damara Orogen, Namibia Contact metamorphism associated with Bushveld Complex, Limpopo Province