Contact & Regional Metamorphism

EENS 211	Earth Materials
Tulane University	Prof. Stephen A. Nelson
Contact & Regional Metamorphism

Contact Metamorphism

As discussed previously, contact metamorphism occurs as a result of a high geothermal gradient produced locally around intruding magma. Contact metamorphism is usually restricted to relatively shallow depths (low pressure) in the Earth because it is only at shallow depths where there will be a large contrast in temperature between the intruding magma and the surrounding country rock. Also, since intrusion of magma does not usually involve high differential stress, contact metamorphic rocks do not often show foliation. Instead, the common rocks types produced are fine grained idioblastic or hypidioblastic rocks called hornfels. The area surrounding an igneous intrusion that has been metamorphosed as a result of the heat released by the magma is called a contact aureole. We will here first discuss contact aureoles, then look at the facies produced by contact metamorphism.

Contact Aureoles

Within a contact metamorphic aureole the grade of metamorphism increases toward the contact with the igneous intrusion.

An example of a contact aureole surrounding the Onawa Pluton in Maine is shown here. The granodiorite pluton was intruded into slates produced by a prior regional metamorphic event. The aureole is a zone ranging in width from about 0.5 to 2.5 km around the intrusion. Two zones representing different contact metamorphic facies are seen within the aureole. The outer zone contains metapelites in the Hornblende Hornfels Facies, and the zone adjacent to the pluton contains metapelites in the Pyroxene Hornfels Facies. The zones are marked by an isograd, which represents a surface along which the grade of metamorphism is equal.

The size of a contact aureole depends on a number of factors that control the rate at which heat can move out of the pluton and into the surrounding country rock. Among these factors are:

The size and temperature of the intrusion. This will control how much heat is available to heat the surrounding country rocks.

The thermal conductivity of the surrounding rocks. This will control the rate at which heat can be transferred by conduction into the surrounding rocks. In general, the rate of heat flow Q, depends on the thermal conductivity, K, and the temperature gradient, (¶T/¶x)

Q = K(¶T/¶x)

Thus, the rate at which heat moves by conduction increases if the thermal conductivity and temperature gradient are higher.

The initial temperature within the country rock. This, in combination with the temperature of the intrusion, will determine the initial temperature gradient, and thus the rate at which heat can flow into the surrounding country rocks.
The latent heat of crystallization of the magma. As you recall, the total amount of heat available in a liquid is not only dependent on the temperature, but also involves the heat released due to crystallization. Thus, if the latent heat of crystallization is large, their will be more heat available to heat the surrounding country rocks.
The heat of metamorphic reactions. In order for a metamorphic reaction to take place some heat is necessary and this heat will be absorbed by the reactions without increasing the temperature in the intrusion.
The amount of water in and the permeability of the surrounding country rock. If the country rock is permeable and contains groundwater, heat will be able to move by convection.

Solutions to the heat equation given above are complicated because most the terms in the equation are functions of temperature, time, and position. Solutions for a simple case are shown below.

In this simple case a basaltic dike is assumed to have intruded at temperature of 1100^oC, into dry country rock at a temperature of 0^oC. The width of dike is assumed to be 100 m, and the latent heat of crystallization is assumed to be released between 1100^o and 800^o. Solidification of the intrusion is thus complete at 800^o, after 10,300 years. Note how the temperature of the country rock near the contact reaches a maximum of about 600^o after about 1600 years, and how the temperature in the country rock at distances greater than about 700 m from the center of the dike continues to rise, while temperatures near the contact drops.

The model above assumes that all heat moves by conduction. If the country rock is saturated with water or the pluton expels water, and if the country rock is permeable, then the heat will move into the country rock by convection. Water will be heated near the contact and carry heat outward and away where it will eventually cool to return to the contact to carry more heat away.

To show these effects, a model was developed for a diabase sill 700 m thick, intruded under 350 m of cover into both dry and wet country rock. The results show that the temperature gradient developed in the country rock will be higher under dry country rock conditions, and the actual temperature attained in the country rock at any position will be slightly less under wet conditions than under dry conditions. Thus, the size of the aureole will be smaller if the heat is removed and distributed by convection.

Facies of Contact Metamorphism

The facies of contact metamorphism progress in temperature at relatively low pressure from the Albite-Epidote Hornfels Facies to the Hornblende Hornfels Facies, to the Pyroxene Hornfels Facies. Xenoliths picked up by the magma may be metamorphosed to the Sanidinite Facies, but such rocks are relatively rare. In this lecture we will look at the mineral assemblages that develop in these contact metamorphic facies.

The hornfels facies are shown in the diagram below. Note that the facies are all relatively low pressure facies. This is because at higher pressures under normal geothermal gradients, the contrast in temperature between the magma and the surrounding country rock is lower than under the low pressure conditions near the surface. Thus, regional metamorphism is likely to overprint the effects of contact metamorphism or the rocks may already be metamorphosed to a regional metamorphic facies.

Albite - Epidote Hornfels Facies
Pelitic rocks will be characterized by an assemblage of

quartz, albite, epidote, muscovite or andalusite, chlorite, biotite

Quartzo-feldspathic rocks will be characterized by an assemblage of

microcline, quartz, muscovite, albite, and biotite.

Basic rocks will contain

actinolite, epidote, chlorite, and/or biotite, and possibly talc, and may contain quartz and albite.

Calcareous rocks will consist of

calcite, epidote and tremolite, with possibly quartz.

Hornblende-Hornfels Facies
Pelitic rocks will be characterized by an assemblage of

quartz, plagioclase, muscovite or andalusite, cordierite, or
quartz, plagioclase cordierite, muscovite, and biotite

Note the absence of epidote and chlorite in these assemblages.

Quartzo-feldspathic rocks will be characterized by an assemblage of

microcline, quartz, muscovite, plagioclase and biotite and possibly almandine.

Basic rocks will likely contain

plagioclase, biotite, and possibly almandine, and may contain quartz, anthophyllite & cordierite

Note the absence of epidote and actinolite.

Calcareous rocks will consist of

plagioclase, grossularite, and tremolite and possibly quartz, or
calcite, diopside, and grossularite with possibly quartz.

Note the absence of epidote.

Pyroxene-Hornfels Facies
Pelitic rocks will be characterized by an assemblage of

quartz, plagioclase, K-spar, andalusite or sillimanite, and cordierite

Note the absence of muscovite.

Quartzo-feldspathic rocks will be characterized by an assemblage of

K-spar, quartz, plagioclase and biotite

Again note the absence of muscovite

Basic rocks will likely contain

plagioclase, cordierite, and biotite and possibly quartz, or
plagioclase, hypersthene, biotite, and diopside, and possibly quartz.

Note the absence of hornblende in the these assemblages.

Calcareous rocks will consist of

plagioclase, grossularite, and diopside and possibly quartz, or
wollastonite, diopside, and grossularite with possibly quartz.

Note the absence of calcite and tremolite in these assemblages.

Sanidinite Facies
The sanidinite facies is relatively rare in contact metamorphic aureoles, although it is somewhat more common in rocks found as xenoliths in igneous rocks. It represents the highest conditions of temperature. The facies is characterized by the absence of hydrous minerals, particularly micas.

Pelitic and quartzo-feldspathic rocks contain unusual phases like mullite (3Al₂O₃^.2SiO₂), along with sanidine, cordierite, anorthite, hypersthene, and sillimanite or corundum. Sometimes tridymite is present in place of quartz.
Basic rocks of the sanidinite facies are more common, and are often found along the conduit walls of dikes. Several assemblages have been reported.

augite, hypersthene, calcic plagioclase, brookite, and tridymite

olivine, augite, plagioclase, magnetite, and ilmenite (similar to an igneous mineral assemblage)

hypersthene, plagioclase, magnetite, ilmenite, psuedobrookite

cordierite, plagioclase, magnetite, hematite, psuedobrookite

some rare aluminous basic rocks have also been found with
corundum and hematite

corundum, mullite, and hematite, sometimes with cristobalite or tridymite

corundum, mullite, hercynite (FeAl₂O₄), sometimes with cordierite and cristobalite or tridymite
Calcareous rocks contain various assemblages with rare minerals. Among the assemblages observed are:

wollastonite, anorthite, and diopside

wollastonite, mellilite ([Ca,Na]₂[Mg,Fe,Al,Si]₃O₇), and

calcite, larnite (Ca₂SiO₄), along with the rare minerals brownmillerite (Ca₂[Al,Fe]₂O₅) and mayenite (Ca₁₂Al₁₄O₃₃)

Skarns

Sometimes when a siliceous magma intrudes carbonate rocks like limestone and dolostone, significant chemical exchange (metasomatism) takes place between the magma and the carbonate rock. Such a metasomatized rock is refered to as skarn. An excellent example of a skarn occurs in the Crestmore quarry near San Diego, California.

Here, quartz monzonite intruded an Mg-rich limestone. Metamorphism and metasomatism produced four zones near the contact three ranging in size from 3 cm to 15 m in width. The outer zone consists of calcite marble or calcite - brucite [MgOH₂] marble, showing little metasomatism.

Closer to the contact is the montecellite zone. This zone consists of calcite, montecellite [Ca(Mg,Fe)SiO₄] and one or more of the minerals clinohumite [Mg(OH,F)₂^.4Mg₂SiO₄], forsterite, mellilite, spurrite [2Ca₂SiO₄.CaCO₃], tilleyite [Ca₃Si₂O₇^.2CaCO₃], and merwinite [Ca₃MgSi₂O₈]

Interior to the montecellite zone is the idocrase zone, consisting of idocrase [Ca₁₉(Al,Fe)₁₀(Mg,Fe)₃Si₁₈O₆₈(OH,F)₁₀] in association with calcite, diopside, wollastonite, phlogopite (Mg-rich biotite), montecellite, and xanthophyllite [Ca₂(Mg,Fe)_4.6Al_6.9Si_2.5O₂₀(OH)₄].

Next to the contact is the garnet zone consisting of grossularite garnet, diopside, wollastonite, and miner calcite and quartz.

A thin zone along the contact shows evidence of assimilation of the limestone by the magma.

The ratio of Si to Ca and the concentration of Al all increase toward the contact, indicating that the limestone received these components from the magma.

Regional Metamorphism

Regional metamorphism is metamorphism that occurs over broad areas of the crust. Most regionally metamorphosed rocks occur in areas that have undergone deformation during an orogenic event resulting in mountain belts that have since been eroded to expose the metamorphic rocks.

The Dalradian and Moinian Series of Scotland
The classic example of a regionally metamorphosed area is the Dalradian series of Scotland. The Dalradian Series occurs in a zone 50 to 80 km wide, north of the Highland Boundary Fault. A similar group of metamorphic rocks occurs to the North of the Great Glenn Fault (a strike-slip fault) and is called the Moinian Series.

The rocks were originally shales, limestones, diabase sills, and basalts that had been emplaced in the Precambrian to early Cambrian. In this area, Barrow (1893) mapped metamorphic zones in pelitic rocks based on mineral assemblages he observed in a small part the area. This mapping was later extended across the Scottish Highlands to cover most of the Dalradian and Moinian Series as shown in the map. The series of metamorphic zones mapped by Barrow has since become known as the Barrovian Facies Series (At the time Barrow did his mapping, the facies concept had not yet been developed). In pelitic rocks, Barrow recognized 6 zones of distinctive mineral assemblages, which he recognized as representing increasing grade of metamorphism.

The boundaries for his zones were based on the first appearance of a particular mineral, called an index mineral, which is characteristic of the zone. These boundaries were later called isograds (equal grade) and likely represent surfaces in a three dimensional sense. He called the zone of lowest grade rocks the "zone of digested clastic mica," but Tilley, mapping the area in 1925, renamed this zone the chlorite zone.

Zone (textural type)	Mineral Assemblage in Pelitic Rocks
Chlorite (slates & phyllites)	quartz, chlorite, muscovite, albite
Biotite (phyllites & schists)	biotite begins to replace chlorite, quartz, muscovite, albite
Garnet (phyllites and schists)	quartz, muscovite, biotite, almandine, albite
Staurolite (schists)	quartz, biotite, muscovite, almandine, staurolite, oligoclase
Kyanite (schists)	quartz, biotite, muscovite, oligoclase, almandine, kyanite
Sillimanite (schists & gneisses)	quartz, biotite, muscovite, oligoclase, almandine, sillimanite

Mineral assemblages for pelitic rocks of the Barrovian Zones are listed in the table above. Note the following important points:

The index mineral that defines a zone, does not necessarily disappear when entering the next higher grade zone. For example the first appearance of biotite is at the biotite isograd where chlorite is seen to be reacting to produce biotite. Biotite does not disappear at the garnet isograd, and, in fact continues to be seen though the garnet, sillimanite, staurolite, kyanite, and sillimanite zones.
Staurolite, kyanite, and sillimanite only occur in the staurolite, kyanite, and sillimanite zones, respectively.
The composition of the plagioclase changes with increasing grade of metamorphism. It is nearly pure albite in the chlorite and biotite zones, and becomes somewhat more calcic (oligoclase) in higher grade zones.
The texture of the rocks changes from slates and phyllites in the chlorite zone to schists in the staurolite and kyanite zones, to schists and gneisses in the sillimanite zone.

As mentioned above, Tilley mapped the area in 1925, and extended the zones across the area between the Highland Boundary Fault and into the Moinian Series, and renamed the chlorite zone. Later, Wiseman (1934), mapped the metabasic rocks and Kennedy (1949) mapped the meta calcareous sediments. Mineral assemblages in these rocks and their correlation with the metamorphic zones in the pelitic rocks are shown in the table below.

Note that the metabasic rocks only define two zones, one corresponding to the chlorite and biotite zones in the pelitic rocks, and the other corresponding to the staurolite, kyanite, and sillimanite zones. The calcareous rocks define four zones. The lowest grade rocks are only metamorphosed in the higher grade parts of the pelitic chlorite zone, another set of minerals occurs in the calcareous rocks throughout the biotite and garnet zones, another mineral assemblage is characteristic of the staurolite and kyanite zone, and a fourth mineral assemblage is found in calcareous rocks of the sillimanite zone.

Note how the Anorthite content of plagioclase increases with increasing grade of metamorphism in the basic rocks and the calcareous rocks.

Index Mineral (Pelitic Rocks)	Basic Rocks	Calcareous Rocks	Facies
Chlorite	Chlorite, albite, epidote, sphene, ± calcite ± actinolite		Greenschist
		qtz, muscovite, biotite, calcite
Biotite		garnet, zoisite, sodic plagioclase, biotite or hornblende
Garnet	Hornblende, plagioclase, ±epidote, ±almandine, ±diopside		Amphibolite
Staurolite		garnet, anorthite or bytownite, hornblende
Kyanite
Sillimanite		garnet, anorthite or bytownite, pyroxene

The facies concept was developed by Eskola in 1939. Recall that the names of Eskola's facies are based on mineral assemblages found in metabasic basic rocks. Applying the facies concept to the Dalradian series of Scotland, one finds only two facies represented. The chlorite and biotite zones represent greenschist facies metamorphism, and the garnet, staurolite, kyanite, and sillimanite zones represent amphibolite facies metamorphism.

The structural history of the Dalradian and Moinian Series is complex. Deformation can be divided into three main stages, based on structures found in the rocks and radiometric dating.

Between about 600 and 550 million years ago was a period of large scale recumbent folding.
Between 550 and 480 million years ago simple anticlinal flooding with fold axes plunging toward the southwest. During this stage, much of the metamorphism took place.
Between 480 and 420 million years ago minor refolding occurred accompanied by minor retrograde metamorphism The retrograde metamorphism consisted of chloritization of biotite and garnet, and seritization of kyanite.

These events occurred during the Caledonian Orogeny, when the European continental block was colliding with the North American continental block, thus, in North America, this event is correlative with the Appalachian Orogenies.

There is apparently no correlation with stratigraphic depth in the Dalradian metamorphic rocks, although there must be a correlation with tectonic depth. Pressure - Temperature estimates for the Sillimanite zone indicate a maximum temperature of about 700^oC and maximum pressure of about 7 kb. This corresponds to a depth of about 25 km, and gives a geothermal gradient of about 28^oC /km (compared to a normal stable continental geotherm of 25^oC /km

Regional Metamorphism of the Southern Appalachians
Metamrophism in the southern Appalachians extends from Central Virginia to Alabama. Interpretation of the relationship between deformation and metamorphism is complicated by two factors:

Three or more orogenic events affected the region. The most recent are:
1. The Taconic Orogeny - Ordovician, - 500 to 400 million years ago.
2. The Acadian Orogeny - Devonian to Mississippian - 400 to 350 million years ago.
3. The Alleghenian Orogeny - Pennsylvanian to Permian - 325 to 260 million years ago.
Thrust faulting during these three orogenic events sliced up the area. Some of these thrust faults, like the Fries-Hayesville Thrust, shown below, occurred after metamorphism.

sappalmeta.gif (59644 bytes)

Zeolite and Prehnite-Pumpellyite Facies Metamorphic Rocks. These occur mostly in the Valley and Ridge Province, located northwest of the Great Smokey (or Blue Ridge) Thrust. The rocks are Precambrian to Lower Cambrian shales and carbonates that were folded during the Alleghenian Orogeny. Pelitic rocks show only recrystallization of clay minerals, carbonate rocks have an assemblage of calcite or dolomite ± quartz, and rare basic rocks have chlorite, calcite, and rare pumpellyite [Ca₄(Mg,Fe)(Al,Fe⁺³)₅Si₄O₂₃(OH)₃^.2H₂O].
Greenschist Facies Rocks. These occur in the Carolina Slate Belt in the east, and near the Great Smokey Fault in the west. Some greenschist facies rocks are also exposed in structural windows within the central Blue Ridge Province. The following mineral assemblages are observed:
- Pelitic Rocks
  Chlorite, sericite, quartz, albite, magnetite, ±garnet (highest grade greenschist facies).
- Quartzo-Feldspathic Rocks
  quartz, sericite, chlorite, K-spar, magnetite ± biotite, ±calcite, ±pyrite.
- Basic Rocks
  chlorite, epidote, albite, quartz, hematite or
  actinolite, chlorite, albite, quartz, magnetite.
- Carbonate Rocks
  dolomite, calcite, quartz, plagioclase, biotite ± sericite
Amphibolite Facies Rocks. These occur throughout the eastern Blue Ridge Province and in the Piedmont Province. Pressure and temperature calculations suggest that the amphibolite facies rocks were metamorphosed between 500 and 850^oC and 5 to 11 kilobars pressure. Migmatites accompany some of the higher grade assemblages, and are expected since the maximum temperatures approach the range of wet granite melting.
- Pelitic Rocks
  quartz, muscovite, k-spar, ±staurolite, ±garnet, ±kyanite
- Quartzo-Feldspathic Rocks
  quartz, muscovite, plagioclase, garnet, ±magnetite ±ilmenite.
- Basic Rocks
  hornblende, plagioclase, garnet, ±quartz, ±epidote, ±biotite, ±magnetite, ±ilmenite, ±pyrite.
- Calcareous Rocks
  rare marbles contain diopside and calcite. More siliceous calcareous rocks are seen as pseudo-diorites (because they look like diorites in hand specimen) with zoisite, garnet, and hornblende.
Granulite Facies Rocks. Granulite Facies rocks occur in a small area near the crest of the Blue Ridge Province in southeastern North Carolina (Winding Stair Gap). The rocks here have suffered some retrograde metamorphism, but original granulite facies assemblages are still discernable. Peak metamorphic conditions were apparently reached in this area during the Taconic Orogeny. Pressure - Temperature calculations indicate that the peak metamorphic conditions reached temperatures of 750 - 775^oC and pressures of 6.5 - 7.0 kb. This indicates a geothermal gradient of about 30^oC/km.
- Pelitic Rocks
  biotite, garnet, sillimanite, K-spar, andesite, quartz, magnetite, ilmenite.
- Quartzo-Feldspathic Rocks
  Quartz, andesine, K-spar, biotite, garnet, magnetite, ilmenite.
- Basic Rocks
  biotite, orthopyroxene, bytownite, quartz, magnetite, ilmenite, ±hornblende (probably retrograde).
- Calcareous Rocks
  calcite, quartz, scapolite [Ca₄Al₃(Al,Si)₃Si₆O₂₄(Cl,CO₃,SO₄)], ±grossularite, ±diopside, ±clinozoisite, ±sphene, ±apatite.
- Ultramafic Rocks
  orthopyroxene, andesine, biotite, and retrograde minerals - hornblende, cummingtonite, & quartz.

High Pressure - Low Temperature Metamorphism in the Franciscan of California
Metamorphism along low geothermal gradients results in a series of rocks that pass through the Zeolite, Prehnite-Pumpellyite, Blueschist, and Eclogite Facies of Regional Metamorphism. The best studied example of this type of metamorphism occurs within the Cretaceous Franciscan Complex of California. The Franciscan Complex contains highly folded and faulted blocks and slabs of both unmetamorphosed or weakly metamorphosed rocks, and smaller tectonic blocks bounded by faults. It is structurally complex, and only in a few places can continuous structures be mapped over large areas. For this reason, the complex is called a mélange (French for mixed).

The nonmetamorphosed to weakly metamorphosed rocks are those that are typically found in ophiolite sequences, such as pillow basalts, shales, radiolarian cherts, and limestones, along with clastic sedimentary rocks usually found in oceanic trenches, such as graywackes. These rock types are also found metamorphosed into the Zeolite, Prehnite-Pumpellyite, Blueschist, and Eclogite Facies.

fransican.gif (49092 bytes)

Typical mineral assemblages are as follows:

Zeolite Facies From the mineral assemblage listed below metamorphism of zeolite facies rocks occurred at temperatures of 100 - 200^oC and pressures of 1 to 3 kb.
- Metagraywackes - retain their clastic sedimentary textures, contain quartz, feldspars and rock fragments with laumontite (CaAl₂Si₄O₁₂^.4H₂O), prehnite [Ca₂Al₂Si₃O₁₀(OH)₂] or pumpellyite [Ca₄(Mg,Fe)(Al,Fe⁺³)₅Si₄O₂₃(OH)₃^.2H₂O in the recrystallized feldspars. Typical rocks have the following mineral assemblages:
  
  quartz, albite, laumontite, chlorite, sericite, hematite
  quartz, albite, prehnite, chlorite, sericite, calcite
  quartz, albite, K-spar, chlorite, sericite, stilpnomelane, calcite
- Metabasalts - again retain their primary igneous textures but contain such minerals as epidote, laumontite, prehnite, and pumpellyite.
Blueschist Facies - most of the rocks of the blueschist facies do not retain their original textures, and are now schistose. Typical mineral assemblages in various compositional groups are as follows: Pressure temperature estimates for the mineral assemblages below show a wide range, with temperatures from 125 to 350^oC and pressures from 4 to 10 kb.
- Pelitic Rocks
  muscovite, chlorite, quartz, albite, lawsonite [CaAl₂Si₂O₇(OH)₂^.H₂O]
  muscovite, chlorite, glaucophane [Na₂Mg₃Al₂Si₈O₂₂(OH)₂], garnet
- Quartzo-Feldspathic (Metagraywackes)
  quartz, muscovite, albite, chlorite, lawsonsite, aragonite, ± jadeitic pyroxene ±glaucophane, ±stilpnomelane ±sphene
- Metacherts
  quartz, crossite [Na₂(Mg,Fe⁺²)₃(Al,Fe⁺³)₂Si₈O₂₂(OH)₂], aegerine, hematite, ±aragonite or
  quartz, stilpnomelane, garnet
- Basic Rocks
  glaucophane, lawsonite, albite, sphene
  glaucophane, lawsonite, stilpnomelane, chlorite, albite, quartz
  glaucophane, albite, quartz, garnet, muscovite
  glaucophane, lawsonite, pumpellyite, chlorite, albite, garnet
  chlorite, lawsonite, jadeitic pyroxene, glaucophane, quartz, sphene
- Calcareous Rocks
  aragonite, lawsonite, glaucophane
Eclogite Facies rocks in the Franciscan are only found as exotic tectonic blocks measuring a few meters in diameter. They are all metabasalts, and are characterized by the presence of omphacitic pyroxene (a complex clinopyroxene rich in the jadeite [NaAlSi₂O₆] component. Besides omphacitic pyroxene, the rocks contain garnet (typically up to 50% pyrope component), rutile, sphene, and sometimes quartz. Maximum temperature - pressure conditions for the eclogite mineral assemblages indicate temperatures between 300 and 540^oC and pressures from 6 to 14 kb.

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