Metamorphic Zones, Index Minerals, Isograds, Facies and Facies Series

the onslaught of terminology to understand how we categorize metamorphic rocks and their conditions of formation!

What textures do you see?

Review: Types of Protolith

1.Pelitic/mudrocks high Al, K, Si 2. Quartzo-feldspathic high Si, Na, K, Al 3. Calcareous high Ca, Mg, CO 2 4. Mafic high Ca, Mg, Fe 5. Ultramafic very high Mg, Fe, low Si, Al WHY IS CHEMICAL COMPOSITION OF PROTOLITH IMPORTANT?

Metamorphic Grade

Refers to maximum T or P of metamorphism

What grades have we talked about?

The idea of grade is general. We can better express the maximum P, T constraints using the concepts of metamorphic

zone

and

facies

. More on this!

Orogenic Regional Metamorphism of the Scottish Highlands: Development of the Index Mineral Concept

• George Barrow (1893, 1912) • SE Highlands of Scotland Caledonian Orogeny ~ 500 Ma • • Lots of folding Granites

Barrow’s Area

Figure 21-8.

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.

Orogenic Regional Metamorphism of the Scottish Highlands

• Barrow studied 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

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 Al 2 SiO 5 )

Each of these minerals is an INDEX mineral.

 Chlorite zone  Biotite zone  Garnet zone  Staurolite zone  Kyanite zone  Sillimanite zone

WHAT IS AN INDEX MINERAL

• 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 • ANOTHER DEFINTION: • Isograd line that separates the zones (a line in the field of constant metamorphic grade). Also reflects the FIRST APPEARANCE of the index mineral.

Figure 21-8.

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.

To summarize: • An isograd 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 • Because 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

A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district • Pelitic compositions are similar, but the sequence of isograds is:  chlorite  biotite  garnet  andalusite  sillimanite

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 1 GPa = 10kbars

Figure 21-9.

The P-T phase diagram for the system Al 2 SiO 5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al 2 SiO 5 to pyrophyllite, which limits the occurrence of an Al 2 SiO 5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

Metamorphic Facies

• Eskola (1915) developed the concept of metamorphic facies: • What is a metamorphic facies?

Fig. 25-2.

Temperature pressure diagram showing the generally accepted limits of the various facies used in this text. Boundaries are approximate and gradational. The “typical” or average continental geotherm is from Brown and Mussett (1993). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Metamorphic Facies

Metamorphic Facies

• The range of temperature and pressure conditions represented by each facies  Eskola aware of the P-T implications and correctly deduced the relative temperatures and pressures of facies he proposed  Can now assign relatively accurate temperature and pressure limits to individual facies

Metamorphic Facies

Eskola (1920) proposed 5 original facies:  Greenschist  Amphibolite  Hornfels  Sanidinite •  Eclogite • Easily defined on the basis of mineral assemblages that develop in mafic rocks More facies have been added since original designations

Metamorphic Facies

Formation of Zeolites Greenschist Facies Temperature Epidote Amphibolite Facies Amphibolite Facies Sanadinite Facies Pyroxene Hornfels Facies Granulite Facies Glaucophane Schist Facies Eclogite Facies

Fig. 25-1 The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After Eskola (1939) Die Entstehung der Gesteine. Julius Springer. Berlin.

Fig. 25-2.

Temperature pressure diagram showing the generally accepted limits of the various facies used in this text. Boundaries are approximate and gradational. The “typical” or average continental geotherm is from Brown and Mussett (1993). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Metamorphic Facies

•

Metamorphic Facies defined for mafic protolith

The definitive mineral assemblages that characterize each facies (for mafic rocks).

Table 25-1

. Definitive Mineral Assemblages of Metamorphic Facies Zeolite

Facies Definitive Mineral Assemblage in Mafic Rocks

zeolites: especially laumontite, wairakite, analcime Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite) Greenschist Amphibolite chlorite + albite + epidote (or zoisite) + quartz ± actinolite hornblende + plagioclase (oligoclase-andesine) ± garnet Granulite Blueschist Eclogite Contact Facies orthopyroxene (+ clinopyrixene + plagioclase ± garnet ± hornblende) glaucophane + lawsonite or epidote (+albite ± chlorite) pyrope garnet + omphacitic pyroxene (± kyanite) Mineral assemblages in mafic rocks of the facies of contact meta morphism do not differ substantially from that of the corresponding regional facies at higher pressure.

After Spear (1993)

It is convenient to consider metamorphic facies in 4 groups: 1) Facies of high pressure  The blueschist and e c l o g i t e facies: low molar volume phases under conditions of high pressure  Blueschist facies occurs in areas of low T/P gradients, characteristically developed in subduction zones  E c l o g i t e s are stable under normal geothermal conditions May develop wherever mafic magmas solidify in the deep crust or mantle: crustal chambers or dikes, sub-crustal magmatic underplates, subducted crust that is redistributed into the mantle

Metamorphic Facies

2) Facies of medium pressure  Most metamorphic rocks now exposed belong to the greenschist , amphibolite , or granulite facies  The greenschist and amphibolite “typical” geothermal facies conform to the gradient

Fig. 25-9.

Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Metamorphic Facies

3) Facies of low pressure  Albite-epidote hornfels , hornblende hornfels , and pyroxene hornfels facies: contact metamorphic terranes and regional terranes with very high geothermal gradient.

 Sanidinite facies is rare- limited to xenoliths in basic magmas and the innermost portions of some contact aureoles adjacent to hot basic intrusives

Metamorphic Facies

4) Facies of low grades  Rocks often fail to recrystallize thoroughly at very low grades, and equilibrium is not always attained  Zeolite and prehnite pumpellyite facies are thus not always represented, and the greenschist facies is the lowest grade developed in many regional terranes

Metamorphic Facies

Review • Metamorphic zone (e.g., chlorite zone) • Index Mineral • Isograd • Metamorphic Facies

Facies Series/Field Gradient

A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients, and, if extensive enough, cross through a sequence of facies

Fig. 25-3.

Temperature pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Field gradient

Pressure-Temperature Time Paths

•

Facies concept leads to idea that metamorphic petrologists try to reconstruct CONDITIONS of metamorphism.

•

Also important is TIME. Time tells us about the RATES of processes.

Regional Metamorphism

3 stages: Burial/crustal thickening- why does trajectory have steep slope?

Heating stage Uplift stage

Regional Metamorphism

What are prograde vs. retrograde metamorphic paths or reactions?

Figure 21-1.

Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw Hill.

Example of Contact Metamorphism

What does this diagram show? ------------------> Explain how the metamorphic grade and assemblages MIGHT change with distance from this dike.