1. 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!
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

2. What textures do you see?

3. Review: Types of Protolith
1.Pelitic/mudrocks - high Al, K, Si
2. Quartzo-feldspathic - high Si, Na, K, Al
3. Calcareous- high Ca, Mg, CO2
4. Mafic- high Ca, Mg, Fe
5. Ultramafic- very high Mg, Fe, low Si, 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
WHY IS CHEMICAL COMPOSITION OF PROTOLITH IMPORTANT?

4. 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!
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

5. 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
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 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

6. Barrow’s Area
Figure 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.

7. 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
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

8. 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)

9. 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

10. 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.
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

11. Figure 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.

12. 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
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)

13. 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

14. The stability field of andalusite occurs at pressures less than 0
The stability field of andalusite occurs at pressures less than GPa (~ 10 km), while kyanite  sillimanite at the sillimanite isograd only above this pressure
1 GPa = 10kbars
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
Figure 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).

15. Metamorphic Facies
Eskola (1915) developed the concept of metamorphic facies:
What is a metamorphic facies?
On the basis of the relationship between rock composition and mineral assemblage, and the worldwide occurrence of virtually identical mineral assemblages, Eskola (1915) developed the concept of metamorphic facies:
At the time Grubenmann’s epizone, mesozone, and catazone were very broad and poorly defined and the isograd-zones were restricted to pelitic compositions, and were too narrow for easy correlation from one locality to another
Eskola’s facies were initially based on metamorphosed mafic rocks
Basaltic rocks occur in ~ all orogenic belts, and the mineral changes in them define broader T-P ranges than those in pelites, so facies provided a convenient way to compare metamorphic areas around the world

16. Metamorphic Facies
Fig 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.
The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed
They are thus separated by mineral reaction isograds
The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase
The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.

17. 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
Eskola was aware of the temperature-pressure implications of the concept, and he correctly deduced the relative temperatures and pressures represented by the different facies that he proposed
Advances in experimental techniques and the accumulation of experimental and thermodynamic data have allowed us to assign relatively accurate temperature and pressure limits to individual facies

18. 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
Each was easily defined on the basis of distinctive mineral assemblages that develop in mafic rocks (clearly reflected in the facies names, most of which correspond to common metamorphic mafic rock types)

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

20. Metamorphic Facies
Fig 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.
The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed
They are thus separated by mineral reaction isograds
The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase
The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.

21. Metamorphic Facies defined for mafic protolith
The definitive mineral assemblages that characterize each facies (for mafic rocks).
The definitive mineral assemblages that characterize each facies (for mafic rocks) are listed in Table (details and variations later)

22. 1) Facies of high pressure
It is convenient to consider metamorphic facies in 4 groups:
1) Facies of high pressure
The blueschist and eclogite 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
Eclogites 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
The name is due to the blue-gray color of most mafic blueschists, imparted by sodic amphiboles

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

24. Fig 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.

25. 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
Because intrusions can stall at practically any depth, and orogenic geotherms are quite variable, there is considerable overlap and gradation between the hornfels facies types and the corresponding medium-pressure facies immediately below them on Fig. 25-2

26. 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
Because intrusions can stall at practically any depth, and orogenic geotherms are quite variable, there is considerable overlap and gradation between the hornfels facies types and the corresponding medium-pressure facies immediately below them on Fig. 25-2

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

28. 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
Miyashiro extended the facies concept to encompass broader progressive sequences: facies series

29. Field gradient
Fig 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.
The high P/T series, for example, typically occurs in subduction zones where “normal” isotherms are depressed by the subduction of cool lithosphere faster than it can equilibrate thermally
The facies sequence here is (zeolite facies) - (prehnite-pumpellyite facies) - blueschist facies - eclogite facies.
The medium P/T series is characteristic of common orogenic belts (Barrovian type)
The sequence is (zeolite facies) - (prehnite-pumpellyite facies) - greenschist facies -amphibolite facies - (granulite facies)
Crustal melting under water-saturated conditions occurs in the upper amphibolite facies (the solidus is indicated in Fig. 25-2)
The granulite facies, therefore, occurs only in water-deficient rocks, either dehydrated lower crust, or areas with high XCO2 in the fluid
The low P/T series is characteristic of high-heat-flow orogenic belts (Buchan or Ryoke- Abukuma type), rift areas, or contact metamorphism
The sequence of facies may be a low-pressure version of the medium P/T series described above (but with cordierite and/or andalusite), or the sequence (zeolite facies) - albite-epidote hornfels facies - hornblende hornfels facies - pyroxene hornfels facies
Sanidinite facies rocks are rare, requiring the transport of great heat to shallow levels

30. 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.

31. Regional Metamorphism
3 stages:
Burial/crustal thickening--why does trajectory have steep slope?
Heating stage
Uplift stage

32. Regional Metamorphism
What are prograde vs. retrograde metamorphic paths or reactions?

33. Figure 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.

34. Example of Contact Metamorphism
What does this diagram show? >
Explain how the metamorphic grade and assemblages MIGHT change with distance from this dike.