1. Progressive Metamorphism
Reading: Winter, Chapter 21

2. Progressive Metamorphism
Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions
Prograde metamorphism: changes in a rock that accompany increasing metamorphic grade
Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event
Retrograde metamorphism: any accompanying changes

3. Progressive 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
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

4. P-T-t Path
The preserved zonal distribution of metamorphic rocks suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature)
All rocks that we now find must also have cooled to surface conditions
At what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate?
If a metamorphosed sedimentary rock experienced a cycle of increasing metamorphic grade, followed by decreasing grade, at what point on this cyclic P-T-t path did its present mineral assemblage last equilibrate?
The zonal distribution of metamorphic rock types preserved in a geographic sequence of increased metamorphic grade suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature) experienced by that rock during metamorphism

5. Prograde Reactions
Retrograde metamorphism is of only minor significance
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

6. Types of Protolith
The common types of sedimentary and igneous rocks fall 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

7. 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

8. Orogenic Regional Metamorphism of the Scottish Highlands
George Barrow (1893, 1912)
SE Highlands of Scotland
Caledonian orogeny ~ 500 Ma
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

9. Barrow’s Area
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.

10. 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

11. Low Grade Barrovian Zones
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

12. High Grade Barrovian Zones
Staurolite zone. Schists with staurolite, biotite, muscovite, quart, 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)

13. 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

14. 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.

15. Summary
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)

16. Banff and Buchan Districts
The pelitic compositions are similar
But the sequence of isograds is:
chlorite
biotite
cordierite
andalusite
sillimanite

17. 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
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).

18. Regional Burial Metamorphism
Otago, New Zealand example
Jurassic graywackes, tuffs, and volcanics in a deep trough metamorphosed in the Cretaceous
The fine grain size and immature nature of the material is highly susceptible to alteration, even at low grades
Voluminous Permian through Jurassic sedimentation and volcanism  graywackes, tuffs, and some volcanics in a deep trough that was metamorphosed in the Cretaceous
The fine grain size and immature nature of the material makes it highly susceptible to metamorphic alteration, even at low grades
The “type locality” of burial metamorphism

19. Otago, New Zealand Section X-Y shows more detail
Geologic sketch map of the South Island of New Zealand showing the Mesozoic metamorphic rocks east of the older Tasman Belt and the Alpine Fault. The Torlese Group is metamorphosed predominantly in the prehnite-pumpellyite zone, and the Otago Schist in higher grade zones. X-Y is the Haast River Section of Figure From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

20. Otago, New Zealand Isograds mapped at the lower grades: 1) Zeolite
2) Prehnite-Pumpellyite
3) Pumpellyite (-actinolite)
4) Chlorite (-clinozoisite)
5) Biotite
6) Almandine (garnet)
7) Oligoclase (albite at lower grades is replaced by a more calcic plagioclase)
The isograds mapped at the lower grades are listed below and are well represented in the Haast River section (Fig next frame)

21. Regional Burial Metamorphism
Metamorphic zones of the Haast Group (along section X-Y in Figure 21-10). After Cooper and Lovering (1970) Contrib. Mineral. Petrol., 27,

22. Otago, New Zealand
Orogenic belts typically proceed directly from diagenesis to chlorite or biotite zones
The development of low-grade zones in New Zealand may reflect the highly unstable nature of the tuffs and graywackes, and the availability of hot water
Whereas pelitic sediments may not react until higher grades

23. Paired Metamorphic Belts of Japan
The Sanbagawa and Ryoke metamorphic belts of Japan. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic Petrology. Oxford University Press.
Shikoku and Honshu in Japan: a pair of parallel metamorphic belts are exposed along a NE- SW axis parallel to the active subduction zone
These belts are of the same age, suggesting that they developed together

24. Paired Metamorphic Belts of Japan
The NW belt (“inner” belt, inward, or away from the trench) is the Ryoke (or Abukuma) Belt
Low P/T Buchan-type of regional orogenic metamorphism
Dominant meta-pelitic sediments, and isograds up to the sillimanite zone have been mapped
A high-temperature-low-pressure belt, and granitic plutons are common

25. Paired Metamorphic Belts of Japan
Outer belt, called the Sanbagawa Belt
It is of a high-pressure-low-temperature nature
Only reaches the garnet zone in the pelitic rocks
Basic rocks are more common than in the Ryoke belt, however, and in these glaucophane is developed (giving way to hornblende at higher grades)
Rocks are commonly called blueschists

26. Paired Metamorphic Belts of Japan
Two belts are in contact along their whole length across a major fault zone (the Median Line)
Ryoke-Abukuma lithologies are similar to seds derived from a relatively mature volcanic arc
Sanbagawa lithologies more akin to the oceanward accretionary wedge where distal arc-derived sediments and volcanics mix with oceanic crust and marine sediment

27. Paired Metamorphic Belts of Japan
The 600oC isotherm could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath the volcanic arc

28. Miyashiro (1961, 1973) suggested that the occurrence of coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt ought to be a common occurrence in a number of subduction zones, either modern or ancient
Some of the paired metamorphic belts in the circum-Pacific region. From Miyashiro (1994) Metamorphic Petrology. Oxford University Press.
Miyashiro (1961, 1973) noted the paired nature of the Ryoke-Sanbagawa belts, and suggested …
Miyashiro called these paired metamorphic belts
May be separated by km of less metamorphosed and deformed material (“arc-trench gap”) or closely juxtaposed (Ryoke-Sanbagawa)
In the latter cases the contact is commonly a major fault
Most of these belts are quite complex, and are not always coeval

29. Contact Metamorphism of Pelitic Rocks
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

30. 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

31. 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.
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

32. The Skiddaw Aureole, UK
Middle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartz
Cordierite-andalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.
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
1 mm

33. The Skiddaw Aureole, UK Inner zone: Thoroughly recrystallized
Lose foliation
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.
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

34. 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

35. Contact Metamorphism of Pelitic Rocks
Inner aureole at Comrie (a diorite intruded into the Dalradian schists back up north in Scotland), the intrusion was hotter and the rocks were metamorphosed to higher grades than at Skiddaw
Tilley describes coarse-grained non-foliated granofelses containing very high-temperature minerals such as orthopyroxene and K-feldspar that have formed due to the dehydration of biotite and muscovite in the country rocks
Orthopyroxene occurs in pelitic and quartzo-feldspathic rocks only at the very highest grades of contact and regional metamorphism, grades that may not be reached prior to melting in many instances
Typical mineral assemblages = hypersthene + cordierite + orthoclase + biotite + opaques
Some very interesting silica-undersaturated rocks also occur in the inner aureole
Contain such non-silicate high-temperature phases as corundum and Fe-Mg spinel
Tilley noted that the low-silica rocks occur only in the inner aureole, and attributed their origin to loss of SiO2 into the diorite
Better explanation is that SiO2 (and H2O) were scavenged by granitic partial melts formed in the sediments adjacent to the contact with the hot diorite

36. Skarn Formation at Crestmore, CA, USA
Crestmore quarry in the Los Angeles basin
Quartz monzonite porphry of unknown age intrudes Mg- bearing carbonates (either late Paleozoic or Triassic)
Brunham (1959) mapped the following zones and the mineral assemblages in each (listed in order of increasing grade):

37. Forsterite Zone: Monticellite Zone: Vesuvianite Zone: Garnet Zone:
calcite + brucite + clinohumite + spinel
calcite + clinohumite + forsterite + spinel
calcite + forsterite + spinel + clintonite
Monticellite Zone:
calcite + forsterite + monticellite + clintonite
calcite + monticellite + melilite + clintonite
calcite + monticellite + spurrite (or tilleyite) + clintonite
monticellite + spurrite + merwinite + melilite
Vesuvianite Zone:
vesuvianite + monticellite + spurrite + merwinite + melilite
vesuvianite + monticellite + diopside + wollastonite
Garnet Zone:
grossular + diopside + wollastonite
In this progression we can see the sequential development of index minerals, such as clinohumite, followed by forsterite, clintonite, monticellite, melilite, spurrite/tillyite, merwinite, vesuvianite, diopside, wollastonite, and finally grossular garnet

38. Skarn Formation at Crestmore, CA, USA
An idealized cross-section through the aureole
Idealized N-S cross section (not to scale) through the quartz monzonite and the aureole at Crestmore, CA. From Burnham (1959) Geol. Soc. Amer. Bull., 70,
The list of zones is at first quite confusing, and again serves to illustrate a common problem faced by petrologists (and probably all scientists)
We can collect quality data, but can become overwhelmed by the quantity at times, and it is often difficult to recognize meaningful patterns
Two approaches are helpful in this case:

39. Skarn Formation at Crestmore, CA, USA
The mineral associations in successive zones (in all metamorphic terranes) vary by the formation of new minerals as grade increases
This can only occur by a chemical reaction in which some minerals are consumed and others produced
If this is so, one ought to be able to relate minerals in the lower and next higher zone by a balanced chemical reaction

40. Skarn Formation at Crestmore, CA, USA
a) Calcite + brucite + clinohumite + spinel zone to the Calcite + clinohumite + forsterite + spinel sub-zone involves the reaction:
2 Clinohumite + SiO2  9 Forsterite + 2 H2O
b) Formation of the vesuvianite zone involves the reaction:
Monticellite Spurrite Merwinite Melilite
+ 15 SiO H2O  6 Vesuvianite + 2 CO2
For example, the step from the first zone to the second zone is (a)
When we address isograds as reactions we can then turn to what variables are involved
In the present case, we discover than the majority of these prograde reactions consume SiO2
Since quartz is not found in the Crestmore aureole, the SiO2 must be added in the form or dissolved silica in hydrothermal fluids
We can thus conclude that diffusion of silica from the monzonite into the country rocks must play a critical role in the aureole development

41. Skarn Formation at Crestmore, CA, USA
2) Find a way to display data in simple, yet useful ways
If we think of the aureole as a chemical system, we note that most of the minerals consist of the components CaO-MgO-SiO2-CO2-H2O (with minor Al2O3)
Addressing the list of mineral assemblages in the zones at Crestmore can be bewildering
Find a way to display…
What if we graphically plot the minerals on a triangular CaO-MgO-SiO2 diagram?

42. Zones are numbered (from outside inward)
CaO-MgO-SiO2 diagram at a fixed pressure and temperature showing the compositional relationships among the minerals and zones at Crestmore. Numbers correspond to zones listed in the text. After Burnham (1959) Geol. Soc. Amer. Bull., 70, ; and Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.
Zones are numbered (from outside inward)
Silica-saturated water escaping from the porphry permeates the silica-free marbles and a gradient in silica content results due to the diffusion
Silica reacts with the carbonates to produce skarns consisting of Ca-Mg silicates, while CO2 is liberated by the reactions
The porphyritic nature of the pluton thus supports the idea of fluid release
The zones at Crestmore could have formed at constant temperature, and reflect a diffusion gradient in SiO2 only
This is probably not the case, however, since temperature should also increase toward the pluton
Only by knowing the pressure-temperature stability ranges of the minerals, and the pressure- temperature dependence of the reactions relating them, could we fully understand the processes at Crestmore