1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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)

8. 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

9. 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.

10. 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)

11. 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

12. 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).

13. 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

14. 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

15. 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

16. Contact Metamorphism of Pelitic Rocks in 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.
1 mm
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

17. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK
Inner zone:
Thoroughly recrystallized
Lose foliation
1 mm
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
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.

18. 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

19. 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

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