Diversity in metamorphic facies Lecture 6

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. 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.

Figure 21-14. 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

Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Middle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartz Figure 21-15. 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

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 Figure 21-16. 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.

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.

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. 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

Paired Metamorphic Belts of Japan Figure 21-12. 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

Lawsonite- glaucophane schist, minor sphene Lawsonite- glaucophane schist, minor sphene. (X10 ppl) picture from Kate Brodie

Blueschist, Corsica, from L. Jolivet

Blueschist, Aegean islands, from L. Jolivet

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. 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

Coesite in garnet, Alps, from L. Jolivet

Coesite in garnet, Alps, from L. Jolivet