ABSTRACT
Retrogression or retrograde metamorphism is a process involving the breakdown of higher P–T assemblages in association with declining P–T conditions. Since the majority of retrograde reactions require hydration or carbonation (Table 7.1), the presence of a fluid phase is essential for these reactions to proceed. Many high-grade metamorphic rocks seen at the Earth’s surface exhibit remarkably fresh assemblages, despite the phases in the assemblage being well outside their stability fields. This indicates either that fluids did not enter the rock to promote the reactions expected with declining P–T conditions, or that uplift rates were far greater than reaction rates. Common retrograde reactions in the main compositional groups of metamorphic rocks.
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Ultramafic rocks |
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Olivine |
→ serpentine → magnesite |
H2O-rich fluids CO2-rich fluids |
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Enstatite Opx and/or olivine |
→ anthophyllite → talc ± serpentine |
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Metabasites |
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Ca-plagioclase |
→ Na-plagioclase + Ep/Zo/Czo → zeolites → sericite/muscovite → calcite → scapolite |
Very common amphibolite facies → greenschist facies retrogression (H20-rich fluids). Common in very low grade burial metamorphism and ocean-floor metamorphism In metabasites this usually requires significant K+ introduction. CO2-rich fluids. Hydrothermal metamorphism CO2-rich fluids. |
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Clinopyroxene Hypersthene Hornblende |
→ hornblende/actinolite → hornblende/actinolite → actinolite → chlorite → biotite |
Usually associated with significant K+ introduction |
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Blue (Na-) amphibole (glaucophane/crossite) Garnet Ilmenite or rutile |
→ green (Ca-) amphibole (actinolite) → chlorite → sphene |
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Granitoid rocks |
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K-feldspar Plagioclase Biotite |
→ sericite/muscovite/pyrophyllite → clay minerals (e.g. kaolinite) → sericite (epidote group minerals) → chlorite |
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Calc-silicate rocks |
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Forsterite Anorthite Diopside Tremolite |
→ serpentine → epidote minerals (± sericite) → carbonate minerals → tremolite–actinolite → talc |
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Metapelites |
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Garnet Staurolite Andalusite, kyanite, sillimanite Cordierite Chloritoid Biotite Ilmenite |
→ chlorite and/or biotite → sericite → sericite + chlorite → sericite/white mica → pinite (fine mix of sericite + chlorite) → chlorite (± sericite) → chlorite → sphene |
