Abstract
Eskola's concept of metamorphic facies, now 50 years old, is reappraised in the light of current knowledge and usage among petrologists. Facies should be defined solely in terms of observable geologic criteria. LikeEskola, we continue to view each facies as a set of mineral assemblages that approximate equilibrium within a definite range of temperature; but this is inference and must be excluded from the definition of facies.
Mutual boundaries between facies are transitional. Division into subfacies has proved unacceptable to many writers, and has led to confusion in the physical interpretation of metamorphic parageneses. We propose henceforth not to recognize subfacies.
Eleven facies are recognized in this paper, and their terminology has been adapted as nearly as possible to current general usage:
-
A.
Low-pressure facies commonly but not exclusively of contact metamorphism. In order of increasing temperature:
-
(1)
Albite-epidote-hornfels.
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(2)
Hornblende-hornfels.
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(3)
Pyroxene-hornfels.
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(4)
Sanidinite.
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(1)
-
B.
High-pressure low-temperature facies of regional metamorphism. In order of increasing pressure:
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(5)
Zeolitic.
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(6)
Greenschist.
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(7)
Glaucophane-lawsonite-schist.
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(5)
-
C.
High-pressure medium- to high-temperature facies of regional metamorphism. In order of increasing temperature:
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(8)
Albite-epidote-amphibolite.
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(9)
Amphibolite.
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(10)
Granulite.
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(8)
-
D.
Facies of extreme pressure and wide temperature range:
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(11)
Eclogite.
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(11)
The pressure-temperature regime of a metamorphic terrane can be discussed in terms of the facies there represented, with detailed gradients inferred from sequences of mineral assemblages in basic, pelitic and calcareous rocks. The general simplicity of metamorphic mineral assemblages and their tendency to recur in space and time suggests a simple relation between such theoretically independent pressure variables as load pressureP l fluid pressureP j partial pressure of water,\(P_{{\text{H}}_{\text{2}} {\text{O}}}\). As a first approximation we accept for non-carbonate, hydrous assemblages a model in which\(P_{{\text{H}}_{\text{2}} {\text{O}}}\)≃P f ≃P l during mineral reactions.
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Fyfe, W.S., Turner, F.J. Reappraisal of the metamorphic facies concept. Contr. Mineral. and Petrol. 12, 354–364 (1966). https://doi.org/10.1007/BF00616820
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DOI: https://doi.org/10.1007/BF00616820