1. Geol 2312 Igneous and Metamorphic Petrology
Lecture 19
Metamorphic Facies and
Metamorphosed Mafic Rocks
April 4, 2016

2. Metamorphic Facies Development of the Concept
V.M. Goldschmidt (1911, 1912a)
Studied contact metamorphosed pelitic (shales), calcareous (limestones), and psammitic (sandstone) hornfels in the Oslo region. Found relatively simple mineral assemblages (< 6 major minerals) in the inner zones of the aureoles around granitoid intrusives
Determined that equilibrium mineral assemblage related to Xbulk of the protolith
Also noted that certain mineral pairs (e.g. anorthite + hypersthene) were consistently present in rocks of appropriate composition, whereas the compositionally equivalent pair (diopside + andalusite) was not
If two alternative assemblages are compositional equivalents, we must be able to relate them by a reaction
In this case the reaction is simple:
MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5
En An Di Als

3. Metamorphic Facies Development of the Concept
Pentii Eskola (1914, 1915) Orijärvi, S. Finland
Rocks with K-feldspar + cordierite at Oslo contained the compositionally equivalent pair biotite + muscovite at Orijärvi
Deduced from thermodynamic principles that the Finnish rocks were more hydrous and lower volume assemblage and they equilibrated at lower temperatures and higher pressures than the Norwegian ones
2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH) SiO2 (Orihjärvi)
Bt Ms Qtz
= 3 Mg2Al4Si5O KAlSi3O8 + 8 H2O
Crd Kfs
(Oslo)

4. Metamorphic Facies Development of the Concept
Eskola (1915) developed the concept of metamorphic facies:
“In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, the mineral composition is controlled only by the chemical composition. We are led to a general conception which the writer proposes to call metamorphic facies.”
Dual basis for the facies concept
Descriptive - relationship between the Xbulk & mineralogy
If we find a specified assemblage (or better yet, a group of compatible assemblages covering a range of compositions) in the field, then a certain facies may be assigned to the area
Interpretive - the range of temperature and pressure conditions represented by each facies
Eskola was aware of the P-T implications and correctly deduced the relative temperatures and pressures of facies he proposed. We can now assign relatively accurate temperature and pressure limits to individual facies

5. Metamorphic Facies Development of the Concept
Eskola (1920) proposed 5 original facies:
Greenschist
Amphibolite
Hornfels
Sanidinite
Eclogite
Easily defined on the basis of mineral assemblages that develop in mafic rocks, which are abundant in most terranes and mineral changes define a broad range of P & T
In his final account, Eskola (1939) added:
Granulite
Epidote-amphibolite
Glaucophane-schist (now called Blueschist)
... and changed the name of the hornfels facies to the pyroxene hornfels facies

6. Metamorphic Facies Development of the Concept
Winter (2001) Fig The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After Eskola (1939) Die Entstehung der Gesteine. Julius Springer. Berlin.

7. Metamorphic Facies Currently Accepted Facies Designations
Boundaries based on isograds (mineral-in)
Winter (2001) Fig 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).

8. Metamorphic Facies Currently Accepted Facies Designations

9. Metamorphic Facies Four Facies Groups
High Pressure – Subduction Zones
Medium Pressure -
Orogenic Regions
Low Pressure -
Contact Auroles
Low Grades –
Burial Metamorphism

10. Metamorphic Facies Isograds and Zones
An isograd 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.
Because 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
BASED ON METAMORPHIC REACTIONS IN PELITIC ROCKS

11. Metamorphic Facies Facies and Zones
Winter (2001) Fig Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison.

12. Metamorphic Facies Series
Relates progressive metamorphism in a particular tectonic regime
Winter (2001) Fig Temperature-pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994).

13. Metamorphism of Mafic Rocks General Considerations
Mineral changes and associations along T-P gradients characteristic of the three facies series
Hydration of original mafic minerals generally required
If water unavailable, mafic igneous rocks will remain largely unaffected, even as associated sediments are completely re-equilibrated
Coarse-grained intrusives are the least permeable and likely to resist metamorphic changes; Tuffs and graywackes are the most susceptible
Plagioclase: More Ca-rich plagioclases become progressively unstable as T lowered
General correlation between T and maximum An-content of the stable plagioclase
The excess Ca and Al ® calcite, an epidote mineral, sphene, or amphibole, etc., depending on P-T-X
Clinopyroxene – Depending on grade CPX breaks down to a number of mafic minerals - chlorite, actinolite, hornblende, epidote, a metamorphic pyroxene, etc.

14. Metamorphism of Mafic Rocks Hydration of Mafic Rocks
Subaerial basalts -
Minimally hydrated
Mafic graywacke – Strongly hydrated
Submarine basalts -
Strong hydrothermal alteration

15. Metamorphism of Mafic Rocks Low Grade Metamorphism
Water-bearing minerals
Hydrated protolith is critical to initation of low grade reactions
Zeolites
Zeolite amygdules in North Shore Volcanics

16. Metamorphism of Mafic Rocks Medium Pressure Series
Greenschist Facies
Ep+Chl+Act+Alb±Qtz
Water-bearing Phases
Amphibolite Facies
Hb+Pl(>An17)±Gt±Cpx
Amphibolite-Granulite Facies: Melting possible if
sufficient H2O
Granulite Facies
Opx+Pl(>An35)+Cpx ±Gt

17. Metamorphism of Mafic Rocks High Pressure Series
Blueschist Facies
Glaucophane/Jadite (Na)
+ Lawsonite/Epidote (Ca-Al)
± Garnet (Fe-Mg)
(± Aragonite, Paragonite, Chlorite,
Stilpnomelane, Qtz, Albite, Sericite)
Eclogite Facies
Omphacite(Na-Cpx)
+ Pyrope-Almandine
((Fe,Mg)Al Garnet)
± Kyanite
± Opx