Geol 2312
1 / 18

Geol 2312 Igneous and Metamorphic Petrology - PowerPoint PPT Presentation

  • Updated On :

Geol 2312 Igneous and Metamorphic Petrology. Lecture 24 Metamorphic Facies and Metamorphosed Mafic Rocks. April 1, 2009. Metamorphic Facies Development of the Concept. V.M. Goldschmidt (1911, 1912a)

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'Geol 2312 Igneous and Metamorphic Petrology' - patrice

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Geol 2312 Igneous and Metamorphic Petrology

Lecture 24

Metamorphic Facies and

Metamorphosed Mafic Rocks

April 1, 2009

Metamorphic facies development of the concept
Metamorphic FaciesDevelopment 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

Metamorphic facies development of the concept1
Metamorphic FaciesDevelopment of the Concept

PentiiEskola (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)2 + 15 SiO2 (Orihjärvi)

Bt Ms Qtz

= 3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O



Metamorphic FaciesDevelopment 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

Metamorphic facies development of the concept2
Metamorphic FaciesDevelopment 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

Metamorphic facies development of the concept3
Metamorphic FaciesDevelopment of the Concept

Winter (2001) Fig. 25-1 The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After Eskola (1939) Die EntstehungderGesteine. Julius Springer. Berlin.

Metamorphic facies currently accepted facies designations
Metamorphic FaciesCurrently Accepted Facies Designations

Boundaries based on isograds (mineral-in)

Winter (2001) 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).

Metamorphic facies currently accepted facies designations1
Metamorphic FaciesCurrently Accepted Facies Designations

Metamorphic facies four facies groups
Metamorphic FaciesFour Facies Groups

High Pressure – Subduction Zones

Medium Pressure -

Orogenic Regions

Low Pressure -

Contact Auroles

Low Grades –

Burial Metamorphism

Metamorphic facies isograds and zones
Metamorphic FaciesIsograds 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


Metamorphic facies facies and zones
Metamorphic FaciesFacies and Zones

Winter (2001) Fig. 25-9.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.

Metamorphic facies series
Metamorphic FaciesSeries

Relates progressive metamorphism in a particular tectonic regime

Winter (2001) Fig. 25-3.Temperature-pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994).

Metamorphism of mafic rocks general considerations
Metamorphism of Mafic RocksGeneral 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.

Metamorphism of mafic rocks hydration of mafic rocks
Metamorphism of Mafic RocksHydration of Mafic Rocks

Subaerial basalts -

Minimally hydrated

Mafic graywacke – Strongly hydrated

Submarine basalts -

Strong hydrothermal alteration

Metamorphism of mafic rocks low grade metamorphism
Metamorphism of Mafic RocksLow Grade Metamorphism

Hydrated protolith is critical to initation of low grade reactions

Zeolite amygdules in North Shore Volcanics

Metamorphism of mafic rocks medium pressure series
Metamorphism of Mafic RocksMedium Pressure Series



Water-bearing Phases

Amphibolite Facies


Amphibolite-Granulite Facies: Melting possible if

sufficient H2O


Opx+Pl(>An35)+Cpx ±Gt

Metamorphism of mafic rocks high pressure series
Metamorphism of Mafic RocksHigh Pressure Series



+ Lawsonite/Epidote (Ca-Al)

± Garnet (Fe-Mg)

(± Aragonite, Paragonite, Chlorite,

Stilpnomelane, Qtz, Albite, Sericite)



+ Pyrope-Almandine

((Fe,Mg)Al Garnet)

± Kyanite

± Opx

Quiz 5
Quiz #5

Friday, April 3

Lectures 20-24