GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 24

Metamorphic Facies and

Metamorphosed Mafic Rocks

April 1, 2009

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 protolithAlso 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 FACIESDEVELOPMENT OF THE CONCEPT

Pentii Eskola (1914, 1915) Orijärvi, S. FinlandRocks 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

Crd Kfs

(Oslo)

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.”

METAMORPHIC FACIESDEVELOPMENT OF THE CONCEPT

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 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 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 Entstehung der Gesteine. Julius Springer. Berlin.

Formation of Zeolites

Temperature

Pre

ssur

e GreenschistFacies

Epidote-Amphibolite

Facies

AmphiboliteFacies

Pyroxene-HornfelsFacies

Glaucophane-Schist Facies Eclogite

Facies

GranuliteFacies

SanadiniteFacies

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 FACIESCURRENTLY ACCEPTED FACIES

DESIGNATIONS

Facies Definitive Mineral Assemblage in Mafic Rocks

Zeolite zeolites: especially laumontite, wairakite, analcime

Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite)

Greenschist chlorite + albite + epidote (or zoisite) + quartz ± actinolite

Amphibolite hornblende + plagioclase (oligoclase-andesine) ± garnet

Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ±

hornblende)

Blueschist glaucophane + lawsonite or epidote (+albite ± chlorite)

Eclogite pyrope garnet + omphacitic pyroxene (± kyanite)

Contact Facies

After Spear (1993)

Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies

Mineral assemblages in mafic rocks of the facies of contact meta-morphism do not differ substantially from that of the corresponding regional facies at higher pressure.

METAMORPHIC FACIESFOUR FACIES GROUPS

High Pressure – Subduction Zones

Medium Pressure -Orogenic Regions

Low Pressure -Contact Auroles

Low Grades – Burial Metamorphism

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

METAMORPHIC FACIESISOGRADS 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 FACIESSERIES

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

Relates progressive metamorphism in a particular tectonic regime

METAMORPHISM OF MAFIC ROCKS

GENERAL CONSIDERATIONS Mineral changes and associations along T-P gradients

characteristic of the three facies seriesHydration 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 ROCKSSubaerial basalts - Minimally hydrated

Mafic graywacke – Strongly hydrated

Submarine basalts - Strong hydrothermal alteration

METAMORPHISM OF MAFIC ROCKS

LOW GRADE METAMORPHISM

Hydrated protolith is critical to initation of low grade reactions

Zeolite amygdules in North Shore Volcanics

Water-bearing Phases

METAMORPHISM OF MAFIC ROCKS

MEDIUM PRESSURE SERIES

Greenschist FaciesEp+Chl+Act+Alb±Qtz

Amphibolite FaciesHb+Pl(>An17)±Gt±Cpx

Granulite FaciesOpx+Pl(>An35)+Cpx ±Gt

Amphibolite-Granulite Facies: Melting possible if sufficient H2O

METAMORPHISM OF MAFIC ROCKS

HIGH PRESSURE SERIES

Eclogite FaciesOmphacite(Na-Cpx)

+ Pyrope-Almandine ((Fe,Mg)Al Garnet)

± Kyanite± Opx

Blueschist FaciesGlaucophane/Jadite (Na)

+ Lawsonite/Epidote (Ca-Al) ± Garnet (Fe-Mg)

(± Aragonite, Paragonite, Chlorite, Stilpnomelane, Qtz, Albite, Sericite)

QUIZ #5

Friday, April 3

Lectures 20-24