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