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) 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 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)2 + 15 SiO2 (Orihjärvi) Bt Ms Qtz = 3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O Crd Kfs (Oslo)

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

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

Metamorphic Facies Development 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.

Metamorphic Facies Currently 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 Designations

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

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

Metamorphic Facies Facies 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 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 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 Subaerial 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

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

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

Quiz #5 Friday, April 3 Lectures 20-24