Geol 2312 Igneous and Metamorphic Petrology

1. Geol 2312 Igneous and Metamorphic Petrology Lecture 24 Metamorphic Facies and Metamorphosed Mafic Rocks April 1, 2009

2. 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

3. 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 CrdKfs (Oslo)

4. 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

5. 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

6. 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.

7. 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).

8. Metamorphic FaciesCurrently Accepted Facies Designations

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

10. 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 BASED ON METAMORPHIC REACTIONS IN PELITIC ROCKS

11. 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.

12. 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).

13. 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.

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

15. Metamorphism of Mafic RocksLow Grade Metamorphism Hydrated protolith is critical to initation of low grade reactions Zeolite amygdules in North Shore Volcanics

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

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

18. Quiz #5 Friday, April 3 Lectures 20-24