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)
Geol 2312 Igneous and Metamorphic Petrology
Metamorphic Facies and
Metamorphosed Mafic Rocks
April 1, 2009
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
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
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
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.
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).
High Pressure – Subduction Zones
Medium Pressure -
Low Pressure -
Low Grades –
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
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.
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).
Subaerial basalts -
Mafic graywacke – Strongly hydrated
Submarine basalts -
Strong hydrothermal alteration
Hydrated protolith is critical to initation of low grade reactions
Zeolite amygdules in North Shore Volcanics
Amphibolite-Granulite Facies: Melting possible if
+ Lawsonite/Epidote (Ca-Al)
± Garnet (Fe-Mg)
(± Aragonite, Paragonite, Chlorite,
Stilpnomelane, Qtz, Albite, Sericite)
Friday, April 3