Medium-Pressure Metamorphism of Pelitic Rocks

See http://plate-tectonic.narod.ru/metpetrographylinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html


I. Barrowvian zones

A. History

-George Barrow
-First study of progressive mineralogical changes associated with regional metamorphism

B. Sequence

-Chlorite zone - rocks are slates & phyllites
-Biotite zone - phyllites and schists
-Garnet zone - porphyroblastic schists
-Staurolite zone - "
-Kyanite zone - "
-Sillimanite zone - schists and gneisses
-Sequence has been observed in many other places

C. Interpretation

-Barrow thought zones were the effect of nearby intrusions.
-It''s now recognized that metamorphism and intrusions are both the results of orogeny due to continental collision
-Specific sequence of isograds varies with: pressure; temperature; chemical composition
-Most isograds occur within the stability field of the mineral, thus not absolute limits of index mineral
II. Reactions in the Barrowvian sequence

A. Greenschist facies

-Chlorite :

-At low T, Al-sil is pyrophyllite (Al mica); mica is phengite (Fe,Mg-Ms)
-Mineralogy is Chl + Ms + Q +/- Kfs +/- Prl
-Within Chlorite zone, Prl = Ky + Q + water (This is a discontinuous reaction - runs to completion at 1 T, causing an abrupt change in mineralogy/Classic Barrowvian kyanite isograd occurs at higher grade, but can occur lower in Al-rich pelites )

-Biotite:

-Reaction is Chl + Kfs = Bt + Ms + Q + water (This is a tie-flipping reaction/Usually Chl >> Kfs, so Kfs disappears here/Most pelites lie outside of where this reaction applies).
-Continuous reaction - occurs over a range of T, while compositions change

-Chloritoid: appears only in high-Al pelites ; reaction is Fe-Chl + Ms = Ann + Fe-Cld + Q + water
-Garnet: garnet may form by one of at least 4 reactions; reaction is Cld + Bt + Q + water = Grt + Chl + Ms (This is an unusual reaction - prograde hydration reaction).

B. Amphibolite facies

-Staurolite: reaction is Cld + Ky = St + Chl + Ms + Q (Affects only high-Al pelites)

-Chloritoid breaks down to Grt + Chl + St (Terminal reaction (Cld-out isograd)
-Chl becomes more magnesian
-Most pelites cross St isograd with reaction Grt + Chl + Ms = St + Bt + Q + water (This is a tie-line flipping reaction)
-With increasing T, Chl range shrinks toward M side, and St-Bt-Chl triangle migrates to the right (continuous reaction)

-Kyanite:

-Reaction is St + Chl + Ms + Q = Ky + Bt + water (Discontinuous - tie-line flip/This is the classic kyanite isograd).
- this reaction is near the wet granite melting curve (If pelites contain free water + Q + feldspar, melting occurs, but most water is bound within micas/Dehydration reactions can trigger melting).
-Chl breaks down to Ky + Bt via continuous reaction, and Ky-Bt-Chl subtriangle migrates right.
-Chl disappears.

-Sillimanite:

-Reaction is Ky = Sil
-St breaks down to Grt + Bt + Sil: St-out isograd.
-At slightly higher P, these reactions occur in reverse order

C. Granulite facies
-Still in Sil zone: Ms-out reaction: Ms + Q = Kfs + Sil + water; Ms > Kfs => rocks less schistose, more gneissic
-Cordierite: Reaction is Phl + Sil + Q = Crd + Kfs + water (appearance of Crd) OR Bt + Sil = Grt + Crd + water (tie-line flip); note that both of these reactions occur at higher T
-Orthopyroxene: Bt + Q = Opx + Kfs + water; this occurs only in upper granulite facies, when rocks don''t melt first! Dehydration + high-T may lead to melting