Quiz:
Geo 406 Lecture 25 –Metabasites & Metapelites
Announcements
Goals for today
Understand facies, zones
Facies & Mineralogy
Blueschist
Above ~0.6 GPa (6 kbar)
Below ~500°C
Mineralogy:
glaucophane
lawsonite
±jadeite
±epidote
±aragonite
Multiple reactions:
Prh  Lws
Act  Glc
Ab  Jd + Qz
{see Keynote)
At what pressure is Jadeite stable?
At what pressure is Jadeite + Quartz stable?
Tectonics of blueschist metamorphism:
Accretionary prisms accumulate by
scraping at the toe
underplating at the base (blueschists must form this way)
Eclogite
High P, High T
Above 1.2 GPa, 500°C
Mineralogy:
Garnet (Pyrope-rich = Mg)
Omphacite (Na-Cpx)
± quartz
± glaucophane
Isograds vs. GIS
Reaction on ground vs. line of constant grade
Metapelites
Protolith [ASK]
Shale
Location: where does this form?
distal part of continental shelf/slope
Mineralogy
clays (e.g., kaolinite)
fine white micas (e.g., sericite)
chlorite
fine-grained quartz
other stuff (fsp, oxides, carbonates, etc.)
Chemistry
high Al2O3, K2O
low CaO
generally KFMASH: K2O, FeO, MgO, Al2O3, SiO2, H2O
Diagrams:
AKF
AFM
Barrovian Facies Series:
Chlorite zone
What facies is this?
Biotite zone
first Bt: Chl + Kf = Bt + Ms
for what rock compositions will this occur?
More typical Bt-in reaction is continuous:
Ms1+Chl1 = Ms2+Chl2+Bio
Mineralogy:
chl+Bt+Ms+Qtz±Ctd
What if you had an unusually Fe-rich protolith?
Would have grt at this point
Garnet zone
What is the reaction?
Cld + Bt = Grt + Chl
Does this make grt in the full range of typical pelites?
No (just like Bt), only for Fe-rich
Staurolite zone
What is the reaction?
Cld + Ky = St + Chl
For what types of compositions does this produce Sta?
Al-rich?
What are all the assemblages you might observe in the sta zone, given the full range
of typical pelite chemistry?
Chl+Bt
Chl + Bt + Grt
Chl
Chl + Cld
Chl + Cld + Sta
Chl + Sta + Ky (!)
Chl + Ky
Next Sta Rxn: loss of Cld [What is that reaction?]
Cld = Sta + Grt + Chl
Puts Sta into Fe-rich high-Al pelites
Third Sta Rxn [What is it?]
Grt + Chl = Sta + Bt
Puts Sta into low-Al pelites
But what happened between second and third?
Continuous reaction added Sta to Fe-poor, high-Al pelites
See T-XMf diagram
Note no kyanite anymore
Kyanite zone
What is the reaction?
St + Chl = Ky + Bt
Puts Ky into Mg-rich pelites
Note that Chl is still stable in some low-Al pelites, but disappears mid-Ky zone
weird that Cld is lost before Chl is!
Sillimanite zone
Reaction?
St = Sil + Grt + Bt
Second Sill Reaction?
Ms + Qz = Sil + Kf