Lecture 2 – Facies, etc.
Facies, etc.
Facies = region of P,T that produces the same set of minerals in
most metamorphosed basalts
Why does this work?
Compare to quartz arenite: no changes with P,T
Compare to pelite: too many changes, and too sensitive to vagaries
of bulk composition.
Metamorphic mineral zone
mostly used in pelites
maps increasing grade (~T), but not comparable between regions
Isograd = line of constant grade (on the ground)
typically mapped based on mineral appearance/disappearance
ideally, via the same discontinuous reaction
Reaction = mechanism by which minerals appear, disappear, or
change composition
Facies series = set of facies found in a region
distinguished by P/T ratio
Very high P/T:
High P/T: Sanbagawa (glc – jd)
Medium P/T: Barrovian (ky – sil)
Low P/T: Buchan / Abukuma (and – sil)
Very Low P/T: Contact
Facies
Eskola: found that the key was the set of minerals (assemblage)
that are linked to PT conditions
Facies = a set of metamorphic mineral assemblages, repeatedly
associated in time and space and showing a regular relationship
between mineral composition and bulk chemical compositions,
such that different metamorphic facies (sets of mineral
assemblages) appear to be related to different metamorphic
conditions, in particular temperature and pressure, although
other variables may also be important.
Facies = region of P,T that produces the same set of minerals in
most metamorphosed basalts
Why does this work?
Compare to quartz arenite: no changes with P,T
Compare to pelite: too many changes, and too sensitive to vagaries
of bulk composition.
Name all facies
Describe parageneses (Qm assemblage of mineral
phases)
Correlate with Pelitic zones (Spear p. 16)
Facies Series (varying P/T ratio)
Facies series = set of facies found in a region
distinguished by P/T ratio
Very high P/T:
High P/T: Franciscan / Sanbagawa (glc – jd)
Subduction-related
Medium P/T: Barrovian (ky – sil)
often accompanied by low-P/T regions (e.g., Barrovian-Buchan)
sometimes with high-P/T region
Low P/T: Buchan / Abukuma (and – sil)
requires shallow heating
regional contact metamorphism
crustal thinning & extension (w/intrusion)
-increased mantle heat flow
Very Low P/T: Contact
Tectonics
Paired metamorphic belts
Miyashiro
high-P/T belt parallel to lower P/T belt
high-P/T = subducted rock
low-P/T = arc volcanism & plutonism
separated by major fault (former plate boundary
implies arc-trench system
Accretionary prisms (e.g., Franciscan)
Older and higher pressure in the east
diachronous-progressive Mtm
vs progressive (=continuous)
160-120 Ma blueschists
100Ma accretion
Late K accretion
early T accretion
Continental collision
medium-P/T due to thickened crust
thrust faulting
If there is intrusion, then shallower crust may have low-P/T
metamorphism as well.
e.g., Acadian in New England