Metamorphism of Pelitic Rocks: Barrovian Zones vs Metamorphic Facies vs Bathozones | GEO GIRL

Metamorphosed shales and claystones contain mineral assemblages with various amounts of Na, K, Ca, Mg, Fe, Al, Ti, Si, & water. With increased temperature and pressure passed the point of diagenesis, but before the point of melting, the original clay minerals transform into higher T and P polymorphs (minerals stable at higher temperatures & pressures). For examples, montmorillonite is replaced by chlorite and illite with low grade zeolite metamorphism, and illite is replaced by muscovite in the greenschist facies, slightly higher grade metamorphism. These mineralogical transitions are made possible by the increase in energy provided by heat which overcomes the activation energy of these chemical reactions.
Barrovian zones (temperature and pressure zones where specific minerals appear) include the chlorite, muscovite, biotite, garnet, staurolite, kyanite, & sillimanite zones each defined by the first appearance of that mineral. These metapelitic mineral assemblages can also be categorized by bathozones or pressure ranges. Remember, metamorphic facies, barrovian zones, and bathozones are all different but are all dependent on pressure & temperature and are often used for a similar purpose (to group common mineral assemblages).

References & helpful resources:
Essentials of Igneous Petrology by Ronald and Carol Frost: https://amzn.to/37Zs9AL
Earth Materials by Cornelis Klein and Anthony Philpotts: https://amzn.to/3z5sCxe
Atlas of Metamorphic Minerals under Thin Section: https://www.earth.ox.ac.uk/~oesis/atl...
Thin Section Rotation + Labeled Hand Specimens Website: https://www.virtualmicroscope.org/col... (just pick a collection, pick a rock sample, and it will show you hand sample pictures, thin sections, and tell you the mineral assemblage)
Thin Section Atlas Book (in color): https://amzn.to/3HYsrrj

GEO GIRL Website: https://www.geogirlscience.com/ (visit my website to see all my courses, shop merch, learn more about me, and donate to support the channel if you'd like!)

0:00 Pelitic protoliths & composition
1:48 Metamorphic grade determines minerals present
4:02 Why minerals transform with increasing T & P
5:46 Barrovian metamorphism & zones
6:24 Biotite zone
7:49 Garnet zone
9:00 Staurolite & Kyanite zones
10:06 Sillimanite zone
11:01 Metamorphic facies vs zones
14:00 Mineral assemblages depend on P, T, & chemistry
14:55 Bathozones
16:41 Relating Bathozones to PT diagrams
18:02 Related videos & references

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