When mafic igneous rocks, basalt & gabbro, become metamorphosed, their mineral assemblages undergo predictable transitions depending on the pressure & temperature of metamorphism. In the greenschist facies, greenschist forms from basalt/gabbro which contains actinolite, chlorite, and epidote giving the rock a green color and it's name. Non-foliated greenschist is called greenstone. Albite (Na-rich plagioclase feldspar) is also present in greenschist assemblages. Blueschist forms at higher pressure than greenschist and has a blue color due to the transition of actinolite and albite to sodic amphiboles like crossite and glaucophane which have a blue / purple color. Blueschist also contains sodic pyroxene rather than plagioclase, lawsonite in addition to epidote, and garnet due to high pressures. Amphibolite forms at higher temperatures but about the same pressures as blueschists. The higher T converts actinolite & glaucophane into hornblende, which gives amphiboles a green color under plane polarized light in thin section and a black color in hand sample. Plagioclase and at high pressures, garnet, are also present in amphibolites. Granulites form at the highest temperatures of metamorphism and contain orthopyroxenes, clinopyroxenes (augite), olivine, & plagioclase, which is basically the same composition as the basalt or gabbro protolith, so we have to look to the granular (granofels) texture to distinguish granulite from its protolith. Eclogite forms at the highest pressures of metamorphism and contains abundant garnet in a groundmass of epidote, & sodic pyroxene (omphacite).
References & helpful resources:
Essentials of Igneous Petrology by Ronald and Carol Frost: amzn.to/37Zs9AL
Earth Materials by Cornelis Klein and Anthony Philpotts: amzn.to/3z5sCxe
Alex Strekeisen site (info + examples!): www.alexstrekeisen.it/english/... (this is the amphibolite page, but if you go there, you can click any metamorphic mineral or rock on the left side to see more examples!)
Rocks PTX: www.rockptx.com/fkm-26-to-fkm... (this site has many sets of thin sections in both PP & CP and identifies the minerals for you; this link will take you to one set, but if you go to the thin sections scans tab, you can look through all the others as well ;)
Atlas of Metamorphic Minerals under Thin Section: www.earth.ox.ac.uk/~oesis/atl...
Thin Section Rotation + Labeled Hand Specimens Website: 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): amzn.to/3HYsrrj
GEO GIRL Website: 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 Metabasite Protoliths
2:22 Greenschist Facies
7:08 Blueschist Facies
11:07 Amphibolite Facies
15:22 Granulite Facies
20:16 Eclogite Facies
22:17 Zeolite & PP Facies
25:57 Fe, Mg, Ca, Al Content vs Facies Table
27:58 Magical Flow Chart of Metabasite Minerals!
31:08 Related videos & references
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16 июн 2024
