Download Metamorphism of Shales (Pelitic Rocks)

Geol 285 - Dr. Helen Lang, West Virginia University, Spring 2010
Metamorphism of Shales (Pelitic Rocks)
So far we have talked about metamorphism of:
Mafic Rocks (metabasalts & metagabbros)
Metamorphic Facies
ACF diagram
Note: One of the major effects of metamorphism is dehydration (or
decarbonation)
Next we’ll look at Metapelites, metamorphosed shales
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They were the first metamorphic rocks to be understood in a modern way (by
Barrow in about 1890)
Probably the most studied metamorphic rocks
Why?
They form index minerals that are porphyroblastic and easily identified in the
field (garnet, staurolite, kyanite, etc.), so field mapping of isograds is possible.
They provide more Pressure information than any other common rocks, based
mostly on Al2SiO5 polymorphs.
We’ll need a new Triangle!
We need a diagram that separates FeO and MgO
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Different minerals have different preferences for Fe and Mg
Some minerals of metapelites are high in Fe, some are high in Mg
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We need to be able to show this
Separates minerals like Garnet and Chlorite, which plot on top of eachother on
ACF
Important chemical components in metapelites (from clay and Qtz silt in
shales)
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K2O - FeO - MgO - Al2O3 - SiO2 - H2O
CaO and Na2O are mostly in Plagioclase, assume it’s present
Quartz is always present, so we can delete SiO2
Assume H2O is present (pure or at constant fraction), so we can delete H2O
The four remaining components can be represented on corners of a tetrahedron
(see handout)
J.B. Thompson in 1957 suggested projecting from Muscovite onto the AFM
triangle
Plotting positions of common Metapelite Minerals on AFM diagram (see
handout)
Al2SiO5 Polymorphs occur in metapelites and are very important as Pressure
indicators
Properties of Al2SiO5 Polymorphs
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Kyanite - blue, bladed, different hardness in different directions, one good
cleavage
Andalusite - white to pink; stubby, square prisms; may be Chiastolite with cross
of graphite inclusions
Sillimanite - long, thin prisms or needles; clear and colorless
Metamorphism of pelitic rocks in the Snow Peak area, northern Idaho
Dr. Lang’s Ph.D. project
Location and General Geology
Snow Peak
Best outcrops are on ridge-tops
Kacey Largent Collecting Samples, August 2009
Pseudomorphs are prominent in many samples
Lowest Grade rocks: Chl-Bt zone
(rocks are still pretty shale-like)
First isograd = Garnet isograd
Next isograd = Staurolite isograd
Staurolite is euhedral, twinned and porphyroblastic
Staurolite (+Garnet) in thin section
First Kyanite found along Snow Peak Trail
Kyanite is most abundant in Pseudomorphs
At the next two isograds, Chlorite and Staurolite finally disappear
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Highest grade rocks contain Garnet, Biotite, Kyanite, Muscovite, Quartz,
Plagioclase, Ilmenite and Graphite
Highest grade rocks are in the southwestern-most corner of the area
Metamorphic grade increases toward the nearest exposures of the Idaho batholith
We originally thought that metamorphism was Cretaceous, but it’s Precambrian!
From progression of assemblages and isograds, we can say that metamorphic
conditions in all zones were generally in this region
We analyze minerals on an electron microprobe to estimate T and P
From minerals analyzed with the electron microprobe, I calculated the
following temperatures for each zone (called geothermometry)
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Garnet zone
Staurolite zone
Transition zone
Staurolite-Kyanite
470oC
490oC
530oC
& Kyanite zones
560oC
Pressure estimate for Transition, St-Ky, and Kyanite Zones is 6-6.5 kbars
(called geobarometry)
A little bit about the larger region
A-S-K rocks at Goat Mountain
Andalusite clearly replaces Kyanite crystals, so
Kyanite +Sillimanite formed first, and Andalusite formed later due to a
Pressure decrease