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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 • • • • • 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 • • Different minerals have different preferences for Fe and Mg Some minerals of metapelites are high in Fe, some are high in Mg • • 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) • • • • 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 • • • 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 • • • • 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) • • • • 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