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| gneiss
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Description
Banded Foliation composed of layers/zones of light colored quartz, feldspar interlayered with layers/zones of dark colored biotite and amphibole. The minerals bands in this specimen are just beginning to develop, but are distinctly separate from each other putting it in the gneissic category.
This specimen is not far removed from a schist seen closeup, but the megascopic banding here puts it in the gneiss category. At the other end of the spectrum the banding is very distinct.
Type of Metamorphism
Slates are part of a metamorphic sequence that begins with shale and progresses through slate, phyllite, schist, and gneiss. At the top of this column you can click through rocks showing the changes. Or go to Barrovian Metamorphic Changes.
Composition
Tectonic Association
gneisses are typically associated with major mountain building events when shales or clay rich sandstones (wackes), or felsic igneous rocks (e.g. granite, granodiorite, etc.) are metamorphosed through depth of burial, and proximity to batholiths (click picture for larger version).
Detail
Description
In this closeup the separation of the dark and light layers is not as clear as it appears in the larger image. Still, clearly the light colored zones are largely uncontaminated with dark minerals, which is characteristic of gneiss.
Another Sample
Description
Banded Foliation where the light- colored quartz and feldspar have segregated into layers from the pods/zones of dark colored amphibole and biotite. The mineral bands in this specimen are just beginning to develop, but are distinctly separate from each other putting it in the gneissic category.
Detail
Description
In this detail the dark and light colored minerals are distinctly separated, and the large garnet masses are also well segretated from the other minerals.
Another Sample
Description
In this gneiss not only have the minerals separated into layers, these layers have been folded. The intensity of the folding shows how plastic the rocks was during de formation.
Another Sample
Description
Banded Foliation composed of layers/zones of light colored quartz, feldspar interlayered with layers/zones of dark colored biotite and amphibole. The minerals bands in this specimen are well developed, although the folding in the rock may reduce some of the clarity of that.
Detail
Description
In this closeup the tight folds can be easily seen. There is a (missing) fold coming in from the top right above the specimen, and then within the rock folding back to the left.
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Thursday, November 11, 2021