phyllite - teaching hand specimen of a silvery gray Paleozoic phyllite
This phyllite is from a broad band of metamorphic rocks on the west side of the Sierra Nevada in California, which originated as sediments deposited in a series of subduction zones that existed during the Paleozoic along the western edge of North America. During the Late Jurassic Nevadan orogeny, they were subjected to a period of intense metamorphism and were added to the continent.
The Shoo Fly Complex rocks are the oldest of the metamorphic belt. The original sediments were derived from the continent, deposited on the ocean floor to the west, and in the mid-Devonian, were subducted. Subsequent subduction shoved slabs of younger sediments under the Shoo Fly, like a stack of pancakes, though with the oldest on top, a reversal of the normal sedimentary sequence of older at the bottom, youngest on top.
Phyllite is from the Latin for leaf. The sheen is from tiny grains of mica, chlorite or graphite. While slate has a flat cleavage, phyllite tends break in wavy sheets. Some of these specimens contain pyrite cubes, or square holes where pyrite crystals have dissolved.
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