gneiss - teaching student specimens of Grenville age orthogneiss from near St. Jerome, Quebec - Unit of 5 specimens
Orthogneiss is formed by the metamorphism of igneous rocks. This gneiss is from a one to two-billion year old orogenic crustal relic of of mid-late Mesoproterozoic age in the Quebec Laurentians near St. Jerome. The absence of multiple metamorphic features suggests that the age is near the younger limit. Pink orthoclase imparts its color to this gneiss. The foliated black mineral is biotite.
A student should identify this as a gneiss because of the banding, or foliation, that is present. The color is something we do not see in U.S. west coast gneisses, which are generally black and white. An interesting rock type, with an age only seen in very old crust.
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