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Typical Minerals
QUARTZ > 20%
PLAGIOCLASE is greater than
Orthoclase (here virtually none)
Mafics - < 10% (here < 5%)
MUSCOVITE - trace (brassy looking)
     A felsic igneous rock from the bottom of Bowen's Reaction Series. Typically light colored (few mafic minerals) with sodium plagioclase the most obvious mineral, but quartz >20% is essential. Orthoclase can be present in plagiogranite, but here there is virtually none. Muscovite it not common in most igneous rocks but this specimen has some; it appears brassy and shiny.
      This rock can be confused with a monzonite which appears superficially similar. The amount of quartz is the key: <20% and the rock is "quartz monzonite". Watch the mafics, however; if they exceed 5-10% then the rock is heading toward a diorite or granodiorite.
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Tectonic Association
      Plagio-granite, is the result of fractional melting above a rapid subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains). Plagio-granite is a common component of the cores (cratons) of continents, especially the most ancient portions which were constructed from numerous small volcanic arcs with rapid subductions in the earth's earliest, hottest phases. Another common name for Plagio-granite is tonolite.
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