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Another specimen

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Typical Minerals
QUARTZ - > 20%
Amphibole/Biotite - 10-25%
Orthoclase - none to trace
     This is a strange, mixed rock. It has a high quartz content, like a granite, but also a high mafic (amphibole/biotite) content (10-25%) more like a diorite.
Granodiorite is typically intermediate colored with a subequal mixture of light colored sodium plagioclase/quartz, and dark colored amphibole and biotite. Appearance, like diorite, is often described as "salt and pepper" because of the mix. In this specimen some of the quartz grains can be seen as smooth light gray grains scattered among the white feldspars.
     Orthoclase may be present in small amounts.
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Tectonic Association
      Granodiorite, like diorite, is the result of fractional melting of a mafic parent rock above a 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). It emplaces in large batholiths (many thousands of square miles) and sends magma to the surface to produce composite volcanoes with andesite lavas.
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