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.

 

orthoclase - none to trace

 

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.

 

 

In this detail we see a mix of clear, glassy, grayish quartz, white, translucent Sodium Plagioclase, and amphibole. It is difficult in the picture to separate the quartz from sodium plagioclase, but the quartz mostly fills the tight little spaces among the feldspar and amphibole crystals.