GRANODIORITES

 

Compared with granites, granodiorites have a slightly different chemistry and mineralogy.  They are somewhat enriched in plagioclase feldspar & somewhat depleted in potassium feldspar.

 


 

Barre Granite - this rock sees widespread use in America as a “gray granite”.  The name is pronounced “berry”.  This material comes from the Barre Granite (Barre Pluton) of Vermont.  It's actually a granodiorite, derived from a calc-alkaline pluton formed by partial melting of Silurian-Devonian sedimentary rocks that had been folded & regionally metamorphosed during the Acadian Orogeny in eastern America.  The Barre Granite forms part of the New Hampshire Plutonic Series/Suite and is ~380 million years old (= early Late Devonian or mid- to late Middle Devonian, depending on which time scale one uses).  This rock contains quartz (gray), feldspar (white), and biotite mica (black).  Barre Granite is extensively quarried throughout the Barre Granite Quarry District in the vicinity of the towns of Websterville and Graniteville, south of Barre, in far-southern Washington County, north-central Vermont, USA.

 


 

Mt. Airy Granite - a nice whitish granodiorite from the Mt. Airy Pluton of North Carolina.  It consists of potassium feldspar & plagioclase feldspar (both whitish), quartz (gray), and biotite mica (black).  This sample comes from a large quarry on the eastern side of the town of Mt. Airy, in northeastern Surry County, northwestern North Carolina, USA.

Mt. Airy Pluton rocks range from monzogranites to granodiorites.  The pluton was emplaced during the Middle Mississippian (331 to 337 million years ago).

Location of quarry (Google Earth coordinates): 36° 30.419’ North latitude, 80° 35.375’ West longitude.

 


 

 

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