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 Ògray graniteÓ.
The name is pronounced ÒberryÓ. 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¡ 30.419Õ North latitude, 80¡ 35.375Õ West
longitude.