Robertson River Igneous Suite - Cobbler Mountain alkali feldspar quartz syenite

Light- to dark-gray, medium- to coarse-grained, porphyritic (mesoperthite phenocrysts) to seriate-equigranular alkali feldspar-quartz syenite composed of microcline mesoperthite, quartz, and plagioclase, with hastingsitic amphibole, biotite, stilpnomelane, zircon, allanite, fluorite, and rare aegirine-augite. Euhedral to subhedral feldspar phe nocrysts are diagnostic. Syenite locally displays miarolitic cavities contain ing quartz. The rock has been dated at 722± 3 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).
State Virginia
Name Robertson River Igneous Suite - Cobbler Mountain alkali feldspar quartz syenite
Geologic age Proterozoic Z
Lithologic constituents
Major
Igneous > Plutonic > Syenitic > Alkali-feldspar-syenitealkali syenite
Comments Blue Ridge Basement Complex, Late Proterozoic Igneous
References

Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.

Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.

NGMDB product
Counties Fauquier - Loudoun