Unit is named the Iron Granodiorite Gneiss of the Iron Mountains Intrusive Suite (new) in the Iron Mountains in the southeast part of the Sheep Hole Mountains 30' x 60' quad. Is extensively exposed in the Iron Mountains. These rocks were formerly called the porphyritic granodiorite gneiss of Iron Mountains by Miller and Howard (1985) and Howard and others (1989). Is porphyritic, medium-grained biotite granodiorite gneiss. Accessory muscovite, garnet, zircon, and sphene. Alkali-feldspar relict phenocrysts occur as augen porphyroclasts 0.5-1.5 cm long. Alignment of deformed and recrystallized micas, quartz, and feldspar produces distinct mylonitic foliation and lineation. Contains rare zones 1 to 10 cm thick, of black mylonite aligned approximately parallel to foliation of gneiss. Protolith and metamorphic ages are Late Cretaceous.

Source: GNU records (USGS DDS-6; Menlo GNULEX).