Idaho Batholith

The overall intrusive event that created the Idaho batholith lasted for around 55 million years from Late Cretaceous to the Eocene (98 to 43 Ma) of magmatism and includes the younger Challis suite which is not considered to be part of the Idaho Batholith. The Challis suite intruded both the Atlanta and Bitterroot lobes of the Idaho Batholith as well as the surrounding areas to the east of the Atlanta lobe. The Challis suite is more petrologically diverse and contains a range of rocks from gabbro to granite.

Atlanta lobe

Age

The Atlanta lobe was formed in the Late Cretaceous, 98 to 68 million years ago. The early metaluminous suite (98-87 Ma) makes up the southeastern edge of the Atlanta pluton. The border zone suite (92-85 Ma) makes up the western edge of the Atlanta pluton. The Atlanta peraluminous suite (83-67 Ma) makes up the majority of the Atlanta lobe that stretches farther south than the early metaluminous suite and ends farther north than the early metaluminous suite.

Structures

The Atlanta lobe lacks uniform deformation and foliation across the lobe but contains small areas of localized deformation.

Petrology

The Atlanta lobe is petrologically homogeneous overall. The magma from the Atlanta peraluminous suite (which makes up the majority of the Atlanta Lobe) is made almost entirely of preexisting continental material and contains biotite-containing granodiorite and two-mica granite. However, the early metaluminous suite (which makes up the southern edge of the Atlanta lobe) is petrologically different from the early metaluminous suite and contains tonalite, granodiorite and quartz diorite. The early metaluminous suite is also distinguished from the Atlanta peraluminous suite by the presence of hornblende and 10 cm long potassium feldspar megachrysts. Like the early metaluminous suite, the border zone suite (which makes up the western edge of the Atlanta Lobe) is made of tonalite, granodiorite and quartz diorite.