Castle Peak, White Cloud Mountains, Idaho

The Idaho Batholith is a

granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz- ...

and granodioritic batholith of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

- Paleogene age that covers approximately of central

Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...

and adjacent

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...

. The batholith has two lobes that are separate from each other geographically and geologically. The Bitterroot lobe is the smaller lobe and the larger lobe is the Atlanta lobe. The Bitterroot lobe is in the north and is separated from the larger Atlanta lobe in the south by the Belt Supergroup

metamorphic rocks Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

that compose the Salmon River Arch. Much of the Atlanta and Bitterroot lobes are in the Idaho Batholith ecoregion. The overall intrusive event that created the Idaho batholith lasted for around 55 million years from

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

to the

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

(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 Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

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 boarder 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 Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defor ...

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 Granodiorite () is a coarse-grained ( phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gr ...

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 Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total ...

and

quartz diorite Quartz diorite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less potassium feldspar. Quartz Quartz is a hard, ...

. 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.

Bitterroot lobe

Age

The Bitterroot lobe was formed in the Late Cretaceous and

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

, 75 to 53 million years ago. The Late metaluminous suite (75-69 Ma) makes up a small portion of the Bitterroot lobe that borders the greater Bitterroot lobe. The Bitterroot lobe is mostly made from the Bitterroot peraluminous suite (66-53 Ma) consisting of the central portion of the Bitterroot lobe.

Structures

The Bitterroot lobe shows regional strain with

foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...

that strikes to the northwest and dips to the northeast, which is parallel with other natural features in the area such as the Lewis and Clark line. The Lewis and Clark line is a area of weakened crust where major

faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

has occurred with steep or vertical dipping and striking to the northwest.Harrison, J.E.; Griggs A.B.; Wells J.D. (1986).
Geologic and Structure Maps of the Wallace 1(degree) x 2 (degree) Quadrangle, Montana and Idaho
. ''U.S. Geological Survey.'' Map I-1509-A: sheet 2 of 2. This deformation is attributed to the western Idaho shear zone.

Petrology

The Bitterroot lobe is petrologically similar to the Atlanta lobe. The Bitterroot peraluminous suite (which makes up the majority of the Bitterroot lobe) mostly consists of biotite

, but with less

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...

-bearing

than the Atlanta lobe. The late metaluminous suite (which borders the Bitterroot lobe) is made of a range of

diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sili ...

from,

References

{{coord missing, Idaho Igneous petrology of Idaho Batholiths of North America Cretaceous magmatism Paleogene magmatism Geology of Idaho

Atlanta lobe

Age

Structures

Petrology

Bitterroot lobe

Age

Structures

Petrology

See also

References