TERTIARY GRANITE AND GRANDIORITE SUITES OF THE BITTERROOT LOBE OF THE IDAHO BATHOLITH

Two distinct suites of epizonal Tertiary granitoid plutons are present in the Bitterroot lobe of the Idaho batholith. The plutons are widely distributed and have sharp intrusive contacts with rocks of the Cretaceous Idaho batholith. Some examples appear to have been emplaced along faults while others form equant plutons. Many of the most silicic are part of the well-known “pink granite” suite that commonly are resistant to erosion and form prominent tors in the otherwise subdued landscape of the batholith. Newly compiled geochemical data on 540 samples of plutonic and metamorphic rocks of the Bitterroot lobe and field work indicates that a second suite of Eocene granodiorite and granite plutons, some of which had been mapped as part of the Cretaceous biotite granodiorite of the Idaho batholith, are far more common than previously recognized.

The Eocene pink granite suite ranges from very coarse- to fine-grained with prominent miarolitic cavities. Potassium feldspar commonly is bright pink, subhedral to euhedral perthite, while plagioclase is fine grained and interstitial. Biotite with or without hornblende is commonly present at a few percent. Silica contents are above 74%, Rb/Sr is >1.2. Rb is 140-300 ppm, and Sr is <120 ppm. In the Eocene granodiorite to granite suite biotite, with or without hornblende, is the common mafic mineral. The K-feldspar content is variable and the crystals commonly, but not always, are pink. In addition to variable amounts of K-feldspar, the granodiorites are distinguished from the Cretaceous biotite granodiorites by being finer grained and having strongly zoned plagioclase. The Eocene granodiorite to granite suite ranges from 65-74% SiO₂, have Rb/Sr<1.2, Rb<180 ppm, and Sr is 180-550 ppm. In the Cretaceous biotite granodiorites of the Idaho batholith, SiO₂ is 65-76%, but Rb/Sr is <0.2, Sr is >500 ppm, Rb is < 100 ppm, and the rocks have a higher Na₂O/K₂O ratio than the pink granodiorite suite at similar silica contents.

Other common Tertiary plutons in the Bitterroot lobe include biotite-rich granodiorites, unusual high-silica granites which commonly are mingled with dioritic inclusions and dikes, syenites, and quartz syenites.