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