Paper No. 2
Presentation Time: 8:40 AM
PETROLOGY AND GEOCHEMISTRY OF CRETACEOUS PLUTONS IN THE CAREYWOOD 7.5' QUADRANGLE, NORTHERN IDAHO
Five plutons from within the Careywood quadrangle in northern Idaho are bound to the west by high-grade metamorphic rocks of the Priest River Complex and to the east by middle Proterozoic rocks of the Belt group. The plutons are notably diverse, comprising the Clagstone tonalite, two-mica monzogranite of Long Mountain, two mica granite with phenocrystic muscovite, the Salee Creek granodiorite, and the granodiorite of Kelso Lake, which has two phases: biotite granodiorite and a more mafic hornblende-biotite granodiorite. Zircon ages on the Clagstone tonalite and the Kelso Lake granodiorite are 90-100 Ma and 88 Ma, respectively (Miller et. al, 1999). Existing Ar dates show a younging trend in the Kelso Lake granodiorite from W-E across the Careywood quadrangle into the Cocallala quadrangle, which could signal tilting or faulting across this zone. Apatite fission track dates within the Careywood quadrangle show ages from ~30-50 Ma and when coupled with zircon data, will constrain uplift rates across this east-west zone in northern Idaho. Field relations demonstrate that the muscovite monzogranite grades into and intrudes the finer- grained two-mica granite. The rocks grade from being metaluminous in the most mafic rocks to strongly peraluminous in the most siliceous rocks. SiO2 contents within the five units range from 57%-75%. The granites have wide ranges of Na2O, K2O, and Al2O3 at constant SiO2 contents, and the variability within the single plutons is nearly as great as that of the entire suite. K2O and Ba contents in the monzogranite of Long Mountain indicate potassium feldspar fractionation, which is supported by the presence of large 2-3cm-long, flow-aligned alkali-feldspar phenocrysts in parts of this unit. Sr has a bimodal distribution in the four plutons. The low-Sr rocks include all samples from the Clagstone tonalite (Kcl) and Salee Creek granodiorite (Kslc), which are the western and easternmost plutons bounding the intrusive complex. Several samples of the hornblende-biotite variety of the Kelso Lake granodiorite are in the low-Sr suite and lie on the west and south margins of the Kelso Lake pluton.