THE FLOOR OF THE IDAHO BATHOLITH: U-PB GEOCHRONOLOGY OF GNEISSIC ROCKS FROM UPPER SELWAY RIVER ALONG THE BOUNDARY OF THE BITTERROOT LOBE

The Idaho batholith is a large granitic mass emplaced in Idaho during the late Cretaceous and early Paleocene. The Bitterroot lobe consists of the northern section of the batholith and covers an ~14,000 km² area, straddling the border with Montana. The upper Selway river cuts through the southern edge of the Bitterroot lobe, exposing a series of poorly understood gneissic and migmatitic units. Wiswall and Hyndman (1987) described this area as the floor of the batholith, as exposed rocks here exhibit qualities suggesting that they formed earlier and deeper than the main mass of the Bitterroot lobe, possibly representing Precambrian basement. Here, we present an analysis of U-Pb ages of zircons and monazites from these gneissic rocks from the Selway River using LA-ICP-MS.

Three samples of gneissic tonalite/quartz diorite and one of migmatite were analyzed. Two of the gneissic samples yielded ²⁰⁶Pb/²³⁸U zircon ages of ~75 Ma, predating the emplacement of the main mass of the Bitterroot lobe (which formed from ~66 to 54 Ma) and in agreeance with Wiswall and Hyndman’s (1987) characterization as the floor of the Bitterroot lobe. The third sample yielded a zircon age of ~51 Ma, indicating that it postdates the batholith completely and is part of the Challis magmatic province. Zircons from a migmatite yielded older ages clustered around 1.6 – 1.7 Ga, which we interpret to be relic detrital zircons from the sedimentary Belt Supergroup protolith of migmatite. Monazite obtained from the migmatite yielded ²⁰⁶Pb/²³⁸U ages of 50 – 55 Ma, with a smaller cluster at 70 – 75 Ma. One possible interpretation is that the older monazite population records an earlier regional metamorphic episode whereas the Eocene population records partial melting from decompression during the exhumation of the Bitterroot metamorphic core complex. No evidence of exposed Precambrian basement was found in the area.