LATEST CRETACEOUS AND PALEOGENE COOLING HISTORY FROM APATITE AND ZIRCON (U-TH)/HE IN THE SYRINGA EMBAYMENT, WESTERN IDAHO

The Syringa embayment refers to the bent geometry of the Mesozoic accreted arc-continent boundary in western Idaho, where it is intermittently exposed below the Columbia River flood basalts. Underlying rocks of the embayment include Precambrian metasedimentary rocks of rift age, Permian-Triassic island arc rocks of the Blue Mountain terranes, and Jurassic to Cretaceous intrusive rocks. Granitoid and quartzite samples were collected where exposed beneath the Columbia River basalts along a transect SW to NE, from the arc terrane igneous rocks to the intrusive rocks within the North American craton. Where possible, 3 single-grain (U-Th)/He analyses per sample were done each for both zircon and apatite. In general, the (U-Th)/He ages in zircon and apatite reveal a latest Cretaceous to Miocene low-temperature cooling history, with both zircon and apatite ages younging towards the craton. The outboard-most intrusive rocks cooled below ~200˚C by 67.4 ± 1.6 Ma and 64.8 ± 3.2 Ma, as recorded by (U-Th)/He zircon ages. The apatite (U-Th)/He ages for these plutonic rocks are 42.7±4.6 Ma and 66.0 ± 2.1 Ma, respectively. Farther contintentward plutonic and metasedimentary rocks yield zircon cooling ages of 44.3 ±3.6 Ma, 45.6 ± 2.2 Ma, and 39.6 ± 2.6 Ma, from SW to NE. Two out of these three samples yielded Oligocene (U-Th)/He apatite ages of 27.9 ±1.3 Ma and 24.9 ± 1.3 Ma.

Cooling on the continental side of the “bend” in the Syringa embayment occurred later than within the western Idaho shear zone (to the S) or Ahsahka shear zone (to the W). Further, this young cooling history is not shared by the outboard plutonic rocks in the Syringa embayment, including those deformed by the right-lateral Mt. Idaho deformation zone, which exhibit a latest Cretaceous to early Paleogene cooling history. The data indicate that deformation and exhumation occurred during the Paleocene-Eocene in the area recognized as the Coolwater Culmination by Lund et al. (2008). We interpret that this area is a syntaxis during latest Cretaceous-Paleogene tectonism, resulting in locally enhanced exhumation and relatively young cooling.