ENIGMATIC PERMIAN U-PB ZIRCON AGES FROM IGNEOUS CLASTS FOUND IN MID-CRETACEOUS DEPOSITS OF THE COLVILLE FORELAND BASIN, NORTHERN ALASKA

In many deeply eroded orogenic systems, the sedimentary fill of adjacent foreland basins represents the best archive of information bearing on the structural history of a fold and thrust belt. This is true for the Brooks Range, where multiple phases of exhumation and erosion have removed higher structural levels, including obducted material associated with a Jurassic-Early Cretaceous arc-continent collision. In order to improve the understanding of the Cretaceous architecture of the Brooks Range, a multi-component provenance study was initiated on the Early Cretaceous Fortress Mountain Formation, a thick syntectonic clastic wedge in the proximal Colville foreland basin. Detailed conglomerate clast count data document an erosional history that can be related to unroofing of specific structural levels within the orogen, including the Endicott Mountains Allochthon.

Although the source of many clasts can be inferred, the uppermost strata at Castle Mountain include distinctive pink igneous clasts (up to 19% of clast population), the origin of which is unclear. Based on preliminary petrographic examination, equigranular plutonic clasts are tonalites (sub-equal quartz and plagioclase, little or no K-spar); less abundant porphyritic volcanic clasts contain plagioclase phenocrysts and appear rhyodacitic in composition. In order to clarify the nature of these igneous clasts, between 25 and 30 zircons were dated from each of seven igneous clasts using laser ablation ICP-MS. U-Pb ages were produced from five of the distinctive plutonic clasts and two from less common volcanic clasts. The tonalites produced concordant U-Pb ages of 258, 265, 265, 267, and 274 Ma. The intermediate(?) volcanic samples yielded ages of 253 and 254 Ma.

The origin of these Permian clasts is enigmatic; there is presently no recognized igneous activity of this age in the Brooks Range. Igneous rocks preserved in higher allochthons of the western Brooks Range are largely thought to be Jurassic, based primarily on Ar/Ar geochronology. However, it remains possible that additional dating, particularly using zircons where possible, might document a phase of Permian magmatism. Regardless, these clasts represent a detrital record of an igneous-bearing structural level that is now completely eroded in the central Brooks Range.