NEW VIEWS OF OLD ROCKS IN THE BROOKS RANGE (ALASKA): RESULTS OF MAPPING AND DETRITAL ZIRCON GEOCHRONOLOGY OF THE ERNIE LAKE AREA

The Brooks Range (Alaska) formed from Cretaceous arc-continent collision. Although stratigraphic relations are well known in the north half of the range, the ages and primary relationships between individual map units of the two metamorphic belts (Coldfoot and Hammond subterranes) to the south are poorly known. The Ernie Lake area (ELA), commonly included in the Hammond subterrane (HST), is significant because: (1) it exposes the oldest rocks in the Brooks Range (~ 970 Ma Ernie Lake granite and host rocks), (2) it may reveal an unconformity beneath rocks mapped as possible Mississippian Kekiktuk conglomerate. Our study includes U-Pb detrital zircon geochronology tied to new mapping and has two goals: (1) refine palinspastic models for pre-orogenic continental margin shortened during Brooks Range orogenesis, and (2) improve our understanding of the stratigraphic packages and events involved in construction of the Arctic Alaska Chukotka terrane prior to the Mississippian.

U-Pb geochronology samples were collected from rocks tentatively correlated with Mississippian Kekiktuk conglomerate (and contiguous map units) in the ELA and from the equivalent contiguous rocks farther west. Samples dated thus far yield results that include ages as young as ~360 Ma and numerous ages around 400-480 Ma, similar to those of known Kekiktuk conglomerate. Five samples from units below the Kekiktuk(?) in the ELA yielded a range of age peaks between ~1.0 and 1.8 Ga, and few older grains. Youngest ages were ~970 Ma in two samples and greater than ~1000 Ma in the others.

The age gap (~360-970 Ma) in the youngest zircons is consistent with (but not definitive) a major pre-Mississippian unconformity. This would suggest that in post-Late Devonian, pre-orogenic margin reconstructions, the ELA could be a part of the North Slope subterrane, rather than part of the HST. The lack of 500-700 Ma zircons in the sub-Kekiktuk(?) rocks suggests either all of this package is pre-Cryogenian or that no source of that age was exposed. In the latter case, if 500-700 Ma ages are indeed diagnostic of the HST, as suggest by several workers, then the Ernie Lake rocks, may have originated in a different geographic position. We discuss plausible models relating ELA rocks, Ordovician arc rocks of the adjacent Doonerak Window, and the Pearya terrane in a peri-Laurentian position.