THE WHALE MOUNTAIN ALLOCHTHON OF THE NORTHEASTERN BROOKS RANGE, ALASKA: AN ARCTIC VESTIGE OF THE IAPETUS OCEAN

The Whale Mountain allochthon is a structural complex composed of lower Paleozoic igneous and marine sedimentary rocks that are exposed within three linear, east–west-trending, fault-bounded belts in the northeastern Brooks Range of Alaska and Yukon. Most of the igneous rocks have basaltic mineralogical and whole-rock geochemical compositions, and occur as massive, pillowed, and brecciated flows. Although field and petrographic features of the volcanic units are similar throughout the allochthon, each belt contains a distinct trace element character. Classification based on concentrations of the incompatible major and trace elements suggests that most samples have a transitional to alkali character, while a distinct suite of samples, confined to the southern belt of the allochthon, have a tholeiitic character. Two volcaniclastic units from within the Whale Mountain allochthon yielded 209 zircon grains that have reliable U-Pb ages. Nearly all 209 grains (98%) fit within an age population that ranges from ~567 to ~474 Ma, with a late Cambrian peak at ~510 Ma. In the southern and central belts of the allochthon, basalt flows are interbedded with discontinuous carbonate units that contain trilobites and agnostoid arthropods that corroborate a late Cambrian age for the associated volcanic rocks. Three distinct trilobite faunas, all almost certainly of late Cambrian (global Furongian Series) age, were recovered from widely separated localities in the present study. The scarcity or absence of uniquely Laurentian genera coupled with the abundance of species that defy assignment to any established Furongian genus argues strongly against models that invoke extrusion of these volcanic rocks onto the autochthonous Laurentian shelf or slope. We favor the interpretation that the rocks of Whale Mountain allochthon represent an ancient oceanic fragment that was scraped from the lower plate of a subduction zone and emplaced onto the Laurentian passive margin during the closure of Iapetus Ocean.