APTIAN THERMAL PEAK AND THE EXTENSIONAL FRAGMENTATION OF THE BROOKS RANGE, ALASKA: GEOCHRONOLOGICAL CONSTRAINTS FROM METAMORPHIC ZIRCON IN THE SCHIST BELT
New U/Pb “rim” ages (LA-ICP-MS and SIMS) of zircons extracted from metapelites ~200 km apart along strike (Cosmos Hills, Ambler River Quad; John River, Wiseman Quad) constrain zircon recrystallization/metasomatism (and/or possibly new zircon growth) at ~114 Ma. Cretaceous rim ages are associated with high (< 240) U/Th, low Lu/Hf, distinct non-oscillatory CL textures embaying relict igneous textures of detrital cores, and an abundance of mineral inclusions (ab, ap, qtz) at the rim-core contact, consistent with a metamorphic/metasomatic origin. Undisturbed zircon cores from a volcanic-rich lithic sandstone or intermediate/mafic volcanic rock mapped as Ambler sequence by Dillon in the John River display a prominent population peak of 550-750 Ma, a broad distribution from 900-1800 Ma, and a small peak around 2700 Ma, but no Devonian grains (complicating mapped relationships). Epsilon-Hf-isotope values (LA-MC-ICP-MS) were assessed for the Cosmos Hills sample on rims (-10.2 to -13.8 with one at -35.7; n=11), cores (-16.5 to -40.0; n=4) and mixed domains (-10.5 to -18.1; n=7). Metamorphic assemblages and graphite crystallinity suggest temperatures were <450 deg C for the John River samples, and likely <550 C for the Cosmos Hills sample.
Our new metamorphic zircon ages from the Schist belt are consistent with previously proposed timing for extension along an orogen-wide top-to-the-south extensional shear zone (e.g. Little et al 1994). Our age barely postdates the onset of magmatism south of the Brooks Range (e.g. Roeske et al 1998), is nearly synchronous with onset of the major pulse of submarine deposition in the Yukon-Koyukuk Basin (e.g. Patton et al 2009), shortly precedes hornblende and mica cooling ages along the range front (e.g. Vogl et al 2002), and postdates proposed timing of sea floor spreading in the Amerasia Basin by at minimum 10 Myr (e.g. Grantz et al 2011).