DETRITAL ZIRCON RECORD OF THE CONFIGURATION AND EVOLUTION OF THE ALASKA RANGE SUTURE ZONE, SOUTH-CENTRAL ALASKA

Collision of a major island arc complex, the Wrangellia composite terrane, with the Mesozoic continental margin of North America marks one of the largest additions of juvenile crust to the Cordillera. New U-Pb detrital zircon ages from Upper Triassic-Upper Cretaceous strata exposed in the northwestern Talkeetna Mountains and Alaska Range define four stages in the development of this suture zone.

Stage 1 is marked by deposition of Upper Paleozoic-Lower Triassic strata. U-Pb detrital zircon ages from the redbeds of the Chulitna terrane show a maximum depositional age of Norian near the top of this unit with prominent age clusters between 300-500 Ma, 1100-1300 Ma, and 1700-2000 Ma (Mz-Pz-Pc - 5%/65%/30%). The oldest grains fall between 2500-3000 Ma. We interpret Stage 1 deposition to represent pre-collision continental margin sedimentation and volcanism adjacent to a continental source during the Upper Paleozoic-Late Triassic time.

Stage 2 is defined by deposition of the lower part of the Upper Jurassic-Lower Cretaceous Kahiltna assemblage. U-Pb detrital zircon ages indicate a maximum depositional ages of Valanginian and show detrital age clusters between 145-165 Ma and 179-199 Ma. The oldest grains in these samples are Mesozoic (212 + 8 Ma). The lower part of the Kahiltna is interpreted as representing deposition associated with exhumation of the Wrangellia island arc (lower Talkeetna arc) during the initial stages of Late Jurassic-Early Cretaceous collision.

Stage 3 is characterized by deposition of the upper part of the Kahiltna assemblage. Maximum depositional ages for these strata are Aptian-Albian. Prominent age clusters occur between 100-140 Ma, 160-240 Ma, and 300-400 Ma with the oldest grains being >2500 Ma (Mz-Pz-Pc - 70%-13%-17%). The upper part of the Kahiltna assemblage is interpreted as syncollisional strata that record erosion of both island arc and contintenal margin sources during Cretaceous time.

Stage 4 is represented by a nonmarine succession of mainly fluvial cross-stratified sandstone, interbedded with fossil leaf-bearing siltstone that overlie the Kahiltna assemblage (locally defined by an angular unconformity). Palynomorphs from this unit indicate a latest Cretaceous to younger age for these strata. Stage 4 is interpreted to represent the latest stages of collision.