Detrital zircon ages from the Chugach terrane, southern Alaska, reveal multiple episodes of accretion and erosion in a subduction complex

Detrital zircon U-Pb ages from the McHugh Complex reveal the Mesozoic history of the Chugach terrane accretionary complex in southern Alaska (United States). The majority of the eight samples of graywacke and argillite from the melange have youngest peaks that are also the dominant peaks on probability distribution diagrams. The youngest groups of zircons are 157 Ma and 146 Ma in two samples and range from 91 to 84 Ma in six samples that also have Jurassic zircons. Minor Proterozoic zircons are present only in samples with Late Cretaceous zircons. A trondhjemite that intrudes the McHugh Complex has a U-Pb zircon age of 120 +/- 3 Ma. We show that the McHugh Complex consists of two packages, a mesomelange of argillite and chert that accreted during the Late Jurassic-Early Cretaceous and a graywacke-conglomerate assemblage that accreted during the Late Cretaceous. We propose that either the Talkeetna arc or Chitina arc in southern Alaska was the principal sedimentary source for the older McHugh Complex, whereas volcanic rocks from the Coast orogen of British Columbia were a probable source for the Late Cretaceous McHugh Complex and the Valdez Group flysch. The age gap between the two packages corresponds to a ridge subduction event recorded by forearc trondhjemite magmatism and regional uplift at 125-120 Ma that likely triggered subduction erosion. Our data suggest that the youngest detrital zircon ages can approximate the time of accretion in subduction complexes because arc detritus is added to the downgoing plate near the trench and is subsequently rapidly accreted.