MULTIPLE EPISODES OF TECTONIC EROSION AND ACCRETION IN THE MCHUGH COMPLEX, CHUGACH TERRANE, SOUTHERN ALASKA, BASED ON DETRITAL ZIRCON AGES

Detrital zircon U-Pb ages from the McHugh Complex of southern Alaska indicate that this Mesozoic accretionary complex is a composite of at least two assemblages that are distinct in age and lithology. In a relatively complete exposure along Turnagain Arm near Anchorage, a mesomelange unit consists of sheared argillite with disrupted chert beds, metavolcanic rocks, and minor greywacke, and forms a distinct lithologic belt adjacent to the Border Ranges fault. The mesomelange unit is juxtaposed against a greywacke-conglomerate unit that forms a broad outcrop belt between the mesomelange and the Eagle River fault, the structural boundary with the younger Valdez Group, but younger faulting cuts out the mesomelange northward and the greywacke-conglomerate lies directly along the Border Ranges fault at Knik River. The greywacke-conglomerate unit consists of (1) massive greywacke; (2) coarse matrix-supported conglomerate with clasts (up to 2 m) of argillite, metavolcanic rocks, plutonic rocks, and limestone in a matrix of conglomerate; and (3) argillite. We dated detrital zircons in 8 samples that have youngest peaks that are also the dominant peaks on probability distribution diagrams. These peaks are 164–151 Ma in two samples from the mesomelange unit and 100–90 Ma in six samples from the greywacke-conglomerate unit that also have the mid-Jurassic grains. Minor Proterozoic zircons are present. A trondjhemite that intrudes the McHugh Complex has a U-Pb zircon age of 120 ± 3 Ma. These data indicate that at least part of the mesomelange is Middle Jurassic but the greywacke-conglomerate assemblage is Upper Cretaceous (Turonian). We propose that the Chitina arc of SE Alaska was the principal sedimentary source for the Jurassic McHugh Complex, but the greywacke-conglomerate assemblage, like the younger Valdez Group, is dominated by Coast Plutonic Complex source. Gaps in McHugh ages (151–105 Ma and 90–80 Ma) are interpreted as the result of subduction erosion. The older period of erosion may have been triggered by a ridge subduction event beginning at 125–115 Ma whereas the younger period at 90–80 Ma may have been triggered by rapid subduction during the Laramide orogeny.