DETRITAL ZIRCON AGES FROM THE CHUGACH TERRANE: A STUDY OF THE McHugh COMPLEX MELANGE AT SELDOVIA, ALASKA

We dated detrital zircons from the Chugach Terrane accretionary complex near Seldovia, Alaska, to investigate the presence and timing of subduction accretion and erosion events. The McHugh Complex mesomelange unit consists of incoherent massive beds of argillite and volcanogenic litharenites (greywacke) interbedded with metavolcanic rocks and chert. We dated 5 samples of mostly volcanogenic litharenites from Seldovia and Tutka Bay. Seldovia is located on the Kenai Peninsula and is ~ 200 km south of Anchorage, and Tutka Bay is ~12 km east of Seldovia. The youngest age population in each sample consists of 10-50 analyses, and the weighted mean of these ages is the maximum depositional age. Four samples from Seldovia yielded youngest age groups of 177 ± 2 Ma, 172 ± 2 Ma, 162 ± 2 Ma, and 146 ± 3 Ma. A sample from Tutka Bay has a youngest age group at 162 ± 2. The samples with 162 Ma and 146 Ma youngest age groups are similar to those from the McHugh mesomelange at Turnagain Arm near Anchorage. Samples with youngest age groups of 177 Ma and 172 Ma are lacking the Late Jurassic/Early Cretaceous populations, suggesting that if the youngest ages approximate accretion age, the Seldovia area exposes an older part of the McHugh Complex that may have been removed by faulting in other areas. Four samples of blueschist from the older, inboard part of the accretionary complex at Seldovia did not yield zircon. The ages of the McHugh Complex zircons are consistent with the Talkeetna arc being a major source for the sediment.