Paper No. 9
Presentation Time: 11:00 AM
DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF THE YUKON-TANANA TERRANE, COAST MOUNTAINS, SOUTHEAST ALASKA
Pericratonal rocks in the western Coast Mountains, southeast Alaska, that have been assigned to the Yukon-Tanana terrane (YTT), include marine clastic strata with mafic to felsic volcanic/plutonic rocks, and are subdivided into three distinct units: 1) Pre-Devonian Tracy Arm Assemblage (TAA), 2) Devonian Endicott Arm Assemblage (EAA), and 3) Mississippian-Pennsylvanian Port Houghton Assemblage (PHA). U-Pb geochronologic and Hf isotopic analyses have been conducted on zircons separated from 26 samples in order to evaluate provenance of the section. TAA samples are dominated by Proterozoic (~.9-1.2, 1.6-2.0 Ga) and Archean (2.5-2.7 Ga) zircons. EAA samples contain either prominent Silurian age peaks (438-424 Ma) with juvenile εHfT values of +15 to +9, or are mainly comprised of Devonian age peaks (403-369 Ma) with variable εHfT values ranging from +13 to -8. PHA samples are dominated by Paleozoic zircons with highly variable εHf values, and consist of either: 1) Silurian peaks (432-425 Ma) with juvenile to intermediate εHf values of +15 to -1, or 2) late Devonian-Mississippian age peaks (379-351 Ma) with Hf mixing arrays displaying intermediate to evolved sources (εHf values of +7 to -20). TAA samples strongly resemble previously published detrital age spectra from the YTT and the northern miogeocline of western North America. Several EAA and PHA samples contain Proterozoic age peaks similar to those of the underlying TAA and the northern miogeocline, indicating a peri-Laurentian affinity for these assemblages as well. Possible connections between the YTT in the Coast Mountains and the Alexander terrane (AXT) may exist, as indicated by the presence of either juvenile Silurian zircons or intermediate to evolved Devonian zircons in EAA and PHA samples. Three options that account for the presence of Silurian-Devonian zircons and their associated εHf values are: 1) YTT and AXT are tectonically interleaved at a scale previously not documented, 2) YTT and AXT both experienced ~360 and ~420 Ma arc magmatism, but formed along different plate boundaries, 3) YTT and AXT evolved in the same 420-360 Ma arc system, with YTT of western Laurentia origin and AXT with primary ties to the Baltica margin.