Constraints on the age and provenance of the Chugach accretionary complex from detrital zircons in the Sitka Graywacke near Sitka, Alaska

Abstract

The Sitka Graywacke is the westernmost and youngest unit of the Chugach accretionary complex in southeastern Alaska. Using laser-ablation inductively coupled plasma mass spectroscopy, we obtained 492 detrital-zircon ages on seven typical samples of Sitka Graywacke turbidites, which were collected in a transect across much of the unit near Sitka, Alaska. Individual grains range in age from 66 to 1,802 m.y. The youngest peak ages on relative-probability plots of the western four samples (74, 72, 74, and 74 m.y., from west to east) are distinctly younger than the youngest peak ages of the eastern three samples (105, 103, and 97 m.y., from west to east). These youngest peak ages set maximum depositional ages for each sample. We suggest that these peak ages are not significantly older (within ∼5 m.y.) than the depositional age of the Sitka Graywacke because the deposits accumulated in a trench along a convergent margin, where magmatic sources likely continuously introduced juvenile zircons. The differences in the youngest cluster of detrital-zircon ages between the eastern and western sample localities is likely due to both a change in provenance and a fault. The similarity of the youngest peak ages in the Sitka Graywacke to fossil ages in the Valdez Group, in Prince William Sound, implies that the western part of the Sitka Graywacke is correlative with the Valdez Group, as previously inferred. However, the eastern part of the Sitka Graywacke has youngest detrital-zircon ages older than fossil ages in the Valdez Group and younger than fossil ages in the McHugh Complex, which in southcentral Alaska is the oldest part of the accretionary complex. The age distribution of zircons in the older, eastern sequence suggests sources along the British Columbia margin. The detrital-zircon ages in the younger, western sequence are similar to igneous ages from south-central Alaska to southern British Columbia. Rightlateral strike slip on various fault systems inboard of the Sitka Graywacke implies that it lay to the south when it was deposited and offscraped. Thus, although source areas as far north as the St. Elias Mountains and south-central Alaska are possible, they were most likely in coastal and interior British Columbia. Department of Geosciences, University of Arizonia, Tucson. Introduction Graywacke turbidites are notoriously poor in fossils, and so dating turbidite sequences can be a great challenge. The Sitka Graywacke, which lies along the west coast of southeastern Alaska, is one such turbidite sequence (Berg and Hinckley, 1963; Loney and others, 1963; Decker and others, 1979; Decker, 1980). The unit is part of a Mesozoic through early Tertiary accretionary complex rimming southern Alaska (fig. 1; Plafker and others, 1977) that is commonly referred to as the Chugach terrane (for example, Plafker and others, 1994). Various workers (Nilsen and Moore, 1979; Nilsen and Zuffa, 1982; Plafker and others, 1994) have proposed that these accretionary-complex turbidites were deposited by northward-flowing trench-parallel turbidity currents derived from erosion of the Coast Mountains of southeastern Alaska and British Columbia. Regional correlations between the Sitka Graywacke and other units of the Chugach accretionary complex are hindered by the absence of fossils in the turbidites. Thus far, only two fossils have been identified from the Sitka Graywacke. Reifenstuhl (1986) obtained a gastropod with a nondiagnostic age ranging from Silurian to Eocene. Also, a small limestone pod, with sharp contacts, in graywacke (at the location of our sample 65, fig. 2) contained a poorly preserved cone-shaped nasselarian radiolarian of possible Jurassic age (C.D. Blome, written commun., 1994). Detrital-zircon ages can provide a maximum depositional age because the host sediment must have been laid down after the youngest detrital-zircon grain crystallized. In this study, we use the youngest ages from sandstone of the Sitka Greywacke to constrain depositional ages, subdivide the unit, and interpret the provenance of the detrital-zircon grains on the basis of a comparison with the ages of potential igneous source rocks along the northern Cordilleran margin.