Southwestern Laurentian zircons in Upper Cretaceous flysch of the Chugach-Prince William terrane in Alaska

Abstract

The Chugach-Prince William (CPW) terrane in southern Alaska is dominated by thick imbricated flysch, mainly Maastrichtian to Paleocene, that represents one of the thickest accretionary complexes in the world. Detrital zircons from sandstones from across the belt are dominated by grains with crystallization ages close to the age of deposition, and hence the source region supported a long-lived Late Cretaceous to Paleocene volcanic arc. The metaplutonic basement that supported this arc was made of rocks with Mesozoic zircons (Jurassic), with a minor fraction of Paleozoic (Devonian) and Precambrian grains. There is continuity of grain ages across the belt from the Shumagin Islands in the west to the eastern localities, including the Yakutat Group. Positive εHf isotopic ratios in zircons (from the arc and basement) are consistent with melting of a relatively juvenile source, and the leading source candidate is the Coast Plutonic Complex that intrudes the Wrangellia composite terrane.

The small fraction of Precambrian detrital zircons, typically <5 percent, reveal two cohorts with distinct histories. A western cohort of Precambrian grains from Shumagins, Kodiak, and Prince William Sound have grain age populations and isotopic signatures consistent with a northern Laurentian source. These grains have a wide range of ages, but are dominated by populations between 1810 to 1870 Ma and 2520 to 2680 Ma. Collectively these grains have εHf (t) from +13.9 to –21.1, but most grains >1800 Ma have negative εHf (t) values consistent with an evolved source. Raman spectroscopy on zircons from this suite indicates total radiation damage and internal disorder is considerable and comparable to 0.5 to 1.0 Gyr of accumulated damage. Thus the source rocks had to have resided high in the crustal column for most of the Phanerozoic. An eastern cohort of Precambrian grains have grain-ages and an isotopic signature consistent with a southern Laurentian source – mainly Yavapai-Mazatzal, Mojave and the Granite-Rhyolite province of the southwest United States. These zircons are dominated by populations at ∼1380 Ma, ∼1485 Ma, and ∼1722 Ma and εHf (t) values are mostly positive, ranging from +11.7 to –3.4. All zircons between 1436 and 1716 Ma have positive εHf (t) values, which indicate juvenile source rocks, a unique and distinctive aspect of SW Laurentia. Total radiation damage and internal disorder in this suite of detrital grains is minimal and comparable to ∼100 Myr of accumulated damage, which requires source rocks to have been heated to amphibolite grade in the Cretaceous. Thus the low radiation damage may require derivation from in and around the arc, and not farther inland.

We hypothesize that the majority of the flysch of the CPW terrane accumulated in an accretionary complex flanking what is now the Coast Plutonic Complex, but that zircons from the eastern cohort of the CPW were derived from rocks of the Sierran-Southern California-Peninsular Arc, which collapsed along a narrow tectonic corridor due to the subduction of the conjugate to the Shatsky Plateau in the Maastrichtian – the Mojave breach. This point source of southwestern Laurentian rocks allowed sediment with a distinct Precambrian signature to spill out into the forearc and thus part of the Chugach flysch shares a common history with part of the Franciscan Complex. We hypothesize that some of the Chugach flysch was deposited in a trench adjacent to this southwestern Laurentian point source and was subsequently displaced ∼3200 km northward to Alaska.