DETRITAL ZIRCON OF THE CHUGACH-PRINCE WILLIAM ACCRETIONARY COMPLEX, ALASKA

The Maastrichtian-Paleocene Chugach-Prince William (CPW) terrane in southern Alaska is dominated by imbricated flysch that represents one of the thickest accretionary complexes in the world. Detrital zircon are dominated by grains with crystallization ages close to the age of deposition, and thus the source included a Late Cretaceous to Paleocene arc. The metaplutonic basement to this arc was made of rocks with Mesozoic zircon (Cretaceous and Jurassic), with a minor fraction of Paleozoic (Devonian) and Precambrian grains. There is remarkable continuity of grain ages across the belt for nearly 2000 km. Positive εHf isotopic ratios in zircon (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 Wrangellia and allied terranes. Precambrian detrital zircon are uncommon in these sediments (<5%) but they reveal two cohorts. A western cohort of Precambrian grains have ages and isotopic signature consistent with a northern Laurentian source. These grains are dominated by populations between 1810-1870 and 2520 to 2680 Ma. Most grains >1800 Ma have negative εHf (t) values consistent with an evolved source. An eastern cohort of Precambrian grains have ages and an isotopic signature consistent with a southern Laurentian source – mainly Yavapai - Mazatzal and the Granite-Rhyolite province of the southwest US. These zircon 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 zircon between 1436 and 1716 Ma have positive εHf (t) values, which indicate juvenile source rocks, a unique and distinctive aspect of SW Laurentia. The majority of the flysch of the CPW terrane accumulated in an accretionary complex flanking what is now the Coast Plutonic Complex, but zircon from the eastern cohort of the CPW were derived from rocks of the Sierran-Southern California-Peninsular Arc, which collapsed in Late Cretaceous time along a narrow tectonic corridor at the Mojave breach. This source of SW Laurentian rocks provided sediment with a distinct Precambrian signature, which is also recognized in the Franciscan Complex. Thus some of the CPW flysch was deposited in a trench adjacent to this point source and was subsequently displaced ~3200 km northward to Alaska.