WESTERN ALASKA RANGE MAGMATIC RESPONSE TO PROGRESSIVE ACCRETION OF THE WRANGELLIA COMPOSITE TERRANE AND EVOLUTION OF THE SOUTHERN ALASKA MARGIN

Late-Cretaceous to Eocene plutons of the western Alaska Range are investigated to interpret magmatic stages before, during, and after accretion of the Wrangellia Composite Terrane (WCT) to the North American Margin (locally, the Mystic-Farewell terrane, MFT). Local basin closure between (mostly) oceanic WCT and (mostly) continental MFT was finished by ~80 Ma based on minimum detrital zircon ages for Kahiltna basin flysch and 80-76 Ma ages of the oldest post-deformational plutons. The flysch sequence obscures the contact between mostly Mesozoic WCT rocks to the south and mostly Paleozoic rocks of the MFT to the north, but the approximate boundary is interpreted from regional magnetic data. Within the study area, plutons predating and contemporaneous with basin closure (~100-76 Ma) are calcalkaline diorite to granite, and are within the WCT, or proximal to the WCT–MFT basement domain margin.

A magmatic flare-up following basin closure (72-56 Ma) is recorded by intrusive rocks emplaced across both basement domains. New U/Pb ages reveal that the flare-up occurred in two pulses; 72-67 (Late Cretaceous) & 63-56 Ma (Paleocene). The Late Cretaceous suite is less evolved, more oxidized, and contains an adakite-like subset. In the Paleocene, magmas are more evolved, reduced, A-type, and commonly intruded by mafic dikes. Both suites are isotopically bimodal, but the Late Cretaceous is typically enriched, Paleocene dikes are always depleted, and Paleocene pluton isotopes strongly correlate with basement domains. They are always enriched in continental (MFT), relative to oceanic (WCT), basement. The Late Cretaceous stage may represent a peak of crustal addition to magmas following shortening and thickening below the Kahiltna basin, consistent with mineralization patterns and isotopic maturity. The Paleocene stage may represent lower percentage mantle melting or shallower slab dip, yielding laterally diffuse melting of heterogeneous crustal sources. A magmatic hiatus of ~10 m.y. began at ~56 Ma, coincident with spreading ridge subduction beneath the region. Mid-Eocene circum-Pacific tectonic reorganization led to subduction geometry resembling the modern southern Alaska configuration and rejuvenation of arc melting that produced a large composite gabbro to granitic pluton and associated rhyodacite deposits.