LATE CRETACEOUS TO EOCENE PROGRESSION OF ARC AND COLLISIONAL MAGMATISM IN SOUTH-CENTRAL ALASKA

A nearly continuous record of Late Cretaceous-Eocene (ca. 84-37 Ma) igneous rocks in the Alaska Range and northern Talkeetna Mountains cross-cut the Wrangellia composite terrane (WCT) and its suture zone. The WCT accreted to southern Alaska during Mesozoic time, culminating at ~62 Ma. We report geochemical data and 31 zircon U-Pb LA-ICPMS ages for these rocks, when combined with other data, define three main magmatic episodes: Late Cretaceous (66-84 Ma, with a peak at 68-70 Ma), Paleocene to early Eocene (52-64 Ma), and middle Eocene (37-42 Ma). The Paleocene episode includes age peaks at 61-63 and 56-59 Ma. The Late Cretaceous rocks are largely magnesian and metaluminous, have moderate La/Yb, and are calc-alkaline to alkalic. The Paleocene rocks are ferroan and peraluminous, reach the highest La/Yb, Sr/Y, and Ga/Al, and are tholeiitic. The Eocene rocks are largely magnesian, peraluminous to metaluminous, have the lowest range in La/Yb, and are tholeiitic to calc-alkaline. The Late Cretaceous and mid-Eocene magmas formed under relatively oxidized hydrous conditions related to subduction processes. Contamination by over-thickened crust led to the high alkali Late Cretaceous magmas during waning of arc magmatism with culmination of WCT accretion. The Eocene rocks are similar in chemistry to the Aleutian arc and represent re-initiation of subduction beneath southern Alaska after WCT accretion and following outboard Paleocene spreading ridge subduction. The Eocene magmas inherited a relatively depleted chemical composition from mantle emplaced through the preceding slab window and/or because of prior mantle melting. The Paleocene rocks were formed under relatively dry ‘intra-plate’ conditions not typical of arc processes. Their magmas were from a combination of WCT crustal and mantle sources during and closely following WCT collision and coeval with outboard near-trench slab window magmatism. Associated migmatite yields a mean zircon U-Pb age of 160 Ma, confirming involvement of suture zone crustal rocks (Kahiltna assemblage). The presence of 56-58 Ma hypabyssal intrusions in the 61-63 Ma plutons as well as clasts of these plutons in late Paleocene sedimentary and pyroclastic rocks indicates rapid emplacement, uplift, and exhumation of early collisional plutons during the culminating phase of WCT accretion.