EOCENE FOREARC MAGMATISM IN THE MATANUSKA BASIN, SOUTHERN ALASKA: REVISITING SLAB WINDOW AND ALTERNATE HYPOTHESES

Forearc basins are unique settings for magmatism. Modern and ancient examples of igneous rocks in forearc and accretionary prism settings have been documented worldwide and have been attributed to subduction initiation, slab rollback, and slab windows (as with spreading ridge subduction). The Matanuska forearc basin (MB) in southern Alaska contains mid-Eocene gabbro sills and rhyo-dacite plugs (U-Pb ages of 46-39.5 Ma), but their tectonic origin is unclear. The gabbros are tholeiitic and depleted with εNd_(t) of 11.7 to 9.7 and rare earth element patterns similar to mid-ocean-ridge basalt (MORB). Rhyodacites are also depleted (εNd_(t) of 11.1 to 5.9). Zircons from two gabbro and two rhyolite samples have εHf_(t) of 15 to 23, above the Nd-Hf mantle array. Some gabbros have mineralogical evidence (e.g., pyroxene not in equilibrium with whole rock) consistent with cumulate origin, and in these cases, their ultra-depleted chemistry is a function of mineral-melt fractionation, rather than a characteristic of primary magmas. Other gabbros are chemically similar to coeval basalt dikes and lavas in the MB which more likely represent equilibrium magma composition. These other gabbros imply a depleted MORB mantle and they have less arc-like chemistry (lower U/Yb, Th/Yb, La/Yb) than preceding late Paleocene lavas in the MB (Arkose Ridge) and the modern Aleutian arc. While spreading ridge subduction has been a viable model to explain Paleocene plutons in the accretionary prism in southern Alaska (Sanak-Baranof belt) and exhumation and sedimentation ~60-56 Ma in the forearc, the MB intrusions were emplaced ~10 million years after these events. Relating the MB intrusions to ridge subduction requires either long-term influence of a Paleocene slab window or a more complicated model with an additional mid-Eocene ridge-trench encounter. Alternatively, mid-Eocene MB magmatism might be related to transtension along the margin in response to dextral-oblique subduction at ~46 Ma. In this case, a source for MB magmas might have been previously melted mantle from earlier subduction that was emplaced beneath the forearc by corner flow. These are testable models and highlight the importance of establishing a clear geochronologic and petrogenetic framework, in the context of regional tectonics, for interpretations of forearc magmatism.