FOREARC INTRUSIONS IN SOUTHERN ALASKA: EXAMPLE OF LONG-TERM INFLUENCE OF A SLAB WINDOW ON CONVERGENT MARGIN MAGMATISM

Eocene intrusions in the Matanuska forearc basin of southern Alaska are the most geochemically depleted near-trench igneous rocks yet known in western North America and among the most depleted worldwide. The intrusions include gabbro sills and rhyodacite plugs with zircon U-Pb ages ranging from 46.0-39.5 Ma. The gabbros are tholeiitic, primitive (Mg# up to 0.72), have εNd(t) of 11.7 to 9.7, and exhibit rare earth element trends the same as mid-ocean-ridge basalt (MORB). The rhyodacites are also strongly depleted (εNd(t) of 11.1 to 5.9) and include adakite. P-T calculations indicate that primary magmas were formed at ~1300-1350^oC and 30-45 km, consistent with sub-crustal depths that existed before underplating of oceanic lithosphere beneath southern Alaska as defined by published seismic velocity profiles.

The Matanuska forearc intrusions signify emplacement of depleted MORB mantle (DMM) through a slab window that formed by spreading ridge subduction beneath southern Alaska. A slab window first opened beneath southern Alaska during Paleocene to early Eocene ridge-trench encounters, triggering near-trench igneous and related events in and outboard of the Matanuska forearc region. We newly propose that during mid-Eocene time there was renewed asthenospheric upwelling after the subduction of one (or more) additional small ridge segment(s) that were offset from earlier ridge segments. In this model, a narrow portion of the southern Alaska margin experienced repeated ridge subduction with prolonged mantle upwelling and heating, driving upper plate extension and ongoing mafic magmatism between ~60-40 Ma.

We envision this to be a process by which anhydrous depleted mantle is added beneath a convergent margin (i.e., depleted mantle growth). Any subsequent magmatism, including re-established arc systems, would inherit this mantle which may explain the relatively depleted and juvenile nature of the eastern Aleutian arc system compared to pre-slab window arc rocks of southern Alaska. With evidence for ridge subduction and near-trench slab-window magmatism along much of western North America this process has likely been an important control on the composition of past and present Cordilleran arc magmas and highlights the long-term role that slab windows can have on mantle evolution beneath convergent margins.