NEW 40AR/39AR AGE CONSTRAINTS FROM EXTENSIVE DREDGING OF MURRAY CANYON, WESTERN ALEUTIANS

Murray Canyon is a 25 km wide by > 3.5 km deep feature located off the southwestern tip of Kiska Island in the western Aleutians. Previous dredging (WAVE cruise dredge 30) and ⁴⁰Ar/³⁹Ar geochronology yielded some of the oldest rocks in the Aleutian arc (Jicha et al., 2006). A concerted dredging effort was conducted in 2016 at Murray Canyon during cruise SO249 of the German-Russian-American BERING project with a goal of obtaining samples from one of the oldest exposed segments of Aleutian arc crust. Eight closely spaced dredges were carried out along the steep, NW facing wall of Murray Canyon in the depth interval of 3430 to 2978 m. All dredges yielded fresh to moderately altered volcanic and plutonic rocks along with hydrothermally altered sediments. New ⁴⁰Ar/³⁹Ar ages range from 14 to 48 Ma and are coincident with previously determined peaks of Aleutian arc volcanism and plutonism. The older Murray Canyon samples have relatively flat rare-earth Element (REE) patterns similar to Eocene rocks on nearby Kiska Island and on Attu Island to the west. These data suggest that, during the initial stages of arc growth in the western Aleutians, magma originated from a source that was subject to a large degree of melting and was minimally modified by components from the subducting slab. This contrasts with the Late Eocene to Oligocene samples in the Delarof and Adak region to the east and with the younger volcanic rocks in the western Aleutians, both of which have more pronounced subduction signatures. Further geochemical and geochronologic analyses on additional SO249 dredges in the western Aleutians should help clarify the age and composition of the early Aleutian arc.