Ultramafic and Mafic Inclusions, Kanaga Island, Alaska, and the Occurrence of Alkaline Rocks in Island Arcs

Dunite nodules, bearing minor amounts of chromite and clinopyroxene, occur as inclusions in an isolated outcrop of alkaline basalt (limburgite) near the northwest coast of Kanaga Island in the Aleutian arc. Gabbroic inclusions with very calcic plagioclase (up to An91) are found in a nearby outcrop on the Kanaga coast, but their geological relationship, if any, to the dunitic material is unknown. The dunite nodules are not typical lherzolites from an alkali-basalt xenolith suite. Instead, the dunite nodules have mineralogical affinities with dunitic members of Alaskan-type zoned ultra-mafic complexes. In particular, olivine compositions (Fo86) and clinopyroxene major-element (Ca47Mg48Fe5) and minor-element (0.10 wt% TiO2, 1.41 wt% Al2O3) compositions are very similar to those of Alaskan-type dunites. Clinopyroxenes in the host limburgite from Kanaga also fall on the trend of calcic augites and accord with previous arguments for a genetic association of Alaskan-type zoned complexes with alkaline magmas. The present-day tectonic settings of nine occurrences of alkaline rocks in island arcs built across oceanic crust, including Kanaga, are of only two types: (1) along or near lateral edges of subduction zones where hinge faulting is occurring (Bering, Grenada, Hispaniola, Fiji), or (2) where a fracture zone or other linear feature approximately perpendicular to the trench is being subducted (Kanaga, Aoba and Ambrym, Malaita, New Georgia, Iwo-Zima). We suggest that these tectonic settings provide pathways for extraction of alkaline magmas from regions beneath (or beside) subducted lithosphere that are not normally accessible in island arcs.