ALEUTIAN ADAKITES: MELTS OF SUBDUCTION-ERODED MAFIC ARC CRUST?

Although the 12 Ma adakites from Adak Is., Central Aleutian oceanic island arc, are neatly modeled as melts of subducted Pacific oceanic plate (Kay, 1978), some nagging inconsistencies make this mechanism increasingly less tenable. Among the inconsistencies are: 1) the absence of any pelagic sediment components (radiogenic Pb, high Ba/La, etc.) in the adakites. Further, Pb isotope ratios in a second adakite locality are less radiogenic than Pacific MORB (their presumed source). Geochemically, adakites are distinct from other Aleutian magmas, which are derived from the mantle wedge. 2) the restriction of the Adak adakites to the time of northward arc relocation--where they are the first magmas erupted at what is now the volcanic front. 3) the subducting slab is not "young" and therefore not "hot" enough to melt. An alternative origin to slab melting is suggested by evidence for extensive Aleutian subduction erosion, coupled with a Miocene northward relocation of the volcanic arc. Analogy is made with recent investigations of Andean magmatic activity, relating some Andean adakite occurrences to subduction erosion that causes migration of magmatic fronts. In the case of Adak, the proposed adakite mafic source was not the subducted slab (depth of 100 km), much less the immediately underlying arc crust (which was undoubtedly too thin at 12 Ma to produce the high-pressure adakite geochemical signatures), but was pre-existing oceanic crust in the Aleutian forearc. At the time of arc migration, mafic crust was plucked from the hanging wall (subduction erosion) and transported together with hanging wall peridotite into the mantle wedge where it melted at high pressure in the mantle under the arc. More generally, the role of subduction erosion has been unwisely ignored in convergent margins; it is an important contributor to mass flux, is important in furnishing components to arc magmas, and its intermittent operation is revealed by relocation of volcanic lines.