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Recently, [Garrison and Davidson (2003)][1] questioned the possibility that the adakites of the Northern volcanic zone of the Andes were generated by slab melting. The conclusions of the authors, involving a lower-crustal source for adakites, are in clear contradiction with ideas already proposed

Slab melting has been suggested as a likely source of adakitic arc magmas (i.e., andesitic and dacitic magmas strongly depleted in Y and heavy rare earth elements). Existing numerical and petrologic models, however, restrict partial melting to very young (≤ 5 Ma) oceanic crust (typically at 60–80 km depth). Paradoxically, most of the known Pliocene-Quaternary adakite occurrences are related to subduction of 10–45 Ma lithosphere, which should not be able to melt under normal subduction-zone thermal gradients. We propose an unusual mode of subduction known as flat subduction, occurring in ∼10% of the world's convergent margins, that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust. Of the 10 known flat subduction regions worldwide, eight are linked to present or recent (

Geologic field relationships observed in the axial valley of the southern Juan de Fuca Ridge suggest that high-effusion-rate sheet-flow volcanism has been active for a maximum of about 16,000 yr. Similar observations in the axial region of the East Pacific Rise near lat 21° N suggest that the duration of the pillow-flow-dominant phase may be 5 × 10 4 yr. Geochemical evidence from axial and off-axis basalt samples from both the Juan de Fuca Ridge and East Pacific Rise 21° N indicates that surficial geologic cycles of volcanism, tectonism, and hydrothermal activity are an expression of changes in the locus of replenishment within the magma chamber that take place with a periodicity of 10 4 −10 5+ yr. Anomalously high rates of spreading due to magmatic replenishment and inflation during the first phases of a volcanic cycle may cause the southern Juan de Fuca Ridge to display a morphology more commonly associated with fast-rate spreading centers.