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10 D. Bradley et al.
isotopes and trace elements, Hill et al. (1981) interpreted the
Kodiak, Shumagin, and Sanak granodiorite plutons as the products
of mixing between MORB from a subducted spreading
center and melted ßysch of the overlying accretionary prism.
Kyanite and garnet xenocrysts in several of the plutons (Hill
et al., 1981) imply that the granodiorite magmas originated at
depths of at least 20 km in aluminous metasediments of the
accretionary prism. Barker et al. (1992), working with the most
complete geochemical and isotopic data set, interpreted three
granodiorite plutons near Cordova as the product of partial
melting of arc-derived ßysch at depth in the accretionary prism.
These plutons have the following isotopic compositions: 87 Sr/
86
Sr = 0.7051–0.7067, 206 Pb/ 204 Pb = 19.04–19.20, 207 Pb/ 204 Pb
= 15.60–15.66, 208 Pb/ 204 Pb = 38.59–38.85, and ε Nd
= +2.1 to
–3.3—values that overlap with isotopic compositions from the
Orca Group ßysch and provide evidence that the granodiorites
formed by melting of ßysch. Barker et al. (1992) suggested that
the most likely source of heat, and of mantle melts that formed
the gabbroic plutons, was a ridge-transform complex similar to
the one presently bounding the Explorer plate off the Cascadia
margin. They interpreted dacite dikes in their study area as the
products of partial melting of basalts at depth in the accretionary
prism. Harris et al. (1996), on the basis of trace-element
and Sr- and Nd-isotopic abundances, similarly interpreted
felsic dikes in the Chugach metamorphic complex in terms of
mixing between melted ßysch and a maÞc source associated
with subduction of the Kula-Farallon ridge. Based on trace elements,
Lytwyn et al. (2000) interpreted dikes from the Seldovia
quadrangle as representing a spectrum from I-type basalts and
andesites to S-type dacites and rhyolites, the more silicic com-
UNCORRECTED PAGE PROOFS