Arc‐like mid‐ocean ridge basalt formed seaward of a trench‐forearc system just prior to ridge subduction: An example from subaccreted ophiolites in southern Alaska

The dismembered Resurrection Peninsula and Knight Island ophiolites, south‐central Alaska, contain pillow basalts interbedded with flysch, indicating that these ophiolites may have formed along an oceanic spreading center proximal to a continental margin and may be relicts of early Tertiary subduction of the Kula‐Farallon ridge beneath North America. New major, minor and trace element data for Resurrection and Knight Island pillow basalts and sheeted dikes support a near‐trench origin and ridge subduction model. The rare earth element abundances are broadly similar to transitional ( T ‐type) mid‐ocean ridge basalt, but extended trace element patterns show high field strength elemental characteristics and alkali enrichments more typical of calc‐alkaline basalts. The lavas and dikes exhibit variable trace element abundances that cannot be related to a common parental magma composition and appear to indicate multiple sources. The multiple parental magmas can be modeled through mixing of compositionally diverse, near‐instantaneous melts derived through near‐fractional (critical) melting of variably depleted mantle source within a subaxial melting column. The unusual calc‐alkaline overprint may reflect variable assimilation of continentally derived, flyschoid sedimentary material from a nearby accretionary prism below which an asthenospheric window opened as the result of ridge subduction. Delivery of the arc‐like assimilant products to the ridge axis in front of the trench may occur by buoyancy driven flow of melt upward and laterally along the base of the accretionary prism. Flow would occur within a channelized zone corresponding to the asthenospheric window (or melting column) that opened during ridge subduction below the accretionary prism. East of Knight Island, metabasic pillow lavas, breccias, and dikes within the Valdez/Orca accretionary complex are compositionally similar to refractory island‐arc tholeiites and are possibly products of parental liquids formed through inefficient mixing of near‐instantaneous melts derived primarily from depleted mantle sources. The Valdez/Orca metabasalts may have been emplaced directly into the accretionary prism forearc environment as refractory melts generated near the top of the asthenospheric window and were possibly associated with the same Kula‐Farallon ridge system that produced the Resurrection and Knight Island ophiolites.