New studies of forearc tonalite-trondhjemite plutons in southern Alaska provide further support for an Early Cretaceous intra-oceanic ridge subduction along the northern Cordilleran margin. The geological setting together with the timing and geochemistry of the plutons strongly suggest the origin of these plutons can be related to a ridge subduction event in the Early Cretaceous. The plutons were emplaced along the Border Ranges fault system, the structural boundary between forearc and arc assemblages in southern Alaska. The plutons were emplaced late in the ductile deformational history of an amphibolite- to greenschist-facies shear zone that developed in a previously accreted mélange at this arc-forearc boundary. New U-Pb zircon dates reported here indicate the five main plutons are indistinguishable in age at ca. 123 Ma. The zircon hafnium (εHf[t] > +12 to +20) and oxygen (δ18O = 4.6‰–6.0‰) isotope compositions strongly suggest that these plutons were derived from a typical depleted mantle source. These Hf-O isotope compositions coupled with the absence of zircon inheritance indicate a minimal contribution from partial melting of either subducting slab or preexisting evolved Jurassic crust. The restricted timing (ca. 123 Ma), zircon Hf-O isotopic compositions, and the whole-rock major and trace-element geochemistry of these plutons strongly support their origin by anatexis of a garnet-free amphibolitic source derived from a mafic rock of mid-ocean-ridge basalt composition, unaffected by seawater interactions, and without significant contribution of continental crust (sediment) components. Therefore, we suggest that during a restricted time period from 126 to 123 Ma, a “slab window” opened up when a ridge segment was subducted underneath the forearc region. By analogy with younger sites of ridge subduction, we infer that decompressing asthenosphere melted lower-oceanic-crust metagabbros that presumably had been underplated by earlier phases of the ridge interaction. The magma thus generated was differentiated and emplaced as near-trench plutons. Our results suggest that these Alaskan forearc plutons represent a primary, relatively unmodified magma type that may be diagnostic of magmas generated during ridge subduction. Because ridge subduction represents a potential modern analog to Archean subduction, comparison to compositionally similar tonalite-trondhjemite suites and oceanic plagiogranites is informative for general models of granitoid petrogenesis. The mantle-like Hf-O isotope compositions of zircons from these forearc tonalite plutons in southern Alaska are comparable to those of oceanic plagiogranites, suggesting partial melting of similar sources such as depleted mantle and unaltered oceanic crust. These forearc tonalite plutons are systematically depleted in rare earth elements (REEs) compared to the suprasubduction-zone ophiolite-related plagiogranites produced by extreme fractional crystallization. However, the REE patterns of these Alaska tonalites exhibit near-complete overlap with those observed in ophiolite plagiogranites produced by partial melting of metamorphosed oceanic crust. Several Archean tonalite-trondhjemite-granodiorite (TTG) rocks have chondritic Hf and mantle-like oxygen isotopic compositions coupled with a REE pattern similar to that observed in experimental melts produced from garnet-absent amphibolite. These similarities suggest that some Archean TTG rocks may have been produced by partial melting of garnet-free amphibolite underplates, possibly in similar ridge subduction tectonic settings.

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