Early Cretaceous ridge subduction beneath southern Alaska: Insights from zircon U-Pb geochronology, hafnium and oxygen isotopic composition of the Western Chugach Tonalite-Trondhjemite Suite

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 plutons were emplaced along the Border Ranges Fault System, the structural boundary between forearc and arc assemblages in southern Alaska. New U-Pb zircon dates reported here indicate the five main plutons are indistinguishable in age at 123Ma. The restricted timing ( 123 Ma), zircon hafnium (ɛHf (t) > +12 to +20), oxygen (δ¹⁸O = 4.6 to 6.0 ‰) isotope compositions, the whole-rock geochemistry of these plutons strongly support their origin by anatexis of a garnet-free amphibolitic source derived from a mafic rock of MORB composition, unaffected by seawater interactions, and without significant contribution of continental crust. 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 crustal metagabbros that presumably had been underplated by earlier phases of the ridge interaction. The magma thus generated differentiated and was emplaced as near-trench plutons. Our results suggest that these Alaska forearc plutons represent a primary, relatively unmodified magma type that may be diagnostic of magmas generated during ridge subduction. The mantle-like Hf-O isotope compositions of zircons from these forearc tonalite plutons in southern Alaska are comparable to those of the oceanic plagiogranites, suggesting partial melting of similar sources such as depleted mantle and unaltered oceanic crust. These forearc tonalite plutons are systematically depleted in REE compared to the suprasubduction zone ophiolite-related plagiogranites produced by extreme fractional crystallization. However, the REE patterns of these Alaska tonalities exhibit near-complete overlap with those observed in ophiolite plagiogranites produced by partial melting of metamorphosed oceanic crust. Also, Hf-O isotopic similaritites suggest that some Archean TTG rocks may have been produced by partial melting of garnet free amphibolite underplates, possibly in a similar ridge subduction tectonic setting.