HF-ND-SR ISOTOPE SYSTEMATICS OF ALASKAN OPHIOLITE COMPLEXES: IMPLICATIONS FOR THE ORIGIN OF MARGINAL OCEAN BASINS IN EAST CENTRAL ALASKA

Alaska reflects a complex history of accretion of disparate allochthonous terranes. Mafic-ultramafic complexes thought to represent variably complete sections of oceanic lithosphere are presumed to define the paleo boundaries and/or demarcate margins of the accreted allochthonous assemblages in Alaska. We present major, trace-element and Sr-Nd-Hf radiogenic isotopic data of gabbro, diabase and pyroxenite samples from four Paleozoic to Jurassic mafic-ultramafic complexes in interior Alaska: Livengood (ca. 550 Ma), Seventymile (ca. 240 Ma), Kanuti (≤ 180 Ma) and Rampart (ca. 250 Ma). The purpose is to provide insights into their magmatic origin and implications for the tectonic evolution of east-central Alaska.

The Livengood and Seventymile samples have ε_Nd(i) and ε_Hf(i) values that generally overlap Pacific MORB (ε_Nd(i)~ 9-11; ε_Hf(i) ~11-15) . Samples from both assemblages show smooth, LREE depleted patterns and lack HFSE depletions, similar to N-MORBs. The Kanuti complex samples have more variable ε_Nd(i) and ε_Hf(i) that extend to values lower than Pacific MORB. The LREE/HREE ratios correlate with ε_Nd(i) and ε_Hf(i), indicative of a mixed magma source. They also lack the distinct Nb-Ta depletions, or Pb enrichments, characteristic of arc magmas. The Rampart complex has ε_Nd(i) and ε_Hf(i) values of ~4-5, and 9-11 respectively, with E-MORB-like LREE enrichments and slight Nb-Ta depletions. The pyroxenites are isotopically more depleted than the diabases, suggesting different magma sources.

We interpret the data to suggest that the Seventymile samples represent a marginal basin between ancestral North America and the Yukon-Tanana allochthon with minimal subduction influence. The older Livengood, with a similar mantle source as the Seventymile, may represent earlier obducted oceanic lithosphere now found within the allochthonous assemblage. Assuming Kanuti is part of the Angayucham Terrane of Arctic Alaska, its MORB-EMORB composition suggests an origin as a marginal basin that formed as a consequence of southward subduction at the Koyukuk terrane margin. Rampart still has an ambiguous origin, but provisional data are generally more elementally and isotopically enriched than for the other three mafic complexes, with more of an arc signature.