PETROGENESIS OF SUPRA-SUBDUCTION ZONE OPHIOLITES FROM THE SEVENTYMILE ULTRAMAFIC COMPLEX, EASTERN ALASKA

We use ophiolitic rocks from eastern Alaska to provide new insight for geodynamic setting interpretations of the Seventymile ultramafic complex (SUC). The SUC, part of the upper Paleozoic Seventymile Terrane, is thought to represent a dismembered ophiolite suite obducted onto western ancestral North America during the late Paleozoic to Triassic. The petrogenesis of the Seventymile Terrane is unclear, but may be related to oceanic rocks from the Slide Mountain Terrane, Canada. We use new bulk rock and mineral analyses to constrain relative roles of hydrous flux-melting (i.e. suprasubduction – SSZ) versus decompression-melting (i.e., “abyssal”) origins for SUC peridotite samples, which include spinel harzburgite and spinel lherzolite. Relict porphyroclastic olivine is commonly 0.5 – 3 mm. Clinopyroxene is typically small (<0.5 mm); rare larger grains (0.5 – 1 mm) occur along olivine-orthopyroxene grain boundaries. Orthopyroxene (2.5 – 5 mm) is mostly bastitized or otherwise extensively altered. Spinels (or spinel-magnetite pairs) exhibit symplectite textures near or along grain boundaries or holly leaf texture.

Preliminary peridotite bulk rock compositions display HREE 0.1 – 0.8x depleted MORB mantle (DMM) and positive REE slopes. Volcanic samples associated with the ultramafics, including a plagioclase-phyric basalt, in contact with peridotite, and a peridotite-capping finely laminated metabasalt, have more variable HREE (up to 37x DMM). Olivine in peridotite is typically Fo₉₁. Average TiO₂ in orthopyroxene, clinopyroxene, and spinel are 0.018%, 0.048%, and 0.023%, respectively. Orthopyroxene has Mg# 90 – 92 and Cr# 0 – 15. Clinopyroxene has Mg# between 93 – 97 and Cr# between 6 – 23. The major element chemistry of ol-opx-cpx is consistent with an SSZ origin. However, Mg# for most spinels range from 60 – 70 and Cr# from 20 – 40, which is more consistent with abyssal peridotite. The concurrence of both abyssal- and SSZ-like peridotites is not unusual in SSZ-derived ophiolites. However, the petrographic texture and spinel compositional ranges may indicate reactions with later melts. In aggregate, these results point toward an SSZ setting for SUC rocks.