PETROGENESIS OF ULTRAMAFIC ROCKS FROM THE SEVENTYMILE OPHIOLITE, EASTERN ALASKA

We investigate the petrography and bulk-rock and mineral major- and trace-element chemistry from a suite of ultramafic rocks from the Seventymile terrane in eastern Alaska. The sampled rocks from the Seventymile river area range from lherzolite to harzburgite and are thought to represent a portion of the ultramafic component of a dismembered ophiolite suite, obducted onto western ancestral North America in the late Paleozoic to Triassic period. The petrogenesis of the Seventymile terrane is proposed to be correlative with the Slide Mountain terrane in Yukon Territory and British Columbia, Canada.

Porphyroclastic olivine (Fo93-90) are commonly 0.5–3 mm. Clinopyroxene (Mg# 93-97, Cr# 9-23) is typically small (<0.5 mm), with rare large grains (up to 1 mm); websteritic clinopyroxene within the peridotite have lower Cr# (~1-10) and Mg# (~91). Relict orthopyroxene (Mg# 90-92, Cr# 2-15) are mostly bastitized in particularly serpentinized samples, and holly leaf spinels (Mg# 60-70, Cr# 20-40) are common but also exhibit symplectite textures near or along grain boundaries. Seventymile bulk compositions are HREE depleted (0.1-0.5x DMM), like Slide Mountain peridotites. Volcanic samples (plagioclase-phyric basalt, metabasalt) have more variable HREE (up to 37x DMM). Clinopyroxene-normalized REE patterns are diverse, ranging from flat to slightly inclined (average La/Lu ~ 1.1), spoon-shaped REE profiles (LREE enriched to 0.2x DMM), j-shaped patterns with enriched HREE (up to 4x DMM) and strongly to ultra-depleted LREE (0.04x DMM).

The mineral chemistry remains ambiguous with regard to the tectonic affinity of the Seventymile ophiolite (e.g., suprasubduction (SSZ) vs. “abyssal”). Pyroxene major-element data are consistent with an SSZ origin, yet spinels are more consistent with abyssal peridotite. However, the symplectite spinels appear more SSZ-like with aberrant Cr-rich compositions, which may indicate reactions with later melts. The clinopyroxene trace-element data also demonstrate a range of tectonic affinities. However, the concurrence of both abyssal- and SSZ-like affinity is not unusual in other known SSZ-derived ophiolites, as the first melts produced through a subduction initiation sequence are decompression melts with abyssal-like compositions, although back-arc origins cannot be ruled out.