MANTLE PERIDOTITES IN THE INGALLS OPHIOLITE

The Jurassic Ingalls Ophiolite, Washington Cascades, is composed predominantly of ultramafic rocks, which are unusually diverse, and unlike other parts of the ophiolite, have not received recent attention. The ophiolite is interpreted to have formed in a back-arc basin setting cut by an oceanic fracture zone. Previous work identified three main units: dunite-harzburgite in the south (Unit 1), lherzolite/cpx-rich harzburgite in the north (Unit 2), and intervening mylonitic lherzolite, hornblende peridotite and plagioclase peridotite (Unit 3). Based on new mapping, petrography and geochemical data, we have extended Units 1 and 3 eastward, and identified a cumulate wehrlite unit also in the eastern part of the ophiolite; its relation to the other units is still uncertain.

New trace element data suggests the units are geochemically quite distinct. Unit 1 shows the most extreme REE depletion relative to chondrite, indicating these rocks are residues. Unit 2 rocks have comparable depletions but are marked by slight positive Eu anomalies, which may be indicative of refertilization. Unit 3 is distinctly less depleted in REE compared to Units 1 and 2, and the cumulate wehrlite unit has higher REE concentrations than all mantle tectonites. Cr-spinels in Unit 1 fall above the field for abyssal peridotites, also indicating the Unit 1 harzburgites-dunites are residues. Unit 2 Cr-spinels are within this field, indicating a lower degree of partial melting. Unit 3 Cr-spinels are lowest in the abyssal peridotite field, consistent with their less depleted REE signatures. Spinels for units 2 and 3 encompass a wider range of Mg #, which is consistent with both MORB and suprasubduction zone origin.

New data presented in this study are compatible with the proposal that at least part of the harzburgitic Ingalls mantle was refertilized by an infiltrating basaltic melt. Ongoing study should clarify the relationship of the peridotites to the polygenetic mafic assemblages within the Ingalls and lead to a better understanding of the tectonic setting of the ophiolite.