Geochemistry and tectonomagmatic affinity of the Misheguk massif, Brooks Range ophiolite, Alaska

Abstract

Petrologic and geochemical data from the type location of the Misheguk igneous sequence (Misheguk massif), clarify the tectonomagmatic origin of the Brooks Range ophiolite. At Misheguk the igneous sequence consists, in ascending order, of (1) mantle peridotite (2) ultramafic cumulates (3) layered gabbro (4) massive gabbro and high level intrusives and (5) out of sequence intrusives. The various units are mostly transitional except for local intrusive and fault relations. Sheeted dikes and volcanic cover sequences associated with Brooks Range ophiolite have not been found at Misheguk, but are documented at the Siniktanneyak body. Distinctive compositional features found at Misheguk and other bodies throughout the ophiolite include a dunite dominate mantle sequence depleted in Ti, Al and Ca, and enriched in Cr and Ni. This sequence is transitional with cumulates with a crystallization sequence of olivine-clinopyroxene-plagioclase that produces a succession of dunite, wehrlite and olivine gabbro. The cumulate sequence is interpreted to reflect source characteristics of melts that are transitional between MORB- and arc-types. Mineral chemistry comparisons show that average compositions of olivine of Fo 90.7, orthopyroxene with 0.5-2.7% Al 2O 3, and Cr-rich spinel with average Cr ∗ of 0.58 are intermediate between those from MORB- and arc-type mantle peridotites. Coexisting cumulate olivine and plagioclase compositions are similar to those found in dunite-rich arc complexes. Cr Mg abundances in spinel from various mantle peridotites cluster in distinct groups that reflect various degrees of partial melting. Models of partial melting paths based on Cr Ti abundances in mantle peridotite predict the mantle sequence experienced around 20% melt removal from a fertile (Ligurian) mantle source. These geochemical characteristics are similar to those predicted for, and documented in interarc basins.