AMPHIBOLE IN ULTRA-DEPLETED PERIDOTITES FROM THE HOROKANAI OPHIOLITE, HOKKAIDO, JAPAN: BONINITE MELT TRAPPED IN THE MANTLE WEDGE

Abstract

Ultra-depleted peridotites have been reported from the mantle sections of a number of ophiolitic complexes (e.g., Papua New Guinea and New Caledonia). Such rocks are residues derived from boninitic melts in supra-subduction zones, particularly in forearcs. In the mantle wedge, slab-derived fluid-melt containing H₂O increases the degree of melting and metasomatizes the peridotites. However, the relationship between high-degree melting under hydrous conditions and metasomatism is not well understood. In this paper, we report petrological features of amphibole-bearing ultra-depleted peridotites from the Horokanai ophiolite, Hokkaido, Japan. The peridotite complex in the Horokanai ophiolite consists of clinopyroxene-free harzburgite, orthopyroxene-poor harzburgite, and dunite. The harzburgites contain high-Cr# spinel (0.70-0.85) and orthopyroxene with very low abundances of incompatible elements (e.g.,
Y < 0.01 µg/g; Ti ~ 2 µg/g). Pargasitic-edenitic amphibole occurs as spinel-hosted inclusions and as an interstitial phase to olivine and orthopyroxene. The harzburgite is a residue formed by the high-degree melting caused by fluid flux. The hydrous conditions cause incongruent melting of orthopyroxene, producing a depleted Si-rich melt (i.e., a boninitic melt). Because such Si-rich melt can coexist with Cr-rich spinel under these conditions, amphibole is ultimately
crystallized in the harzburgite. Such amphibole formation is important in producing secondary sodic amphibole during alteration of ultra-depleted peridotites