Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite

Abstract

The Massif du Sud is a large ophiolitic complex that crops out in the southern region of New Caledonia (SW Pacific). It is dominated by harzburgite tectonite that locally shows a transitional gradation to massive dunite up section. Clinopyroxene, orthopyroxene and plagioclase progressively appear in dunite up to the transition to layered wehrlite and orthopyroxene-gabbro. Highly depleted modal, mineral and whole-rock compositions indicate that harzburgites are residues after high degrees (20-30%) of partial melting mainly in the spinelstability field. Dunite formed in the Moho transition zone by reaction between residual mantle harzburgite and olivinesaturated melts that led to pyroxene dissolution and olivine precipitation. Locally, rare clinopyroxene and plagioclase crystallized in interstitial melt pores of dunite from primitive, low-TiO2, ultra-depleted liquids with a signature transitional between those of island arc tholeiites and boninites. Ascending batches of relatively high-SiO2, ultra-depleted melts percolated through the Moho transition zone and generated wehrlite by olivine dissolution and crystallization of clinopyroxene, orthopyroxene and plagioclase in variable amounts. Similar but distinct magmas ascended to the lower crust where they generated ultra-depleted gabbroic cumulates with subduction-related affinity. Thus, the ultramafic and mafic rocks in the Moho transition zone and lower crust of the Massif du Sud ophiolite are results of percolation and accumulation of different liquids in a multi-stage evolution. Extreme partial melting in the mantle section, and migration and accumulation of ultra-depleted subductionrelated melts in the Moho transition zone and lower crust support that the Massif du Sud ophiolite is a portion of forearc lithosphere generated in an extensional regime during the early phases of the subduction zone evolution.