Formation of discordant chromitite at the initiation of sub-arc mantle processes: Observations from the northern oman ophiolite

Abstract

Discordant chromitite, one of the structural types of ophiolitic chromitite, and surrounding peridotites, were examined in the mantle section of the northern Oman ophiolite along Wadi Hilti, to constrain its origin. Spinels from chromitite and adjacent peridotites show high Cr# (= Cr/(Cr + Al) atomic ratio), around 0.65 to 0.75, indicating an arc-related feature. Clinopyroxenes from adjacent harzburgites show low HREE contents, indicating a highly depleted character. The calculated melt composition in equilibrium with clinopyroxene in the dunite envelope indicates that the chromitite and surrounding peridotites were derived from arc-related magma, such as boninite. The arc-related magma was probably generated by the flux melting of depleted mantle harzburgite on the subducted slab, in terms of incompatible element enrichment in clinopyroxene from dunite and harzburgite. The highly depleted harzburgite surrounding discordant dunite and chromitite shows transitional changes in spinel and REE chemistry toward the surrounding oceanic harzburgites. This feature indicates that pre-existing oceanic harzburgite was partially modified by arc-related magma. Discordant chromitite, dunite, and highly depleted harzburgites are probably formed at the supra-subduction zone environment just before ophiolite obduction. Such arc-related features in mantle peridotite may be produced at a young sub-arc mantle. © 2013 Brown University.