Abstract
We found a harzburgite–dunite–orthopyroxenite suite from the West Jizi block in the northern Oman ophiolite as a record of supra-subduction zone magmatism in the mantle. The orthopyroxenite has features of parallel dikes, each dike being 10–30 cm thick and in direct contact with the host harzburgite. The orthopyroxenite is composed of orthopyroxene (> 95 vol.%), clinopyroxene (< 5 vol.%) and trace spinel (< 1 vol.%), and is free from olivine. All rocks from the suite are characterized by high Cr# [= Cr / (Cr + Al) atomic ratio] of spinel (Cr# ≥ 0.65). The Fo content of olivine is relatively low, 89.9 to 91.1, in harzburgite and dunite. Clinopyroxenes are very low in incompatible elements and show LREE enrichment in orthopyroxenite and host harzburgite. Similar trace element concentrations of clinopyroxenes in the harzburgite and orthopyroxenite suggest their genetic linkage: the harzburgite is a residue of a partial melting, and its extracted melt formed orthopyroxenite dike. The melt in equilibrium with clinopyroxene is similar in trace-element characteristics to boninite. The mineral chemistries indicate that the harzburgite experienced higher degree of partial melting than abyssal peridotite. LREE-enriched melt and/or fluid assisted the high-degree melting. These lines of evidence indicate an involvement of arc magma in the rock suite genesis, i.e. a supra-subduction zone setting experienced by the Oman ophiolite mantle.
Highlights
Evidence of melt-rock reaction between suprasubduction zone (SSZ) peridotites and island arc boninititc and tholeiitic melts are identified
Influence of boninitic melt in peridotites of South Sandwich island arc leads to increasing of TiO2 and Cr-number (Cr#) in spinels [Pearce et al, 2000] e.g. REE patterns of clinopyroxene from Voykar are equilibrium to boninitic melts [Belousov et al, 2009]
Cr# increasing without FeO and Fe2O3 growth is typical of upper-amphibolite facies metamorphism [Muntener et al, 2000]
Summary
Pyroxenite veins within SSZ peridotites – evidence of melt-rock interaction (Egiingol massif), major and trace element composition of minerals. The Egiingol rocks include the serpentinized harzburgite and dunite, which are geochemicaly similar to SSZ peridotites and evidence of interaction with boninite-like melts [Gornova et al, 2008, 2010]. Spinels of harzburgite and pyroxenite correspond (Cr#-Mg# ratio, TiO2 and Fe2O3 content) to the spinel composition of peridotites from SSZ settings and differ from boninitic spinels (Fig. 1). Orthopyroxenes from thin orthopyroxenite veins and host harzburgites have similar composition (Mg#, Al2O3 and Cr2O3 content), they correspond to the field of SSZ peridotites (Fig. 2, A, B). Comparing with Opx from orthopyroxenites, orthopyroxenes of websterite have lower Mg# and wider Mg#, Al2O3 and Cr2O3, they partially fall into the high-Ca boninites field (Fig. 2, A, B).
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