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Abstract Pyroxenite veins and dikes are commonly observed in the mantle section of ophiolites. Because of their mantle occurrence, these pyroxenites are free from crustal contamination and offer a unique opportunity for studying mantle compositions and melt–rock interaction processes. We conducted an integrated petrological and geochemical study of a suite of composite orthopyroxenite, websterite, and pyroxene-bearing dunite veins from the Xiugugabu ophiolite located on the western segment of Yarlung–Zangbo Suture Zone. The dunite is separated from the host peridotite by a layer of pyroxenite, forming a composite vein system. Systematic variations in major, minor, and trace element compositions in minerals across the composite veins are observed. Two generations of orthopyroxenes in the pyroxenites are characterized by high Mg#, low TiO2 concentrations, and depleted patterns of incompatible trace elements. Clinopyroxenes in the pyroxenites are characterized by high Mg#, low contents of TiO2 and Na2O, spooned shaped REE patterns, and a negative Zr anomaly. Through major and trace element modeling, we showed that both orthopyroxene and clinopyroxene were in equilibrium with melts with different compositions. This hypothesis is further confirmed by distinct initial Nd and Hf isotope ratios in the two pyroxenes. A model for the formation of composite pyroxenite veins is developed, whereby hydrous and silica-rich melts percolate along the margins of a dunite channel. The orthopyroxenite was formed by the reaction between a hydrous, silica-rich melt and the surrounding peridotite. The websterite is formed by reactive crystallization of a hybrid melt produced by mixing silica-rich melt and the melt formed by remelting of previously depleted peridotite in the deeper part of the mantle column. The extremely enriched Nd–Hf isotope compositions of the pyroxenite veins (εNd = −20.3 to +11.5 and εHf = −13.2 to +25.3, 125 million years ago) can be explained by the addition of ancient, recycled sediments to the mantle source in a supra-subduction setting. Based on the low-Cr# spinel in the Xiugugabu dunites (Cr# = 19–50) and the depleted nature of the parental melt of the Xiugugabu pyroxenites, we deduced that the formation of pyroxenites postdate the formation of the Xiugugabu ophiolite at ~125–130 Ma. Collectively, results from this study have provided support to the hypothesis that the Xiugugabu ophiolite experience a two-stage evolution, i.e., firstly formed in a mid-ocean ridge setting and subsequently modified in a supra subduction zone. 

Abstract The architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2 gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-meter IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-meter shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe-Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe-Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.