Origins of orogenic dunites: Petrology, geochemistry, and implications

Abstract

Orogenic dunites are common in high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belts. This paper reviews the origins of orogenic dunites from over 40 localities around the world and identifies three major types: residual dunite, replacive dunite and cumulative dunite. These orogenic dunites, which were generated from different formation processes, are expected to have diverse compositional characteristics and geodynamic histories. Residual dunites are the products of high degrees of partial melting (25–60%) of the primitive mantle and exhibit high Mg# values (commonly >92) and low FeOT values (<8wt.%) in bulk-rock compositions, which are similar to ancient cratonic peridotites. These dunites contain olivines with high Fo values [atomic 100∗Mg/(Mg+Fe)] (mostly 92.0–93.2) and spinels with high Cr# values [atomic 100∗Cr/(Cr+Al)] (50–90) and low TiO2 concentrations (<0.2wt.%). The Re–Os isotopic data indicate that these dunites were mainly derived from the Archean–Paleoproterozoic subcontinental lithospheric mantle (SCLM). In contrast, replacive dunites were formed from the reaction of ambient pyroxene-rich peridotite with silica-undersaturated melt. The typical texture of the melt–rock reaction results from the dissolution of orthopyroxene associated with the precipitation of olivine neoblasts. Compared with the residual dunites, replacive dunites generally have lower SiO2 and higher FeOT concentrations (>8wt.%) for a given MgO concentration in the bulk compositions. These patterns obviously deviate from the partial melting curves. Olivines from replacive dunites show relatively low Fo values (mostly 90.4–92.4) with variable NiO contents (0.18–0.53wt.%), and spinels yield a positive correlation between the Cr# (<65) and TiO2 contents (up to 0.6wt.%). The osmium isotopic compositions of the replacive dunites, ranging from subchondritic to suprachondritic, are higher than those of the residual dunites and yield young or future rhenium-depletion ages (TRD). Additionally, the replacive dunites display variable platinum group element (PGE) abundances ranging from slight enrichment to extreme depletion relative to the ambient pyroxene-rich peridotites. The cumulative dunites were formed via crystal cumulation from mantle magmas and only occupy a small proportion of the orogenic dunites. These dunites generally exhibit a large range in bulk compositions that overlap the values of residual and/or replacive dunites due to the diverse compositions of the original magma, various crystallization environments, and late-stage crustal metasomatism. Nevertheless, the olivines in the cumulative dunites have the lowest Fo values (mostly 88.5–90.9) among the orogenic dunites and feature a rapid decrease in the NiO content with decreasing Fo values. Moreover, the spinels have remarkably lower Mg# values and higher TiO2 contents than those from residual and replacive dunites, making them a useful mineral for distinguishing cumulative dunite from the other two types.Compared with garnet lherzolite, harzburgite and pyroxenite, which were subjected to strong modification by crust-derived fluids/melts in the subduction channel, orogenic dunite experienced the weakest metasomatism and adequately preserved the initial petrological and geochemical characteristics of orogenic peridotite before being incorporated into the subduction zone. Therefore, orogenic dunite offers an opportunity to better understand the origins of orogenic peridotites and the evolutionary processes that they underwent before subduction, metamorphism, and the crust–mantle interactions during subduction and exhumation process. Note that the silica-undersaturated melt–rock reactions recorded in orogenic dunites were widespread in the lithospheric mantle peridotites before they were involved in the subduction zone. These reactions have markedly changed the compositions of the orogenic peridotites and may result in controversy regarding the crust–mantle interactions during subduction. Thus, the application of in situ analysis methods to orogenic peridotites should be attractive and significant methods for better understanding these geodynamic processes.