Multistage petrogenesis and suprasubduction metasomatism of orthopyroxenites in the Ab-Bid ultramafic complex (Iran): Insights from open-system mantle melting

Abstract

The Ab-Bid ultramafic complex, located within the Esfandagheh–Hadji Abad ophiolite mélange in southern Iran, hosts numerous orthopyroxenite intrusions emplaced as dykes and irregular bodies within mantle peridotites. These orthopyroxenites exhibit sharp contacts with the host rocks and display cumulate textures characterized by early-crystallizing orthopyroxene and spinel, followed by intercumulus olivine and clinopyroxene. While the Ab-Bid ultramafics show textural and compositional similarities to abyssal mantle residues, they also record subtle geochemical signatures consistent with limited slab-derived metasomatism. This study presents the first detailed petrographic and geochemical dataset from orthopyroxenites in the Ab-Bid massif, including field observations, mineral chemistry, whole-rock and in-situ trace element analyses. Petrogenetic modeling of clinopyroxene trace element distributions in lherzolite, harzburgite, and orthopyroxenite supports a multistage evolutionary scenario involving open-system melting of a depleted oceanic mantle source variably modified by suprasubduction fluids. The resulting high-Mg tholeiitic melts triggered localized remelting, melt-peridotite interaction, and sequential lithological transformation across the complex. Our results indicate a genetic link among the lithological units and demonstrate that localized metasomatic processes—without the need for boninitic melt involvement—can produce high-Mg tholeiitic magmas in suprasubduction settings. These findings provide new constraints on mantle heterogeneity, melt evolution, and the complex dynamics of subduction-modified ultramafic systems within the Tethyan ophiolite belt.