Transitional chromitites within the Maqsad crust-mantle transition zone of the Semail ophiolite formed above the nascent forearc setting

Abstract

The Semail ophiolite is a classic example of a well-preserved ophiolite, hosting over 450 documented podiform chromite deposits. This study investigates a transitional chromitite occurrence in the crust-mantle transition zone (CMTZ) of the Maqsad area within the Sumail massif of the Semail ophiolite. The chromitites, enveloped by dunites, exhibit massive and disseminated textures. Host harzburgites, representing upper mantle residues, underwent ∼20–35 % partial melting followed by metasomatism from subducting slab-derived melts/fluids. These peridotites show geochemical affinities with Izu-Bonin-Mariana (IBM) forearc peridotites, indicating a forearc tectonic setting. The Maqsad chromitites, with Cr# values of 54.3–66.2, are classified as transitional chromitites. Their geochemical features resemble chromites crystallized from mid-ocean ridge basalt (MORB) melts. Elevated oxygen fugacity (f_O2) values compared to abyssal peridotites further support their formation in a supra-subduction zone (SSZ)environment. Based on the petrogeological and geochemical characteristics of the host rocks, we propose that the parental magma of the Maqsad transitional chromitites derived from a nascent forearc mantle above an SSZ setting, with MORB-like affinity.