article · Island Arc
The Khoy ophiolitic complex in Northwestern Iran comprises two distinct sections: an older Eastern ophiolite and a Late Cretaceous Western ophiolite. Mineral analysis separates the chromitites within the complex into two distinct groups. High-aluminium chromitites from the Eastern section likely originated from interactions between a mid-ocean-ridge basalt-like melt and depleted harzburgite in a back-arc basin. These rocks show evidence of moderate partial melting. In contrast, the Western section contains high-chromium chromitites formed during two stages of arc development, involving island-arc-tholeiite and boninitic melts reacting with highly depleted harzburgite in a supra-subduction zone. Western chromitites also contain inclusions of clinopyroxene, olivine, and platinum-group mineral-bearing sulfides. These mineral and geochemical differences demonstrate that the Khoy complex represents a tectonic aggregation of two separate ophiolitic units that developed in different tectonic settings and geological time periods.
Understanding how ophiolite complexes assemble offers insight into the tectonic history and evolution of ancient ocean basins. By identifying the geochemical signatures of chromitites and associated minerals, geologists can decipher complex plate boundary processes, mantle melting histories, and the mechanisms that bring together diverse fragments of oceanic crust and upper mantle over geological time.
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Abstract The K hoy ophiolitic complex in N orthwestern Iran is a part of the T ethyan ophiolite belt, and is divided into two sections: the E astern ophiolite in Q eshlaq and K alavanes ( J urassic– C retaceous) and the W estern ophiolite in B arajouk, C huchak and H essar ( L ate C retaceous). Our chromitites can be clearly classified into two groups: high‐ A l chromitites ( Cr # = 0.38–0.44) from the E astern ophiolite, and high‐ C r chromitites ( C r# = 0.54–0.72) from the W estern ophiolite. The chromian spinels in high‐ A l chromitite include primary mineral inclusions mainly as N a‐bearing diopside and pargasite with subordinate rutile and their formation was probably related to reaction between a MORB (mid‐ocean‐ridge basalt)‐like melt with depleted harzburgite, possibly in a back‐arc setting. Their host harzburgites contain clinopyroxene with higher contents of A l 2 O 3 , N a 2 O , C r 2 O 3 , and T i O 2 relative to W estern harzburgites and are possibly residue after moderate partial melting (~15 %) whereas the Western harzburgite is residue after high partial melting (~25 %). The chromian spinel in the W estern K hoy chromitites contains inclusions such as clinopyroxene, olivine and platinum group mineral‐bearing sulfides. These W estern chromitites were possibly formed at two stages during arc growth and are divided into the moderately high‐ C r# chromitites ( B arajouk and H essar) and the high‐ C r# chromitites ( C huchak A and C ). The former crystallized from island‐arc‐tholeiite ( IAT ) melts during reaction with the host depleted harzburgites, whereas the latter crystallized from boninitic melts (second stage melt) during reaction with highly depleted harzburgite in a supra‐subduction‐zone environment. Based on the mineral chemistry of chromian spinels, pyroxenes, and mineral inclusions, the chromitites and the host peridotites from the E astern and W estern K hoy ophiolites were formed in a back‐arc basin and arc‐related setting, respectively. The K hoy ophiolitic complex is a tectonic aggregate of the two different ophiolites formed in two different tectonic settings at different ages.
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DOI: 10.1111/iar.12211
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