Petrogenesis and PGE distribution in the Al- and Cr-rich chromitites of the Qalander ophiolite, northeastern Iraq: Implications for the tectonic environment of the Iraqi Zagros Suture Zone

Abstract

Abstract The Qalander ophiolite is a small, poorly preserved and incomplete melange-type complex situated within the Eocene–Oligocene Walash–Naopurdan Group in the Iraqi Zagros Suture Zone (IZSZ). It is one of six fragmented ophiolite complexes emplaced in the IZSZ during the Cretaceous and Tertiary. Within the Qalander ophiolite, serpentinized dunite and harzburgite hosts small lens-shaped podiform high-Al (North Qalander) and high-Cr (North Shitna) chromitite bodies. The average range of Cr-, Mg-, and Fe 3 -numbers for North Qalander chromitites is 39–53, 75–77, and 3–5, respectively; meanwhile those for North Shitna chromitites are 78–80, 65–73, and 6–8, respectively. The ranges of Al 2 O 3 wt.% and FeO/MgO for the North Qalander and North Shitna chromitites are 15–15.5 and 0.8–1.0, and 9.0–10.5 and 0.4–1.0, respectively. In addition to pyroxene and olivine, inclusions of laurite, millerite and galena were detected within the chromitites of Qalander ophiolite. The matrix minerals between chromite grains are serpentinized olivine and pyroxene, chlorite, and calcite; grains of magnetite, pyrite and ilmenite are also common accessories. The concentrations of platinum-group elements (PGE) in both varieties are typical for the ophiolitic chromitites, but they show two distinct patterns of PGE enrichment. The high-Cr chromitites have relatively uniform mantle-normalized PGE distribution patterns with a steep slope, positive Ru and negative Pt anomalies, and they show enrichment of PGE and depletion in Pt relative to the average upper mantle. The high-Al chromitites show relatively gently sloping patterns with slight positive Ru and negative Pt anomalies, high iridium-group PGE (IPGE) abundances relative to the high platinum-group PGE (PPGE), and are distinctly enriched in Pt and Pd relative to the upper mantle and the average abundances in the high-Cr chromitites. The differences in the PGE content, geochemistry, rare earth element (REE), mineral chemistry and petrographical characteristics of the chromitites suggests two stages of magmatic activity which were responsible for the chromitite genesis in the region. The first stage involved low degree partial melting of S-saturated peridotite melt that produced high-Al chromitites of MORB affinity, which crystallized in the upper mantle close to the Moho. The second stage involved a high degree partial melting of S-undersaturated boninitic melt that produced high-Cr chromitites of arc affinity and crystallized in a deeper mantle section. The presence of low-Cr MORB-type chromitites with high-Cr boninitic-type chromitites in the same mantle section suggests their formation in aback-arc supra-subduction zone tectonic setting.