Genesis and tectonic setting of high-Cr podiform chromitites of the Rayat ophiolite in the Zagros Suture Zone, northeastern Iraq

Abstract Chromitite pods hosted by serpentinized harzburgites of the Rayat ophiolite in the Zagros suture tectonic zone, northeasten Iraq, were studied petrographically and geochemically. These chromitites comprise high-Cr podiform ophiolitic chromite deposit and consist of magesiochromite in a matrix dominated by serpentine with minor carbonate minerals. The chromitites contain 40.2 wt.% Cr2O3, 15.8 wt.% MgO, 11.4 wt.% Al2O3, and 16.7 wt.% Fe2O3t, and lesser amounts of the other major oxides and traces of Ba, Co, Cu, Ga, Mn, Ni, Rb, Sc, Sr, Ti, V, and Zn. Magnesiochromites of chromitites contain Cr2O3 (55.6 wt.%), MgO (15.2 wt.%), FeOt (13.8 wt.%), and Al2O3 (14.3 wt.%), and minor amounts of Ni, Ti, Mn and V; their average Cr# and Mg# are 0.72 and 0.71, respectively. Accessory chromite in the serpenitinized harzburgite contains 56.9 wt.% Cr2O3, 9.9 wt.% MgO, 19.6 wt.% FeOt, and 12.7 wt.% Al2O3; with Cr# and Mg# of 0.75 and 0.49, respectively. The relatively high concentrations of compatible trace elements which have strong affinity to Cr and Mg such as Ni, V, Co, and the low concentrations of incompatible LILE (K, Rb, Sr, and Ba) elements as well as the high concentrations of mantle rock-forming elements (MRFE: Si, Al, Fe, Mg, Ca, Cr, Ni, Zn, Co, and Cu) is a characteristic feature of the Rayat chromitite and indicates mantle origin. The chromitites also contain very low Au (3 ppb) and PGE (53 ppb). The IPGEs (Os, Ir, and Ru) are enriched and are ∼8 times higher than the PPGEs (Rh, Pt, and Pd), with a positive peak of Ru and a negative peak of Pt. The chondrite-normalized PGE pattern has generally lower concentrations but similar trend to most high-Cr chromitites of the world. The chromitites belong to Group-I “normal ophiolite type” indicated from the Ptn/Pdn vs. Ptn/Irn digram. Various tectonic discrimination diagrams including the calculated parental melt compositions of Al2O3, TiO2 and FeOt/MgO for the magnesiochromites and chromites suggest that the Rayat chromitites were derived from boninitic magmas within suprasubduction zone in a fore-arc tectonic setting during the formation of the Taurus (Turkey) - Zagros (Iran/Iraq) Ophiolite Belt.