Spinel compositions of mantle-hosted chromitite from the Eastern Anatolian ophiolite body, Turkey: Implications for deep and shallow magmatic processes

Abstract

The Eastern Anatolian Accretionary Complex includes several ophiolitic megablocks and/or tectonic slivers within a 150–180km long east–west trending complex formed during the Late Cretaceous–Tertiary in Eastern Turkey. The Alabayır, Mollatopuz, Yukarıbalçıklı and Mehmetalan ophiolites are megablocks or tectonic slivers containing locally massive, nodular or schlieren banded chromitite layers. These podiform chromitites formed in mantle sequences and are classified as high-Cr chromitites (Cr# 0.63–0.88; Mg# 0.50–0.64; 0.01–0.5wt.% TiO2; 5.7–18.8wt.% Al2O3). Calculated parental melt compositions of these chromitites indicate boninite magma characteristics (8.2–13.4wt.% Al2O3; 0.64–1.50 FeO/MgO). The PGE patterns also support the view that they crystallized from a boninitic melt. The total PGE contents of chromitites vary between 79 and 390ppb. Pd/Ir ratios of chromitites vary from 0.07 to 0.28 and are consistent with an IPGE fractionated nature.The examined chromitites were divided into two groups in terms of their mineral chemistry (Group-I: Alabayır, Mollatopuz, Yukarıbalçıklı and Mehmetalan-I chromitites; Group-II: Mehmetalan-II chromitites). Mineral chemistry of these chromitites and their parental melt composition indicate that Group-I chromitites were probably formed at shallow mantle depths and that Group-II chromitites formed in deeper parts of the mantle. Mineral chemistry of these Eastern Anatolia high-Cr chromitites indicate that they formed not only in the deeper mantle but also in shallower parts of the MOHO transition zone.