Abstract
The Bulqiza ultramafic massif, which is part of the eastern Mirdita ophiolite of northern Albania, is world renowned for its high‐Cr chromitite deposits. High‐Cr chromitites hosted in the mantle section are the crystallized products of boninitic melts in a supra‐subduction zone (SSZ). However, economically important high‐Al chromitites are also present in massive dunite of the mantle‐crust transition zone (MTZ). Chromian‐spinel in the high‐Al chromitites and dunites of the MTZ have much lower Cr# values (100Cr/(Cr+Al)) (47.7–55.1 and 46.5–51.7, respectively) than those in the high‐Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The chemical differences in these two types of chromitites are reflected in the behaviors of their platinum‐group elements (PGE). The high‐Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high‐Al chromitites have relatively higher PPGE/IPGE ratios between 1.20 and 7.80. The calculated melts in equilibrium with the high‐Cr chromitites are boninitic‐like, and those associated with the high‐Al chromitites are MORB‐like but with hydrous, oxidized and TiO2‐poor features. We propose that the coexistence of both types of chromitites in the Bulqiza ultramafic massif may indicates a change in magma composition from MORB‐like to boninitic‐like in a proto‐forearc setting during subduction initiation.
Published Version
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