Petrogenesis and PGE Abundances of High‐Cr and High‐Al Chromitites and Peridotites from the Bulqiza Ultramafic Massif, Eastern Mirdita Ophiolite, Albania

Abstract

AbstractThe Bulqiza ultmafic massif, which belongs to the eastern Mirdita ophiolite of northern Albania, is world renowned for its high‐Cr chromite resource. The high‐Cr chromitites commonly host in the mantle section, while high‐Al chromitites also present in massive dunite of the mantle‐crust transition zone (MTZ) in this massif. Chromian‐spinel in the MTZ high‐Al chromitites and MTZ dunites have much lower Cr# values [Cr/(Cr+Al)×100] (47.7–55.1 and 46.5–51.7, respectively) than those of chromian‐spinel in the high‐Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). 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 higher PPGE/IPGE ratios between 1.20 and 7.80. The partial melting degrees of parental magmas for the high‐Cr chromitites are beyond the critical interval (> 25%) and thus prevented sulfide saturation and diluted Pt and Pd in melts, producing high‐Cr chromitites barren of Pt and Pd. However, the degrees for the high‐Al chromitites just enter the critical interval (20–25%) for the effective extraction of PGE from mantle sulfides, which may account for the enrichments of PPGE in high‐Al chromitites. The parental melts of the high‐Cr chromitites have Al2O3 and TiO2 contents of ∼10.6–11.4 wt.% and 0.14–0.31 wt.%, whereas the calculated Al2O3 and TiO2 for the high‐Al chromitites are ∼14.9–15.9 wt.% and 0.07–0.61 wt.%, respectively. The calculated melts in equilibrium with the high‐Cr chromitites are boninitic‐like, and those with high‐Al chromitites are MORB‐like but with hydrous, oxidized and TiO2‐poor affinities. To make a compromise between the inconsistence above, we proposed that coexistence of both types of chromitites in the Bulqiza ultramafic massif may reflect that their magma compositions transited from MORB‐like to boninitic‐like in a proto‐forearc setting during subduction initiation. Key words: Chromian‐spinel, Platinum‐group elements, high‐Cr and high‐Al chromitite, Mirdita ophiolite, Albania.