Accessory and rock forming minerals monitoring the evolution of zoned mafic–ultramafic complexes in the Central Ural Mountains

This study describes major and trace element compositions of accessory and rock forming minerals from three Uralian–Alaskan-type complexes in the Ural Mountains (Kytlym, Svetley Bor, Nizhnii Tagil) for the purpose of constraining the origin, evolution and composition of their parental melts. The mafic–ultramafic complexes in the Urals are aligned along a narrow, 900 km long belt. They consist of a central dunite body grading outward into clinopyroxenite and gabbro lithologies. Several of these dunite bodies have chromitites with platinum group element mineralization. High Fo contents in olivine (Fo 92–93) and high Cr/(Cr + Al) in spinel (0.67–0.84) suggest a MgO-rich (> 15 wt.%) and Al2O3-poor ultramafic parental magma. During its early stages the magma crystallized dominantly olivine, spinel and clinopyroxene forming cumulates of dunite, wehrlite and clinopyroxenite. This stage is monitored by a common decrease in the MgO content in olivine (Fo 93–86) and the Cr/(Cr + Al) value of coexisting accessory chromite (0.81–0.70). Subsequently, at subsolidus conditions, the chromite equilibrated with the surrounding silicates producing Fe-rich spinel while Al-rich spinel exsolved chromian picotite and chromian titanomagnetite. This generated the wide compositional ranges typical for spinel from Uralian–Alaskan-type complexes world wide. Laser ablation analyses (LA-ICPMS) reveal that clinopyroxene from dunites and clinopyroxenite from all three complexes have similar REE patterns with an enrichment of LREE (0.5–5.2 prim. mantle) and other highly incompatible elements (U, Th, Ba, Rb) relative to the HREE (0.25–2.0 prim. mantle). This large concentration range implies the extensive crystallization of olivine and clinopyroxene together with spinel from a continuously replenished, tapped and crystallizing magma chamber. Final crystallization of the melt in the pore spaces of the cooling cumulate pile explains the large variation in REE concentrations on the scale of a thin section, the REE-rich rims on zoned clinopyroxene phenocrysts (e.g. LaRim/LaCore ∼ 2), and the formation of interstitial clinopyroxene with similar REE enrichment. Trace element patterns of the parental melt inferred from clinopyroxene analyses show negative anomalies for Ti, Zr, Hf, and a positive anomaly for Sr. These imply a subduction related geotectonic setting for the Uralian zoned mafic–ultramafic complexes. Ankaramites share many petrological and geochemical features with these complexes and could represent the parental melts of this class of mafic–ultramafic intrusions. Diopside from chromitites and cross cutting diopside veins in dunite has similar trace element patterns with LREE/HREE ratios (e.g. La/Lu = 5–60) much higher than those in diopside from all other lithologies. We suggest that the chromitites formed at high temperatures (800–900 °C) during the waning stages of solidification as a result of the interaction of an incompatible element-rich melt or fluid with the dunite cumulates.