The Significance of Hypermagnesian Clinopyroxene in the Yanisvaara Ultrabasic Complex, Kola Peninsula, Russia

Abstract Two generations of hypermagnesian clinopyroxene [92 < Mg# < 98] are found in the Yanisvaara ultrabasic complex in the northwestern Lapland–Belomorian Belt, Kola Peninsula, Russia. Grains of a first generation (Wo47.3–50.7En46.5–49.4Fs2.5–3.5), 0.2–0.4 mm across, are attributed to a reaction in which tremolite + clinopyroxene replaced orthopyroxene. The second generation generally attains even more strongly magnesian compositions (Wo46.5–50.9En47.5–50.8Fs1.0–3.2). It is sporadically developed as submicrometric rim- or veinlet-like grains associated with tremolite and domains of fresh olivine with Mg# in the range 80.5–82.5 to 87.7–88.0. The later generation of clinopyroxene crystallized at elevated values of fO2 under closed-system autometasomatic conditions rather than during regional metamorphism. The compositions attained in three representative samples are highly magnesian, but characteristically poor in Cr compared to primary clinopyroxene. Compositions of the accompanying silicate, oxide, and sulfide minerals are provided. A novel Ni-Mn oxyspinel may be related to hausmannite. Yanisvaara and coeval subvolcanic komatiitic complexes are inferred to have degassed, which promoted the rapid rise in fO2 and indirectly promoted the buildup of Mg in the late clinopyroxene.