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1 Department of Geological Sciences, Jadavpur University, Kolkata–700032, India
2 Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany
3 Institute of Geochemistry, Siberian Branch, RAS, Favorski st., 1a, Irkutsk–664033, Russia
Correspondence: * E-mail: m.raith{at}uni-bonn.de
This study investigates the compositional characteristics, parageneses, and stability relations of some högbomite-bearing assemblages in coarse-grained corundum-garnet amphibolites from a blackwall zone that separates troctolitic metagabbro from kyanite-bearing paragneiss at Diadina Mountain, Belomorian Belt, Russia. The blackwall zone presumably was formed through infiltration driven metasomatism during the Svecofennian metamorphic event at ~1.9 Ga. Euhedral högbomite grains (up to 15 mm in size) occur in domains of coarse tschermakitic amphibole, biotite, and spinel with minor rutile and ilmenite in the two blackwall varieties having contrasting bulk compositions. The other minerals in the two associations include corundum + garnet ± cordierite + chlorite + plagioclase + carbonate and spinel ± gedrite + sapphirine + chlorite + carbonate.
The studied högbomite belongs to the magnesiohögbomite-2N
S polysome type [P3–m1 with a = 5.721(1) Å, c = 23.045(1) Å] and exhibits compositions that are poor in Zn (0.05–0.42 wt% ZnO) and Ni (0.20–0.50 wt% NiO) [(Fe2.7–3.12+ Mn0.01Ni0.04–0.1Zn0.01–0.1)Mg4.8–3.7(Al18.1–18.8Cr0.1–0.2Fe0.4–0.93+)Ti1.6–1.2O38(OH)2]. Compositional variation is controlled by the substitution Ti4+ + R2+ 
2R3+. Systematic partitioning data for Fe2+, Mg, and Zn indicate attainment of chemical equilibrium between magnesiohögbomite and the associated minerals (Spl, Hbl, Ged, Grt, Spr, Bt) on the thin section scale. Textural relations suggest growth of magnesiohögbomite under amphibolite-facies conditions (6 ± 1 kbar, 600 ± 50 °C) through complex mineral-fluid equilibria involving oxide (Crn, Spl, Rt, Ilm), silicate (Am, Bt, Spr), and carbonate (Cal, Dol, Mgs) phases. A partial logfO2-logfS2 diagram for the system FeO-Al2O3-TiO2-O2-S2-H2O shows that growth of magnesiohögbomite from Crn + Ilm and Spl + Rt is restricted to a narrow fO2-window and low fS2. The topological constraints, together with petrological data, suggest that magnesiohögbomite is formed in titanian and aluminous protoliths under greenschist-to amphibolite-facies conditions if fH2O is high, fS2 low, and fO2 is defined by the paragenesis ilmenite + rutile + magnetite.
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