Compositional characteristics and paragenetic relations of magnesiohögbomite in aluminous amphibolites from the Belomorian complex, Baltic Shield, Russia

Abstract

This study investigates the compositional characteristics, parageneses, and stability relations of some hogbomite-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 hogbomite 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 magnesiohogbomite-2N3S polysome type [P3̅m1 with a = 5.721(1) Å , c = 23.045(1) Å ] and exhibits compositions that are poor in Zn (0.05̅0.42 wt% ZnO) and Ni (0.20̅0.50 wt% NiO) [(Fe_2.7̅3.1²⁺Mn_0.01Ni_0.04̅0.1Zn_0.01̅0.1)Mg_4.8̅3.7(Al_18.1̅18.8Cr_0.1̅0.2Fe_0.4̅0.9³⁺)Ti_1.6̅1.2O₃₈(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 magnesiohogbomite 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 logf_O2-logf_S2 diagram for the system FeO-Al₂O₃-TiO₂-O₂-S₂-H₂O shows that growth of magnesiohogbomite from Crn + Ilm and Spl + Rt is restricted to a narrow f_O2-window and low f_S2. The topological constraints, together with petrological data, suggest that magnesiohogbomite is formed in titanian and aluminous protoliths under greenschist-to amphibolite-facies conditions if f_H2O is high, f_S2 low, and f_O2 is defined by the paragenesis ilmenite + rutile + magnetite.