- © 2009 American Mineralogist
Layered anorthosite, chromiferous pyroxenite, and spatially associated mafic and felsic rocks of the 2.9 Ga Sittampundi layered complex (SLC), South India, underwent high-grade metamorphism at ca. 2.5 Ga and were subjected to amphibolite-facies metamorphism accompanied by intrusion of granitoid plutons during late-Neoproterozoic tectonothermal activity (0.72–0.45 Ga). During the latter event, anorthosite developed millimeter- to centimeter-thick compositional layers rich in clinopyroxene+amphibole+clinozoisite+chlorite±chromite and corundum+spinel+chlorite. Tiny grains of högbomite replaced only the grains of corundum and spinel and are in textural equilibrium with chlorite. The studied högbomite contains appreciable TiO2 (>4.4 wt%) but insignificant NiO and ZnO. Cr2O3 content reaches up to 0.35 wt% only in chromite-bearing samples. Systematic partitioning of Fe and Mg between högbomite and associated Fe-Mg minerals demonstrate attainment of chemical equilibrium among these phases. Integrating textural relations and algebraic analyses of the phase compositions, several reactions were constructed involving the associated oxide phases (spinel, corundum, högbomite), calcite, and silicates (amphibole, chlorite, anorthite, clinozoisite). Interpretation of the reaction reveals that (1) Mg2+ and Ti4+ were mobile for more than 2 cm during the formation of högbomite and chlorite, and (2) chloritization of amphibole in the clinopyroxene+amphibole-bearing layers released Ti that was transported to the spinel+corundum-bearing layers to develop högbomite. Stability fields of some critical mineral assemblages in P-XCO2 and T-XCO2 space combined with geothermobarometry in the associated rocks tightly constrain the growth of högbomite in the presence of aqueous fluids (XCO2 < 0.15) to the P-T range of 7 ± 1 kbar, 650 ± 50 °C. These aqueous fluids, presumably derived from the Pan-African granitoid batholiths, chloritized amphibole grains, and transported the released Ti4+ to the spinel+corundum-bearing layers to develop högbomite. Topological relations in isothermal-isobaric fugacity diagrams (log fO2-log fS2 and log fO2-log fH2O) in the system FeO-Al2O3-TiO2-O2-S2-H2O-CO2 (+MgO, Cr2O3) indicate that the stability and compositional characteristics of natural högbomite are strongly influenced by fO2, fS2, fH2O, and concentrations of other soluble species (Ti, Mg, Cr, etc.) in the metamorphic fluids.