Chemical substitutions, paragenetic relations, and physical conditions of formation of hogbomite in the sittampundi layered anorthosite complex, South India

Abstract

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 hogbomite replaced only the grains of corundum and spinel and are in textural equilibrium with chlorite. The studied hogbomite 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 hogbomite 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, hogbomite), 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 hogbomite 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 hogbomite. Stability fields of some critical mineral assemblages in P-X_CO2 and T-X_CO2 space combined with geothermobarometry in the associated rocks tightly constrain the growth of hogbomite in the presence of aqueous fluids (X_CO2 < 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 hogbomite. Topological relations in isothermal-isobaric fugacity diagrams (log f_O2-log f_S2 and log f_O2-log f_H2O) in the system FeO-Al₂O₃-TiO₂-O₂-S₂-H₂O-CO₂ (+MgO, Cr₂O₃) indicate that the stability and compositional characteristics of natural hogbomite are strongly influenced by f_O2, f_S2, f_H2O, and concentrations of other soluble species (Ti, Mg, Cr, etc.) in the metamorphic fluids.