Original paper

Cation distribution in calcic amphiboles from the Nuggihalli Greenstone belt, S. India and its geological significance

Mitra, Sachinath; Bidyananda, Maibam


Amphiboles from Nuggihalli schist belt are analyzed and found to be mainly magnesio-hornblende, ferro-hornblende and actinolitic hornblende. Mössbauer study of actinolitic hornblende shows presence of iron in the M1, M2, M3 and M4 sites. A lower QS value (1.66mm/s) at M4 site for Fe2+ indicates a faster rate of crystallization of the studied sample whereas Fe3+/Fe2+ ratio (0.14) indicates that the amphiboles were formed in an oxidizing environment (high fO2). Based on plagioclase-amphibole geothermometry, equilibration temperature of meta-anorthosite is seen to range between 648 °C (921 K) and 704 °C (977 K). The amphibolite is found to be equilibrated in a water rich environment (XH2O ranging from 0.81 to 0.89). Considering the pressures as 5 and 7 kbar, and the amphibolite subassemblages such as, calcite-rutile-quartz and epidote-plagioclase-calcite, in the T-XCO2 section (derived from THERMOCALC program) the invariant points are seen to be at (i) XCO2 = 0.11, T = 493 °C and (ii) XCO2 = 0.19, T = 562 °C at the respective pressures. The calculated M2-M4 distribution coefficient (KD) in Nuggihalli actinolitic hornblende is found to be 0.0034, which corresponds to a temperature of equilibration at around 435 °C.


greenstone beltdharwar cratonnuggihalliamphibolemossbauermeta-anorthositeamphibolitegeothermobarometryassemblagethermocalcequilibrium temperature