Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Milan Novák

Blue dravite as an indicator of fluid composition during subsolidus replacement processes in Li-poor granitic pegmatites in the Moldanubicum, Czech Republic

Journal of the Czech Geological Society, volume 43 (1998), issue 1-2, 24 - 30

Fibrous to needle-like blue dravite characterized by low and variable Fe/Mg ratios, low Ti, Mn, Li and F contents and locally elevated Ca contents was found at several localities of Li-poor granitic pegmatites in Moldanubicum, Czech Republic. These pegmatites commonly penetrate Ca, Mg-rich rocks such as dolomite-calcite marble and serpentinite. Blue dravite from Strážek and Věžná replaces cordierite in early stages of subsolidus crystallization at temperature above 450 to 500 °C, before relics of cordierite were completely replaced by a mixture of micaceous minerals. Simplified reaction may be expressed: 3Na_0.4(Mg,Fe)₂Al₄Si₅O₁₈.nH₂O + 3B(OH)₃ + yH₂O + zCaO = 2(Na,Ca)_0.7(Mg,Fe)₃Al₆Si₆O₁₈(BO₃)₃(OH)₄ + 3SiO₂. Blue dravite from Blizna formed in relatively late stage of subsolidus process and it is closely associated with Ca, Mg-rich elbaite. Pegmatite-derived fluids in Li-poor pegmatites are characterized by high activity of B and low activities of F and Li. It is in a contrast with highly evolved Li-rich complex granitis pegmatites, where high activities of B, F and alkalis (Li, K, Rb, Cs) in pegmatite-derived fluids were observed. Metamorphic fluids are Ca, Mg-enriched, but their participation in the reaction is negligible to moderate. Significant influx of metamorphic fluids is assumed particularly during a low temperature formation of blue dravite in Bližná, but it was small during replacement of cordierite at localities Věžná and particularly Strážek.