Abstract
Terra Nova, 22, 166–171, 2010 Abstract Sapphirine [(Mg3.41Fe0.11Al4.45)(Al4.55Si1.45)O20] and corundum are found as inclusions (<0.2 mm) in Cl-rich scapolite (Me24-25) and plagioclase (An20-24) in the metasomatised parts of the Ødegården gabbro, Bamble sector, South Norway. Microtextural relationships suggest that the overall reaction plagioclase + Mg + Cl = sapphirine + corundum + scapolite + Ca + Fe took place in two stages possibly related to infiltration of two metasomatic fronts. Gresen analyses support that the scapolitisation was combined with a strong Mg metasomatism and removal of Fe and Ca on a whole-rock scale. The ferromagnesian minerals associated with sapphirine in the scapolite metagabbro have especially high Mg ratios (orthopyroxene #Mg = 0.96; phlogopite #Mg = 0.95; amphibole #Mg = 0.81–0.87; clinopyroxene #Mg = 0.95). This is much higher than in biotite (#Mg = 0.60) and clinopyroxene (#Mg = 0.78–0.81) in the unscapolitised parts of the gabbro and comparable with that found in sapphirine-bearing, orthoamphibole–cordierite schists in the region, suggesting that they also formed by metasomatism. A metasomatic origin for the sapphirine has consequences for models of crustal growth and metamorphic history.
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