Abstract
A minor outcrop of a sapphirine-bearing rock within migmatites is geologically associated with the large mafic-ultramafic complex at Pien, State of Parana, S, Brazil. The rock, petrographycally classified as a sapphirine meta-norite, is characterized by a fine-grained equilibrium mosaic texture, with plagioclase (An 60%), bronzite, sapphirine and green spinel as main minerals. The sapphirine is observed as simplectitic intergrowths with orthopyroxene and as isolated grains associated with_ plagioclase and orthopyroxene. Accessory minerals, not associated texturally with sapphirine crystallization, are garnet and some retrograde minerals. Chemically, the rock presents high Al 2 O 3 , and MgO contents and low SiO 2 , ratios, with a strongly undersaturated normative mineralogy. It is suggested that the original material was a spinel-rich ultramafic rock. The sapphirine probably crystallized by a combination of reactions involving mainly Al-pyroxenes, olivine and spinel. A 2-pyroxene geothermometer, applied to associated mafic and ultramafic rocks, indicates for the granulitic metamorphism a temperature of crystallization between 750 and 880°C and minimum pressures of 7 Kb, while temperatures estimated from the Al-content of orthopyroxenes coexisting with sapphirine are much too high. Similar conclusions are drawn from the coexistence of sapphirine, bronzite and spinel, which suggests a crystallization temperature of about 800°C or more, at a minimum pressure of 4 Kb.
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