Geologica Acta, Vol.11, Nº 2, June 2013, 133-148 DOI: 10.1344/105.000001836 Available online at www.geologica-acta.com Spinel group minerals in metamorphosed ultramafic rocks from Río de Las Tunas belt, Central Andes, Argentina 1 1 2 M.F. GARGIULO E.A. BJERG A. MOGESSIE 1 INGEOSUR (Universidad Nacional del Sur – CONICET) San Juan 670, B8000ICN Bahía Blanca, Argentina Gargiulo E-mail:
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[email protected] ABS TRACT In the Río de Las Tunas belt, Central Andes of Argentina, spinel group minerals occur in metaperidotites and in reaction zones developed at the boundary between metaperidotite bodies and their country-rocks. They comprise two types: i) Reddish-brown crystals with compositional zonation characterized by a ferritchromite core surrounded by an inner rim of Cr-magnetite and an outer rim of almost pure magnetite. ii) Green crystals chemically homogeneous with spinel (s.s.) and/or pleonaste compositions. The mineral paragenesis Fo+Srp+Cln+Tr+Fe-Chr and Fo+Cln+Tr+Tlc±Ath+Fe-Chr observed in the samples indicate lower and middle grade amphibolite facies metamorphic conditions. Nonetheless, the paragenesis (green)Spl+En+Fo±Di indicates that granulite facies conditions were also reached at a few localities. Cr-magnetite and magnetite rims in zoned reddish-brown crystals and magnetite rims around green-spinel/pleonaste grains are attributed to a later serpentinization process during retrograde metamorphism.