The Sarmiento Complex of southern Chile is a Mesozoic ophiolite that underwent hydrothermal “ocean-floor metamorphism” prior to its uplift and exposure in the Andean Cordillera. The overprint of this metamorphism exhibits a steep vertical metamorphic gradient although the extent of metamorphic replacement decreases markedly within the gabbro unit of the complex, probably because of restricted access of circulating sea-water at deeper levels of the complex. Even for rocks of the same facies, pseudostratigraphic variations in the proportions of major metamorphic minerals and the presence or absence of accessory phases such as biotite, magnetite, ilmenite, and orthoclase indicate that conditions other than the temperature of metamorphism, particularly fO2, varied pseudostratigraphically. Detailed textural, mineralogic, and geochemical studies indicate that ocean-floor metamorphism is a complicated interaction between the various rock units that make up the igneous pseudostratigraphy of the ocean floor and circulating seawater; the interaction is strongly dependent on PH2O and the thermal gradient.

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