First occurrence and significance of the staurolite-bearing amphibolites between marbles and garnet micaschists of the Silgará Formation at the Central Santander Massif

Abstract

The Silgara Formation metamorphic rocks have been affected by a Barrovian-type of metamorphism, which has occurred under medium-pressure and high-temperature conditions. Scarce intercalations of metabasites from millimeter up to centimeter scale occur in reaction bands observed in the gradational contact between garnet-bearing pelitic and calc-silicate rocks. In this study, we report for the first time the presence of staurolite metabasites in the Santander Massif (Colombian Andes), which is of particular interest since it is an unusual occurrence, taking into account that staurolite is most commonly regarded as an index mineral in metapelites and is not very well known from other bulk compositions and pressure and temperature conditions. Staurolite metabasites contain plagioclase, hornblende and staurolite, suggesting a history of prograde metamorphism up to amphibolite facies conditions. The origin of staurolite can be associated to aluminium-rich metabasites and, therefore, it is strongly affected by bulk rock chemistry. Taking into account mineral assemblages and geothermobarometric calculations in pelitic rocks, we suggest that the staurolite + hornblende association can be formed at least at 400 to 600 °C and 6 kbar at the peak of prograde metamorphism. Retrograde reactions suggest that these rocks experienced nearly isobaric cooling accompanied by retrograde metamorphism. © 2014. Acad. Colomb. Cienc. Ex. Fis. Nat. All rights reserved.