Abstract
The Lower Paleozoic Silgara Formation in the northern Santander Massif was affected by Caledonian prograde regional metamorphism, which varies from low to medium grade and is represented by greenschist, epidote-amphibolite and amphibolite facies. The Silgara Formation shows evidence of a regional metamorphism of Buchan type (andalusite-cordierite) attributed to a crust that was affected by a considerable addition of heat due to magmas which was overimposed on the Barrovian metamorphism that characterize this massif. An anticlockwise PT path reveals that the initial conditions are represented by the mineral assemblage of chloritoid+pryrophyllite+chlorite (all assemblages with quartz+muscovite) in greenschist facies and the final conditions correspond to the mineral assemblage of sillimanite+biotite+cordierite (+muscovite+quartz+garnet if sufficient MnO and CaO are present) in amphibolite facies due to the reaction andalusite = sillimanite occurred at 600 °C and 2.4 kbar. Additionally, in these rocks several deformation events and evidences of shearing and intracrystalline deformation were found.
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