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Abstract
The massive occurrence (18 and 52%) of sillimanite (Sil) in schists and quartzites of the Silgará Schits Unit in Cerro La Ventana and La Cuchilla Frailejones sectors is a unique case in the Santander Massif, Eastern Cordillera of the Colombian Andes. Two types of sillimanite were recognized: Sil1 of the fibrolite and/or prismatic variety and Sil2 of the fibrolite variety (which developed in a radial form associated with quartz). The association of the metamorphic peak consists in bioti te+muscovite+quartz+plagioclase+sillimanite 1 with the abundant formation of sillimanite of the fibrolite variety allowed determining the following conditions: T=650-720 °C and P=5.5-6.9 Kb. These conditions suggest that the rocks suffered a Barrovian metamorphism. Sil 2 was formed in the slightly metamorphic post-peak stage in a weak deformation environment and possibly by the thermal effect produced by the ortogneisses emplacement during the Late Ordovicic. The geochemical characteristics evidence a sedimentary pelitic protolith for the schists and quartzites that formed an active continental margin.
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