OCURRENCE OF GRANULITES IN THE NORTHERN PART OF THE WESTERN CORDILLERA OF COLOMBIA

The following paper introduces a new unit composed of high-grade metamorphic rocks in the northern part of the Colombian Andes: the Pantanillo Granulite. The unit is exposed near Santa Fe de Antioquia (Antioquia Province), where the Cauca River Valley divides the Western and Central Cordilleras. The granulites are exposed in the western side of the Cauca-Almaguer Fault, which is the westernmost fault of the Cauca-Romeral system. This fault represents the main boundary between continental and oceanic lithospheres in the Colombian Andes. The chief mineral parageneses determined in these rocks are Pl + Amp (hbl?) + Ol + Opx + Spl (hercynite) + opaques, Pl + Cpx + Opx, and Pl + Qtz + Opx + Cpx. These mineral associations are indicative of high-grade, granulite-facies conditions at relatively low P. The chemical composition of Pantanillo Granulite ranges between high-Mg tholeiitic basalt and komatiitic basalt. Trace-element diagrams are flat, similar to N-MORB, but somewhat depleted, a feature indicative of a plutonic protolith, although a volcanic one cannot be precluded. Tectonic discrimination diagrams based on trace elements suggest the granulite's protolith may have been formed in a tholeiitic arc. Whole-rock 40Ar/39Ar dating of the Pantanillo Granulite yielded 216.2 ± 14.2 Ma and K/Ar dating of amphibole yielded 360.7 ± 12.4 Ma. However, the very low content of potassium in these samples renders the ages practically useless. The possibility of excess argon in the samples is quite probable, so the geological meaning of the datings is uncertain at the moment