Granulitic rocks at the Western Cordillera of Colombia: Evidence of metamorphism in the Colombian Caribbean Oceanic Plateau

摘要

The geological basament of the Western Cordillera of Colombia has been interpreted as an igneous province derived from the Colombian-Caribbean Oceanic Plateau (CCOP) with no high-grade metamorphic rocks. The Mistrato Quartz-diorite and Sabanalarga Batholith, however, as part of the CCOP, contain some rocks that evidence granulite facies metamorphism. Here we define the Mistrato Granulite as a different unit from the Mistrato Quartz-diorite and correlate with the Pantanillo Granulite, which has been previously defined as a different unit from the Sabanalarga Batholith. We use field relationships, petrography, mineral chemistry and whole-rock chemistry analyses to evidence the metamorphism. The Mistrato Granulite, which outcrops into the inner region of the Colombian Western Cordillera, hosts a Ca-richer recrystallized assemblage, consisting of plagioclase + clinopyroxene + ortopyroxene + quartz + ilmenite, and a Ca-poorer relict assemblage, which is composed of plagioclase + clinopyroxene + orthopyroxene + ilmenite + actinolite. The anhydrous nature and polygonal granoblastic texture of the recrystallized assemblage suggest metamorphism of a gabbroic protolith that reached granulitic conditions (900 +/- 50 degrees C and 1.1 GPa). Geochemical analyses reveal that these rocks may have formed in an island volcanic arc linked to the CCOP during the Late Cretaceous. We have found that the geochemistry of the Mistrato and Pantanillo granulites, together with regional trends, are comparable, allowing us to propose that both units could have derived from similar mafic oceanic rocks and formed under similar tectonic settings. This differs from previous interpretations that explain the Pantanillo Granulite as part of the continental basament of the Central Cordillera.