Tectonomagmatic evolution of the Northwestern Gondwana margin during the Upper Triassic: Insights from the Pueblito Pluton (Colombia)

The breakup of Pangea along the northwestern margin of Gondwana was accompanied by widespread magmatic activity driven by slab rollback processes and the progressive opening of back-arc basins. Within this tectonic framework, several mafic–ultramafic bodies and granitoids were emplaced along the western flank of the Central Cordillera of Colombia during the Triassic. Among them, the Pueblito Pluton was emplaced as an elongated N-S trending body approximately 55 km long and up to 5.2 km wide, consisting of gabbros in tectonic contact with peridotites, mainly harzburgites. The harzburgites record moderate degrees of partial melting (9–13%) and are composed of forsteritic olivine, enstatite, and minor spinel and diopside, later affected by serpentinization accompanied by magnetite and chlorite formation. The gabbros, locally containing hornblende, preserve primary igneous textures and consist mainly of calcic plagioclase (now completely saussuritized) and augite. These assemblages are overprinted by greenschist-facies metamorphism, characterized by actinolite, albite, epidote, and chlorite. Whole-rock major and trace element geochemistry of the metagabbros reveals two compositional groups, one slightly enriched and the other depleted in light rare earth elements (LREE), indicating affinities ranging from E -MORB to N-MORB. Zircon U-Pb dating yields a crystallization age of 228.6 ± 1.4 Ma (Carnian, Upper Triassic), consistent with the emplacement ages of other Triassic mafic-ultramafic bodies in the basement of the Central Cordillera, including the El Picacho Gabbro and La Espadera Amphibolite. Positive zircon εHf(t) values (+7.3 to +10.9) indicate derivation from a juvenile asthenospheric mantle source. The lack of evidence for prograde metamorphism in harzburgite, like serpentine dehydration or chemical zoning in high-temperature minerals, suggests that metamorphism most likely occurred during exhumation. Hydrothermal alteration of the gabbros resulted in pervasive saussuritization of plagioclase and uralitization of pyroxene, reflecting the circulation of moderate-temperature fluids during pluton cooling. Altogether, petrological, geochemical, geochronological and isotopic data support a supra-subduction origin for the Pueblito Pluton, representing part of the magmatic systems generated during slab rollback of the Farallon plate and the associated back-arc extension linked to the early stages of Triassic fragmentation of Pangea along northwestern margin of Gondwana.