GEOCHEMISTRY AND GEOCHRONOLOGY OF PLUTONIC ROCKS FROM THE COLOMBIAN WESTERN CORDILLERA: NEW INSIGHTS INTO THE CENOZOIC TECTONICS OF THE NW SOUTH AMERICAN PLATE MARGIN

The Western Cordillera of Colombia is composed of two allochthonous crustal blocks of oceanic affinity, the Western Cordillera Block (WCB) and the Panama-Choco Block (PCB), separated by the Uramita Fault. The WCB block was accreted to South America during the Late Cretaceous, while the accretion time of the PCB is still a matter of debate. The Western Cordillera is intruded by Cenozoic plutons that record the magmatic response to subduction and strike-slip faulting along: 1) the PCB, 2) the boundary between the WCB and PCB and 3) the current subduction zone. This magmatism is key to understand the paleogeography and geologic evolution in northern South America during Cenozoic time. New geochemistry (major and trace elements), geochronology (zircon U/Pb) and isotopic (zircon Hf) data from several plutons and dikes from the Western Cordillera define four magmatic periods (ca. 60-42, 22-17, 12-11 and 9-7 Ma). Geochemically, the first period is characterized by tholeiitic to calc-alkaline magmas the second and third periods are dominated by calc-alkaline to high-K calc-alkaline magmas and the fourth period yields high-K calc-alkaline to shoshonitic magmas. Hf Isotopic analysis yielded the highest ƐHf values for the first magmatic period up to 15.4 and the lowest values up to 4 for the last period. All four magmatic stages are characterized by trace elements patterns with magmatic arc signatures. These results suggest that PCB and southern WCB magmatism between ca. 60-42 Ma occurred within an immature oceanic island arc setting. The 22-17, 12-11 and 9-7 Ma magmatic stages are more evolved and derived from the older same volcanic arc rocks or partly from a continental margin of the WCB. These data reveal the maturation of a single magmatic arc through time and/or the replacement of the mantle wedge by a homogeneous and relatively undepleted asthenospheric mantle source after the formation of a subduction zone since ~22 Ma, after collision of the PCB with the WCB.