U-PB GEOCHRONOLOGY AND LOW TEMPERATURE THERMOCHRONOLOGY FROM PLUTONIC BODIES OF NORTHWESTERN COLOMBIA: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF NW ANDES

The NW region of Colombia is composed of two allochthonous crustal blocks of oceanic affinity, the Western Cordillera-Cauca Patia Valley Block (WCCPVB), and the Panama-Choco Block (PCB), separated by the Dabeiba Fault. The WCCPVB block, was accreted to South America during the Late Cretaceous, while the accretion time of the PCB is still a matter of debate. The WCCPVB block is intruded by Cretaceous-Cenozoic plutons, while the PCB is intruded by a Paleogene pluton. These plutonic bodies record exhumation in response to collision events such as the (PCB) accretion to the WCCPVB block and changes in subduction-accretion dynamics at the continental margin. Zircon U/Pb geochronology and low-temperature thermochronology (AHe, AFT, and ZFT) data were used on five plutonic bodies (four from the WCCPVB block and one from the PCB) to date the crystallization age, and to reconstruct exhumation rates. These rates were determined using vertical profiles (3 plutons) and multiple thermochronometers on the same sample (2 plutons). Our results show weighted crystallization ages of 79 Ma, 17 Ma, 12 Ma, and 8 Ma (plutons from the WCCPVB block) and 42 Ma (pluton from the PCB). Three periods of rapid exhumation were defined 1) Middle Eocene-Late Eocene (0.92-1.15) mm/yr) recorded by both blocks; 2) Middle Miocene (4.35 mm/yr); and 3) Late Miocene (0.64-0.48 mm/yr). Events 2 and 3 were recorded only by the WCCPVB block. In contrast, slow exhumation rates during Late Eocene-Pliocene (0.12-0.14 mm/yr) were found for the PCB and the oldest plutonic body of the WCCPVB block. These data suggest spatially heterogeneous exhumation probably due to partitioned deformation along the continental margin. A rapid exhumation event at 25 Ma proposed by several authors as the initial docking of the Panama-Choco Block is not recorded within the oldest plutonic bodies. This may imply that the initial accretion was later, progressive, or the rocks were not at the appropriate depth to record cooling.