DETRITAL ZIRCON DOUBLE DATING AS A TOOL FOR IDENTIFYING VOLCANIC ZIRCONS AND EXHUMATION PATTERNS IN THE EASTERN CORDILLERA OF COLOMBIA

Analysis of lag time (the difference between closure age of a thermochronologic system and depositional age of host strata), in sedimentary basin fill provides insights into the exhumation history of adjacent eroded orogens. However, near-zero lag times can be produced either by syndepositional volcanism or rapid exhumation. We apply the results of combined zircon U-Pb (ZPb) geochronology and (U-Th)/He (ZHe) thermochronology from the Paleogene Floresta basin in the Eastern Cordillera of Colombia to determine sediment provenance, identify volcanic zircons, and calculate lag time.

Results show that the lower Paleocene Guaduas Formation lacks cooling ages < 140 Ma and crystallization ages < 930 Ma. Both ZPb and ZHe age spectra are consistent with derivation from the eastern craton. The Paleocene – lower Eocene Socha Group records the first arrival of syndepositional volcanic zircons, characterized by overlapping ZPb and ZHe ages (within 2s error). These zircons represent the first arrival of orogenic detritus, derived from the westernmost potential sediment source: the Central Cordilleran magmatic arc. The middle Eocene Picacho Formation is characterized by ZHe cooling ages which are younger than the youngest observed volcanic zircons a maximum lag time of < 10 Myr. The ZPb age spectra, showing affinities with the Eastern Cordillera, and low lag times point to basement highs of the Magdalena Valley east of the Central Cordillera as the possible source for this detritus. In the upper Eocene-Oligocene Concentración Formation, lag time increases and the ZHe age of exhumed grains are within error of volcanic zircon ages. We interpret the increase in lag time as exhumation of supra-PRZ foreland basin strata deposited in the region presently occupied by the Eastern Cordillera.

The presence of volcanic zircons with ages within error of exhumationally cooled zircons in the Concentración Formation indicates that only double dating provides a robust method for discriminating these two groups of ages. An attempt to exclude mineral cooling ages from lag time analyses based on observations of other volcanic minerals will likely result in false positives and incorrect results. The data also point to a tectonic history of the Eastern Cordillera of Colombia characterized by systematic eastward migration of the deformation front.