U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF PALEOZOIC TO MESOZOIC STRATA IN THE COASTAL CORDILLERA, NORTHERN CHILE

This study evaluates the variation in U-Pb detrital zircon (DZ) ages across the Paleozoic to Mesozoic (meta)sedimentary units exposed along the Coastal Cordillera near Taltal, Chile. Zircon grains obtained from seven samples were analyzed using LA-ICP-MS to collect U-Pb isotopic data. Petrographic analysis of each sample was used to describe textures and mineralogical composition. DZ ages were plotted as frequency curves, where peaks reflect magmatic sources and the distribution of peaks reflect sedimentation patterns associated with subduction along the western margin of South America. Maximum depositional ages calculated using the youngest population of 3-5 zircons in each sample are ~328 Ma in the basal Chañaral Melange (CM)/Las Tortolas Formation (LT) and ~182 Ma in the Cifuncho Formation (CF), which unconformably overlies the CM/LT and records a lithological transition from quartz-rich metasandstones in the CM/LT to lithic sandstones in the CF. Detrital zircon ages from the CF record a major increase in arc activity at ~200 Ma. A gradational boundary between CF and the overlying Pan de Azucar Formation (PdA) yielded a maximum depositional age of ~193 Ma with major peaks at ~270 Ma and ~380 Ma. The older maximum depositional age for the overlying PdA can be explained by a lull in volcanic activity, in which sedimentation patterns shifted from a single volcanic source in the CF to multiple older sources after the ~200 Ma magmatic flare-up subsided. The ~270 Ma peak is present in all overlying units, but ~380 Ma zircons are only present in CM and a feldspathic sandstone just below the base of La Negra Formation (LN). The base of the LN contains a ~190 Ma peak, a large peak at ~380 Ma, and several zircons older than 465 Ma, which together with rounded grains and less clay matrix suggests a similar provenance as the CF. At the top of the section, the base of the Aeropuerto Formation has a maximum depositional age of ~125 Ma, corresponding to Early Cretaceous arc activity. Unit thicknesses and depositional ages were combined to estimate 0.19 and 0.17 km/Myr accumulation rates for LN and CF/PdA, respectively. Together these data record punctuated periods of sedimentation in the Coastal Cordillera coeval with Jurassic arc activity and intervening periods of sedimentation that tapped pre- arc sources as magmatism waned.