DETRITAL APATITE U-PB AGES AND TRACE AND RARE EARTH ELEMENT GEOCHEMISTRY RECORD CA. 100 MA CRUSTAL THICKENING IN THE SOUTHERN ANDES

The mid-temperature (~350-550°C) partial retention zone of the apatite U-Pb system offers an exciting window into mid- to upper-crustal processes such as exhumation, metamorphism, and hydrothermal activity. Recent studies characterizing trace and rare earth element behavior in apatite suggests that apatite geochemistry can be used to determine source rock lithology and to estimate whole rock geochemistry. In the southern Andes, widespread Cenozoic exhumation has overprinted low-temperature thermochronometers in the hinterland and fold-thrust belt. We present new detrital apatite U-Pb ages and trace and rare earth geochemistry from the uppermost Cretaceous-Paleogene Dorotea Formation and Eocene Man Aike Formation of the Magallanes-Austral foreland basin, southern Andes, to explore the utility of detrital apatite for tracking cryptic regional tectonic events.

Previous work on the same samples yields cosmopolitan zircon U-Pb age spectra (with peaks between 1200–80 Ma, and limited 70-40 Ma ages). Despite the diversity of zircon U-Pb dates, apatite U-Pb dates yield remarkably consistent ages, tightly clustered between ca. 110-85 Ma (²⁰⁷Pb discordia-based correction). Within the seemingly unimodal apatite dates, geochemical discriminators resolve the presence of at least two contemporaneous but geochemically distinct apatite populations, derived from primary igneous and thermally perturbed sources respectively. Tightly clustered apatite U-Pb ages temporally overlap a number of independent proxies for mid-Cretaceous crustal thickening including: (1) an increase in U/Th ratios in detrital zircon, (2) onset of coarse-clastic sedimentation in the Magallanes-Austral foreland basin, (3) structural restorations, and (4) an increase in Sr/Y and La/Yb from compiled whole rock geochemistry. We propose that the most likely reason for the lack of detrital apatite populations older than ca. 110 Ma is due to a regional thermal perturbation related to the onset of crustal thickening. If true, crustal thickening drove high heat flow and thermally reset pre-100 Ma apatite while crystallizing new igneous apatite during pluton emplacement. Finally, apatite show elevated La/Yb ratios at ca. 100 Ma, hinting at the potential to use apatite geochemistry and U-Pb ages to reconstruct changes in crustal thickness.