ZIRCON AND APATITE U-PB AND (U-TH)/HE GEO- AND THERMOCHRONOMETRY OF THE CRYSTALLINE BASEMENT IN THE CHICXULUB’S PEAK RING STRUCTURE (IODP EXP. 364)

IODP-ICDP Expedition 364 drilled the peak ring of the K-Pg Chicxulub impact structure and recovered 580 m of shocked granitic basement. We used zircon U-Pb depth-profile LA-ICP-MS analysis to constrain the pre-impact tectono-magmatic evolution of the impact site at the northern edge of the Yucantan/Maya block, while apatite U-Pb depth-profile LA-ICP-MS and zircon and apatite (U-Th)/He thermochronometry were employed to elucidate the thermal evolution of the granitic basement in the NW portion of the Chicxulub peak ring. These plutonic rocks yielded ~320-350 Ma zircon U-Pb crystallization ages with evidence for earliest Paleozoic inherited zircon cores and Permian zircon rims associated with younger E Mexican arc magmatism. These data document Carboniferous arc magmatism, associated with southward Rheic ocean subduction beneath Yucatan, intruding into peri-Gondwanan Maya block basement (~540 Ma). Detailed LA-ICP-MS depth profiling also revealed that only high-U metamict zircon domains were affected by partial or complete Pb loss attributable to the K-Pg Chicxulub impact. In contrast, apatite U-Pb depth-profiling ages only yielded late Carboniferous ages in response to rapid exhumation associated with post-magmatic cooling or Ouchita orogenic uplift. The apatite U-Pb ages are not reset by the impact and clearly show that the bulk of the felsic rocks in the peak ring were not heated >400C. Zircon (U-Th)/He data for the drilled basement section mostly gave earliest Paleocene ages, clustering around 58-66 Ma, demonstrating impact resetting of the ZHe ages and possible minor age rejuvenation due to post-impact hydrothermal heating. He diffusion experiments on zircon with variable degrees of impact shock microstructures showed ZHe kinetics similar to un-shocked zircons, with the exception of zircon with granular textures. Apatite (U-Th)/He ages range from 15 to 30 Ma and are mostly younger than the impact and likely (partially) reset in response to younger tectonic and deposition events in S Mexico. Together these zircon and apatite U-Pb and (U-Th)/He data from the felsic basement of the Chicxulub in the peak ring allow the reconstruction of its pre-impact tectono-magmatic and thermal evolution, the estimation of the maximum heating during impact (<400C), and post-impact thermal history reconstruction.