New U–Pb radiometric dates for K-bentonite horizons within the Lower Cambrian to Middle
Ordovician platform carbonates from the Precordillera terrane of NWArgentina provide further constraints on
models for the allochthonous or parautochthonous accretion of this terrane. Two K-bentonite layers from the
Talacasto section yield indistinguishable sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon
dates of 469.5 3.2 Ma and 470.1 3.3 Ma respectively. These are within uncertainty of the U–Pb SHRIMP
zircon date of 468.3 3.4 Ma for a porphyritic rhyolite from the Famatinian magmatic arc, Sierra de las
Planchadas, near Rio Chaschuil. Geochemical and isotope data also demonstrate the similarity of the Kbentonite
and Chaschuil rhyolite parent magmas. Thus, it is highly probable that the Famatinian arc volcanoes
provided the ash for the K-bentonite horizons, suggesting proximity to the Precordillera terrane during the
deposition of the Lower Cambrian to Middle Ordovician platform carbonates. This implication supports a
mid-Ordovician collision model, but could also be compatible with a parautochthonous model for docking of
the Precordillera terrane, by movement along the Pacific margin of Gondwana, rather than across the Iapetus
Ocean
