K-Bentonites in the Argentine Precordillera Contemporaneous with Rhyolite Volcanism in the Famatinian Arc

FANNING, C.M.; PANKHURST, R.J.; RAPELA, C.W.; BALDO, E.G.; CASQUET, C.

JOURNAL OF THE GEOLOGICAL SOCIETY

The Geological Society of London

Año: 2004 vol. 161 p. 747 - 756

0016-7649

New U–Pb radiometric dates for K-bentonite horizons within the Early Cambrian to Middle Ordovician platform carbonates from the Precordillera terrane of NW Argentina provide further constraints on models for the allochthonous or para-autochthonous accretion of the this terrane. Two K-bentonite layers from the Talacasto section yield indistinguishable SHRIMP U–Pb 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 K-bentonite 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 Early Cambrian to Middle Ordovician platform carbonates. This implication appears to support a mid-Ordovician collision, but could also be compatible with a para-autochthonous model for docking of the Precordillera terrane, by movement along the Pacific margin of Gondwana, rather than across the Iapetus Ocean.     Keywords: U–Pb Zircon, SHRIMP, Ordovician, Terranes, Gondwana