Abstrakt: 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.
Abstrakt: The pink clays from the Tagus basin, Spain, were characterized by X-ray difraction (XRD),
infrared spectroscopy (IR), scanning electron microscopy (SEM), and transmission electron microscopy
(TEM). Chemical data were obtained by plasma emission spectroscopy and analytical electron microscopy
(AEM), and specific surface and cation-exchange capacity
were measured also. The data indicate that
these pink clays are primarily stevensite. This Mg-rich smectite is characterized by poor crystallinity, a
high degree of structural disorder, trioctahedral character (pure magnesian), a very low cation-exchange
capacity, a very small crystal size (which generates an abnormally high specific surface area), and a
deficiency of octahedral cations. On the basis of the very small crystal size, a large number of edge
dislocations, the lack of periodicity (turbostratic) in the structure, and a cellular (spherical) texture observed
by TEM, we consider this occurrence to be an early stage of crystallization. Unlike other precursor
clay materials described in the literature, this clay is not an alteration of volcanic ash, but it was generated
by precipitation from a Si- and Mg-saturated medium.
