NEODYMIUM-GRAPTOLITE STRATIGRAPHY OF ORDOVICIAN SHALES FROM THE PRECORDILLERA TERRANE, ARGENTINA: LAURENTIAN OR GONDWANAN SOURCE?

The provenance of Ordovician foredeep (?) clastic sequences deposited within and around the periphery of the Precordillera terrane has implications for the rifting, drifting and docking history of this far-traveled microcontinent. An origin in the North American Ouachita embayment is now widely accepted, but competing models disagree on the timing and nature of the rifting event(s), duration of drift, and ensuing collisional events that led to the suturing of this Laurentian terrane to Gondwana. Our sampling is designed to address this problem by comparing graptolite age-calibrated neodymium isotopic variations over a wide geographic area, including the western and eastern tectofacies of the Precordillera, and potential source terranes including the Famatina arc and the Cordillera Oriental. The provenance of Ordovician shales from the Ouachita and southern Appalachian Laurentian margin has already been characterized by this method. Preliminary data (12 samples) from the Precordillera show that Ordovician graptolitic shales of the eastern tectofacies (n=8), deposited upon the Cambro-Ordovician platform carbonate succession (San Juan Formation), are remarkably homogeneous in composition (e_Nd=-11 ± 0.5 present; -6.5 ± 0.5 initial). This includes the Middle to Upper Ordovician sequence at Don Braulio Creek consisting of the Gualcamayo, La Cantera, and Don Braulio formations (D.murchisonito upper N.gracilis). A similar section at La Chilca shows slightly more variation. Such homogeneity would appear to favor an orogenic source (e.g., Famatina Range) rather than localized sources. In contrast, late Middle and Upper Ordovician graptolitic shales (N.gracilisand younger) of the western tectofacies, consisting of the Yerba Loca and Alcaparossa formations (n=4), show greater variation (e _Nd=-8 to -12 present; -4 to -8 initial), indicating that less well-mixed material was entering the western basin (trench foredeep assemblage?). This material evidently does not represent direct distal deposits of the orogenic sources supplying sediment to the eastern part of the Precordillera, nor does it match the more homogeneous composition of late Middle and Upper Ordovician shales of the Laurentian margin. The most likely source was an approaching arc of continental affinity (Chilenia terrane).