Geochemical characterization of the Río Bonete mafic rocks, northernmost Argentine Precordillera: implications for the Early Paleozoic evolution of the western margin of Gondwana

MARTINA, F.; ASTINI, R.A.; KAY, S.M.

Córdoba

Congreso; Congreso Geológico Argentino; 2014

Asociación Geológica Argentina

The Neoproterozoic-Early Paleozoic mafic and ultramafic rocks of western Argentine Precordillera are usually interpreted as suture zones between the Precordillera and Chilenia terranes (Ramos et al., 1986; Astini et al., 1995; Davis et al., 1999). The Río Bonete region constitutes the northernmost exposure where mafic rocks are present both in the strongly foliated metamorphic basement and in the Late Ordovician volcano-sedimentary cover (Martina and Astini, 2009). Ordovician rocks are undoubtedly the most studied (e.g., Kay et al., 1984; Fauque and Villar, 2003), but little is known about the mafic igneous rocks from the basement. Here we present a new set of geochemical data and a broad review for all known Neoproterozoic to Early Paleozoic mafic rocks in NW Argentina in order to determine their origin, evolution and possible stratigraphic correlations. Geochemistry shows they are tholeiitic E-MORBs with high TiO2, Nb, Th and low MgO, Ni and Cr concentrations. The FeO/MgO ratios range from 1.42 to 3.33, which are higher than typical N-MORBs (1-1.5). The total alkali content is variable, but basement mafic rocks usually have higher K2O concentrations. Large ion lithophile elements (LILE) display random behaviors due to the intense hydrothermal alteration that affected these units. As such, interpretations are only based on more immobile high field strength elements (HFSE). Chondrite-normalized rare earth elements (REE) plots exhibit light REE enriched patterns with (Sm/Yb)N > 2, which are different from N-MORBs. In the Th/Yb vs Nb/Yb diagram of Pearce (2008), the Río Bonete mafic rocks plot within the mantle array near the E-MORB field with the basement samples shifted toward the alkaline OIB edge. These data reveal the oceanic character of these rocks and the absence of major crustal contamination. Few differences between the analyzed samples are also observed in the TiO2/Yb vs Th/Yb diagram (Pearce, 2008) where the basement mafic rocks plot toward the alkaline OIB field suggesting a deeper mantle source. Compared with typical volcanic-arc basalts, the Río Bonete mafic rocks have higher TiO2 concentrations and lower LILE/HFSE ratios. Furthermore, none of the analyzed samples show the typical negative Nb-Ta anomalies that characterize subduction related basalts. Compared with other E-MORB type basalts from modern tectonic settings, the studied mafic rocks show great similarities with the enriched oceanic basalts from southern Mid-Atlantic Ridge (MAR) and Iceland. The Late Ordovician basalts may correlate with synchronous rocks exposed along western Precordillera (e.g., González Menendez et al., 2013; Boedo et al., 2013), as previously suggested. On the other hand, the basement mafic rocks may alternatively be compared with 1) the Neoproterozoic E-MORB rocks tectonically juxtaposed within the southwest Precordillera described by Davis (1999) or 2) with mafic rocks from the basement of Sierra de Umango and Cerro Valdivia in the Western Sierras Pampeanas. Both alternatives may be reconciliated within the allochthonous model for the Precordillera terrane, with the basement mafic suite providing a potential record of the Blue Ridge rifting event (Thomas, 2011) within the present Precordillera. An interesting comparison with the Neoproterozoic Catoctin volcanics in the central Appalachians, which show similar geochemical compositions including slightly enriched light REE and HFSE concentrations, low MgO contents and low Nb/Zr and Nb/Y ratios (Badger and Sinha, 2004), supports this interpretation.