Basement evolution of the Sierra de la Ventana fold belt: New evidence for Cambrian continental rifting along the southern margin of Gondwana

Journal of the Geological Society, Jul 2003 by Rapela, C W, Pankhurst, R J, Fanning, C M, Grecco, L E

The rocks of the Sierra de la Ventana Fold Belt consist of (1) a thick Palaeozoic sedimentary pile that constitutes most of the belt, and (2) a poorly exposed basement complex consisting of small outcrops (1-1.5 km^sup 2^) of acid igneous rocks located on the SW side of the orogen (Fig. 1).

The stratigraphy of the Palaeozoic sequences was described by Harrington (1947), who divided the clastic sedimentary pile into three groups, ranging in age from Late Cambrian? to Permian (Fig. 1). Siliciclastic marine sedimentary rocks of the Upper Cambrian?-Ordovician Curamalal Group (thickness c. 1150 m) dominate the lower part of the sequence. The coarse-grained basal clastic rocks of the Curamalal Group, known as the La Lola conglomerate (see cross-section, Fig. 1), contain pebbles of predominant quartzite and rare rhyolite. It has been assumed that the rhyolites represent the uppermost part of the basement complex (von Gosen et al. 1990). There are no fossils in the Curamalal Group, and its age has been assigned to the Cambrian-Silurian interval only on the basis of stratigraphic evidence (see Limarino et al. (1999) for references). The Devonian(?) Ventana Group (thickness c. 1500 m) is also dominated by marine siliciclastic sedimentary rocks. An Early Devonian age has been traditionally assigned to this group from brachiopods found in its uppermost part (Lolen Formation, Harrington 1980). However, the finding of Skolithos and new trace fossils in the Naposta Formation in the lower levels of the group suggests an Ordovician to Silurian age (Buggisch 1987). These mid-Palaeozoic formations are unconformably overlain by the Carboniferous-Permian Pillahuinco Group (thickness c. 2800 m) (Fig. 1). The sequence starts with Upper Carboniferous-Lower Permian diamictites associated with the Gondwana glaciations, followed by post-glacial marine petites (Harrington 1980). The latter sediments are covered by pelites and sandstones carrying marine invertebrate fossils and deltaic facies with a Glossopteris flora, which indicates an Early Permian age (see Limarino et al. (1999) for references). The sedimentary sequence culminates with sandstones and pelites with increasingly continental provenance, which also contain interlayered pyroclastic levels (Iniguez et al. 1988) that have been associated with the widespread Permian volcanism of northern Patagonia (Limarino et al. 1999).

Basement complex

The basement complex is dominated by: (1) granites (those of Cerro Colorado, Agua Bianca, Pan de Azucar-Cerro del Corral and San Mario being the largest); (2) subordinate bodies of rhyolite that are also considered an integral part of the basement, such as those that crop out at La Mascota, La Ermita and Cerro del Corral; (3) minor occurrences of paragneiss (Fig. 1). Gravimetric and magnetic studies indicate that the Cerro Colorado granite is subcircular in shape with a diameter of 12 km, whereas the Agua Bianca granite is 5 km wide and 25 km long north-south (Schillizzi & Kostadinoff 1985). Earlier K-Ar and Rb-Sr geochronological studies have been carried out on both granitic and rhyolitic rocks of the basement (Cingolani & Varela 1973; Varela & Cingolani 1976;Varela et al. 1990, and references therein). A summary of these results recalculated according to new decay constants by Varela et al. (1990) indicates a range of ages from Neoproterozoic to Devonian: 678 or - 30 Ma for the Cerro del Corral rhyolites (Rb-Sr, average of three samples assuming an initial ^sup 87^Sr/^sup 86^Sr ratio of 0.7090); 613 or - 30 Ma for the Pan de Azucar porphyritic dolerite (K-Ar); 594 or - 10 Ma for the Agua Bianca and Las Lomitas granites (Rb-Sr isochron, error at 1[sigma]); 487 or - 15 Ma for the Cerro Colorado granite (Rb-Sr isochron, MSWD = 20, error at 1[sigma]); 360 or - 21 Ma for the La Mascota-La Ermita rhyolites (Rb-Sr isochron, error at 1[sigma]). Palaeozoic ages of