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

U-Pb geochronology

U-Pb dating was carried out using a sensitive high-resolution ion microprobe (SHRIMP II) at The Australian National University, Canberra, following the procedures of Williams (1998). Zircons were hand-picked from mineral concentrates, mounted in epoxy resin together with chips of a reference zircon (AS-3), and polished. Cathodoluminescence images were used to target the magmatic rims and tips of euhedral grains. Analysis spots, mostly within the well-zoned ends of grains, were chosen to avoid cracks and inclusions. Data for the SHRIMP analyses were processed using SQUID (Ludwig 2001) and Isoplot/Ex (Ludwig 1999), and ages were calculated from the ^sup 206^Pb/^sup 238^U ratios after correction for the appropriate composition of common Pb, based on the measured ^sup 207^Pb. Analytical data can be obtained from the Society Library or the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, UK as Supplementary Publication No. SUP. 18185 (3 pages). They are also available online at http://www.geolsoc.org.uk/SUP18185. Errors on the final ages are reported as 95% confidence limits.

All granitic and rhyolitic samples from the basement complex analysed in this work exhibit, to differing degrees, the physical effects of the Late Palaeozoic deformation (von Gosen et al. 1990). This includes kinked and broken plagioclase, K-feldspar and muscovite replaced by sericite along S^sub 1^ planes, epidote-filled extension veins and lattice deformation of quartz visible as undulose extinction, followed by recrystallization along grain boundaries. Sampling was carried out in the least deformed sectors of the various units, where the original fabric and mineralogy of the igneous units are well preserved. An exception is the strongly deformed Cerro del Corral area (Fig. 1), where the local granitic and rhyolitic rocks have a mylonitic texture, with recrystallization of quartz resulting in a fine-grained matrix along relict crystals, and newly formed sericite and albite.

Samples were analysed from the La Ermita rhyolite, and the San Mario, Cerro Colorado and Cerro del Corral granites, and the results are shown in Tera-Wasserburg diagrams (Fig. 7). Sixteen grains of the Cerro del Corral granite were analysed, and 15 of them define a precise Neoproterozoic age of 607.0 or - 5.3 Ma (Fig. 7a). A grain with an older age and a large error (reflecting inheritance?) was not used for the age calculation. From the 16 analysed grains of the Cerro Colorado granite, two show Neoproterozoic inheritance at 640 and 710 Ma (Fig. 7b); the remaining 14 grains define an Early Cambrian age of 531.1 or - 4.1 Ma. Potentially inherited zircon cores of the same sample (SLV001), analysed using the less precise SHRIMP I ion microprobe, produced ages similar to the rims. Twelve grains of the San Mario granite also gave an Early Cambrian age of 524.3 or - 5.3 Ma, with inheritance at 550-580 Ma and Pb loss (one grain) at 495 Ma (Fig. 7c). It was not possible to separate zircon from samples of the Agua Blanca granite; for the purpose of this paper it is assumed that this has the same age as the fluorite-bearing Cerro Colorado granite, with which it shares many geochemical characteristics (see above). Finally, 19 weakly zoned zircons from a sample of the La Ermita rhyolite produced a Mid-Cambrian age of 509.0 or - 5.3 Ma (Fig. 7d).