U-Pb detrital ages on a tuffaceous sandstone sheet in the Vinchina Formation, La Rioja, Argentina: Deposition and exhumation implications

DÁVILA, F.M., COLLO, G., ASTINI, R.A., GEHRELS, G.

Jujuy

Congreso; Congreso Geológico Argentino; 2008

Between the High Andes and the basement-involved Sierras Pampeanas province, the Tertiary Vinchina basin exposes a superb, >10-km thick, alluvial synorogenic succession represented by the Vinchina and Toro Negro Formations. These units were correlated with coeval successions exposed in the Bermejo basin, further south, in the Argentine Precordillera (eg., Jordan et al., 1993) allowing to understand the along-strike evolution of the Andean foreland system (Jordan et al., 2001). Although the Vinchina basin fill was studied by diverse methods, its geochronology is still debated. Initially, Tabbutt (1986) proposed a zircon fission-track age of ~7.3 Ma for the lower member of the Vinchina Fm (uncertain stratigraphic position). More recently, Ciccioli et al. (2005) obtained an older age (~8.3 Ma) for the overlying Toro Negro Fm, using the whole-rock K-Ar method. Magnetostratigraphic studies, in turn (correlating Tabbutt ages), constrained the sedimentation of the whole basin between ~14.5 and ~3.4 Ma (Reynolds et al., 1990; Re and Barredo, 1993). Though methodologically the ages of Tabbutt (1986) on zircons are much more accurate than whole-rock K-Ar ages, the uncertain stratigraphic position of Tabbutt lowermost tuff hampers a correct correlation. An additional problem is the tuffaceous beds in the Vinchina basin show rarely primary volcaniclastic features. Thus, whole-rock or single zircons ages must be analyzed carefully. To avoid the detrital influence and test the proposed ages from different authors, we carried out a U-Pb geochronologic study on zircons (laser ablation) of the lowermost volcanogenic layer (~0.5-m thick) exposed in the lower section of the Vinchina Fm, in the Sierras de los Colorados (28°43’47.8’’ SL, 68°14’20.6’’ WL, 3790 masl), west La Rioja, Argentina. This not only may assist to constrain the depositional age but also may help define the origin and provenance of the tuffaceous level. The analyzed layer, composed of grayish and laminated tuffaceous sanstones, has a high percentage of primary volcaniclastic components (mainly formed by shard quartz, plagioclase, volcanic fragments, biotite, muscovite, and zircons, immerse in a poorly developed glassy matrix), and also shows reworking features and detrital contamination. The detrital ages range from ~1912.6 Ma to ~19.1 Ma (Proterozoic to Cenozoic, n=75, Fig. 1a), supporting an origin by stream reworking. Six distinct major populations are at 19 Ma, 288 Ma, 463 Ma, 504 Ma, 547 Ma and 1141 Ma. Minor peaks are recorded at 1464 Ma and 1842 Ma. The Proterozoic-Paleozoic zircons are rounded to subrounded, whereas Tertiary zircons are sharp and likely cogenetic with volcanism. It is important to note that the sample only supplied four 19-Ma zircons (Fig. 1b), and that another volcaniclastic horizons located up section in the Toro Negro Fm did not provide zircons. This is likely indicating that synchronous volcanism yielded a low-quantity of zircons to pyroclastic deposits. Although the youngest detrital age in a sedimentary rock is by definition a minimum age, the lack of