U-PB detritical ages on a tuffaceous sandstone sheet in the Vinchina Formation, La Rioja: Deposition and exhumation.

DÁVILA, F.M.; COLLO, G.; NOBILE, JULIETA CAROLINA; ASTINI, R. A; GEHRELS, G.

Jujuy

Congreso; Congreso Geológico Argentino; 2008

&amp;amp;amp;amp;amp;lt;!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-1610611985 1107304683 0 0 159 0;} @font-face {font-family:Calibri; panose-1:2 15 5 2 2 2 4 3 2 4; mso-font-charset:0; mso-generic-font-family:swiss; mso-font-pitch:variable; mso-font-signature:-1610611985 1073750139 0 0 159 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin-top:0mm; margin-right:0mm; margin-bottom:10.0pt; margin-left:0mm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Calibri; mso-fareast-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Calibri; mso-fareast-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;} .MsoPapDefault {mso-style-type:export-only; margin-bottom:10.0pt; line-height:115%;} @page Section1 {size:595.3pt 841.9pt; margin:70.85pt 30.0mm 70.85pt 30.0mm; mso-header-margin:35.4pt; mso-footer-margin:35.4pt; mso-paper-source:0;} div.Section1 {page:Section1;} --&amp;amp;amp;amp;amp;gt; 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 Formation (uncertain stratigraphic position). More recently, Ciccioli et al. (2005) obtained an older age (~8.3 Ma) for the overlying Toro Negro Formation, 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 analyzedcarefully.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 Formation, 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 onlysupplied four 19-Ma zircons (Fig. 1b), and that another volcaniclastic horizons located up section in the Toro Negro Formation 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 <19-Ma grains (given that volcanism has been very active since Early Miocene to Pliocene times in this region) together with their euhedral shapes indicate the ca. 19 Ma (Early Miocene) peak is a depositional age. This is consistent with magnetostratigraphic studies on correlative sections (Reynolds et al. 1990), which constrained the base of the Vinchina Formation at ~18 Ma.The lack of Mesozoic and Early Tertiary detrital zircons suggests that Choiyoi (Triassic) and the earliest Andean (Paleogene) records exposed along the main cordillera were not sources for the lowermost Vinchina Formation. Although paleocurrent studies show general paleoslopes to the east, given the detrital ages we cannot discard that the playa-lake assemblage in the lower section of the Vinchina Formation (from where the tuffaceous sandstone was collected) has different sources, somelike consistent with older detrital ages in the Precordillera and Sierras Pampeanas. Alternatively, accepting regional paleocurrents are toward the east, as proposed by Ramos (1970) and Tripaldi et al. (2001), detrital ages demonstrate the Triassic to Paleogene interval was not unroofed along the High Cordillera, although this seems in contradiction with thermochronological analysis in Chile (eg., Nalpas et al. 2005) that show three major cooling ages (Mid-Cretaceous to Paleogene in the coast, Eocene in the Central Valley region and Oligo-Miocene close to the Argentina-Chile border). Yet, according to our results, the exposition of Mesozoic to Paleogene rocks had to occur <19 Ma, after deposition of the analyzed tuffaceous sandstone, and likely during the deposition of the Mio-Pliocene coarse conglomerates of the overlying Toro Negro Formation. A third alternative, is a source area located to the north, in the southern Puna plateau, as previously suggested by Coughlin (2000) and Carrapa et al. (2005) by detrital apatite fission track analysis. Our detrital zircon ages are consistent with such an interpretation.