Título: The history of a continent from U-Pb ages of zircons from Orinoco River sand and Sm-Nd isotopes in Orinoco Basin river sediments
Autor: Goldstein, S. L.; Arndt, N. T.; Stallard, R. F.
Fuente: Chemical Geology vol. 139 no. 1-4; p. 271-286
Editor: Hawkesworth, Chris; Arndt, N. T.
Evento: Goldschmidt meeting
Lugar y Fecha del Evento: Heidelberg, Federal Republic of Germany, - March 31-April 4, 1996
Resumen: We report SHRIMP U-Pb ages of 49 zircons from a sand sample from the lower Orinoco River, Venezuela, and Nd model ages of the fine sediment load from the main river and tributaries. The U-Pb ages reflect individual magmatic or metamorphic events, the Sm-Nd model ages reflect average crustal-residence ages of the sediment sources. Together they allow delineation of the crust-formation history of the basement precursors of the sediments. The U-Pb ages range from 2.83 to 0.15 Ga, and most are concordant or nearly so. Discrete age groupings occur at differs from 2.8, differs from 2.1, and differs from 1.1 Ga. The oldest group contains only three samples but is isolated from its closest neighbors by a differs from 600 Ma age gap. Larger age groupings at differs from 2.1 and differs from 1.1 Ga make up about a third and a quarter of the total number of analyses, respectively. The remaining analyses scatter along concordia, and most are younger than 1.6 Ga. The differs from 2.8 and differs from 2.1 Ga ages correspond to periods of crust formation of the Imataca and Trans-Amazonian provinces of the Guyana Shield, respectively, and record intervals of short but intensive continental growth. These ages coincide with differs from 2.9 and differs from 2.1 Ga Nd model ages of sediments from tributaries draining the Archean and Proterozoic provinces of the Guyana Shield, respectively, indicating that the U-Pb ages record the geological history of the crystalline basement of the Orinoco basin. Zircons with ages corresponding to the major orogenies of the North Atlantic continents (the Superior at differs from 2.7 Ga and Hudsonian at 1.7-1.9 Ga) were not found in the Orinoco sample. The age distribution may indicate that South and North America were separated throughout their history. Nd model ages of sediments from the lower Orinoco River and Andean tributaries are differs from 1.9 Ga, broadly within the range displayed by major rivers and dusts. This age does not coincide with known thermal events in the region and reflects mixing of sources with different crust-formation ages. The igneous and metamorphic history of these sources, as recorded by the detrital zircons, is that of the Orinoco basin basement. This implies that, despite evidence of fast sedimentary recycling, global similarities in Nd crustal-residence ages, and the probability of cross-continent mixing through continental drift, the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin. The global semblance in Nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large-scale sampling of the continents, which are heterogeneous in age on smaller regional scales. A large portion of the continental crust in the Orinoco basin formed during the Trans-Amazonian orogeny at 2.0-2.1 Ga, and smaller portions formed both earlier, at differs from 2.8 Ga, and later, after 1.6 Ga. These observations, which are consistent with the relative sizes of crustal age provinces in the Orinoco basin, indicate that sediments from the lower Orinoco and Andean tributaries contain 25-35% of material added to the crust since Trans-Amazonian times. Nd model ages of these sediments underestimate the average crust-formation age of the basement of the Orinoco basin by only about 10%. If this relationship holds in other river basins, then Nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is differs from 1.9-2 Ga.
Enviar Comentarios |
Bibliografía Geológica de Venezuela
Código Geológico de Venezuela
© PDVSA-Intevep, 1997