Krustales Recycling am paläozoischen Kontinentalrand Südamerikas mittels U-Pb-Alter und Hf-Isotopenzusammensetzungen detritischer Zirkone
Final Report Abstract
The south-central Proto-Andes at 22-25°S was part of an active continental margin of west Gondwana almost throughout the complete Palaeozoic era. Only the Silurian to Devonian time was marked by tectonic quiescence. It can be assumed that the plate-tectonic conditions and its changes had an immense impact on composition, transfer, and deposition of siliciclastic detritus. Furthermore, juvenile mantle input is assumed to take place along active continental margins. Nevertheless Palaeozoic juvenile material barely has been demonstrated before. Therefore, we analysed detrital zircons for their morphology, U-Pb ages and Hf-isotope composition. We traced variations in source areas during Cambrian to Permian time and we compared apparent changes in zircon transportation paths as an effect of variations in the tectonic setting. We also analysed the detrital zircons to trace juvenile input during the complete Palaeozoic era. Throughout Palaeozoic time the zircon populations are dominated by subangular to subrounded grains of Cambrian to Ordovician age, which can be related to the Pampean and Famatinian orogenies. Pampean material in combination with juvenile Hf-isotope signatures of Grenvillian-age zircons indicate further transport from the Sierras Pampeanas in the south. Only the Silurian is marked by a different transportation system, which encompassed the Río de la Plata craton as well as the Eastern Sierras Pampeanas. The present-day location of the Silurian strata east of the main belt of Famatinian arc plutons indicates that the variation in provenance is due to the geotectonic position of the basins rather than due to true variations in transportation paths. We conclude that the zircon signature could not detect variations in tectonic setting during the Palaeozoic because the Famatinian arc remained the main source throughout this time. Hence, zircon investigations will not necessarily be a guide for the tectonic setting. Furthermore, erroneous interpretations due to variations in zircon age spectra can be avoided if zircon data are combined with other geological information, such as sedimentary facies or the location in relation to potential positive areas. The only major cust-forming events we recognized from the Hf-isotope data are the Precambrian Transamazonian and Grenvillian orogenies. This is indicated by the main groups of model ages that encompass these and older ages. The trend towards older ages can be explained by magma mixing. No major crust was formed during Palaeozoic time as indicated by mainly negative hafnium epsilon values for the Palaeozoic zircons. A similarity in model ages for Palaeozoic and Grenvillian-age zircons reveals a common origin for the two age groupings. The younger zircons crystallized from remelted material of the Grenvillian-age orogen. Instead of juvenile addition, the Hfepsilon mixing trends and model ages probably reflect times of major crustal thickening along the convergent margin.
Publications
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(2009): Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America, Earth-Science Reviews 97, 227-253
Bahlburg, H., Vervoort, J. D., Du Frane, S. A., Bock, B., Augustsson, C. & Reimann, R. C.
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(2011): Detrital quartz and zircon combined: the production of mature sand with short transportation paths along the Cambrian west Gondwana margin, northwestern Argentina, Journal of Sedimentary Research 81, 284-298
Augustsson, C., Rüsing, T., Adams, C. J., Chmiel, H., Kocabayoğlu, M., Büld, M., Zimmermann, U., Berndt, J. & Kooijman, E.
