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Nd isotopic signatures of Ordovician sequences from Central Iberia: Unclear origin for older TDM along Paleozoic times

The Rodada Formation is one of the Ordovician sequences that crops out in the eastern domain of the Iberian Central System (Central Iberian Zone, CIZ). This formation is mainly composed of Middle Ordovician (Darriwillian) fine-grained shales. An isotopic study (Sr and Nd) in 15 shale samples of this formation reveals negative εNd(460) values, which indicate a high cortical affinity and relatively older isotopic signatures (TDM= 1.6-1.8 Ga). These Nd model ages prove to be older than those associated with Ediacaran and Cambrian sequences in southern CIZ. Altogether, TDM values represent an inverse chronological evolution compared to the ages of the stratigraphic sequences. The causes for this evolution are yet unclear, however it should be considered that, during the Middle Ordovician, the North African margin of Gondwana shares the characteristics of a passive margin affected by a pronounced extension. In this margin, the location of the CIZ is subject to an extensive debate. Even so, it is commonly accepted that its sedimentary sequences were deposited in an elongated section of the continental shelf adjacent to the Sahara Metacraton and Trans-Saharan Orogen. The evolution followed by the TDM record may be compatible with a progressive variation in the source areas and their shifting towards regions closer to the West African Craton. In addition, it may be also explained by an increased exposure or progressive exhumation of the cratonic domains of northern African continent. An extended post-glacial period may have contributed to both processes, either by the retreat of the ice sheet or by the subsequent isostatic compensation.

Details

Author
Inés Blázquez Blázquez1, José Manuel Fuenlabrada1, Francisco Javier Rubio Pascual1, Ricardo Arenas1
Institutionen
1Universidad Complutense de Madrid, Spain
Veranstaltung
GeoSaxonia 2024
Datum
2024
DOI
10.48380/rqhz-fn41