A Sm-Nd isochron on pelites 1 Ga in excess of their depositional age and its possible significance

Details A Sm-Nd isochron on pelites 1 Ga in excess of their depositional age and its possible significance A Sm-Nd isochron on pelites 1 Ga in excess of their depositional age and its possible significance

Feb 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56..789n&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 56, Issue 2, pp.789-795

Computer Science

6

Scientific paper

Fine-grained Tremadocian (490 Ma) sediments of the Almaden syncline (Central Iberian Zone, Spain) were analyzed for Sm-Nd and Rb-Sr isotopes in order to get information on the crustal evolution of the Iberian Peninsula. These shallow-water platform sediments consist mainly of 2 M illite/ muscovite and quartz, plus kaolinite, chlorite and minor quantities of plagioclase. Neodymium contents fall into a range of 11-76 ppm. Nd (490 Ma ) range from +1 to -4 and 147 Sm / 144 Nd ratios from 0.1135-0.1504. The rocks yield an isochron age of 1524 ± 173 Ma (95% confidence level, Nd [ init ] = +7.1), more than three times older than the sedimentation age, but in line with the mean neodymium crustal residence age of the samples (1.51 Ga). A comparison of crustal residence ages calculated in a two-step model with those calculated in a single-step model indicate that syn- or post-sedimentary fractionation of 147 Sm / 144 Nd ratios is not very likely. Rather, the data seem to indicate an inherited isochron relationship reflecting the composition and mean age of the provenance. Thus, the Sm-Nd system was not disturbed during erosion, sedimentation, or diagenesis. A Rb-Sr errorchron of the Tremadocian samples indicates a postdepositional age of 335 ± 15 Ma. It probably reflects the main folding (between Lower Namurian and Upper Westphalian) or the deepest burial of the strata.

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