The Ediacaran–Cambrian transition in the Cantabrian Zone (northern Spain): sub-Cambrian weathering, K-metasomatism and provenance of detrital series

Abstract

The Upper Ediacaran detrital succession in the Cantabrian Zone shows geochemical and mineralogical changes resulting from sub-Cambrian weathering during the Late Ediacaran worldwide sea-level fall. Relative to the unaltered rocks the altered ones show crosscutting rubefaction of varying thickness and remarkable increase in illite, K 2 O, Rb and Cs indicating K-metasomatism, and also depletion in MgO, CaO, Na 2 O, Be and Sr, but not in Zr, Nb, Y or Sc. The basal Cambrian siliciclastic rocks mostly consist of detritus derived from the Ediacaran materials, as demonstrated by the geochemical and Nd-isotope data ( ε Nd ( t ) ranges are −3.4 to −2.1 and −3.6 to −1.8, respectively). However, these geochemical features of the basal Cambrian change upwards to more evolved compositions with lower ε Nd ( t ) values (−4.9 to −5.8). This change coincides with the Ediacaran–Cambrian unconformity in the Central Iberian Zone, and the Ediacaran siliclastic rocks of this zone and their unaltered equivalents of the Cantabrian Zone share the same geochemical features. This geochemical homogeneity rules out a significant coeval juvenile contribution to the Upper Ediacaran series. Thus, the juvenile supply was from a geological setting different from that in which the Ediacaran series were finally deposited. Supplementary material: Details of sampling, sample location, analytical techniques, diffractograms, tables of analyses and figures are available at https://doi.org/10.6084/m9.figshare.c.2850016 .