Provenance of Upper Precambrian-Lower Cambrian shales in the Central Iberian Zone, Spain: evidence from a chemical and isotopic study

Geochemical and isotopic (Sr, Nd, O) data are presented for Upper Precambrian and Lower Cambrian shale sequences from the Central Iberian Zone, Spain, in order to define geochemical discriminants between them, and to assess the nature of the source regions supplying these basins. Elemental data, especially Th/Sc ratios, coupled with εNdt values of ∼ −2.5 suggest that the Upper Precambrian rocks contain a greater proportion of a much younger, more mafic component than the Lower Cambrian sequence which has high Th/Sc and εNdt of ∼ −7.3. Coherent trends between elemental and O isotope data imply a homogeneous, recycled source as the dominant provenance for the Upper Precambrian shales, whereas a lack of such coherence for the Lower Cambrian sediments suggests a more heterogeneous provenance. The sedimentological, geochemical and isotopic data are consistent with the Upper Precambrian succession being derived ultimately from a polycyclic mixture of young (pre-Cadomian), mafic sources, and older (> 2 Ga) crust. In contrast, the Lower Cambrian sequence was mainly derived from old, more felsic crust which would have contributed increasingly greater proportions of material as the younger sources were progressively removed by erosion.