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 Prec ambrian and Lower Cambrian shale sequences from the Central Iberian Zo ne, 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 epsilon(Nd)(t) values of similar to-2.5 suggest that the Upper Precambrian rocks cont ain a greater proportion of a much younger, more mafic component than the Lower Cambrian sequence which has high Th/Sc and epsilon(Nd)(t) of similar to-7.3. Coherent trends between elemental and O isotope data imply a homogeneous, recycled source as the dominant provenance for th e Upper Precambrian shales, whereas a lack of such coherence for the L ower Cambrian sediments suggests a more heterogeneous provenance. The sedimentological, geochemical and isotopic data are consistent with th e Upper Precambrian succession being derived ultimately from a polycyc lic mixture of young (pre-Cadomian), mafic sources, and older (>2 Ga) crust. In contrast, the Lower Cambrian sequence was mainly derived fro m old, more felsic crust which would have contributed increasingly gre ater proportions of material as the younger sources were progressively removed by erosion.