Criterios geoquímicos de correlación y posible área fuente de las pelitas del Precámbrico superior-Cámbrico inferior de la Zona Centro-Ibérica (Macizo Ibérico, España)

Zusammenfassung

Upper Precambrian shales show geochemical characteristics (low CaO: 0,47; Al2O3/Na2O: 8,71; correlation Al2O3,-K2O: r=0,958) that are compatible with their derivation from a chemically mature source area, therefore excluding the direct contribution of andesitic/basaltic compositions to the sediments. The K2O, Rb, Sr, Ba, Be, La, Th and ∑REE and the ∑LREE/ ∑HREE ratios are higher in Lower Cambrian shales than in those of Precambrian age. This indicates an increased contribution of granitic compositions to the Lower Cambrian shales, at the same time that excludes a source area rich in basic igneous rocks. On the other hand, the low Th/Sc and high La/Th ratios indicate an important participation of mafic/ultramafic compositions in generating these sediments. These characteristics are considered as inherited by canibalistic recycling from previous sediments, and preserved at least since Lower Proterozoic times or even before. It is suggested that both groups of shales were deposited in a margin lacking significant vulcanism and their source area might have been the W Africa craton. Qualitative (positive correlations: SiO2-Zr, Al2O3,-K2O, Al2O3,-TiO2, AI2O3,-Fe2O3+MgO in Upper Precambrian shales; negative correlation: (La/Yb)N-Zr in Lower Cambrian shales) and quantitative (higher contents in large-ion lithophile elements above referred to; higher Rb/Zr, (La/Yb)N, (Gd/Yb)N, Th/Yb, La/Sc and Th/Sc ratios in Lower Cambrian shales) geochemical criteria are proposed as a guide for correlating the Upper Precambrian and Lower Cambrian sediments in other areas of the Central Iberian Zone.