Diagenesis-very low-grade metamorphism of clastic Cambrian and Ordovician sedimentary rocks in the Iberian Range (Spain)

Cambrian and Ordovician pelites and greywakes from the Iberian Range were studied using XRD, EMPA and AEM/TEM techniques. Illite and chlorite are the main phases in Cambrian <2 μm fractions. Illites show crystallinities (IC) from 0.28 to 0.23°2θ: the 2M1 is almost the only polytype; they do not have expandable layers, and they are heterogeneous in composition. Chlorites show two compositional types, both containing variable proportions of smectite-like layers. Clinochlores growing over bedding surfaces are also present. An evolutionary trend from anchizone to near the anchizone-epizone limit has been proposed for Cambrian samples. In the Ordovician <2 μm fractions, illites are associated with minor chlorites and kaolinites. The ICs range from 0.68-0.28°2θ; the 2M1 polytype dominates over 1M, and it is associated with illite-smectite mixed-layering (R3 ordering). An evolutionary trend from diagenesis to low grade anchizone has been inferred. Chemical analyses of coarser detrital micas indicate muscovite and phengite compositions. The compositional heterogeneity exhibited by authigenic and detrital phases suggests that chemical equilibrium was not attained.