Comparative study of the transition between very low-grade and low-grade metamorphism in siliciclastic and carbonate sediments: Early Cretaceous, Cameros basin (northern Spain)

The transition between very low-grade and low-grade metamorphism has been studied in the Urbion and Enciso Groups of the Late Jurassic-Early Cretaceous Cameros basin (NE Spain). The 'illite crystallinity' (IC) values do not appear to be controlled by burial depth but rather by: (1) XRD overlap with other phases (e.g. paragonite and mixed-layered muscovite-paragonite) in the siliciclastic samples; (2) the presence of carbonates, which delay illitization, resulting in an increase in the IC values, that becomes more pronounced as the P-T conditions decrease towards the diagenesis zone; and (3) the permeability of the sediments, which controls the circulation of metamorphic fluids and therefore the distribution of 'crystallinities' and mineral assemblages within the basin. The anchizone is represented by a narrow range of 'chlorite crystallinity' (ChC) values. Therefore, ChC is a less sensitive parameter than IC for estimating changes from diagenetic to low-grade metamorphic conditions. However, ChC can be a useful tool when there is a great heterogeneity of facies types, as it is not affected by the presence of carbonates.