K-AR SYSTEMATICS OF CLAY-TO-MICA MINERALS IN A MULTISTAGE LOW-GRADE METAMORPHIC EVOLUTION

The behavior of the Ar isotopic system of clay-to-mica fractions was s tudied in Cambrian greywacke samples along a transect of northwestern Morocco. These rocks were affected by a progressively increasing metam orphic overprint spanning from diagenetic to epizonal metamorphic cond itions. K-Ar isotopic data of < 0.2-10-mu m size fractions recovered f rom seven samples allowed to recognize three episodes of illite crysta llization: (1) during an Upper Ordovician thermal event at similar to 445 Ma which is characterized by an illite + chlorite mineral paragene sis in the northernmost part of the transect; (2) during a Carbonifero us low-grade overprint at similar to 305 Ma which is characterized by a similar mineral paragenesis; and (3) during a Triassic-to-Liassic hy drothermal activity at similar to 205 Ma which generated a smectite-do minated paragenesis. The ages were examined, in combining X-ray minera logic and K-Ar isotopic data, in terms of linear arrays in both isochr on and Harper diagrams to differentiate true isochrons from mixing lin es. It could also be shown that mixtures of two generations of micaceo us minerals can induce different trends in the K-Ar dates depending wh ether the first generation of minerals was either or not affected by t he event inducing the later generation.