KAr systematics of clay-to-mica minerals in a multi-stage low-grade metamorphic evolution

The behavior of the Ar isotopic system of clay-to-mica fractions was studied in Cambrian greywacke samples along a transect of northwestern Morocco. These rocks were affected by a progressively increasing metamorphic overprint spanning from diagenetic to epizonal metamorphic conditions. KAr isotopic data of < 0.2–10-μm size fractions recovered from seven samples allowed to recognized three episodes of illite crystallization: (1) during an Upper Ordovician thermal event at ∼ 445 Ma which is characterized by an illite + chlorite mineral paragenesis in the northernmost part of the transect; (2) during a Carboniferous low-grade overprint at ∼ 305 Ma which is characterized by a similar mineral paragenesis; and (3) during a Triassic-to-Liassic hydrothermal activity at ∼ 205 Ma which generated a smectite-dominated paragenesis. es were examined, in combining X-ray mineralogic and KAr isotopic data, in terms of linear arrays in both isochron and Harper diagrams to differentiate true isochrons from mixing lines. It could also be shown that mixtures of two generations of micaceous minerals can induce different trends in the KAr dates depending whether the first generation of minerals was either or not affected by the event inducing the later generation.