NEODYMIUM AND STRONTIUM ISOTOPIC DATING OF DIAGENESIS AND LOW-GRADE METAMORPHISM OF ARGILLACEOUS SEDIMENTS

The behaviour of the Rb-Sr and Sm-Nd isotopic systems with increasing degree of Hercynian metamorphic overprint was studied along a transect in Cambrian shales of northwestern Morocco. Clay fractions of <0.2 to 2-6 mum size from five samples were investigated, representing a rang e from non-metamorphic to epizonal metamorphic conditions. The samples were washed in cold 1 N HCl prior to digestion to separate soluble/ex changeable Rb, Sr, Sm, and Nd from amounts of these elements fixed in the crystallographic sites of the minerals and to analyze both compone nts separately. The results reveal that the Rb-Sr isotopic system is d ominated by Sr hosted by clay mineral phases (both detrital and authig enic illite and chlorite) and carbonate-hosted soluble Sr. Isotopic ho mogenization of Sr occurred during Hercynian metamorphism, yielding ag es between 309 and 349 Ma. The Sm-Nd isotopic system, on the other han d, is dominated by cogenetic apatite and Fe oxide/hydroxide, both havi ng high contents of leachable REEs. The leachates yield a Sm-Nd isochr on age of 523 +/- 72 Ma, indicating diagenetic equilibrium between apa tite and Fe-oxide/hydroxide. Fine-grained clay fractions of <0.2 mum s ize plot onto this reference line, suggesting isotopic equilibrium wit h the leachates. Size fractions >0.2 mum show inheritance of a detrita l Nd component. The study demonstrates that the diagenesis of the inve stigated argillaceous sediments can be dated by the Sm-Nd chronometer in authigenic cement phases. The isotopic system of these minerals (ap atite, Fe hydroxide/oxide) was homogenized during authigenic mineral g rowth in a sediment that was flushed by diagenetic fluids and had abun dant primary or secondary interconnected pore space. The Hercynian met amorphic over-print caused partial isotopic rehomogenization of the ad sorbed and clay-hosted portion of the Sr as well as of the carbonate-h osted Sr. The Sm-Nd system in the cement phases survived this metamorp hism. This results in decoupling of the two isotopic systems and allow s the dating of diagenesis on the one hand (Sm-Nd) and metamorphism on the other hand (Rb-Sr).