SELECTIVE DOLOMITIZATION BY SYNTAXIAL OVERGROWTH AROUND DETRITAL DOLOMITE NUCLEI: A CASE FROM THE JURASSIC OF THE LIGURIAN BRIANCONNAIS (LIGURIAN ALPS)

Dolomite crystals, occurring in Middle Jurassic dolostones and dolomitic limestones of the Ligurian Brianc¸onnais Domain (French–Italian Ligurian Alps), show irregular, complex zoning evidenced by cathodoluminescence (CL) and backscattered electron imaging. Comparable zoning patterns from other study areas have been attributed alternatively to recrystallization or dissolution–precipitation processes. However, none of these mechanisms can account for the range of observations from the Ligurian Brianc¸onnais Domain, which include: 1) the presence of multiple core types, each with different CL characteristics; 2) marked compositional differences between cores and surrounding rims (ferroan vs. nonferroan, Mg/Ca ratio); 3) a marked shape difference between cores with irregular outlines, and rims which progressively approach a welldeveloped rhombohedral habit. These features are most reasonably interpreted as dolomitic rims that formed as syntaxial overgrowths around detrital dolomite nuclei, which in turn originated from erosion of underlying Triassic rocks and triggered selective replacement of fine-grained calcareous sediment. Failure to recognize the exact nature of this type of dolomite crystal may lead to overestimating the degree of dolomitization and to overlooking possible detrital inputs in a basin otherwise supplied with pure allochemical sediments, with consequent loss of information on the tectonosedimentary evolution.