Pervasive calcite veins and cleavage dilation in low-grade metamorphic rocks as a marker of lower Jurassic rift-basin margins: A signature of microbial colonization

We report herein for the first time the occurrence of low-grade metamorphic rocks dilated by the growth of calcite veins induced by microbial communities inhabiting the cleavage planes. This process took place at the rock/sea-water interface in a rocky shore environment when the Hercynian basement rocks of the European-Iberian continental margin in Calabria (Italy) experienced stepwise flooding by a shallow tropical sea in the Early Jurassic, a process which was accompanied by synsedimentary extension, resulting in the birth of the Longobucco Basin. The veins exhibit a multiphase filling history, as, in an early phase, microstromatolites lining the walls of cleavage planes document the initial development of microbial biofilms. Subsequently, the growth of bands of radiaxial fibrous calcite enlarged the cavities through force of crystallization, also producing fissures at a high angle with respect to cleavage, which were in turn enlarged by crystal growth. Carbonate clumped isotope analysis indicates crystallization of calcite from sea water, at temperatures (T₄₇) ranging from c. 33 to c. 44 °C. The inferred palaeoenvironment is that of a rocky coastline, locally with tide pools, where the seawater lapped cliffs made of Palaeozoic metasandstone. Microbially induced mineralization was a very rapid process, as clasts of veined metamorphic rocks are found in only slightly younger Early Jurassic deposits, like the sub-reefal carbonate bodies that grew in the Pliensbachian attached to the Palaeozoic bedrock. The occurrence of low-grade metamorphic rocks bearing calcite veins can be mapped in the field for kilometers, following the high-angle unconformity that separates the shallow-water carbonates from the basement. This microbial overprint therefore is a marker of the margins of a rift basin, where the exposure of basement rock along steep submarine surfaces was the result of footwall unroofing.