Amygdaloidal basalts are found in the Lower Devonian Upper Gaspe Limestones in the Gaspe Peninsula. These basalts were extruded on a distal outer shelf, as evidenced by field relationships, embedded fragments of metazoans, and high vacuole content. The lava flows were fractured and brecciated; the network of fractures allowed connections of some vesicles with seawater. Vesicles in the nonfractured basalt are filled by silicates, whereas those opened to seawater are filled by carbonates. The latter comprise, in stratigraphic order, geopetal micrite, botryoidal calcite, and coarse anhedral calcite. Petrographic attributes, including microdolomite inclusions and trace-element distribution in the botryoids, indicate former high-magnesium calcite mineralogy. The delta 18 O PDB (-9.2 per thousand to -10.0 per thousand and delta 13 C PBD (-3.0 per thousand to -5.3 per thousand ) ratios support botryoid precipitation from heated marine waters that were contaminated by magmatic CO 2 volatiles. High radiogenic Sr values originated from compaction waters expelled from the host succession. Submarine high-magnesium calcite agrees with the aragonite-inhibiting stage for the Early Devonian. The botryoid fabric should be the norm for early calcite in such an odd diagenetic setting.