Felsic segregation during crystallization of a subaqueous lava field (ODP-IODP Site 1256, East Pacific Rise) : Inferences from structure and petrography

Massive basalt flows have been mainly observed and studied in subaerial environments, but data collected during sonar surveys and drilling cruises in ocean highlighted that thick lava fields (up to 70 m in thickness) are widespread in the oceanic crust produced along fast spreading ridges. Most of the thickest lava flows include felsic differentiates consisting of Na-plagioclase + quartz. Drilling at ODP-IODP Site 1256 encountered a >30 m and >70 m-tick massive lava flow near the top of the basement in Holes 1256C and D respectively. Rare (1 to 3%) late magmatic veins (LMVs) and late magmatic domains (LMDs) of felsic material occur within this very thick lava flow. LMVs and LMDs occur in a range of different orientations, attitudes and textures and cut the basalt magmatic assemblage inducing a deformation ranging from brittle to ductile. Fine-scale structural, microstructural, and petrographic analyses from the giant lava flow suggest that segregation and migration of felsic melt through host basalt were strictly related to the cooling and crystallization of the lava flow, which represents a single stage magmatic event.