Fluid flow characteristics recorded in continental eclogites of the Münchberg Massif

Eclogite-facies metamorphic veins provide records of fluid flow dynamics during high-pressure metamorphism. Here, we evaluate the inventory of quartz-rich metamorphic veins and segregations in Variscan eclogites from the Münchberg Massif (NE Bavaria) to investigate the fluid origin and the scale of fluid flow. These eclogites experienced peak P-T conditions around 3 GPa and 700°C, and a continental rather than oceanic mafic igneous protolith setting is proposed.

Isolated, mm-sized quartz pockets with euhedral high-pressure minerals are common in the Münchberg eclogites, but continuous veins that may have allowed focused fluid flow to beyond specimen scale are rare. Nevertheless, where such veins occur, they can contain eclogite assemblages including minerals such as omphacite, garnet, rutile, kyanite, and phengite. Oxygen isotope thermometry of quartz-garnet, quartz-phengite, and quartz-kyanite pairs yield temperatures around 700°C. δ18O values of quartz (+6.1 to +10.5‰) from almost all the veins are identical to predicted (from mass balance modelling at 700°C based on host rock δ18O values from +4.0 to +7.9‰) equilibrium δ18O values of host rock quartz.

The veins and pockets are mostly in equilibrium with their host eclogites regarding texture, mineralogy, mineral chemistry, and oxygen isotopes. They seem to represent former cavities filled by internally-derived fluids, whereas indications for fluid transport beyond specimen scale are rare. The abundance of isolated quartz pockets and scarcity of more extended veins suggest that rather low quantities of fluid were produced. This may be typical for continental eclogites, which are supposed to have experienced little pre-subduction hydration compared to oceanic eclogites.