Eclogitization of lower crustal granulites by fluid migration through shear zones

The granulite facies assemblages of the anorthositic rocks of the Bergen Arcs (stable at 800 -900°C and 10 kbar) have been transformed to eclogite facies assemblages (stable at 700 -750°C and 16-19 kbar) in the vicinity of Caledonian shear zones. This section of the root zone of the Caledonian mountain chain reveals a deep polymetamorphic crust where Precambrian granulites (mean density 3.02 g / c m 3) and Caledonian eclogites (mean density 3.19 g / c m ~) alternate on a scale of meters over a min imum area of 3 × 12 km. Detailed mapping of three localities shows that eclogites account for up to 30-45% of the rock volume. The stabilitization of the eclogite mineralogy is controlled by fluids penetrating these deep crustal shear zones. The eclogitization is independent of preexisting compositional variation in this anorthosite-norite complex. The Bergen Arcs example suggests that the amount of eclogite versus granulites in the lowermost crust is a function of deformation and fluid access, rather than being controlled by T, P and rock composition alone. These relationships may explain the gradual increase in seismic velocity observed in some deep crustal sections and also the complex reflection pattern obtained from the lowermost crust in many areas.