Mass Transfer Related to Ductile Shear Zone Development in a Metagabbro

Abstract

Metagabbros from the Rouergue area (French Massif Central), show the synmetamorphic development of ductile shear zones under amphibolite facies conditions. Shearing results in three kinds of rocks (preserved, foliated, and mylonitic metagabbros) in which different stages of deformation and reaction progress can be studied. Geochemical methods and mass balance calculations have been applied to the analyses of the three types in order to study the mass transfer controlled by deformation and metamorphic processes. The mobility of some major and trace elements have been shown in the shear zones only. The mobility occured without significant changes in volume. The chemical changes are related to progressive deformation and amphibolitization. Mechanisms and limiting processes of mass transfer are discussed. The scale and the character of this mass transfer requires the general percolation of fluids through the metagabbros and the surrounding gneisses. The limiting process of mass transfer seems to be the reaction kinetics which are increased by deformation and grain size reduction in the shear zones and not in the undeformed rocks.