The role of microcracking and grain-boundary dilation during retrograde reactions

Abstract

Abstract Under conditions of sub-greenschist and low greenschist facies retrogression, both intragranular microcracking and grain-boundary dilatancy are important processes for facilitating fluid access into domains between fractures in metamorphic rocks. Both processes play a crucial role in promoting retrogression of high-temperature minerals. In a quartz-rich schist from Norway, garnet retrogression is clearly linked to microcracking in the vicinity of a minor fracture. Intense microcracking of quartz extends up to 12 mm from the fracture, and permits fluid infiltration. This causes extensive (70–100%) garnet retrogression to chlorite. At distances >16 mm from the fracture there is no sign of fluid infiltration and garnets are entirely unaltered. The spatial distribution of grain-boundary reaction products in a scapolite-dominated rock from Scotland provides evidence for the significant role played by transient grain-boundary dilatancy in promoting grain-scale fluid access and resultant retrogression. By applying distinct element modelling (UDEC code) the geometrical differences in distribution of grain-boundary reaction products are directly related to subtle differences in the magnitude and orientation of applied stresses and patterns of transient grain-boundary dilatancy.