Low-grade metamorphism in contrasting geodynamic settings

Staff: R.E. Bevins with D. Robinson (University of Bristol), N. Rubinstein (Universidad de Buenos Aires), L. Aguirre (Universidad de Chile, Santiago)

The Lower Palaeozoic rock sequences of Wales have been affected by low-grade metamorphism. In the clay-dominated fine-grained sedimentary rocks the effects produced very subtle mineralogical changes, especially in the character and crystallinity of the clay minerals. In the chemically more variable volcanic rocks the original high temperature igneous minerals are replaced by hydrous calcium-aluminium silicate minerals, in particular pumpellyite, prehnite and epidote.

Evidence from the clay mineralogy and calcium-aluminium silicate minerals has helped to constrain the pressures and temperatures to which the Lower Palaeozoic rocks have been subjected. This allows comparisons to be made with other low-grade metamorphic regions world-wide in order to establish if the studies in Wales represent the norm, or whether they are anomalous. What has been demonstrated convincingly is that the metamorphism in Wales is linked to crustal extension, and this model has been applied to explain the origins of metamorphism in the Andean mountain chain in Chile, and also the metamorphic patterns seen in the Precordillera of Argentina. Research is now focussing on the extent to which it is possible to identify the extent to which chemical equilibrium has occurred in rocks at these low grades of metamorphism. This is demonstrated in part by the systematic partitioning of elements between different metamorphic minerals and requires detailed analysis of the chemistry of the minerals, determined by electron microprobe methods.