Subtle Zones Research Articles

Metamorphic studies: research in progress

A Metamorphic Studies Group meeting on ‘Metamorphic Studies: Research in Progress’ and the seventh Annual General Meeting of the Group were held on 9 March 1988. It was well attended, and 15 papers were presented on a range of metamorphic topics, including an invited contribution from J. W. Valley (University of Wisconsin, USA).Four poster contributions were also presented. The meeting was organized by G. T. R. Droop. The morning session concentrated on techniques and low-grade regional metamorphic topics. The first talk was given by G. E. Lloyd who, in a joint presentation with B. W. D. Yardley, promoted the application of cathodoluminescence microscopy to the study of metamorphic rocks. The technique is particularly suited to carbonate minerals, and its usefulness was demonstrated with reference to marble samples from Connemara in which subtle zoning and replacement textures, invisible by conventional light microscopy, could be observed. The authors stressed the paucity of information on the applicability of the technique to metamorphic silicates and urged petrologists to try it their own samples. In the first of two papers on very low-grade metamorphism, D. Robinson (jointly with R. E. Bevins) presented a new model in which sub-greenschist metamorphism develops in extensional settings. Building published thermal models of extensional sedimentary basins, the authors argued that this type of metamorphism should be characterized by pre-tectonic peak assemblages of high T/P facies and ‘anticlockwise’ P–T paths. Quoted examples of low–grade terranes which may have formed in this way included the Welsh Basin and some