The transition from diagenesis to metamorphism in the past 20 years ? providing a different perspective on the metamorphic process

Abstract

The last 20 years have seen major re-appraisal in the understanding of metamorphic processes operating at very low-grade metamorphic conditions. Identification of the increase in crystallite size as the cause of change in the illite crystallinty index was a major step on the road to consideration of processes in terms of reaction progress. In addition, the analysis of the ubiquitous layer-silicate minerals in terms of reaction progress has provided new insights into documenting the metamorphic process and identifying the mainly kinetic controlling factors. This style of metamorphic process provides a contrast and different perspective to the ``normal' metamorphic style at higher grades. Petrogenetic grid analysis, and mineral projections of VLGM assemblages in metabasites have rationalised the documentation of the relationships between whole rock and P-T controls on mineral distributions and facies designation. This work has shown that recognition of the prehnite-pumpellyite, prehnite-actinolite and pumpellyite-actinolite facies directly overlap in P-T space. As such, to use these facies to identify differing P-T conditions is unjustified and it is better to use sub-greenschist facies if it wished to designate the P-T conditions between the zeolite, blueschist and greenschist facies. New methods of dating utilising highly characterized, neoformed minerals (e.g. illite and monazite) are now able to provide precise ages of VLGM thermal events. This, coupled with the new understanding of very low grade metamorphic processes, has great potential to constrain timings of discrete orogenic events and rates of tectonic processes in regions peripheral to main areas of orogenic activity.