Ultra High Pressure and Temperature Metamorphism
"Understanding a metapelite is like trying to have an in-depth life conversation in a club.
Too much going on to understand anything one thing in much detail.
Metabasic rocks are like having going to a quiet country pub.
Not too much going on, but good beer, good friends, and deep conversations."
The Western Gneiss Region of Norway serves as a perfect test area to understand how ultra-high pressure rocks are able to return from depth.
My group and I use geochemical evidence contained in the breakdown microstructures of high pressure minerals, such as omphacite, which act as a tape recorder of the pressure and temperature conditions these rocks experienced on their return journey to the surface.
Omphacite breaking down to pyroxene, plagioclase, and minor amphibole.
We can use these three minerals to calculate the P/T path of the symplectite as it grew.
For a much more thorough resource on omphacite symplectites please visit Dave Waters' wonderful website.
The Lewisian Complex in Scotland contains the oldest rocks in the UK, but more importantly the ages of these rocks span the time period where plate tectonics as we know it began.
As such these rocks have a lot to tell us about how the Earth worked in the Archean, and how and when modern style subduction and plate tectonics began.
Unfortunately these rocks are grizzled and worn and have experienced multiple metamorphic overprints. It is up to us microstructural clues to see past the scars, and try to build a picture of what these rocks looked like in their youth.
QEMSCAN image of the breakdown microstructure of
garnet (dark-red) to plagioclase (light blue), orthopyroxene (purple),
and oxides (light-red and orange).
We can use exchange thermometers to calculate the temperature
at which this breakdown happened,
as well as recombining the breakdown minerals to calculate the peak P/T.
For a much more thorough resource on granulites please visit Dave Waters' webpage.