High and ultra-high pressure terranes are a common feature of Phanerozoic mountain belts, and their formation and preservation may be regarded as a normal part of plate tectonics.
Research up until now has tended to concentrate on the high pressure rocks themselves, aiming to answer questions such as "what were the rocks before they were metamorphosed?", "what pressure and temperature conditions did they form under?" and "when did they form?".
Recent data from high presure regions such as the Alps and the Himalaya suggest that these terrains exhumed and cooled very quickly after their formation. In other regions, such as Norway, available data suggests slower exhumation and cooling.
The aim for this research project is to date part of the decompression path using Ar-Ar muscovite/phengite geochronology. The relatively low closure temperature for Ar in white micas means that we should be able to fill in gaps in the low temperature part of the retrogression path. In this way we should be able to map out whether juxtaposed high and low grade units share part of a common tectonic history, when they were juxtaposed and how long they took too cool from their peak temperature.
In Oman, for example, high pressure eclogites are exposed alongside slightly lower pressure, similar temperature, garnet blueschists. This high-grade package was exhumed beneath, and juxtaposed against, an epidote-blueschist grade terrane, after which the entire package was juxtaposed under continental margin material which never experienced equivalently high pressures. We are aiming to date part of the cooling path of the high pressure rocks, and the formation of micas within the shear zones which separate the different packages.
The parallel, but equally important strand to this research is to unravel the systematics of excess argon contamination in high pressure rocks. Excess argon is argon which has been decoupled from its parent potassium and has been incorporated into a K-rich mineral at some point after crystallisation. This is a particular problem in high pressure micas and leads to ages which appear older than those calculated by other isotopic techniques such as U-Pb. It is currently not well understood where the excess argon sits within the mica lattice (only in the rims, or spread throughout the whole grain for example).
Using a UV laser coupled with the NU mass spectrometer available here at the OU we hope to map argon concentrations within high pressure mica grains, and hence calculate grain-by-grain age maps. Not only will these hopefully provide detailed information about the siting of excess argon, they may also finally yield "true" eclogite cooling ages which are useful for tectonic reconstructions.
The image on the left shows aluminium concentrations in a mica within an eclogite-facies calc schist from Oman. Zoning can be seen; what is uncertain is whether there is a link between zoning of major elements and relative argon concentrations.
Close-up of an eclogite from the beach in Oman.
The eclogites are enclosed within highly sheared calc schists. These show fabulous c-s fabrics and mica fish made of bright green phengite.
Major recumbent fold in the epidote-blueschist zone. The lowest unit is a quartz mica schist (pale grey), then the main mafic unit (green) and the uppermost unit is a dolomite schist (orange). Oman can be mapped from satellite images as there is little vegetation and the rock units are all such different colours!
