THE CURRENT STATE OF PLAY IN LINKING ACCESSORY MINERAL GROWTH AND BREAKDOWN TO MAJOR MINERAL EVOLUTION IN METAMORPHIC ROCKS

One of the challenges that remain in investigating the metamorphic evolution of rocks and terranes is to thoroughly understand their temporal evolution. Attempts to extract the meaning of geochronological data collected from accessory minerals (zircon, monazite) have been plentiful. One of the primary reasons this field of research is extremely so active is because being able to link the ages collected from metamorphic rocks to their thermal and physical evolution is able to provide us with useful clues as to the duration and rates of tectonic processes.

In tacking this ‘link’ essentially two types of approaches have emerged. The first involves comparing the trace element chemistry of, for example, garnet (or some other major mineral) or with zircon and/or monazite to establish whether accessory minerals grew in equilibrium with garnet (e.g. Rubatto 2002, Chem Geol, 184, 123-138), and to then infer pressure-temperature (P-T) conditions (utilising garnet) for the geochronology. This approach is valuable as establishing equilibrium between minerals is crucial to the second approach. The second involves a systematic and adaptable equilibrium thermodynamic approach in which Zr- and REE-phopshate- bearing phases are included in phase diagram calculations (Kelsey et al 2008, J Met Geol, 26, 199-212; Spear & Pyle 2010, Chem Geol, 273, 111-119; Kelsey & Powell 2010, J Met Geol, in press). As the output of phase diagram calculations is graphical, the growth and dissolution of zircon and monazite can be monitored with respect to the total (and evolving) mineral assemblage as a function of changing P, T and bulk rock composition. In addition, 1) thermometers based on the trace element content of minerals (e.g. Zr-in-rutile) have the capacity to now be output as part of routine phase diagram calculations; and 2) the trace element zoning (if present) of minerals such as garnet and rutile, can provide constraint on the P-T path trajectory additional to that offered by the major minerals.

This presentation will review the two approaches to linking accessory phase geochronology to major metamorphic mineral assemblage evolution. In particular, how the information we have gleaned thus far aids our understanding and interpretation of the meaning of isotopic geochronological data will be discussed.