ESTABLISHING PEAK TEMPERATURE IN UHT METAMORPHIC TERRANES

Establishing the tectonic drivers that lead to the formation of high-grade (T> 850 °C; P< 12kbar) metamorphic conditions at a regional scale requires a detailed understanding of the P–T trajectories and timescales involved. Obtaining these constraints can be problematic as, in most cases, the P–T evolutions and durations involved result in the destruction of the evidence required to build the case. This has lead to a number of scenarios being proposed for the generation of high-temperature terrains that have little quantitative basis and a rather tenuous link to the actual observed geology. Advances in the application of in situ geochronological techniques coupled with trace and REE geochemistry and development of activity composition models for key high-temperature minerals is enabling more detailed histories for the development of realistic P–T–t evolutions of these terrains. In this presentation examples from two of the hottest pieces of crust, the Eastern Ghats and the Napier Complex, will be discussed. In theses examples a combination of trace element thermometry and P–T pseudosection analysis allows the peak temperatures to be constrained. These datasets also highlight how some of these systems may not be as robust as others in recording peak conditions as they tend to be reset to lower temperatures during extended periods of residence at high temperatures.