Paper No. 4
Presentation Time: 1:45 PM
RARE EARTH ELEMENT BEHAVIOR IN ORTHOPYROXENE, ZIRCON AND GARNET DURING UHT METAMORPHISM
The extreme crustal conditions of ultra-high temperature metamorphism (T> 900°C; P< 12kbar) provide an opportunity to observe the behavior of trace elements with the potential for reaching chemical equilibrium over prolonged timescales. Rare earth element equilibrium, and disequilibrium, can be investigated by looking at the partitioning of these elements between minerals extensively used for P-T
estimates and geochronology. The application of in situ
techniques such as SIMS and LA-ICP-MS enables analysis of trace element and isotopic systems in minerals that may preserve complex metamorphic histories. Orthopyroxene, garnet and zircon are key minerals in unraveling and interpreting the P-T-t
histories of high-grade metamorphic terranes, and all are repositories for the heavy REE during metamorphism. Whilst zircon-garnet trace element partitioning has been used in many studies of high-grade metamorphism, the role of orthopyroxene in the distribution of the rare earth element budget is less well understood.
The Trivandrum Block and Eastern Ghats in India, and Enderby Land in Antarctica, provide examples of UHT granulites that have attained extreme crustal conditions. The importance of orthopyroxene as a host for the REE can be examined not only in competition with garnet, but also in garnet-absent granulites where orthopyroxene can be the most abundant HREE-bearing phase. Examples from each of these terranes, compared with experimental data, provide an insight into the behavior of trace elements during varying metamorphic processes, including the growth and recrystallization of metamorphic and accessory phases. Combined with in situ geochronology the observed REE distributions can be key in identifying what stage of the P-T-t path the ages are recording.