Behaviour of uranium-thorium minerals in the ultra-high temperature granulites of Rogaland

In the continents, the main radioelements, U and Th, are concentrated in a few accessory minerals, like monazite and zircon. The behaviour of these minerals through geological processes is in the centre of several important questions in geosciences: the h eat budget of continents, the geochemical cycle of heavy metals, and the measurement of time in geology, i.e. geochronology. Two leading institutions in geosciences in France and Norway join force to address the mineralogy and geochronology of accessory m inerals in rocks that underwent uncommonly hot and dry metamorphism and now superbly exposed in Rogaland. The specific goals of this proposal are to exchange knowledge, meet in the field in Rogaland and provide an international environment for a PhD stude nt. The granulites in Rogaland are famous in that they formed in "ultra-high temperature" metamorphic conditions above 1000°C. They contain zircon and monazite with complex internal textures. These microtextures are attributed to a variety of post-growth processes and complicate interpretation of U-Th-Pb geochronological data. We propose to study the transformations leading to these textures and to link them with the evolutions of pressure-temperature conditions, fluid activity, irradiation and petrology of their host rocks. We will produce images and generate chemical and geochronological data at microscopic scale in monazite, zircon and their hosts, using microanalytical methods. The data will serve to solve a geological conundrum. The ultra-high tempe rature domain in Rogaland follows the contact of the Rogaland Anorthosite Complex in the field, and therefore it has been interpreted as contact metamorphism caused by anorthosite magmatism. However, recent data suggest that metamorphism is some 80 m.y. o lder than magmatism challenging this interpretation. The time relationship between magmatism and metamorphism will be established, as well as implications on what controls the heat budget in "ultra-hot" orogens.