Séminaire de Antonin Laurent

Titre: Metamorphism at extreme conditions, what information can be gathered from monazite and zircon behaviour?

Résumé:

Ultrahigh-temperature (UHT) and ultrahigh-pressure (UHP) metamorphic rocks have been recognized in a variety of geodynamic setting and are considered to play an important role in deciphering lithosphere evolution. Unravelling the evolution of such terrains in terms of melting duration, fluid-flow and ore genesis usually requires the use of the U–(Th)–Pb chronometers in zircon and monazite. Both minerals show complex chemical zoning at the µm- to nm-scale that can be used to identify growth processes and link them to monazite and zircon crystallization environment like mineral paragenesis and P–T–fluids conditions. However, the wide diversity of internal texture observed in zircon and monazite also largely reveals modification of pre-existing structures during metamorphism without changing the external shape of the grain. This presentation will highlight some complexities where chemical zoning do not always coincide with age zoning derived from U-Th-Pb isotopes. In a first part, we compare in detail the mineralogical and U–Pb isotopic response of zircon and monazite during repeated granulite facies events reaching ultra-high temperature in the Rogaland basement (South Norway). In a second part, this presentation will discuss the potential for S-bearing monazite to record the redox condition of its crystalizing medium through time and to probe the presence of sulfur in geological fluids at UHT and UHP.