Influence of deformation mechanisms on deformation-induced resetting of radiogenic isotope systems (U-Pb, Rb-Sr, Sm-Nd) and homogenization on the grain-scale

Direct dating deformation processes in metamorphic rocks still remains difficult, though substantial advances in geochronological dating techniques have been made in recent years. In particular in-situ techniques facilitate the correlation of radiometric ages to petrology, and the selection of isotopically homogeneous sample volumes. For incomplete homogenization, ages may diverge from the true age to variable extent. However, isotope heterogeneities do occur at the nano-scale being always smaller than sampling volumes for in-situ dating techniques and have to be avoided. Thus, to obtain reliable and accurate deformation ages, it is essential to understand how deformation processes influence isotopic resetting and over which distances on the mineral grain-scale they act. For this purpose, we will investigate deformed natural rock samples showing microfabrics characteristic for deformation-induced mineral reactions and fluid availability. Our samples from structurally well defined tectonic settings allow the extrapolation from grain-scale to map-scale phenomena. The expected results of this project have important implications for determining the timing, duration and rates of geodynamic processes. Such data are a requirement for the quantitative understanding and modelling continent-scale tectono–metamorphic processes, e.g. burial and exhumation of metamorphic rocks.

During deformation of granitoid rocks (Thassos Metamorphic Core Complex, Greece), grain-boundary fluids are channelized along cleavage planes of recrystallized mica. Fluids support dissolution and precipitation of pre-deformative magmatic zircon, resulting in the zone with high porosity. In contrast, monazite more readily recrystallizes during deformation. This process is highly efficient to completely reset the U-Th-Pb isotopic system of monazite, and the monazite age will date deformation. Secondary electron image of rock thin section with insets of backscattered electron illustrations of zircon and monazite. Abbreviations: Zrn zircon, Mnz monazite, Wm mica, Pl plagioclase, Ap apatite.