Wijbrans, Jan1, Uunk, Bertrum1, de Paz Alvarez,Manuel1, Huybens, Rosanne1, Brouwer, Fraukje1
1Department of Earth Sciences, Vrije Universiteit Amsterdam, the Netherlands
Time scales of tectonic and metamorphic processes in the greenschist-blueschist domain are essential to better understand the dynamics of accretionary wedges. However, such time scales are difficult to obtain because the number of geochronometers available to us is limited. For these geochronometers only partial setting, or resetting, of the isotopic clocks can be expected at temperatures reached in the blueschist and greenschist facies, whilst shear zone deformation may not cause full recrystallization. New, or more refined geochronological approaches, can thus shed additional light on the processes in accretionary wedges.
The Aegean subduction complex has emerged over the years as a key natural laboratory to study subduction processes. Our focus in recent years has been on the subduction related processes as experienced by the rocks of Syros and Sifnos islands. Here, we report (1) our efforts to further refine dating approaches: 40Ar/39Ar by dating of suites of single phengite crystals on an outcrop to section scale, and (2) our work exploring new approaches of dating of minerals free of lattice-bound potassium such as garnet, amphiboles and epidotes by dating the signal derived from the fluid inclusions by stepwise crushing.
Dating complete 100+ m sections by single grain phengite dating has the added benefit that all available lithologies in the section contribute to a histogram or PD- plot. Where previously a multigrain average from a single rock specimen was obtained, now is revealed that different units record an age range often as much as 10 Myrs wide, the oldest ages recording crystallization and the younger resetting following crystallization. Dating fluid inclusions by stepwise crushing allows the identification of multiple fluid reservoirs that contribute to the obtained signal sequentially. Typically, the first crushing steps reveal a reservoir that contains large amounts of excess 40Ar, whereas following exhaustion of this reservoir other distinctly different sources of argon are revealed. Age signals thus obtained are interpreted as documenting periods of increased fluid mobility during and following mineral crystallisation.
Combination of these two approaches provides new insights into the tectonic processes during deep subduction and subsequent exhumation to shallower depths.
Jan Wijbrans is currently professor of Argon Geochronometry at the Vrije Universiteit Amsterdam. Jan was an ANU postgraduate research scholar at the Research School of Earth Sciences, ANU from 1981 until 1985, when he worked under the guidance of prof. Ian McDougall on timescales of metamorphic processes using the 40Ar/39Ar method.