Forster, Marnie1, Lister, Gordon2
1Research School of Earth Sciences, Australian National University, Canberra, Australia; 2The Virtual Explorer
There is confusion in the argon geochronology world as to what allows movement in a ductile shear zone to be dated. Some assert that all that is necessary is to data mica and to obtain a ‘plateau‘. But this is not at all sufficient to make the argument. 40Ar/39Ar geochronology (like all radiometric dating techniques) does not have the ability to date movement. It is the microstructural modification of existing grains that must be dated, e.g., growth or regrowth during movement. Otherwise it may be that the fabric forming minerals are remnant: namely, relicts of an earlier formed mineralogy, and the ages obtained not at all relevant to the timing of movement in a later shear zone.
An ideal circumstance would be clear and unambiguous demonstration that growth of a particular mica had taken place immediately prior to or during the operation of a ductile shear zone, and that mineral separation (or laser spots) had focussed on those volumes during measurement. An example would be sudden growth of mica porphyroblasts that were then rotated and aligned in a developing shear zone fabric, as occurs in retrograde shear zones in the Cycladic Eclogite-Blueschist Belt. Movement has not been dated, but an estimate has been obtained as to the timing of growth during or immediately preceding movement. Another optimal circumstance would be if the operation of the ductile shear zone had shredded mica, progressively reducing its diffusion dimension. This behaviour leads to staircase spectra characteristic of fractal diffusion. Such age spectra appear to be able to allow the distinction of the start and the end of shear zone operation. In some cases, the age of relict mica microstructures is also evident, e.g., in mica from the Main Central Thrust of the Himalaya.
A more difficult circumstance occurs when age spectra from fabric forming minerals appear to be unrelated to the timing of movement, e.g., for mica from greenschist facies ductile shear zones in the Cap de Creus, Spain. We inverted data from 40Ar/39Ar geochronology step-heating experiments, using potassium feldspar, after conjoint inversion of data from simultaneous ultra-high-vacuum (UHV) 39Ar diffusion experiments. The resultant temperature-time curves imply that these mylonites formed in Eocene to Oligocene time, and therefore that they are not Variscan or Jurassic, as previously argued. Their tectonic significance is likely to be as right-lateral strike-slip shear zones formed in transfer faults accommodating roll-back of the Tethyan subduction zone as it dragged Sardinia and Corsica away from the Palaeo-European margin during opening of the Gulf of Lyon.
These examples suggest caution needs to be exerted in the dating of movement in ductile shear zones. Laser-step heating (or laser spot analysis) is not suited for this purpose, since this method provides no information in respect to Arrhenius data. In consequence the retentivity of relevant minerals in respect to argon diffusion cannot be assessed. In addition, laser methods do not produce consistent detail in age spectra. In contrast, robotic methods applied to resistance-furnace step-heating experiments offer a cheap, efficient, and reliable way to obtain the detailed age spectra that are necessary (in conjunction with Arrhenius data) to characterise the pattern of argon release.
Gordon Lister and Marnie Forster are structural geologists with a particular interest in the theory and practice of argon geochronology, in particular in the study of the dynamics of the evolution of orogenic architecture and its impact on metallogenesis.