Dating mylonitic overprinting of ancient rocks

Deformation in shear zones is difficult to date because mylonites can preserve partially reset pre-existing isotopic signatures. However, mylonites can be key structural elements in terrane recognition, so an accurate estimation of their age is important. Here we determine the in situ Rb-Sr isotopic composition of mica from major NE-SW trending mylonitic zones in the Archean Akia Terrane of Greenland and complement this information with inverse thermal history modelling. Rb-Sr isochrons indicate a dominant age of radiogenic-Sr accumulation in biotite of around 1750 million years (Ma) ago. Yet, magmatic titanite is unreset yielding a U-Pb age of around 2970 Ma. These constraints require that biotite Rb-Sr directly dates mylonitic fabric generation. The 1750 Ma mylonites, associated with the Proterozoic Nagssugtoqidian Orogeny, overprint Archean crust widely regarded as preserving evidence of early Earth horizontal tectonics. The timing of deformation in ancient shear zones of the Akia Terrane, Greenland, is recorded by the Proterozoic Rb-Sr age of mylonitic biotite which is determined through collision cell laser ablation and inverse thermal history modelling.

Automated summary

The age of ancient shear zones in the Akia Terrane, Greenland, was determined using inverse thermal history modelling and collision cell laser ablation. The results indicate that mylonitic biotite in the shear zones has a radiogenic-Sr accumulation age of approximately 1.75 billion years ago. This research is important for terrane recognition and understanding early Earth tectonic activities.