Dating mylonitic overprinting of ancient rocks

Abstract

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.