Dating Low‐Grade Deformation: Role of Lithology and Strain Partitioning on Ar Isotope Records in the Alpi Apuane of Northern Apennines (Italy)

Abstract

AbstractThe Alpi Apuane (northern Apennines) represent a key area for understanding the processes related to the Apennines building and geodynamic evolution of the central Mediterranean, but the timing of tectono‐metamorphic events is still debated. White mica from cover rocks of the Massa and Apuane units was investigated by the 40Ar‐39Ar in situ and step‐heating techniques. Samples recorded several episodes of deformation developed under low‐grade conditions and resulting in a D1 composite schistosity followed by a variably developed D2 retrograde crenulation. In the Massa Unit, D2 crenulation was accompanied by replacement of muscovite‐phengite by paragonite, whereas the composition of phengite from the Apuane Unit does not vary noticeably, irrespective of whether phengite is aligned along D1 or D2 schistosities. Both units exhibit a significant intrasample scatter of in situ ages and variably discordant age spectra from step‐heating data, due to partial reequilibration of white mica along a deformation path lasting ≥10 Ma, with dates approaching the time of younger events as retrogression becomes more pervasive. The onset of deformation has a minimum age of ∼20 Ma, as documented by 40Ar‐39Ar data for the composite D1 foliation in the Massa Unit. D2 deformation shows a common temporal evolution at the scale of the region and is recorded for at least ∼2.5 Ma, starting not after ∼12.5 Ma and ending at ∼10.5 Ma. Results highlight the importance of a detailed microscale characterization and of a careful preselection of samples, as compositionally different domains a few millimeters apart may yield different results because of compositionally driven strain partitioning.