Mineral chemistry and 40Ar/39Ar dating of muscovite from the East Kemptville leucogranite, Southern Nova Scotia: Evidence for localized resetting of 40Ar/39Ar systematics in a shear zone

The East Kemptville topaz-muscovite leucogranite, part of the 370 Ma peraluminous South Mountain Batholith of southern Nova Scotia, experienced a complex thermal evolution. A variety of geochronological studies (Pb-Pb, U-Pb, Rb/Sr 40Ar/39Ar) indicate that after initial emplacement at ca. 370 Ma, variable resetting of radiometric systems occurred until cn. 240 Ma. The presence of mylonitic zones, C-S fabrics and spaced cleavage within the leucogranite indicate that it is located within a high-strain zone, the East Kemptville - East Dalhousie Fault, one of several regionally extensive fault zones transecting the South Mountain Batholith. The present study combines results of electron-microprobe analyses, X-ray diffraction and 40Ar/39Ar step-heating experiments on seven grain-size fractions (550 to 175 mu m) of muscovite from a sample of leucogranite to determine the influence of chemistry and grain size on argon diffusivity. The similar composition of the size fractions, presence of a single polytype (2M(1)) and discordant but comparable 40Ar/39Ar age spectra indicate that argon diffusion was not controlled by differences in mineral chemistry, a mixture of polytypes or differences in apparent grain-size. Although a regional thermal overprinting event and mixing of grain-size fractions of different ages are considered as potential explanations for the discordances in the age spectra, the most likely reason is diffusive loss of argon controlled by an effective grain-size smaller than the finest size-fraction prepared (<175 mu m); the effective grain-size was controlled by development of microstructures and subtle chemical heterogeneity. Thus, the variably discordant 40Ar/39Ar age spectra of muscovite from this area are considered to reflect structural focusing of heated fluids that was facilitated by the inherent anisotropy of the host leucogranite (i.e., shear zones).