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
DANIEL J. KONTAK
Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, Nova Scotia B3J 2T9
EDWARD FARRAR and SANDRA McBRIDE
Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6
ROBERT F. MARTIN
Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7
Abstract
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 ca. 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 µm) 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 (2M1) 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 µm); 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).
Key words: 40Ar/39Ar dating, tin mineralization, muscovite, chemical composition, shear zones, leucogranite,
East Kemptville, Nova Scotia.