COMPARISON OF 4039ARAR CONVENTIONAL AND LASER DATING OF BIOTITES FROM THE NORTH TREGOR BATHOLITH

Bulk samples and single grains of biotites from the North Tregor Batholite (Armorican Massif, France) were analysed by the 40Ar/39Ar method. Apparently pure, single biotite grains, analysed by the step heating procedure with the laser probe, displayed plateau ages or slightly disturbed age spectra. One strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages. The bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps. These disturbances are explained by 39Ar recoil from the biotite into the chlorite which is clearly evidenced by HRTEM observations, and not by excess 40Ar. The more concordant results obtained on single biotites are probably due to a more careful selection of the minerals. In most cases, we obtain good concordance of the ages (600-610 Ma) measured on the apparently pure, single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks.