Abstract

B. Auvray, J.-J. Peucat & J.-P. Brun write: Dallmeyer et al. (1991a) present new geochronological data (U-Pb zircons, Ar-Ar minerals, Sm-Nd whole rocks) concerning several quartz-diorite stocks outcropping in Guernsey and Sark sometimes referred to as the Perelle quartz diorites. The authors claim that their intrusion is the result of two successive geological events separated by about 100 Ma. The first stage, close to 700 Ma, is interpreted as the age of genesis and emplacement of the quartz-dioritic magmas; the second one, at about 600 Ma, is considered as a tectonometamorphic event which corresponds to the gneissification of the plutonites and is followed in a few later times (570-580 Ma) by a hydrothermal alteration of regional overprint related to the emplacement of post-kinematic plutons.

A first point of disagreement previously raised by one of us (J.J.P.) in his review of the manuscript, is the age of 700 Ma interpreted as the emplacement age of the magmatic protoliths of the quartz diorites. This interpretation seems to us highly questionable considering the published data and concordia diagram (fig. 4, p. 695) provided by the authors. Among the seven points plotted in the diagram, only two are used to define the discordia and the upper intercept at 709 Ma; the five other being largely scattered in the diagram. We agree with the authors that this scattering is due to a contamination process related to the intrusion and emplacement of the quartz-diorite magmas in the Icartian basement dated at about 2.0 Ga. As noted

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