Comments on “Precambrian Rocks South of Erquy and around St Cast, Côtes-du-Nord”: Reply

Abstract

IT was not our intention1 to question the accuracy of the isotopic determinations of Vidal et al.2 but rather to stress the difficulties in an interpretation of their Rb/Sr isochron age of 466 ± 10 m.y. for the Erquy Spilites. Vallance3 has argued that the paragenesis of ancient spilites is one due to alteration in a hydrothermal environment which may be either diagenetic or in response to the lower grades of metamorphism. The typical feldspar of these ancient spilitic pillow lavas is albite with low-temperature optics in contrast to the more calcium-rich plagioclase with high or transitional optics found in pillow lavas of Recent age3. We reassert that the Erquy volcanics contain a metamorphic paragenesis (albite, chlorite and epidote) overprinting the original igneous assemblage—glassy pillow margins and crystalline pillow centres. The clinopyroxene present within these rocks is metastable and in no way contradictory with alteration under lower greenschist facies conditions. The presence of forked albites quoted by Auvray et al.4 as evidence for the presence of a primary assemblage derived from the crystallization of a sodium-rich basaltic magma we suggest is evidence of the response of these rocks to a low grade of metamorphism and is a common feature of spilitic rocks within proven greenschist facies terrains5.