Evidence for old Precambrian relics shown by UPd zircon dating of eclogites and associated rocks in the Hercynian belt of South Brittany, France

Abstract

UPb dating of zircon from an eclogite of the Champtoceaux Complex, south Brittany, has yielded an upper intercept age of 2208 ± 39 Ma. This age is interpreted as representing contamination processes or the emplacement of an eclogite protolith having geochemical affinities with intermediate rocks of talc-alkaline magmatic series. The lower intercept on the concordia diagram is interpreted as the age of the high-pressure-high-temperature metamorphic event at 413 ± 16 Ma. Results obtained on a serpentinite from the Baie d'Audierne Complex indicate an ultrabasic protolith crystallization age older than 840 Ma. Taken in conjunction with published upper intercept ages of 1300 Ma for the eclogite protoliths from the Vendée and the Baie d'Audierne, the present data would suggest the existence of relics of a Precambrian basement in south Brittany. The geological history of this basement can be summarized as follows: 1. (1) a magma of intermediate composition emplaced during the early Proterozoic, around 2.2 Ga ago, in an active margin-type tectonic environment. This date is in agreement with the possible existence of continental crust as old as Archaean in south Brittany, as shown by the presence of inherited zircons in Palaeozoic formations with upper intercept ages around 2.5 Ga; 2. (2) eclogite protoliths were preserved in a continental crustal environment as far as 440-380 Ma; 3. (3) around 400 Ma eclogite-facies metamorphism affected the ancient basement rocks of south Brittany, probably as a result of continental collision during the early stage of the Hercynian orogeny. The regional zonation of the Siluro-Devonian high-pressure event (436-413-384 Ma) throughout the south Brittany area could be due to a succession of small collisions or related to the convergence of plates with irregular outlines.