Bernard Auvray

Precadomian relicts in the Armorican Massif: Their age and role in the evolution of the western and central European Cadomian-Hercynian belt

Abstract Well-dated Precambrian is mostly developed in the north of the Armorican Massif, in which area voluminous Cadomian magmatism is dated at between 650 and 550 Ma. Much older relicts occur at Cap de la Hague, in Guernsey and in the Tregor, and are also found in the northern continental margin of the Iberian Peninsula. In all these occurrences, whole-rock systems have been opened, so that the true ages cannot be determined by the Rb-Sr whole rock isochron method. Four U-Pb zircon ages are between 1.8 and 2 Ga (in Guernsey: Icart orthogneisses; in Tregor: Port Beni, Trebeurden, Morguignen orthogneisses). There is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 Ma to 270 Ma ago. On the contrary, the evolution of initial 87Sr/86Sr ratios with time shows that the observed mid- and west European continental crust is probably not older than 700 Ma. The increase of the initial 87Sr/86Sr ratios of the magmas with time suggests that, after its formation in Cadomian times, this segment of continental crust evolved virtually as a closed system and Hercynian magmatism arose principally from re-melting of relatively young sialic components.

Late Proterozoic zircon ages from a basic-ultrabasic complex: A possible Cadomian orogenic complex in the Hercynian belt of western Europe

The basic-ultrabasic complex of Belle-IsIe-en-Terre (northwestern part of the Armorican Massif, France) has been dated at 602 +4 −1 m.y. by the U-Pb zircon method. The calc-alkaline trend of the metabasic rocks and the geologic context suggest that this ophiolite-like complex is related to a back-arc orogenic setting similar to the geodynamic environment that has been proposed for Pan-African calc-alkaline ophiolite-like complexes of late Proterozoic age around the West African craton. Our results are compatible with the closing, in late Proterozoic time, of a “Channel ocean” north of the basement and formed during the Cadomian orogeny, rather than with possible Caledonian or Hercynian subduction.

Differential behaviour of the Rb-Sr and K-Ar systems of spilitic flows and interbedded metasediments: the spilite group of Erquy (Brittany, France). Paleomagnetic implications

The Erquy series (Côtes du Nord, France) consists, in its upper part, of spilitic pillow lavas with some interbedded volcano-sedimentary horizons.The Rb-Sr system of the pillows allowed the construction of a whole-rock isochron at 482±10 M.a. with an initial87Sr/86Sr ratio of 0.7055±0.0002. These rocks and the associated keratophyres give, on the other hand, K-Ar ages of 285±16 M.a. interpreted as the consequence of late-hercynian tectonism.A volcano-sedimentary horizon interbedded with such pillow flows has been studied from petrographic, mineralogical, geochemical and isotopic (Rb-Sr and K-Ar) points of view. The sequence keeps a sedimentary “memory”. Its clay fractions <2 μm and corresponding whole-rocks fit an isochron which is identical to that of the volcanic rocks: 494±11 M.a. with an initial87Sr/86Sr ratio of 0.7052±0.0005. The clay fractions give K-Ar data at about 450 M.a., but those which contain important amounts of volcanic glass, at the top of the horizon, have K-Ar values as low as 400 M.a., and those which contain almost no glass have a K-Ar age close to the Rb-Sr age at 480 M.a.This study emphasizes the possibility of a complete reset of the K-Ar system of spilitic rocks by a tectonic event without notice-able temperature increase. This result may have important implications on combined paleomagnetic and K-Ar studies: it seems that a least for spilites and keratophyres, the Curie point and Ar blocking temperature can be very different.