Detailed petrographic and geochemical studies conducted on zircons from the Lys-Caillaouas pluton reveal their igneous and metamorphic affinities. The igneous zircons constrain the emplacement of the pluton to 300±2 Ma. By contrast, the metamorphic zircons yield an older age of 307±3 Ma, which probably dates the thermal peak of the HT/LP Variscan metamorphism. Therefore, a short time lag of c. 7 Ma emerges between the metamorphic climax and emplacement of the pluton in the Axial Zone (Pyrenees).

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A N-S trending, narrow zone of crystalline basement occurs from Biscayarhalv@ya to Hultedahlfonna in northwestern Spitsbergen and is composed of various metasedimentary and igneous rocks, including granites. Previous isotopic age determinations on these rocks are by the K-Ar. Rb-Sr, '"Ad9Ar and conventional zircon U-Pb method and yielded the Caledonian and Grenvillian ages. The single-grain zircon Pb evaporation method has recently been applied to solve complex problems and this is the first report by the method. A granitic rock. syntectonically intruded into the phyllitic metasediments of the Biscayarhuken formation. which is the uppermost lithotectonic unit in the metamorphic rocks of the zone, was dated on four zircon grains, yielding a narrow age range from 955 f 4 to 968 + 9 Ma, 961 f 4 Ma in average. This age of ca. 960 Ma is considered to be the age of intrusion, based on the occurrence and zircon morphology, which is roughly simultaneous with the formation of the phyllitic cleavages of the surrounding metasediments. The data obtained imply that the Caledonian events did not reset the Ph isotope system of zircon and major metamorphism occurred during the Grenvillian time in the Biscayarhuken fomiation, accordingly, the protolith age of the metasediinents is Mesoproterozoic.

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