U-PB SHRIMP ZIRCON AGES AND ND SIGNATURE OF VARISCAN BASEMENT, EASTERN PYRENEES (FRANCE) AND CATALAN COASTAL RANGES (SPAIN): RIFT-RELATED GRANITOIDS ABOVE CRYPTIC CADOMIAN BASEMENT

To test whether orthogneisses represent peri-Gondwanan Cadomian basement or pre-Variscan rift-related intrusions, SHRIMP U-Pb zircon ages and ¹⁴³Nd/¹⁴⁴Nd ratios were determined on granitic samples from the Núria massif, the southern tip of the Canigo dome in the Eastern Pyrenean axial zone, and from the Guilleries massif in the Catalan Coastal Ranges. From the core of the Núria massif, two-mica granite gneiss and granitic augen gneiss have crystallization ages of 457±4 and 457±5 Ma, respectively, and a metagranophyre from the base of Caradocian metaconglomerates crystallized at 458±3 Ma. Orthogneisses from the Guilleries massif are strongly foliated, concordant tabular bodies which intruded Neoproterozoic to Upper Ordovician, upper amphibolite facies metasedimentary rocks; six separate leucocratic bodies have crystallization ages of 487±3, 469±3, 462±8, 457±4, 453±4, 452±4, and 451±7 Ma. Inherited cores in zircons from both groups of orthogneisses are Neoproterozoic, ranging between about 905 and 620 Ma. ¹⁴³Nd/¹⁴⁴Nd ratios of the dated rocks from the Núria massif range between 0.0512260 and 0.512640, and from the Guilleries massif between 0.0512101 and 0.0512313, with all samples having ε_Nd values between -2.1 and -5.2 at 450 Ma, similar to values of volcanics of central Brittany and northwest Iberia. These data indicate that (1) magmatism lasted about 35-40 m.y. (from 477 to 446 Ma in the Canigou dome and from 487 to 452 Ma in the Guilleries massif), similar to the time span of granitic magmatism recorded in the Montagne Noire, Sardinia, and Alpine basement related to rifting of Gondwanan crust and opening of the Rheic Ocean; and (2) Nd isotopes record a mixture of juvenile Cadomian material and significantly older components, also indicated by the presence of detrital zircons as old as 2.74 Ga in Guilleries metapelites. In the Variscan chain in Iberia and the Pyrenees, there apparently is a crustal level below the Neoproterozoic-Paleozoic metasedimentary cover where melting occurred in the Ordovician to produce granitic magmas; in areas that have been studied, these source rocks do not outcrop. In these areas, the Variscan chain is basement-absent, in contrast to modern collisional orogens. The mechanism of Variscan shortening did not allow for lower crust to overthrust its sedimentary cover.