EVOLUTION OF THE WESTERN-EUROPEAN CONTINENTAL-CRUST - IMPLICATIONS FROM ND AND PB ISOTOPES IN IBERIAN SEDIMENTS

This paper presents Sm-Nd and geochemical data on fine-grained sedimen ts of the Iberian Massif, Spain, together with new and published U-Pb results of selected sedimentary rocks. Profiles from two major zones ( Cantabrian and Central Iberian Zone) have been investigated. Both prof iles exhibit almost complete stratigraphic successions from uppermost Precambrian to Lower Carboniferous. Paleozoic shallow-depth platform s ediments are essentially unmetamorphosed; the Precambrian syn-orogenic succession underwent a metamorphic overprint up to biotite zone. Sm-1 47/Nd-144 ratios range from 0.0988 to 0.1457 and Nd contents from 6 to 112 ppm. epsilon(Nd)[today]-values vary from -16.5 to -7.6, Pb conten ts from 1.7 to 97 ppm. Samples with high epsilon(Nd)[T] (> -3) are fou nd to be low in mu(2)[330Ma] (< 9.75). Mean Nd crustal residence ages (T-DM) are 1.92 Ga (Cantabrian Zone) and 1.96 Ga (Central Iberian Zone ). As juvenile Cadomian crust is present in the Iberian Massif, the ol dest components have to be significantly older than 2 Ga. Within the E uropean Variscides comparably old T-DM were so far only described from the Armorican Massif, France. An active margin of the Gondwana contin ent is the most probable provenance of the uppermost Precambrian strat a as indicated by their Cadomian age, syn-orogenic facies and juvenile components. Precambrian as well as samples from the Precambrian-Cambr ian boundary gave epsilon(Nd)[T]-values > -4.8, significantly higher t han average Paleozoic rocks. Contributions of juvenile Cadomian crust to the syn-orogenic sediments must be in the order of 30-50% to fit th e Nd and Pb data. There is no evidence for any addition of mafic mater ial according to major or trace elements, or to Sm/Nd ratios. Lower Ca mbrian samples are already below -7.3 in epsilon(Nd)[T]. Thus, compare d to the total pre-existing crust, contribution of juvenile crust was very small. epsilon(Nd)[T]-values of almost all samples ranging in str atigraphic age from 515 to 410 Ma are strikingly negative (-12.9 to -1 0.1) and thus indicate the absence of newly differentiated continental crust in the provenance(s). Only the Tremadocian of the southern Cent ral Iberian Zone gave high epsilon(Nd)[T]-values (-4.0 to +1.0). Again there is no evidence for addition of mafic material to the strata. As Precambrian and Tremadocian epsilon(Nd)[T] peaks are separated by the lowermost Cambrian of epsilon(Nd)[T] > -7, juvenile crust may have be en added related to the pre-Ordovician unconformity (Sardic phase). Du ring the Middle Paleozoic (410-330 Ma), epsilon(Nd)[T]-values increase from similar to -12 to similar to -7, indicating a continuous contrib ution of juvenile continental crust of similar to 20%. As major terran es of the Iberian Massif were not amalgamated until Late Paleozoic, it can be assumed that the respective juvenile crust was formed within t he regime of one or several small subduction zones. Nd isotope data of this study compared with published results of Brittany, France, sugge st that the Medio-North Armorican-Centro-Iberian domain persisted from uppermost Precambrian to the Variscan.