Evolution of the Western European continental crust: implications from Nd and Pb isotopes in Iberian sediments

Abstract

This paper presents Smz.sbnd;Nd and geochemical data on fine-grained sediments of the Iberian Massif, Spain, together with new and published Uz.sbnd;Pb results of selected sedimentary rocks. Profiles from two major zones (Cantabrian and Central Iberian Zone) have been investigated. Both profiles exhibit almost complete stratigraphic successions from uppermost Precambrian to Lower Carboniferous. Paleozoic shallow-depth platform sediments are essentially unmetamorphosed; the Precambrian syn-orogenic succession underwent a metamorphic overprint up to biotite zone. 147 Sm 144 Nd ratios range from 0.0988 to 0.1457 and Nd contents from 6 to 112 ppm. ϵ Nd[today]-values vary from −16.5 to −7.6, Pb contents from 1.7 to 97 ppm. Samples with high ϵ Nt[ T] ( > −3) are found to be low in μ 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 oldest components have to be significantly older than 2 Ga. Within the European Variscides comparably old ( T DM) were so far only described from the Armorican Massif, France. An active margin of the Gondwana continent is the most probable provenance of the uppermost Precambrian strata as indicated by their Cadomian age, syn-orogenic facies and juvenile components. Precambrian as well as samples from the Precambrian-Cambrian boundary gave ϵ Nt[ T]-values > −4.8, significantly higher than average Paleozoic rocks. Contributions of juvenile Cadomian crust to the syn-orogenic sediments must be in the order of 30–50% to fit the Nd and Pb data. There is no evidence for any addition of mafic material according to major or trace elements, or to Sm Nd ratios. Lower Cambrian samples are already below −7.3 in ϵ Nd[ T]. Thus, compared to the total pre-existing crust, contribution of juvenile crust was very small. ϵ Nd[ T]-values of almost all samples ranging in stratigraphic age from 515 to 410 Ma are strikingly negative ( −12.9 to −10.1) and thus indicate the absence of newly differentiated continental crust in the provenance (s). Only the Tremadocian of the southern Central Iberian Zone gave high ϵ Nt[ T]-values ( −4.0 to + 1.0). Again there is no evidence for addition of mafic material to the strata. As Precambrian and Tremadocian ϵ Nt[ T] peaks are separated by the lowermost Cambrian of ϵ Nt[ T] > -7, juvenile crust may have been added related to the pre-Ordovician unconformity (Sardic phase). During the Middle Paleozoic (410–330 Ma), ϵ Nt[ T]-values increase from — − 12 to ∼ − 7, indicating a continuous contribution of juvenile continental crust of ∼20%. As major terranes of the Iberian Massif were not amalgamated until Late Paleozoic, it can be assumed that the respective juvenile crust was formed within the regime of one or several small subduction zones. Nd isotope data of this study compared with published results of Brittany, France, suggest that the Medio-North ArmoricanCentro-Iberian domain persisted from uppermost Precambrian to the Variscan.