U–Pb geochronology and Hf isotope ratios of magmatic zircons from the Iberian Pyrite Belt

A geochronology and Hf isotope study, using laser ablation-ICP-MS analysis of zircon grains, has been conducted to date felsic volcanic rocks from the Portuguese sector of the Iberian Pyrite Belt and to establish possible sources for these rocks. The ages obtained range from the Famennian to the Tournaisian, with the oldest ages reported in the Belt so far being identified in its southwestern part (Cercal area). Results also indicate that within each area, volcanism may have extended for significant periods of time. This suggests that caution is needed in interpreting possible migration trends for the volcanism, as the exact stratigraphic position of the sampled rocks is not always clear. Despite of this, the new data, coupled with previously reported information, suggests that volcanism migrated within the basin from the southwest to the northeast (present day coordinates). Projection from initial zircon ɛHf values towards the depleted mantle evolution curve, via an intermediate reservoir, allows the calculation of Hf protolith model ages that are predominantly Meso-Proterozoic. This is compatible with acid magmas resulting from the fusion of Phyllite–Quartzite (PQ) Formation metasedimentary rocks, which are beneath the volcanic rocks. This is because zircon grains from one PQ Formation sample provided Late Neo-Proterozoic ages and Paleo-Proterozoic to Late Archean U–Pb ages, and the Hf isotope signatures of these zircons can be expected to mix during fusion and result in protolith model ages that would be intermediate between the two U–Pb age populations, as recorded. Further supporting this source for the magmas, the distribution of U–Pb ages of (pre-Variscan) inherited zircon grains in the volcanic rocks is very similar to that shown by the detrital zircon grains from a PQ sample

The multistage crystallization of zircon in calc-alkaline granitoids: U–Pb age constraints on the timing of Variscan tectonic activity in SW Iberia

CL imaging and U–Th–Pb data for a population of zircons from two of the Évora Massif granitoids(Ossa-Morena Zone, SW Iberia) show that both calcalkaline granitoids have zircon populations dominated by grains with cores and rims either showing or not showing differences in Th/U ratio, and having ages in the range ca. 350–335 Ma (Early Carboniferous). Multistage crystallization of zircon is revealed in two main growth stages(ca. 344–342 Ma and ca. 336–335 Ma), well represented by morphologically complex zircons with cores and rims with different ages and different Th/U ratios that can be explained by: (1) crystallization from melts with different compositions (felsic peraluminous to felsic-intermediate metaluminous; 0.001 837 °C; granulite facies)and/or because they were derived from inheritance-poor felsic and mafic rocks from a previous cycle, as suggested by the internal structures of zircon cores. These Variscan magmatic rocks with crystallization ages estimated at ca. 336–335 Ma are spatially and temporally related to hightemperature metamorphism, anatexis, processes of interaction between crustal- and mantle derived magmas and intra-orogenic extension that acted in SW Iberia during the Early Carboniferous

Detrital zircon geochronology of the Cretaceous succession from the Iberian Atlantic Margin: palaeogeographic implications

Detrital zircon U–Pb data performed on eight Cretaceous sandstone samples (819 age isotopic results) from the Lusitanian basin (west Portugal) constrain the history of uplift and palaeodrainage of western Iberia following break-up of Pangaea and opening of the North Atlantic Ocean. We examined the links between shifts in provenance and known basinwide unconformities dated to the late Berriasian, Barremian, late Aptian and Cenomanian–Turonian. The detrital zircon record of sedimentary rocks with wider supplying areas is relatively homogenous, being characterized by a clear predominance of late Palaeozoic ages (c. 375–275 Ma) together with variable proportions of ages in the range c. 800–460 Ma. These two groups of ages are diagnostic of sources within the Variscan Iberian Massif. A few samples also reveal significant amounts of middle Palaeozoic (c. 420–385 Ma) and late Mesoproterozoic to early Neoproterozoic (c. 1.2–0.9 Ga) zircon, which are almost absent in the basement to the east of the Lusitanian basin, but are common in terranes with a Laurussia affinity found in NW Iberia and the conjugate margin (Newfoundland). The Barremian unconformity marks a sudden rise in the proportion of c. 375–275 Ma zircon ages accompanied by a decrease in the abundance of the c. 420–385 Ma and c. 1.2–0.9 Ga ages. This shift in the zircon signature, which is contemporaneous with the separation of the Galicia Bank from Flemish Cap, reflects increased denudation of Variscan crystalline rocks and a reduction in source material from NW Iberia and adjoining areas. The late Aptian unconformity, which represents the largest hiatus in the sedimentary record, is reflected by a shift in late Palaeozoic peak ages from c. 330–310 Ma (widespread in Iberia) to c. 310–290 Ma (more frequent in N Iberia). It is considered that this shift in the age spectra resulted from a westward migration of catchment areas following major uplift in northern Iberia and some transport southward from the Bay of Biscay under the influence of a well-established Atlantic circulation