U-Pb ion microprobe dating and Sr and Nd isotope geology of the Galiñeiro Igneous Complex: A model for the peraluminous/peralkaline duality of the Cambro-Ordovician magmatism of Iberia

Abstract

The pre-Variscan basement of Iberia contains two contrasting associations of silicic igneous rocks, one composed of intermediate to felsic peraluminous rocks with ages clustering around 486 Ma, and the other dominated by felsic peralkaline rocks considered to be of similar age. The most voluminous manifestation of peralkaline magmatism is the Galiñeiro Igneous Complex, mainly consisting of A1-type granitoids locally extremely enriched in HFSE and REE, which underwent medium-grade metamorphism during the Variscan. Previous attempts at dating these rocks using zircon concentrates have not succeeded. Using zircon U-Pb ion microprobe we found these granitoids have a crystallization age of 482 ± 2 Ma, and they are therefore coeval with the spatially related peraluminous magmas. Although the Rb-Sr isotope system composition was totally reset at 325 Ma, coincidental with the peak of the Variscan metamorphism, the Nd isotope composition has not been perturbed, and yielded εNd 482 Ma ≈ 1.6 and T DM ≈ 0.9–1.25 Ga, thus indicating mantle-derived magmas with a perceptible crustal component. The chemical features of these rocks: A1-type chemistry, elevated fluorine, exceptionally high HFSE and REE with a large Eu negative anomaly, constant Nb/Ta ≈ 9.4, and (Th/Ta) PM increasing from ca. 0.1 to > 3 as Th and Ta increase, cannot be explained by magmatic differentiation of mantle magmas plus bulk-assimilation of continental crust. They, however, can be reasonably explained as the result of the percolation through lower crustal materials of highly mobile F-HFSE-REE-rich alkaline hydrous fluids released from a mafic alkaline magma in an extensional environment. Since previous work based on zircon inheritance and dissolution kinetics has revealed that the magmas of the coeval peraluminous association were formed by fast melting caused by underplating on mafic magmas in an extensional environment, we conclude that the duality between peralkaline and peraluminous Cambro-Ordovician magmas of Iberia does not reflect different geodynamic settings, as previously believed, but is the result of the same phenomenon of rifting and crustal melting caused by the arrival of small batches of hot mantle magmas at the base of the crust. If these magmas were tholeiitic or moderately alkaline, they rapidly induced crustal melts that simply inherited the geochemical signature of the protolith. If, on the other hand, the underplating magmas were more alkaline, they released hydrothermal alkaline fluids that percolated through the lower crust to form such compositionally anomalous rocks as the peralkaline Galiñeiro gneisses.