U-Pb ion microprobe dating and Sr and Nd isotope geology of the Galiñeiro Igneous Complex. A model for the peralkaline/calc-alkaline duality of the Cambro-Ordovician magmatic rocks of Iberia
|Location||International Geological Congress,oslo 2008|
|Author||Montero, Pilar۱; Bea, Fernando۱; Corretgé, Guillermo۲; Floor, Peter۳; Whitehouse, Martin J.۴|
|Holding Date||08 October 2008|
The basement of the Iberian Variscides contains peralkaline and calc-alkaline to peraluminous felsic igneous rocks with ages between 495 Ma and 475 Ma. The most voluminous manifestation of peralkaline magmatism is the Galiñeiro Igneous Complex, consisting of HFSE and REE enriched rift-related A1-type granitoids with a zircon ion-microprobe U-Pb crystallization age of 481 ± 2 Ma and åNd481Ma between +1.4 and +1.8. The Sr isotope composition of these rocks was totally reseted at 325 Ma, coincidental with the peak of the Variscan metamorphism, though an indirect estimation indicates that 87Sr/86Sr481Ma was very high, probably around 0.7129, a value that suggests a marked decoupling between Nd and Sr isotopes and indicates the contribution of crust-derived components, which is in excellent agreement with the trace element evidence. The duality between peralkaline and calc-alkaline Cambro-Ordovician magmas of Iberia does not reflect different geodynamic settings but is a consequence of the same phenomenon of rifting and crustal melting caused by the arrival of small batches of hot mantle magmas to the basis of the crust. If these magmas were tholeiitic or moderately alkaline, they rapidly induced crustal melts that inherited the geochemical signature of the protolith and would be for that reason calc-alkaline or peraluminous. If, on the other hand, the underplating magmas were nepheline-syenitic or more alkaline, they released a substantial volume of hydrothermal alkaline fluids that mixed with the induced crustal melts, resulting in peralkaline compositions and leading to such anomalous rocks as the Galiñeiro gneisses.