Pre-variscan magmatism of the central iberian zoneChemical and isotope composition, geochronology and geodynamic significance

Resumen

Growing geochronological evidence indicates that the Cambro-Ordovician magmatism of Central and South Europe was more voluminous than earlier believed. In Iberia, concretely in the Central Iberian Zone, pre-Variscan magmatism is located in three NW-SE roughly parallel curved lineaments which probably have a Cambro-Ordovician age: Metavolcanic rocks and metagranites of the Ollo de Sapo Domain. Metavolcanic rocks and metagranites of the northern Schist-Graywacke Complex Domain. Carrascal-Portalegre granitoids and metavolcanic rocks of the Urra Formation, located in Portugal. This Thesis has concentrated on the two first lineaments. Its main objectives has been to date and to study chemical composition and Sr-Nd isotope geology of these rocks. From these data, a geodynamic model of the Cambro-Ordovician magmatism is proposed. Determination of crystallization ages of these rocks has been found very difficulties due to: (i) the lack of initial isotope homogeneity and disturbances produced during Variscan metamorphism cause many difficulties when applying Rb-Sr and Sm-Nd methods; and (ii) the complexity of their zircon which have a high proportion of inherited cores yielding highly discordant and very imprecise ages when applying conventional zircon U-Pb methods. In this thesis, 30 igneous bodies of the Ollo de Sapo Formation, the Tormes Dome, Sierra de Gredos, Sierra de Guadarrama and the Anatectic Complex of Toledo have been dated using single crystal zircon methods. The chemical and Sr-Nd isotope composition of these rocks have also studied. Previously, more than 2000 zircons were studied using cathodoluminiscence revealing the presence of an elevated proportion of pre-magmatic cores and then most of these zircons were systematically dated using U-Pb SIMS and LA-ICPMS and Pb-Pb stepwise evaporation. Locally, Rb-Sr methods were used. These bodies have crystallization ages ranging from 498 to 462 Ma. They also have different populations of restitic cores, which are Ediacaran (between 605-625 Ma), Cryogenian, Tonian, Orosirian, Neoarchean and Mesoarchean. The chemical and isotope composition of the Cambro-Ordovician rocks from two lineaments is very similar. They are peraluminous, magnesian and alkaline-calcic and are characterized by an enrichment in the most incompatible elements, with a positive Li anomaly and a negative Sr anomaly and a typical composition of granites derived from old crustal materials. They have (87Sr/86Sr)490Ma ¿ 0.707 - 0.713, and ¿Nd490Ma ¿ -5.8 a -3.3, which indicates that these rocks derived from old continental materials with little, if any, input of juvenile mantle materials, except in Miranda do Douro and San Sebastián metagranites which are more primitive with (87Sr/86Sr)490Ma ¿ 0.704, and ¿Nd490Ma between -3.40 and 2.1. From chemical and isotope composition and inherited ages of their zircons and their cathodoluminiscence imaging, the source of the Cambro-Ordovician rocks could have been felsic igneous Panafrican rocks located in the northern margin of west Gondwana. As mention before, the Cambro-Ordovician rocks have an high inheritance and to explain this fact, the peak temperature of Cambro-Ordovician magmas has been estimated to be 826 ºC, with some values of 950ºC, which exceeds the zircon saturation temperature, estimated using the Ti-in-zircon geothermometer at 815ºC. The zircon dissolution of these magmas have been also studied using Watson's equation (1996). From these calculations, Cambro-Ordovician magmas have been interpreted as magmas generated by swift magmatic pulses which lasted just a few thousand years from the beginning of melting to final emplacement and solidification. From geochronological data, chemical and isotope composition and zircon studies, the Cambro-Ordovician magmas have been interpreted as magmas generated during rifting of continental crust caused by an up-welling mantle plume which probably occurred during early Paleozoic fragmentation and dispersal of terranes from northern margin of west Gondwana and eventually led to the formation of Iberian microplate. Finally, the combination of all available geochronological data of Central Iberian enables the determination of the timing of Cambro-Ordovician rifting of Iberia. In Central Iberian and Galicia Tras-os-Montes Zones, the magmatic activity started at 500-495 Ma, has a maximum at 495-483 Ma, and finished at 470 Ma. In the Ossa Morena Zone, the situation is not so clear. Our estimate indicates that the magmatic activity began at 515 Ma and finished at 460 Ma. In this zone, a maximum of magmatic activity has not been observed.