Development of new analytical procedures of isotope geochemistry by mass spectrometry. Application to the study of high-pressure metamorphic rocks from the Iberian Massif

Abstract

The development of Isotope Geochemistry/Geochronology has triggered many technical improvements in the past decades and has revolutionised the potential information available on the tectonics of metamorphic belts. As a consequence, parameters such as precision, accuracy and sensitivity of the used techniques have reached levels that were inconceivable years ago, thus permitting to date effectively minerals and rocks at small spatial scales. These advances, along with the concerns of researchers have opened a wide range of possibilities in the field of rock dating.The University of the Basque Country UPV/EHU has acquired the specific analytical equipment and constructed clean laboratories in the last years which has allowed the implementation and improvement of several of the worldwide most used geochronological methods in the course of this PhD Thesis. Additionally, successful application of those methods on natural samples was also carried out. One of the objectives of the present study was the development of ID-MC-ICP-MS analyses by applying the Rb-Sr and Lu-Hf systems. The Rb-Sr method has been implemented using a 86Sr tracer for the first time in geochronological studies. The procedure of fussion-dissolution set up for the treatment of geological samples, as well as the chromatographic isolation of each isotope in both systems, have proven to work fine in all the cases considered and are regarded as fully reliable for geochemical/geochronological research. Furthermore, since it has been possible to find a solution for all the possible uncertainties associated with the measurements of the Rb-Sr and Lu-Hf systems, it can be stated with certainty that both methods have been implemented correctly being now available for the scientific community.On the other hand, refined techniques for analysing samples in situ, an area of increasing interest in geology, have been also set up. In this PhD Thesis, the methods for measurements of the U-Th-Pb and Lu-Hf systems on zircons have been developed successfully. The results obtained on international standards are in agreement with the values of reference, which demonstrates that all the problems inherent to the analytical procedures are solved properly and therefore the methods are fully operational for their use. Finally, these techniques have been employed to date and trace the origin of high-pressure rocks from the Malpica-Tui and Cabo Ortegal Allochthonous Complexes of the NW Iberian Massif. The results obtained for the Upper Allochthon suggest that those rocks were formed as a consequence of the subduction of Ordovician protoliths during middle to late Devonian times. The subduction of the regional Lower Allocthon took place subsequently, 15-20 Ma after. The Hf isotopic data on zircons suggest that the system was partially open during the maximum burial of the Upper Allochthon. Rb-Sr ages for both allochthons, interpreted as reset ages, along with new zircon growth, point to an important activity of fluids during the exhumation of these units.