GEOCHEMICAL AND GEOCHRONOLOGICAL CROSS-SECTION OF THE DEEP VARISCAN CRUST - THE CABO-ORTEGAL HIGH-PRESSURE NAPPE (NORTHWESTERN SPAIN)

The high-pressure rocks of the Cabo Ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite-facies recrystallization at ~ 380 Ma. The occurrence of final cooling is recorded by 40Ar/39Ar and Rb-Sr mica ages of up to 350 Ma. U-Pb zircon dating and Sm-Nd and Rb-Sr isotope data together with REE and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths. The earliest tectonothermal event occurred at 490-480 Ma and corresponds to the emplacement of LREE-enriched magmas of probable calc-alkaline affinity and their metamorphism in the granulite facies. This occurred at approximately the same time as the eclogite-facies metamorphism of MORB-like protholiths associated with basic to intermediate granulites. Together, this suggests formation of syn-accretion granulites in an active margin setting. A second high-grade metamorphic event is recorded at ~ 420 Ma in metasedimentary formations associated with eclogite-like assemblages (garnet-omphacite ± plagioclase). This event is possibly related to a collisional plate tectonic setting in a back-arc basin. The age and origin of the associated ultrabasic rocks remains unclear, but a late pyroxenite vein has yielded an emplacement age of ~ 390 Ma. © 1990.