Rare Earth Element Distribution in Some Hercynian Granitoids from the Finisterre Region, NW Spain

Two series of granitoids are present in the Finisterre region. One is formed mainly by syn-collision two-mica leucogranites. The other is composed of syn- and post-collision granodiorites and monzogranites of calc-alkaline affinity. The emplacement of these granitoids took place during and after the main phases of regional deformation. Petrographic and geochemical studies on the leucogranites suggests a common crustal origin with the surrounding metamorphic series, which includes metasediments and orthogneisses of medium to high grade amphibolite facies. The syn- and post-collision "calc-alkaline" series is associated with basic to intermediate rocks, suggesting a lower crustal origin. REE data have been used to characterize the two types of granitoids and to model the source areas of generation. There is no evidence for an origin of the granitoids by melting of mantle-type igneous rocks. Instead, the sequences of granitoids can be explained assuming different degrees of partial melting from a heterogeneous crustal source region. In some instances, however, in situ magmatic differentiation may also have occurred to account for variations in REE fractionation patterns observed in the Hercynian granitoids when compared with patterns in postulated source rocks. The source compositions were pelites and greywackes or mixtures of these rocks, although the leucogranites may possibly be better explained by partial melting of orthogneisses analogous to those outcropping in the region studied. The formation of two series of granitoids supposes an essentially intracontinental orogenesis (continent-continent collision) with the development of granitoids by recycling older continental crust. The coeval emplacement of mantle-derived magmas into the lower crust could enhance the melting process to form the biotite-rich calc-alkaline granitoids. By comparison with petrological and geochemical data from other granitoids of north Portugal and east Galicia, it is suggested that toward the west an increasingly younger crust was involved in partial melting processes in the northwestern part of the Iberian Massif.