Sr-Nd isotope evidence for mixing in the mafic rocks from the Reguengos de Monsaraz Variscan Pluton (Ossa Morena Zone, Portugal)

Abstract

The late-post-kinematic Reguengos de Monsaraz pluton is located in the Ossa Morena Zone, one of the major geotectonic units of the Iberian Variscan Belt. At the presently exposed level, the intrusion is dominated by tonalite-granodiorite rocks, spatially associated with minor bodies of gabbro-diorite composition, particularly in the central part of the area. Field evidence shows that the different intrusive facies define mixing/mingling contact relationships, suggesting a coeval emplacement for mafic and felsic magmas. The Rb-Sr age obtained for the pair amphibole-feldspar in one granodiorite sample (297.5 ± 2.9 Ma, initial 87/86 Sr = 0.70766 ± 0.00011) provides a minimum estimate for the time of original crystallization (Antunes et al., 2009). Petrographic and geochemical studies reveal that the different members of the suite have compositions ranging from metaluminous to slightly peraluminous and a distinctive calc-alkaline signature. Furthermore, the occurrence of systematic rectilinear correlations in Harker variation diagrams supports a mixed provenance for this sequence (Antunes et al., 2009). The εSr300-εNd300 co-variation displayed by the least evolved members of the suite indicates that these rocks do no longer represent the mafic mixing end-member. Their relatively low εNd300 values [εNd300=-5.1 to -7.8; 87Sr/86Sr300= 0.70871 to 0.71037] point to a significant interaction with time-integrated LREE-enriched upper crustal source components, comparable to the Late Proterozoic metasediments of the “Série Negra”. A scenario where the gabbro-diorite magmas from Reguengos de Monsaraz correspond to contaminated basic liquids is therefore proposed. Further hybridization of mafic magmas with anatectic crustal melts would have generated the entire range of granitoid types present in the massif.