The source of mafic post-collisional magmatism in the Southern Variscan domain: Insights from the Permian dyke swarms of northern Sardinia

Abstract

Early Permian post-collisional magmatism is widely spread in southern Europe and it is represented by both felsic and mafic occurrences. One of the best places to observe such magmatism is northern Sardinia, where felsic Late Carboniferous-Early Permian batholiths are intruded by a large number of mafic dyke swarms. This study focuses on several dykes of mafic and intermediate composition from the Gallura region (northern Sardinia, Italy) that were emplaced during the extensional event(s) related to the collapse of the Variscan orogen. Based on textural, chemical and isotopic data the dykes are divided into three groups. Group 1 and Group 3 generally show calc-alkaline affinity, moderate to high enrichment in LREE over HREE, respectively, strong enrichments in LILE, Nb and Ta negative anomalies. Group 1 samples show more radiogenic Nd and less radiogenic Sr isotopic values compared to Group 3, while the two groups show similar Pb isotopic values. Group 2 shows depleted LREE and depleted SrNd isotopic composition, relatively close to those of MORBs. Nonetheless, the negative Nb anomaly and Pb isotopic composition clearly distinct from those of MORBs highlight the subduction signature also in dykes from this group.

Our modelling suggest that the distinctive geochemical features of the three groups are not related to closed-system fractional crystallization or to crustal assimilation, but they are the result of melting of a heterogeneous mantle source. Group 1 and 2 could have formed by melting of a peridotite at the transition of the spinel and garnet stability, with Group 1 possibly requiring a more enriched mantle composition and generally lower melting temperature. By contrast, a peridotite-pyroxenite mixture or an amphibole peridotite is the most likely source of Group 3 dykes.

The widespread coeval Early Permian mafic dykes and intrusive bodies from Southern Europe show geochemical features that are quite similar to those of northern Sardinia dykes. The comparison shows that the most common magma-types are similar to Group 1 and subordinately to Group 3 from Sardinia, suggesting that most of the mafic Early Permian post Variscan magmatism was formed by melting a mantle that was enriched during the Variscan orogeny. On the contrary, Group 2-like magmatism is very scarce in southern Europe and its more depleted compositions are probably the result of decompression melting of asthenospheric mantle with a subtle subduction signature.