Sr, Nd isotope evidence for an enriched mantle component in the origins of the Hercynian gabbro-granite series of the "Serie dei Laghi" (Southern Alps, NW Italy).

In the “Serie dei Laghi”, Western Southern Alps, large Permian granite plutons and mafic-intermediate stocks and dykes (“Appinite suite”) were emplaced in a post-collision environment. The magmatic rocks of “Serie dei Laghi” are metaluminous, and mainly exhibit petrographic and geochemical characters typical of medium-K calc-alkaline series. Granites and leucocratic "Appinites" show LREE and LILE enriched patterns and Nb, P, and Ti negative spikes typical of calc-alkaline series. In the mafic “Appinite” samples, the above characteristics become progressively less pronounced with decreasing acidity. The gabbro-noritic “Appinites” (Mg# = 67-75, Ni and Cr contents up to 163 ppm and 882 ppm, respectively) are the most primitive reported up to now among the Permian magmatic rocks from Southern Alps east of the Ivrea Verbano Zone. On the basis of their geochemical and isotope patterns, they can be considered with reasonable confidence as mantle derivatives that underwent very little, if any, crustal contamination. Their overall geochemical and isotope characteristics (eNd = –0.06/-2.45, (87Sr/86Sr)280 = 0.7044-0.7072), suggest an enriched mantle as the source of “Serie dei Laghi” magmatic series. Large-scale correlation of the whole rock series in the Nd and Sr covariation diagrams indicates the prominence of mantle-crust interaction in the evolution of the Permian plutonic rocks from “Serie dei Laghi ”. The correlation between isotope ratios and SiO2 suggests evolution of the magmas in crustal chambers. The entire Permian intrusive series was generated through complex crust-mantle interaction mechanisms, starting from a mantle-derived magma isotopically similar to gabbro-noritic “Appinites”. The following frame can be depicted: a basaltic magma rising from the mantle resided in a magma chamber at the base of the crust and underwent combined assimilation and crystal fractionation (AFC): the amount of assimilated crust at this stage is about 10%. A late-orogenic transtensive regime allowed small magma batches to rise from the chamber directly to the surface, without further crustal exchange (“Appinites”). Continued magma input, crustal assimilation, crystallization, and tapping, extended the duration of the magma chamber until post-orogenic stage. Crustal extension permitted the formation of shallower magma chambers in which large volumes of hybrid magmas (Alzo-Roccapietra, Montorfano and Baveno-Mottarone plutons) rested and further assimilated crustal material (20-25%).