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)

摘要

In the "Serie dei Laghi" of the Southwestern Alps, large Permian granite plutons and mafic-intermediate stocks and dykes (the "Appinite suite") were emplaced in a post-collision environment. The magmatic rocks of the Serie dei Laghi are metaluminous and exhibit petrographic and geochemical characteris typical of medium-K, calc-alkaline series. Granites and leucocratic Appinites show LREE and LILE enriched patterns and negative Nb, P and Ti spikes typical of calcalkaline series. In the mafic Appinite samples, the above characteristics become progressively less pronounced, as their acidity decreases. The gabbro-noritic Appinites (Mg# = 67-75, Ni and Cr contents up to 163 ppm and 882 ppm, respectively) are the most primitive of the Permian magmatic rocks reported to date in the Southern Alps east of the Ivrea Verbano Zone. On the basis of their geochemical and isotope patterns, they can, with a reasonable degree of confidence, be considered mantle derivatives that underwent very little, if any, crustal contamination. Their overall geochemical and isotope characteristics (ε_(Nd) = -0.06/-2.45, (~(87)Sr/~(86)Sr)_(280) = 0.7044-0.7072) suggest an enriched mantle as the source of the Serie dei Laghi magmatic series. The strong overall correlation of the whole rock series in the Nd and Sr covariation diagrams indicates the prominence of mantle-crust interactions in the evolution of the Permian plutonic rocks from the Serie dei Laghi. Moreover, the correlation between isotope ratios and SiO_2 suggests that the magmas evolved in crustal chambers. The entire Permian intrusive series was generated through complex crust-mantle interaction mechanisms which began with a mantle-derived magma isotopically similar to gabbro-noritic Appinites. The following picture can thus be envisioned: 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 was about 10%. A late-orogenic transtensive regime allowed small magma batches to rise from the chamber directly to the surface without further rustal exchange (Appinites). Continued magma input, crustal assimilation, crystallization and tapping extended the duration of the magma chamber up until the post-orogenic stage. Crustal extension enabled the formation of shallower magma chambers in which large volumes of hybrid magmas (the Alzo-Roccapietra, Montorfano and Baveno-Mottarone plutons) resided and further assimilated crustal material (20-25%).