Attilio Boriani

Paleozoic evolution of southern Alpine crust (northern Italy) as indicated by contrasting granitoid suites

Abstract The western southalpine crust of northern Italy and southern Switzerland consists of high-grade (Ivrea Verbano) and medium-grade rocks (Serie dei Laghi). The latter contains Ordovician (OG) and Permian granitoids (PG). The OG, which range in composition from diorite to leucogranite, show a calcalkaline metaluminous character. The felsic lithologies yield a Rb Sr W.R. isochron of 466 ± 5 Ma with I.R. of 0.7087. The most mafic lithologies do not plot on the isochron, but arrange on a mixing line indicating mantle-crust interaction ( ( 87 Sr 86 Sr ) i = 0.704−0.709 ) Their initial Pb isotope ratios ( ( 206 Pb 204 Pb ) i = 17.47−18.00 ; ( 207 Pb 204 Pb ) i = 15.55−15.63 ; ( 208 Pb 204 Pb ) i = 37.66−38.12 ) fall within typical values of the lower crust. Linear trends in the Pb-Pb diagrams are in agreement with a mantle-crust interaction. The PG consist of mafic to intermediate dykes and granitic plutons with calcalkaline and weakly peraluminous character. Two felsic plutons give two whole rock isochrons at 276 ± 7 and 277 ± 8 Ma with the same I.R. of 0.710. The mafic dykes show Sr isotopic variations indicating mantle-crust interaction ( ( 87 Sr 86 Sr ) i = 0.705−0.710 ) The Pb isotope ratios of PG [ ( 206 Pb 204 Pb ) i = 18.12–18.53 ; ( 207 Pb 204 Pb ) i = 15.63−15.73 ; ( 208 Pb 204 Pb ) i = 38.08–38.73 ] are distinctly higher than those of OG and similar to those of the Ivrea Verbano metasediments. OG are depleted in both LILE and HFSE, show less pronounced negative anomalies of Sr, P and Ti and have lower Sr and Pb isotope ratios than PG. For both OG and PG, we suggest that the most mafic representatives were generated through interaction of mantle-derived magma and crustal material, while the granitoids derived from the resulting hybrid magma mainly through crystal fractionation. The ultimate sources of the Ordovician and Permian magmatisms were different both geochemically and isotopically, reflecting two different geodynamic environments: a convergent plate boundary in an accretionary terrane in the Ordovician, a post-orogenic, extensional regime in cratonized continental crust in the Permian. Geological data are compatible with such environments.

The Pb isotopic systematics during crustal contamination of subcrustal magmas: the Hercynian magmatism in the Serie dei Laghi (Southern Alps, Italy)

Abstract The Pb isotopic composition and Pb, U and Th concentrations of a series of Hercynian granite plutons and mafic-intermediate stocks and dykes belonging to the Serie dei Laghi (Western Southern Alps) have been analyzed. The initial Pb isotope ratios of both the granites and the dykes are typical of the lower crust, and are spread over a significant range [( 206 Pb 204 Pb ) 280 =17.883–18.530 , ( 207 Pb 204 Pb ) 280 =15.597–15.710 and ( 208 Pb 204 Pb ) 280 =38.077–38.708 ]. Four dykes, all at the same locality, display evidence of post-emplacement fluid circulation, with mobilization of U and Pb. In spite of the Pb isotopic crustal signature of the studied rock series, the 87 Sr 86 Sr ratios reach values as low as 0.704 in the dykes and as low as 0.707 in the granites, suggesting a mantle contribution in the genesis of both lithologies. Indeed, when compared with the metasediments from the same area, the magmatic rocks have somewhat lower initial 207 Pb 204 Pb ratios, which weakly trend towards the less radiogenic Pb isotopic composition of the mantle. The relationships between the Pb isotopic composition and the Pb content, along with the Pb and Sr isotopic covariation, suggest that the suite under examination was generated from a mantle-derived primary magma that underwent extensive crustal contamination. A primary magma similar to the neighbouring Ivrea mafic complex has been inferred. Further assimilation of crustal material is particularly evident in the dykes, while the granites appear to have been generated mainly by crystal fractionation processes from the intermediate magma, already contaminated.

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” 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 characters 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 (ϵNd = −0.06/-2.45, (87Sr/86Sr)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 mantlecrust interactions in the evolution of the Permian plutonic rocks from the Serie dei Laghi. Moreover, the correlation between isotope ratios and SiO2 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 crustal 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%).