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 $\text{Rb}\text{Sr}$ 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 ($\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.704−0.709}$) Their initial Pb isotope ratios ($\text{(}{}^{206}\text{Pb}{}^{204}\text{Pb}\text{)}{}_{\mathrm{<mtext>i</mtext>}}\text{= 17.47−18.00}$; (${}^{207}\text{Pb}{}^{204}\text{Pb}\text{)}{}_{\mathrm{<mtext>i</mtext>}}\text{= 15.55−15.63}$; (${}^{208}\text{Pb}{}^{204}\text{Pb}\text{)}{}_{\mathrm{<mtext>i</mtext>}}\text{= 37.66−38.12}$) 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 ($\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.705−0.710}$) The Pb isotope ratios of PG [$\text{(}{}^{206}\text{Pb}{}^{204}\text{Pb}\text{)}{}_{\mathrm{<mtext>i</mtext>}}\text{= 18.12–18.53}$; (${}^{207}\text{Pb}{}^{204}\text{Pb}\text{)}{}_{\mathrm{<mtext>i</mtext>}}\text{= 15.63−15.73}$; (${}^{208}\text{Pb}{}^{204}\text{Pb}\text{)}{}_{\mathrm{<mtext>i</mtext>}}\text{= 38.08–38.73}$] 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.