Transition from “orogenic-like” to “anorogenic” geochemical affinity in Mesozoic post-collisional magmatism: evidence from alkali-rich dykes from Ivrea-Verbano Zone (Southern Alps)

Dyke swarms intruding the mantle–continental crust transition of the Adria plate as documented by the Ivrea-Verbano Zone (IVZ, Southern Alps) represent a unique opportunity to investigate the evolution of mantle melts from Late Paleozoic to Mesozoic in the post-collisional Variscan realm. Thus, we present new petrological and geochemical data of dyke swarms cropping out in the Finero Phlogopite Peridotite mantle unit. Dykes are from a few cm to >1 m thick and cut at a high angle the mantle foliation.

The dyke swarms are composed of cumulus phlogopite-bearing amphibole peridotite, hornblendite, diorite and anorthosite. Many dykes are composite, showing variable proportions of melanocratic and leucocratic layers. Volatiles overpressure during the late magmatic stage is testified by plastic flow and development of a porphyroclastic structure by deformation of early cumulates and by the widespread segregation of a fine-grained mica matrix. The dyke swarms show mineralogical and geochemical features varying between two end-member series.

A dyke series is characterized by Al-rich pargasite (Al₂O₃ up to 18 wt.%) and phlogopite, associated with apatite, calcite, sulphides and sometimes sapphirine. The amphiboles show i) large LILE and LREE contents, ii) negative Nb, Ta, Zr and Hf anomaly and iii) isotopic oxygen composition heavier than the mantle interval, which support the occurrence of recycled continental crust components in the parent melts and impart an overall “orogenic” affinity.

The second series mainly consists of Al-poorer pargasite, phlogopite and albite (An 8-10), associated with apatite, monazite, ilmenite, zircon, Nb-rich oxides and carbonates. Mineral compositions and assemblages indicate that the parent melts were strongly enriched in Fe, Na, H₂O, P and C. Amphiboles are still enriched in LILE and LREE, but show extreme enrichments in Nb, Ta, Zr and Hf. As a whole, the petrochemical features point to an “anorogenic” alkaline affinity. Zircons from the “anorogenic” dykes are mostly anhedral, with homogenous internal structure or sector zoning. The strongly positive εHf_t (average of +10) of zircons and the Sr isotopic composition of amphiboles (0.7042) point to a derivation of such “anorogenic” melts from mildly enriched mantle sources. Concordant ²⁰⁶Pb/²³⁸U zircon ages for “anorogenic” dykes vary from 221 ± 9 Ma to 192 ± 8 Ma. Some dykes show both “orogenic” and “anorogenic” affinities, thus recording different pulses of mantle melts and metasomatic overprinting. As a whole, the dyke swarms show a transition from “orogenic” to “anorogenic” affinity indicating re-opening of dykes’ conduits for the melt ascending, pointing to a progressive change of the mantle sources of the Mesozoic magmatism of the Southern Alps.