Is the Finero Complex (Ivrea Verbano Zone) originally Proterozoic or even older? Inferences from in-situ Sr isotope disequilibria

The Finero Complex in the northern Ivrea-Verbano Zone represents one of the most enigmatic cross-sections through the mantle and lower continental crust. The mantle peridotite unit (Phlogopite Peridotite, PP) consists of pervasively metasomatized spinel-harzburgite enriched with abundant phlogopite and amphibole, locally exhibiting Cr-diopside veins (Zanetti et al., 1999). This unit is in contact with a mafic complex of lower crustal origin and Permian age (Lu et al., 1997), composed of three distinct units (Siena and Coltorti, 1989): i) the Layered Internal Zone (LIZ), which includes lherzolites, garnet-hornblendites, pyroxenites, anorthosites, and garnet-gabbros; ii) the Amphibole Peridotite (AP); iii) the External Gabbro (EG), which borders the Kinzigite Formation. The genetic relationships among these units remain a subject of debate.

We carried out in-situ Sr-isotope analyses of clinopyroxene, amphibole, and plagioclase across all lithologies of the Finero Complex to identify isotopic disequilibria that might indicate multiple, genetically distinct igneous events. In-situ Sr-isotope measurements were performed using LA-MC-ICP-MS at the Geochemistry Laboratory (GL@M) of the University of Milan.

In the PP samples, clinopyroxene and amphibole are in isotopic equilibrium but ⁸⁷Sr/⁸⁶Sr is highly variable, ranging from around 0.7040 to 0.7080. In the LIZ lithologies, amphibole shows a narrow variation in ⁸⁷Sr/⁸⁶Sr (0.7020 - 0.7030) and it is in isotopic equilibrium with clinopyroxene. An exception is observed in the anorthosite, where clinopyroxene exhibits a significantly more radiogenic signature (⁸⁷Sr/⁸⁶Sr > 0.7040). Plagioclase in all LIZ lithologies is more radiogenic than amphibole, with ⁸⁷Sr/⁸⁶Sr values ranging from around 0.7030 to 0.7040. In the AP, clinopyroxene and amphibole are isotopically equilibrated, with values close to 0.7035 ± 0.0001 (2SD). In the EG, plagioclase displays a significantly less radiogenic ⁸⁷Sr/⁸⁶Sr values (around 0.7030) compared to amphibole and clinopyroxene (>0.7070), showing isotopic disequilibrium.

Amphibole and clinopyroxene from the PP show variable crustal signatures supporting differences in the metasomatic agents (Zanetti et al. 1999). In both the AP and LIZ, plagioclase consistently shows more radiogenic values than amphibole, which is in isotopic equilibrium with clinopyroxene, except in the anorthosites. The highly unradiogenic ⁸⁷Sr/⁸⁶Sr signature of plagioclase in EG, close to that found in LIZ, reveals that plagioclases are remnants of a former reacted lithology. These observed isotopic disequilibria suggest that present-day Finero Complex is the result of infiltration and reaction of melts with different ⁸⁷Sr/⁸⁶Sr signatures into a pre-existing igneous complex. The less reacted lithologies, preserving similar ⁸⁷Sr/⁸⁶Sr signatures are lherzolites, pyroxenites and wehrlites. This peculiar rock association, the layered structure and the extremely depleted initial ⁸⁷Sr/⁸⁶Sr signature (down to 0.7020) closely resemble that of some Proterozoic, or even older, layered mafic-ultramafic complexes.

 

Lu, M. Hoffman H.W., Mazzucchelli M., Rivalenti G. 1997. The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone). Chem. Geo. 140, 207-222. 

Siena, F., and M. Coltorti. 1989. The petrogenesis of a hydrated mafic-ultramafic complex and the role of amphibole fractionation at Finero (Italian Western Alps)." Neues Jahrbuch für Mineralogie Monatshefte 6, 255-274.

Zanetti, A., Mazzucchelli, M., Rivalenti, G., & Vannucci, R. 1999. The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism. Contributions to Mineralogy and Petrology, 134(2), 107-122.