New Insights into the Formation of Phanerozoic Continental Crust: Evidence from Metamorphosed Metabasites in the Ivrea-Verbano Zone, Italy

The Ivrea-Verbano Zone (IVZ, western Alps) exposes an iconic middle-lower continental crustal section of the Adriatic Plate. This metamorphic sequence mainly consists mainly of metapelites/metapsammites and metabasites, with numerous lenses of metacarbonate rocks. The metamorphic grade of the crustal section increases progressively in P-T conditions, from amphibolite to granulite facies, with increasing crustal paleo-depth moving from East to West. The metamorphic overprinting renders particularly difficult determining the origin of metabasites (Leake, 1964). The amphibolite facies metabasites occur as numerous layers intercalated between siliciclastic metasediments. The mineralogical composition of metabasitesis is variable. These variations may be related to the original variability of the protolith or to interaction with surrounding metasediments and marble lenses. Despite this heterogeneity, pioneering studies identified a possible MORB signature from geochemistry (Sills & Tarney, 1984; Mazzucchelli & Siena, 1986).

In this contribution, we report new constraints on the geochemical affinity of the protoliths for all the amphibolite-facies metabasite paragenesis using major and trace elements and isotopes of the bulk rocks. To understand the primary signature of amphibolite and the metamorphic changes, we collected amphibolites from borehole 5071_1_B, drilled in Ornavasso (Val d’Ossola, Italy) in the frame of the DIVE-ICDP project (Pistone et al., 2017). We selected 13 samples representative of amphibolite sequences covering all different mineralogical parageneses, and one representative of siliciclastic metasediments. The selected metabasite samples are divided in the following four groups by mineralogy: (i) amphibolite s.s. (Amph+Cpx+Pl±Qz), (ii) garnet-bearing amphibolite, (iii) biotite-bearing amphibolite, and (iv) carbonate-rich amphibolite.

The results obtained from the characterization of bulk trace elements provide insights into the geochemical affinity of the sequence, revealing that it derives from two different protoliths with N-MORB and E-MORB affinities, respectively. Moreover, highly incompatible elements, which reflect the original signature in some samples, have been partially or totally modified by various high-temperature metamorphic events and contaminated by fluids migrating from surrounding metaterrigenous and metacarbonate lithologies. To better discriminate the original magmatic signature from the metamorphic overprinting, we used the isotopes data. The protholith signature is well-preserved in the garnet-bearing amphibolites and amphibolite s.s. by the Nd isotopes, which show a range of εNd values between 5.16 and 6.86, indicating a mantle-derived source for the parental melt that formed the amphibolite. In contrast, the ⁸⁷Sr/⁸⁶Sr₄₀₀ (0.70492–0.71562) and ²⁰⁶Pb/²⁰⁴Pb₄₀₀(18.4200–18.9536) ratios precisely track the remobilization of crustal signatures in all lithologies, reflecting the overprinting that occurred during collisional and post-collisional metamorphism.

The mineralogical evidence, in combination with trace element and geochemical data, suggests that the metabasites are volcanic sequence of rocks erupted in an extensional basin, being successively buried at ~20 km depth in the continental crust.

References:

Leake BE (1964), J Petrol, 5, 238-254

Mazzucchelli M & Siena RC (1986), TMPM, 35, 99-116

Pistone et al. (2017), Sci Dril, 23, 47-56

Sills JD & Tarney J (1984), Tectonophysics, 107, 187-206