The peridotite-pyroxenite sequence of Rocca d'Argimonia (Ivrea-Verbano Zone, Italy): Evidence for reactive melt flow and slow cooling in the lowermost continental crust

• Published in: Chemical geology 2023-03, Vol.619, p.121315, Article 121315
• Main Authors: Tribuzio, Riccardo, Renna, Maria Rosaria, Antonicelli, Marta, Liu, Tong, Wu, Fu-Yuan, Langone, Antonio
• Format: Article
• Language: English
• Subjects: Isotopic disequilibrium; Melt-peridotite reaction; Nd-Hf-Sr isotopes; Post-collisional Variscan evolution; Slow cooling evolution; U-Pb zircon geochronology
• ISSN: 0009-2541
• DOI: 10.1016/j.chemgeo.2023.121315

The present study investigates the origin of a ∼ 400 m thick peridotite-pyroxenite sequence, locally known as Rocca d'Argimonia, which is encased within the lowest levels of the ∼8 km thick Mafic Complex, a gabbro-dioritic batholith from the lower continental crust section of the Ivrea-Verbano Zone (Southern Alps). The broad purpose of this work is to elucidate the evolution of mantle-derived magmas emplaced in the lowermost continental crust. We also wish to provide new geochronological constraints on the intrusion and the cooling evolution of the Lower Mafic Complex, which are still mostly unknown. Zircon grains separated from two gabbronorites enclosing the peridotite-pyroxenite sequence document a whole reset of the original UPb isotopic system at 286 ± 2 Ma. A Rocca d'Argimonia peridotite provided a consistent age of 296 ± 19 Ma, based on a LuHf clinopyroxene-amphibole alignment (initial εHf = +7.9). These results are interpreted to date the early cooling evolution of the Lower Mafic Complex. The clinopyroxene-amphibole peridotite pair also gave a RbSr alignment corresponding to an age of 250 ± 16 Ma (initial 87Sr/86Sr = 0.7045), which is reconciled with the subsequent, slow cooling evolution of the Lower Mafic Complex. The Rocca d'Argimonia peridotites (dunites to harzburgites and lherzolites) have whole-rock εNd(286 Ma) and εHf(286 Ma) values ranging from +0.4 to −1.1 and from +8.8 to +3.4, respectively, and 87Sr/86Sr(286 Ma) ratios varying from 0.7048 to 0.7060. The peridotites are crosscut by gabbronorite dykes and associated with olivine-free orthopyroxene-dominated pyroxenites. The gabbronorite dykes and the pyroxenites share the Nd-Hf-Sr signature of the peridotites. The gabbronorites enclosing the peridotite-pyroxenite sequence differ in the relatively enriched Nd-Hf-Sr isotopic fingerprint (εNd(286 Ma) = −4.5 to −5.6, εHf(286 Ma) = +0.2 to −2.5, 87Sr/86Sr(286 Ma) = 0.7073 to 0.7086). Remarkably, pyroxenes from the pyroxenites typically contain higher Cr2O3 than the peridotite counterparts (e.g., 0.23–0.37 wt% Cr2O3 in peridotite orthopyroxenes and 0.43–0.50 wt% Cr2O3 in pyroxenite orthopyroxenes). We propose that the peridotite-pyroxenite sequence formed by reactive melt percolation through an olivine-rich spinel-bearing matrix. In particular, the relatively high Cr2O3 content of the pyroxenitic pyroxenes is attributed to redistribution of Cr released by the breakdown of accessory spinel that was originally present in the olivine-dominated ma