Metasomatism by Boron-Rich Fluids along Permian Low-Angle Normal Faults (Central Southern Alps, N Italy)

Abstract

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper Carboniferous-Permian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.

Evidence of Early Permian extension during the post-Variscan evolution of the central Southern Alps (N Italy)

“…6, 7). The occurrence of tourmalinites along Early Permian fault zones represents a common feature already described for other fault systems of the same age in the Aga-Vedello area (Zanchi et al 2019;Zanchetta et al 2022). Preserved sectors of the Fig.…”

“…In the study area, as in other parts of this sector of the Alps (Zhang et al 1994;Slack et al 1996;De Capitani et al 1999;Zanchi et al 2019;Zanchetta et al 2022) the low-angle normal fault planes are invariably decorated with cataclasites frequently transformed into tourmalinites, precipitated from Boron-rich fluids, during or shortly after fault activity.…”

“…Aga Conglomerate overlying the Variscan basement along the exceptionally preserved Aga-Vedello LANF. This fault, together with the Masoni LANF, documents the Early Permian extension (Zanchi et al 2019;Zanchetta et al 2022), generating asymmetric half-grabens deepening toward the basins depocenters. This low-angle fault system interacted with high-angle normal faults active in the hanging wall during the deposition of the lowermost Pizzo del Diavolo Formation, where co-seismic soft-sediment deformation structures are abundant (Berra and Felletti 2011;Zanchi et al 2019Zanchi et al , 2021Zanchetta et al 2022).…”

“…The fault planes related to the Permian extensional system are often decorated with cryptocrystalline to aphanitic tourmalinites impregnating cataclasites formed along with the basement-cover fault contacts (Zhang et al 1994;De Capitani et al 1999;Zanchi et al 2019;Zanchetta et al 2022). Tourmalinites were likely formed after the circulation of Boron-rich hydrothermal fluids released by the emplacement of granitic and granodioritic bodies in the upper crust (De Capitani et al 1999) in the 290-275 Ma time interval (Pohl et al 2018).…”

“…Tourmalinites play a significant role in our reconstruction since they can be considered as proxies of the coeval Early Permian magmatism and tectonic extensional regime in the central Southern Alps (Zanchetta et al 2022). Samples coming both from preserved and from reactivated low-angle normal fault' segments have been studied to provide a full characterization of their microstructure and mineralogical composition.…”