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Título

Evidence for interaction of mantle-derived magmas and crustal metasedimentary rocks in the Sesia Magmatic System, Ivrea Verbano Zone, Italy

Texto do resumo

Processes that lead to the formation of granitic magmas are currently a topic of discussion. Some suggest that they originate mainly from fractional crystallization of mantle-derived magmas, while others argue instead for predominance of crustal melts at the origin of granites and related rocks. Indeed, plutonic rocks emplaced in the upper continental crust are often hybrid, with compositions that lie between those typically associated with mantle-derived and (meta-)sedimentary origins. The crustal section exposed in the Ivrea Verbano Zone (IVZ) is an excellent natural laboratory to study how magma is generated in the lower crust and subsequently evolve in composition to form granites in the upper crust. In this study we combine field observations at kilometre to metre scale with petrography and accessory mineral (zircon and apatite) petrochronology at the macro- to micro-scale to unravel how hybrid magmas are created and evolve over time.
The IVZ and Strona Ceneri Zone expose lower (to the NW) to upper (to the SE) crustal rocks that were affected by Carboniferous metamorphism. Although partial melting started as early as 310 Ma, granites are only emplaced at upper levels in the Lower Permian (ca. 285 Ma), coeval with mantle-derived magma intrusion in the lower crust (Mafic Complex). Within the Mafic Complex, south of the area, a region of interaction between mafic magmas and crustal lithologies (granulitic paragneiss) is observed, where charnockites occur spatially at the interface between both lithologies.
Locally, charnockite intermingle with gabbro and are also in textural continuity with leucosome veins, both concordant and discordant relative to the foliation of the host migmatitic paragneiss. Zircon and apatite chemical-isotopic compositions corroborate the hypothesis that charnockites are intermediate between mafic melts and metasedimentary rocks. Furthermore, zircons display a variety of textures that, along with chemical composition, indicate a long-lived history of injection of magma, interaction, and possibly re-injection. The detailed study of a lower crustal segment of the IVZ thus reveals the existence of a hybridization zone where mantle-derived magma interacts with crustal melts, resulting in the formation of charnockites. In turn, these are now hybrid cumulates resulting from the extraction of residual melt that was transported upwards to form granitic plutons. As such, they represent the missing link between crustal- and mantle-derived magmatic end-members, and upper crustal granites/surface volcanism.

Palavras Chave

Hybridization; Lower crust; Magmatism