Granite Genesis by Magma Hybridisation: New Insights from the Sesia Magmatic System (Ivrea Zone, Italy)
Creators: name: Sieber, Veronique Pascale Janine
Publisher: name: ETH Zurich
Year: 2023
Subjects: Extraction channels
Granite genesis
Opx-dominated series
Sesia magmatic system
Trans-lithospheric magmatic system

description: The Ivrea-Verbano Zone, located in a Permian post-collisional tectonic setting, provides an opportunity to study the complexities of a trans-lithospheric magmatic system, which are the major producers of new felsic crust. These systems typically exhibit gabbroic rocks in the lower, intermediate rocks in the middle and granitic rocks in the upper crust. The chemical formation of granites in trans-lithospheric magmatic systems can occur through different processes, including the differentiation of mantle-derived melts in a closed-system or with an interaction of crustal materials in an open-system. The melt extraction and the migration between the lower and upper crust are defined and connected by intra-mush processes or extraction channels.

This study focuses on understanding the granite genesis within the Val Sessera in the Ivrea-Verbano Zone, which is part of the paragneiss-bearing belt. This area comprises gabbroic rocks, intermediate rocks (orthopyroxene-dominated rocks) and metasediments. Based on field measurements, the observation of the magmatic contacts between the different lithologies, the lens-shaped and non-uniform distribution of the opx-dominated series, the absence of cumulate texture, the presence of fine- and coarse-grained variation in the opx-dominated series, as well as the embedded metasediments, it can be inferred that the melt extraction and the migration in Val Sessera occurred through extraction channels and the intermediate rocks have undergone differentiation within them.

Geochemical analyses of bulk rock compositions reveal a clear differentiation trend, with the orthopyroxene-dominated series acting as a transitional link between the gabbros and the granites. Neodymium isotopes show similarities among the amphibole-bearing gabbronorites, orthopyroxene-dominated series and granites, suggesting an early hybridisation. However, the isotopic data also support the presence of a lithospheric mantle and hence, a possible source contamination. Nevertheless, the mineral geochemistry reveals assimilation processes, evidenced by the enrichment of anorthite during the differentiation pathway and K-enrichment in amphibole. Combined with the ternary diagram of the bulk rock major elements, the evidence points towards early assimilation of crustal materials in mantle-derived melts.

However, further investigation of an uncontaminated amphibole-bearing gabbronorite would provide additional insights. Comparison with hybridisation experiments suggests a proportion of more than 25% of assimilated pelitic material into the mantle-derived melts and that the orthopyroxene-dominated series represent a mixture of cumulate opx + plag + differentiated, more or less granitic interstitial liquid. The comprehensive investigation of the Val Sessera has provided valuable insights into the magmatic processes and crustal evolution within a post-collisional setting. The multidisciplinary analysis has revealed an early interaction between mantle-derived melts and crustal components, leading to hybridisation effects that significantly influenced the petrogenesis of the studied rocks.