Magma transfer between lower and upper continental crust is a fundamental process linking the dominantly mafic composition of the lower crust with the more felsic composition of the upper crust. We explored the mechanisms of mafic magma ascent and emplacement in the middle crust by studying a midcrustal gabbroic to dioritic magmatic system (Sondalo complex, eastern central Alps, northern Italy). We characterized the structure and anisotropy of magnetic susceptibility (AMS) fabric of concentric gabbroic to dioritic intrusions. The significance of AMS fabrics is discussed using anisotropy of anhysteretic remanent magnetization (AARM) and crystallographic preferred orientation (CPO) data acquired on different test sites. The magmatic and magnetic fabrics of the pluton were acquired during its emplacement in the Permian and were not subsequently tilted: The fabrics are essentially vertical, indicating vertical magma transfer through the crust with a two-phase intrusion history. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggests that the first magma ascent phase occurred along pathways subparallel to the vertical fabric of the host metasedimentary rocks. (2) The second magma ascent phase was controlled by a change in the rheology of the host rock and the mafic magma. Heat dissipation to the contact aureole induced partial melting, thereby lowering the mechanical strength of the host rocks, whereas the viscosity of the mafic magma increased due to cooling and associated fractional crystallization. This caused an en-masse rise of the pluton, resulting in the formation of a structural aureole, i.e., a vertical foliation in the contact aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase accounts for the pressure-temperature evolution recorded by metasedimentary rocks in the contact aureole of the pluton.
Mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust: The Sondalo magmatic system (N Italy)
Benoît Petri, Etienne Skrzypek, Geoffroy Mohn, Tsvetomila Mateeva, Philippe Robion, Karel Schulmann, Gianreto Manatschal, Othmar Müntener; Mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust: The Sondalo magmatic system (N Italy). GSA Bulletin doi: https://doi.org/10.1130/B31693.1
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