Mechanical Anisotropies and Mechanisms of Mafic Magma Ascent in Middle Continental Crust: The Sondalo Gabbroic Complex (N Italy)
Abstract
The ascent mechanisms of magma through the continental crust remain a long standing controversy. The pathways of intermediate to felsic magmas can be continuously traced through the crust, however mafic magma transfer between lower and upper crustal levels is rarely documented. To fill this gap, we explore the mechanisms of mafic magma ascent and emplacement in middle continental crust. We characterize the structure and anisotropy of magnetic susceptibility (AMS) fabrics of a mid-crustal mafic complex (Sondalo gabbroic complex, N-Italy) together with Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Crystallographic Preferred Orientation (CPO) data. Field data indicate concentric gabbroic to dioritic intrusions emplaced in sub-vertically foliated metasedimentary host-rocks. The petrofabrics and magnetic fabrics of the pluton (foliations and lineations) are coaxial, syn-magmatic and sub-vertical. U-Pb dating of zircons along with the structural record of the plutonic rocks indicate two major pulses of magma emplacement in sub-vertical channels. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggest that the early emplacement phase occurred through magma fracture opening subparallel to the vertical fabric of the host rocks at 289-288 Ma. (2) The second magma ascent phase was controlled by a change in the rheology of the host-rock and the mafic magma. The temperature increase in the contact aureole induced partial melting and decreased its mechanical strength, whereas the viscosity of the mafic magma increased due to progressive cooling and crystallization. This caused an en-masse rise of the crystal mush and drag forces resulting in the formation of a vertical foliation in the metamorphic aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase is slightly younger (288-285 Ma) and accounts for the contrasted P-T evolution recorded by metasedimentary rocks in the contact aureole.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.V41E..07P
- Keywords:
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- 1036 Magma chamber processes;
- GEOCHEMISTRYDE: 1037 Magma genesis and partial melting;
- GEOCHEMISTRYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGYDE: 8486 Field relationships;
- VOLCANOLOGY