Airborne magnetic data compared to petrology of crustal scale shear zones from southern Madagascar: A tool for deciphering magma and fluid transfer in orogenic crust,
Résumé
The southern part of Madagascar consists of a granulitic metamorphic belt with a complex Proterozoic shear zone network. Aeromagnetic maps reveal sharp magnetic spatial gradients, especially across shear zones. All shear zones are associated with high magnetic values, except one, the Beraketa shear zone. Based upon relationships between rock magnetic properties, petrographic and aeromagnetic data, we show that the magnetic signal is controlled by variations in proportions of iron-rich oxides. Their nature and texture are variable and complex. Magnetite and ilmenite are often observed together showing intergrowths texture, suggesting possible lamellar magnetism. Detailed petrographic observations of the Zazafotsy shear zone show that a strong magnetic signal is correlated with metamorphic reactions and especially with biotite breakdown to peritectic phases such as orthopyroxene and iron-rich oxides (metamorphic charnockitization). Magmatic material can migrate easily inside the Zazafotsy shear zone and inside the fold hinges close to the shear zone, increasing the kinetics of charnockitic reaction. In opposition, inside the Beraketa shear zone, lower magnetic values are correlated with the absence of iron-rich oxides. This is interpreted as back reaction melting. Thus, peritectic phases, such as iron-rich oxides, react with the water released when magmas crystallise to produce biotite. Inside the Zazafotsy shear zone, iron-rich oxides are stable because part of the migmatite was segregated and escaped with dissolve H2O. In this case, back reaction was no longer possible.