I don’t believe that the solid crust contributes to the creation of the magnetic field. I think it’s creation is due to the motion of the molten conductive iron in the liquid portion of the Earth’s core.
Tectonic action would create enormous pressures on crystalline rock, generating electrical forces such as those seen in geologically active earthquake zones, and volcanic zones.
Depending on the makeup of the local crust, conductivity and possibly even "semiconductivity" ( silicon diodes ) could be possible. ( Semiconductivity would be akin to conduction of electricity in one direction, but high resistance in the other, resulting in a sort of directional current flow. )
If electromagnetic forces were being created from the frictional and pressure forces of tectonic subduction at the threshold of mantle and crust, it's possible the two dissimilar layers transfer that electrical force into the mantle much like a transformer uses a solid metal core.
The result is the core producing an amplified electrical field on a planetary scale.