Looks like you will need to remove both the broken stanchion and the attached cast aluminium base from your boat, to your workshop. There you can apply lots of heat to the base in order to break up the corrosion that seems to have resulted in one part gripping the other. Normally there is a piece of plastic inserted between the stainless steel stanchion and the aluminium base to prevent or reduce the problem of corrosion between the aluminium and the stainless steel stanchion, so any heat you apply will melt the plastic if it still remains. Don’t forget to replace the plastic when you reassemble with new parts.
