If you are dead set on having meta magic create new nodes, perhaps additionally require that it be the highest tier of geomancer (e.g. archmage equivalent) and consume the unit. Lore-wise, one could argue, the amount of energy required to crystallize all that mana into a new node simply burns the person out. Perhaps also limit it by level, even if the geomancer unit type follows the same upgrade restrictions as other spellcaster units.
It's still pretty spammable, but even so giving up a level 6 or even level 8 or 10 unit is arguably a fair cost.