Say a player casts a card that returns a permanent to the top of owner's library, for example Banishment Decree, and the targeted creature card he chooses has two Auras enchanting it. Where do the Aura enchantments go, to the players hand, graveyard or library?
Assuming when you say 'Enchantment' you specifically mean 'Aura', they are sent to the graveyard.
Bold is mine.
Some popular cards, like Rancor and Angelic Destiny, may appear to violate this rule. There are a number of cards that have triggers that will pop the Aura out of the graveyard and send it to your hand. Many players will take the shortcut of simply taking the card from the battlefield to their hand instead of taking the intermediate step of putting it in the graveyard. If there's any confusion about such a short cut, make sure to ask your opponent or call a judge, because you do have an opportunity to respond before the trigger resolves while the card is still in the graveyard.
Also, if you were referring to another enchantment type you'll have to clarify. As far as I'm aware, only Auras can attach to creatures and other permanents.
If an object ceases to exist, and creatures can only exist on the battlefield, any Auras attached to the creature are placed in their owner's graveyard as a state-based action.
It goes to the graveyard.
You seem to be under the misconception that Auras follow the permanent to which they are attached to the graveyard. That's not the case. It's just a coincidence that both end up in the same place.
When an object changes zone (e.g. goes from the battlefield to the graveyard), it becomes a brand new object. The original object ceases to exist, and a new one is created in the new zone.
No matter whether the enchanted permanent goes to the graveyard or to the player's library, the object the Aura was enchanting has ceased to exist. The game doesn't tolerate that since Auras are suppose to enchant something, so it sends the Aura to the graveyard.
If it was an Equipment or Fortification instead, it would remain on the battlefield unattached.