Not in any part I can find, either. Nor can I find anything in the Constitution which allows secession without the consent of states. Perhaps you can point that part out?
Sure, no problem. It's right there in the Tenth Amendment:
Amendment 10Unless you can point out where the power of approving secession was delegated to the United States or prohibited to the states, then it is a power they retain individually.The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.