Doesn't need to be a secession amendment. From the Declaration of Independence:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.--That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, --That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.The U.S. Constitution is part and parcel of the Form of Government of the USA -- the government the people have the right to alter or abolish, when it becomes destructive of securing their rights. Secession is a form of altering the government -- by removing from its jurisdiction some part of the population and territory it governs, or, alternately by abolishing it within certain areas -- and instituting a new one.
This means the Constitution is subordinate to the right of the people to alter or abolish the government it authorizes. A Constitutional amendment is unnecessary.