WASHINGTON, Dec. 1--Neil H. McElroy, the first space age secretary of defense, resigned today and Philadelphia banker Thomas Sovereign Gates, Jr. was promoted to the top Pentagon post, McElroy is leaving after 26 months at the helm of the Defense Department.

Meeting with newsmen a few hours after the White House announced his resignation, McElroy said he planned to end his government task during the evening and witness the swearing in of Gates tomorrow at a ceremony in the executive mansion.

Gates, an investment banker in private life, recently has been serving as deputy secretary after a previous stint as secretary of the Navy.

'Antarctic Pact' Signed

WASHINGTON, Dec. 1--The Antarctic Pact--a pledge to keep the great frozen continent at peace--was signed today by 12 nations, including the United States and the Soviet Union. The treaty bans war bases, nuclear explosions and missile sites forever from a vast south polar region covering five million square miles.

One of the pact's most significant aspects, U.S. officials said, is its provision for a revolutionary system of international inspection in Antarctica. It gives each of the 12 nations the right, on mere advance notice, to check the other's installations, equipment, ships and planes in the Antarctica at any time.

Cuban Trial Opens

HAVANA, Dec. 1--An American exassociate of Fidel Castro appeared before a military tribunal late today to defend himself against a possible death sentence. He is accused of counter-revolutionary activity.

Rafael del Pine, 33, of Miami, Fla., a Cuban-born naturalized U.S. citizen wounded by police bullets in his capture last July, went on trial with Luciano Lineras Gastell, who was a Havana policeman during Fulgencio Batista's dictatorship.

The military prosecutor has demanded death for both defendants.