Against the backdrop of the Soviet brigade in Cuba and turmoil in Iran, Marine leaders disclosed yesterday that they had been ordered to organize a 50,000-man spearhead for President Carter's Rapid Deployment Force.

The first of three brigades of 16,500Marines will be ready for airlifting to distant trouble spots by 1983, Maj.Gen. Paul X. Kelly, Marine planning chief, said yesterday in describing the corps' role in the quick reaction force.

Although planning for this outfit, which the Army calls the Unilateral Corps, preceded the current crisis in Iran, Pentagon officials said that rising concern about the oil-rich Persian Gulf is providing a sense of urgency to implement these plans. The basic idea is to give the president more military options in the Third World than he has now.

Each brigade would be supported by five supply ships deployed ahead of time near likely trouble spots around the world. Kelley said a Marine brigade could operate on its own, though presumably not fight any kind of major battle, for 30 days before having to depend on the ships for resupply. o

White House budget chiefs, in reviewing the Pentagon's fiscal 1981 money request, to be sent to Congress next month, have given their blessing to the Rapid Deployment Force. They have approved about $80 million to start developing a successor to the C5 transport plane for airlifting troops and supplies and $220 million for the first two of a fleet of cargo ships.

Defense officials said yesterday thatover the next several years about $6 billion will go for the new CX transport aircraft, with about 50 planes costing $180 million each. Another $3 billion in the quick reaction force account is earmarked for 16 ships loaded with Marine gear and pre-positioned near likely areas of conflict.

The Army's 82nd Airborne Division and armored units from Ft. Hood, Tex., and Ft. Carson, Colo. are expected to be designated as part of the force. Their heavy tanks and other weaponry are expected to take up most of the space on the fleet of CX planes.

Navy leaders are cool to the idea of building special cargo ships for the Marines and deploying them near likely trouble spots. The admirals complain that such ships would take money out of their already-strained shipbuilding budget and predict that warships ultimately would have to be assigned to protect them.

Defense Secretary Harold Brown has overridden these objections. Not only has he directed the Marine Corps to bring its 50,000-member special airlift force into being by the mid-1980s, but he approved extra money so the corps does not have to go through with its planned cut of 10,000 men.

To ease the corps' budget crunch, Marine Commandant Robert Barrow had decided to reduce his force from 189,000 to 179,000 by the end of fiscal 1981. The extra money approved by Brown, Kelley said, will enable the Marines to field a force of 185,200, a cut of only 3,800.

Kelley said the Marines wanted to brief reporters on the Rapid Deployment Force because some "of our friends" had gotten the idea that the corps' mission was being changed. He said that, on the contrary, the corps is enthusiastic about its new role.

"In looking at the 1980s," Kelley said, "It becomes obvious that we need a sharper focus for the Third World."

The United States would "do well to sharpen that focus before we let it [Third World] slip through our fingers," he said.

Kelley said the Marines have traditionally designed their forces to go into battle in a hurry but that a "glaring deficiency" is the shortage of long-range planes to carry them to crises.

He said the present airlift force of 70 C5 transports and 234 C141s is not enough to carry troops and weaponry to distant points. The new CX cargo plane, he added, will help close that gap.

To fly one brigade of 16,500 Marines in a quick reaction unit would require 130 round trips of the following aircraft: 14 C5s; 102 C141s and 14 widebodied civilian tranport planes pressed into service as part of what is called the Civil Reserve Air Fleet.

Kelley said the Marine brigade would not be targeted on any specific area of the world but could go anywhere. Asked about getting to the Persian Gulf, he said that one option would be to fly Marines from Okinawa to Diego Garcia in the Indian Ocean.

The transport planes, as well as the fighter-bombers which would support the Marine ground troops, would have to be refueled en route by Air Force flying tankers.

Although he did not volunteer it, when pressed by reporters Kelley said Diego Garcia could indeed become the "linchpin" as the United States increases its military presence for conflicts in the Mideast and Africa.

In the past, some members of Congress have been reluctant to increase the U.S. military presence on the island for fear of making the Indian Ocean a new area of confrontation for the superpowers, the United States and Soviet Union. Sen. Sam Nunn (D-Ga.), is leading a delegation of the Senate Armed Services Committee to the Persian Gulf early next year to assess the need for addition U.S. military facilities in the area, including Diego Garcia.

One staffer of the Senate Foreign Relations Committee, who observed the Senate effort of recent years restrict military activities on Diego Garcia, said: "That was a different era. Iran has changed all that."