SUDDENLY, IF NOT entirely unexpectedly, the makings of a major crisis in southern Africa have been dumped into the new administration's lap. This is the evident meaning of Britain's announcement that its efforts to negotiate a peaceful transition to majority rule have collapsed. The outlines, but only the outlines, of a policy of the Carter administration's own were disclosed yesterday by Ambassador to the United Nations Andrew Young.

Attributing Britain's announcement to its ambassador's "fatigue," Mr. Young said the parties still had enough common interest to push towards a negotiated settlement. South Africa, he said, would be expected to deliver Rhodesia back to the table. Implicit was a determination to prevent Prime Minister Ian Smith from taking his preferred route of dealing with certain "moderate" blacks in a manner of his own choosing. At the same time, Mr. Young made clear that the United States does not accept the African front-line states' recognition of the "Patriotic Front," which includes the guerrillas, as in effect the coming government of Zimbabwe - the African name for Rhodesia. Talks leading to a transitional administration, in turn leading to a popularly chosen government, remain the route the United States prefers.

What is unclear is whether the Carter administration will press something like the original Kissinger plan, which Ian Smith broadly accepted, or the later British proposals, which were devised to satisfy the frontline states when forces in the Patriotic Front rejected the Kissinger plan. It was that plan's offer of guarantees to the white minority that won it Ian Smith's favor. It was the Front's fear that Ian Smith would exploit those guarantees to consolidate his own power that produced the Front's rejection. Ambassador Young indicated yesterday that it is Ian Smith who must to bend and accept the British proposals, rather than the Front bending and accepting the Kissinger plan. But this point, which is the crucial one, has not yet been made explicit. Mr. Smith's argument is that since he has accepted the essentials of the Kissinger plan, the United States cannot let him down.

So Mr. Carter is, very quickly, getting a taste of what the real world offers: tough choices. A white regime with a well-earned reputation for neglecting legitimate black demands is posing to him what it bills as a test of American good faith. The nationalists pose their case as a test of the new administration's professed devotion to liberation and majority rule. The administration's first response, namely, the careful statement of Ambassador Young, attempted to navigate between these shoals, but the administration is still far from shore.