A few minutes into Saturday morning's SALT meeting around a polished table at the Soviet mission in Geneva, Foreign Minister Andrei Gromyko turned with full force to an issue of treaty timing that had not seemed to be a serious problem in the past.

Before his speech was over, U.S. negotialors knew that their high hopes for early completion of the strategic arms limitation treaty and a January summit conference had been abruptly dashed.

Gromyko had brought up the timing question briefly during Friday's meetings with Secretary of State Cyrus Vance, and it had been mentioned to reporters later that day in a Washington background briefing by presidential assistant Zbigniew Brzezinski.

At that point, thought it had been considered a matter to be resolved at a summit meeting of President Carter and Soviet leader Leonid Brezhnev, rather than an issue that might be employed to postpone the summit.

U.S. officials cannot be certain why the Soviets introducted a complicating problem just when the SALT negotiations were on the verge of success. But some of them have a compelling theory. In this view, the extraneous issue was carefully chosen by Kremlin leaders midway in the Geneva talks to block the SALT agreement and the January summit without raising fundamental questions of nuclear arms control.

The reasons, according to a view that has gained considerable acceptance in U.S. officialdom, lies in a bolt-from-the-blue: normalization of Washington's realtions with Peking and the announcement of a U.S. trip by Chinese Vice Premier Teng Hsiao-ping to begin Jan. 29.

Despite Carter's assertion that the China initiative will have no influence on the SALT talk, evidence is piling up that the Soviets feel hemmed in, taken for granted, possibly even tricked by Washington's sudden move.

Before the China announcement, Moscow had let it be known that it was interested in an early Carter Brezhnev summit to sign the SALT treaty, with mid-January the earliest possible date if all wen well at Geneva. After receiving that word, however, Washington announced a late January Carter-Teng meeting which could upstage the Brezhnev visit.

One sign of Moscow's concern was the highly unusual Tass statement of last Thursday that amplified Brezhnev's private message to Carter, and which, in effect, corrected Carter's public rendition of it.

Brezhnev made it clear, according to Tass, that while normalization of diplomatic relations is legitimate, "the Soviet Union will most closely follow what the development of American-Chinese relations will be in practice and from this will draw appropriate conclusions."

Despite major progress on several issues during the Geneva meetings, the new problem raised by Moscow Saturday is expected to put off final agreement on a SALT treaty until after Teng's visit to the United States a month from now.

Moscow, therefore, will be more than an interested onlooker to the Washington-Peking goings-on next month. Given the new status of the SALT negotiations, Moscow will be able not only to "draw appropriate conclusions," but also to act quickly on these conclusions in ways that touch the central nerve of U.S.-Soviet relations.

The surprise issue brought up by Gromyko, it was learned, has to do with the expiration date of the SALT treaty "protocol" that covers several of the most sensitive nuclear arms issues. Th United States maintains that the deadline by which the Soviets must dismantle 150 of their strategic nuclear missiles under the SALT accord should be no later than the expiration of the protocol.

Orginally the protocol was to run for three years after the end of the U.S.-Soviet interim agreement, which would make its termination date Oct. 1, 1980. Since the treaty has been so long delayed, the Soviets want the protocol extended to three years after the ratification of SALT, which would take it to mid-1982 under the most optimistic assumptions.

The United States some time ago proposed Jan. 1, 1981, for termination of the protocol, and recently offered to extend this to June 30, 1981, still about a year short of the Soviet demand.

The Soviets have agreed to a fixed end date for dismantling their missiles, according to informed sources. This date is reported to be between June 30, 1981, and June 30, 1982.

The matter of these dates involves no fundamental issue, and until Gromyko spoke strongly on Saturday there seemed no reason the question could not be easily resolved. Now the problem is more serious, and it will be up to Moscow to decide when to move.

What was considered a more serious problem before Geneva-the question of Soviet encoding of some missile testing data, or telemetry-is reported to have been resolved by the two sides at Geneva. This was made known yesterday in response to pleas in the U.S. press for the united states to insist on the issue.

The basis for the solution appears to have been a pledge by Moscow not to encode missile testing data when such action would interfere with verification of the U.S.-Soviet agreements under SALT II. It is uncertain, however, whether the two sides fully agree on the details of just when encoding would be permitted, and when it would be forbidden.

Tentative Soviet acceptance of a deal on the missile data issue-and possibly on some other outstanding questions-is what caused American officials to be almost euphoric last Friday about the chances for immediate agreement on a SALT treaty. Some of the most cautious and usually skeptical officials, at that point, were convinced that the Soviets had come to Geneva to wind up the six years of SALT II negotiations.

Saturday morning they learned differently when Gromyko suddenly turned uncooperative and brought new problems to the fore. With nearly all the big issues now resolved, however, the Soviets can decide to be reasonable on some smaller ones and permit completion of the treaty and a Carter Brezhnev summit whenever-and if ever-they decide the time is right.