Pope John Paul II, assuming the role of papal diplomat, announced today that he will attend an important conference of Latin American bishops in Mexico next month and will immediately send a special emissary to mediate a border dispute between Chile and Argentina.

In his Christmas message to cardinals assembled here, the Pope said he would travel to Mexico City and nearby Puebla at the end of January to open the conference of bishops from throughout Latin america, where nearly half of the world's 700 million Roman Catholics live.

It will be the first papal journey outside Italy since November 1970, when Pope Paul VI spent nine days visiting seven countries in the Far East. Paul VI, who captured world wide attention with his travels, also made the first papal trip to Latin American to open the last conference of Latin American bishops in Colombia in 1968.

It was at the 1968 conference that the Roman Catholic hierarchy decided to depart from its historical alignment with the ruling class of Latin America and to press for political, economic and social reforms. Next month's conference which begins Jan. 27, is to explore the role of the clergy in pressing for social justice.

The church's new role in Latin America has brought it into conflict with many governments there, especially those in Brazil, the world's largest Roman Catholic country, and in Chile, where the church has been active in fighting for human rights and tracing missing persons since the right-wing military takeover in 1973.

Pope John Paul II said yesterday he was taking on the "pilgrim's staff of peace" from Paul I and stressed there cannot be peace without justice and respect for human rights.

He announced that he was sending a personal envoy with "all necessary urgency" and with the acceptance of both Chile and Argentina to try to mediate their dispute over several tiny islands in the Beagle Channel area at the tip of South America.

The area is of strategic value because it controls the Strait of Magellan, the passage around SouthenAmerican, is a rich fishing ground, and may contain significant undersea of deposits. The Beagle Channel Islands were awarded to Chile by British arbitration, but Argentina has disputed that ruling.

The pope acted after tension following the collapse of negotiations this week between Chile and Argentina had produced fears and rumors of armed clashes. Troops of both nations have been placed on alert and Chilean and Argentine naval units were reported in the area.

Chile requested an emergency meeting of the Organization of American States tody in Washington to discuss the problem. The Chilean Foreign Ministry also asked today that the 1947 Inter-American Reciprocal Assistance Treaty be put into effect. The treaty commits most Latin American nations and the United States to defend any one of hem coming under attack.

"I let both countries know of my desire to send a personal representative," the pope said, "to have direct contact with tthe situation, understand the positions, and perhaps arrive at an honourable and peaceful solution."

The pope will begin his trip to Mexico in late January with a visit to Our Lady of Guadalupe just northwest of Mexico City, one of the most famous shrines in Latin America. Thousands of pilgrims, many of whom walk hundreds of miles, visit the shrine each month.

Mexico has about 46 million Catholics among its population of 61 million, although church and state were separated there in the 19th century and the clergy is prohibited from wearing vestments in the streets.

"There will be an awakening of faith, but I don't tahink there will be any other modification" in government policy from the Pope's visit, said Msgr. Guillermo Schulenburg, abbott of the Bascilica of Guadalupe.