Chief Justice Warren E. Burger refused yesterday to block an abortion contemplated for a 19-year-old unmarried McLean woman who, because of an apparent brain ailment, can't say whether she wants one.

The woman's mother, who was appointed the daughter's legal guardian when the daughter became ill a month ago, is said in court documents to be planning an abortion for the young woman this morning. A court-appointed lawyer for the daughter sought Burger's intervention on the grounds that her rights would be violated by an abortion to which she did not consent.

The lawyer, Steven Garver, has spent the last few days racing from courthouse to courthouse trying to limit the mother's authority over the daughter. He went to Burger after being turned down by the Fairfax County Circuit Court and a Virginia State Supreme Court panel. The chief justice, without comment, also rejected his plea late yesterday.

The 19-year-old, called "Jane Doe" in court papers, has been in a semicatatonic state in a Fairfax hospital since contracting an as-yet-undiagnosed illness.Garver said she is conscious but cannot talk and may not be able to hear.

But "she has a personal right of control over her body," he told Burger in the emergency appeal. "There are certain fundamental or basic rights which are so personal and peculiar to the individual" that even a legal guardian, like the woman's mother, should not be able to assume, he said. l

Legal guardians are generally appointed when a person, even an adult, is adjudged incompetent by a court. A court-appointed "guardian ad litem," Garver in this case, also is often used in legal proceedings to insure that the interests of the "incompetent" are represented.

Legal guardianship confers broad powers on the guardian. In one highly publicized case recently, a Maine father obtained guardianship over his mental-patient son and had his son's marriage annulled. The guardianship in the Fairfax case gave the mother full authority to make medical decisions on behalf of the 19-year-old.

It made no exemptions for abortion or any other procedure. When Garver heard that an abortion was being contemplated, he sought such an exemption. Martin E. Morris, the lawyer for the mother, declined to say whether an abortion would be performed. But he said that the mother's guardianship permitted "any medical procedure recommended by a doctor."

Morris said the daughter was a perfectly normal, "healthy teen-ager" until she fell ill and was hospitalized Feb. 2. He said he believes she may have a brain disease like encephalitis.

Garver said the woman was 20 weeks pregnant and the fetus will reach viability on March 21, after which an abortion might not be feasible. In his petition to Burger, Garver said he was not arguing that an abortion be prohibited. He said that before one is allowed, however, there should be "competent medical evidence that it is necessary for her medical or psychological welfare."