Often considered not quite part of the real Virginia - is, economically, a distinctly wealthy enclave in the state of widely varying incomes according to an analysis of state income tax returns.

In Fairfax County, for instance, 46.2 per cent of families filing returns in 1975, reported incomes of $25,000 or more.

In Grayson County, on the North Carolina-Tennessee border in Southwest Virginia, only 1.4 per cent of families reported incomes of $25,000 or more, the lowest in the state, according to a study of income distribution by the Tayloe Murphy Institute at the University of Virginia in Chorlottesville.

The percentage of families earning $25,000 or more was 36.5 in Fairfax City, 35.7 in Arlington, 32.6 in Fall Church; 30.3 in Alexandria; 24.7 in London County and 22.0 in Prince William County, according to the study.

No other jurisdiction on the state topped 20 per cent in that high income bracket. Richmond had 14 per cent Norfolk 10.5 Virginia Beach 17.1 Charlottesville 13.3 and Henrico County 17.0.

The study directed by John L. Knapp, used adjusted gross incomes from all state tax returns from 1975. It does not include nontaxable itemslike Social Security payments, welfare or persons with income below taxing requirements.

K'rapp said yesterday that the retruns of married people represent "a fair proxy for family income, but not a perfect one."

Its figures slightly from the statistics reported by the Census Bureau and the U.S. Department of Commerce but have the advantage of coming directly from consistently state records a opposed to surveys in which people are asked how much they earn. Knapp said.

For the state, the median family income was $14.219 with Northern Vir (See INCOME, C3, Col.1> ginia having the five highest localities: Fairfax County, $23,875: Fairfax City, $21,348; Arlington County, $19.936; Falls Church, $19,374; and Alexandria, $18,613.

The median is that point at which half the returns are higher and half lower. The figures used refer to adjusted gross income of married people.

The five lowest median family income figures were recorded in Highland County with $7,868; Grayson County; $7,901; Charlootte County, $8,582 and Galax $8,6140.

Statewide, 5.4 per cent families had income less than $3,000; 5.8 per cent had $3,000 to $4,999; 19.5 per cent made $5,000 to $9,999; 22.8 per cent $10,000 to $14,999; 19 per cent had $15,000 to $19,999; 115 per cent had $20,000 to $24,99; and 15.9 per cent made $25,000 or more.

In Fairfax County, while 46.2 per cent made more than $25,000 only 2.2 cent made less than $30,000 and 1.9 per cent fell in the $3,000-$4,999 bracket. Some 10.6 per cent made $10,000-$14,999; 15.2 per cent had $15,000-$19,999; and 16.9 per cent had $20,000-$24,999.

By contrast, only 2.7 per cent Grayson County families made $20,000 or more; 6.1 per cent had $15,000-$19.999; 24.7 per cent made $10,000-$14,999; 39.1 per cent made $5,000-999; 12.6 per cent ade $3,000-4.999; and 12.7 per cent made less than $3,000. Other N.Va. Income Statistics

The percentage for other Washington-area jurisdictions:(TABLE) (COLUMN)LESS(COLUMN)$3,000(COLUMN)$5,000(COLUMN)$10,000(COLUMN)$15,000(COLUMN)$20,000(COLUMN)$25,000 (COLUMN)THAN(COLUMN)TO(COLUMN)TO(COLUMN)TO(COLUMN)TO(COLUMN)TO(COLUMN)HOR (COLUMN)$3,000(COLUMN)$4,999(COLUMN)$9,999(COLUMN)14,999(COLUMN)$19,999(COLUMN)$24,999(COLUMN)MORE SLAlexandria 3.5(COLUMN)3.6(COLUMN)13.0(COLUMN)17.4(COLUMN)17.4(COLUMN)14.9(COLUMN)30.3 Arlington(COLUMN)3.5(COLUMN)3.6(COLUMN)12.7(COLUMN)14.4(COLUMN)16.0(COLUMN)14.1(COLUMN)25.7 Fairfax City(COLUMN)2.5(COLUMN)2.4(COLUMN) 8.7(COLUMN)13.4(COLUMN)18.1(COLUMN)18.5(COLUMN)36.5 Falls Church(COLUMN)4.0(COLUMN)3.8(COLUMN)12.9(COLUMN)14.2(COLUMN)17.4(COLUMN)15.1(COLUMN)32.6 Loudoun(COLUMN)3.7(COLUMN)3.3(COLUMN)13.1(COLUMN)15.7(COLUMN)20.6(COLUMN)18.7(COLUMN)24.7 Prince William(COLUMN)2.8(COLUMN)2.8(COLUMN)10.0(COLUMN)16.2(COLUMN)25.2(COLUMN)21.0(COLUMN)22.0(END TABLE)