The Virginia Senate approved a constitutional amendment early today that would freeze state taxes at about 6.5 percent of total personal income. The Senate action came seven hours after the House of Delegates rejected a similar constitutional proposal by a vote of 52 to 43.

The Senate, by a vote of 27 to 11, approved a proposal that assembly leaders said only weeks ago had almost no chance of passage. It would apply a stringent limit on spending in a state that has prided itself on decades of fiscal conservatism. The measure now goes back for reconsideration by the House.

Also in the Senate, a measure was approved last night that would reduce the penalties for possession and personal use of marijuana, eliminating jail terms for first offenders. The marijuana penalty bill now goes to the House.

The chief patron of the tax limitation amendment, Sen. Elliot S. Schewel (D-Lynchburg), told the Senate that the proposed ceiling is based on a carefully thought-out formula that offers a safeguard against free spending General Assemblys in future years.

However, Sen. Hunter B. Andrews (D-Hampton) said, "Even the proponents of this amendment have admitted that the consequences of this resolution could be a little fuzzy. But they say, 'Let's send it out to the people.'" Then, dropping his voice, he added, "Let's get by this election."

Sen. Virgil H. Goode Jr. (D-Rocky Mount) called on the Senate not to hide behind an Andrews motion to recommit the amendment to the Senate Finance Committee. "I'm not going to hide. We've hid on enough things this year. We shouldn't be sending this back to some committee where there's going to be a lot of secret goings-on."

Of the eight Northern Virginia senators, Omer L. Hirst (D-Fairfax) and Joseph V. Gartlan (D-Fairfax) were the only two voting against the amendment. Sen. Charles L. Waddell (D-Loudoun) abstained, and the other five voted in favor.

The tax limitation proposal was generated by a group of Lynchburg business leaders and was given momentum in this assembly session by passage of Proposition 13 in California. The California law, enacted by voters, imposed a ceiling on property taxes.

To become a part of the state constitution, the amendment would have to be approved by both houses of the assembly twice, with an election of members of the House of Delegates falling between the two votes. Then, the amendment must be approved in a statewide referendum.The earliest effective date would be Jan. 1, 1981.

The marijuana measure, approved by a vote of 23 to 15 last night in the Senate, would eliminate jail terms for the first offense of growing the substance for personal use and would provide for lower maximum sentences for distribution of the drug.

Sen. Frederick C. Boucher (D- Abingdon), who headed a special study commission that drafted the bill, assured some leery legislators to night that the measure "does not legalize marijuana. It is not being decriminalized."

Instead, Boucher said, the bill is intended to lessen penalties for more minor marijuana abuses in order to encourage law enforcement officials to concentrate their efforts on arresting those who traffic in the drug.

Virginia's current marijuana laws provide for a maximum penalty of one year in jail and a $1,000 fine for possession of marijuana for personal use and sentences of up to 40 years for cultivation for distribution of the drug.

The proposed maximum sentence for the first conviction of possession would be a fine of $500. Ten other states have eliminated jail terms for such first offenses.

In other assembly action yesterday, the Senate hotly debated then approved creation of a special judicial nominating commission designed to emphasize merit over political connections in filling bench vacancies. The measure passed 23 to 17 and goes to the House where it is given little chance of passage.

Democratic majorities in both the House and Senate now elect all state judges following the recommendations of lawmakers from those areas where a judgeship vacancy exists. But the method has been highly criticized for perpetuating a political approach to picking judges.

As proposed, the General Assembly would still make the final selection of judges but would do so only after candidates were nominated and screened by a special commission composed of one member from each congressional district in the state and five lay members. The commission would, in turn, rely on recommendations from local advisory boards.