DUBAI, Sept 26 (Reuters) - Saudi banking shares rallied on Monday after the central bank said it would deposit about 20 billion riyals at commercial lenders and introduce two new money market instruments to fight a surge in market interest rates caused by low oil prices. urn:newsml:reuters.com:*:nL8N1C10K5

Riyadh's main index .TASI was up 0.6 percent after 15 minutes of trade as four-fifth of the banking shares rose. Samba Financial Group 1090.SE was the top gainer in the industry, up 2.3 percent to 18.20 riyals ($4.85).

Banking shares have been underperforming the general index in several months, hit by an illiquid construction sector, which makes up a large portion of corporate loans. Samba, for example, is trading at a 26 percent discount to its average fair value as estimated by 13 analysts, according to Reuters data.

The petrochemical sector was also positive, with its largest producer Saudi Basic Industries 2010.se up 0.9 percent as Brent futures LCOc1 climbed back over $46 a barrel.

Dubai's main index .DFMGI was down 0.4 percent with nine-tenth of the traded shares declining in low volumes. Gulf General Investment GGIC.DU , an industrial conglomerate, was the biggest loser, dropping 2.6 percent. Emaar Properties EMAR.DU lost 0.6 percent.

Blue chips weighed on Abu Dhabi's index .ADI which retreated 0.4 percent. Etisalat ETEL.AD , the telecommunication giant, dropped 0.3 percent and Aldar Properties ALDR.AD lost 1.1 percent.

($1 = 3.7494 riyals)

(Reporting by Celine Aswad; Editing by Andrew Heavens) ((celine.aswad@thomsonreuters.com)(+9715 62247653)(Reuters Messaging: celine.aswad.thomsonreuters.com@reuters.net))