MIDEAST STOCKS-Petchems lift Saudi, other Gulf markets mixed

* Saudi's trading volume highest since last August * Dar Al Arkan surges despite decline in earnings * Abu Dhabi Islamic Bank gains after strong results * Lowest Egyptian volume since mid-March devaluation * Qalaa Holdings drops after 2015 earnings By Celine Aswad DUBAI, April 24 (Reuters) - Saudi Arabia's stock market rose strongly in heavy trade on Sunday as firm oil prices boosted petrochemical shares, while other markets in the region were mixed. Riyadh's main index jumped 1.7 percent to 6,6