MIDEAST STOCKS-Saudi index retreats from multi-year peak; Gulf rivals also fall

* Petrochemicals and banks weigh on Saudi index * SHUAA Capital surges on merger talks * Banking shares fall in Abu Dhabi DUBAI, March 24 (Reuters) – Saudi Arabia’s stock market closed lower on Sunday, along with other Gulf markets, having ended the previous week at its highest in nearly four years after inclusion in the FTSE Russell’s emerging-market index. Saudi’s Tadawul index will have a weighting of 2.9 percent in the FTSE Emerging All Cap Index and this year will j