** Shares of Australia's Magellan Financial MFG.AX fall 4.1% to A$11.9

** Stock set for weakest trading session since early Dec

** Says veteran Gerald Stack is stepping down from the asset management firm after 18 years of running its crucial infrastructure business

** Stack ran A$16.6 bln ($10.35 bln) infrastructure unit and retained funds and flows despite the rocky global equities business

** Infrastructure accounts for 41% of Magellan's A$38.6 bln of funds under management

** The move comes along with MFG's global portfolio manager Nikki Thomas moving from global equities to investment strategy

** Stock up 7% YTD

($1 = 1.6036 Australian dollars)

(Reporting by Rishav Chatterjee in Bengaluru)

((Rishav.Chatterjee@thomsonreuters.com;))