* Investa now recommends Oxford's A$3.35 bln buyout
(Adds economist quote, updates shares to close):
SYDNEY (Reuters) - Private equity giant Blackstone Group (BX.N) has quit a takeover battle for Australian office-block owner Investa Office Fund (IOF.AX) after it was gazumped by Canadian landlord Oxford Properties Group’s A$3.35 billion ($2.4 billion) bid. Blackstone told Investa it would not match the offer, Investa said on Thursday, all but handing the sought-after target to Oxford as office rents boom, especially in Sydney where Investa’s 20 towers are concentrated.