* Watchdog offers no reason for early exit by Melanie Aitken
* Deals under review include TMX-Maple, Viterra asset sales
* In job since 2009, Aitken says she accomplished her goals
OTTAWA, June 28 (Reuters) - The head of Canada's antitrust watchdog will quit two years before her term ends, a surprise move announced on Thursday as her office re views a C$3.8 billion ($3.7 billion) bid to buy the operator of the Toronto Stock Exchange, among other deals.
Melanie Aitken, head of the Competition Bureau, will step down on Sept. 21, her office said in a statement. She joined the bureau in 2005 and took on the top job there in August 2009 for a five-year term.
The Bureau declined to comment on the reason for her decision. Aitken said in the statement that she and her staff had accomplished their goal of enhancing enforcement at the Bureau.
"Her many contributions to the Competition Bureau over the last seven years have made a lasting impression, " said Industry Minister Christian Paradis.
The Bureau is an independent law enforcement agency charged with reviewing major business deals to assure fair competition in the marketplace. It has the power to challenge proposed acquisitions and refer them to a special competition tribunal or negotiate conditions with the companies involved in the deal.
The bureau is still studying a bid by a consortium of Canadian banks, called Maple Group, to buy stock exchange operator TMX Group Inc. It has said rules under which the Ontario securities regulator said it would allow the deal to proceed would allay its concerns if accepted.
The bureau is also looking a proposal by Glencore International Plc to sell some of the assets owned by Canadian grain handler Viterra - which Glencore aims to acquire - to two other Canadian companies, Agrium and Richardson International Ltd. [ID: nL1E8HDIYV]
Bell Canada's parent BCE Inc also needs clearance from the watchdog on its purchase of Astral Media Inc in a C$3 billion deal.
The bureau is currently seeking to address competition concerns in the airline, telecommunications and credit card industries, according to the statement.