The board may establish a retirement system for the officers and employees of the district and provide for the payment of annuities, pensions, retirement allowances, disability payments, and death benefits or any of them, provided that the adoption, terms and conditions of any retirement system covering employees of the district, a majority of who are represented by a labor organization in accordance with Chapter 4 (commencing with Section 25051) of this part, shall be pursuant to a collective bargaining agreement between such labor organization and the district.