4k-50hz rMacbook Pro 15" early 2013

Hey guys,

i own an early 2013 retina macbook with dedicated graphics. if i run bootcamp with windows 7, i am able to run 4k resolution at 50hz through the hdmi port. but if i run el capitan i am only able to select a max of 25hz in 4k. any idea how i can run those 50hz in mac os?.

any help is appreciated.