UHD display problem

hi there,

just connect my new Dell UP3216Q monitor to my mac book pro.

As shown here (Use 4K displays and Ultra HD TVs with your Mac - Apple Support ), it should work with the display port at 60hz (monitor is SST )

However i didn't manage to get this frequency and be stuck at 30z .( no possibility of changing in the display preferences )

Is anybody use this kind of monitor or managed to change the Hz ?

thanks

MacBook Pro (Retina, mi-2012)

2,6 GHz Intel Core i7

16 Go 1600 MHz DDR3

NVIDIA GeForce GT 650M 1024 Mo

OSX 10.11.1 (15B42)