Looks like some kind of intrusion of magma (the pale rock) into the darker rock. you can see how all the veins seem to originate from the pale rock, also the broken-of dark part in the pale rock seems to indicate it, could be part of the original rock, that surrounded the magma, before breaking of.

Some actual geologist might want to give their opinion though. I only had like two years of geology at university before shifting my studies toward crystallography and crystal growth.