It's most likely your mast collar is anodized/cast aluminum
. The loads imposed on the individual block shaft housings would have to be great to break/crack unless there was a specific point loading at the break during a traumatic event(s). I would check your alignment from the halyard
exit plate holes on the mast to be certain they are in alignment with the mast blocks rather than at an aggressive angle. If you are not the original owner of the boat, the block base may have been broken when the stainless shaft of the block became corroded by galvanic corrosion
in the collar and someone tried to force it to remove the block. This was done on my boat by the PO. Hope this has given you a couple of ideas. Good luck and good sailing.