without detailed photographs, I'm in doubt between:
- Breccia of tectonic origin, probably due to an old cemented fault. The cement looks to me rich in iron, hence the colour.
- High energy conglomerate, so the hardened sediments of a mass movement (something like a landslide).
Nevertheless, I believe it's the second option. There are few blocks that seem too big for a tectonic breccia (at this scale), and I can't see any alignment of the blocks, which would be expected. For a conglomerate of high energy, alignment isn't needed. Besides, a cement rich in iron would imply that the rock has been subject to the action of hydrothermal fluids, which would mean that has been affected by two not-so-common processes.
So, just with these photos, I think it's a conglomerate.
Answering your last sentence, it can happen, but I've never seen it in that volume. Usually are small (and scarce) pieces - xenoliths - amongst the igneous mass.